The tumor suppressor p53 has recently been shown to regulate energy metabolism through multiple mechanisms. However, the in vivo signaling pathways related to p53-mediated metabolic regulation remain largely uncharacterized. By using mice bearing a single amino acid substitution at cysteine residue 305 of mouse double minute 2 (Mdm2 C305F ), which renders Mdm2 deficient in binding ribosomal proteins (RPs) RPL11 and RPL5, we show that the RP-Mdm2-p53 signaling pathway is critical for sensing nutrient deprivation and maintaining liver lipid homeostasis. Although the Mdm2 C305F mutation does not significantly affect growth and development in mice, this mutation promotes fat accumulation under normal feeding conditions and hepatosteatosis under acute fasting conditions. We show that nutrient deprivation inhibits rRNA biosynthesis, increases RP-Mdm2 interaction, and induces p53-mediated transactivation of malonyl-CoA decarboxylase (MCD), which catalyzes the degradation of malonyl-CoA to acetyl-CoA, thus modulating lipid partitioning. Fasted Mdm2 C305F mice demonstrate attenuated MCD induction and enhanced malonylCoA accumulation in addition to decreased oxidative respiration and increased fatty acid accumulation in the liver. Thus, the RPMdm2-p53 pathway appears to function as an endogenous sensor responsible for stimulating fatty acid oxidation in response to nutrient depletion.T he dynamic process of cell growth and division is under constant surveillance. As one of the primary "gatekeepers" of the cell, p53 plays a major role in sensing and responding to a variety of internal and external stressors to maintain cellular homeostasis. In addition to its conventional roles in promoting cell cycle arrest, senescence, and apoptosis, p53 has recently been shown to regulate metabolism through the transcriptional activation of genes involved in glucose transport, glycolysis, oxidative phosphorylation (OXPHOS), and glutamine hydrolysis as well as in the activation of genes upstream of the mammalian target of rapamycin and autophagic pathways (reviewed in ref.1). Virtually all cancers show metabolic changes that result in enhanced glucose consumption and elevated glycolytic activity known as the Warburg effect (2). Because cells continuously undergo metabolic perturbations as a result of constantly changing physiological and environmental cues such as daily feeding/fasting cycles, p53 may act in this context as a metabolic stress regulator altering cellular metabolic programs under nonlethal or "low-stress" conditions. Recent studies have shown that inhibition of ribosomal biogenesis can activate p53 through the ribosomal protein (RP)-mediated suppression of mouse double minute 2 (Mdm2) in the RP-Mdm2-p53 stress response pathway (3). Detailed analysis revealed that several RPs require the Mdm2 central zinc finger motif for efficient Mdm2 binding. Interestingly, cancer-associated MDM2 mutations have been reported to affect the central zinc finger motif (4, 5) and can specifically disrupt RP binding (6). To study the physiolog...

Mitochondrial outer membrane permeabilization (MOMP) is a critical control point during apoptosis that results in the release of pro-apoptotic mitochondrial contents such as cytochrome c. MOMP is largely controlled by Bcl-2 family proteins such as Bax, which under various apoptotic stresses becomes activated and oligomerizes on the outer mitochondrial membrane. Bax oligomerization helps promote the diffusion of the mitochondrial contents into the cytoplasm activating the caspase cascade. In turn, Bax is regulated primarily by anti-apoptotic Bcl-2 proteins including Bcl-xL, which was recently shown to prevent Bax from accumulating at the mitochondria. However, the exact mechanisms by which Bcl-xL regulates Bax and thereby MOMP remain partially understood. In this study, we show that the small CHCH-domain-containing protein CHCHD2 binds to Bcl-xL and inhibits the mitochondrial accumulation and oligomerization of Bax. Our data show that in response to apoptotic stimuli, mitochondrial CHCHD2 decreases prior to MOMP. Furthermore, when CHCHD2 is absent from the mitochondria, the ability of Bcl-xL to inhibit Bax activation and to prevent apoptosis is attenuated, which results in increases in Bax oligomerization, MOMP and apoptosis. Collectively, our findings establish CHCHD2, a previously uncharacterized small mitochondrial protein with no known homology to the Bcl-2 family, as one of the negative regulators of mitochondria-mediated apoptosis.

SUMMARY Mdm2 E3 ubiquitin ligase-mediated p53 degradation is generally accepted as the major mechanism for p53 regulation; nevertheless, the in vivo significance of this function has not been unequivocally established. Here, we have generated an Mdm2Y487A knock-in mouse; Mdm2Y487A mutation inactivates Mdm2 E3 ligase function without affecting its ability to bind its homologue MdmX. Unexpectedly, Mdm2Y487A/Y487A mice were viable and developed normally into adulthood. While disruption of Mdm2 E3 ligase function resulted in p53 accumulation, p53 transcriptional activity remained low; however, exposure to sub-lethal stress resulted in hyperactive p53 and p53-dependent mortality in Mdm2Y487A/Y487A mice. These findings reveal a potentially dispensable nature for Mdm2 E3 ligase function in p53 regulation, providing insight that may affect how this pathway is targeted therapeutically.

The transcription factor p53 plays a critical role in maintaining homeostasis as it relates to cellular growth, proliferation, and metabolism. In an effort to identify novel p53 target genes, a microarray approach was utilized to identify DHRS3 (also known as retSDR1) as a robust candidate gene. DHRS3 is a highly conserved member of the short chain alcohol dehydrogenase/reductase superfamily with a reported role in lipid and retinoid metabolism. Here, we demonstrate that DHRS3 is an endoplasmic reticulum (ER) protein that is shuttled to the ER via an N-terminal endoplasmic reticulum targeting signal. One important function of the ER is synthesis of neutral lipids that are packaged into lipid droplets whose biogenesis occurs from ER-derived membranes. DHRS3 is enriched at focal points of lipid droplet budding where it also localizes to the phospholipid monolayer of ER-derived lipid droplets. p53 promotes lipid droplet accumulation in a manner consistent with DHRS3 enrichment in the ER. As a p53 target gene, the observations of Dhrs3 location and potential function provide novel insight into an unexpected role for p53 in lipid droplet dynamics with implications in cancer cell metabolism and obesity.