[Abstract unavailable

No abstract

Fibrosis is the major determinant of morbidity and mortality in patients with nonalcoholic steatohepatitis (NASH) but has no approved pharmacotherapy in part because of incomplete understanding of its pathogenic mechanisms. Here, we report that hepatocyte Notch activity tracks with disease severity and treatment response in patients with NASH and is similarly increased in a mouse model of diet-induced NASH and liver fibrosis. Hepatocyte-specific Notch loss-of-function mouse models showed attenuated NASH-associated liver fibrosis, demonstrating causality to obesity-induced liver pathology. Conversely, forced activation of hepatocyte Notch induced fibrosis in both chow- and NASH diet–fed mice by increasing Sox9-dependent Osteopontin (Opn) expression and secretion from hepatocytes, which activate resident hepatic stellate cells. In a cross-sectional study, we found that OPN explains the positive correlation between liver Notch activity and fibrosis stage in patients. Further, we developed a Notch inhibitor [Nicastrin antisense oligonucleotide (Ncst ASO)] that reduced fibrosis in NASH diet–fed mice. In summary, these studies demonstrate the pathological role and therapeutic accessibility of the maladaptive hepatocyte Notch response in NASH-associated liver fibrosis.

The mechanistic target of rapamycin complex 1 (MTORC1) is a critical negative regulator of general autophagy. We hypothesized that MTORC1 may specifically regulate autophagic clearance of damaged mitochondria. To test this, we used cells lacking tuberous sclerosis complex 2 (TSC2 -/-), which show constitutive MTORC1 activation. TSC2 -/- cells show MTORC1-dependent impaired autophagic flux after chemical uncoupling of mitochondria, increased mitochondrial protein aging and accumulation of p62/SQSTM1 positive mitochondria. Mitochondrial autophagy (mitophagy) was also deficient in cells lacking TSC2, associated with altered expression of PTEN-induced kinase 1 (PINK1) and PARK2 translocation to uncoupled mitochondria, all of which were recovered by MTORC1 inhibition or expression of constitutively active FoxO1. These data prove the necessity of intact MTORC1 signaling to regulate two synergistic processes required for clearance of damaged mitochondria: 1) general autophagy initiation, and 2) PINK1/PARK2-mediated selective targeting of uncoupled mitochondria to the autophagic machinery.

Hyperactivation of the mammalian target of rapamycin complex 1 (mTORC1) in b-cells is usually found as a consequence of increased metabolic load. Although it plays an essential role in b-cell compensatory mechanisms, mTORC1 negatively regulates autophagy. Using a mouse model with b-cell-specific deletion of Tsc2 (bTsc2 2/2 ) and, consequently, mTORC1 hyperactivation, we focused on the role that chronic mTORC1 hyperactivation might have on b-cell failure. mTORC1 hyperactivation drove an early increase in b-cell mass that later declined, triggering hyperglycemia. Apoptosis and endoplasmic reticulum stress markers were found in islets of older bTsc2 2/2 mice as well as accumulation of p62/SQSTM1 and an impaired autophagic response. Mitochondrial mass was increased in b-cells of bTsc2 2/2 mice, but mitophagy was also impaired under these circumstances. We provide evidence of b-cell autophagy impairment as a link between mTORC1 hyperactivation and mitochondrial dysfunction that probably contributes to b-cell failure.Nutrient overload is one of the main causes of insulin resistance. This triggers the compensatory mechanisms leading to b-cell mass increase and hyperinsulinemia. Hyperactivation of the mammalian target of rapamycin complex 1 (mTORC1) is elicited under nutrient overload conditions (1-3). mTORC1 plays a positive role in b-cell mass expansion (3-6), and rapamycin treatment impairs b-cell mass adaptation (7,8). On the other hand, mTORC1 hyperactivation is also a cause of insulin resistance (1) and endoplasmic reticulum (ER) stress (9), conditions linked with b-cell dysfunction and diabetes progression (10,11). We previously described how Tsc2 deletion in b-cells (bTsc2 2/2 ) results in chronic mTORC1 hyperactivation leading to a biphasic phenotype with early b-cell mass increase, hyperinsulinemia, and hypoglycemia. This was followed by b-cell failure and hyperglycemia in older mice (3). We also found how the first phase in bTsc2 2/2 mice is characterized by both an increase of mitochondrial mass and enhanced glucose-stimulated insulin secretion (12). Autophagy is a cytoprotective mechanism also found essential for b-cell homeostasis (13,14). Autophagy plays a protective role under stress conditions, such as ER stress (15), and we and others have described its positive role in b-cells under these conditions (16,17). Autophagy is also responsible for the turnover of mitochondria by the specific elimination of defective or damaged organelles. mTORC1 is a critical negative regulator of autophagy (18), and studies have shown how TSC deficiency leads to impaired autophagy in human tumors and cell lines (19,20). In this study, we explored the intriguing possibility that mTORC1 hyperactivity in b-cells, apart from being essential for b-cell compensatory mechanisms, might