Effects of lipid overload on Hepatic Progenitor Cells

Abstract

One of the causes for damage to the liver is lipid overload, like in obesity, leading to fatty liver disease. As with other causes of liver damage, lipid overload can cause activation of hepatic progenitor cells(HPCs). HPCs can differentiate towards hepatocytes. Both the topics of HPC activation and fatty liver disease are subject to many studies, but nothing is known about the effect of lipid overload on HPCs. In this study HepaRG cells and organoïd culture were used as models for HPCs. The lipid overload was simulated by supplementing the culture medium with 0.2mM oleate and 0.1mM palmitate. During differentiation (as measured with qRT-PCR samples) measurements were taken to assess the viability with a MTT assay, the lipid droplet formation with fluorescent staining and the lipid content with mass spectrometry. Both the cells with and without extra fatty acids showed an increase in hepatocyte markers like HNF4a and albumin. Cyp3a4 increased gradually in the control without extra fatty acids, in contrast 24 hours after the addition of fatty acids the mRNA levels of CYP3a4 were reduced. The HepaRGs cultured with extra fatty acids for a long time changed in morphology. It looked like a mesenchymal transition, but no significant difference in the expression of mesenchymal genes was found. HPC models (both HepaRG and organoids) had more lipid droplets and triacylglycerol in differentiated than in undifferentiated state. When extra fatty acids were added the amount of TAG and lipid droplets increased further in the next 24 hours. After a week the HepaRG cells and their media showed almost no trace of the extra fatty acids. After adding etomoxir (inhibitor of the carnitine shuttle) the amounts of triacylglycerol increased even more, whereas orlistat (lipase inhibitor) had almost no effect. This might indicate that the HPCs use fatty acids for energy. Our results suggest that HPCs are protected from lipid overload by rapid lipid metabolism and less uptake than hepatocytes. Unravelling the molecular mechanisms of these protective pathways might give new ways of protecting hepatocytes against lipid overload.