Effect of inhibition of the protein tyrosine phosphatase 1B in lipotoxicity in liver progenitor oval cells

Abstract

Oval cells are progenitor cells with an emerging role in liver regenerative responses. However, their susceptibility to lipotoxicity in obesity-associated insulin resistance is unknown. Inhibition of protein tyrosine phosphatase 1B (PTP1B), a negative modulator of insulin signaling, is a pharmacological strategy against insulin resistance/obesity. Ours aims were to identify whether oval cells are insulin target cells and to analyze the impact of PTP1B deficiency in their susceptibility to lipotoxicity in the context of obesity-associated non-alcoholic fatty liver disease (NAFLD). For lipotoxicity studies, oval cells were treated with palmitic acid (PA) for different time-periods, after which oxidative/endoplasmic reticulum (ER) stress responses and autophagy/apoptosis markers were analyzed. A lipidomic study was conducted to characterize the intracellular lipid species upon PA stimulation. Oval cells responded to insulin by inducing insulin receptor tyrosine phosphorylation, this effect being higher in the absence of PTP1B. Treatment with PA induced apoptotic cell death in PTP1B+/+ oval cells in parallel with a blockade of the autophagy flux. Cell death was not found in PTP1B-/- cells that accumulated lipid droplets and increased perilipin2 levels upon PA treatment. Cell death in PA-treated PTP1B+/+ oval cells was prevented by cotreatment with sulforaphane, a Nrf2 activator. Also, PTP1B-/- oval cells showed higher basal levels of HO-1 and catalase, suggesting constitutive anti-oxidant defense. An early activation of ER stress was found in both genotypes but this response was enhanced in PTP1B-/- cells in both basal and upon PA treatment. Lipidomics revealed that PTP1B-/- oval cells accumulated triglyceride species and cholesterol esters in response to PA. By contrast, PTP1B+/+ oval cells showed higher diacylglycerides, more toxic lipid species. In conclusion, we have revealed that oval cells are insulin target cells susceptible to lipotoxicity, an effect mediated at least in part by the induction of oxidative stress. PTP1B deficiency protects against lipotoxic cell death by mechanisms including enhancement of anti-oxidant defences and capacity to accumulate TG containing lipid droplets, suggesting a potential benefit in cellular regenerative therapies against NAFLD.