p53-binding proteins in squamous cell carcinoma and normal human keratinocytes

In squamous cell carcinomas (SCCs), the tumor suppressor protein p53 is frequently overexpressed. The overexpression of p53 is often due to a mutation in the p53 gene; however, increased levels of p53 protein can be observed in the tumors without p53 gene mutations. In normal human keratinocytes, p53 is a multiconformational protein. The different conformations of p53 can be identified by their reactivity with epitope-specific, anti-p53 monoclonal antibodies. This study provides evidence that the different p53 conformations seen in human keratinocytes bind to distinct cellular proteins. Proteins that bind p53 in normal human keratinocytes were compared with p53-binding proteins from cells derived from SCC tumors by immunoprecipitation of [35S]methionine-labeled and 32P(i)-labeled cell lysates using a panel of anti-p53 monoclonal antibodies. In one tumor, the SCC cells contained a protein of Mr 30,000 bound to p53 that was not seen in normal human keratinocytes. Cells derived from a separate SCC did not have the Mr 30,000 protein but did contain two proteins of Mr 15,000 and Mr 16,000, which were not seen in normal human keratinocytes. The immunofluorescent staining pattern of cultured normal human keratinocytes, cells derived from two SCCs, as well as the original tumors from which the cells were derived, was also examined. The immunofluorescent staining of the cells derived from the tumors and the tumors themselves was different from that seen in normal cultured keratinocytes and normal epidermis. These studies suggest that there are alterations in the proteins that bind to p53 in SCCs.