
<ArticleTitle>An alternatively spliced <prot>HDM2</prot> product increases <prot>p53</prot> activity by inhibiting <prot>HDM2</prot>.</ArticleTitle>
<AbstractText>The human counterpart <prot>hdm2</prot> of the <prot>murine double-minute 2</prot> (<prot>mdm2</prot>) gene encodes a 90-kD protein (<prot>HDM2</prot>) that inhibits the function of the <prot>p53</prot> tumor suppressor. <prot>Hdm2</prot> is amplified in approximately 30% of sarcomas, leading to overproduction of <prot>HDM2</prot> and inactivation of <prot>p53</prot>. Using immunohistochemistry to screen a panel of human tumors for <prot>HDM2</prot> overproduction, we detected high levels of <prot>HDM2</prot> in the cytoplasm in 25% of lung tumors as opposed to its normal localization in the nucleus. These samples contained full-length <prot>hdm2</prot> and several alternate-splice forms of <prot>hdm2</prot> mRNA. Sequence analysis revealed deletions in the alternate-splice forms of the <prot>p53</prot> binding domain and absence of a nuclear localization signal. In transient transfection assays, one of the alternate-splice forms, <prot>HDM2</prot>(<prot>ALT1</prot>), bound and sequestered full-length <prot>HDM2</prot> in the cytoplasm. In addition, the binding of <prot>HDM2</prot>(<prot>ALT1</prot>) to <prot>HDM2</prot> inhibited the interaction of <prot>HDM2</prot> with <prot>p53</prot>, thus enhancing <prot>p53</prot> transcriptional activity. These data suggest the existence of another level of regulation of <prot>HDM2</prot> which increases the activity of <prot>p53</prot>.</AbstractText>
