An inhibitor of nuclear export activates the p53 response and induces the localization of HDM2 and p53 to U1A-positive nuclear bodies associated withthe PODs

Leptomycin B is a cytotoxin which directly interacts with and inhibits the action of CRM1, an essential mediator of the nuclear exit of proteins conta ining nuclear export signals (NES) of the HIV1 REV type. We show that addit ion of leptomycin B to human primary fibroblasts increased the levels of th e p53 tumor suppressor protein. This was accompanied by the induction of p5 3-dependent transcriptional activity in cultured cells and an increase in t he levels of the products of two p53-responsive genes) the p21(CIP1/WAF1) a nd HDM2 proteins. Leptomycin B induced the accumulation of p53 and HDM2 in the nucleus and the appearance of discrete nuclear aggregates containing bo th proteins. It has been reported that the transcriptional activity of p53 is modulated by its interaction with the HDM2 protein which also targets p5 3 for rapid degradation. Using a model cell. line conditionally expressing MDM2, the murine analogue of HDM2, we present evidence indicating that lept omycin B abrogates MDM2's role in p53 degradation and that the accumulation of p53 in distinct nuclear bodies is mediated by MDM2. Since HDM2 has rece ntly been shown to contain a functional NES of the REV type, the most likel y explanation for our results is that the effect of leptomycin B on HDM2 an d p53 is due to the inhibition of nuclear export. The ability to visualize sites where p53 and HDM2 colocalize provides a new approach to study the as sociation between the two proteins in vivo. These p53/HDM2-positive nuclear foci were found to also contain the U1A snRNP A and to be juxtaposed to th e PML oncogenic domains. (C) 1999 Academic Press.