Cross-talk between epidermal growth factor receptor and protein kinase C during calcium-induced differentiation of keratinocytes

The induction of epidermal differentiation by extracellular Ca2+ involves a ctivation of both tyrosine kinase and protein kinase C (PKC) signaling casc ades. To determine if the differentiation-dependent activation of tyrosine kinase signaling can influence the PKC pathway, we examined the tyrosine ph osphorylation status of PKC isoforms in primary mouse keratinocytes stimula ted to terminally differentiate with Ca2+ Elevation of extracellular Ca2+ i nduced tyrosine phosphorylation of PKC-delta, but not the other keratinocyt e PKC isoforms (alpha,epsilon, eta, zeta). We have previously demonstrated that activation of the epidermal growth factor receptor (EGFR) pathway indu ces PKC-delta tyrosine phosphorylation in basal keratinocytes (Denning M F, Dlugosz A A, Threadgill D W, Magnuson T, Yuspa S H (1996)J Biol Chern 271: 5325-5331). When basal keratinocytes were stimulated to differentiate by C a2+, the level of cell-associated transforming growth factor-alpha (TGF-alp ha) increased 30-fold, while no increase in secreted TGF-alpha was detected . Furthermore, Ca2+-induced tyrosine phosphorylation of PKC-delta and phosp hotyrosine-association of the receptor adapter protein Shc was diminished i n EGFR -/- keratinocytes, suggesting that EGFR activation may occur during keratinocyte differentiation. Tyrosine phosphorylated PKC-delta was also de tected in mouse epidermis, suggesting that this differentiation-associated signaling pathway is physiological. These results establish a requirement f or the EGFR in Ca2+-induced tyrosine phosphorylation of PKC-delta, and docu ment the production of cell-associated TGF-alpha in differentiated keratino cytes which may function independent of its usual mitogenic effects.