Absence of p21 partially rescues Mdm4 loss and uncovers an antiproliferative effect of Mdm4 on cell growth

Abstract

Mdm4 (MdmX) is a p53-binding protein that shares structural similarities with Mdm2 and has been proposed to be a negative regulator of p53 function. Like Mdm2, the absence of Mdm4 has recently been found to induce embryonic lethality in mice that is rescued by p53 deletion. Mdm4-null embryos are reduced in size and die at midgestation, and Mdm4-deficient embryos and embryonic fibroblasts displayed reduced rates of cell proliferation. The p53-induced, cyclin-dependent kinase inhibitor p21 is strongly upregulated in Mdm4-null embryos and cells. Here, we report that deletion of p21 delays the midgestation lethality observed in Mdm4-null mice, suggesting that Mdm4 downregulates p53-mediated suppression of cell growth. Surprisingly, the absence of p21 also uncovers an antiproliferative effect of Mdm4 on cell growth in vitro and in Mdm4-heterozygous mice. These results indicate that p21 is a downstream modifier of Mdm4, and provides genetic evidence that Mdm4 can function to regulate cell growth both positively and negatively.

Rescue of Mdm4-deficient mice by Mdm2 reveals functional overlap of Mdm2 and Mdm4 in development

“…Mdm2-deficient embryos display excess numbers of apoptotic cells proceeding the time of embryonic lethality (Chavez-Reyes et al, 2003), whereas Mdm4-deficient embryos and embryonic fibroblasts display a reduction in cell cycling and increased levels of p21 without displaying increased apoptosis in most tissues. Furthermore, deletion of Bax, but not p21, has been reported to modestly delay the embryonic lethality of Mdm2 null embryos (Montes de Oca Luna et al, 1997;Chavez-Reyes et al, 2003), whereas deletion of p21 partially rescues Mdm4 null mice (Steinman et al, 2004). These results suggest that Mdm2 specifically regulates p53-mediated cell death and Mdm4 specifically regulates p53-mediated inhibition of cell growth during development.…”

“…We have previously reported that deletion of p21 partially rescues Mdm4-null mice (Steinman et al, 2004), and that haploinsufficiency of Mdm4 increases the rate of cell proliferation of p21-null MEFs. These results indicate that Mdm4 has an antiproliferative effect on cell growth when p53 is functionally compromised, either by the absence of p21 or when Mdm2 is overexpressed.…”

Phosphorylation of Ser312 contributes to tumor suppression by p53 in vivo

“…A reduction in p21 levels alongside altered p53 expression has also been observed in human hepatocellular carcinomas (36). Reduced p21 expression could also explain the partial rescue of the Mdm4 phenotype, as Mdm4 −/− p21 −/− embryos have a similar phenotype to that seen in the Mdm4 −/− p53 312A/+ mice (37). However, the number of genes whose expression varies between p53 WT and p53 312A/A mice (Fig.…”

Jmjd5, an H3K36me2 histone demethylase, modulates embryonic cell proliferation through the regulation of Cdkn1a expression

“…Notably, the elevation of Cdkn1a in Brca1-or Mdm4-deficient mice has been observed (Hakem et al, 1996;Migliorini et al, 2002), and, importantly, intercrossing with Cdkn1a-deficient mice partially rescued the phenotype, suggesting that the aberrant expression of Cdkn1a is a major, but not exclusive, reason for the embryonic death Steinman et al, 2004). Interestingly, the phenotype of histone deacetylase 1 (Hdac1)-deficient mice was similar, although more severe, than with Jmjd5 / mice.…”