EXPRESSION OF THE RETINOID-INDUCIBLE POLYPEPTIDE, MIDKINE, IN HUMAN EPIDERMAL-KERATINOCYTES

Midkine (MK) is a retinoid-inducible and potent cell growth/differenti ation factor active during mouse embryogenesis. We studied MK expressi on in various cell strains established from the skin. MK and its mRNA were detected in cultured keratinocytes but not in melanoma cell lines or dermal fibroblasts by both Western blot analysis and reverse trans cription-polymerase chain reaction (RT-PCR). Treatment of cultured ker atinocytes with retinoic acid (10(-5) M, 24 h) resulted in an increase in the level of MK mRNA. When added to cultured keratinocytes, MK sti mulated cell proliferation fourfold. Immunohistochemistry revealed MK to be present at the epidermal-dermal junction in embryonic mouse skin and in normal human skin. The limited expression of MK in epidermal k eratinocytes indicates that this polypeptide may be involved in the di fferentiation/proliferation of keratinocytes.