Metabolic regulation of stem cell behavior and implications for aging

Caloric intake influences metabolic homeostasis, somatic maintenance, tissue regeneration, and longevity in?metazoans. Recent studies indicate that nutrient-dependent changes in stem cell populations play an important role in these effects. Here, we review the emerging picture of how nutrient-sensing pathways affect stem cell behavior, providing a mechanism to influence life span.     

Metabolic alteration in tumorigenesis

Altered metabolism in cancer was first discovered by Otto Warburg early last century.Although the Warburg Effect has been widely used in tumor detection,relatively little progress had been made in mechanistic understanding of cancer metabolism in the subsequent eight decades.Genetic studies have recently identified mutations in human cancer targeting multiple enzymes involved in intermediate metabolism.One emerging mechanism common to these mutant enzymes is the accumulation of a metabolite that alters the epigenetic control.     

Metabolic regulation by lactate

For more than a century, the metabolic role of lactate has intrigued physiologists and biochemists. Yet, for the first half of the last century lactate had been designated as a waste product, and assigned no additional significance besides its controversial role in muscle fatigue. The decline of the lactate hypothesis for the onset of muscle fatigue and the defining of some modulatory properties attributed to lactate have increased the interest on this molecule. The present critical review aimed at evaluating some recent publications concerned with unveiling the regulatory actions of lactate in cellular function. Lactate has been described to modulate enzymes catalytic properties to affect hormonal release and responsiveness, and to control body homeostasis. Moreover, these properties are directly related to the genesis and the sustainability of pathological conditions, such as diabetes and cancer. In the end, we concluded that lactate should not be regarded as simply an anaerobic metabolite, but should be considered as a regulatory molecule that modulates the integration of metabolism.     

Localization of mouse mitochondrial SIRT proteins: shift of SIRT3 to nucleus by co-expression with SIRT5

Yeast silent information regulator 2 (SIR2) is involved in extension of yeast longevity by calorie restriction, and SIRT3, SIRT4, and SIRT5 are mammalian homologs of SIR2 localized in mitochondria. We have investigated the localization of these three SIRT proteins of mouse. SIRT3, SIRT4, and SIRT5 proteins were localized in different compartments of the mitochondria. When SIRT3 and SIRT5 were co-expressed in the cell, localization of SIRT3 protein changed from mitochondria to nucleus. These results suggest that the SIRT3, SIRT4, and SIRT5 proteins exert distinct functions in mitochondria. In addition, the SIRT3 protein might function in nucleus.     

Cyclin D1 and molecular chaperones: implications for tumorigenesis

We recently investigated the mechanisms of cyclin D1 action in human cancer using global analyses of gene expression. With an experimentally-determined expression signature for cyclin D1 overexpression, gene expression data from human tumors, and a novel data-mining method, we were able to reveal a previously unappreciated and apparently predominant functional interdependency between cyclin D1 and C/EBPbeta. Many of the genes we found to be affected by cyclin D1 overexpression are recognized as molecular chaperones or their regulators. Might this provide insights to the role of the cyclin D1-C/EBPbeta axis in carcinogenesis?     

Analysis of dystroglycan regulation and functions in mouse mammary epithelial cells and implications for mammary tumorigenesis

Abnormalities in the interactions of cells with the extracellular matrix (ECM) play an important role in the development and progression of many types of cancer and are a hallmark of malignant transformation. The dystroglycan (DG) complex is a transmembrane glycoprotein that forms a continuous link from the ECM to the actin cytoskeleton, providing structural integrity and perhaps transducing signal, in a manner similar to integrins. Deregulated expression of DG has been reported in a variety of human malignancies and related to tumor differentiation and aggressiveness. In breast cancer, reduced DG expression has been associated with patient survival and with loss of differentiation of tumor cells. Limited data are available on DG physiology in epithelial cells. In this study, we used the HC11 spontaneously immortalized murine mammary epithelial cells to study DG function(s) and regulation in normal cells. We found that expression of DG protein and mRNA is cell-cycle and cell-density regulated in these cells. Moreover, expression of both DG subunits increased upon lactogenic differentiation of the HC11 cells. The turnover of cell-surface-expressed DG was evaluated in the same cells and half-life of DG subunits was evaluated to be about 12 h. DG-specific small inhibitory RNAs were used to analyze the effects of a reduced expression of DG in these cells. Cells in which DG expression was suppressed were growth inhibited, accumulated in the S-phase of the cell cycle, failed to undergo lactogenic differentiation, and displayed an increase in the percentage of apoptotic cells. Moreover, changes were observed in the expression and/or activity of several molecules involved in cell growth control. These results demonstrate that DG expression is tightly regulated in normal mammary epithelial cells and support the hypothesis that DG is involved in several functions other than structural integrity in these cells. This finding provides new insight into the roles played by DG in epithelial cell physiology and will contribute to our understanding of its involvement in the process of epithelial cell transformation.     

Acetylation-dependent regulation of mitochondrial ALDH2 activation by SIRT3 mediates acute ethanol-induced eNOS activation

Moderate alcohol consumption has beneficial effects on endothelial nitric-oxide synthase (eNOS) activation, which can engender an array of anti-atherogenic actions. Here we show that in human aortic endothelial cells (HAECs), rapid activation of mitochondrial aldehyde dehydrogenase 2 (ALDH2) mediates ethanol-induced eNOS activation by preventing reactive oxygen species (ROS) accumulation. Furthermore, activation of ALDH2 by ethanol is due to its hyperacetylation by SIRT3 inactivation. These data suggest that ethanol-induced eNOS activation in HAECs may be dependent on ALDH2 hyperacetylation by SIRT3 inactivation.     

Transactivation-deficient Delta TA-p73 inhibits p53 by direct competition for DNA binding: implications for tumorigenesis

The p53 family member p73 displays significant structural and functional homology to p53. However, instead of mutational inactivation, overexpression of wild-type p73 has been reported in various tumor types compared with normal tissues, arguing against a classical tumor suppressor function. Recently, N-terminally truncated, transactivation-deficient p73 isoforms (DeltaTA-p73) have been identified as a second class of p73 proteins. Because overexpression of p73 in tumors includes DeltaTA-p73, we further characterized these novel p73 isoforms. We show that DeltaTA-p73 retains DNA-binding competence but lacks transactivation functions, resulting in an inability to induce growth arrest and apoptosis. Importantly, DeltaTA-p73 acts as a dominant-negative inhibitor of p53 and full-length p73 (TA-p73). We demonstrate that inhibition of p53 involves competition for DNA binding, whereas TA-p73 can be inhibited by direct protein-protein interaction. Further, we show that up-regulation of endogenous p73 just like ectopic overexpression of DeltaTA-p73 confers resistance to p53-mediated apoptosis induced by the chemotherapeutic agent H-7. Because inhibition of p53 is a common theme in human cancer, our data strongly support a role of DeltaTA-p73 expression for tumor formation.     

Metabolic memory: mechanisms and implications for diabetic vasculopathies

In the past decades, a persistent progression of diabetic vascular complications despite reversal of hyperglycemia has been observed in both experimental and clinical studies. This durable effect of prior hyperglycemia on the initiation and progression of diabetic vasculopathies was defined as “metabolic memory”. Subsequently, enhanced glycation of cellular proteins and lipids, sustained oxidative stress, and prolonged inflammation were demonstrated to mediate this phenomenon. Recently, emerging evidence strongly suggests that epigenetic modifications may account for the molecular and phenotypic changes associated with hyperglycemic memory. In this review, we presented an overview on the discovery of metabolic memory, the recent progress in its molecular mechanisms, and the future implications related to its fundamental research and clinical application.     

The BRCA2-EMSY connection: implications for breast and ovarian tumorigenesis

In this issue, Hughes-Davies et al. describe a novel gene product, EMSY, which suppresses the transactivational activity of BRCA2. EMSY is located within an amplicon in sporadic breast and ovarian cancers, suggesting that its overexpression may mimic the effects of BRCA2 inactivation. The implications for BRCA2 function are discussed.     

Inactivation of glycogen synthase kinase-3 by protein kinase C delta: implications for regulation of tau phosphorylation

The role of the phosphatidylinositol 3-kinase (PI3K) pathway in the hyperphosphorylation of tau was investigated in SY5Y human neuroblastoma cells. Wortmannin, an inhibitor of PI3K, induced transient (after 1 h) activation of glycogen synthase kinase-3 (GSK-3), hyperphosphorylation of tau and dose-dependent cytotoxicity. However, continuous inactivation of protein kinase (PK) B was observed from 1 to 24 h, suggesting the involvement of protein kinase(s) other than PKB in the phosphorylation and inactivation of GSK-3 after 3 h. In cells treated with wortmannin, PKC delta fragments were observed, and the PKC activity increased after 3 h, whereas treatment of cells with z-DEVD-fmk, an inhibitor of caspase 3, also inhibited fragmentation of PKC delta and induced continuous activation of GSK-3. It is suggested that fragmentation of PKC delta during the process of apoptosis results in the phosphorylation and inactivation of GSK-3 and consequently inhibition of the phosphorylation of tau.     

Structure of DNA-dependent protein kinase: implications for its regulation by DNA

DNA double-strand breaks are created by ionizing radiation or during V(D)J recombination, the process that generates immunological diversity. Breaks are repaired by an end-joining reaction that requires DNA-PKCS, the catalytic subunit of DNA-dependent protein kinase. DNA-PKCS is a 460 kDa serine-threonine kinase that is activated by direct interaction with DNA. Here we report its structure at 22 A resolution, as determined by electron crystallography. The structure contains an open channel, similar to those seen in other double-stranded DNA-binding proteins, and an enclosed cavity with three openings large enough to accommodate single-stranded DNA, with one opening adjacent to the open channel. Based on these structural features, we performed biochemical experiments to examine the interactions of DNA-PKCS with different DNA molecules. Efficient kinase activation required DNA longer than 12 bp, the minimal length of the open channel. Competition experiments demonstrated that DNA-PKCS binds to double- and single-stranded DNA via separate but interacting sites. Addition of unpaired single strands to a double-stranded DNA fragment stimulated kinase activation. These results suggest that activation of the kinase involves interactions with both double- and single-stranded DNA, as suggested by the structure. A model for how the kinase is regulated by DNA is described.