Control of the p53 tumor suppressor protein is critical because p53 can induce cell death. The key inhibitor of p53 appears to be the p90DM2 protein, based on experiments in which inhibition of p90MDM2 leads to high levels of p53 activity in cultured cells. Basal levels of p90MDM2 in cultured cells are determined by transcription of the mdm2 gene by p53. p90MDM2's ability to inhibit p53's function and p53's ability to determine P90MDM2's levels together constitute an autoregulatory feedback loop proposed to be critical for maintaining low, basal levels of p53's activity. This laboratory has made two fundamentally important discoveries that provide insight into the mechanisms regulating the balance between p53 and mdm2. First, whereas p53 constitutively stimulates mdm2 expression in cultured cells, it does not constitutively regulate mdm2's expression in intact tissues. Instead, p53 requires a stress such as whole body ionizing irradiation to stimulate expression of mdm2 in vivo. Thus at least one arm of the p53/mdm2 autoregulatory loop is not functional in unstressed tissues. It is not known whether the second arm of the autoregulatory loop the regulation of p53's activity by P90MDM2 is constitutively functional in vivo. We hypothesize that p90 DM2 regulates p53 only under conditions in which p53 regulates expression of mdm2. That is, the two arms of the p53/mdrn2 autoregulatory loop are coordinately regulated. To test this hypothesis, we have generated a line of mice carrying a conditionally null allele of mdm2 so that we can determine whether mdm2 regulates p53 constitutively in vivo. Our second key discovery is a new regulatory interaction between p53 and mdm2. We found that a smaller MDM2 protein antagonizes p90MDM2's function in cultured cells, thereby activating p53. Both MDM2 proteins are expressed in vivo. Here we propose to test the hypothesis that the smaller MDM2 protein contributes to the increase in p53 'S activity in response to genotoxic stress in vivo. Another goal of this proposal is determine whether the induction of mdm2 by p53 in the response to genotoxic stress induced by ultraviolet and ionizing irradiation is important for the recovery of low, basal levels of p53 in the later stages of the damage response. The results from these experiments will be of interest not only to the basic biologist, but also to the clinician, since the D53/p90MDM2 interaction has become a potential target for chemotherapy.
