An ER Stress Inducible START Domain Cholesterol Transport Protein, StarD5; and Unique Role in Fatty Liver Disease

Fatty liver disease is an inflammatory response to accumulation of toxic metabolites within the liver. In the United States, the progression of fatty liver disease to liver inflammation → fibrosis → cirrhosis will soon be the leading cause of liver transplantation, especially within the Veteran’s

population; creating a challenge for our country’s Health Care budget. The molecular basis of nonalcoholic fatty liver disease (NAFLD) is incompletely understood. Recent findings provide strong evidence abnormal cholesterol metabolism drives the transition from fatty liver to inflammation initiated by ER stress pathways. StarD5, a member of the StarD4 subfamily of

proteins that contains a characteristic START domain, is a soluble protein that binds and translocates cholesterol within the cell. StarD5 is ‘upregulated’ by endoplasmic reticulum (ER) stress, which makes it a candidate to play a key role in the carrying of cholesterol out of the ER to maintain ER integrity. We have shown in preliminary observations shown that lack of StarD5

in hepatocytes can lead to a decrease in plasma membrane (PM) cholesterol and fluidity, an increase in cholesterol and its potential inflammatory metabolites, a decrease in neutral lipid (triglyceride) secretion, an increase in neutral lipid accumulation (triglycerides), subsequent development of insulin resistance, and progression to steatohepatitis and ensuing transcriptional

activation of fibrotic pathways. Furthermore, the storing of higher levels of cholesterol and triglycerides in StarD5-/- mouse livers, which is markedly exasperated with Western diet feeding, leads to altered expression of cholesterol-regulated genes. Conversely, preliminary studies show StarD5’s restoration in StarD5-/- livers, using an AAV9-StarD5 liver-selective overexpression

vector, is capable of reversing changes and protecting against ER stress. These observations demonstrate an ‘adaptive protective role’ of StarD5 in NAFLD. Lipids, including cholesterol, have been shown to be moved between membranes by non-vesicular lipid transfer proteins (LTP), transfer that it is fueled by Phosphatidylinositols (PIPs). LTPs that target the ER have an FFAT

motif (diphenylalanine [FF] in an acidic tract [AT]) that interact with ER proteins. We have identified a potential FFAT motif in the StarD5 protein that is conserved among species. In cells transfected with the FFAT motif mutated, StarD5 does not interact with the PM, cells have less accessible PM cholesterol, and, upon PIPs incubation, cells do not increase PM cholesterol as

they do in cells transfected with the WT StarD5. We hypothesize that StarD5 functions in the physiologic maintenance of cholesterol homeostasis by moving cholesterol out of the ER. In doing so, in cells with high or readily inducible levels of cholesterol, like hepatocytes, it prevents accumulation of cholesterol and neutral lipids by moving cholesterol to the PM and potentiating

lipoprotein assembly and secretion. We propose to 1) characterize the role of StarD5 in the intracellular movement of cholesterol in hepatocytes, including its role in VLDL assembly and secretion; 2) characterize the molecular mechanisms involved in the transfer of StarD5-mediated cholesterol; 3) characterize the in vivo role of StarD5 in the development of fatty liver disease.

The proposed studies will likely unveil a novel biologic pathway showing StarD5 plays a key role in maintaining physiological cholesterol levels within the ER, inhibiting lipid accumulation, and providing protection against the development of fatty liver disease.