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 i nvolved 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 keratinocy tes in vitro and in situ. Immunohistochemical staining for PKC-alpha i n frozen neonatal human foreskin exhibited intermittent 2-3+ staining along the basal cell layer consistent with melanocytes, and 0-1+ stain ing of keratinocytes (on a scale of 0-3), Microscopic densitometry of the intermittent cellular staining was at least 3-fold greater than th at 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 melan ocytes. Northern blot analysis with a specific cDNA probe for PKC-alph a showed strong PKC-alpha mRNA expression in cultured melanocytes, whe reas PKC-alpha mRNA in cultured non-stratifying keratinocytes was expr essed at low levels, Western blot analysis revealed a prominent PKC-al pha band at approximately 80 kDa in melanocytes as opposed to a weak b and in keratinocytes. Densitometry of the northern and western blots r evealed that melanocytes had at least 1.0-fold more PKC-alpha mRNA and approximately 6-fold more PKC-alpha protein expression than keratinoc ytes. Total PKC activity measured in vitro revealed that melanocytes h ad 5-fold more activity than keratinocytes. The marked difference in m elanocyte and keratinocyte expression of PKC-alpha provides further ev idence for cell type specificity in the balance of PKC-alpha expressio n and may implicate differential PKC isoform signaling pathways in neu ro-ectodermally derived cells.