Altered Lipid Droplet Trafficking: Role in Alcoholic Fatty Liver Disease

摘要

ABSTRACT This proposal is a competitive renewal of a multiple-PI R01 funded through NIAAA. The goal of the application is to examine how ethanol exposure contributes to fat accumulation in the liver due to altered dynamic properties of the fat storage organelle, the lipid droplet (LD). Specifically we will examine how alcohol exposure compromises both the lipolytic and lipophagic machineries in hepatocytes via disrupting signaling cascades and membrane trafficking effectors, ultimately leading to a fatty liver. During alcoholic liver disease, almost all heavy drinkers develop fatty liver, which is marked by the aberrant and significant accumulation of intrahepatocellular triglycerides in the form of LDs. Understanding the cellular processes contributing to this fat accumulation will provide important information for preventing further progression of injury, as it is known that alcoholic fatty liver is the initial, but entirely reversible stage of liver injury. Our research in the previously- funded R01 examined how LD dynamics in hepatocytes are regulated by several GTPases (dynamins and Rabs in particular) that can act as molecular switches to regulate membrane traffic. We showed that ethanol- induced disruption of these GTPases dramatically increases accumulation of LDs in the liver cell. Preliminary data in support of this continuation application show that the lipolytic machinery of hepatocytes, is activated by agonists of cAMP kinase (isoproterenol, forskolin) but this response is markedly inhibited by ethanol exposure. Additionally, we show that the hepatocyte utilizes sequential mechanisms to catabolize LDs that entail lipolysis followed by lipophagy. Further, it appears that activation of non-receptor tyrosine kinases that reside on the LD-autophagosome (AP) surface function to drive lipophagy, and that alcohol impairs this process. Finally, in addition to engulfment of LDs by APs, removal or “sampling” of lipids away from LDs seems to occur by a transient, measurable interaction that is sensitive to ethanol exposure. These recent, novel findings provide an excellent foundation for this proposal, and support our central hypothesis that ethanol exposure compromises both the lipolytic and lipophagic machineries in the hepatocyte by disrupting signaling cascades and membrane trafficking events, leading to hepatic steatosis. The two principal investigators directing this project have complementary strengths: Dr. Casey is a biochemist whose expertise is in alcoholic-induced liver damage. Dr. McNiven is a cell biologist whose specialty is membrane-cytoskeleton dynamics. This unique collaborative effort has proven very beneficial to the field of alcoholic liver disease and will continue to result in outcomes otherwise unattainable by individual efforts. The proposed investigation will utilize state-of-the-art membrane trafficking and imaging technologies to quantify specific molecular events that contribute to alcohol- induced fatty liver. Successful completion of these studies will provide novel insights as to how ethanol affects LD dynamics in liver cells, and important information for therapeutic strategies aimed at reducing or eliminating the severity of steatosis and blocking its further progression to alcoholic steatohepatitis, fibrosis and cirrhosis.