p73 expression modulates p63 and Mdm2 protein presence in complex with p53 family-specific DNA target sequence in squamous cell carcinogenesis

The expression of p73 and p63 isoforms is frequently deregulated in human epithelial tumors. We previously showed that loss of p73 protein expression associates with malignant conversion in vivo and ionizing radiation (IR) resistance in vitro in a clonal model of mouse epidermal carcinogenesis. Here we show that loss of endogenous p73 expression in squamous cell carcinoma (SCC) cells and tumors was concomitant with preferential DNA binding of the inhibitory ΔNp63α isoform and reduction of transcriptionally active p63γ isoforms binding to a p21 promoter sequence in vitro. Reconstitution of TAp73α in malignant cells increased the steady state DNA-binding capabilities of the endogenous transcriptionally active TAp63γ and ΔNp63γ isoforms, correlating with restoration of tumor suppression but not IR sensitivity. Loss of p73 in malignant cells also coincided with increased presence of p53 family inhibitor Mdm2 in p53-specific DNA-bound complexes, whereas reconstitution of TAp73α expression resulted in exclusion of Mdm2 from these complexes. These results suggest a dual mechanism for TAp73α to foster tumor suppression through enhancement of the DNA-binding activity of p63γ isoforms, and through inhibition of transcriptional repressors Mdm2 or ΔNp63α.