Nuclear degradation of p53 occurs during down‐regulation of the p53 response after DNA damage

Abstract

The principal regulator of p53 stability is HDM2, an E3 ligase that mediates p53 degradation via the ubiquitin-26S proteasome pathway. The current model holds that p53 degradation occurs exclusively on cytoplasmic proteasomes and hence has an absolute requirement for nuclear export of p53 via the CRM-1 pathway. However, proteasomes are abundant in both cytosol and nucleus, and no studies have been done to determine under what physiological circumstances p53 degradation might occur in the nucleus. We analyzed HDM2-mediated degradation of endogenous p53 in the presence of various nuclear export inhibitors of CRM-1, including leptomycin B (LMB), a noncompetitive, specific, and fast-acting inhibitor; and HTLV1-Rex protein, a potent competitive inhibitor. We found that significant HDM2-mediated p53 degradation took place in the presence of LMB or HTLV1-Rex, indicating that endogenous p53 degradation occurs locally in the nucleus, in parallel to cytoplasmic degradation. Moreover, p53 null cells that coexpressed export-defective mutants of p53 and HDM2 retained partial competence for p53 degradation. It is important that nuclear degradation of p53 occurred during the poststress recovery phase of a p53 response, after DNA damage ceased. We propose that the capability of local p53 degradation within the nucleus provides a tighter and faster control during the down-regulatory phase, when an active p53 program needs to be turned off quickly.