mdm2 Is Critical for Inhibition of p53 during Lymphopoiesis and the Response to Ionizing Irradiation

Abstract

The function of the p53 tumor suppressor protein must be highly regulated because p53 can cause cell death and prevent tumorigenesis. In cultured cells, the p90 MDM2 protein blocks the transcriptional activation domain of p53 and also stimulates the degradation of p53. Here we provide the first conclusive demonstration that p90 MDM2 constitutively regulates p53 activity in homeostatic tissues. Mice with a hypomorphic allele of mdm2 revealed a heretofore unknown role for mdm2 in lymphopoiesis and epithelial cell survival. Phenotypic analyses revealed that both the transcriptional activation and apoptotic functions of p53 were increased in these mice. However, the level of p53 protein was not coordinately increased, suggesting that p90 MDM2 can inhibit the transcriptional activation and apoptotic functions of p53 in a manner independent of degradation. Cremediated deletion of mdm2 caused a greater accumulation of p53, demonstrating that p90 MDM2 constitutively regulates both the activity and the level of p53 in homeostatic tissues. The observation that only a subset of tissues with activated p53 underwent apoptosis indicates that factors other than p90 MDM2 determine the physiological consequences of p53 activation. Furthermore, reduction of mdm2 in vivo resulted in radiosensitivity, highlighting the importance of mdm2 as a potential target for adjuvant cancer therapies.

MDM2 restrains estrogen-mediated AKT activation by promoting TBK1-dependent HPIP degradation

“…To investigate whether MDM2 negatively regulates HPIP protein levels in vivo, we assessed HPIP levels in mice expressing hypomorphic Mdm2 levels. 37 As expected, Mdm2 deficiency results in increased p53 levels in vivo (Figure 7f). Interestingly, although TBK1 protein levels remained unchanged, HPIP expression was markedly elevated on Mdm2 deficiency (Figure 7f), most likely because of both enhanced p53-dependent transcription and defective Mdm2-mediated degradation of HPIP.…”

“…Mdm2 hypomorphic mice were previously described. 37 Mammary glands were isolated from 8 to 10-week-old virgin control or MDM2 hypomorphic females. Mammary epithelial cell (MEC) isolation was conducted by mincing mammary glands into small pieces with razor blades in a sterile manner, followed by a digestion with collagenase/hyaluronidase (STEMCELL Technologies, Grenoble, France) for 6 h at 37 1C under shaking at 125 r.p.m.…”

The codon 72 polymorphism-specific effects of human p53 are absent in mouse cells: implications on generation of mouse models

“…They are many reports that support this notion. Firstly, haploinsufficiency effects were observed in cells from the p53 þ /À mice and the opposite effect was observed from mice carrying the hypomorphic mdm2 allele (Harvey et al, 1993;Mendrysa et al, 2003Mendrysa et al, , 2006. Cells from these mice were shown to have reduced or elevated p53 levels, which consistently affected proliferation and apoptotic rates accordingly.…”

“…Cells from these mice were shown to have reduced or elevated p53 levels, which consistently affected proliferation and apoptotic rates accordingly. Moreover, p53 haploinsufficieny affected tumorigeneic rates and the mdm2 hypomorphic mice were more resistant to tumours (Venkatachalam et al, 2001;Mendrysa et al, 2003). Secondly, although transient transfection studies such as those presented here have inherent limitations, the amount of transfected p53 was also shown by other investigators to correlate with the extent of its function, in many systems, including direct transfection of p53 alone, in combination with Mdm2 or Epstein-Barr virus-infection, in response to drug treatment or in cells that inducibly express varying amounts of p53 (Chen et al, 1996(Chen et al, , 1998Haupt et al, 1996;Rowan et al, 1996).…”

GC-selective DNA-binding antibiotic, Mithramycin A, reveals multiple points of control in the regulation of Hdm2 protein synthesis

“…However, the sensitivity of the p53-Hdm2 regulatory module is such that even small changes in the levels of Hdm2/Mdm2 can have profound effects on cellular biology. Most notably, the transgenic manipulation of mice to incorporate a hypomorphic allele of mdm2 that results in as little as a 20% reduction in Mdm2 protein levels, renders the animals markedly more sensitive to ionizing radiation (Mendrysa et al, 2003). In humans, a single nucleotide polymorphism (SNP309) in the hdm2 gene which results in increased Hdm2 expression has been shown to correlate with accelerated rates of tumorigenesis in a subset of individuals (Bond et al, 2004.…”