Autophagy promotes hepatic cystogenesis in polycystic liver disease by depletion of cholangiocyte ciliogenic proteins

NAMPT Overexpression Drives Cell Growth in Polycystic Liver Disease through Mitochondrial Metabolism Regulation

A group of genetic diseases known as polycystic liver disease (PLD) are distinguished by the gradual development of fluid-filled hepatic cysts formed from cholangiocytes and commonly related to primary cilia defects. The NAD salvage pathway, which sustains cellular bioenergetics and supplies a required substrate for tasks important to rapidly multiplying cells, has a rate-limiting phase that is mediated by nicotinamide phosphoribosyltransferase (NAMPT). In this study, the efficacy and mechanisms of action of FK866, a novel, high-potency NAMPT inhibitor with a good toxicity profile, were assessed. NAMPT-siRNA and FK866 reduced NAD levels and inhibited the proliferation of PLD cells in a dose-dependent manner. Notably, this pharmacologic and siRNA-mediated suppression of NAMPT was less effective in normal cells at the same concentrations. The addition of nicotinamide mononucleotide (NMN), a byproduct of NAMPT that restores NAD concentration, rescued the cellular viability of PLD cells and verified the on-target action of FK866. In FK866-treated PLD cells, mitochondrial respiration and ATP production were impaired and reactive oxygen species production was induced. Importantly, FK866 treatment was associated with improved effects of octreotide, a drug used for PLD treatment. As a result, the use of NAMPT inhibitors, including FK866 therapy, offers the possibility of a further targeted strategy for the therapeutic treatment of PLD.

Clinical manifestation, epidemiology, genetic basis, potential molecular targets, and current treatment of polycystic liver disease

Polycystic liver disease (PLD) is a rare condition observed in three genetic diseases, including autosomal dominant polycystic liver disease (ADPLD), autosomal dominant polycystic kidney disease (ADPKD), and autosomal recessive polycystic kidney disease (ARPKD). PLD usually does not impair liver function, and advanced PLD becomes symptomatic when the enlarged liver compresses adjacent organs or increases intra-abdominal pressure. Currently, the diagnosis of PLD is mainly based on imaging, and genetic testing is not required except for complex cases. Besides, genetic testing may help predict patients' prognosis, classify patients for genetic intervention, and conduct early treatment. Although the underlying genetic causes and mechanisms are not fully understood, previous studies refer to primary ciliopathy or impaired ciliogenesis as the main culprit. Primarily, PLD occurs due to defective ciliogenesis and ineffective endoplasmic reticulum quality control. Specifically, loss of function mutations of genes that are directly involved in ciliogenesis, such as Pkd1, Pkd2, Pkhd1, and Dzip1l, can lead to both hepatic and renal cystogenesis in ADPKD and ARPKD. In addition, loss of function mutations of genes that are involved in endoplasmic reticulum quality control and protein folding, trafficking, and maturation, such as PRKCSH, Sec63, ALG8, ALG9, GANAB, and SEC61B, can impair the production and function of polycystin1 (PC1) and polycystin 2 (PC2) or facilitate their degradation and indirectly promote isolated hepatic cystogenesis or concurrent hepatic and renal cystogenesis. Recently, it was shown that mutations of LRP5, which impairs canonical Wnt signaling, can lead to hepatic cystogenesis. PLD is currently treated by somatostatin analogs, percutaneous intervention, surgical fenestration, resection, and liver transplantation. In addition, based on the underlying molecular mechanisms and signaling pathways, several investigational treatments have been used in preclinical studies, some of which have shown promising results. This review discusses the clinical manifestation, complications, prevalence, genetic basis, and treatment of PLD and explains the investigational methods of treatment and future research direction, which can be beneficial for researchers and clinicians interested in PLD.

XIAP-mediated degradation of IFT88 disrupts HSC cilia to stimulate HSC activation and liver fibrosis

Activation of hepatic stellate cells (HSCs) plays a critical role in liver fibrosis. However, the molecular basis for HSC activation remains poorly understood. Herein, we demonstrate that primary cilia are present on quiescent HSCs but exhibit a significant loss upon HSC activation which correlates with decreased levels of the ciliary protein intraflagellar transport 88 (IFT88). Ift88-knockout mice are more susceptible to chronic carbon tetrachloride-induced liver fibrosis. Mechanistic studies show that the X-linked inhibitor of apoptosis (XIAP) functions as an E3 ubiquitin ligase for IFT88. Transforming growth factor-β (TGF-β), a profibrotic factor, enhances XIAP-mediated ubiquitination of IFT88, promoting its proteasomal degradation. Blocking XIAP-mediated IFT88 degradation ablates TGF-β-induced HSC activation and liver fibrosis. These findings reveal a previously unrecognized role for ciliary homeostasis in regulating HSC activation and identify the XIAP-IFT88 axis as a potential therapeutic target for liver fibrosis.

A historical perspective of macroautophagy regulation by biochemical and biomechanical stimuli

Macroautophagy is a lysosomal degradative pathway for intracellular macromolecules, protein aggregates, and organelles. The formation of the autophagosome, a double membrane-bound structure that sequesters cargoes before their delivery to the lysosome, is regulated by several stimuli in multicellular organisms. Pioneering studies in rat liver showed the importance of amino acids, insulin, and glucagon in controlling macroautophagy. Thereafter, many studies have deciphered the signaling pathways downstream of these biochemical stimuli to control autophagosome formation. Two signaling hubs have emerged: the kinase mTOR, in a complex at the surface of lysosomes which is sensitive to nutrients and hormones; and AMPK, which is sensitive to the cellular energetic status. Besides nutritional, hormonal, and energetic fluctuations, many organs have to respond to mechanical forces (compression, stretching, and shear stress). Recent studies have shown the importance of mechanotransduction in controlling macroautophagy. This regulation engages cell surface sensors, such as the primary cilium, in order to translate mechanical stimuli into biological responses.

Liver-restricted deletion of the biliary atresia candidate gene Pkd1l1 causes bile duct dysmorphogenesis and ciliopathy

BACKGROUND AND AIMS

A recent multicenter genetic exploration of the biliary atresia splenic malformation syndrome identified mutations in the ciliary gene PKD1L1 as candidate etiologic contributors. We hypothesized that deletion of Pkd1l1 in developing hepatoblasts would lead to cholangiopathy in mice.

APPROACH AND RESULTS

CRISPR-based genome editing inserted loxP sites flanking exon 8 of the murine Pkd1l1 gene. Pkd1l1Fl/Fl cross-bred with alpha-fetoprotein-Cre expressing mice to generate a liver-specific intrahepatic Pkd1l1 -deficient model (LKO). From embryonic day 18 through week 30, control ( Fl/Fl ) and LKO mice were evaluated with standard serum chemistries and liver histology. At select ages, tissues were analyzed using RNA sequencing, immunofluorescence, and electron microscopy with a focus on biliary structures, peribiliary inflammation, and fibrosis. Bile duct ligation for 5 days of Fl/Fl and LKO mice was followed by standard serum and liver analytics. Histological analyses from perinatal ages revealed delayed biliary maturation and reduced primary cilia, with progressive cholangiocyte proliferation, peribiliary fibroinflammation, and arterial hypertrophy evident in 7- to 16-week-old LKO versus Fl/Fl livers. Following bile duct ligation, cholangiocyte proliferation, peribiliary fibroinflammation, and necrosis were increased in LKO compared with Fl/Fl livers.

CONCLUSIONS

Bile duct ligation of the Pkd1l1 -deficient mouse model mirrors several aspects of the intrahepatic pathophysiology of biliary atresia in humans including bile duct dysmorphogenesis, peribiliary fibroinflammation, hepatic arteriopathy, and ciliopathy. This first genetically linked model of biliary atresia, the Pkd1l1 LKO mouse, may allow researchers a means to develop a deeper understanding of the pathophysiology of this serious and perplexing disorder, including the opportunity to identify rational therapeutic targets.

Multi-omics profiling of cholangiocytes reveals sex-specific chromatin state dynamics during hepatic cystogenesis in polycystic liver disease

BACKGROUND & AIMS

Cholangiocytes transit from quiescence to hyperproliferation during cystogenesis in polycystic liver disease (PLD), the severity of which displays prominent sex differences. Epigenetic regulation plays important roles in cell state transition. We aimed to investigate the sex-specific epigenetic basis of hepatic cystogenesis and to develop therapeutic strategies targeting epigenetic modifications for PLD treatment.

METHODS

Normal and cystic primary cholangiocytes were isolated from wild-type and PLD mice of both sexes. Chromatin states were characterized by analyzing chromatin accessibility (ATAC sequencing) and multiple histone modifications (chromatin immunoprecipitation sequencing). Differential gene expression was determined by transcriptomic analysis (RNA sequencing). Pharmacologic inhibition of epigenetic modifying enzymes was undertaken in PLD model mice.

RESULTS

Through genome-wide profiling of chromatin dynamics, we revealed a profound increase of global chromatin accessibility during cystogenesis in both male and female PLD cholangiocytes. We identified a switch from H3K9me3 to H3K9ac on cis-regulatory DNA elements of cyst-associated genes and showed that inhibition of H3K9ac acetyltransferase or H3K9me3 demethylase slowed cyst growth in male, but not female, PLD mice. In contrast, we found that H3K27ac was specifically increased in female PLD mice and that genes associated with H3K27ac-gained regions were enriched for cyst-related pathways. In an integrated epigenomic and transcriptomic analysis, we identified an estrogen receptor alpha-centered transcription factor network associated with the H3K27ac-regulated cystogenic gene expression program in female PLD mice.

CONCLUSIONS

Our findings highlight the multi-layered sex-specific epigenetic dynamics underlying cholangiocyte state transition and reveal a potential epigenetic therapeutic strategy for male PLD patients.

IMPACT AND IMPLICATIONS

In the present study, we elucidate a sex-specific epigenetic mechanism underlying the cholangiocyte state transition during hepatic cystogenesis and identify epigenetic drugs that effectively slow cyst growth in male PLD mice. These findings underscore the importance of sex difference in the pathogenesis of PLD and may guide researchers and physicians to develop sex-specific personalized approaches for PLD treatment.

Secondary bile acids and the biliary epithelia: The good and the bad

The biliary tract has been considered for several decades a passive system just leading the hepatic bile to the intestine. Nowadays several researches demonstrated an important role of biliary epithelia (i.e. cholangiocytes) in bile formation. The study of biliary processes therefore maintains a continuous interest since the possible important implications regarding chronic cholestatic human diseases, such as primary biliary cholangitis or primary sclerosing cholangitis. Bile acids (BAs), produced by the liver, are the most represented organic molecules in bile. The physiologic importance of BAs was initially attributed to their behavior as natural detergents but several studies now demonstrate they are also important signaling molecules. In this minireview the effect of BAs on the biliary epithelia are reported focusing in particular on secondary (deriving by bacterial manipulation of primary molecules) ones. This class of BAs is demonstrated to have relevant biological effects, ranging from toxic to therapeutic ones. In this family ursodeoxycholic and lithocholic acid present the most interesting features. The molecular mechanisms linking ursodeoxycholic acid to its beneficial effects on the biliary tract are discussed in details as well as data on the processes leading to lithocholic damage. These findings suggest that expansion of research in the field of BAs/cholangiocytes interaction may increase our understanding of cholestatic diseases and should be helpful in designing more effective therapies for biliary disorders.

Unraveling the actin cytoskeleton in the malignant transformation of cholangiocyte biology

Correct actin cytoskeleton organization is vital in the liver organ homeostasis and disease control. Rearrangements of the actin cytoskeleton may play a vital role in the bile duct cells cholangiocytes. An abnormal actin network leads to aberrant cell morphology, deregulated signaling networks and ultimately triggering the development of cholangiocarcinoma (CCA) and paving the route for cancer cell dissemination (metastasis). In this review, we will outline alterations of the actin cytoskeleton and the potential role of this dynamic network in initiating CCA, as well as regulating the course of this malignancy. Actin rearrangements not only occur because of signaling pathways, but also regulate and modify cellular signaling. This emphasizes the importance of the actin cytoskeleton itself as cause for aberrant signaling and in promoting tumorigenic phenotypes. We will highlight the impact of aberrant signaling networks on the actin cytoskeleton and its rearrangement as potential cause for CCA. Often, these exact mechanisms in CCA are limited understood and still must be elucidated. Indeed, focusing future research on how actin affects and regulates other signaling pathways may provide more insights into the mechanisms of CCA development, progression, and metastasis. Moreover, manipulation of the actin cytoskeleton organization highlights the potential for a novel therapeutic area.

Polycystic Liver Disease: Pathophysiology, Diagnosis and Treatment

Polycystic liver disease (PLD) is a clinical condition characterized by the presence of more than 10 cysts in the liver. It is a rare disease Of genetic etiology that presents as an isolated disease or assoc\iated with polycystic kidney disease. Ductal plate malformation, ciliary dysfunction, and changes in cell signaling are the main factors involved in its pathogenesis. Most patients with PLD are asymptomatic, but in 2–5% of cases the disease has disabling symptoms and a significant reduction in quality of life. The diagnosis is based on family history of hepatic and/or renal polycystic disease, clinical manifestations, patient age, and polycystic liver phenotype shown on imaging examinations. PLD treatment has evolved considerably in the last decades. Somatostatin analogues hold promise in controlling disease progression, but liver transplantation remains a unique curative treatment modality.