Jekyll and Hyde, the p53 protein, pleiotropics antagonisms and the thrifty aged hypothesis of senescence

Mdm2 is required for survival of hematopoietic stem cells/progenitors via dampening of ROS-induced p53 activity

Mdm2 is an E3 ubiquitin ligase that targets p53 for degradation. p53(515C) (encoding p53R172P) is a hypomorphic allele of p53 that rescues the embryonic lethality of Mdm2(-/-) mice. Mdm2(-/-) p53(515C/515C) mice, however, die by postnatal day 13 resulting from hematopoietic failure. Hematopoietic stem cells and progenitors of Mdm2(-/-) p53(515C/515C) mice were normal in fetal livers but were depleted in postnatal bone marrows. After birth, these mice had elevated reactive oxygen species (ROS) thus activating p53R172P. In the absence of Mdm2, stable p53R172P induced ROS and cell cycle arrest, senescence, and cell death in the hematopoietic compartment. This phenotype was partially rescued with antioxidant treatment and upon culturing of hematopoietic cells in methycellulose at 3% oxygen. p16 was also stabilized because of ROS, and its loss increased cell cycling and partially rescued hematopoiesis and survival. Thus, Mdm2 is required to control ROS-induced p53 levels for sustainable hematopoiesis.

Caveolin-1 regulates the antagonistic pleiotropic properties of cellular senescence through a novel Mdm2/p53-mediated pathway

We show that caveolin-1 is a novel binding protein for Mdm2. After oxidative stress, caveolin-1 sequesters Mdm2 away from p53, leading to stabilization of p53 and up-regulation of p21(Waf1/Cip1) in human fibroblasts. Expression of a peptide corresponding to the Mdm2 binding domain of caveolin-1 is sufficient to up-regulate p53 and p21(Waf1/Cip1) protein expression and induce premature senescence. Oxidative stress-induced activation of the p53/p21(Waf1/Cip1) pathway and induction of premature senescence are compromised in caveolin-1 null mouse embryonic fibroblasts (MEF). We also show that reintroduction of caveolin-1 in oncogenic Ras (Ras(G12V))-transformed fibroblasts, which express residual levels of caveolin-1, is sufficient to promote cellular senescence. Moreover, caveolin-1 expression in MEFs is required for senescent fibroblast-induced stimulation of cell growth and tumorigenesis of both Ras(G12V)-transformed fibroblasts and MDA-MB-231 breast cancer epithelial cells both in vitro and in vivo. Thus, our results propose caveolin-1 as a key mediator of the antagonistic pleiotropic properties of cellular senescence.