Characterization of lipid droplet-associated proteins in chronic steatotic liver disease and in cell culture models of steatosis

Fatty change is the most frequent liver pathology in western countries. It is caused by a broad range of disorders such as alcohol abuse and metabolic syndrome and is characterized by intracellular lipid droplet (LD) accumulation. The surface layer of LDs contains amphiphilic proteins of the PAT-family, named after their constituents, perilipin, adipophilin, and TIP47, which are fundamental for the stabilization of LDs. We recently showed that PAT-proteins are differentially expressed in liver and that perilipin is de novo expressed in human steatotic hepatocytes. In the last years, other proteins have been described to play a role in the biogenesis of LDs and to be enriched at LDs, e.g. Abhd5 (CGI58), caveolin-1 and -2, CIDE proteins, and rab18. Thus, we characterized the expression pattern of these proteins, together with PAT-proteins, in livers of different species, in human steatotic liver and in cell culture models of hepatocyte steatosis using immunofluorescence microscopy and immunoblot analysis. In liver in situ, Abhd5, caveolin-2 and rab18 localized to the cytoplasm of hepatic stellate cells, and caveolin-1 to the cytoplasm of hepatocytes. In contrast to perilipin, adipophilin, and TIP47, we could not detect Abhd5, caveolin-1 and -2, or rab18 at LDs of bovine or human hepatocytes. In hepatocellular tumor-derived cells of the lines PLC, HuH7, Hep3B and HepG2, adipophilin and TIP47 surrounded small LDs. Upon lipid loading, Abhd5, caveolin-2 and CIDEB colocalized with growing sizes of LDs as well. Perilipin however, which has been implicated in long term lipid storage, was not detected at LDs in normal or induced cultured cells. In conclusion, commonly used cell culture models for hepatocyte steatosis seem to be more amenable for the study of acute-onset steatosis, but of minor value for the study of chronic steatosis as it is most frequent in non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) patients.