THE MELANOMA DIFFERENTIATION-ASSOCIATED GENE-6 (MDA-6), WHICH ENCODESTHE CYCLIN-DEPENDENT KINASE INHIBITOR P21, MAY FUNCTION AS A NEGATIVEREGULATOR OF HUMAN-MELANOMA GROWTH AND PROGRESSION

Defects in differentiation are common occurrences in human cancers. In vitro growth of human melanoma cells in medium containing human fibro blast interferon (IFN-beta) and the antileukemic compound mezerein (ME Z) results in a rapid and irreversible loss in proliferative ability a nd terminal cell differentiation. To define the molecular basis of ter minal differentiation a subtraction hybridization approach was used. c DNA libraries prepared from proliferating human HO-1 melanoma cells we re subtracted from cDNA libraries generated from IFN-beta + MEZ-treate d HO-1 cells. This strategy resulted in the identification and cloning of melanoma differentiation associated (mda) genes displaying elevate d expression in terminally differentiated human melanoma cells. One di fferentially expressed cDNA, mda-6, is the ubiquitous inhibitor of cyc lin-dependent kinases p21. The expression of mda-6 increases in growth arrested, DNA damaged and differentiation inducer treated human melan oma cells in a p53-independent manner. The de novo level of mda-6 is h igher in normal melanocytes vs. metastatic human melanoma cells and in less progressed vs. progressed early vertical growth phase (VGP) prim ary human melanoma cells. These results suggest that mda-6 (p21) may f unction as a negative regulator of human melanoma growth, development, and progression. The studies supporting this hypothesis are presented in the context of our current understanding of p21 action.