Phorbol esters increase insulin binding in astrocytic glial but not neuronal cells in primary culture from the brain.

In this study we used differential culturing techniques to study the effects of phorbol esters on insulin receptors on neuronal and astrocytic glial cells in primary culture from the brain. 12-O-Tetradecanoyl-phorbol-13-acetate (TPA), a potent activator of protein kinase C (PKC), increased [125I]insulin binding in a time- and concentration-dependent manner with a maximally effective dose of 50 nM TPA for 2 h in glial cells. Treatment with TPA did not affect [125I]insulin binding in neuronal cells. The TPA effect on glial [125I]insulin binding was specific as evidenced by the observation that potencies of phorbol ester analogs to increase [125I]insulin binding were similar to their abilities to stimulate PKC. Competitive-inhibition experiments indicated that this effect of TPA was due primarily to an increase in the number of high affinity insulin binding sites on glial cells. Removal of the TPA after pretreatment resulted in a recovery from its effects within 6 h. The increase in glial insulin binding was not accompanied by an increase in insulin-sensitive glucose uptake, suggesting that TPA inactivates the glial cell receptors as it increases their numbers.