Blackwell Publishing Journal Backfiles 1879-2005
Abstract: The induction of epidermal differentiation by extracellular Ca2+ involves activation of both tyrosine kinase and protein kinase C (PKC) signaling cascades. To determine if the differentiation-dependent activation of tyrosine kinase signaling can influence the PKC pathway, we examined the tyrosine phosphorylation status of PKC isoforms in primary mouse keratinocytes stimulated to terminally differentiate with Ca2+. Elevation of extracellular Ca2+ induced tyrosine phosphorylation of PKC-δ, but not the other keratinocyte PKC isoforms (α,ε, η, ζ). We have previously demonstrated that activation of the epidermal growth factor receptor (EGFR) pathway induces PKC-δ tyrosine phosphorylation in basal keratinocytes (Denning M F, Dlugosz A A, Threadgill D W, Magnuson T, Yuspa S H (1996) J Biol Chem 271: 5325–5331). When basal keratinocytes were stimulated to differentiate by Ca2+, the level of cell-associated transforming growth factor-α (TGF-α) increased 30-fold, while no increase in secreted TGF-α was detected. Furthermore, Ca2+-induced tyrosine phosphorylation of PKC-δ and phosphotyrosine-association of the receptor adapter protein Shc was diminished in EGFR −/− keratinocytes, suggesting that EGFR activation may occur during keratinocyte differentiation. Tyrosine phosphorylated PKC-δ was also detected in mouse epidermis, suggesting that this differentiation-associated signaling pathway is physiological. These results establish a requirement for the EGFR in Ca2+-induced tyrosine phosphorylation of PKC-δ, and document the production of cell-associated TGF-α in differentiated keratinocytes which may function independent of its usual mitogenic effects.
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