Lack of protein kinase C (PKC)-β and low PKC-α, -δ, -ε, and -ζ isozyme levels in proliferating human melanoma cells

Protein kinase C (PKC), a calcium and phospholipid-dependent kinase, has been implicated in carcinogenesis of melanocytic cells. However, its role in melanoma cell growth remains controversial. We therefore investigated the growth dependence of PKC isozyme expression in human normal melanocytes and melanoma cells. Logarithmic and stationary growth phases in culture were clearly distinguished by nuclear cell staining with the proliferation marker Ki-67. PKC-βI and -βII were expressed exclusively in normal melanocytes but not in melanoma cells, whereas PKC-γ was not found in any of the cultures studied. Low PKC-δ, -ε and -ζ mRNA levels were detected by RT-PCR in proliferating melanoma cells and higher in confluent non-proliferating cells, whereas levels of PKC-α mRNA remained rather stable. Subcellular fractionation and immunoblotting revealed accordingly low expression of PKC-α, -δ, -ε, and -ζ in the logarithmic growth phase of melanoma cells, with subsequent increase of expression and of membrane association in the stationary phase. Only weak differences were detected between the growth phases in normal melanocytes for the respective PKC isozymes, except for membrane-associated PKC-βI and -βII which were clearly elevated in confluent melanocyte cultures. These data suggest that certain PKC isozymes are involved in the intracellular signalling that regulates melanoma cell proliferation, and may function as suppressors of tumour cell growth.