Anna Mae Diehl
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Researcher at Duke University
Background & Aims: While altered host-microbe interactions are implicated in non-alcoholic fatty liver disease/non-alcoholic steatohepatitis (NAFLD/NASH), specific contributions of microbially derived metabolites remain obscure. We investigated the impact of altered bile acid (BA) populations on intestinal and hepatic phenotypes in a rodent model of NAFLD/NASH. Methods: Wistar rats fed a choline-deficient high-fat diet (CDAHFD) were assessed for altered intestinal permeability after dietary intervention. Cecal and portal venous BA composition were assessed via mass spectrometry. BA-mediated effects on epithelial permeability were assessed using Caco2 epithelial monolayers. Micelle formation was assessed using fluorescent probes and electron microscopy. Bile salt hydrolase (BSH) activity was inhibited with a gut-restricted small molecule in CDAHFD-fed rats and intestinal and hepatic phenotypes were assessed. Results: Increased intestinal permeability and reduced intestinal conjugated BAs were early phenotypes of CDAHFD-fed rats preceding hepatic disease development. Similar intestinal BA pool changes were observed in rats and human NAFLD/NASH patients with progressive disease. Conjugated BAs protected epithelial layers from unconjugated BA-induced damage via mixed micelle formation. The decrease in intestinal conjugated BAs was mediated by increased activity of bacterial BSHs and inhibition of BSH activity prevented the development of pathologic intestinal permeability and hepatic inflammation in the NAFLD/NASH model. Conclusions: Conjugated BAs are important for the maintenance of intestinal barrier function by sequestering unconjugated BAs in mixed micelles. Increased BSH activity reduces intestinal conjugated BA abundance, in turn increasing intestinal permeability and susceptibility to the development of NAFLD/NASH. These findings suggest that interventions that shift the intestinal bile acid pool toward conjugated BAs could be developed as therapies for NAFLD/NASH.
Liver is one of the few organs with immense regenerative potential even at adulthood in mammals. It is composed of primarily two cell types: hepatocytes and cholangiocytes, that can trans-differentiate to one another either directly or through intermediate progenitor states, contributing to remarkable regenerative potential of the liver. However, the dynamical features of decision-making between these cell-fates during liver development and regeneration remains elusive. Here, we identify a core gene regulatory network comprising c/EBP, TGFBR2 and SOX9 that underlies liver development and injury-induced reprogramming. Dynamic simulations for this network reveal its multistable nature, enabling three distinct cell states - hepatocytes, cholangiocytes and liver progenitor cells (hepatoblasts/oval cells) - and stochastic switching among them. Predicted expression signature for these three states are validated through multiple bulk and single-cell transcriptomic datasets collected across developmental stages and injury-induced liver repair. This network can also explain the experimentally observed spatial organisation of phenotypes in liver parenchyma and predict strategies for efficient cellular reprogramming among these cell-fates. Our analysis elucidates how the emergent multistable dynamics of underlying gene regulatory networks drive diverse cell-state decisions in liver development and regeneration.
Background and Aims: Liver fibrosis develops in the context of excessive oxidative stress, cell death and accumulation of myofibroblasts (MFs) derived from hepatic stellate cells (HSCs). Ferroptosis is a type of regulated cell death that can be caused by inhibiting the cystine/glutamate antiporter xCT. However, while xCT is induced in various liver diseases, its role in HSC activation and liver fibrosis is unknown. We hypothesized that xCT is required for HSCs to antagonize ferroptosis and remain myofibroblastic. Methods: xCT activity was disrupted by siRNA or pharmacological inhibitors in MF-HSC cell lines to determine its effect on redox homeostasis, growth, myofibroblastic activity and viability. xCT expression was then determined by RNA sequencing and RT-PCR during primary HSC activation, and its role in HSC trans-differentiation was assessed. For comparison, xCT expression and function were also determined in primary hepatocytes. Finally, the roles of xCT in HSC accumulation and liver fibrosis were assessed in mice treated acutely with CCl4. Results: Inhibiting xCT in MF-HSCs decreased intracellular glutathione (GSH), suppressed growth and fibrogenesis, and induced cell death. These effects were rescued by antioxidants, an iron chelator, and a canonical ferroptosis inhibitor, but not by inhibitors of apoptosis or necrosis. xCT was dramatically up-regulated during primary HSC activation, and inhibiting xCT suppressed myofibroblastic trans-differentiation and induced ferroptosis. In contrast, healthy hepatocytes were relatively insensitive to ferroptosis induced by xCT inhibition. In vivo, inhibiting xCT systemically reduced MF-HSC accumulation and liver fibrosis after a single dose of CCl4 without exacerbating liver injury or reducing hepatocyte regeneration. Conclusion: Compared to healthy hepatocytes, MF-HSCs are exquisitely sensitive to ferroptosis induced by inhibiting xCT. In acutely injured livers, systemic inhibitors of xCT can inhibit fibrosis without worsening liver injury. Further research is needed to determine if this therapeutic window remains sufficiently robust to safely target MF-HSCs and inhibit fibrogenesis in chronically injured liver.