GANGLIOSIDE GQ1B-INDUCED TERMINAL DIFFERENTIATION IN CULTURED MOUSE KERATINOCYTES - PHOSPHOINOSITIDE TURNOVER FORMS THE ONSET SIGNAL

Investigations were undertaken to see whether mouse keratinocyte differentiation was elicited by gangliosides. Among the gangliosides tested only G(Q1b), a tetrasialoganglioside containing two disialosyl residues, induced keratinocyte differentiation, as indicated by the formation of cornified envelopes, enhancement of transglutaminase activity and suppression of DNA synthesis. Upon stimulation with G(Q1b) the mass content of Ins(1,4,5)P3 and the intracellular Ca2+ levels were markedly enhanced in a time- and dose-dependent manner, whereas no significant changes were observed with other gangliosides, thereby indicating activation of phospholipase C for the onset of keratinocyte differentiation. Furthermore, only G(Q1b) promoted the translocation of protein kinase C (PKC) from cytosol to membrane. Inhibition of PKC with H-7 or down-regulation of the enzyme by prolonged pre-treatment with phorbol 12,13-dibutyrate greatly suppressed transglutaminase activity and formation of cornified envelopes induced by G(Q1b). These results demonstrate that the tetrasialoganglioside G(Q1b) generates the initial differentiation signal in mouse keratinocytes through phosphoinositide turnover, and also suggest that PKC activation may act at certain, as yet unidentified, stages of differentiation processes.