Regulation of hdm2 by stress-induced hdm2^alt1 in tumor and nontumorigenic cell lines correlating with p53 stability

Alternative and aberrant splicing of hdm2 occurs in tumor and normal tissues. However, the factors that induce these splice variants and whether they are translated to protein products in vivo is unknown, making it difficult to decipher which of these hdm2 transcripts have a normal physiologic function or contribute to carcinogenesis. We investigated the conditions that induce this post-transcriptional modification of hdm2 in tumor and nontumorigenic cell lines. We showed that UV and γ radiation as well as cisplatin treatment induced alternative splicing of hdm2, which resulted in a single splice variant, hdm2^alt1, irrespective of the cell type. Interestingly, the mechanism of UV-induced splicing is independent of p53 status. Immunoanalysis revealed that, after UV radiation, HDM2^ALT1 protein was expressed and interacted with HDM2 that correlated to increased p53 protein levels and its accumulation in the nucleus, whereas HDM2 localized more to the cytoplasm with a decrease in its RNA and protein level. We propose that stress-induced HDM2 ^ALT1 regulates HDM2 at two levels, RNA and protein, further modulating the p53-HDM2 interaction or interactions of HDM2 with other cell cycle regulatory proteins. This kind of regulation may possibly restrict oncogenic functions of HDM2 and contribute to the many protective responses triggered by certain stress signals. Our data imply that HDM2^ALT1 possesses a normal physiologic function in damaged cells, perhaps facilitating cellular defense.