WTp53 induction does not override MTp53 chemoresistance and radioresistance due to gain-of-function in lung cancer cells

New molecular cancer treatment strategies aim to reconstitute wild-type p53 (WTp53) function in mutant p53 (MTp53)-expressing tumors as a means of resensitizing cells to chemotherapy or radiotherapy. The success of this approach may depend on whether MTp53 proteins are acting in a dominant-negative or independent gain-offunction mode. Herein, we describe an isogenic, temperature-sensitive p53 model (p53^A138V) in p53-null human H1299 lung cancer cells in which WTp53 can be selectively coexpressed with a temperature-sensitive MTp53 allele (A138V) during initial DNA damage and subsequent DNA repair. Cells expressing MTp53 alone or coexpressing induced WTp53 and MTp53 were tested for p53 transcription, G₁ and G ₂ cell cycle checkpoints, apoptosis, and long-term clonogenic survival following DNA damage. Transient transfection of WTp53 into H1299 cells, or shift-down of H1299-p53^A138V stable transfectants to 32°C to induce WTp53, led to increased p21^WAF1 expression and G₁ and G₂ arrests following DNA damage but did not increase BAX expression or apoptosis. In contrast, both transient and stable expression of the p53^A138V mutant in p53-null H1299 cells (e.g. testing gain-of-function) at 37°C blocked p21^WAF1 induction following DNA damage. Cell death was secondary to mitotic catastrophe and/or tumor cell senescence. Overexpression of WTp53 did not resensitize resistant MTp53-expressing cells to ionizing radiation, cisplatinum, or mitomycin C. Our results suggest that human MTp53 proteins can lead to resistant phenotypes independent of WTp53-mediated transcription and checkpoint control. This should be considered when using p53 as a prognostic factor and therapeutic target.