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Protein kinase C (PKC) plays a pivotal role in modulating the growth ofmelanocytic cells in culture. We have shown previously that a majorphysiological substrate of PKC, the 80 kDa myristoylated alanine-rich C-kinasesubstrate (MARCKS), can be phosphorylated in quiescent, non-tumorigenicmelanocytes exposed transiently to a biologically active phorbol ester, butcannot be phosphorylated in phorbol ester-treated, syngeneic malignant melanomacells. Despite its ubiquitous distribution, the function of MARCKS in cellgrowth and transformation remains to be demonstrated clearly. We report herethat MARCKS mRNA and protein levels are down-regulated significantly in thespontaneously derived murine B16 melanoma cell line compared with syngeneicnormal Mel-ab melanocytes. In contrast, the tumourigenicv-Ha- ras -transfonned melan-ocytic line, LTR Ras 2, showed ahigh basal level of MARCKS phosphorylation which was not enhanced by treatmentof cells with phorbol ester. Furthermore, protein levels of MARCKS in LTR Ras 2cells were similar to those expressed in Mel-ab melanocytes. However, in fourout of six murine tumour cell lines investigated, levels of MARCKS protein werebarely detectable. Transfection of B16 cells with a plasmid containing theMARCKS cDNA in the sense orientation produced two neomycin-resistant clonesdisplaying reduced proliferative capacity and decreased anchorage-independentgrowth compared with control cells. In contrast, transfection with the antisenseMARCKS construct produced many colonies which displayed enhanced growth andtransforming potential compared with control cells. Thus, MARCKS appears to actas a novel growth suppressor in the spontaneous transformation of cells ofmelanocyte origin and may play a more general role in the tumour progression ofother carcinomas.

MARCKS functions as a novel growth suppressor in cells of melanocyte origin