P53-independent downregulation of p73 in human cancer cells treated with Adriamycin

Abstract

P73, a new p53 homologue, has been recently identified as a candidate tumor suppressor gene. We studied the alterations in p73 in a panel of human cancer cell lines treated with the chemotherapeutic agent, Adriamycin (ADR), in comparison with the changes in p53. P73 and p53 mRNA and protein were determined using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting, respectively. ADR cytotoxicity was examined by a trypan blue dye exclusion assay. The cell lines bearing wild-type p53 were more susceptible to ADR than the cell lines bearing mutant p53. ADR treatment resulted in a significant accumulation of p53 protein and mRNA expression in the wild-type p53 cell lines and caused little (slight increase) or no influence on p53 expression in the cell lines with p53 mutation and deletion. However, in striking contrast to the alterations in p53, a decline in p73 at both the protein and mRNA levels was observed in all the cell lines examined following ADR treatment. Further studies indicated that this p53-independent downregulation of p73 was induced by ADR in a dose- and time-dependent manner. Moreover, the p73 protein decline was abrogated by the presence of proteasome inhibitors. Our findings revealed that although p73 shares a similar structural and functional composition with p53, there is a significant difference in the mechanisms that govern the responses of p53 and p73 to ADR-induced DNA damage.