Polycystic liver disease is genetically heterogeneous and occurs alone or in combination with polycystic kidney disease.1 Autosomal dominant polycystic liver disease (ADPLD) displays no renal involvement and is caused by a mutation of the gene protein kinase substrate 80K-H (PRKCSH) on chromosome 19p13 that encodes the protein, hepatocystin. PCLD is characterized by an overgrowth of the biliary epithelium and supportive connective tissue.2–4
Polycystic kidney disease (PKD) is a multiorgan disorder and is the most common genetic, life threatening, disease affecting 12.5 million people worldwide and more than 600,000 Americans. PKD may be inherited as autosomal dominant (ADPKD) or autosomal recessive (ARPKD). ADPKD has an incidence of 1:800 and is linked to a mutation in either PKD1 or PKD2, genes that encode the proteins, polycystin 1 and polycystin 2, respectively. PKD1 is localized to the short arm of chromosome 16p13.3-p13.12, and causes a more frequent (85%-90% of cases) and severe form of ADPKD. Mutations in PKD2 located to chromosome 4q21-q23, account for 10% to 15% of cases. Liver involvement is the most frequent extrarenal manifestation. The prevalence and number of hepatic cysts in patients with ADPKD increase with age, female sex, severity of renal cystic disease and degree of renal dysfunction. Approximately 80% of ADPKD patients by age 60 develop hepatic cysts.1, 5–7
ARPKD is less common than ADPKD, with an incidence of 1:20,000 to 1:40,000 but with a high mortality rate. Disease presentation is highly variable, but genetic linkage studies indicate mutations in a single gene, PKHD1, localized to chromosome 6p21-23 that encodes the protein fibrocystin/polyductin.8, 9 In ARPKD, approximately 30% of affected neonates die within hours after birth because of respiratory difficulties due to greatly enlarged kidneys. The prognosis of those who survive the first month is much better. In surviving patients, kidneys do not increase in size with age and even a progressive decrease in kidney size has been reported.10 However, hepatic lesions become progressively more severe with age, and liver disease is the major cause of morbidity and mortality. Liver involvement is characterized by biliary dysgenesis resulting in congenital hepatic fibrosis, intrahepatic bile duct dilation (Caroli's disease) and/or cyst development.1, 6, 11, 12
Liver symptoms and complications in all the polycystic liver diseases include abdominal distension or discomfort, fullness, back pain, cyst infection, hemorrhage, and rupture; and in ADPLD and ARPKD, portal hypertension and jaundice. To date, no curative or preventive therapy for polycystic liver disease exists. For symptomatic relief, interventional or surgical options include aspiration of cyst fluid, cyst fenestration, liver resection and isolated liver or combined liver-kidney transplantation.1, 12
While the polycystic liver diseases are genetically and clinically heterogeneous, they all lead to similar end points: bile duct dilation and cystogenesis. As noted above, while the genetic defects that are involved in initiation of cyst formation have been identified, the mechanisms underlying the pathogenesis of cyst growth and expansion are still unclear. Recent advances have suggested that abnormalities in at least three clearly distinguishable cellular events occur: (1) cell proliferation, apoptosis and differentiation; (2) cell-matrix interactions; and (3) fluid secretion. Different hormones, growth factors, cytokines (individually or in combination) could impact these processes and promote cyst formation and expansion.
In this issue of the HEPATOLOGY, Fabris and colleagues13 describe the role of angiogenic factors, in particular vascular endothelial growth factor (VEGF), angiopoetin-1 (ang-1) and angiopoetin-2 (ang-2) and their receptors (VEGFR-1, VEGFR-2, Tie-2) in hepatic cystogenesis in ADPKD. They showed that VEGF, VEGFRs, Ang-1 and Tie-2 are all upregulated in cholangiocytes of polycystic livers. Moreover, VEGF stimulates proliferation of normal and cystic (derived from pkd2ws25/− mice, an animal model of ADPKD) cholangiocytes with a more prominent effect in the latter. Furthermore, expression of VEGF in cholangiocytes strongly correlates with vascular density. This is an important study that answers some questions but, as with all well-done good science, generates more.
There are several ways by which angiogenic factors might contribute to liver cyst progression. First, they might accelerate a cholangiocyte hyperplastic response. Evidence for this mechanism include: (1) liver cystic fluid from ADPKD patients contains VEGF in physiological ranges sufficient for binding to VEGFRs14; and (2) as shown in the article by Fabris et al.,13 ADPKD cystic epithelium overexpress VEGFR-1 and VEGFR-2. Second, it is known that VEGF is the most prominent angiogenic factor. Thus, cholangiocytes might secrete VEGF leading to an increased arterial blood supply since, in polycystic livers, an expanded mass of proliferated bile ducts requires additional nutrients and oxygen. Indeed, Fabris et al. have shown that upregulation of VEGF in cystic epithelium is strongly correlated with an increased number of portal vascular structures in close proximity to liver cysts. Why VEGF and VEGFR are overexpressed in polycystic livers is unclear; perhaps their expression is triggered by other factors. For example, IL-6 is known to be present in cystic fluid of ADPKD patients and can increase VEGF expression promoting angiogenesis.15
Extracellular matrix remodeling involves secretion of critical extracellular proteins and metalloproteases that play an important role in cystogenesis. Cholangiocytes from patients with ADPKD show elevated metalloprotease activity and secretion.16 Cytokines might also contribute to these processes since cystic fluid from ADPKD livers contains significant amounts of IL-614 and IL-6 induces proliferation and collagen synthesis from hepatic stellate cells.17
Hepatic cystic epithelium is derived from cholangiocytes and retains a biliary phenotype. In ADPKD, human liver cysts continue to secrete fluid in response to intravenous administration of secretin. Increased rates of fluid secretion generate positive pressure on the cystic luminal wall and may directly trigger cholangiocyte proliferation.18 Indeed, accelerated cell proliferation in response to intraluminal pressure has been shown in cell culture models.1, 19 The rate of progressive cyst expansion is determined to a significant extent by a variety of hormones, lipids, cytokines and growth factors. For example, EGF has been shown to play an important role in the expansion of renal cysts. Moreover, EGF is secreted into the cystic lumen in amounts that can induce cell proliferation, and the EGFR in cystic cells is overexpressed and mislocalized to the apical surface.6 Furthermore, inhibitors of the EGFR are able to reduce the number of cysts in bpk mice, a model of ARPKD.20 It has been recently reported that cultured cholangiocytes from the PCK rat, another well-characterized model of ARPKD,21 are hyperresponsive to EGF and their increased proliferation was accompanied by activation of the MEK5/ERK5 cascade.22 While inhibitors of the EGFRs can suppress bile duct proliferation in vitro, in vivo in the PCK rat, EGFR inhibitors have a completely opposite effect. In contrast to bpk mice and cultured PCK cholangiocytes, EGFR inhibitors worsen the disease progression in the PCK rat.23
It is likely that disruption of several different pathways that control a wide range of highly coordinated cellular processes such as proliferation, differentiation and morphogenesis may lead to the cystic phenotype. PKD-related proteins have been shown to localize to different cellular sites: in the plasma membrane, junctional complexes, intracellular compartments, and recently, in primary cilia and basal bodies.6,24-26 There is strong evidence that primary cilia are linked to the development of polycystic kidney and liver diseases.25–27 In kidney epithelia, primary cilia function as mechanoreceptors responding to luminal flow mediated bending by an increase in intracellular calcium with involvement of two ciliary-associated proteins, polycystin 1 and polycystin 2.28 Mutation in PKD1 disrupts this flow-cilia-calcium axis and cells no longer respond by an increase in calcium influx. Moreover, recent data from our laboratory demonstrate that cholangiocytes have primary cilia that express two PKD-associated proteins, polycystin 1 and polycystin 2. Under normal conditions, cholangiocyte cilia, like cilia in renal epithelia, function as mechanosensors responding to changes in luminal bile flow by activation/inhibition of at least two second messenger systems: intracellular calcium and cAMP.29 We have also shown that primary cilia play a significant role in liver cyst development. In the PCK rat, cholangiocyte cilia are morphologically and functionally abnormal; they are shorter, malformed and do not express fibrocystin.26 Thus, structural and functional abnormalities in primary cilia may result in inability of cholangiocytes to sense mechanical or chemical stimuli. As a result, many pathways underlying normal cell functions may be disrupted leading to cyst progression. However, the mechanisms by which cilia control signaling cascades that are essential for cell proliferation, differentiation and secretion are currently unknown.
While substantial progress has been made in our understanding of the pathogenesis of the renal disease in PKD at the molecular and cellular levels, we still have a lot to learn about factors underlying cyst growth and expansion in the liver. Given the development of new models, methods and probes to study cholangiocyte function and dysfunction, including the ability to isolate cilia from cholangiocytes,30 as well as recent increased interest in cystic disease, clarification of the factors that promote hepatic cyst development and progression can be anticipated.