Abstract 045: SR-BI and PCPE2 Modulate Lipid Trafficking in Adipocytes

Adipose tissue undergoes distinct structural remodeling in response to increases in fat mass, as in obesity. During this process, the extracellular matrix (ECM) plays a pivotal role with respect to both adipocyte receptor signaling and structural remodeling. One ECM protein, procollagen endopeptidase enhancer 2 (PCPE2), has been shown by our lab to work in partnership with the scavenger receptor class B type I (SR-BI) to regulate adipocyte cholesterol homeostasis. To investigate the functional impact of the partnership between SR-BI and PCPE2, we studied mature adipocytes differentiated from murine embryonic fibroblasts (MEF) and compared to our adipose specific PCPE2 knockout mice. Our studies showed the presence of an SR-BI-PCPE2 protein complex based on the co-immunoprecipitation of PCPE2 with SR-BI in both differentiated MEFs and mouse adipose tissue. Furthermore, stimulated emission depletion (STED) microscopy showed SR-BI and PCPE2 localized on the lipid droplet surface separated by distances ranging from 24 – 90 nm, suggesting that the SR-BI-PCPE2 complex may contain other proteins. Most significantly, a loss of SR-BI function was shown in PCPE2-deficient adipocytes by the SR-BI mediated HDL binding and cholesteryl ester (CE) uptake assays. Compared to differentiated MEFs from Ldlr -/- mice, binding of HDL to differentiated MEFs from Ldlr -/- Pcpe2 -/- mice was reduced by 75%, and HDL cholesteryl oleyl ether uptake was reduced by 50%. Disruption of adipocyte SR-BI function in the absence of PCPE2 also reduced free cholesterol efflux indicating substantial disruption to intracellular cholesterol homeostasis. Moreover, analysis of RNAseq from several human clinical studies show that PCPE2 mRNA expression is positively correlated with fat mass in specific adipose depots, while negatively correlated with plasma insulin, glucose and triglyceride levels, with the latter two more robust in type 2 diabetic populations (FDR < 0.004). Overall, these results suggest that the partnership between SR-BI and PCPE2 in adipocytes plays an important role in cholesterol trafficking and homeostasis which influences fat mass expansion, glucose tolerance and insulin resistance.