Two-dimensional gel analysis of apoptosis-specific p53 isoforms induced by 2-methoxyestradiol in human lung cancer cells

The natural metabolic byproduct of estradiol, 2-methoxyestradiol (2-MeOE2), induces apoptosis in human lung cancer cells by a p53-dependent mechanism. The expression of wild-type p53 isoforms was investigated in H1299 non-small cell lung carcinoma cells induced into apoptosis by 2-MeOE2. H1299 cells lack endogenous p53 and undergo predominantly a G1 arrest when infected with a recombinant wild-type p53 adenovirus. However, when H1299 cells transfected with p53 were treated with 2-MeOE2, they underwent rapid and extensive apoptosis. H1299 cells expressing mutant his273 p53 were unaffected by 2-MeOE2, indicating a dependence of 2-MeOE2-mediated apoptosis on the presence of a functional p53. Analysis of wild-type p53 phosphoisoforms in H1299 cells by two-dimensional gel electrophoresis revealed that 2-MeOE2 induced a unique group of acidic p53 isoforms. Although most of the wild-type p53 in untreated H1299 cells migrated as at least five diffuse species with isoelectric points from pH 5.5–6.3, as many as nine additional forms migrating toward the acidic region with pI values from 4.4–5.3 were detected in 2-MeOE2-treated apoptotic cells. Two other agents known to induce apoptosis, vinblastine and actinomycin D, induced a similar pattern of acidic p53 species as that observed for 2-MeOE2. The results indicated that the induction of apoptosis in H1299 cells by 2-MeOE2 is dependent on the upregulation of specific p53 isoforms. Identification of the specific p53 phosphoisoforms induced by MeOE2 will be an important step in understanding the regulation and function of p53 in apoptosis.