Effect of the transcriptional repressor Mad1 on proliferation of human melanoma cells.

Mad1 is a Myc antagonist that heterodimerizes with Max and functions as a transcriptional repressor. We studied the effects of Mad1 on cell growth and malignant phenotype in human melanoma cells. To perturb the activity of c-Myc, which is involved in the progression of melanoma, we overexpressed Mad1 protein with liposomal-mediated transfection of cytomegalovirus promoter-driven expression vector containing the human Mad1 gene, pcMad-1. The growth characteristics and malignant potential of two Mad1 transfectants of the FEM human melanoma cell line, overexpressing Mad1 stably and the respective vector control were analysed both in vitro and in a nude mice xenograft model in vivo. Two Mad1 transfectants exhibited up to 2.8 times longer doubling time, less proliferation rate (50% inhibition), increased G0/G1 accumulation in cell-cycle distribution, and active melanin synthesis, compared with vector controls in vitro. In the mice model, the volume of tumors that arose from Mad1 transfected clones was 4-5 times less than those arising from vector control FEM cells. Histopathologically, tumors that arose from Mad1 transfectants showed altered round-shaped morphology with less pleomorphism compared with control FEM cells. Our results indicating that Mad1 gene transfer inhibits the proliferation of human melanoma cells suggest that Mad1 could be a potentially useful candidate for the modification of genes against malignancies.