Accumulation of an inactive form of p53 protein in cells treated with TNFα

In MCF-7 cells, TNFa induces a G1 arrest with an increased expression of p21/Waf1, an activation of NF-κB and an accumulation of p53. NF-κB and p53 are two transcriptional factors known to activate p21/Waf1 gene expression. Here we show that p53 inhibition has no effect on p21/Waf1 mRNA accumulation following TNFx treatment. In contrast, inactivation of NF-κB inhibits p21/Waf1 expression without affecting G1 arrest. The fact that p21/Waf1 gene expression is still stimulatedwhen p53 is inactivated strongly suggests that TNFα induces accumulation of an inactive form of p53 protein. This assumption was further supported by the following observations: (i) the p53 DNA-binding activity to its consensus sequence was not stimulated following TNFα treatment, (ii) phosphorylation at Ser-15, -20 or -392 was not detected in response to TNFa, (iii) the transcription rate of Ddb2, another p53 target gene, was not stimulated by TNFa. Finally, the accumulation of p53 in the nuclei of TNFα-treated MCF-7 cells was concomitant with an increase in p53 mRNA level, suggesting a regulation at the transcription level.