In the present study we have investigated the effect of increased serine/threonine phosphorylation of insulin receptor substrates-1 and -2 (IRS-1 and IRS-2) by okadaic acid pretreatment on brown adipocyte insulin signalling leading to glucose transport, an important metabolic effect of insulin in brown adipose tissue. Okadaic acid pretreatment before insulin stimulation decreased IRS-1 and IRS-2 tyrosine phosphorylation in parallel to a decrease in their sodium dodecyl sulfate–polyacrylamide gel electrophoresis mobility. IRS-1/IRS-2-associated p85α and phosphatidylinositol (PI) 3-kinase enzymatic activity were partly reduced in brown adipocytes pretreated with okadaic acid upon stimulation with insulin. Furthermore, insulin-induced glucose uptake was totally abolished by the inhibitor in parallel with a total inhibition of insulin-induced protein kinase C (PKC) ζ activity. However, activation of Akt/PKB or p70 S6 kinase (p70s6k) by insulin remained unaltered. Our results suggest that downstream of PI 3-kinase, insulin signalling diverges into at least two independent pathways through Akt/PKB and PKC ζ, the PKC ζ pathway contributing to glucose transport induced by insulin in fetal brown adipocytes.
