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Keywords:

  • stroma;
  • invasion;
  • myofibroblast;
  • cross-signaling;
  • therapy

Abstract

  1. Top of page
  2. Abstract
  3. Characterization and origin of a myofibroblast
  4. Are myofibroblast changes unique for cancer?
  5. The tumor stroma regulates invasion and metastasis
  6. How do myofibroblasts react to cancer management?
  7. Conclusion
  8. Acknowledgements
  9. References

Tissue integrity is maintained by the stroma in physiology. In cancer, however, tissue invasion is driven by the stroma. Myofibroblasts and cancer-associated fibroblasts are important components of the tumor stroma. The origin of myofibroblasts remains controversial, although fibroblasts and bone marrow-derived precursors are considered to be the main progenitor cells. Myofibroblast reactions also occur in fibrosis. Therefore, we wonder whether nontumorous myofibroblasts have different characteristics and different origins as compared to tumor-associated myofibroblasts. The mutual interaction between cancer cells and myofibroblasts is dependent on multiple invasive growth-promoting factors, through direct cell–cell contacts and paracrine signals. Since fibrosis is a major side effect of radiotherapy, we address the question how the main methods of cancer management, including chemotherapy, hormonotherapy and surgery affect myofibroblasts and by inference the surrogate endpoints invasion and metastasis. © 2008 Wiley-Liss, Inc.

Malignant tumors consist of founder cancer cells and tumor-associated host cells. The latter do receive increasing attention because of their participation at tumor development,1 including invasion and metastasis,2 and of their response to therapy.3, 4 The list of tumor-associated host cells comprises: blood and lymph endothelial cells, inflammatory cells, immunocytes and macrophages, and fibroblasts.5, 6 Together, the stromal mass makes up about half of most malignant tumors.7 Fibroblasts contributing to the tumor stroma have been termed peritumoral fibroblasts, reactive stroma, cancer-associated fibroblasts (CAF) and myofibroblasts. In general, fibroblastic cells adjacent to cancer cell nests express α-smooth muscle actin (α-SMA), an important marker for differentiated myofibroblasts. The term myofibroblasts encompasses heterogeneous and multifunctional cell populations exhibiting different phenotypes.8 Myofibroblasts were originally described in skin wounds where they contract the stroma, bringing the epithelial borders closer together and, so, facilitate wound healing.9 Myofibroblasts modulate the stroma in physiology and pathology through direct cell–cell contacts and through secretion of matrix metalloproteinase (MMP)'s, tissue inhibitors of metalloproteinase (TIMP)'s, extracellular matrix (ECM) components, growth factors, cytokines, chemokines, and lipid products and through the expression of specific receptors. Some normal tissues, such as the gastro-intestinal tract and the lungs, also harbor myofibroblasts.10, 11 Besides their role in wound healing, myofibroblasts are essential for tissue morphogenesis, assist at stem cell niches and at mucosal immunity.10, 11 In cancer, a wound that does not heal,12 myofibroblasts are either deficient or fullfil other functions such as the production of proinvasive proteinases.13 Early evidence in favor of a role for myofibroblasts in invasion was obtained by Dimanche-Boitrel et al.14 using a chemically induced rat colon cancer. Epithelial cancer cell lines isolated from such tumor failed to invade in vitro, whereas fresh tumor suspensions containing both cancer cells and tumor-associated host cells were invasive in vitro and in vivo. Similarly, isolated myofibroblasts and CAF's conveyed invasive capacity upon the cancer cells. These observations have been amply confirmed and mechanistically underpinned.15, 16

It is the aim of the present review to broaden our understanding of the molecular switch from the noninvasive to the invasive stage of cancer progression, taking the myofibroblast and its soluble mediators as a presumptive central regulator. Since myofibroblast reactions also occur in fibrosis, for example, of the lung after radiotherapy,17, 18 we wonder whether noncancerous myofibroblasts have different characteristics and different origins as compared to tumor-associated myofibroblasts. Since fibrosis is a major side effect of radiotherapy, we address the question how the main forms of cancer diagnosis and treatment, including chemotherapy, hormonotherapy and surgery, affect myofibroblasts and by inference the surrogate endpoints invasion and metastasis.

Following editorial restrictions, we shall limit references to publications not cited in our 2003 review.2

Characterization and origin of a myofibroblast

  1. Top of page
  2. Abstract
  3. Characterization and origin of a myofibroblast
  4. Are myofibroblast changes unique for cancer?
  5. The tumor stroma regulates invasion and metastasis
  6. How do myofibroblasts react to cancer management?
  7. Conclusion
  8. Acknowledgements
  9. References

Myofibroblasts are large spindle-shaped cells defined by indented nuclei, stress fibers and well-developed cell-matrix interactions (fibronexus). Unfortunately, there is no myofibroblast-specific immunocytochemical marker. Therefore, characterization of human tumor-associated myofibroblasts is based on a combination of positive markers such as actin isoforms specialized in cellular contraction such as α-SMA and γ-SMA; the stress fiber controlling protein paladin 4Ig, the mucin-type transmembrane glycoprotein podoplanin, the intermediate filaments vimentin and desmin, the sialylated transmembrane molecule endosialin; the cell–cell adhesion molecule cadherin-11, the collagen Type I maturation enzyme prolyl-4 hydroxylase (P4H) and negative markers such as the epithelial marker cytokeratin, the monocyte marker CD14, the endothelial marker CD31, the fibrocyte marker CD34 and the smooth muscle cell marker smoothelin (Fig. 1). Besides these cytoplasmic and membrane markers, myofibroblast and CAF's express integrin α11, ECM components such as collagens (Type I, III and IV), MMPs (mainly MMP-3 and MMP-9) and TIMP's as well. In agreement, in this review the term myofibroblast will be used when the authors used the α–SMA marker in combination with at least 3 other markers (e.g., positive for α–SMA, vimentin and P4H, and negative for cytokeratin).2 If this criterium is not fulfilled, the term CAF is used.

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Figure 1. Markers of tumor stromal fibroblasts. Cellular localization of markers used to identify tumor stromal fibroblasts. Markers are underlined, synonyms are between brackets. Its presence in a specific cell type is indicated below marker. The presence in myofibroblasts is indicated in red. CAF, cancer-associated fibroblast; EnC, endothelial cell; ME, myo-epithelial cell; MF, myofibroblast; P4H, prolyl-4 hydroxylase; SMA, smooth muscle actin; SMC, smooth muscle cell. Data from Refs.6, 10, 19–27.

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Myofibroblasts are found in the embryo from gestational week 13 on, and are co-opted by the remodeling tissues. In postembryonic stages, differentiation from resident stromal fibroblasts into myofibroblasts is induced by paracrine signals generated by repairing or inflamed tissues. Among those signals, transforming growth factor (TGF)-β is the most potent one. Coreceptors like heparan sulfate and decorin facilitate myofibroblast triggering by TGF-β.10 In the adult, the stroma present in invasive tumors consists of both bone marrow-derived and non-bone marrow-derived myofibroblasts. Bone marrow-derived myofibroblasts originate either from circulating mesenchymal stem cells (MSC's) or CD34+ fibrocytes (which have CD14+ monocytes as precursor).28 Non-bone marrow-derived myofibroblasts may originate from multiple resident precursors such as endothelial cells, myoepithelial cells, epithelial (cancer) cells, fibroblasts, smooth muscle cells, adipocytes and stellate cells (Fig. 2). It is important to understand that the precursor cells mentioned in Figure 2 do not give rise exclusively to myofibroblasts. For example, resident (r)CD34+fibrocytes may develop α-SMA-negative contractile bundles, and therefore do not meet the criteria of myofibroblasts. The contribution of non-bone marrow-derived precursors in myofibroblast differentiation of the tumor stroma has been extensively reviewed by others.2, 8 Accurate extravasation and homing of bone marrow-derived circulating cells to find their stromal niche is not completely understood, but appears to be mediated by chemokine interactions. One example is the influx of CD34+ fibrocytes into the pulmonary stroma via interaction of their CXCR4 receptor with locally produced CXCL12.6 Fibrocyte expression of α–SMA-positive contractile bundles (and consequently myofibroblast differentiation) from CD14+ peripheral blood monocytes is prohibited by serum amyloid P.31 Ascites from ovarian cancer patients and the secretome (soluble factors present in the conditioned medium) from ovarian cancer cells stimulate differentiation of MSC's into myofibroblasts in a lysophosphatic acid dependent way,32 suggesting that the cancer environment is capable of differentiating MSC's into myofibroblasts. Bone marrow-derived MSC's contribute to 25% of the total myofibroblast population in the tumor stroma in a mouse model of pancreatic insulinoma33 and in a subcutaneous pancreatic xenograft tumor.34 Furthermore, these bone marrow-derived MSC-derived myofibroblasts actively participate in the production of matrix proteins, such as collagen Type I, in xenograft tumors.35 Once integrated in the stroma, human bone marrow-derived MSC's cause the cancer cells to increase their metastatic potency. Cultured bone marrow-derived MSC's subcutaneously coinjected with weakly metastatic breast cancer cells form highly metastatic xenografts. The breast cancer cells stimulate de novo secretion of the chemokine CCL5 from bone marrow-derived MSC's, which then acts in a paracrine fashion on the cancer cells to enhance their motility, invasion and metastasis. This enhanced metastatic ability is reversible and is dependent on CCL5 signaling through the chemokine receptor CCR5.36 Interestingly, these observations have paved the way for the use of bone marrow-derived MSC's in gene therapy. Genetically modified MSC's carrying the human interferon-β gene and intravenously injected into immunodeficient mice with established xenografted human local tumors and metastasis, resulted in a significant improvement in survival compared with controls.37

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Figure 2. Origin of adult myofibroblasts. Schematic of cells that may transit to (arrows) myofibroblasts in normal and pathological adult tissues. Abbreviations: MSC, mesenchymal stem cell; c, circulatory; HSC, hepatic stellate cells; PSC, pancreatic stellate cell; r, resident; SMC, smooth muscle cell. Data from Refs.6, 10, 28–30.

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Are myofibroblast changes unique for cancer?

  1. Top of page
  2. Abstract
  3. Characterization and origin of a myofibroblast
  4. Are myofibroblast changes unique for cancer?
  5. The tumor stroma regulates invasion and metastasis
  6. How do myofibroblasts react to cancer management?
  7. Conclusion
  8. Acknowledgements
  9. References

Pathological analysis of human cancers suggests that tumor-associated changes in the stromal myofibroblasts occur early during cancer development. A number of review articles covering this topic in more detail have been published recently.2, 10 For example, in breast invasive ductal carcinoma (IDC) and micropapillary carcinoma (a rare and aggressive variant of IDC), CD34+ stromal cells disappear and α-SMA+ myofibroblasts are abundant.38, 39 This cancer-associated loss of CD34 expression is not reported in lobular breast carcinoma and diffuse gastric carcinoma.40, 41

In normal wound healing myofibroblast appearance is transient, but in pathology this process may lead to persistent accumulation of these contractile cells with excessive production of collagen in the ECM. This phenomenon, which is described in many organs and tissues as fibrosis, can occur in the absence of cancer cells. Fibrosis is usually permanent and resistant to treatment.11 Among the organs most frequently affected by fibrosis are liver, lungs, kidneys, pancreas, skin and cardiovascular walls, but the phenomenon has also been observed in bile ducts, mammary glandular epithelium, fascias, stomach, ileum, synovium, peritoneal mesothelium, eye structures (e.g., conjunctiva, sclera, lens capsule after glaucoma surgery and in vitreo-retinopathies).42, 43 Although many cell types have been implied in these different tissue fibrosis sites, and numerous recruitment mechanisms were described, a common pathophysiological mechanism appears from recent studies. Most informative have been gene expression profiles with deduction of “fibrosis signatures,” and these can be the result of, for example, epigenetic mechanisms, such as histone acetylation, DNA methylation and transcription of noncoding microRNA's.44, 45 In a comparative micro-array investigation between Peyronie's and Dupuytren's diseases, 2 examples of fibrosis of the penile tunica albuginea and the palmar fascia respectively, emphasis was put on the similarities in the profiles (molecules implicated in collagen degradation, ossification and myofibroblast differentiation).46 Other comparisons, like those between nodular fasciitis and desmoid tumors, offered evidence of different pathway activations (implicated in signal transduction, transcription and inflammation/ECM remodeling).47 Although both bone marrow-derived MSC's and resident precursors can be at the origin of nearly all types of fibrosis, some mechanisms are typical for certain locations. So, epithelial mesenchymal transition is “classical” for renal fibrosis, and typical resident stellate cells transdifferentiate to myofibroblasts in liver and pancreatic fibrosis.29, 30 In lung fibrosis, epithelial and endothelial transitions are frequent events, but in vitreo-retinopathies the myofibroblasts are recruited into the epiretinal membranes from circulating fibrocytes43 (see Fig. 2). It is noteworthy that the tendency for development of fibrosis in multiple organs can be an individual trait, as illustrated by gastric fibrosis in systemic sclerosis patients.48 In these cases, at least a common pathogenesis is very likely.

In the molecular signaling of fibrosis, TGF-β plays a pivotal role as it does in myofibroblast co-optation in invasive cancers, and virtually all known cytoplasmic/nuclear pathways associated with its Type I and Type II receptors are activated in fibrosis. This is the case for Smad (similar to mothers against decapentaplegic),49 nuclear factor (NF)-κB,50 phosphatidyl inositol-3 kinase (PI-3K), mitogen-activated protein kinase, semaphorin 7A receptors (plexin c1 and β-integrins),51 the focal adhesion protein Hic-552 and β-catenin translocation to the nucleus, and this is confirmed in lung and liver fibrosis.53 In these 2 types, the renin–angiotensin system appears to sensitize both organs for fibrosis development, and angiotensine II receptor blockers can interrupt the autocrine TGF-β/angiotensine II loop in the myofibroblast.54 Platelet-derived growth factor B (PDGF-B), whose hepatic overexpression was shown to induce liver fibrosis,55 has caught clinical interest since the demonstration of stimulatory auto-antibodies to the PDGF receptor in patients suffering from systemic sclerosis (scleroderma).56 Remarkably, phagocytosis by pancreatic stellate cells of necrotic cell debris is a potent suppressor of differentiation to myofibroblasts.30

Fibrosis is not only the result of myofibroblast differentiation after tissue injury and cell stress, but it can also persist in the absence of reversion mechanisms that impair granulation tissue resolution. This inapproriate myofibroblast reaction is at least partly sustained by a defective apoptosis mechanism, which creates a permanent niche for these cells.57 Several mechanisms have been invoked to explain the prolonged survival of myofibroblasts, and escape from immune surveillance may be a plausible one. The CD34+ stellate precursors behave as antigen-presenting cells, and their extensions are associated with lymphocytes and plasmocytes. Differentiation to myofibroblasts, however, induces apoptosis of the associated Fas-ligand positive T-lymphocytes, so that the myofibroblasts do not receive Fas-ligand dependent signals anymore that would induce their own apoptosis. This escape from immune surveillance consolidates myofibroblast recruitment, and sustains fibrosis.6, 58 Watson et al.50 reported on the antiapoptotic effect of increased levels of NF-κB in liver fibrosis myofibroblasts, an additional mechanism that can prolong their survival.

It is a puzzling clinical observation that metastases from colon cancer occur less frequently in fibrotic livers than in normal ones. Apart from immunologic mechanisms, the quality of the collagenous interstitial ECM and the sparseness of blood vessels are thought to create an unfavorable “soil” for colon metastases.59

Insight into the molecular mechanisms of myofibroblast accumulation in fibrosis has revealed potential targets for therapy. When accelerated wound closure is envisaged, extending the life span of myofibroblasts may be an option. One target is then tumor necrosis factor (TNF)-α, which is overexpressed in Crohn's disease myofibroblasts as compared to fibroblasts.60 Experiments in vitro have shown that infliximab, a blocking antibody against TNF-α, was able to downregulate MMPs, upregulate TIMPs, increase cell motility and accelerate wound healing.61 However, when differentiation from fibroblasts has to be blocked, targeting the TGF-β receptor looks an attractive approach. SM16, a small kinase inhibitor of activin receptor-like kinase (ALK)5/ALK4, inhibits TGF-β signaling, prevents activation of receptor Smads (2/3) and reduces development of fibrosis in blood vessel walls and in the lungs.62 As mentioned earlier, another promising target is the angiotensin II receptor, for which safe and selective blockers are available for oral treatment of hypertension.54

The tumor stroma regulates invasion and metastasis

  1. Top of page
  2. Abstract
  3. Characterization and origin of a myofibroblast
  4. Are myofibroblast changes unique for cancer?
  5. The tumor stroma regulates invasion and metastasis
  6. How do myofibroblasts react to cancer management?
  7. Conclusion
  8. Acknowledgements
  9. References

Clinical and experimental data support the hypothesis that the tumor stroma regulates invasion and metastasis. Extensive changes in the expression of genes that encode invasion-associated secreted proteins and receptors are found in myofibroblasts in breast cancer63 and CAF's during basal cell carcinogenesis.64 SPARC (secreted protein, acidic and rich in cysteine) expression by peritumoral fibroblasts portends a poorer prognosis for patients with resectable pancreatic cancer65 and α–SMA expression by myofibroblasts predict disease recurrence in colorectal cancer.66 The proinvasive activity of human myofibroblasts in vitro was shown by De Wever et al.16 using human colon cancer cells and myofibroblasts isolated from surgical colon cancer fragments. In 48-hr cultures, the colon cancer cells invaded the collagen gels only when myofibroblasts were added. The proinvasive activity was found also with conditioned media from myofibroblast cultures, suggesting the involvement of soluble mediators. Furthermore, using a coimplantation tumor xenograft mouse model, myofibroblasts were shown to stimulate invasive growth of breast and colon cancer cells.16, 67 Several transgenic mouse models clarify the complex signaling between the epithelium and the stroma and suggest that normal fibroblasts may prevent epithelia from becoming tumorigenic. Conditional inactivation of the bone morphogenic protein Type II receptor in the stroma increases the myofibroblast cell population, causing epithelial hyperplasia throughout the colon.68 Furthermore, loss of TGF-β responsiveness in fibroblast specific protein-1-positive fibroblasts, by conditional inactivation of the TGF-β Type II receptor, resulted in an increase of scatter factor/hepatocyte growth factor (SF/HGF)-secreting stromal fibroblasts causing intraepithelial neoplasia in prostate and invasive squamous cell carcinoma of the forestomach.69 In another model, lowering Foxf gene dosage by inactivation of the Foxf1 and Foxf2 mesenchymal forkhead transcription factors results in accumulation of Wnt5a secreting α–SMA positive myofibroblasts. As expected, increased Wnt5a expression in the surrounding stroma activates the canonical Wnt pathway with stabilized β-catenin in epithelial cells leading to epithelial hyperproliferation and resistance to apoptosis.70

Oncogene-induced hyperplasia (the efferent signal) triggers a stromal response initiating a vicious cycle of paracrine afferent signals leading to tumor invasion and loss of tissue integrity. A key question is to identify the efferent signals (cancer cell-derived) that have an impact on myofibroblast attraction, differentiation, proliferation and production of proinvasive signals. Myofibroblasts infiltrate collagen matrices upon treatment with TGF-β71 and invasion of myofibroblasts into subcutaneously implanted ovarian cancer spheroids mark the exit of tumors from dormancy. Ex vivo labeling of myofibroblasts with either magnetic resonance or near-infrared and fluorescent stains render them detectable for in vivo imaging and reveals the alignment of these invading myofibroblasts in the outer rim of the tumor, colocalizing with the angiogenic neovasculature.72 Once stromal cells are attracted, they differentiate into myofibroblasts. Orthotopic transplantation into a nude mouse pancreas of TGF-β1 transfected Panc-1 cells induced a myofibroblast-rich stroma, suggesting that the transfer of a single growth factor conveys the ability to induce a fibroblast response similar to that seen in human pancreas tumors (cited in Ref.2). In vitro coculture experiments show that cancer cell-derived TGF-β modulates myofibroblast differentiation and promotes SF/HGF dependent invasion of colon, breast and squamous carcinoma cells.16, 25, 73 TGF-β-induced responses require Smad2 and the smooth muscle transcriptional regulator myocardin.74 The above-mentioned observations support the opinion that TGF-β is a dominant indirect proinvasive factor for epithelial cancer cells, as it converts α-SMA negative fibroblasts that do not stimulate invasion into α-SMA positive myofibroblasts that do stimulate invasion. We note that other factors acting in combination may also be implicated in this maintained conversion75 (Fig. 3).

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Figure 3. Molecular cross-signaling between myofibroblasts and cancer cells. Efferent factors (black window) produced by cancer cells regulate the conversion from stromal precursors into myofibroblasts (black dotted arrow), which produce afferent factors (red window) regulating invasive cancer growth (red dotted arrows). Abbreviations: ADAM, a disintegrin and metalloproteinase; COX, cyclooxygenase; EGF, epidermal growth factor; EMMPRIN, extracellular matrix metalloproteinase inducer; FGF, fibroblast growth factor; GF, growth factor; GRO, growth-regulated oncogene; IGF, insulin-like growth factor; IL, interleukin; MMP, matrix metalloproteinase; NGF, nerve growth factor LPA, lysophosphatic acid; PDGF, platelet-derived growth factor; RANTES, regulated on activation, normal T cell expressed and secreted; SDF, stromal cell-derived factor; SF/HGF, scatter factor/hepatocyte growth factor; SFRP, secreted frizzled-related proteins; TGF, transforming growth factor; TNF, tumor necrosis factor; VEGF, vascular endothelial growth factor.

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The cancer-induced formation of a myofibroblast network may serve as guidance structure that direct the migration of epithelial cancer cells. Imaging of collectively invading cocultures of squamous cell carcinoma cells and CAF's reveals that the leading cell is of stromal origin and that the cancer cells move within tracks in the ECM generated by the leading CAF.15 To achieve this, the invading CAF's trigger both proteolytic and structural modifications of the ECM to create channels that precede progressively widening chains of following cancer cells. Myofibroblasts and CAF's may induce invasive growth by transient heterotypic cell–cell contacts (reviewed in Ref.2) or by paracrine diffusible signals. The afferent proinvasive growth signals that are identified by screening the secretome from myofibroblasts or CAF's are summarized in Figure 3. Interestingly, during invasive growth, cancer cells have an inherent tendency to turn to anaerobic glycolysis even in the presence of high oxygen tension. Therefore, the tumor stroma expresses complementary metabolic pathways to buffer and recycle products of anaerobic metabolism in order to sustain invasive cancer growth.98

Altered responsive cross-signaling by stromal cells may also be initiated by mutations. In a mouse model of prostate carcinoma, Hill et al.99 showed that cancer cells induce upregulation of p53 through a paracrine mechanism in stromal CAF's. This process creates a selective pressure that promotes the expansion of a subpopulation of CAF's that lack p53. Stromal CAF's that lack p53 then contribute to cancer invasion. Mutation including loss of heterozygosity is a common feature and occurs in both the carcinoma epithelium and stroma from the tumor ecosystem. In over one-fourth of sporadic breast cancers, the stromal cells appeared to harbor p53 mutations. Moreover, the presence of p53 mutations in the stroma, but not in the cancer cells, was significantly associated with lymph-node metastases.100 How mutations are acquired by tumor stroma is currently not known. A possible mechanism may be the induction of stromal-derived MMP-3 that is frequently upregulated in breast cancer, and that induces genomic instability through upregulation of reactive oxygen species (ROS).101 Alternatively, mutated stromal cells might be cancer cells that have undergone epithelial-to-mesenchymal transition.102

It is important that, in the natural situation in vivo, the tumor is not composed solely of cancer cells and tumor-associated host myofibroblasts (Fig. 4). The latter may recruit (lymph) endothelial cells that organize into newly formed (lymph) vessels80, 114 and monocytes which, next to their immunomodulatory activity, may also stimulate invasion. Conversely, myofibroblasts may also suppress the function of cancer-killing T-cells.104

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Figure 4. Molecular cross-signaling between myofibroblasts and other tumor-associated host cells within the tumor ecosystem. Factors produced by myofibroblasts (red windows) signal to other cells present in the tumor ecosystem, which produce factors (black windows) that modulate myofibroblast function. Abbreviations: COX, cyclooxygenase; FGF, fibroblast growth factor; GM-CSF, granulocyte macrophage-colony stimulating factor; IGF, insulin-like growth factor; IL, interleukin; MCP, monocyte chemoattractant protein; NGF, neural growth factor; PDGF, platelet-derived growth factor; SF/HGF, scatter factor/hepatocyte growth factor; TGF, transforming growth factor; TN, tenascin; TNF, tumor necrosis factor; VEGF, vascular endothelial growth factor. Data from Refs.10, 19, 103–113.

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How do myofibroblasts react to cancer management?

  1. Top of page
  2. Abstract
  3. Characterization and origin of a myofibroblast
  4. Are myofibroblast changes unique for cancer?
  5. The tumor stroma regulates invasion and metastasis
  6. How do myofibroblasts react to cancer management?
  7. Conclusion
  8. Acknowledgements
  9. References

Myofibroblasts have been proposed as putative targets for therapy.2, 5, 64 The issue raised here is slightly different, discussing whether or not routine methods of cancer management may stimulate myofibroblasts and by inference enhance invasion and metastasis. We did not find prospective randomized clinical trials providing Level 1 evidence in favor or against such side-effect of management so that the present discussion is admittedly speculative.4, 115

Surgical interventions make wounds and will inevitably elucidate myofibroblast stimulation as part of the healing process. This makes a better niche for growth and invasion of cancer cells that were eventually spillled in the operation field.116 During wound healing, proinvasive cytokines, such as epidermal growth factor, TGF-α, TGF-β, basic fibroblast growth factor, insulin-like growth factor and proteinases, such as MMPs are released, some of these by myofibroblasts. Biopsies, routinely practiced for histopathological diagnosis, make a wound in the core of the tumor. In areas surrounding the biopsy track in breast cancers, CAF's express higher amounts of the proinvasive proteinase urokinase plasminogen activator (uPA) complex than in intact parts of the tumor.117 Tumor seeding in the biopsy tract is very rare, except for sarcomas. Nevertheless, surgeons are well aware of the preclinical evidence for proinvasive and prometastatic potential of wound reactions. To counteract this potential drawback, they are considering minimal surgical trauma and postoperative anti-inflammatory treatment.115, 118 Comparison between conventional and laparoscopic-assisted surgery for colorectal cancer illustrates the multiple variables that are implicated in such analysis and that make conclusions difficult.115, 119 Apart from differences in the extent of the surgical trauma, the effect of the carbon dioxide pneumoperitoneum, applied for laparoscopy, upon the metabolism of the cancer cells including invasive and metastatic stimuli should be taken into consideration.120 Breast prostheses are implanted during mastectomy for esthetical reasons. Such prostheses enhance fibrosis around the site of implantation. Although prospective randomized trials are lacking, there is no evidence that, in breast cancer, immediate expander-implant reconstruction, followed by adjuvant chemotherapy, radiotherapy and eventual hormonal treatment enhances locoregional relapse or distant metastasis.121, 122

The effects of radiotherapy on invasion and metastasis, including the putative role of ionizing radiation (IR)-induced stimulation of myofibroblasts, have been recently reviewed.4 As part of an elaborate reaction, also implicating inflammation and angiogenesis, IR-induced fibrosis in noncancerous tissue is a side effect of tumor radiotherapy. For some organs such as the rectum,123 the small intestine124 and the lungs,18 implicated in irradiation of prostate, gynecological and thoracic cancers respectively, fibrosis constitutes a major constraint. TGF-β is a master switch for the initiation and the persistence of IR-induced fibrosis. ROS generated within milliseconds after irradiation, cause oxidation of specific amino acids in the latent TGF-β complex and release of its active form (reviewed in Ref.2). In vitro, IR converts fibroblasts into myofibroblasts,125 and stimulates the proinvasive activity of MRC-5 embryonic lung fibroblasts and pancreatic CAF's. It is likely that IR is responsible for irreversible DNA mutations in these CAF's. Pancreatic cancer cells are more invasive into Matrigel in vitro or after orthotopic implantation in vivo when they are cocultured with irradiated (5–10 Gy) as compared to nonirradiated MRC5 fibroblasts.126 Experiments with Smad3−/− mice show that TGF-β acts also as a chemoattractant, since less myofibroblasts invade irradiated skin wounds of the transgenic as compared to wild type mice.127

The reaction of tumor-associated myofibroblasts can be evaluated in patients that receive neo-adjuvant treatment, exemplified by colorectal cancer, since in such cases material is available both before (diagnostic biopsy) and after (surgical resection) the neoadjuvant treatment. It should be taken into account that neoadjuvant radiotherapy is frequently associated with chemotherapy. In irradiated tumor-free tissue, such as the distal resection margins of the colorectal specimens, the fibrogenic effect of the radiotherapy is quite obvious (Fig. 5). In the area of the tumor, evaluation of therapy-induced changes of the myofibroblasts is less easy because of extensive desmoplasia before therapy. Proposals for grading of tumor regression take into account fibrotic changes and irradiation vasculopathy, though the emphasis is on the residual mass of cancer cells.128

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Figure 5. Immunohistochemical staining of colorectal cancers. Paraffin sections were made from resection specimens of colorectal cancers at the level of the tumor (panels A and B) or of the tumor-free distal end of the specimen (panels C and D). Patients underwent neo-adjuvant radio-chemotherapy during 5 weeks followed by surgery 6 weeks later (panels B and D) or were operated immediately after diagnosis (panels A and C). Sections were immunostained with an antibody against α-SMA (smooth muscle actin). Scale bar = 100 μm (Our unpublished results).

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Chemotherapeutic agents like cis-platinum or alkylating agents activate TGF-β like IR18; they cause local chronic inflammation and submucosal fibrosis in human cancers, for example, esophageal squamous cell carcinoma.129 We found no literature concerning the amount or the nature of fibrosis as compared before and after therapy.

Hormonal treatment is used mostly in breast (tamoxifen) and prostate cancer (androgen blockade), but their effect on the tumor stroma has not been subject to clinical studies. Experimental data show that androgens may stimulate proliferation of myofibroblasts. The WPMY-1, human prostate cancer-derived myofibroblast cell line immortalized by SV40 large T-antigen is AR (androgen receptor)-positive and its growth is stimulated by the synthetic androgen mibolerone.130 AR is expressed in Dupuytren's disease and in cell cultures derived therefrom. These cells respond to 5α-dihydrotestosterone, the testosterone metabolite that binds to the AR, by increased proliferation and expression of α-SMA with disappearance of the AR.129 The desmoplastic reaction is abolished by 17-β estradiol in ras-transfected MCF-7 nude mouse tumors (cited in Ref.2). This effect is explained through excessive stimulation of MCF-7 cancer cell growth that may outgrow the myofibroblast population, as stimulation of myofibroblast proliferation by MCF-7 in vitro is unaltered by estradiol, and since the murine myofibroblasts that participate at the formation of the xenografted tumor do not express estrogen receptor (ER). PCR analysis of hormone receptors in microdissected archival paraffin embedded tissue from human breast cancers shows a decrease in PR (progesterone receptor) expression, an increase in glucocorticoid receptor expression and no difference in AR, nor ER-α and ER-β expression between cancer stroma and control tissue.131 Hormone receptors have been demonstrated also in benign myofibroblast tumors of the breast, namely PR in some cases of PASH (pseudoangiomatous stromal hyperplasia) in women132, 133 and AR in myofibroblastoma of elderly men.134

Conclusion

  1. Top of page
  2. Abstract
  3. Characterization and origin of a myofibroblast
  4. Are myofibroblast changes unique for cancer?
  5. The tumor stroma regulates invasion and metastasis
  6. How do myofibroblasts react to cancer management?
  7. Conclusion
  8. Acknowledgements
  9. References

Despite increasing numbers of publications illustrating the role of tumor-associated stromal cells in cancer progression, there still exists a significant ambiguity with respect to the identification of CAF's, myofibroblasts and peritumoral fibroblasts in the cancer tissue. We have, therefore, included in the present review stringent criteria for myofibroblast markers. These stromal myofibroblasts appear early during cancer development. The origin of myofibroblasts remains controversial although fibroblasts and bone marrow-derived precursors are considered to be the main progenitor cells. The mutual interaction (through direct cell–cell contacts and paracrine signals) between cancer cells and myofibroblasts is essential for invasive growth and is translated into a poor clinical prognosis. Myofibroblast reactions and fibrosis also occur as a side effect of routine methods of cancer management such as surgery, radio- and chemotherapy. A better understanding of the induction of stromal myofibroblast reactions as a response to current methods of cancer management might reveal the combinatorial signals that support and promote invasive cancer growth. Future methods of cancer management should consider the monitoring of the stromal reaction and the opportunities for therapeutic prevention of the formation of a myofibroblast rich stroma.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Characterization and origin of a myofibroblast
  4. Are myofibroblast changes unique for cancer?
  5. The tumor stroma regulates invasion and metastasis
  6. How do myofibroblasts react to cancer management?
  7. Conclusion
  8. Acknowledgements
  9. References

Mr. G. De Bruyne is gratefully acknowledged for reference management. O. De Wever is a post-doctoral researcher supported by Fund for Scientific Research-Flanders.

References

  1. Top of page
  2. Abstract
  3. Characterization and origin of a myofibroblast
  4. Are myofibroblast changes unique for cancer?
  5. The tumor stroma regulates invasion and metastasis
  6. How do myofibroblasts react to cancer management?
  7. Conclusion
  8. Acknowledgements
  9. References
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    De Wever O,Mareel M. Role of tissue stroma in cancer cell invasion. J Pathol 2003; 200: 42947.
  • 3
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