Expression of keratins in cutaneous epithelial tumors and related disorders – distribution and clinical significance

Authors


Ichiro Kurokawa, Department of Dermatology, Mie University Graduate School of Medicine, 2-174, Edobashi, Tsu, Mie 514-8507, Japan, Tel.: +81 59 232 1111x6421, Fax: + 81 59 231 5206, e-mail: kuroichi@clin.medic.mie-u.ac.jp

Abstract

Please cite this paper as: Expression of keratins in cutaneous epithelial tumors and related disorders – distribution and clinical significance. Experimental Dermatology 2010.

Abstract:  Keratins are a highly diverse family of cytoskeletal proteins and important markers of epithelial cell differentiation. In this review, applying the new keratin nomenclature recently introduced, we summarize and discuss the distribution and significance of keratin patterns in cutaneous epithelial tumors in relation to the epithelial structures of normal human skin. The available literature data show that the analysis of keratin profiles broadens our understanding of the differentiation, nature and histogenetic origin of the various, highly singular epithelial tumors arising in the skin. Moreover, keratins may aid in histological diagnosis and, in certain instances, may be helpful for the recognition of tumor malignancy and aggressiveness. Furthermore, we briefly address the topic of keratin-related skin disorders.

Definition

Keratins (K) are a large family of cytoskeletal fibrous proteins present in epithelial cells, and keratin filaments represent one type of intermediate sized filaments, that is, long and unbranched filaments of ∼10 nm diameter (1–6). Keratin filaments are an essential component of normal tissue structure and play an important function in epithelial cells. Keratins not only develop one part of the cytoskeleton of epithelial cells, but also serve as an important system of differentiation markers able to determine the epithelial cell type and the state of differentiation during embryonic development and, in epithelial self-renewing tissues, during their terminal differentiation. Keratins may also aid in determining the origin of epithelial tumors and may be useful diagnostic and, in some instances, prognostic markers of benign and malignant epithelial tumors. Furthermore, mutations in keratin genes are associated with specific tissue-fragility disorders.

History of keratins

Epithelial keratins have first been described as tonofilament-forming proteins related to hard α-keratin of hair and have also been called cytokeratins (CK) to distinguish them from the hair keratins (1,2). In 1982, Moll et al. (3) have classified the epithelial keratins into 19, and later (7) 20 subtypes according to their molecular weight and isoelectric pH in an attempt to clarify the diversity and expression patterns of keratins from normal human skin and internal epithelia, tumors and cultured cells. These 20 keratins have been designated K1–K20 (CK1–CK20) and have been classified into two groups (now known to represent sequence homology groups): type I and II. Type I keratins (K9–K20) are acidic keratins, whereas type II keratins (K1–K8) are basic-to-neutral keratins. The distinction of these two types also holds for the ‘hard’ alpha-keratins of hair and nail (hair keratins), and previously the two hair keratin families have been designated as Ha (acidic, type I) or Hb (basic-to-neutral, type II) (8). As a unique feature, among cytoskeleton and intermediate filament proteins, type I and type II keratins are always expressed in pairs and indeed keratin filaments are constituted via heteropolymeric pair formation of type I and type II (1:1) molecules [see Ref. (6)].

New keratin nomenclature

Keratin genes account for most of the intermediate filament genes (9), and in the human genome not <54 functional keratin genes have been identified (4). In 2006, a new consensus nomenclature for mammalian keratins adhering to the guidelines of the Human and Mouse Genome Nomenclature Committees was proposed (4). This consensus nomenclature, which includes both epithelial and hair keratins, revises and extends the 1982 system. The keratin genes are numbered KRT1, KRT2 etc., the corresponding keratin proteins K1, K2 etc. Among the 54 functional human keratin genes there are 28 type I genes and 26 type II genes (Table 1). In the human genome, all type I keratin genes with the exception of the K18 gene are located on chromosome 17q21.2, whereas all type II keratin genes and the K18 gene are located on chromosome 12q13.13. For some of them, the function and significance of the encoded keratin proteins are still unclear (4). The cell and tissue type-dependent expression and distribution of the human keratins is summarized in Table 2, and a more detailed overview on the hair keratins and the hair follicle-specific epithelial keratins and their expression specificities is presented in Table 3. In the following, the most salient features of keratins relevant in skin are outlined [for more details and references, see ref. (6)].

Table 1.   Classification of keratins encoded by human functional keratin genes according to Schweizer et al. (4)
Human functional genes54   
Type I28Epithelial keratins17K9,K10,K12–K20,K23–K28
Hair keratins11K31,K32,K33a,K33b,K34–K40
Type II26Epithelial keratins20K1–K5,K6a,K6b,K6c,K7,K8,K71–K80
Hair keratins6K81–K86
Table 2.   Human epithelial (non-hair follicle) keratins and major sites of their tissue- and cell type-specific expression
Type I epithelial keratinType II epithelial keratin
Former NomenclatureNew NomenclatureExpression siteFormer NomenclatureNew NomenclatureExpression site
K9K9Spinous-granular layers in palmoplantar epidermis   
K10K10Suprabasal epidermal keratinocytesK1K1Suprabasal epidermal keratinocytes
K2eK2Upper spinous and granular epidermal layers
K12K12CorneaK3K3Cornea
K13K13Suprabasal layer in oral mucosaK4K4Suprabasal layer in oral mucosa
K14K14Basal keratinocytes in the epidermis, adnexal glandK5K5Basal keratinocytes in the epidermis, adnexal gland
K15K15Hair follicle bulge   
K16K16Spinous layer in palmoplantar epidermis, suprabasal layer in oral mucosa, sweat gland, lower hair follicle, outer root sheath, wound K6aK6aSpinous layer in palmoplantar epidermis, suprabasal layer in oral mucosa, sweat gland, lower hair follicle, outer root sheath, wound
K17K17Myoepithelial cells in sweat gland, lower hair follicle, outer root sheath, woundK6bK6b 
K6e/hK6c 
K7K7Simple (ductal) epithelia, secretory portion in sweat gland in skin
K18K18Simple epithelia, secretory portion in sweat gland in skinK8K8Simple epithelia, secretory portion in sweat gland in skin
K19K19Simple epithelia, secretory portion in sweat gland, outermost layer of hair bulge and outer root sheathK2pK76Gingiva and hard palate
K20K20Merkel cells, gastrointestinal epithelia, urotheliumK1bK77Eccrine sweat gland duct
K5bK78 
K6lK79 
Kb20K80 
Table 3.   Hair keratins and hair follicle-specific epithelial keratins according to Schweizer et al. (11)
Former nomenclatureNew nomenclatureExpression siteFormer nomenclatureNew nomenclatureExpression site
Hair keratins (Hair fibre keratins)
Type IType II
Ha1K31Entire cortexHb 1K81Mid cortex
Ha2K32CuticleHb 2K82Cuticle
Ha3-I, Ha3-IIK33a, K33bMid cortexHb 3K83Mid cortex
Ha4K34Upper cortexHb 4K84Absent from the hair follicle, present in filiform papillae of the tongue
Ha5K35Matrix, cuticleHb 5K85Matrix, cuticle
Ha6K36Mid cortexHb 6K86Mid cortex
Ha7K37Cortex of vellus hairs, medulla of sexual hairs   
Ha8K38Single cortex cells   
Ka35K39Cortex, upper cuticle   
Ka36K40Upper cuticle   
Hair follicle-specific epithelial keratins
Type IType II
K25irs1K25IRS (Henle, Huxley, cuticle), medullaK6irs1K71IRS (Henle, Huxley, cuticle), medulla
K25irs2K26IRS (cuticle)K6irs2K72IRS (cuticle)
K25irs3K27IRS (Henle, Huxley, cuticle), medullaK6irs3K73IRS (cuticle)
K25irs4K28IRS (Henle, Huxley, cuticle), medullaK6irs4K74IRS (Huxley)
 K6hfK75Companion layer, medulla

Type I keratins

Type I genes encode 17 epithelial keratins and 11 hair keratins (Tables 1–3).

Human epithelial keratins:  With regard to human epithelial type I keratins, the classical and well-known set comprises K9–K20, their designations being maintained in the new nomenclature (Table 2). K9 is specifically expressed in the palmoplantar epidermis whereas K10 is the common keratinization-associated keratin of suprabasal epidermal keratinocytes of all body sites (K11 does not exist as encoded by an own gene but is a variant of K10). K14 is a major keratin of basal keratinocytes, whereas K15 has been associated with stem cells of the hair follicle bulge. Hyperproliferative epidermal suprabasal keratinocytes typically express K16. K17 is inducible in activated keratinocytes and also is characteristic of myoepithelial cells. K18 is a widely distributed keratin of simple epithelia, which within the skin is mainly expressed in secretory cells of eccrine and apocrine glands. The same is true for the smallest keratin, K19, which also occurs in the outermost (basal) layer of the outer root sheath of the hair follicle. K20 is highly specific for Merkel cells. K25–K28 were assigned to K25irs1-4 (original names) and are differentially expressed in the inner root sheath of human hair follicles (10,11) (Table 3).

Human hair keratins:  The type I human hair keratins, formerly Ha1-8, Ka35 and Ka36, have now been numbered K31–K40. K33a and K33b are isoforms. These more sulphur-rich, ‘hard’ keratins exhibit differential expression patterns in the cuticle and cortex of the hair fibre (Table 3).

Type II keratins

Type II keratins comprise 20 epithelial and six hair keratins (Tables 1–3).

Human epithelial keratins:  Type II epithelial keratins were originally classified into eight subtypes (K1–K8) (3); most of them have kept their designations in the new nomenclature. Further type II keratins have been described more recently and to them new numbers have been given. K1 is the main keratinization-associated epidermal type II keratin, being the partner of the type I keratin K10. The recently discovered keratin K1b, which is expressed specifically in eccrine sweat gland ducts, has been renamed as K77 (12). K2e, now redesignated as K2, is correlated with advanced keratinocyte maturation. Palatal keratin K2p has been renamed K76. K5 is expressed in basal keratinocytes and a partner of the type I keratin K14. K6 is – together with K16 – associated with epidermal hyperproliferation. Variants of the K6 gene are now designated K6a (KRT6A) (which is the main variant), K6b (KRT6B) and K6c (KRT6C). K71–K74 have been assigned to the four type II inner root sheath keratins K6irs1-4 (13) (Table 3). The previous K6hf, which is expressed specifically in the companion layer of the hair follicle root sheath, has now been assigned as K75. K7 and K8 are simple-epithelial keratins which in the skin characteristically appear in secretory cells of eccrine and apocrine glands.

Human hair keratins:  The six human type II hair keratins, formerly Hb1-6, have been renamed as K81–K86 (Table 3). Similar to the type I hair keratins, they exhibit differential expression sites in the hair. As an exception, K84 appears to be restricted to the filiform papillae of the tongue.

Expression of a wide variety of keratins during vertebrate evolution

The complete sequencing of genomes of a wide range of animal species has enabled to compare information regarding the evolution of keratin genes during the course of vertebrate evolution; also, these data helped to better understand the variety of keratins. In the evolution of the early vertebrates, specifically fish and amphibians, the variety of type I and type II keratins was not as great as in mammals. The number of keratin genes, particularly those specific for hair follicles and hair shaft, has increased during the course of mammalian evolution. Specifically, compared with 16 type I and 7 type II keratins identified in zebra fish, and 19 type I and 6 type II keratins identified in puffer fish, 28 type I and 26 type II functional keratins have been described in two keratin gene clusters of the human chromosomes 17 and 12 (14–19). The wide diversity of keratins is unique to the keratin cytoskeleton and is not observed among other intermediate filament proteins. Although the number and types of keratin genes in the genome vary greatly among animal species, the genetic structure of all keratins is conserved across all species. The type I and II keratin gene clusters are thought to have been derived from common ancestral keratin genes through repeated gene duplication during the course of vertebrate evolution; these considerations are based on the fact that gene clusters with highly homologous genetic structures and sequences are adjacent to each other and clustered on narrow domains of chromosomes (4,10,18).

According to the diagram of evolutionary lineage of keratins (20), the primitive K8/K18 genes developed at early evolutionary stages and then differentiated into keratins expressed in stratified squamous epithelia derived from ectoderm, such as the epidermis, oral mucosa and the cornea. Hard keratins subsequently diverged from these keratins. Keratins corresponding to K8/K18 in humans are, in fact, shared by sharks and ancient fish such as lungfish, as well as by amphibians (Xenopus) and reptiles (lizard species). These observations suggest that the K8/K18 genes are the ancestral genes and that they served as templates from which the 54 current human keratin genes have evolved.

Many of the evolutionarily recent keratins are specifically expressed in epidermal appendages such as the sebaceous gland, sweat gland, nails, hair follicles and hair shaft. These new keratin genes appear to have been acquired in response to the increasing complexity and diversity of the mammalian epidermis and its appendages. Keratins expressed in hair follicles are particularly diverse, and many of them are specifically expressed in each differentiation stage in each type of hair follicle tissue including the hair shaft, Huxley’s and Henle’s layer of the inner root sheath, companion layer and outer root sheath. In fish and amphibians, no site specificity is present in the skin, and because the lack of hair and nail hard keratins are inevitably absent in the genomes of these early vertebrates (10,11,21,22).

Distribution and significance of keratin expression in human normal skin

In human normal skin, epithelial cells are usually present in epidermis, pilosebaceous units and sweat glands. Monospecific monoclonal antibodies which specifically recognize individual keratin proteins made possible the clarification of keratin distribution in epithelial tissues of the normal skin.

Epidermis

When basal cells differentiate into squamous cells and lose their capacity to divide, they downregulate K5 and K14, and express K1 and K10. K2 keratin is expressed later during terminal differentiation, that is, in upper spinous and granular layers of the epidermis. K9 is almost exclusively expressed in suprabasal layers of the palmoplantar epidermis, that is, in glabrous skin. It lacks a special type II partner but rather forms a pair with K1. K6 and K16 are expressed in suprabasal layers in glabrous skin (23).

Pilosebaceous unit

The diagram of pilosebaceous unit is shown in Fig. 1a. The epithelial keratin expression in pilosebaceous unit is shown in Fig. 1b (22,24).

Figure 1.

 (a) Diagram of the pilosebaceous unit. (b) Epithelial keratin expression in the pilosebaceous unit. K1 and 10 are expressed in suprabasal keratinocytes of the infundibulum and epidermis. K10 can be found in mature sebocytes of sebaceous glands. K5 and K14 are expressed in the basal layer of the epidermis and infundibulum, in the whole layers of the outer root sheath and in the sebaceous gland. K6 is expressed suprabasally in the outer root sheath below the opening of the sebaceous duct as well as in cells of the sebaceous gland and duct. K15 is expressed in the outermost layer of the hair bulge. K16 is expressed suprabasally in the outer root sheath below the opening of the sebaceous duct and in the companion layer. K17 is expressed suprabasally in the infrainfundibulum, sebaceous duct, the outer root sheath below the opening of the sebaceous ducts and in the companion layer. K19 may be expressed in the basal outer root sheath including the bulge region. (c) K1 is expressed in the suprabasal layers of the epidermis and infundibulum (original magnification, × 75). (d) K16 is expressed in suprabasal layers of the outer root sheath below the opening of the sebaceous duct. Trichilemmal keratinization is observed in this area (original magnification, × 75). (e) K15 is expressed in the outermost cells of the hair bulge (original magnification, × 75). (f) The hair keratin and hair follicle-specific epithelial keratin expression in the companion layer, inner root sheath, hair cuticle and hair cortex as reported by Schweizer et al. (11) (with permission). The left half contains the type I keratins, the right half the type II keratins. ORS, outer root sheath; cl, companion layer; IRS, inner root sheath; He, Henle’s layer; Hu, Huxley’s layer; iCu, inner root sheath cuticle.

Epithelial keratins

Infundibulum:

Continuous with the interfollicular epidermis, the infundibulum comprises the outermost portion of the hair follicle up to the orifice of the sebaceous duct. Keratin expression in the infundibulum is similar to that of the interfollicular epidermis. K5 and K14 are found in the basal layer, and K1 and K10 in the suprabasal layers of the infundibular epithelium (Fig. 1c). However, suprabasal K2 expression has got lost (22) whereas K17 is expressed suprabasally in the infrainfundibulum (lower portion of the infundibulum).

Sebaceous gland:

Sebaceous glands contain two elements: sebaceous cells and sebaceous duct cells. The boundary between sebaceous duct cells and undifferentiated (basal) sebaceous cells is not clear. The three-dimensional appearance of the sebaceous gland resembles a cauliflower.

Sebaceous cells: Sebaceous cells contain keratin filaments. When undifferentiated sebocytes differentiate to mature sebocytes, they accumulate lipid droplets. K5 and K14 are expressed in most sebaceous cells (Fig. 1b) with K5 staining being pronounced in undifferentiated peripheral cells. In addition, K10 is positive in mature sebocytes, whereas the undifferentiated peripheral cells are negative (25,26). Simple-epithelial keratins generally are absent in sebaceous glands, although heterogeneous staining of mature sebocytes may be observed with a monoclonal antibody against K7, clone OV-TL12/30 (27).

Sebaceous duct cells: Sebaceous ducts open into the infundibulum and may encompass sebocytes. K5 and K14 are found in the entire epithelium of the sebaceous ducts, whereas K6, K10 and K17 appear in inner layers (25,28).

Outer root sheath (including hair bulge):

K5 and K14 are expressed in the entire length and thickness of the outer root sheath beneath the opening of the sebaceous ducts. K6, K16 and K17 appear in the suprabasal layers (Fig. 1d). K16 is a marker of trichilemmal keratinization. Of particular interest is the localization of K15 in outermost (basal) cells of the bulge region (Fig. 1e) where pluripotent stem cells are assumed to reside (6,22,25). For K19, variable and heterogeneous staining patterns of the outer root sheath have been described, with predominance of the basal cell layer and partly being similar to the K15 (stem cell) pattern (22,25). Furthermore, K8 (but not K18) may be detected in basal cells of the lower portion of the outer root sheath (25). K1 and 10 are not detectable in the outer root sheath beneath the sebaceous gland orifice.

Lower portion of anagen hair follicles:

Anagen hair follicles are, from outside to inside, composed of outer root sheath expressing epithelial keratins (see above paragraph), the companion layer exhibiting a characteristic keratin (see below paragraph), the inner root sheath (consisting of inner root sheath cuticle, Huxley’s layer and Henle’s layer) expressing inner root sheath keratins, and the hair proper (consisting of hair cuticle, hair cortex, hair medulla and hair matrix) expressing hair keratins, as well as the mesenchymal dermal papilla lacking any keratin expression.

Companion layer keratin

In the companion layer, a thin layer between the outer and the inner root sheath, the expression of K75 is a highly characteristic and specific feature (29). Additionally, K6, K16 and K17 are present in this layer (22).

Inner root sheath type keratins

The inner root sheath type keratins (Fig. 1f), which are ‘novel’ members of the epithelial keratins, are divided into type I inner root sheath keratins (K25–K28) and type II inner root sheath keratins (K71–K74) (6,11,13,30).

Hair keratins

Hair or hard keratins (Fig. 1f) are divided into type I hair keratins (K31–K40) and type II hair keratins (K81–K86) (6,10,31,32).

Details on the differential expression patterns of the ‘hard’ type I and type II hair keratins in cuticle, cortex, medulla and matrix of the hair fibre and of the inner root sheath type keratins in the inner root sheath layers can be found in the Langbein’s reports (6,13,22,30–32); a summary is presented in Table 3 and a schematic illustration in Fig. 1f. In Langbein’s studies, immunoreactivities were confirmed by in situ hybridization. In the hair medulla, next to hair keratins, also some inner root sheath type keratins and other epithelial keratins have been found (6).

Sweat glands

The diagram of the cell types constituting eccrine sweat glands is shown in Fig. 2a. Keratin expression in eccrine sweat gland is complex (12,25,33–36). A particularly broad, unique keratin profile is displayed by the luminal cells of eccrine sweat gland ducts which exclusively express K77 (12,35). The immunoreactivity of K77 was confirmed by in situ hybridization (12). There are some differences in the literature concerning the expression of certain keratins, notably K1 and K10, which may be due to the different monoclonal antikeratin antibodies recognizing different epitopes, and by the use of different immunohistochemical methods. Results of two groups are schematically illustrated in Fig. 2b,c (33) and Fig. 2d–f (12,35). The expression of the following keratins has been described in the various segments and cell types of eccrine sweat glands (12,25,33–36).

Figure 2.

 (a) Diagram of eccrine sweat gland. (a, b) Keratin expression in eccrine sweat gland as reported by Kurokawa et al. (33) (with permission). (b) K1 is expressed only in periluminal cells of the acrosyringium. K10 is expressed in luminal cells and periluminal cells of the acrosyringium, and luminal cells of the intradermal ducts. K14 is expressed in luminal cells of acrosyringium, luminal cells and peripheral cells of the intradermal ducts, and myoepithelial cells of the secretory portion. (c) K7, K8 and K18 are expressed in secretory cells of the secretory portion. K19 is expressed in luminal cells of the acrosyringium and intradermal duct, and secretory cells of the secretory portion. K17 is expressed only in myoepithelial cells of the secretory portion. (d–f) Keratin expression in eccrine sweat glands as reported by Langbein et al. (12,35) (with permission): (d) K1 and K10 are expressed in luminal cells of the acrosyringium and in luminal cells (K10 only) and partly (upper portion) in peripheral cells of the intradermal ducts. K5 and K14 are expressed in peripheral and luminal cells of the intradermal ducts and in myoepithelial cells of the secretory portion. K77, present exclusively in sweat glands, is expressed in luminal cells of the acrosyringium and intradermal ducts. (e) K7, K8 and K18 are expressed in secretory cells of the secretory portion. K6 and K16 are expressed in luminal cells of the acrosyringium and intradermal ducts, and focally in secretory cells of the secretory portion. K15 is expressed in some cells of the secretory portion. (f) K17 is expressed in luminal cells of the acrosyringium and intradermal ducts, and myoepithelial cells of the secretory portion. K19 is expressed in luminal cells of the acrosyringium and intradermal ducts, and secretory cells of the secretory portion.

Acrosyringium

The acrosyringium is the intraepidermal portion of the eccrine sweat gland duct. It is composed of luminal and periluminal cells.

Luminal cells:

K6/K16, K10, K14, K19 and, notably, the specific K77 are expressed in luminal cells (the literature data on K17 are non-uniform).

Periluminal cells:

K1 and 10 are expressed in periluminal cells.

Dermal ducts

The dermal duct is composed of luminal and peripheral cells.

Luminal cells:

The major keratins of luminal cells are K5/K14, K6/K16, K19 and the specific K77. Further keratins, for which immunostaining may be weaker or more variable, include K8 (25), K10 and K17. Weak staining for K15 has been found in few luminal cells of the proximal intraglandular duct segment (12).

Peripheral cells:

K5 and K14 are expressed in peripheral cells as their main keratins. Focal immunostaining for K1 and K10 in peripheral cells of the upper ductal segment has been described by Langbein et al. (12,35).

Secretory portion

The secretory portion consists of secretory and myoepithelial cells.

Secretory cells:

The simple-epithelial keratins K7, K8, K18 and K19 are expressed in secretory cells. In addition, strong staining of the clear cell population of the secretory portion for K15, and variable and focal expression of keratins K6/K16 in secretory cells has been described (12,35).

Myoepithelial cells:

Myoepithelial cells are characterized by the presence of myofibrillar structures containing smooth muscle-type α-actin that induces contraction of the secretory portion resulting in the discharge of sweat (Fig. 3a). K5, K14 and 17 are expressed in myoepithelial cells. Among these, K17 is an especially valuable myoepithelial marker. These cells also co-express the mesenchymal intermediate filament protein vimentin.

Figure 3.

 (a) In normal sweat glands, K17 is consistently expressed in myoepithelial cells of the secretory portion (arrowhead), but in this specimen it is not detected in the dermal ducts (arrow), reported by Kurokawa et al. (105) (with permission) (original magnification, × 100). (b) K16 expression in keratoacanthoma: K16 was detectable in the suprabasal layers of tumor cells at the bottom of keratoacanthoma, suggesting that keratoacanthoma differentiates towards the outer root sheath beneath the opening of the sebaceous duct, reported by Ito et al. (39) (with permission) (original magnification, × 100). (c) K17 expression in trichofolliculoma: K17 is expressed in inner cell layers of secondary follicles, suggesting that trichofolliculoma differentiates to outer root sheath, reported by Kurokawa et al. (45) (with permission) (original magnification, × 100). (d) K17 expression in steatocystoma multiplex: K17 was positive in the suprabasal layers of the steatocystoma multiplex, suggesting that steatocystoma multiplex differentiates to sebaceous duct, reported by Kurokawa et al. (54) (with permission) (original magnification, × 100). (e) K17 expression in hidroacanthoma simplex: K17, a keratin typical of activated and hyperproliferative keratinocytes, was strongly expressed in tumor cells of hidroacanthoma simplex, reported by Kurokawa et al. (69) (with permission) (original magnification, × 100). (f) K17 expression in eccrine spiradenoma: K17, a keratin that also is characteristic of contractile myoepithelial cells, was intensely expressed in tumor cells (together with α-smooth muscle-type actin), suggesting that eccrine spiradenoma differentiates to myoepithelial cells, reported by Kurokawa et al. (76) (with permission) (original magnification, × 100). (g) K10 expression in intraepidermal eccrine porocarcinoma: K10 is strongly expressed in the intraepidermal tumor cells, reported by Kurokawa et al. (100) (with permission) (original magnification, × 40). (h) K18 expression in intradermal invasive eccrine porocarcinoma: K18 is strongly expressed in tumor nests, reported by Kurokawa et al. (100) (with permission) (original magnification, × 100).

Distribution and significance of keratin expression in cutaneous epithelial tumors

A broad panel of monoclonal antibodies monospecific for individual keratins meanwhile is available. These antibodies allow precise evaluation of the keratin patterns of the various skin epithelial tumors and their cellular subpopulations, providing information on the type and stage of differentiation and possibly the origin of these tumors. Thus, immunohistochemical procedures are useful in making more precise diagnosis of skin epithelial tumors. In certain instances, they are also useful in evaluating the malignancy and prognosis of these tumors. The immunohistochemical data on the individual keratins (grouped according to their differentiation specificity) detected in the various skin tumors have been taken from relevant references and are summarized in Table 4.

Table 4.   Keratin patterns in cutaneous epithelial tumors as detected by monospecific monoclonal antibodies
TumorStratified-epithelial keratinsSimple-epithelial keratins
Basal keratinocyte keratinsHyperproliferative keratinocyte keratins (K6, K16)Myoepithelial/activated keratinocyte keratin (K17)Keratinization-associated keratins (K1, K10)Mucosal keratins (K4, K13)Hair keratinsPrimary simple-epithelial keratins (K8, K18)Secondary simple-epithelial keratins“Merkel cell keratin” (K20)
K5, K14K15K7K19
  1. +, staining of a significant proportion of tumor cells in most cases studied; (+), weak staining or staining of a few tumor cells or positive reaction in only a minor proportion of cases; –, no (or essentially no) staining of tumor cells. For K75 in trichoblastoma and basal cell carcinoma, see ref. 47.

  2. 1K16 expression suggested by the differential staining patterns with monoclonal antibody AE1 and other monoclonal antibodies (38).

  3. 2The epithelial lip at the edge of keratoakanthoma is positive for K1 and K10.

  4. 3Only in keratinizing ductal structures.

  5. 4In one study, positive K7 staining was found in trichoblastomas (small nodular type) but not in trichoepitheliomas (47).

  6. 5Intratumoral neuroendocrine (Merkel) cells were K20-positive.

  7. 6In pilomatricoma, K17 exhibited weak staining in infundibular-type and trichilemmal keratinization-type epithelium (53), and this keratin could also be detected by gel electrophoresis (49) .

  8. 7K1 and K10 were expressed in cells of keratinizing squamous epithelium associated with syringocystadenoma papilliferum, as was K17; the latter was also present in the basal cells of the adenomatous portion (61).

  9. 8Only in inner squamoid cells of keratinous cysts.

  10. 9K4 staining was unexpectedly found (68).

  11. 10Simple-epithelial keratins were detected in cuboidal luminal cells of some tumor tubules (72).

  12. 11Expression associated with keratinous cysts.

  13. 12Only in duct-like structures in the case reported (83).

  14. 13Delayed expression with onset only in upper epithelial zones in all cases reported in ref. (87) and in the hypertrophic type and hypertrophic areas of the hypertrophic-atrophic type described in ref. (86).

  15. 14Heterogeneous expression between and within cases.

  16. 15In well differentiated carcinomas or carcinoma areas.

  17. 16In poorly differentiated carcinomas or carcinoma areas.

  18. 17Cells of primary extramammary Paget’s disease were K7+/K20– while cells of secondary extramammary Paget’s disease (spread of colorectal and urothelial carcinomas) had a K7(+)/K20+ phenotype like the corresponding primary tumors (92).

  19. 18Basal cells positive.

Benign tumors
Seborrheic keratosis+ + +  
Clear cell acanthoma+ +1   
Keratoacanthoma+ +(+)(+)/–2  
Nevus comedonicus+ +   
Dilated pore (Winer)+   +  (+)
Trichoadenoma++(+)++     
Trichofolliculoma++++     
Trichilemmoma+++++3     
Trichoepithelioma/trichoblastoma+(+)++(+)/–+/–4+5
Pilomatricoma+++6(+)/–++/–(+)+ 
Steatocystoma multiplex+ ++/–     
Sebaceoma++++  +/–+/–+ 
Apocrine hidrocystoma+    + 
Apocrine cystadenoma+    +++ 
Syringocystadenoma papilliferum+  +7+7  +++ 
Apocrine mixed tumor    (+)8 +++ 
Eccrine syringofibroadenoma+ + + +/–+/– 
Eccrine hidrocystoma+ + (+)+9 +/–+/–+ 
Hidroacanthoma simplex+  +    
Eccrine poroma++/–(+)+ +/–+/–+
Syringoma+ ++ +
Papillary eccrine adenoma+   +10+10+
Cylindroma+ +++/– +++
Spiradenoma+ ++ +++
Hidradenoma+ +++/– +++
Malignant tumors           
Basal cell carcinoma++/–(+)11+(+)11/– +/–+/–+/(+)
Pinkus tumor (fibroepithelioma)+ +12++12/ (+)  +/–+12/+ 
Actinic keratosis+ + +13  
Bowen’s disease+ + (+) (+)/–+14 
Squamous cell carcinoma of skin+ +++15/–+16/– +16(+)16/–+16 
Extramammary Paget’s disease    ++/(+)17+–/+17
Merkel cell carcinoma     +(+)++
Trichilemmal carcinoma+++   
Malignant pilomatricoma+  +(+) +    
Sebaceous carcinoma+(+)/–+/(+)3(+)/–  +/–++/– 
Eccrine porocarcinoma, intraepidermal18+   
Eccrine porocarcinoma, intradermal invasive++  +++ 

Solid tumors

Benign tumors

Tumors of epidermis:

Seborrheic keratosis: Keratin profiles suggest that seborrheic keratosis is a hyperproliferative variant (reflected by K6/K16) of the epidermal differentiation process (indicated by K1/K10) (37).

Clear cell acanthoma: Keratin expression of clear cell acanthoma also reflects a hyperproliferative epidermal state, which the authors interpreted as abnormal differentiation or maturation reminiscent of inflammatory dermatoses (38).

Keratoacanthoma: Based on the profile of keratin expression, keratoacanthoma can be considered as differentiating towards the outer root sheath beneath the follicular infundibulum (Fig. 3b) (39,40). The keratin pattern of this tumor appears to be simpler as compared with those of both well and poorly differentiated squamous cell carcinomas (SCC), which express higher levels of keratinization-associated keratins (well-differentiated carcinomas) or abnormally express mucosal or simple-epithelial keratins (poorly differentiated carcinomas) (40). This may aid in the differential diagnosis between keratoacanthoma and SCC.

Adnexal tumors

Tumors of hair follicles:

Nevus comedonicus: Keratin expression in nevus comedonicus is identical to that of normal epidermis and infundibulum, lacking keratin-based signs of hyperproliferation (41). Filament aggregating protein (filaggrin), a marker of terminal differentiation of epidermal keratinocytes (42), is involved in the formation of closed comedo (41).

Dilated pore of Winer: Dilated pore is a follicular tumor differentiating mainly towards the infundibulum (as reflected by the predominance of epidermis-type keratins) and partly towards isthmus (43).

Trichoadenoma: Keratin expression in trichoadenoma suggests differentiation mainly towards the follicular infundibulum, the keratinization of most horn cysts being of the epidermoid type (44).

Trichofolliculoma: The secondary follicles of trichofolliculoma lacked keratinization-associated keratins, and the keratin expression pattern suggests differentiation to hair bulge and the outer root sheath of the isthmus (45) (Fig. 3c).

Trichilemmoma: Trichilemmoma differentiates mainly towards two directions: the main tumor formations with peripheral palisading show an outer root sheath type keratin pattern, whereas keratinizing ductal structures correspond to infundibular (epidermoid) keratinization (24).

Trichoepithelioma/Trichoblastoma: Trichoepitheliomas, including the desmoplastic variant and trichoblastomas, have been reported to exhibit keratin expression phenotypes corresponding to the outer root sheath (25,46,47) (Table 4). Furthermore, the expression of K75 in inner cells of most trichoblastomas indicates focal differentiation towards the companion layer (47). The absence of K7 in trichoepitheliomas may be helpful in the differential diagnostic delineation of these tumors from basal cell carcinomas (46). Trichoepitheliomas/trichoblastomas frequently contain intratumoral neuroendocrine (Merkel) cells specifically detected by K20 antibodies (25,47,48).

Pilomatricoma: Up to now, pilomatricomas are the only tumors in which expression of hair keratins has been demonstrated (6,49–51), confirming the true hair cortex-type differentiation of these tumors. Other hair follicle-derived tumors such as trichoepithelioma, trichoblastoma and basal cell carcinoma do not express hair keratins (50). Basaloid tumor cells of pilomatricoma correspond to the lowermost and hair keratin-free germinative cells of hair follicles. Different specific hair keratins appear in sequential pattern in transitional cells of the tumors (50). Next to the predominant trichocytic differentiation, some areas of pilomatricoma undergo squamous cell or outer root sheath differentiation as indicated by the expression of various epithelial keratins (49,52), and there is even focal companion layer differentiation reflected by K75 (M. Divo, L. Langbein and R. Moll, unpublished data). Patterns of epithelial keratins also suggest focal differentiation towards infundibulum and hair bulge (53).

Tumors of sebaceous glands:

Steatocystoma multiplex: Steatocystoma multiplex shares the characteristics of sebaceous ducts in terms of keratin expression (54,55) (Fig. 3d).

Sebaceoma: K14, K17 and K19 (56) as well as K15 (57) are expressed in sebaceoma. These results are consistent with the notion that sebaceoma is a neoplasm of sebaceous germinative cells with predominantly immature characteristics (56–59).

Tumors of sweat glands:

Apocrine hidrocystoma: According to the keratin expression profile, apocrine hidrocystoma may represent cystic tumor of the apocrine ducts (60).

Apocrine cystadenoma: As per keratin expression, which includes simple-epithelial keratins, apocrine cystadenoma differentiates to secretory coil of apocrine sweat glands (60).

Syringocystadenoma papilliferum: Syringocystadenoma papilliferum is assumed to arise from pluripotent cells giving rise to several epithelial cell types, reflected by diverse keratin composition. Luminal cells of secretory or ductal type express simple-epithelial keratins, basal or myoepithelial cells mainly exhibit stratified-epithelial keratins, and associated keratinizing squamous epithelium reveals a stratified-epithelial-type keratin pattern suggestive of infrainfundibular differentiation (61,62).

Mixed tumor of the skin: Based on keratin analysis, the apocrine type of mixed tumor demonstrates differentiation of the epithelial elements towards all components of apocrine units, including secretory cells and the intrafollicular portion of apocrine ducts (63).

Eccrine syringofibroadenoma: According to the expression patterns of keratins and other markers including filaggrin, eccrine syringofibroadenoma appears to differentiate towards the dermal eccrine duct and the acrosyringium (64–67).

Eccrine hidrocystoma: Discordant data on keratins in eccrine hidrocystoma have been reported. It may represent a cystic tumor of the eccrine sweat duct (60). Another report found that eccrine hidrocystoma largely expresses the keratin set of eccrine sweat gland acini, with some commitment to ductal epithelium (68).

Hidroacanthoma simplex: Hidroacanthoma simplex exhibits a keratin immunophenotype different from that of acrosyringeal cells (69) (Fig. 3e), rather suggesting hyperproliferative and undifferentiated character.

Eccrine poroma: In terms of keratin profile (25,34–36,70), most poroma cells resemble, with their plain keratin composition (K5/K14), the basal cells of both dermal eccrine ducts and the epidermis. Minor subpopulations of poroma cells, however, have revealed more specific keratins including K10, K17 and K19 (25,36). Ductal and cystic structures interspersed between the poroma cell formations express K6 and keratins of the simple-epithelial type, in agreement with differentiation towards luminal cells of dermal eccrine ducts and focally towards secretory cells. These keratin data, although providing no definite proof of histogenesis (34), may suggest that eccrine poroma originates from basal cells of eccrine ducts (34) or the transitional zone between the dermal and the intraepidermal segments of the eccrine duct (70). In a recent study (35), the novel keratin K77 specific for luminal cells of the eccrine ductal system was not detected in poroid cells and also nearly absent in cells of cystic structures, and the authors proposed that eccrine poromas are derived from the basal keratinocytes of the sweat duct ridge and the lowermost acrosyringium.

Syringoma: Keratin phenotyping in syringoma indicates differentiation corresponding to the eccrine ductal system but not the secretory portion (25,35,71). That the luminal cells or inner cell layers of the tubules and cords of syringoma have undergone true eccrine ductal-luminal cell differentiation has recently been proven by the detection of the specific K77 keratin in these cells (6,35), which also express further ductal-luminal type keratins such as K6/K16 and K19 (25,35,71). K1/K10, expressed in the upper portion of the intradermal duct and the sweat duct ridge of normal glands, appear in intermediate cells of syringoma. Thus, syringoma seems to differentiate to the transitional portion existing between the acrosyringium and the dermal ducts, that is, the sweat duct ridge (71).

Papillary eccrine adenoma: Limited data available on keratins in papillary eccrine adenoma suggest that this tumor differentiates to dermal duct and the transitional portions between the dermal ducts and the secretory segments of eccrine glands (72).

Dermal cylindroma: Dermal cylindroma appears to arise from pluripotential cells, and the expression of a broad spectrum of keratins as well as α-smooth muscle-type actin points at (rudimentary) secretory, myoepithelial and ductal differentiation (25,73–75). In a recent study, immunostaining of cylindroma cells for hair follicle-specific keratins K71, K75 and K82 has been reported, and from this a hair follicle origin was proposed (74). On the contrary, the expression of K77 in tubular structures of cylindroma (6) can be considered as evidence for true eccrine ductal differentiation.

Eccrine spiradenoma: Spiradenoma exhibits a marker spectrum of keratins and α-smooth muscle-type actin similar to that of cylindroma (25,73,75–77), suggesting differentiation pathways of secretory, myoepithelial (Fig. 3f) (76) and ductal epithelial cells. One of these reports suggested that eccrine spiradenoma differentiates to the transitional portion between the secretory portion and coiled ducts of eccrine glands (77).

Hidradenoma: Hidradenomas are complex and heterogeneous both in terms of histopathology and keratin distribution (25,78). The keratin patterns suggest differentiation of tumor cells towards secretory cells and the dermal duct in varying proportions. The variant, solid cystic hidradenoma, revealed differentiation to secretory cells at higher extent (78). As to clear cells in hidradenoma, keratin data in the literature are discordant (25,78). Areas of epidermoid differentiation exhibited increased keratinization-associated keratins (25).

Generally, it should be remembered that keratin analysis of adnexal skin tumors arising from the secretory portion of sweat glands may be able to demonstrate secretory glandular differentiation but cannot distinguish between eccrine and apocrine type. However, when differentiation towards ductal portions occurs one now can specifically identify the eccrine type on the basis of the eccrine duct-specific K77.

Malignant tumors

Keratinocytic tumors:

Basal cell carcinoma: There is a large body of literature on keratins in basal cell carcinoma (46,47,79–81). It is clear that the main keratins expressed in this tumor are K5, K14 and K17, as already shown in our early biochemical analyses (3). Additionally, albeit controversially reported in the literature, simple-epithelial keratins may be present in focal and variable patterns. K15 was not detected in one study (47) but was found in some cases by other investigators (6). The same study reported that basal cell carcinomas may stain for the companion layer-typical K75 (47). Altogether, most studies emphasize the similarities in keratin pattern between basal cell carcinoma and the outer root sheath of hair follicles (3,46,47,79) or the germinative cells of developing foetal hair follicles (80). This clearly indicates follicular differentiation and suggests possible follicular (outer root sheath) origin of basal cell carcinoma. Recently established bona fide cell lines from mouse basal cell carcinoma expressed K14 as shown by Western blotting, similar to that of basal cell carcinomas in vivo (82).

Pinkus tumor (fibroepithelioma): In Pinkus tumor, basaloid strands exhibit a plain basal keratinocyte pattern including K14 and K17 (83). This is in good agreement with previous biochemical analyses revealing K5, K14, K15 and K17 as the main keratins (84). In addition, as minor components, the simple-epithelial keratins K8 and K19 may be heterogeneously expressed whereas sparse scattered cells may express K10 (I. Moll, unpublished data). By these features, Pinkus tumor resembles both basal cell carcinoma and trichoblastoma (Table 4). Tumor-associated K20-positive Merkel cells are commonly detected (85). The presence of duct-like structures with a keratin pattern similar to that of normal acrosyringium suggested that Pinkus tumor may originate from intraepidermal eccrine ducts and proliferate in the dermis (83).

Actinic keratosis: Keratin expression in actinic keratosis was often characterized by delayed onset of expression of K1/K10 in only upper zones together with suprabasal expression of the hyperproliferation-associated K16 (86,87). The atrophic type, however, showed preserved suprabasal K1/K10 staining similar to normal epidermis (86).

Bowen’s disease (squamous cell carcinomain situ): Heterogeneous keratin expression patterns were observed in Bowen’s disease (86,87). Besides frequent reduction or even loss of K1 and 10 and suprabasal expression of K16 (87) the atypical keratinocytes exhibited rather variable neoexpression of K19 (86), a simple-epithelial keratin never expressed in adult benign epidermis.

Squamous cell carcinoma (SCC): The main keratins of cutaneous SCC, as apparent by two-dimensional gel electrophoresis, are the basal cell type K5/K14, the hyperproliferation-associated K6/K16, as well as K17, whereas minor amounts of the keratinization-associated K1/K10 can in most cases also be detected (84). In immunohistochemical analyses, K5/K14 often predominate in the peripheral and K6/K16 in the more central, differentiating layers of tumor cell nests, respectively (6,88). K1 and K10 are expressed in well-differentiated SCC in association with keratinization (40,81,84). Notably, in poorly differentiated SCC or SCC areas, there is inappropriate expression of simple-epithelial keratins K8/K18 and K19, and sometimes even K7 (40,87). Simple-epithelial keratin expression could be related to tumor invasion and to changes in epithelial–mesenchymal interactions (87). Interestingly, the expression of certain genes of human papilloma viruses, which have been associated with the development of cutaneous SCC, has been shown to cause changes in the keratin expression pattern of keratinocytes in vitro including the overexpression of K8, K18 and K19 (89). In organotypic three-dimensional explant models of human cutaneous SCC, abundant expression of K6, K16 and K17 and distorted expression of K1 and K10 was found (90). In differential diagnostic respect, there is no sharp distinction between the keratin patterns of SCC and basal cell carcinomas, although there are certain differences in direction such as higher levels of K6/K16 in the former (Table 4). As compared with SCC of cutaneous origin, SCC of internal organs display essentially the same keratin patterns [except for higher levels of K7 observed in SCC of the cervix uteri (88)]. Therefore, generally, keratin analysis does not allow to distinguish between primary cutaneous SCC and skin metastasis of an internal SCC. The possible value of keratins in the differential diagnosis of SCC and keratoacanthoma has already been addressed above.

Extramammary Paget’s disease: In sharp contrast to the surrounding keratinocytes, K7, K8, K18 and K19 are expressed in Paget cells, suggesting that these tumor cells undergo glandular differentiation (91). K7 and K20 are useful for the diagnosis of primary and secondary extramammary Paget’s disease (92).

Merkel cell carcinoma: K20 is a specific and useful marker of Merkel cells, and is usually positive in Merkel cell carcinomas (93,94). This feature is very valuable for differential diagnosis, for example, the distinction from metastatic small cell carcinoma.

Adnexal tumors

Tumors of hair follicles:

Trichilemmal carcinoma: Histogenetically, trichilemmal carcinoma originates from the outer root sheath (95,96). Absence of K15 and K16 in trichilemmal carcinoma may be related to transformation from trichilemmoma to trichilemmal carcinoma (95).

Malignant pilomatricoma: Concerning the hair keratin expression in malignant pilomatricoma, a case has been reported recently in which the pathway of shadow cell differentiation was present but opposed by a dominant epithelial-type differentiation pathway (97).

Tumors of sebaceous glands

Sebaceous carcinoma: Literature data on keratins in sebaceous carcinomas are limited and partly inconsistent. In one case report, a basal keratinocyte-type keratin pattern (mainly K14) was found, similar to that of normal sebaceous glands (98). There was no or little K15 immunostaining in sebaceous carcinomas (57). Some authors described the expression of simple-epithelial keratins in these neoplasms (27,99) which may be attributable to the stage of differentiation or to neoplastic transformation (98,99). Positive staining with a K7 antibody was a feature of both sebaceous carcinoma and normal sebaceous glands (27).

Tumors of sweat glands:

Eccrine porocarcinoma (EPC): Intraepidermal EPC expresses terminally differentiated K1 and K10 (Fig. 3g). In contrast, intradermal EPC expresses, next to K14 and K17, the simple-epithelial keratins K7, K8, K18 and K19 (Fig. 3h). Keratin profiles appear to correlate with the invasive degree and may indicate clinical prognosis of EPC (100). Co-expression of stratified- and simple-epithelial keratins has also been noted in another case of invasive malignant eccrine poroma (36).

Cutaneous cysts

Epidermal cyst:  Keratin immunoreactivity of epidermal cyst is similar to normal epidermis or infundibulum, with prominent suprabasal K1/K10 expression (55,68,101), whereas the hyperproliferation-associated K16 is additionally present suprabasally (68). Filaggrin expression in epidermal cysts is more pronounced than normal epidermis or infundibulum (102).

Milia:  Keratin expression of milia is identical to epidermal cysts, thereby also indicating epidermal-type differentiation and hyperproliferation (68).

Trichilemmal cyst:  Keratinization of trichilemmal cyst corresponds to the keratinization of anagen-phase hair follicle in the region of the isthmus (68,101), primarily reflected by suprabasal expression of K17 (55) together with K16 (68). Moreover, the simple-epithelial K19 may be expressed in the peripheral cell layer (68). Conversely, K1/K10 are markedly reduced as compared with epidermal cysts (68).

Vellus hair cyst:  Eruptive vellus hair cyst expresses K17 but not K10, suggesting that it is a distinct entity from steatocystoma multiplex and trichilemmal cyst (55).

Dermoid cyst:  The cyst wall and associated sebaceous gland-like structures of dermoid cyst differentiate to follicular infundibulum (K14, K1/K10) and sebaceous glands (K14, K17), respectively, based on keratin expression (103).

Pilonidal sinus:  The usual, infundibular-like, orthokeratinizing type of lining epithelium of pilonidal sinus revealed an infundibular-like keratin pattern but without K17, whereas the less common non-infundibular-like epithelium resembled in its keratin composition (including K16 and K17) the outer root sheath (104) (Fig. 4a,b).

Figure 4.

 (a) K17 expression in normal pilosebaceous unit: K17 is expressed in suprabasal layers of the lower portion of the infundibulum, reported by Kurokawa et al. (104) (with permission) (original magnification, × 100). (b) K17 staining of pilonidal sinus: K17 was not detectable in orthokeratinizing sinus epithelium, reported by Kurokawa et al. (104) (with permission) (original magnification, × 100).

In summary, the differentiating pathways of tumors derived from pilosebaceous units and sweat glands are described in Fig. 5a and b, respectively. In conclusion, because their molecular diversity and differentiation specificity, keratins are marker proteins highly meaningful for all kinds of epithelial tumors but in particular for skin tumors. Analysis of keratin profiles broadens our understanding of the differentiation, nature and histogenetic origin of the various, highly singular tumors arising in the skin. Moreover, in certain instances, keratins may be helpful in recognition of tumor malignancy and aggressiveness.

Figure 5.

 (a) The differentiation pathway of tumors derived from the pilosebaceous unit. (b) The differentiation pathway of tumors derived from eccrine sweat glands.

Distribution and significance of keratin expression in other diseases

Wound healing:  In healing skin ulcers, K14, K16 and K17 are elevated or induced in human wound edge epidermis, hair follicles and sweat glands (105). Expression of K14 and K17 in regenerating epithelium arising from dermal sweat gland ducts and K16 in regenerating epithelium arising from the outer root sheath of hair follicles in pathological wound healing may be important for epidermal replacement (105). During skin injury and onset of physiological wound healing, K6 and K16 are rapidly induced in epidermal keratinocytes at the wound edge and are correlated with a reorganization of keratin filaments from a pan-cytoplasmic to a juxtanuclear distribution, favouring keratinocyte migration and reepithelialization (106). Signals from lymphocytes (interferon-γ) induce K17 to make the keratinocytes contractile (Fig. 6a), and signals from fibroblasts such as TGF-β induce K5 and K14 to revert keratinocytes to healthy basal cells and complete the activation cycle (107).

Figure 6.

 (a) K17 expression in wound edge: K17 was expressed in the epidermal tongue (arrow) and epithelial islands at the wound edge, reported by Kurokawa et al. (105) (with permission) (original magnification, × 40). (b) K18 expression in poorly differentiated squamous cell carcinoma: K18 is expressed in tumor nests of poorly differentiated squamous cell carcinoma that arose from hidradenitis suppurativa. The overlying epidermis is K18 negative, reported by Kurokawa et al. (109) (with permission) (original magnification, × 40).

Hidradenitis suppurativa (HS):  In contrast to the normal infundibular epithelium (Fig. 4a), K17 is absent from infundibular-like epithelium in HS, suggesting fragility of this epithelial type (108). Keratin expression in SCC arising from HS indicates that changes of keratin profile such as neoexpression of simple-epithelial keratins in poorly differentiated SCC (Fig. 6b; cf. Section ‘Distribution and significance of keratin expression in cutaneous epithelial tumors’) are related to malignant transformation (109).

Acne vulgaris:  K6, K16 and K17 are increased in acne lesions of normal follicular infundibulum, that is, comedone walls (110). Also filaggrin expression is more pronounced in acne lesions, suggesting that disorder of terminal differentiation is related to abnormal keratinization of the infundibulum (111).

Functions of keratins and keratin diseases

In accordance with the morphological appearance of keratin filaments as more or less dense bundles attached to desmosomes and hemidesmosomes, suggesting their role as structural stabilizers, one major established function of keratins is the maintenance of mechanical stability and rigidity of epithelial cells and tissues and their protection against mechanical stress (112–116). Ample proof for this notion has been provided by various keratin knock-out mouse models as well as by human keratin diseases. Such autosomal-dominant familial diseases, many of which are blistering skin diseases, are pathogenetically related to point mutations acting in a dominant negative manner. These keratin diseases have been extendedly reviewed elsewhere (6,112,116,117). In short, 19 different keratin genes including hair keratins and hair follicle-specific epithelial keratins have up to now been identified as being involved in pathogenic keratin mutations. The prototype is epidermolysis bullosa simplex (EBS) which is caused by a spectrum of point mutations of K5 and K14 leading to cytolysis of epidermal basal cells and epidermal blistering. Gene mutations of keratins expressed in upper epidermal layers or at specific body sites, such as K1/K10, K2 and K9, give rise to diseases characterized by corresponding, layer- or site-specific epidermal disturbances. In general, the granular degeneration and epidermolysis that are histologically observed in lesions of the majority of keratin diseases including EBS are thought to result from impairment of function of keratin filaments during physical stress (112).

Pachyonychia congenita and steatocystoma multiplex are caused by mutations in the K6, K16, and K17 genes, which are specifically expressed in hair follicles, sebaceous gland, nail bed epithelium, the spinous cell layer of the palms and soles, and in wounded epidermis (118,119). These diseases are characterized by acanthosis, hyperplasia of sebaceous glands, hair follicles, nails and cyst formation; pathological findings such as reticular degeneration and cell lysis observed in other keratin diseases are absent.

Despite the importance of the structural functions of keratins, a variety of other, regulatory and signalling functions of keratins have got recognized more recently (6,113–115,120–122). This concerns certain stratified-epithelial keratins such as K17 but especially simple-epithelial keratins, notably K8 and K18, and thus often applies to simple internal epithelia in which rigidity as required in the epidermis is not essential. For example, such regulatory roles include the protection from apoptosis, and the protection of the liver against metabolic/toxic stress. Mutations in the K8 and K18 genes may predispose to certain diseases of the liver and the intestinal tract (120–122). K17 plays a role in the regulation of cell size and in the modulation of hair follicle cycling (115). Thus, it has become clear that keratins may exert various regulatory and signalling functions beyond their mechanical roles, and this also provides some clues for better understanding of the biological significance of the molecular diversity of this protein family.

Transgenic mouse experiments involving gene knock-out or ectopically expressed normal or chimeric keratins suggest a role of K10 in keratinocyte turnover and the development of chemically induced skin papillomas. However, a causal role in tumorigenesis is not established for any keratin (115).

A major issue that remains completely unsolved is the mechanism by which type I and II keratin genes, which evolved independently, share the ability to regulate the tissue- and differentiation-specific expression and acquire the coordinate promoter activity in various epithelial tissues (123).

Acknowledgements

We thank Prof. Hitoshi Mizutani and all colleagues in Department of Dermatology, Mie University Graduate School of Medicine, Prof. Airo Tsubura, Department of Pathology, Kansai Medical University for discussion, supplying samples and technical help, and Ms Gudrun Jurat, Giessen for preparing the figure plates.

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