Role of protein kinase C alpha in calcium induced keratinocyte differentiation: defective regulation in squamous cell carcinoma

Calcium induces both involucrin and transglutaminase-K in normal keratinocytes (NHK) but not in squamous carcinoma cell lines (SCC). The protein kinase C (PKC) agonist phorbol myristoyl acetate potentiates and the PKC antagonist Ro31-8220 blocks the ability of calcium to stimulate the involucrin promoter in normal human keratinocytes but not in SCC4. We thus examined the ability of calcium to regulate the levels of five PKC isozymes in NHK and two SCC. In the normal keratinocytes, the levels of PKC alpha], PKC delta], PKC eta], and PKC zeta] increased over the first one to two weeks in a calcium-and time-dependent manner. PKC epsilon] decreased in a time-and calcium-dependent fashion over the three-week period. All five isozymes showed little change during culture in SCC4 at any calcium concentration. Calcium and time of culture had partial effects on SCC12B2, a carcinoma that shows partial differentiation characteristics. Since PKC alpha] is the only calcium responsive PKC isozyme in keratinocytes and most likely to be directly involved in calcium induced differentiation, we evaluated the effect of inhibiting its production with antisense oligonucleotides on calcium-regulated markers of differentiation. We found that the PKC alpha] specific antisense oligonucleotide blocked calcium stimulated involucrin promoter activity as well as PKC alpha], involucrin, and transglutaminase protein production, whereas the sense oligonucleotide control did not. We conclude that although a number of PKC isozymes are regulated during calcium-induced differentiation, PKC alpha] plays a necessary role in mediating calcium-induced differentiation. Failure to regulate PKC alpha] in SCC4 may underlie at least part of the failure of calcium to promote differentiation in these cells.