Octreotide inhibits hepatic cystogenesis in a rodent model of polycystic liver disease by reducing cholangiocyte adenosine 3',5'-cyclic monophosphate

Disease stage characterization of hepatorenal fibrocystic pathology in the PCK rat model of ARPKD

The rat Pck gene is orthologous to the human PKHD1 gene responsible for autosomal recessive polycystic kidney disease (ARPKD). Both renal and hepatic fibrocystic pathology occur in ARPKD. Affected humans have a variable rate of progression, from morbidly affected infants to those surviving into adulthood. This study evaluated the PCK rat, a model of slowly progressive ARPKD. This model originated in Japan and was rederived to be offered commercially by Charles River Laboratories (Wilmington, MA). Previous studies have described the basic aspects of PCK pathology from privately held colonies. This study provides a comprehensive characterization of rats from those commercially available. Rats were bred, maintained on a 12:12 hr light/dark cycle, fed (7002 Teklad), and water provided ad libitum. Male and female rats were evaluated from 4 through 35 weeks of age with histology and serum chemistry. As the hepatorenal fibrocystic disease progressed beyond 18 weeks, the renal pathology (kidney weight, total cyst volume) and renal dysfunction (BUN and serum creatinine) tended to be more severe in males, whereas liver pathology (liver weight as % of body weight and hepatic fibrocystic volume) tended to be more severe in females. Hyperlipidemia was evident in both genders after 18 weeks. Bile secretion was increased in PCK rats compared with age-matched Sprague Dawley rats. The PCK is an increasingly used orthologous rodent model of human ARPKD. This characterization study of hepatorenal fibrocystic pathology in PCK rats should help researchers select stages of pathology to study and/or monitor disease progression during their longitudinal studies.

Activation of Trpv4 reduces the hyperproliferative phenotype of cystic cholangiocytes from an animal model of ARPKD

BACKGROUND & AIMS

In polycystic liver diseases, cyst formation involves cholangiocyte hyperproliferation. In polycystic kidney (PCK) rats, an animal model of autosomal-recessive polycystic kidney disease (ARPKD), decreased intracellular calcium [Ca(2+)](i) in cholangiocytes is associated with hyperproliferation. We recently showed transient receptor potential vanilloid 4 (Trpv4), a calcium-entry channel, is expressed in normal cholangiocytes and its activation leads to [Ca(2+)](i) increase. Thus, we hypothesized that pharmacologic activation of Trpv4 might reverse the hyperproliferative phenotype of PCK cholangiocytes.

METHODS

Trpv4 expression was examined in liver of normal and PCK rats, normal human beings, and patients with autosomal-dominant polycystic kidney disease or ARPKD. Trpv4 activation effect on cell proliferation and cyst formation was assessed in cholangiocytes derived from normal and PCK rats. The in vivo effects of Trpv4 activation on kidney and liver cysts was analyzed in PCK rats.

RESULTS

Trpv4 was overexpressed both at messenger RNA (8-fold) and protein (3-fold) levels in PCK cholangiocytes. Confocal and immunogold electron microscopy supported Trpv4 overexpression in the livers of PCK rats and ARPKD or autosomal-dominant polycystic kidney disease patients. Trpv4 activation in PCK cholangiocytes increased [Ca(2+)](i) by 30%, inhibiting cell proliferation by approximately 25%-50% and cyst growth in 3-dimensional culture (3-fold). Trpv4-small interfering RNA silencing blocked effects of Trpv4 activators by 70%. Trpv4 activation was associated with Akt phosphorylation and beta-Raf and Erk1/2 inhibition. In vivo, Trpv4 activation induced a significant decrease in renal cystic area and a nonsignificant decrease in liver cysts.

CONCLUSIONS

Taken together, our in vitro and in vivo data suggest that increasing intracellular calcium by Trpv4 activation may represent a potential therapeutic approach in PKD.

Polycystic kidney disease: inheritance, pathophysiology, prognosis, and treatment

Both autosomal dominant and recessive polycystic kidney disease are conditions with severe associated morbidity and mortality. Recent advances in the understanding of the genetic and molecular pathogenesis of both ADPKD and ARPKD have resulted in new, targeted therapies designed to disrupt cell signaling pathways responsible for the abnormal cell proliferation, dedifferentiation, apoptosis, and fluid secretion characteristic of the disease. Herein we review the current understanding of the pathophysiology of these conditions, as well as the current treatments derived from our understanding of the mechanisms of these diseases.

Renin-angiotensin system activation in congenital hepatic fibrosis in the PCK rat model of autosomal recessive polycystic kidney disease

OBJECTIVES

Congenital hepatic fibrosis (CHF) is an important cause of morbidity and mortality in patients with autosomal recessive polycystic kidney disease (ARPKD). The pathogenesis of CHF remains undefined. Several recent studies suggest that the renin-angiotensin system (RAS) is an important mediator of progressive hepatic fibrosis through activation of profibrotic mediators, such as transforming growth factor-beta (TGF-beta). RAS activation has not previously been studied in patients with CHF or in animal models. The aim of the present study was to characterize RAS expression during the course of CHF in the PCK rat.

MATERIALS AND METHODS

Studies were conducted in the PCK rat, an orthologous ARPKD/CHF model, and age-matched normal control Sprague-Dawley rats. Expression of the RAS components, renin, angiotensinogen, angiotensin-converting enzyme (ACE), and angiotensin II type 1 receptor (AT1R), as well as the profibrotic mediator TGF-beta, was examined in cystic PCK and control rat livers at 2, 4, and 6 months of age by quantitative real-time polymerase chain reaction (qRT-PCR). Angiotensin II (ANG II) was examined by immunohistochemistry (IHC). Fibrosis was assessed by IHC using reticulin staining and Masson trichrome. Collagen content was determined by hydroxyproline analysis.

RESULTS

Progressive fibrosis and increased hepatic collagen content occurred in PCK rats with age. In 4- and 6-month-old PCK rat livers, ACE gene expression was markedly increased, 8- and 17-fold, respectively, compared with age-matched control livers. Expression of the other RAS components, renin, angiotensinogen, and AT1R were not significantly different. IHC demonstrated prominent ANG II protein expression in periportal regions in PCK rats. In contrast, no expression was noted in control livers. TGF-beta expression was also increased in PCK rat livers with progressive disease.

CONCLUSIONS

The present study demonstrates, for the first time, RAS upregulation in an orthologous rat ARPKD/CHF model. Increases in ACE and ANG II, as well as the downstream target, the profibrotic mediator TGF-beta, suggest that RAS activation may be an important mediator of CHF disease progression. The findings also suggest that treatment with RAS inhibitors, specifically ACE inhibitors or AT1R blockers, could be therapeutic in slowing disease progression in CHF.

Octreotide reduces hepatic, renal and breast cystic volume in autosomal-dominant polycystic kidney disease

A 43-year-old woman with autosomal-dominant polycystic kidney disease (ADPKD) received octreotide for 12 months, and this was associated with a 6.3% reduction in liver volume, an 8% reduction in total kidney volume and stabilization of renal function. There was also a reduction of cyst size in fibrocystic disease of breast. These data suggest that the cyst fluid accumulation in different organs from patients with ADPKD is a dynamic process which can be reversed by octreotide. This is the first report of a case of simultaneous reduction in hepatic, renal and breast cystic volume with preservation of renal function in a patient with ADPKD receiving octreotide.

Randomized clinical trial of long-acting somatostatin for autosomal dominant polycystic kidney and liver disease

There are no proven, effective therapies for polycystic kidney disease (PKD) or polycystic liver disease (PLD). We enrolled 42 patients with severe PLD resulting from autosomal dominant PKD (ADPKD) or autosomal dominant PLD (ADPLD) in a randomized, double-blind, placebo-controlled trial of octreotide, a long-acting somatostatin analogue. We randomly assigned 42 patients in a 2:1 ratio to octreotide LAR depot (up to 40 mg every 28+/-5 days) or placebo for 1 year. The primary end point was percent change in liver volume from baseline to 1 year, measured by MRI. Secondary end points were changes in total kidney volume, GFR, quality of life, safety, vital signs, and clinical laboratory tests. Thirty-four patients had ADPKD, and eight had ADPLD. Liver volume decreased by 4.95%+/-6.77% in the octreotide group but remained practically unchanged (+0.92%+/-8.33%) in the placebo group (P=0.048). Among patients with ADPKD, total kidney volume remained practically unchanged (+0.25%+/-7.53%) in the octreotide group but increased by 8.61%+/-10.07% in the placebo group (P=0.045). Changes in GFR were similar in both groups. Octreotide was well tolerated; treated individuals reported an improved perception of bodily pain and physical activity. In summary, octreotide slowed the progressive increase in liver volume and total kidney volume, improved health perception among patients with PLD, and had an acceptable side effect profile.

Polycystic liver diseases

Polycystic liver diseases (PCLDs) are genetic disorders with heterogeneous etiologies and a range of phenotypic presentations. PCLD exhibits both autosomal or recessive dominant pattern of inheritance and is characterized by the progressive development of multiple cysts, isolated or associated with polycystic kidney disease, that appear more extensive in women. Cholangiocytes have primary cilia, functionally important organelles (act as mechanosensors) that are involved in both normal developmental and pathological processes. The absence of polycystin-1, 2, and fibrocystin/polyductin, normally localized to primary cilia, represent a potential mechanism leading to cyst formation, associated with increased cell proliferation and apoptosis, enhanced fluid secretion, abnormal cell-matrix interactions, and alterations in cell polarity. Proliferative and secretive activities of cystic epithelium can be regulated by estrogens either directly or by synergizing growth factors including nerve growth factor, IGF1, FSH and VEGF. The abnormalities of primary cilia and the sensitivity to proliferative effects of estrogens and different growth factors in PCLD cystic epithelium provide the morpho-functional basis for future treatment targets, based on the possible modulation of the formation and progression of hepatic cysts.

Isolated polycystic liver disease

Isolated polycystic liver disease (PCLD) is an autosomal dominant disease with genetic and clinical heterogeneity. Apart from liver cysts, it exhibits few extrahepatic manifestations, and the majority of patients with this condition are asymptomatic or subclinical. However, a small fraction of these patients develop acute liver cyst-related complications and/or massive cystic liver enlargement, causing morbidity and mortality. Currently, the management for symptomatic PCLD is centered on palliating symptoms and treating complications.

Treatment strategies and clinical trial design in ADPKD

More frequent utilization and continuous improvement of imaging techniques has enhanced appreciation of the high phenotypic variability of autosomal dominant polycystic kidney disease, improved understanding of its natural history, and facilitated the observation of its structural progression. At the same time, identification of the PKD1 and PKD2 genes has provided clues to how the disease develops when they (genetic mechanisms) and their encoded proteins (molecular mechanisms) are disrupted. Interventions designed to rectify downstream effects of these disruptions have been examined in animal models, and some are currently tested in clinical trials. Efforts are underway to determine whether interventions capable to slow down, stop, or reverse structural progression of the disease will also prevent decline of renal function and improve clinically significant outcomes.

Extrarenal manifestations of autosomal dominant polycystic kidney disease

Although asymptomatic in most patients, extrarenal manifestations of ADPKD may become more clinically relevant with the increasing life expectancy of affected patients. They mainly encompass cysts in other organs than the kidney (liver: 94%, seminal vesicle: 40%, pancreas: 9%, arachnoid membrane: 8%, and spinal meningeal, 2%) and connective tissue abnormalities (mitral valve prolapse: 25%, intracranial aneurysms: 8%, and abdominal hernia: 10%). Their recognition may spare the patient from other, useless investigations (eg, when an arachnoid cyst is incidentally found) or lead to the implementation of prophylactic or therapeutic measures (eg, screening, sometimes followed by the treatment of an asymptomatic intracranial aneurysm in at-risk patients, or, in the presence of a severe polycystic liver disease, avoidance from estrogens and treatment aimed to slow cyst growth).

Novel targets for the treatment of autosomal dominant polycystic kidney disease

IMPORTANCE OF THE FIELD

Autosomal dominant (AD) polycystic kidney disease (PKD) is the most common life-threatening hereditary disorder. There is currently no therapy that slows or prevents cyst formation and kidney enlargement in humans. An increasing number of animal studies have advanced our understanding of molecular and cellular targets of PKD.

AREAS COVERED IN THE REVIEW

The purpose of this review is to summarize the molecular and cellular targets involved in cystogenesis and to update on the promising therapies that are being developed and tested based on knowledge of these molecular and cellular targets.

WHAT THE READER WILL GAIN

Insight into the pathogenesis of PKD and how a better understanding of the pathogenesis of PKD has led to the development of potential therapies to inhibit cyst formation and/or growth and improve kidney function.

TAKE HOME MESSAGE

The results of animal studies in PKD have led to the development of clinical trials testing potential new therapies to reduce cyst formation and/or growth. A vasopressin V2 receptor antagonist, mTOR inhibitors, blockade of the renin-angiotensin system and statins that reduce cyst formation and improve renal function in animal models of PKD are being tested in interventional studies in humans.

Reducing polycystic liver volume in ADPKD: effects of somatostatin analogue octreotide

BACKGROUND AND OBJECTIVES

No medical treatment is available for polycystic liver disease, a frequent manifestation of autosomal-dominant polycystic kidney disease (ADPKD). In a prospective, randomized, double-blind, crossover study, 6 months of octreotide (40 mg every 28 days) therapy limited kidney volume growth more effectively than placebo in 12 patients with ADPKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In this secondary, post hoc analysis of the above study, octreotide-induced changes in liver volumes compared with placebo and the relationship between concomitant changes in liver and kidney volumes were evaluated. Those analyzing liver and kidney volumes were blinded to treatment.

RESULTS

Liver volumes significantly decreased from 1595 +/- 478 ml to 1524 +/- 453 ml with octreotide whereas they did not appreciably change with placebo. Changes in liver volumes were significantly different between the two treatment periods (-71 +/- 57 ml versus +14 +/- 85 ml). Octreotide-induced liver volume reduction was fully explained by a reduction in parenchyma volume from 1506 +/- 431 ml to 1432 +/- 403 ml. Changes in liver volumes were significantly correlated with concomitant changes in kidney volumes (r = 0.67) during octreotide but not during placebo treatment. Liver and kidney volume changes significantly differed with both treatments (octreotide: -71 +/- 57 ml versus +71 +/- 107; placebo: +14 +/- 85 ml versus +162 +/- 114), but net reductions in liver (-85 +/- 103 ml) and kidney (-91 +/- 125 ml) volume growth on octreotide versus placebo were similar.

CONCLUSIONS

Octreotide therapy reduces liver volumes in patients with ADPKD and is safe.

Hepato-renal pathology in pkd2ws25/- mice, an animal model of autosomal dominant polycystic kidney disease

Polycystic liver diseases, the most important of which are autosomal dominant and autosomal recessive polycystic kidney diseases, are incurable pathological conditions. Animal models that resemble human pathology in these diseases provide an opportunity to study the mechanisms of cystogenesis and to test potential treatments. Here we demonstrate that Pkd2ws25/- mice, an animal model of autosomal dominant polycystic kidney disease, developed hepatic cysts. As assessed by micro-computed tomography scanning of intact livers and by light microscopy of hepatic tissue, hepatic cystic volumes increased from 12.82+/-3.16% (5- to 8-month-old mice) to 21.58+/-4.81% (9- to 12-month-old mice). Renal cystogenesis was more severe at early stages of disease: in 5- to 7-month-old mice, cystic volumes represented 40.67+/-5.48% of kidney parenchyma, whereas in older mice cysts occupied 31.04+/-1.88% of kidney parenchyma. Mild fibrosis occurred only in liver, and its degree was unchanged with age. Hepatic cysts were lined by single or multiple layers of squamous cholangiocytes. Cystic cholangiocyte cilia were short and malformed, whereas in renal cysts they appeared normal. In Pkd2ws25/- mice, mitotic and apoptotic indices in both kidney and liver were increased compared with wild-type mice. In conclusion, Pkd2ws25/- mice exhibit hepatorenal pathology resembling human autosomal dominant polycystic kidney disease and represent a useful model to study mechanisms of cystogenesis and to evaluate treatment options.

ERK1/2-dependent vascular endothelial growth factor signaling sustains cyst growth in polycystin-2 defective mice

BACKGROUND & AIMS

Severe polycystic liver disease can complicate adult dominant polycystic kidney disease, a genetic disease caused by defects in polycystin-1 (Pkd1) or polycystin-2 (Pkd2). Liver cyst epithelial cells (LCECs) express vascular endothelial growth factor (VEGF) and its receptor, VEGFR-2. We investigated the effects of VEGF on liver cyst growth and autocrine VEGF signaling in mice with Pkd1 and Pkd2 conditional knockouts.

METHODS

We studied mice in which Pkd1 or Pkd2 were conditionally inactivated following exposure to tamoxifen; these mice were called Pkd1(flox/-):pCxCreER (Pkd1KO) and Pkd2(flox/-):pCxCreER (Pkd2KO).

RESULTS

Pkd1KO and Pkd2KO mice developed liver defects; their LCECs expressed VEGF, VEGFR-2, hypoxia-inducible factor (HIF)-1alpha, phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2), and proliferating cell nuclear antigen (PCNA). In Pkd2KO but not Pkd1KO mice, exposure to the VEGFR-2 inhibitor SU5416 significantly reduced liver cyst development, liver/body weight ratio, and expression of pERK and PCNA. VEGF secretion and phosphorylation of ERK1/2 and VEGFR-2 were significantly increased in cultured LCECs from Pkd2KO compared with Pkd1KO mice. Inhibition of protein kinase A (PKA) reduced VEGF secretion and pERK1/2 expression. Addition of VEGF to LCECs from Pkd2KO mice increased phosphorylated VEGFR-2 and phosphorylated mitogen signal-regulated kinase (MEK) expression and induced phosphorylation of ERK1/2; this was inhibited by SU5416. Expression of HIF-1alpha increased in parallel with secretion of VEGF following LCEC stimulation. VEGF-induced cell proliferation was inhibited by the MEK inhibitor U1026 and by ERK1/2 small interfering RNA.

CONCLUSIONS

The PKA-ERK1/2-VEGF signaling pathway promotes growth of liver cysts in mice. In Pkd2-defective LCECs, PKA-dependent ERK1/2 signaling controls HIF-1alpha-dependent VEGF secretion and VEGFR-2 signaling. Autocrine and paracrine VEGF signaling promotes the growth of liver cysts in Pkd2KO mice. VEGF inhibitors might be used to treat patients with polycystic liver disease.

Cyclic nucleotide signaling in polycystic kidney disease

Increased levels of 3'-5'-cyclic adenosine monophosphate (cAMP) stimulate cell proliferation and fluid secretion in polycystic kidney disease. Levels of this molecule are more sensitive to inhibition of phosphodiesterases (PDEs), whose activity far exceeds the rate of cAMP synthesis by adenylyl cyclase. Several PDEs exist, and here we measured the activity and expression of PDE families, their isoforms, and the expression of downstream effectors of cAMP signaling in the kidneys of rodents with polycystic kidney disease. We found a higher overall PDE activity in kidneys from mice as compared with rats, as well as a higher contribution of PDE1, relative to PDE4 and PDE3, to total PDE activity of kidney lysates and lower PDE1, PDE3, and PDE4 activities in the kidneys of cystic as compared with wild-type mice. There were reduced amounts of several PDE1, PDE3, and PDE4 proteins, possibly due to increased protein degradation despite an upregulation of their mRNA. Increased levels of cGMP were found in the kidneys of cystic animals, suggesting in vivo downregulation of PDE1 activity. We found an additive stimulatory effect of cAMP and cGMP on cystogenesis in vitro. Cyclic AMP-dependent protein kinase subunits Ialpha and IIbeta, PKare, the transcription factor CREB-1 mRNA, and CREM, ATF-1, and ICER proteins were upregulated in the kidneys of cystic as compared with wild-type animals. Our study suggests that alterations in cyclic nucleotide catabolism may render cystic epithelium particularly susceptible to factors acting on Gs-coupled receptors. This may account, in part, for increased cyclic nucleotide signaling in polycystic kidney disease and contribute substantially to disease progression.

Treatment of polycystic liver disease

A range of options

Lanreotide reduces the volume of polycystic liver: a randomized, double-blind, placebo-controlled trial

BACKGROUND & AIMS

Therapy for polycystic liver is invasive, expensive, and has disappointing long-term results. Treatment with somatostatin analogues slowed kidney growth in patients with polycystic kidney disease (PKD) and reduced liver and kidney volume in a PKD rodent model. We evaluated the effects of lanreotide, a somatostatin analogue, in patients with polycystic liver because of autosomal-dominant (AD) PKD or autosomal-dominant polycystic liver disease (PCLD).

METHODS

We performed a randomized, double-blind, placebo-controlled trial in 2 tertiary referral centers. Patients with polycystic liver (n = 54) were randomly assigned to groups given lanreotide (120 mg) or placebo, administered every 28 days for 24 weeks. The primary end point was the difference in total liver volume, measured by computerized tomography at weeks 0 and 24. Analyses were performed on an intention-to-treat basis.

RESULTS

Baseline characteristics were comparable for both groups, except that more patients with ADPKD were assigned to the placebo group (P = .03). The mean liver volume decreased 2.9%, from 4606 mL (95% confidence interval (CI): 547-8665) to 4471 mL (95% CI: 542-8401 mL), in patients given lanreotide. In the placebo group, the mean liver volume increased 1.6%, from 4689 mL (95% CI: 613-8765 mL) to 4895 mL (95% CI: 739-9053 mL) (P < .01). Post hoc stratification for patients with ADPKD or PCLD revealed similar changes in liver volume, with statistically significant differences in patients given lanreotide (P < .01 for both diseases).

CONCLUSIONS

In patients with polycystic liver, 6 months of treatment with lanreotide reduces liver volume.

Autosomal dominant polycystic kidney disease: the last 3 years

Autosomal dominant polycystic kidney disease is the most prevalent, potentially lethal monogenic disorder. It has large inter- and intra-familial variability explained to a large extent by its genetic heterogeneity and modifier genes. An increased understanding of its underlying genetic, molecular, and cellular mechanisms and a better appreciation of its progression and systemic manifestations have laid out the foundation for the development of clinical trials and potentially effective therapies. The purpose of this review is to update the core of knowledge in this area with recent publications that have appeared during 2006-2009.

Follicle-stimulating hormone increases cholangiocyte proliferation by an autocrine mechanism via cAMP-dependent phosphorylation of ERK1/2 and Elk-1

Sex hormones regulate cholangiocyte hyperplasia in bile duct-ligated (BDL) rats. We studied whether follicle-stimulating hormone (FSH) regulates cholangiocyte proliferation. FSH receptor (FSHR) and FSH expression was evaluated in liver sections, purified cholangiocytes, and cholangiocyte cultures (NRICC). In vivo, normal female and male rats were treated with FSH or immediately after BDL with antide (a gonadotropin-releasing hormone antagonist blocking FSH secretion) or a neutralizing FSH antibody for 1 wk. We evaluated 1) cholangiocyte proliferation in sections and cholangiocytes and 2) changes in secretin-stimulated cAMP (functional index of cholangiocyte growth) levels, and ERK1/2 and Elk-1 phosphorylation. NRICC were stimulated with FSH before evaluation of proliferation, cAMP/IP(3) levels, and ERK1/2 and Elk-1 phosphorylation. To determine whether FSH regulates cholangiocyte proliferation by an autocrine mechanism, we evaluated the effects of 1) cholangiocyte supernatant (containing FSH) on NRICC proliferation and 2) FSH silencing in NRICC before measuring proliferation and ERK1/2 and Elk-1 phosphorylation. Cholangiocytes and NRICC express FSHR and FSH and secrete FSH. In vivo administration of FSH to normal rats increased, whereas administration of antide and anti-FSH antibody to BDL rats decreased 1) ductal mass and 2) secretin-stimulated cAMP levels, proliferation, and ERK1/2 and Elk-1 phosphorylation in cholangiocytes compared with controls. In NRICC, FSH increased cholangiocyte proliferation, cAMP levels, and ERK1/2 and Elk-1 phosphorylation. The supernatant of cholangiocytes increased NRICC proliferation, inhibited by preincubation with anti-FSH antibody. Silencing of FSH gene decreases cholangiocyte proliferation and ERK1/2 and Elk-1 phosphorylation. Modulation of cholangiocyte FSH expression may be important for the management of cholangiopathies.

MicroRNAs in cholangiociliopathies

This review is focused on current findings implicating miRNAs in the polycystic liver diseases, which we categorized as cholangiociliopathies. Our recent data suggest that deregulation of miRNA pathways is emerging as a novel mechanism in the development of cholangiociliopathies. Experimental evidence demonstrates that miRNAs (i.e., miR-15a) influence hepatic cyst growth by affecting the expression of the cell cycle regulator, Cdc25A. Given that abnormalities in many cellular processes (i.e., cell cycle regulation, cell proliferation, cAMP and calcium signaling, the EGF-stimulated mitogen-activated protein kinase (MAPK) pathway and fluid secretion) contribute to the hepatic cystogenesis, the potential role of miRNAs in regulation of these processes is discussed.

Cholangiociliopathies: genetics, molecular mechanisms and potential therapies

PURPOSE OF REVIEW

The present review summarizes recent knowledge on polycystic liver diseases (PCLDs), mechanisms of hepatic cystogenesis and potential therapies for these conditions.

RECENT FINDINGS

PCLD may be classified as cholangiociliopathies. In PCLD associated with polycystic kidney disease, cell proliferation is one of the major mechanisms of cystogenesis, whereas in isolated PCLD (autosomal dominant polycystic liver disease), disrupted cell adhesion may be more important in cyst progression. In cystic cholangiocytes, overexpression of ion transporters and water channels facilitates fluid secretion into the cystic lumen, and growth factors, estrogens and cytokines promote cholangiocyte proliferation. With age, cholangiocytes lining liver cysts acquire features of mesenchymal cells contributing to hepatic fibrocystogenesis. A novel mechanism of liver cyst expansion in PCLD involves microRNA regulatory pathways. Hyperproliferation of cystic cholangiocytes is linked to abnormalities in cell cycle progression and microRNA expression. Decreased levels of miR-15a are coupled to upregulation of its target--the cell cycle regulator, Cdc25A. Cholangiocyte cilia in liver cysts are structurally abnormal. Somatostatin analogues and sirolimus reduce liver cyst volume in PCLD patients.

SUMMARY

Clarification of molecular mechanisms of hepatic cystogenesis provides an opportunity for the development of targeted therapeutic options in PCLD.

Advances in the pathogenesis and treatment of polycystic kidney disease

PURPOSE OF REVIEW

Polycystic kidney disease (PKD) is the most common genetic cause of chronic renal failure. Mouse models of PKD, especially those with mutations in genes that are orthologous to human disease genes, have provided insights into the pathogenesis of cyst formation and advanced the preclinical testing of new drugs.

RECENT FINDINGS

PKD is a ciliopathy that arises from abnormalities in the primary cilium, a sensory organelle present on the surface of most cells. The primary cilium is required for the maintenance of planar cell polarity, which regulates tubular diameter. Acute kidney injury stimulates cell proliferation and promotes cyst formation in a mouse model of PKD. Studies of signaling pathways that are perturbed in PKD have identified new potential therapeutic targets. Drugs that have shown beneficial effects in orthologous animal models of PKD include tolvaptan, octreotide, src inhibitors, CFTR inhibitors, pioglitazone, etanercept, and triptolide.

SUMMARY

Abnormalities in the primary cilium perturb signaling pathways that regulate renal epithelial cell growth and differentiation and lead to the formation of kidney cysts. Acute kidney injury promotes cyst formation and may underlie the variability in disease progression that is observed in affected individuals. Several promising new therapeutic agents that have been validated in orthologous animal models have entered clinical trials in humans.

Cholangiocyte proliferation and liver fibrosis

Cholangiocyte proliferation is triggered during extrahepatic bile duct obstruction induced by bile duct ligation, which is a common in vivo model used for the study of cholangiocyte proliferation and liver fibrosis. The proliferative response of cholangiocytes during cholestasis is regulated by the complex interaction of several factors, including gastrointestinal hormones, neuroendocrine hormones and autocrine or paracrine signalling mechanisms. Activation of biliary proliferation (ductular reaction) is thought to have a key role in the initiation and progression of liver fibrosis. The first part of this review provides an overview of the primary functions of cholangiocytes in terms of secretin-stimulated bicarbonate secretion--a functional index of cholangiocyte growth. In the second section, we explore the important regulators, both inhibitory and stimulatory, that regulate the cholangiocyte proliferative response during cholestasis. We discuss the role of proliferating cholangiocytes in the induction of fibrosis either directly via epithelial mesenchymal transition or indirectly via the activation of other liver cell types. The possibility of targeting cholangiocyte proliferation as potential therapy for reducing and/or preventing liver fibrosis, and future avenues for research into how cholangiocytes participate in the process of liver fibrogenesis are described.

Matrix proteins of basement membrane of intrahepatic bile ducts are degraded in congenital hepatic fibrosis and Caroli's disease

Congenital hepatic fibrosis (CHF) and Caroli's disease are though to result from ductal plate malformation, and the basal laminar components play important roles in biliary differentiation during development. To clarify the involvement of basal laminar components in the ductal plate malformation, this study examined the immunohistochemical expression of laminin and type IV collagen in the livers of CHF and Caroli's disease. Using the polycystic kidney (PCK) rat, an animal model of Caroli's disease with CHF, in vivo and in vitro experiments were also performed. Immunostaining showed that basement membrane expression of laminin and type IV collagen around intrahepatic bile ducts was degraded in CHF, Caroli's disease, and the PCK rats. The degradation of laminin and type IV collagen around bile ducts was also observed in foci of cholangiocarcinoma in situ of Caroli's disease. In vitro, PCK cholangiocytes were found to overexpress plasminogen and a serine proteinase, the tissue-type plasminogen activator (tPA). When PCK cholangiocytes were cultured in Matrigel, the amounts of laminin and collagen in the gel were significantly reduced, and addition of alpha2-antiplasmin in the culture medium inhibited the degradation of laminin and collagen in Matrigel. These results suggest that biliary overexpression of plasminogen and tPA leads to the generation of excessive amounts of plasmin, and subsequent plasmin-dependent lysis of the extracellular matrix molecules may contribute to the biliary dysgenesis in CHF and Caroli's disease, including progressive cystic dilatation of the intrahepatic bile ducts in Caroli's disease. In addition, it is suggested that once cholangiocarcinoma in situ develops in the biliary epithelium of CHF and Caroli's disease, it tends to transform into invasive carcinoma, due to instability of the basement membrane of the bile ducts.

Effect of calcium-sensing receptor activation in models of autosomal recessive or dominant polycystic kidney disease

BACKGROUND

Antagonists of relevant Gs protein-coupled and agonists of relevant Gi protein-coupled receptors lower renal cAMP and inhibit growth of renal cysts in animal models of human ARPKD (PCK rat) and/or ADPKD (Pkd2(-/WS25) mouse). A calcium-sensing receptor (CaR) is expressed in various tubular segments and couples to Gq, thereby activating phospholipase Cgamma, InsP3 generation and calcium mobilization from intracellular stores, and Gi proteins. By both mechanisms, CaR activation could lower intracellular cAMP and inhibit renal cyst growth.

METHODS

PCK rat and Pkd2(-/WS25) mouse littermates were fed rodent chow without or with R-568, a type 2 calcimimetic, at a concentration of 0.05% or 0.1% between 3 and 10 or 16 weeks of age. Histomorphometric analysis was performed with Meta-Morph software. Western analysis and immunohistochemical staining were performed using antibodies for aquaporin-2, urea transporter UT-A1 and CaR. Northern blot hybridization was used to quantify the expression of vasopressin V2 receptor and aquaporin 2 mRNAs. Cyclic AMP was measured using an enzyme immunoassay kit.

RESULTS

R-568 had no effect on kidney weight, cyst volume, plasma BUN concentration or severity of the polycystic liver disease. A significant reduction in renal interstitial fibrosis was detected in PCK rats, but not in Pkd2(-/WS25) mice. R-568 administration, as anticipated, resulted in hypocalcemia and hyperphosphatemia, and significant increases in urine output, osmolar clearance, and urinary excretions of sodium, potassium and calcium.

CONCLUSIONS

CaR activation had no detectable effect on cystogenesis in models of autosomal recessive or dominant polycystic kidney disease. The lack of protective effect could be due to the absence of CaR in the outer medullary and cortical collecting ducts, the reduction in extracellular calcium and the unaffected levels of renal cAMP and renal expression of cAMP-dependent genes. A possible beneficial effect on interstitial fibrosis deserves further study at more advanced stages of the disease.

Characterization of PKD protein-positive exosome-like vesicles

Proteins associated with autosomal dominant and autosomal recessive polycystic kidney disease (polycystin-1, polycystin-2, and fibrocystin) localize to various subcellular compartments, but their functional site is thought to be on primary cilia. PC1+ vesicles surround cilia in Pkhd1(del2/del2) mice, which led us to analyze these structures in detail. We subfractionated urinary exosome-like vesicles (ELVs) and isolated a subpopulation abundant in polycystin-1, fibrocystin (in their cleaved forms), and polycystin-2. This removed Tamm-Horsfall protein, the major contaminant, and subfractionated ELVs into at least three different populations, demarcated by the presence of aquaporin-2, polycystin-1, and podocin. Proteomic analysis of PKD ELVs identified 552 proteins (232 not yet in urinary proteomic databases), many of which have been implicated in signaling, including the molecule Smoothened. We also detected two other protein products of genes involved in cystic disease: Cystin, the product of the mouse cpk locus, and ADP-ribosylation factor-like 6, the product of the human Bardet-Biedl syndrome gene (BBS3). Our proteomic analysis confirmed that cleavage of polycystin-1 and fibrocystin occurs in vivo, in manners consistent with cleavage at the GPS site in polycystin-1 and the proprotein convertase site in fibrocystin. In vitro, these PKD ELVs preferentially interacted with primary cilia of kidney and biliary epithelial cells in a rapid and highly specific manner. These data suggest that PKD proteins are shed in membrane particles in the urine, and these particles interact with primary cilia.

The cAMP effectors Epac and protein kinase a (PKA) are involved in the hepatic cystogenesis of an animal model of autosomal recessive polycystic kidney disease (ARPKD)

PCK rats, an animal model of autosomal recessive polycystic kidney disease (ARPKD), develop cholangiocyte-derived liver cysts associated with increased intracellular adenosine 3',5'-cyclic adenosine monophosphate (cAMP), the inhibition of which suppresses cyst growth. We hypothesized that elevated cAMP stimulates cholangiocyte proliferation via two downstream effectors, exchange proteins activated by cAMP (Epac1 and Epac2 isoforms) and protein kinase A (PKA), and that intracellular calcium is also involved in this process. Assessment of Epac isoforms and PKA regulatory subunits at the messenger RNA and protein level showed that cultured normal rat cholangiocytes express Epac1, Epac2, and all regulatory PKA subunits. Epac isoforms and the PKA RIbeta subunit were overexpressed in cultured PCK cholangiocytes. Proliferation analysis in response to Epac and PKA activation indicated that both normal and PCK cholangiocytes increase their growth upon Epac-specific stimulation, while PKA-specific stimulation results in differential effects, suppressing proliferation in normal cholangiocytes but accelerating this process in PCK cholangiocytes. On the other hand, both PKA and Epac activation of cystic structures generated by normal and PCK cholangiocytes when cultured under three-dimensional conditions resulted in increased cyst growth, particularly in PCK-cholangiocyte derived cysts. Pharmacological inhibitors and small interfering RNA-mediated gene silencing demonstrated the specificity of each effector activation, as well as the involvement of MEK-ERK1/2 signaling in all the observed effector-associated proliferation changes. Hyperproliferation of PCK cholangiocytes in response to PKA stimulation, but not to Epac stimulation, was found to be associated with decreased intracellular calcium, and restoration of calcium levels blocked the PKA-dependent proliferation via the PI3K/AKT pathway.

CONCLUSION

Our data provide strong evidence that both cAMP effectors, Epac and PKA, and the levels of intracellular calcium are involved in the hepatic cystogenesis of ARPKD.

Polycystic kidney disease

A number of inherited disorders result in renal cyst development. The most common form, autosomal dominant polycystic kidney disease (ADPKD), is a disorder most often diagnosed in adults and caused by mutation in PKD1 or PKD2. The PKD1 protein, polycystin-1, is a large receptor-like protein, whereas polycystin-2 is a transient receptor potential channel. The polycystin complex localizes to primary cilia and may act as a mechanosensor essential for maintaining the differentiated state of epithelia lining tubules in the kidney and biliary tract. Elucidation of defective cellular processes has highlighted potential therapies, some of which are now being tested in clinical trials. ARPKD is the neonatal form of PKD and is associated with enlarged kidneys and biliary dysgenesis. The disease phenotype is highly variable, ranging from neonatal death to later presentation with minimal kidney disease. ARPKD is caused by mutation in PKHD1, and two truncating mutations are associated with neonatal lethality. The ARPKD protein, fibrocystin, is localized to cilia/basal body and complexes with polycystin-2. Rare, syndromic forms of PKD also include defects of the eye, central nervous system, digits, and/or neural tube and highlight the role of cilia and pathways such as Wnt and Hh in their pathogenesis.

Structural and functional analyses of liver cysts from the BALB/c-cpk mouse model of polycystic kidney disease

Liver cysts arising from hepatic bile ducts are a common extra-renal pathology associated with both autosomal dominant and recessive polycystic kidney disease in humans. To elucidate the functional and structural changes inherent in cyst formation and growth, hepatic bile duct epithelia were isolated from the BALB/ c-cpk mouse model of polycystic kidney disease. Light and transmission electron microscopy revealed substantial fibrosis in the basal lamina surrounding hepatic bile duct cysts isolated from heterozygous (BALB/c-cpk/+) and homozygous (BALB/c-cpk/cpk) animals. Scanning electron microscopy and length analysis of normal, precystic and cystic bile ducts provided the unique observation that primary cilia in cholangiocytes isolated from bile ducts and cysts of animals expressing the mutated cpk gene had lengths outside the minimal and maximal ranges of those in cells lining bile ducts of wild-type animals. Based on the hypothesis that PKD is one of several diseases characterized as ciliopathies, this abnormal variability in the length of the primary cilia may have functional implications. Electrophysiological analyses of freshly isolated cysts indicate that the amiloride-sensitive epithelial Na(+) channel (ENaC) is inactive/absent and cAMP-mediated anion secretion is the electrogenic transport process contributing to cyst fluid accumulation. Anion secretion can be stimulated by the luminal stimulation of adenylyl cyclase.

Vasopressin antagonists in polycystic kidney disease

Increased cell proliferation and fluid secretion, probably driven by alterations in intracellular calcium homeostasis and cyclic adenosine 3,5-phosphate, play an important role in the development and progression of polycystic kidney disease. Hormone receptors that affect cyclic adenosine monophosphate and are preferentially expressed in affected tissues are logical treatment targets. There is a sound rationale for considering the arginine vasopressin V2 receptor as a target. The arginine vasopressin V2 receptor antagonists OPC-31260 and tolvaptan inhibit the development of polycystic kidney disease in cpk mice and in three animal orthologs to human autosomal recessive polycystic kidney disease (PCK rat), autosomal dominant polycystic kidney disease (Pkd2/WS25 mice), and nephronophthisis (pcy mouse). PCK rats that are homozygous for an arginine vasopressin mutation and lack circulating vasopressin are markedly protected. Administration of V2 receptor agonist 1-deamino-8-D-arginine vasopressin to these animals completely recovers the cystic phenotype. Administration of 1-deamino-8-D-arginine vasopressin to PCK rats with normal arginine vasopressin aggravates the disease. Suppression of arginine vasopressin release by high water intake is protective. V2 receptor antagonists may have additional beneficial effects on hypertension and chronic kidney disease progression. A number of clinical studies in polycystic kidney disease have been performed or are currently active. The results of phase 2 and phase 2-3 clinical trials suggest that tolvaptan is safe and well tolerated in autosomal dominant polycystic kidney disease. A phase 3, placebo-controlled, double-blind study in 18- to 50-yr-old patients with autosomal dominant polycystic kidney disease and preserved renal function but relatively rapid progression, as indicated by a total kidney volume >750 ml, has been initiated and will determine whether tolvaptan is effective in slowing down the progression of this disease.

Hepatic cystogenesis is associated with abnormal expression and location of ion transporters and water channels in an animal model of autosomal recessive polycystic kidney disease

Polycystic kidney (PCK) rats are a spontaneous model of autosomal recessive polycystic kidney disease that exhibit cholangiocyte-derived liver cysts. We have previously reported that in normal cholangiocytes a subset of vesicles contain three proteins (ie, the water channel AQP1, the chloride channel CFTR, and the anion exchanger AE2) that account for ion-driven water transport. Thus, we hypothesized that altered expression and location of these functionally related proteins contribute to hepatic cystogenesis. We show here that under basal conditions and in response to secretin and hypotonicity, cysts from PCK rats expanded to a greater degree than cysts formed by normal bile ducts. Quantitative reverse transcriptase-polymerase chain reaction, immunoblot analysis, and confocal and immunoelectron microscopy all indicated increased expression of these three proteins in PCK cholangiocytes versus normal cholangiocytes. AQP1, CFTR, and AE2 were localized preferentially to the apical membrane in normal rats while overexpressed at the basolateral membrane in PCK rats. Exposure of the cholangiocyte basolateral membrane to CFTR inhibitors [5-nitro-2-(3-phenylpropylamino)-benzoic acid and CFTRinh172], or Cl(-)/HCO(3)(-) exchange inhibitors (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid disodium salt hydrate and 4-acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid disodium salt hydrate) blocked secretin-stimulated fluid accumulation in PCK but not in normal cysts. Our data suggest that hepatic cystogenesis in autosomal recessive polycystic kidney disease may involve increased fluid accumulation because of overexpression and abnormal location of AQP1, CFTR, and AE2 in cystic cholangiocytes. Therapeutic interventions that block the activation of these proteins might inhibit cyst expansion in polycystic liver disease.

MicroRNA15a modulates expression of the cell-cycle regulator Cdc25A and affects hepatic cystogenesis in a rat model of polycystic kidney disease

Hyperproliferation of bile duct epithelial cells due to cell-cycle dysregulation is a key feature of cystogenesis in polycystic liver diseases (PCLDs). Recent evidence suggests a regulatory role for microRNAs (miRNAs) in a variety of biological processes, including cell proliferation. We therefore hypothesized that miRNAs may be involved in the regulation of selected components of the cell cycle and might contribute to hepatic cystogenesis. We found that the cholangiocyte cell line PCK-CCL, which is derived from the PCK rat, a model of autosomal recessive polycystic kidney disease (ARPKD), displayed global changes in miRNA expression compared with normal rat cholangiocytes (NRCs). More specific analysis revealed decreased levels of 1 miRNA, miR15a, both in PCK-CCL cells and in liver tissue from PCK rats and patients with a PCLD. The decrease in miR15a expression was associated with upregulation of its target, the cell-cycle regulator cell division cycle 25A (Cdc25A). Overexpression of miR15a in PCK-CCL cells decreased Cdc25A levels, inhibited cell proliferation, and reduced cyst growth. In contrast, suppression of miR15a in NRCs accelerated cell proliferation, increased Cdc25A expression, and promoted cyst growth. Taken together, these results suggest that suppression of miR15a contributes to hepatic cystogenesis through dysregulation of Cdc25A.

Cholangiocyte primary cilia in liver health and disease

The epithelial cells lining intrahepatic bile ducts (i.e., cholangiocytes), like many cell types in the body, have primary cilia extending from the apical plasma membrane into the bile ductal lumen. Cholangiocyte cilia express proteins such as polycystin-1, polycystin-2, fibrocystin, TRPV4, P2Y12, AC6, that account for ciliary mechano-, osmo-, and chemo-sensory functions; when these processes are disturbed by mutations in genes encoding ciliary-associated proteins, liver diseases (i.e., cholangiociliopathies) result. The cholangiociliopathies include but are not limited to cystic and fibrotic liver diseases associated with mutations in genes encoding polycystin-1, polycystin-2, and fibrocystin. In this review, we discuss the functions of cholangiocyte primary cilia, their role in the cholangiociliopathies, and potential therapeutic approaches.

Advances in management of polycystic liver disease

The focus of this review is polycystic liver disease, a genetic disorder characterized by multiple macroscopic liver cysts that initially bud from biliary epithelium but subsequently lack communication with the biliary tree. There are two main clinical presentations: polycystic liver associated with autosomal dominant polycystic kidney disease and isolated polycystic liver disease. Both of these forms of polycystic liver disease exhibit an autosomal dominant pattern of inheritance. Clinical manifestations of polycystic liver disease are related to either mass effect of the volume of hepatic cysts or to complications arising within the cysts. Polycystic liver disease rarely progresses to hepatic failure or clinical complications of portal hypertension. Management is directed at counseling patients and families, treating complications and reducing cyst load by surgical techniques: cyst fenestration, hepatic resection or, rarely, hepatic transplantation. Recent research suggests that blockade of cyst secretion or inhibition of epithelial cells might be useful in halting progression of disease--these observations are discussed in this review.

Progress and prospects in rat genetics: a community view

The rat is an important system for modeling human disease. Four years ago, the rich 150-year history of rat research was transformed by the sequencing of the rat genome, ushering in an era of exceptional opportunity for identifying genes and pathways underlying disease phenotypes. Genome-wide association studies in human populations have recently provided a direct approach for finding robust genetic associations in common diseases, but identifying the precise genes and their mechanisms of action remains problematic. In the context of significant progress in rat genomic resources over the past decade, we outline achievements in rat gene discovery to date, show how these findings have been translated to human disease, and document an increasing pace of discovery of new disease genes, pathways and mechanisms. Finally, we present a set of principles that justify continuing and strengthening genetic studies in the rat model, and further development of genomic infrastructure for rat research.

Morphological and functional features of hepatic cyst epithelium in autosomal dominant polycystic kidney disease

We evaluated the morphological and functional features of hepatic cyst epithelium in adult autosomal dominant polycystic kidney disease (ADPKD). In six ADPKD patients, we investigated the morphology of cyst epithelium apical surface by scanning electron microscopy and the expression of estrogen receptors (ERs), insulin-like growth factor 1 (IGF1), IGF1 receptors (IGF1-R), growth hormone receptor, the proliferation marker proliferating cell nuclear antigen, and pAKT by immunohistochemistry and immunofluorescence. Proliferation of liver cyst-derived epithelial cells was evaluated by both MTS proliferation assay and [(3)H]thymidine incorporation into DNA. The hepatic cyst epithelium displayed heterogeneous features, being normal in small cysts (<1 cm), characterized by rare or shortened cilia in 1- to 3-cm cysts, and exhibiting the absence of both primary cilia and microvilli in large cysts (>3 cm). Cyst epithelium showed marked immunohistochemical expression of ER, growth hormone receptor, IGF1, IGF1-R, proliferating cell nuclear antigen, and pAKT. IGF1 was 10-fold more enriched in the hepatic cyst fluid than in serum. Serum-deprived liver cyst-derived epithelial cells proliferated when exposed to 17beta-estradiol and IGF1 and when exposed to human cyst fluid. ER or IGF1-R antagonists inhibited the proliferative effect of serum readmission, cyst fluid, 17beta-estradiol, and IGF1. Our findings could explain the role of estrogens in accelerating the progression of ADPKD and may suggest a potential benefit of therapeutic strategies based on estrogen antagonism.