Insulin and platelet-derived growth factor acutely stimulate glucose transport in 3T3-L1 fibroblasts independently of protein kinase C.

Insulin and platelet-derived growth factor (PDGF) are mitogenic for murine 3T3-L1 fibroblasts. Both these mitogens acutely stimulate glucose transport by 2-4-fold in these cells, evident within minutes of agonist exposure. The tumour promoter and protein kinase C activator, phorbol 12-myristate 13-acetate (PMA) also stimulates glucose transport by 2-3-fold over a similar time frame, suggesting that protein kinase C may be involved in the mitogenic action of insulin and PDGF in this cell line. In an attempt to address this, we have measured intracellular sn-1,2-diacylglycerol (DAG) levels in response to insulin, PDGF and PMA. We show that PDGF and PMA induce a rapid elevation in intracellular diacylglycerol levels, but insulin was without effect. In addition, we have shown that PMA and PDGF, but not insulin, stimulate protein kinase C activity. However, depletion of protein kinase C by overnight exposure to PMA, abolished PMA-stimulated glucose transport but had no effect on insulin- and PDGF-stimulated glucose transport, suggesting that the stimulation of glucose transport by these mitogens does not involve protein kinase C. The use of the selective protein kinase C inhibitor, Roche 31-8220, which inhibited PMA-stimulated glucose transport, but was without effect on insulin- and PDGF-stimulated glucose transport further supports this conclusion. Taken together, these data argue against a role for protein kinase C in the stimulation of glucose transport in 3T3-L1 fibroblasts caused by acute exposure to insulin or PDGF.