Consequences of lipid droplet coat proteins down –regulation in liver cells: Abnormal lipid droplet metabolism and induction of insulin resistance

Abstract

Objective: Accumulation of intracellular lipid droplets (LD) in non-adipose tissues is recognized as a strong prognostic factor for the development of insulin resistance in obesity. LD are coated with PAT proteins that are thought to regulate LD turnover by modulating lipolysis. Our hypothesis is that PAT proteins modulate LD metabolism and therefore insulin resistance.

Research Design and Methods: We used a cell culture model (murine AML12 loaded with oleic acid) and siRNA to directly assess the impact of PAT proteins on LD accumulation, lipid metabolism and insulin action. PAT proteins associated with excess fat deposited in livers of DIO mice were also measured.

Results: Cells lacking PAT proteins exhibited a dramatic increase in LD size and a decrease in LD number. Further, the lipolytic rate increased by ∼2-2.5- fold in association with increased adipose triglyceride lipase (ATGL) at the LD surface. Down-regulation of PAT proteins also produced insulin resistance as indicated by decreased insulin stimulation of Akt phosphorylation (p<0.001). PDK-1 and PI3 kinase decreased; IRS-1 307 phosphorylation increased. Increased lipid in DIO mice livers was accompanied by changes in PAT composition, but also increased ATGL, suggesting a relative PAT deficiency.

Conclusions: These data establish an important role for PAT proteins as surfactant at the LD surface packaging lipids in smaller units and restricting access of lipases, preventing insulin resistance. We suggest that a deficiency of PAT proteins relative to the quantity of ectopic fat could contribute to cellular dysfunction in obesity and T2D.