AN INVESTIGATION OF NUCLEAR LIPID DROPLET PROTEINS AND NUCLEAR LIPID DROPLET PROTEIN DYNAMICS

Abstract

Aberrant lipid storage is a hallmark of many pathologies, including non-alcoholic fatty liver disease (NAFLD). NAFLD is characterized by an accumulation of lipids stored in lipid droplets (LDs) in the cytoplasm. A subpopulation of nuclear LDs (nLDs) has been reported to form in response to excess fatty acids in hepatocytes and other cells. nLDs associate with nuclear proteins, including the nuclear body-forming promyelocytic leukemia (PML) proteins to form lipid-associated PML structures (LAPS), along with the lipid metabolic enzymes LIPIN1 and CTP:phosphocholine cytidylyltransferase α (CCTα). My research aimed to characterize protein association with nLDs over time, investigating the mechanisms regulating CCTα and LIPIN1 translocation to nLDs and determining whether apolipoprotein L6 (APOL6) associates with LAPS. PML recruitment to nLDs and thus LAPS formation was found to precede CCTα and LIPIN1 recruitment. The ability for CCTα to associate with nLDs was found to be inhibited by the degree of phosphorylation in its C-terminal phosphorylation domain. Interferon (IFN) γ was found to stimulate LAPS formation and APOL6 expression. Furthermore, the combination of IFNγ and fatty acids were found to stimulate greater LAPS formation and APOL6 expression and recruitment to LAPS under conditions which simulate NAFLD. These results demonstrate that nLDs are a heterogenous population of nuclear structures that dynamically recruit PML, CCT, LIPIN1 and APOL6 in response to fatty acids and IFNγ stimulus, possibly to quell the lipotoxic effects of fatty acid overload. Additionally, the establishment of connections between IFNγ, APOL6 and LAPS creates a paradigm in which NASH may be stimulated in hepatic tissue by fatty acids and inflammatory signalling driving the expression of pro-apoptotic APOL6.