Increased diacylglycerol content with phospholipase C or hormone treatment: inhibition of phorbol ester binding and induction of phorbol ester-like biological responses.

The purpose of these studies was to determine whether increased cellular diacylglycerol could modulate phorbol ester receptor properties, in order to demonstrate that diacylglycerol can interact with and modulate the phorbol ester receptor in intact cells. Treatment of GH4C1 cells with bacterial phospholipase C caused an increase in cellular diacylglycerol. This was accompanied by increased PRL secretion and decreased epidermal growth factor (EGF) binding, two responses that also occur with phorbol ester treatment of GH4C1 cells. Phospholipase C treatment led to decreased apparent affinity for phorbol esters with no change in receptor number when measured in intact cells. This is consistent with increased concentrations of a competitive inhibitor of phorbol ester binding in treated cultures. Phospholipase C treatment caused a change in subcellular distribution of phorbol ester receptors, another response characteristic of phorbol ester treatment. TRH is known to activate endogenous phospholipase C activity in these cells, leading to a transient increase in diacylglycerol levels. TRH treatment also led to a transient change in subcellular distribution of phorbol ester receptors. In addition, a coordinate change in subcellular distribution of protein kinase C was observed. These data suggest that diacylglycerol is an endogenous ligand for the target for phorbol ester action in GH4C1 cells.