Mechanistic studies on Bif-1 function in obesity and Atg9 trafficking

Open Access
Author:
Liu, Ying
Graduate Program:
Pharmacology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 07, 2016
Committee Members:
  • Hong-Gang Wang, Dissertation Advisor
  • Hong-Gang Wang, Committee Chair
  • Richard Bernard Mailman, Committee Member
  • Thomas E Spratt, Committee Member
  • Yoshinori Takahashi, Outside Member
  • Yuguang (Roger) Shi, Outside Member
Keywords:
  • Bif-1
  • Atg9
  • obesity
  • autophagy
  • lipid metabolism
  • lipophagy
Abstract:
Autophagy, the catabolic process whereby intracellular components are degraded by the lysosome, is long recognized for its roles in maintaining cellular energetic balance by breaking down dysfunctional proteins and recycling the amino acids. Adding to this classic view, recent studies have found that autophagy also directly impacts energetics by degrading lipids. This process, referred to as lipophagy, serves to prevent excessive fat accumulation in multiple tissues such as the liver and skeletal muscle, and modulates the hormone production in hypothalamus to affect food intake. The molecular mechanisms of lipophagy, particularly those related to the biogenesis of autophagosome to sequester the substrates, are unclear due to poor understanding of the responsible molecular interactions. Moreover, in the largest lipid storage organ - adipose tissue – it is unknown whether autophagy plays a role in fat degradation. It has been established that autophagy is essential for early development including embryonic adipogenesis, but it is not known if impairment of this catabolic process in an individual with mature adipose tissue leads to obesity. Bif-1 is a previously identified positive regulator of autophagy that interacts with UVRAG, and modulates Atg9 trafficking to promote the formation of autophagosomes. The current study identified Bif-1 as a novel regulator in lipid catabolism that will prevent the development of obesity and insulin resistance upon aging or dietary challenge. My data show that Bif-1 deficiency promotes the expansion of adipose tissue mass without altering food intake or physical activities. Although Bif-1 is dispensable for adipose tissue development, its deficiency reduces the rate of adipose tissue lipolysis, lowers the degradation of autophagy adaptor and substrate p62, and results in adipocyte hypertrophy upon aging. This function of Bif-1 in lipid turnover is not limited to adipose tissue since lipid droplet clearance is also attenuated by Bif-1 loss in the liver of mice that were starved and re-fed. Interestingly, obesity induced by a high fat-diet or Bif-1 deficiency down-regulates the protein levels of Atg9 and a lysosomal protein Lamp1 in the adipose tissue. Together, my research discovered a new role for Bif-1 in regulating lipid metabolism likely via lipophagy. Many questions remain, for example, does Bif-1-mediated lipid degradation depend on Atg9, and what other potential interactors of the Atg9-Bif-1 complex promote the biogenesis of autophagosomes? In order to address this question, we performed an Atg9 interactome profiling using inducible protein crosslinking coupled with affinity purification and proteomics. This identified VCP/p97 as a novel interactor with Atg9. Moreover, genetic knockdown of VCP in cells appears to disrupt autophagosome maturation. Although further studies are needed to precisely dissect the function of VCP in the autophagy machinery, I propose a potential role for VCP in the retrieval of autophagy proteins from the autophagosome. In summary, my research has increased our understanding in the molecular mechanisms of autophagy and lipophagy. I identified Bif-1 as a novel player in lipid homeostasis and obesity, and I hypothesize that targeting lipophagy may provide new avenues for the treatment of obesity and its related metabolic complications. My findings also broaden our knowledge of the Atg9 interaction network through the identification of VCP as an Atg9 interactor.