The MDM2 oncoprotein promotes apoptosis in p53-deficient human medullary thyroid carcinoma cells

The MDM2 oncoprotein has been shown to inhibit p53-mediated growth arrest a nd apoptosis. It also confers growth advantage to different cell lines in t he absence of p53. Recently, the ability of MDM2 to arrest the cell cycle o f normal human fibroblasts has also been described. We report a novel funct ion for this protein, showing that overexpression of MDM2 promotes apoptosi s in p53-deficient, human medullary thyroid carcinoma cells. These cells, d evoid of endogenous MDM2 protein, exhibited a significant growth retardatio n after stable transfection with mdm2. Cell cycle distribution of MDM2 tran sfectants [medullary thyroid tumor (MTT)-mdm2] revealed a fraction of the c ell population in a hypodiploid status, suggesting that MDM2 is sufficient to promote apoptosis. This circumstance is further demonstrated by annexin V labeling. MDM2-induced apoptosis is partially reverted by transient trans fection with p53 and p19(ARF). Both MTT and MTT-mdm2 cells were tumorigenic when injected into nude mice. However, the percentage of apoptotic nuclei in tumor sections derived from MDM2-expressing cells was significantly high er relative to that in the parental cell line. MDM2-mediated programmed cel l death is at least mediated by a down-regulation of the antiapoptotic prot ein Bcl-2. Protein levels of caspase-2, which are undetectable in the paren tal cell line, appear clearly elevated in MTT-mdma cells. Caspase-3 activat ion does not participate in MDM2-induced apoptosis, as determined by protei n levels or poly(ADP-ribose) polymerase fragmentation. The results observed in this medullary carcinoma cell line show for the first time that the pro duct of the mdma oncogene mediates cell death by apoptosis in p53-deficient tumor cells.