No abstract

Mitochondrial calcium has been postulated to regulate a wide range of processes from bioenergetics to cell death. Here, we characterize a mouse model that lacks expression of the recently discovered mitochondrial calcium uniporter (MCU). Mitochondria derived from MCU-/- mice have no apparent capacity to rapidly uptake calcium. While basal metabolism appears unaffected, the skeletal muscle of MCU-/- mice exhibited alterations in the phosphorylation and activity of pyruvate dehydrogenase. In addition, MCU-/- mice exhibited marked impairment in their ability to perform strenuous work. We further show that mitochondria from MCU-/- mice lacked evidence for calcium-induced permeability transition pore (PTP) opening. The lack of PTP opening does not appear to protect MCU-/- cells and tissues from cell death, although MCU-/- hearts fail to respond to the PTP inhibitor cyclosporin A (CsA). Taken together, these results clarify how acute alterations in mitochondrial matrix calcium can regulate mammalian physiology.

Cancer cells are known to undergo metabolic reprogramming to sustain survival and rapid proliferation, however, it remains to be fully elucidated how oncogenic lesions coordinate the metabolic switch under various stressed conditions. Here we show that deprivation of glucose or glutamine, two major nutrition sources for cancer cells, dramatically activated serine biosynthesis pathway (SSP) that was accompanied by elevated cMyc expression. We further identified that cMyc stimulated SSP activation by transcriptionally upregulating expression of multiple SSP enzymes. Moreover, we demonstrated that SSP activation facilitated by cMyc led to elevated glutathione (GSH) production, cell cycle progression and nucleic acid synthesis, which are essential for cell survival and proliferation especially under nutrient-deprived conditions. We further uncovered that phosphoserine phosphatase (PSPH), the final rate-limiting enzyme of the SSP pathway, is critical for cMyc-driven cancer progression both in vitro and in vivo, and importantly, aberrant expression of PSPH is highly correlated with mortality in hepatocellular carcinoma (HCC) patients, suggesting a potential causal relation between this cMyc-regulated enzyme, or SSP activation in general, and cancer development. Taken together, our results reveal that aberrant expression of cMyc leads to the enhanced SSP activation, an essential part of metabolic switch, to facilitate cancer progression under nutrient-deprived conditions.

Calcium is thought to play an important role in regulating mitochondrial function. Evidence suggests that an increase in mitochondrial calcium can augment ATP production by altering the activity of calciumsensitive mitochondrial matrix enzymes. In contrast, the entry of large amounts of mitochondrial calcium in the setting of ischemia-reperfusion injury is thought to be a critical event in triggering cellular necrosis. For many decades, the details of how calcium entered the mitochondria remained a biological mystery. In the past few years, significant progress has been made in identifying the molecular components of the mitochondrial calcium uniporter complex. Here, we review how calcium enters and leaves the mitochondria, the growing insight into the topology, stoichiometry and function of the uniporter complex, and the early lessons learned from some initial mouse models that genetically perturb mitochondrial calcium homeostasis. (Circ Res.

No abstract

DNA damage response (DDR) acts as a tumorigenesis barrier, and any defects in the DDR machinery may lead to cancer. SOX4 expression is elevated in many types of tumors; however, its role in DDR is still largely unknown. Here, we show that SOX4, a new DNA damage sensor, is required for the activation of p53 tumor suppressor in response to DNA damage. Notably, SOX4 interacts with and stabilizes p53 protein by blocking Mdm2-mediated p53 ubiquitination and degradation. Furthermore, SOX4 enhances p53 acetylation by interacting with p300/CBP and facilitating p300/CBP/p53 complex formation. In concert with these results, SOX4 promotes cell cycle arrest and apoptosis, and it inhibits tumorigenesis in a p53-dependent manner. Therefore, these findings highlight SOX4 as a potential key factor in regulating DDR-associated cancer.Mdm2 ͉ ubiquitination ͉ tumorigenesis D NA damage response (DDR), a highly conserved response to genotoxic stresses, is the guardian of genomic integrity (1, 2). It has been shown that DDR serves as a barrier to constrain tumor progression in its early stages by inducing cell cycle arrest, DNA repair, or apoptosis (3). A number of components are involved in cellular DDR machinery, in which ATM-Chk2-p53 and ATRChk1-p53 cascade are the key signaling pathways involved (2). A central component of DDR, p53, is one of the most important tumor suppressor proteins (4-8). The major consequence of p53 activation upon DNA damage is the induction of specific target genes, such as p21 WAF , Bax, and Puma, to initiate cell cycle arrest, apoptosis, and DNA repair (4). Cells lacking functional p53 exhibit a partial deficiency in DNA damage repair, resulting in uncontrolled cell proliferation and malignancy. Indeed, p53 gene is either lost or mutated in more than half of all human cancers (9). Around p53 there is a highly regulated network consisting of numerous proteins that interact with p53 and regulate its activity by protein stabilization, posttranscriptional modifications, protein-protein interaction, and protein subcellular localization (10), among which stabilization of p53 is presumed to play a major role in its activation. Under normal conditions, amount and activity of p53 are maintained at low levels by Mdm2, a ubiquitin E3 ligase, which binds to the N terminus of p53 and targets its C-terminal lysine residues for ubiquitination and degradation (11,12). However, in response to DNA damage, p53 protein is rapidly stabilized and activated mostly through multiple posttranslational modifications, such as phosphorylation and acetylation of specific residues in the N-terminal and C-terminal domains. DNA damage-induced p53 phosphorylation, which is mediated by ATM kinase (13, 14), contributes to p53 stability (15). Acetylation of p53 C-terminal lysine residues in p53 stabilizes the protein by preventing Mdm2-mediated ubiquitination of the same residues (16,17). In addition, the activity of p53 is also modulated by its recruitment of transcriptional coactivators or corepressors.SOX4 is a member of the SOX (SRY-re...

Although eukaryotic translation initiation factor 5A (eIF5A) was originally designated as an "initiation factor," recent data have shown it to be also involved in apoptosis. However, the actual function of eIF5A in apoptosis is still unknown. In this study, we performed yeast two-hybrid screens to identify eIF5A-interacting proteins to help us understand the mechanisms of eIF5A. Our results demonstrated that eIF5A and syntenin could engage in a specific interaction both in vitro and in vivo and functioned collaboratively to regulate p53 activity. Our findings, for the first time, revealed a new biological activity for eIF5A as the regulator of p53. Overexpression of eIF5A or its EFP domain resulted in up-regulation of p53, and silencing eIF5A by small interfering RNA reduced the p53 protein level. Further analysis by reverse transcription PCR showed eIF5A-activated p53 transcription. The effect of eIF5A on p53 transcriptional activity was further demonstrated by the increasing expressions of p21 and Bax, well known target genes of p53. In contrast, a point mutant of eIF5A, hypusination being abolished, was revealed to be functionally defective in p53 up-regulation. Overexpression of eIF5A led to a p53-dependent apoptosis or sensitized cells to induction of apoptosis by chemotherapeutic agents. However, when eIF5A interacted with its novel partner, syntenin, the eIF5A-induced increase in p53 protein level was significantly inhibited. Therefore, eIF5A seems to be a previously unrecognized regulator of p53 that may define a new pathway for p53-dependent apoptosis, and syntenin might regulate p53 by balancing the regulation of eIF5A signaling to p53 for apoptosis.

Although many long noncoding RNAs (lncRNAs) have been identified in muscle, their physiological function and regulatory mechanisms remain largely unexplored. In this study, we systematically characterized the expression profiles of lncRNAs during C2C12 myoblast differentiation and identified an intronic lncRNA, SYISL (SYNPO2 intron sense-overlapping lncRNA), that is highly expressed in muscle. Functionally, SYISL promotes myoblast proliferation and fusion but inhibits myogenic differentiation. SYISL knockout in mice results in significantly increased muscle fiber density and muscle mass. Mechanistically, SYISL recruits the enhancer of zeste homolog 2 (EZH2) protein, the core component of polycomb repressive complex 2 (PRC2), to the promoters of the cell-cycle inhibitor gene p21 and muscle-specific genes such as myogenin (MyoG), muscle creatine kinase (MCK), and myosin heavy chain 4 (Myh4), leading to H3K27 trimethylation and epigenetic silencing of target genes. Taken together, our results reveal that SYISL is a repressor of muscle development and plays a vital role in PRC2-mediated myogenesis.