In situ and in vitro expression of protein kinase C alpha in human melanocytes

Protein kinase C (PKC) is a multigene family of at least 12 isoforms involved in the transduction of extracellular signals. We investigated whether PKC-alpha, a major isoform known to be relatively abundant in brain tissue, is increased in human melanocytes relative to keratinocytes in vitro and in situ. Immunohistochemical staining for PKC-alpha in frozen neonatal human foreskin exhibited intermittent 2-3 + staining along the basal cell layer consistent with melanocytes, and 0-1 + staining of keratinocytes (on a scale of 0-3). Microscopic densitometry of the intermittent cellular staining was at least 3-fold greater than that of adjacent keratinocyte cell cytoplasm. Sequential frozen sections revealed similar intermittent cell staining with PKC-alpha and Mel-5 (tyrosinase related protein-1), known to specifically react with melanocytes. Northern blot analysis with a specific cDNA probe for PKC-alpha showed strong PKC-alpha mRNA expression in cultured melanocytes, whereas PKC-alpha mRNA in cultured non-stratifying keratinocytes was expressed at low levels. Western blot analysis revealed a prominent PKC-alpha band at approximately 80 kDa in melanocytes as opposed to a weak band in keratinocytes. Densitometry of the northern and western blots revealed that melanocytes had at least 10-fold more PKC-alpha mRNA and approximately 6-fold more PKC-alpha protein expression than keratinocytes. Total PKC activity measured in vitro revealed that melanocytes had 5-fold more activity than keratinocytes. The marked difference in melanocyte and keratinocyte expression of PKC-alpha provides further evidence for cell type specificity in the balance of PKC-alpha expression and may implicate differential PKC isoform signaling pathways in neuro-ectodermally derived cells.