Multifaceted regulation of hepatic G0/G1 Switch Gene 2 expression and relevance to lipid metabolism

Abstract

Evidence from our laboratory has established a role for the G0/G1 Switch Gene 2 (G0S2) as the primary inhibitor of adipose lipolysis through inhibition of adipose triglyceride lipase (ATGL), the rate‐limiting enzyme in triglyceride hydrolysis. We have demonstrated that differential expression of G0S2 plays a crucial role in regulating hepatic lipid content during fasting and feeding. Hepatic G0S2 expression is also significantly increased in a pathological condition in mice with obesity‐induced hepatic steatosis or non‐alcoholic fatty liver disease (NAFLD), major metabolic health issues in today's society. Based on preliminary evidence we hypothesized there is a dynamic relationship between adipose lipolysis and hepatic G0S2 expression during physiological fasting, a relationship that has not been previously described. To better understand this relationship we evaluated the effect of free fatty acids (FFA) on G0S2 expression in liver. Interestingly, hepatic G0S2 expression during fasting results from increased free fatty acid (FFA) delivery to the liver from adipose tissue. Moreover, we identified that increases in hepatic FFA lead to increased G0S2 expression in a bi‐mechanistic manner. We investigated and subsequently determined that both transcriptional activation of G0S2 and inhibition of protein degradation are occurring in response to increased hepatic FFA levels. Utilizing a luciferase based reporter system and in vivo chromatin immunoprecipitation we found transcriptional activation of G0S2 in liver occurs in response to activation of liver‐X‐receptor α (LXRα). LXRα is a known regulator of pro‐lipogenic signaling in liver and plays a major role in cholesterol homeostasis. It has also been shown that LXRα can be activated in response to FAs most likely through its dimerization partner RXRα that is a known FA responsive transcription factor. We demonstrate G0S2 is a direct downstream target gene for LXRα. Additionally, the direct repeat 4 (DR4) LXR response element (LXRE) found 2kb upstream of the transcriptional start site in the G0S2 promoter is required for G0S2/ LXRα binding. This DR4 binding site is conserved between the human and mouse isoforms of G0S2 and is required in each for LXRα binding. Furthermore, we found that G0S2 is subject to ubiquitin modification and is degraded in a proteasome dependent pathway. Mutation analysis of the six‐lysine residues found in G0S2 provided key evidence that the lysine at position 25 is the prominent site for ubiquitylation. The G0S2 K25 mutant is more stable than wild‐type G0S2, which is consistent with the accompanying decrease in ubiquitylation. Additionally, fatty acids were able to abrogate G0S2 degradation and increase protein stability. These findings indicate that two mechanisms of expression regulation in combination with adipose lipolysis are functioning concomitantly to regulate hepatic G0S2 expression under physiological conditions. Additionally, aberrant G0S2 expression may contribute to the pathological setting with metabolically linked diseases including NAFLD.Support or Funding InformationThis work was supported by NIDDK grants DK089178 and 2R56DK089178 from the National Institutes of Health. Pre‐doctoral funding support for B.H. was generously provided by the Mayo Graduate School and Mayo Foundation for Medical Education & Research.