PHOSPHATIDIC-ACID AND PHOSPHOLIPASE-D BOTH STIMULATE PHOSPHOINOSITIDETURNOVER IN CULTURED HUMAN KERATINOCYTES

Phosphatidic acid (PA) induced a rapid dose-dependent increase in prod uction of inositol phosphates in cultured adult human keratinocytes, p eaking at 30 s. Natural and dioleoyl PA were equally effective, while other phospholipid classes had no effect. Lipid A was also active. Lys o-PA also induced inositol phosphate production, but contamination of the PA preparation by lyso-PA could not account for the effect of PA. The effect of PA could not be reproduced by treatment of cells with ca lcium ionophore. PA-induced inositol phosphate production could be inh ibited ( > 50%) by pre-treatment of cells with either pertussis toxin or 12-0-tetradecanoylphorbol 13-acetate, suggesting the involvement of a GTP-binding protein and a protein kinase C-mediated negative feedba ck mechanism. PA also stimulated release of arachidonic acid from kera tinocytes. Treatment of cells with exogenous phospholipase D similarly induced inositol phosphate production in the keratinocytes. Since PA may be formed by receptor-mediated activation of phospholipase D, or b y phosphorylation of diacylglycerol, the results suggest that PA may p lay a significant role in signalling mechanisms of human keratinocytes .