The effects of intracellular calcium depletion on insulin signaling in 3T3-L1 adipocytes

We have examined the requirement for intracellular calcium (Ca2+) in insulin signal transduction in 3T3-L1 adipocytes. Using the Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, sodium (BAPTA-AM), we find both augmentation and inhibition of insulin signaling phenomena. Pretreatment of cells with 50 muM BAPTA-AM did not affect tyrosine phosphorylation of insulin receptor substrate (IRS)1/2 or insulin receptor (IR)beta. The decreased mobility of IRS1 normally observed after chronic stimulation with insulin, due to serine phosphorylation, was completely eliminated by Ca2+ chelation. Correlating with decreased insulin-induced serine phosphorylation of IRS1, phosphotyrosine-mediated protein-protein interactions involving p85, IRS1, IRbeta, and phosphotyrosine-specific antibody were greatly enhanced by pretreatment of cells with BAPTA-AM. As a result, insulin-mediated, phosphotyrosine-associated PI3K activity was also enhanced. BAPTA-AM pretreatment inhibited other insulin-induced phosphorylation events including phosphorylation of Akt, MAPK (ERK1 and 2) and p70 S6K. Phosphorylation of Akt on threonine-308 was more sensitive to Ca2+ depletion than phosphorylation of Akt on serine-473 at the same insulin dose (10 nM). In vitro 3'-phosphatidylinositol-dependent kinase 1 activity was unaffected by BAPTA-AM. Insulin-stimulated insulin-responsive glucose transporter isoform translocation and glucose uptake were both inhibited by calcium depletion. In summary, these data demonstrate a positive role for intracellular Ca2+ in distal insulin signaling events, including initiation/maintenance of Akt phosphorylation, insulin-responsive glucose transporter isoform translocation, and glucose transport. A negative role for Ca2+ is also indicated in proximal insulin signaling steps, in that, depletion of intracellular Ca2+ blocks IRS1 serine/threonine phosphorylation and enhances insulin-stimulated protein-protein interaction and PI3K activity.