PHOSPHOLIPASE C-MEDIATED SIGNALING IS ALTERED DURING HACAT CELL-PROLIFERATION AND DIFFERENTIATION

To elucidate the signaling mechanisms associated with keratinocyte dif ferentiation, we studied in vitro phospholipase C-mediated signal tran sduction, which results in the generation of inositol phosphates, comp aring proliferating versus differentiated HaCaT cells, a human keratin ocyte line, Bradykinin- or A23187-induced formation of inositol 1,4,5- trisphosphate, inositol 1,4-bisphosphate, and inositol monophosphates, as determined by anion exchange high performance liquid chromatograph y, were found to be highest in the early logarithmic growth phase of t he cells, In more highly differentiated HaCaT cells, which expressed m aximal amounts of the differentiation marker involucrin, inositol phos phate formation was reduced to about one third of that in proliferatin g cells. Thin layer chromatography of membrane phosphatidylinositol ph osphates revealed that this reduction was associated with a steady dec rease in phospholipase C substrates, Immunoblot analysis of phospholip ase C isozymes, however, and of expression of Gq alpha, the G protein subunit that activates phospholipase C beta, revealed no decrease duri ng the differentiation phase. The results suggest that the inositol-ph ospholipid signal transduction pathway is involved in keratinocyte pro liferation and in the induction of differentiation, with attenuated si gnal transduction activity via phospholipase C-coupled receptors in mo re differentiated keratinocytes.