Proteomic Analysis of GLUT4 Storage Vesicles Reveals Tumor Suppressor Candidate 5 (TUSC5) as a Novel Regulator of Insulin Action in Adipocytes

Insulin signaling augments glucose transport by regulating glucose transporter 4 (GLUT4) trafficking from specialized intracellular compartments, termed GLUT4 storage vesicles (GSVs), to the plasma membrane. Proteomic analysis of GSVs by mass spectrometry revealed enrichment of 59 proteins in these vesicles. We measured reduced abundance of 23 of these proteins following insulin stimulation and assigned these as high confidence GSV proteins. These included established GSV proteins such as GLUT4 and insulin-responsive aminopeptidase, as well as six proteins not previously reported to be localized to GSVs. Tumor suppressor candidate 5 (TUSC5) was shown to be a novel GSV protein that underwent a 3.7-fold increase in abundance at the plasma membrane in response to insulin. siRNA-mediated knockdown of TUSC5 decreased insulin-stimulated glucose uptake, although overexpression of TUSC5 had the opposite effect, implicating TUSC5 as a positive regulator of insulin-stimulated glucose transport in adipocytes. Incubation of adipocytes with TNFα caused insulin resistance and a concomitant reduction in TUSC5. Consistent with previous studies, peroxisome proliferator-activated receptor (PPAR) γ agonism reversed TNFα-induced insulin resistance. TUSC5 expression was necessary but insufficient for PPARγ-mediated reversal of insulin resistance. These findings functionally link TUSC5 to GLUT4 trafficking, insulin action, insulin resistance, and PPARγ action in the adipocyte. Further studies are required to establish the exact role of TUSC5 in adipocytes.     

The inhibition of the human cholesterol 7alpha-hydroxylase gene (CYP7A1) promoter by fibrates in cultured cells is mediated via the liver x receptor alpha and peroxisome proliferator-activated receptor alpha heterodimer

In previous work, we showed that the binding of the liver x receptor α:peroxisome proliferator-activated receptor α (LXRα:PPARα) heterodimer to the murine Cyp7a1 gene promoter antagonizes the stimulatory effect of their respective ligands. In this study, we determined if LXRα:PPARα can also regulate human CYP7A1 gene promoter activity. Co-expression of LXRα and PPARα in McArdle RH7777 hepatoma cells decreased the activity of the human CYP7A1 gene promoter in response to fibrates and 25-hydroxycholesterol. In vitro, the human CYP7A1 Site I bound LXRα:PPARα, although with substantially less affinity compared with the murine Cyp7a1 Site I. The binding of LXRα:PPARα to human CYP7A1 Site I was increased in the presence of either LXRα or PPARα ligands. In HepG2 hepatoblastoma cells, fibrates and 25-hydroxycholesterol inhibited the expression of the endogenous CYP7A1 gene as well as the human CYP7A1 gene promoter when co-transfected with plasmids encoding LXRα and PPARα. However, a derivative of the human CYP7A1 gene promoter that contains a mutant form of Site I that does not bind LXRα:PPARα was not inhibited by WY 14,643 or 25-hydroxycholesterol in both McArdle RH7777 and HepG2 cells. The ligand-dependent recruitment of LXRα:PPARα heterodimer onto the human CYP7A1 Site I can explain the inhibition of the human CYP7A1 gene promoter in response to fibrates and 25-hydroxycholesterol.