Towards unravelling the molecular mechanisms of human hepatocyte steatosis in situ

Hepatocellular steatosis is one of the most common liver pathologies in western countries, and is characterized by lipid droplet (LD) accumulation in hepatocytes. Members of the PAT-protein family (perilipin, adipophilin, TIP47;=PERILIPIN 1–3) mainly govern the LD-surface. Interestingly, perilipin A, the major LD-associated protein in mature adipocytes, could be detected in human steatotic hepatocytes as well. Yet, the transcriptional basis of PAT-expression in hepatocyte steatogenesis has not been finally clarified.

In this study, we measured the triacylglyceride (TAG) content of over 30 human livers and analyzed PAT-proteins, transcription factors of adipogenesis (PPARγ, CEBPs), lipogenesis (SREBP1) and other adipocyte specific genes with immunoblot, rtPCR analysis, histochemistry and immunofluorescence microscopy. In our collective, perilipin and adipophilin expression correlated positively with TAG content. Besides perilipin A, also other perilipin isoforms could be detected in livers with severe hepatocyte steatosis. Furthermore, the active form of SREBP1 was stronger expressed in livers with a higher TAG content. Surprisingly, PPARγ and CEBPβ expression did neither change significantly with altered TAG content nor with perilipin A expression. In contrast, TIP47 and MLDP were evenly expressed in all liver specimens but not always localized at LDs, in agreement with the notion that these two PAT-members constitute exchangeable LD-associated proteins. To analyze the transcriptional regulation of PAT-expression in a dynamic model, we induced cells of the lines PLC, HuH7, HepG2 and Hep3B with oleate and other lipids as well as factors known to induce adipogenesis and in part succeeded in simulating the in vivo situation.

In patients with severe fatty change, steatotic hepatocytes undergo changes typically present in adipo- or rather lipogenesis such as expression of perilipin and its isoforms and the respective transcription factors namely active SREBP1.

Hepatocyte Steatosis - PAT-protein - Steatogensis