Author(s): Wang H, Wang AX, Barrett EJ
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Abstract As insulin's movement from plasma to muscle interstitium is rate limiting for its metabolic action, defining the regulation of this movement is critical. Here, we address whether caveolin-1 is required for the first step of insulin's transendothelial transport, its uptake by vascular endothelial cells (ECs), and whether IL-6 and TNFα affect insulin uptake or caveolin-1 expression. Uptake of FITC-labeled insulin was measured using confocal microscopy in control bovine aortic ECs (bAECs), in bAECs in which caveolin-1 was either knocked down or overexpressed, in murine ECs from caveolin-1(-/-) mice and in bAECs exposed to inflammatory cytokines. Knockdown of caveolin-1 expression in bAECs using specific caveolin-1 siRNA reduced caveolin-1 mRNA and protein expression by ∼ 70\%, and reduced FITC-insulin uptake by 67\% (P < 0.05 for each). Over-expression of caveolin-1 increased insulin uptake (P < 0.05). Caveolin-1-null mouse aortic ECs did not take up insulin and re-expression of caveolin-1 by transfecting these cells with FLAG-tagged caveolin-1 DNA rescued FITC-insulin uptake. Knockdown of caveolin-1 significantly reduced both insulin receptor protein level and insulin-stimulated Akt1 phosphorylation. Knockdown of caveolin-1 also inhibited insulin-induced caveolin-1 and IGF-1 receptor translocation to the plasma membrane. Compared with controls, IL-6 or TNFα (20 ng/ml for 24 h) inhibited FITC-insulin uptake as well as the expression of caveolin-1 mRNA and protein (P < 0.05 for each). IL-6 or TNFα also significantly reduced plasma membrane-associated caveolin-1. Thus, we conclude that insulin uptake by ECs requires expression of caveolin-1 supporting a role for caveolae mediating insulin uptake. Proinflammatory cytokines may inhibit insulin uptake, at least in part, by inhibiting caveolin-1 expression.
This article was published in Am J Physiol Endocrinol Metab
and referenced in Endocrinology & Metabolic Syndrome