hMad4, c-Myc endogenous inhibitor, induces a replicative senescence-like state when overexpressed in human fibroblasts.

Mad family proteins have an antagonistic action on Myc-dependent cell proliferation and transformation. We isolated a human cDNA clone, human Mad4 (hMad4), encoding a polypeptide of 209 amino acid residues, exhibiting 90% identity with mouse Mad4. Northern blot analysis shows that hMad4 probe hybridizes to a 3.8 kb message; its expression is highest in quiescent human WI38 fibroblasts. Among tissues, hMad4 mRNA is most abundant in brain, lung, and muscle. Consistent with other members of the Mad family, hMad4 can repress the transactivation activity of Myc/Max heterodimers on an E-box chloramphenicol acteyl transferase (CAT) reporter plasmid; inhibition of both proliferation and clonogenic formation of hMad4-infected cells correlates with the in vitro reporter repression. Moreover, infection of young human fibroblasts induces a replicative senescence-like state. This phenotype was accompanied by s-beta-galactosidase and PAI-1 expression. These results suggest that hMad4 might be an important regulator of replicative senescence in human cells.