Ptpn1 deletion protects oval cells against lipoapoptosis by targeting cellular processes that favour lipid droplet formation and dynamics

Abstract

Nonalcoholic fatty liver disease (NAFLD), the most common cause of chronic liver disease worldwide, ranges from benign and reversible steatosis that is accompanied by insulin resistance and elevated circulating free fatty acids (FAs), to nonalcoholic steatohepatitis (NASH), cirrhosis and culminates in some cases in hepatocarcinoma. The main drivers of NAFLD-associated lipotoxicity are saturated FAs (SFAs), although the molecular mechanisms involved in liver cell death have not been fully defined. Lipid droplets (LDs) buffer the excess of toxic lipid species, including SFAs. The enzyme stearoyl CoA desaturase 1 (SCD1 (encoded by the Scd1 gene) catalyzes desaturation of SFAs such as palmitic acid (PA) and stearic acid (SA) and plays an essential role in the formation of hepatic triglycerides (TGs) easily stored in the LDs. Of relevance for this Thesis, the LDs emerge from the endoplasmic reticulum (ER) and during their life cycle contact with other organelles, not only the ER and mitochondria, but also autophagosomes, lysosomes and peroxisomes through membrane contact sites. Hepatic progenitor cells in humans (HPCs) or oval cells (OCs) in mice (HPCs/OCs) constitute 1-3% of the cell population in the normal adult liver. They are progenitor hepatic stem cells that, upon liver damage, proliferate and differentiate into hepatocytes or cholangiocytes in a process known as ductular reaction, due to their ability to express markers of both biliary and hepatocyte lineages. Activation of HPCs/OCs has been related to hepatocyte injury during chronic liver diseases including NAFLD. However, whether the lipotoxic milieu of the NAFLD affects HPCs/OCs´s viability has not been addressed. Protein tyrosine phosphatase 1B (PTP1B), encoded by the Ptpn1 gene, plays a fundamental role in modulating signaling cascades triggered by hormones/growth factors including insulin, insulin-like growth factor-1 (IGF-1), epidermal growth factor (EGF) and hepatocyte growth factor (HGF), all relevant for liver regeneration. In this Thesis, we have investigated the molecular and cellular processes involved in the susceptibility of HPCs/OCs expressing or not PTP1B to lipotoxicity. We demonstrated that PA induced apoptotic cell death in wild-type (Ptpn1+/+) HPCs/OCs in parallel to oxidative stress and impaired autophagy. This lipotoxic effect was attenuated in HPCs/OCs lacking Ptpn1 that showed up-regulated antioxidant defences, increased unfolded protein response (UPR) signaling, higher ER content and elevated Scd1 expression/activity. These effects in Ptpn1-/- HPCs/OCs concurred with an active autophagy, higher mitochondrial efficiency and a molecular signature of starvation, favoring LD formation and catabolism. Autophagy blockade in Ptpn1-/- HPCs/OCs reduced Scd1 expression, mitochondrial fitness, LD formation and restored lipoapoptosis, an effect also mimicked by Scd1 silencing. The identification of LDs in HPCs/OCs in the livers from Ptpn1-/- mice with NAFLD that concurred with attenuated disease features opens new therapeutic perspectives to ensure HPCs/OCs viability and plasticity under lipotoxic liver damage