RETINOIC ACID ATTENUATES PHOSPHOLIPASE C-MEDIATED SIGNALING IN HACAT KERATINOCYTES

Release of inositol(1,4,5)trisphosphate (Ins (1,4,5)P-3) generated by phospholipase C (PLC) upon receptor stimulation plays an important rol e in the regulation of cell growth and differentiation. A second, comp letely different, signal transduction system involves retinoic acid (R A) and related derivatives. Binding to intracellular receptor sites ca n modulate keratinocyte growth and inhibits differentiation. The prese nt study was aimed at characterizing possible interactions between the two signalling pathways in Ha-CaT keratinocytes. As determined by ani on exchange chromatography and HPLC analysis, HaCaT keratinocytes trea ted with 1 mu W RA for up to 72 h showed a marked decrease in Ins(1,4, 5)P, release upon stimulation with 10 mu M bradykinin or 10 mu M ionom ycin. Thin-layer chromatography of phosphatidylinositol phosphates, th e substrates of PLC, revealed no differences between RA-treated and un treated cells. Western blot analysis of the PLC isozymes present in Ha CaT cells, PLC beta(3) and PLC gamma(1), showed no alterations in the expression of these proteins in RA-treated cells as compared to vehicl e-treated controls. In addition, expression of the PLC-activating G pr otein Gag was not affected by RA treatment. Our results show that RA d ownregulates the PLC-mediated signaling system. The point of interfere nce of this signal transduction crosstalk has yet to be elucidated. Ou r results suggest, furthermore, that RA-induced attenuation of keratin ocyte differentiation might be mediated at least in part by the downre gulation of Ins(1,4,5)P-3 release.