Increased YAP Activation Is Associated With Hepatic Cyst Epithelial Cell Proliferation in ARPKD/CHF

Dysregulation of the Scribble/YAP/β-catenin axis sustains the fibroinflammatory response in a PKHD1-/- mouse model of congenital hepatic fibrosis

Congenital hepatic fibrosis (CHF), a genetic cholangiopathy characterized by fibropolycystic changes in the biliary tree, is caused by mutations in the PKHD1 gene, leading to defective fibrocystin (FPC), changes in planar cell polarity (PCP) and increased β-catenin-dependent chemokine secretion. In this study, we aimed at understanding the role of Scribble (a protein involved in PCP), Yes-associated protein (YAP), and β-catenin in the regulation of the fibroinflammatory phenotype of FPC-defective cholangiocytes. Immunohistochemistry showed that compared with wild type (WT) mice, in FPC-defective (Pkhd1^del4/del4 ) mice nuclear expression of YAP/TAZ in cystic cholangiocytes, significantly increased and correlated with connective tissue growth factor (CTGF) expression and pericystic fibrosis, while Scribble expression on biliary cyst cells was markedly decreased. Cholangiocytes isolated from WT mice showed intense Scribble immunoreactivity at the membrane, but minimal nuclear expression of YAP, which conversely increased, together with CTGF, after small interfering RNA (siRNA) silencing of Scribble. In FPC-defective cholangiocytes, inhibition of YAP nuclear import reduced β-catenin nuclear expression, and CTGF, integrin β6, CXCL1, and CXCL10 mRNA levels, whereas inhibition of β-catenin signaling did not affect nuclear translocation of YAP. Notably, siRNA silencing of Scribble and YAP in WT cholangiocytes mimics the fibroinflammatory changes of FPC-defective cholangiocytes. Conditional deletion of β-catenin in Pkhd1^del4/del4  mice reduced cyst growth, inflammation and fibrosis, without affecting YAP nuclear expression. In conclusion, the defective anchor of Scribble to the membrane facilitates the nuclear translocation of YAP and β-catenin with gain of a fibroinflammatory phenotype. The Scribble/YAP/β-catenin axis is a critical factor in the sequence of events linking the genetic defect to fibrocystic trait of cholangiocytes in CHF.

Inflammatory pathways and cholangiocarcinoma risk mechanisms and prevention

Cholangiocarcinoma (CCA), a neoplasm burdened by a poor prognosis and currently lacking adequate therapeutic treatments, can originate at different levels of the biliary tree, in the intrahepatic, hilar, or extrahepatic area. The main risk factors for the development of CCA are the presence of chronic cholangiopathies of various etiology. To date, the most studied prodromal diseases of CCA are primary sclerosing cholangitis, Caroli's disease and fluke infestations, but other conditions, such as metabolic syndrome, nonalcoholic fatty liver disease and obesity, are emerging as associated with an increased risk of CCA development. In this review, we focused on the analysis of the pro-inflammatory mechanisms that induce the development of CCA and on the role of cells of the immune response in cholangiocarcinogenesis. In very recent times, these cellular mechanisms have been the subject of emerging studies aimed at verifying how the modulation of the inflammatory and immunological responses can have a therapeutic significance and how these can be used as therapeutic targets.

Notch-Hes1 signaling activation in Caroli disease and polycystic liver disease

The Notch signaling pathway plays a key role in the morphogenesis of the biliary tree, but its involvement in cystic biliary diseases, such as Caroli disease (CD) and polycystic liver disease (PLD), has yet to be determined. Immunostaining was performed using liver sections of CD and PLD, and the results were compared with those of congenital hepatic fibrosis (CHF) and von Meyenburg complex (VMC). The expression of Notch receptor 1 (Notch1) was increased in the nuclei of biliary epithelial cells in all cases of CD and PLD, whereas it remained at a low level in CHF and VMC. In addition, Notch2 and Notch3 were preferably expressed in the nuclei of biliary epithelial cells of PLD. Accordingly, the Notch effector Hes1 was highly expressed in biliary epithelial cells of CD and PLD, and the cell proliferative activity was significantly higher in CD and PLD. The expression of the Notch ligand Delta-like 1 was significantly increased in biliary epithelial cells of CD and PLD, which may be causally associated with the nuclear overexpression of Notch1 and Hes1. These results indicate that aberrant activation of the Notch-Hes1 signaling pathway may be responsible for the progression of biliary cystogenesis in CD and PLD.

TAZ/Wnt-β-catenin/c-MYC axis regulates cystogenesis in polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) is the most common genetic renal disease, primarily caused by germline mutation of PKD1 or PKD2, leading to end-stage renal disease. There are few cures for ADPKD, although many researchers are trying to find a cure. The Hippo signaling pathway regulates organ growth and cell proliferation. Transcriptional coactivator with PDZ-binding motif (TAZ) is a Hippo signaling effector. In this study, we demonstrated that the PKD1–TAZ–Wnt–β-catenin–c-MYC signaling axis plays a critical role in cystogenesis. Endo IWR1 treatment, which inhibited β-catenin activity via AXIN stabilization, reduced cyst growth in an ADPKD model. Our findings provide a potential therapeutic target against ADPKD and would be important for clinical translation.

Autosomal-dominant polycystic kidney disease (ADPKD) is the most common genetic renal disease, primarily caused by germline mutation of PKD1 or PKD2, leading to end-stage renal disease. The Hippo signaling pathway regulates organ growth and cell proliferation. Herein, we demonstrate the regulatory mechanism of cystogenesis in ADPKD by transcriptional coactivator with PDZ-binding motif (TAZ), a Hippo signaling effector. TAZ was highly expressed around the renal cyst-lining epithelial cells of Pkd1-deficient mice. Loss of Taz in Pkd1-deficient mice reduced cyst formation. In wild type, TAZ interacted with PKD1, which inactivated β-catenin. In contrast, in PKD1-deficient cells, TAZ interacted with AXIN1, thus increasing β-catenin activity. Interaction of TAZ with AXIN1 in PKD1-deficient cells resulted in nuclear accumulation of TAZ together with β-catenin, which up-regulated c-MYC expression. Our findings suggest that the PKD1–TAZ–Wnt–β-catenin–c-MYC signaling axis plays a critical role in cystogenesis and might be a potential therapeutic target against ADPKD.

Hippo signaling-a central player in cystic kidney disease?

Cystic transformation of kidney tissue is a key feature of various disorders including autosomal dominant polycystic kidney disease (ADPKD), autosomal recessive polycystic kidney disease (ARPKD), and disorders of the nephronophthisis spectrum (NPH). While ARPKD and NPH typically affect children and adolescents, pediatric onset of ADPKD is less frequently found. While both ADPKD and ARPKD are characterized by formation of hundreds of cysts accompanied by hyperproliferation of tubular epithelia with massive renal enlargement, NPH patients usually show kidneys of normal or reduced size with cysts limited to the corticomedullary border. Recent results suggest the hippo pathway to be a central regulator at the crossroads of the renal phenotype in both diseases. Hippo signaling regulates organ size and proliferation by keeping the oncogenic transcriptional co-activators Yes associated protein 1 (YAP) and WW domain containing transcription regulator 1 (TAZ) in check. Once this inhibition is released, nuclear YAP/TAZ interacts with TEAD family transcription factors and the consecutive transcriptional activation of TEA domain family members (TEAD) target genes mediates an increase in proliferation. Here, we review the current knowledge on the impact of NPHP and ADPKD mutations on Hippo signaling networks. Furthermore, we provide an outlook towards potential future therapeutic strategies targeting Hippo signaling to alleviate cystic kidney disease.

Epithelial Vasopressin Type-2 Receptors Regulate Myofibroblasts by a YAP-CCN2-Dependent Mechanism in Polycystic Kidney Disease

BACKGROUND

Fibrosis is a major cause of loss of renal function in autosomal dominant polycystic kidney disease (ADPKD). In this study, we examined whether vasopressin type-2 receptor (V2R) activity in cystic epithelial cells can stimulate interstitial myofibroblasts and fibrosis in ADPKD kidneys.

METHODS

We treated Pkd1 gene knockout (Pkd1KO) mice with dDAVP, a V2R agonist, for 3 days and evaluated the effect on myofibroblast deposition of extracellular matrix (ECM). We also analyzed the effects of conditioned media from primary cultures of human ADPKD cystic epithelial cells on myofibroblast activation. Because secretion of the profibrotic connective tissue growth factor (CCN2) increased significantly in dDAVP-treated Pkd1KO mouse kidneys, we examined its role in V2R-dependent fibrosis in ADPKD as well as that of yes-associated protein (YAP).

RESULTS

V2R stimulation using dDAVP increased the renal interstitial myofibroblast population and ECM deposition. Similarly, conditioned media from human ADPKD cystic epithelial cells increased myofibroblast activation in vitro, suggesting a paracrine mechanism. Renal collecting duct-specific gene deletion of CCN2 significantly reduced cyst growth and myofibroblasts in Pkd1KO mouse kidneys. We found that YAP regulates CCN2, and YAP inhibition or gene deletion reduces renal fibrosis in Pkd1KO mouse kidneys. Importantly, YAP inactivation blocks the dDAVP-induced increase in myofibroblasts in Pkd1KO kidneys. Further in vitro studies showed that V2R regulates YAP by an ERK1/2-dependent mechanism in human ADPKD cystic epithelial cells.

CONCLUSIONS

Our results demonstrate a novel mechanism by which cystic epithelial cells stimulate myofibroblasts in the pericystic microenvironment, leading to fibrosis in ADPKD. The V2R-YAP-CCN2 cell signaling pathway may present a potential therapeutic target for fibrosis in ADPKD.

The Hippo Pathway Is Essential for Maintenance of Apicobasal Polarity in the Growing Intestine of Caenorhabditis elegans

Although multiple determinants for establishing polarity in membranes of epithelial cells have been identified, the mechanism for maintaining apicobasal polarity is not fully understood. Here, we show that the conserved Hippo kinase pathway plays a role in the maintenance of apicobasal polarity in the developing intestine of Caenorhabditis elegans We screened suppressors of the mutation in wts-1-the gene that encodes the LATS kinase homolog, deficiency of which leads to disturbance of the apicobasal polarity of the intestinal cells and to eventual death of the organism. We identified several alleles of yap-1 and egl-44 that suppress the effects of this mutation. yap-1 encodes a homolog of YAP/Yki, and egl-44 encodes a homolog of TEAD/Sd. WTS-1 bound directly to YAP-1 and inhibited its nuclear accumulation in intestinal cells. We also found that NFM-1, which is a homolog of NF2/Merlin, functioned in the same genetic pathway as WTS-1 to regulate YAP-1 to maintain cellular polarity. Transcriptome analysis identified several target candidates of the YAP-1-EGL-44 complex including TAT-2, which encodes a putative P-type ATPase. In summary, we have delineated the conserved Hippo pathway in C. elegans consisting of NFM-1-WTS-1-YAP-1-EGL-44 and proved that the proper regulation of YAP-1 by upstream NFM-1 and WTS-1 is essential for maintenance of apicobasal membrane identities of the growing intestine.

Pancreatic Ductal Deletion of Hnf1b Disrupts Exocrine Homeostasis, Leads to Pancreatitis, and Facilitates Tumorigenesis

BACKGROUND & AIMS

The exocrine pancreas consists of acinar cells that produce digestive enzymes transported to the intestine through a branched ductal epithelium. Chronic pancreatitis is characterized by progressive inflammation, fibrosis, and loss of acinar tissue. These changes of the exocrine tissue are risk factors for pancreatic cancer. The cause of chronic pancreatitis cannot be identified in one quarter of patients. Here, we investigated how duct dysfunction could contribute to pancreatitis development.

METHODS

The transcription factor Hnf1b, first expressed in pancreatic progenitors, is strictly restricted to ductal cells from late embryogenesis. We previously showed that Hnf1b is crucial for pancreas morphogenesis but its postnatal role still remains unelucidated. To investigate the role of pancreatic ducts in exocrine homeostasis, we inactivated the Hnf1b gene in vivo in mouse ductal cells.

RESULTS

We uncovered that postnatal Hnf1b inactivation in pancreatic ducts leads to chronic pancreatitis in adults. Hnf1bΔ^duct mutants show dilatation of ducts, loss of acinar cells, acinar-to-ductal metaplasia, and lipomatosis. We deciphered the early events involved, with down-regulation of cystic disease-associated genes, loss of primary cilia, up-regulation of signaling pathways, especially the Yap pathway, which is involved in acinar-to-ductal metaplasia. Remarkably, Hnf1bΔ^duct mutants developed pancreatic intraepithelial neoplasia and promote pancreatic intraepithelial neoplasia progression in concert with KRAS. We further showed that adult Hnf1b inactivation in pancreatic ducts is associated with impaired regeneration after injury, with persistent metaplasia and initiation of neoplasia.

CONCLUSIONS

Loss of Hnf1b in ductal cells leads to chronic pancreatitis and neoplasia. This study shows that Hnf1b deficiency may contribute to diseases of the exocrine pancreas and gains further insight into the etiology of pancreatitis and tumorigenesis.

Inhibition of Mast Cell Degranulation With Cromolyn Sodium Exhibits Organ-Specific Effects in Polycystic Kidney (PCK) Rats

Autosomal recessive polycystic kidney disease (ARPKD) is a monogenic disease characterized by development of hepatorenal cysts, pericystic fibrosis, and inflammation. Previous studies show that mast cell (MC) mediators such as histamine induce proliferation of cholangiocytes. We observed robust MC accumulation around liver cysts, but not kidney cysts, in polycystic kidney (PCK) rats (an animal model of ARPKD). Therefore, we hypothesized that MCs contribute to hepatic cyst growth in ARPKD. To test this hypothesis, we treated PCK rats with 1 of 2 different MC stabilizers, cromolyn sodium (CS) or ketotifen, or saline. The CS treatment decreased MC degranulation in the liver and reduced serum tryptase (an MC granule component). Interestingly, we observed an increase in liver to body weight ratio after CS treatment paralleled by a significant increase in individual cyst size. Hepatic fibrosis was not affected by CS treatment. The CS treatment increased hepatic cyst wall epithelial cell (CWEC) proliferation and decreased cell death. Ketotifen treatment also increased hepatic cyst size. In vitro, CS treatment did not affect proliferation of isolated hepatic CWECs from PCK rats. In contrast, CS decreased kidney to body weight ratio paralleled by a significant decrease in individual cyst size. The percentage of kidney to body weight ratio was strongly correlated with serum renin (an MC granule component). Ketotifen did not affect kidney cyst growth. Collectively, these data suggest that CS affects hepatic and renal cyst growth differently in PCK rats. Moreover, CS may be beneficial to renal cystic disease but may exacerbate hepatic cyst growth in ARPKD.