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.