EXPRESSION OF PROLIFERATING CELL NUCLEAR ANTIGEN (PCNA) - PROLIFERATIVE PHASE FUNCTIONS AND MALIGNANT TRANSFORMATION OF MELANOCYTES

The UV-dependent G2-phase functions of melanocytes include dendricity, the expression of melanocyte stimulating hormone (MSH) receptors and neural differentiation. The present report studied highly dendritic me lanocytes in epidermis overlying tumours, seborrhoeic keratosis, basal cell carcinoma and melanomas. The expression of the proliferative pro tein PCNA was studied by immunohistochemistry, as this indicates cells in S/G2-phase. In the non-neoplastic dendritic melanocytes, PCNA is r etained in the cytoplasm, resulting in the arrest of the cells in the S/G2-phase for prolonged periods, as indicated by the length and compl exity of the dendritic processes. In melanomas, this barrier is overco me with rapid proliferation of the cells and loss of dendricity. PCNA is produced in the cytoplasm and transported into the nucleus during t he S-phase, as observed in melanomas. The arrest of melanocytes in the S/G2-phase for long periods associated with UV responsiveness makes t hese cells vulnerable to DNA damage and neoplasia. Pools of PCNA in th e cytoplasm, when transported into the nucleus, would support the rapi d proliferation observed in melanomas.