Impaired glucose transport and protein kinase B activation by insulin, butnot okadaic acid, in adipocytes from subjects with Type II diabetes mellitus

Aims/hypothesis. To study the effects of insulin and okadaic acid, a serine /threonine phosphatase inhibitor which does not increase PI3-kinase activit y, on the rare of glucose transport and protein kinase B activation in adip ocytes from healthy subjects and subjects with Type II (non-insulin-depende nt) diabetes mellitus. Methods. Adipocytes were incubated with or without insulin or okadaic acid or both and glucose transport, protein kinase B activity, phosphorylation a nd protein expression measured. Results. Insulin and okadaic acid alone increased glucose uptake to a simil ar degree in adipocytes from healthy subjects and, when combined, exerted a partial additive effect. The effect of insulin was reduced by about 60 % i n adipocytes from Type II diabetic patients, whereas the effect of okadaic acid was essentially unchanged and no further increase was seen when okadai c acid and insulin were combined. Okadaic acid increased protein kinase B a ctivity to a greater extent (two to threefold) than insulin but only slight ly increased the serine phosphorylation of protein kinase B. Adipocytes fro m Type II diabetic subjects exhibited both an impaired sensitivity as well as a reduced total serine phosphorylation and activation of protein kinase B in response to insulin but protein kinase B activity in response to okada ic acid was intact. Conclusion/interpretation. These results show that the ability of insulin t o increase glucose transport and activate protein kinase B is reduced in fa t cells from Type II diabetic subjects. Protein kinase B can, however, be a ctivated by agents like okadaic acid which bypass the upstream defects in t he insulin signalling pathway in Type II diabetic cells and, thus, increase glucose uptake.