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Articles by S Oka
Total Records ( 5 ) for S Oka
  C. P Hsu , P Zhai , T Yamamoto , Y Maejima , S Matsushima , N Hariharan , D Shao , H Takagi , S Oka and J. Sadoshima

Silent information regulator 1 (Sirt1), a class III histone deacetylase, retards aging and protects the heart from oxidative stress. We here examined whether Sirt1 is protective against myocardial ischemia/reperfusion (I/R).

Methods and Results—

Protein and mRNA expression of Sirt1 is significantly reduced by I/R. Cardiac-specific Sirt1–/– mice exhibited a significant increase (44±5% versus 15±5%; P=0.01) in the size of myocardial infarction/area at risk. In transgenic mice with cardiac-specific overexpression of Sirt1, both myocardial infarction/area at risk (15±4% versus 36±8%; P=0.004) and terminal deoxynucleotidyl transferase dUTP nick end labeling–positive nuclei (4±3% versus 10±1%; P<0.003) were significantly reduced compared with nontransgenic mice. In Langendorff-perfused hearts, the functional recovery during reperfusion was significantly greater in transgenic mice with cardiac-specific overexpression of Sirt1 than in nontransgenic mice. Sirt1 positively regulates expression of prosurvival molecules, including manganese superoxide dismutase, thioredoxin-1, and Bcl-xL, whereas it negatively regulates the proapoptotic molecules Bax and cleaved caspase-3. The level of oxidative stress after I/R, as evaluated by anti-8-hydroxydeoxyguanosine staining, was negatively regulated by Sirt1. Sirt1 stimulates the transcriptional activity of FoxO1, which in turn plays an essential role in mediating Sirt1-induced upregulation of manganese superoxide dismutase and suppression of oxidative stress in cardiac myocytes. Sirt1 plays an important role in mediating I/R-induced increases in the nuclear localization of FoxO1 in vivo.


These results suggest that Sirt1 protects the heart from I/R injury through upregulation of antioxidants and downregulation of proapoptotic molecules through activation of FoxO and decreases in oxidative stress.

  C. P Hsu , S Oka , D Shao , N Hariharan and J. Sadoshima

Rationale: NAD+ acts not only as a cofactor for cellular respiration but also as a substrate for NAD+-dependent enzymes, such as Sirt1. The cellular NAD+ synthesis is regulated by both the de novo and the salvage pathways. Nicotinamide phosphoribosyltransferase (Nampt) is a rate-limiting enzyme in the salvage pathway.

Objective: Here we investigated the role of Nampt in mediating NAD+ synthesis in cardiac myocytes and the function of Nampt in the heart in vivo.

Methods and Results: Expression of Nampt in the heart was significantly decreased by ischemia, ischemia/reperfusion and pressure overload. Upregulation of Nampt significantly increased NAD+ and ATP concentrations, whereas downregulation of Nampt significantly decreased them. Downregulation of Nampt increased caspase 3 cleavage, cytochrome c release, and TUNEL-positive cells, which were inhibited in the presence of Bcl-xL, but did not increase hairpin 2–positive cells, suggesting that endogenous Nampt negatively regulates apoptosis but not necrosis. Downregulation of Nampt also impaired autophagic flux, suggesting that endogenous Nampt positively regulates autophagy. Cardiac-specific overexpression of Nampt in transgenic mice increased NAD+ content in the heart, prevented downregulation of Nampt, and reduced the size of myocardial infarction and apoptosis in response to prolonged ischemia and ischemia/reperfusion.

Conclusions: Nampt critically regulates NAD+ and ATP contents, thereby playing an essential role in mediating cell survival by inhibiting apoptosis and stimulating autophagic flux in cardiac myocytes. Preventing downregulation of Nampt inhibits myocardial injury in response to myocardial ischemia and reperfusion. These results suggest that Nampt is an essential gatekeeper of energy status and survival in cardiac myocytes.

  M Zubair , S Oka , K. L Parker and K. i. Morohashi

Deficiency of adrenal 4 binding protein/steroidogenic factor 1 (Ad4BP/SF-1; NR5A1) impairs adrenal development in a dose-dependent manner, whereas overexpression of Ad4BP/SF-1 is associated with adrenocortical tumorigenesis. Despite its essential roles in adrenal development, the mechanism(s) by which Ad4BP/SF-1 regulates this process remain incompletely understood. We previously identified a fetal adrenal enhancer (FAdE) that stimulates Ad4BP/SF-1 expression in the fetal adrenal gland by a two-step mechanism in which homeobox proteins initiate Ad4BP/SF-1 expression, which then maintains FAdE activity in an autoregulatory loop. In the present study, we examined the effect of transgenic expression of Ad4BP/SF-1 controlled by FAdE on adrenal development. When Ad4BP/SF-1 was overexpressed using a FAdE-Ad4BP/SF-1 transgene, FAdE activity expanded outside of its normal field, resulting in increased adrenal size and the formation of ectopic adrenal tissue in the thorax. The increased size of the adrenal gland did not result from a corresponding increase in cell proliferation, suggesting rather that the increased levels of Ad4BP/SF-1 may divert uncommitted precursors to the steroidogenic lineage. The effects of FAdE-controlled Ad4BP/SF-1 overexpression in mice provide a novel model of ectopic adrenal formation that further supports the critical role of Ad4BP/SF-1 in the determination of steroidogenic cell fate in vivo.

  S Oka , S Kimura , T Toshida , R Ota , A Yamashita and T. Sugiura

Lysophosphatidylinositol (LPI) is an endogenous ligand for GPR55, a putative novel type of cannabinoid receptor. In this study, we first examined the effects of LPI on p38 mitogen-activated protein kinase in HEK293 cells expressing GPR55. LPI induced the rapid phosphorylation of p38 mitogen-activated protein kinase in GPR55-expressing cells. No apparent effect was observed in the vector-transfected cells. The exposure of GPR55-expressing cells to LPI also triggered the phosphorylation of activating transcription factor 2 downstream of the p38 mitogen-activated protein kinase. Treatment of the cells with Y-27632 [a Rho-associated kinase (ROCK) inhibitor] blocked the LPI-induced phosphorylation of p38 mitogen-activated protein kinase and activating transcription factor 2, suggesting that the Rho-ROCK pathway is involved in these cellular responses. Notably, GPR55 was found to be abundantly expressed in lymphoid organs such as the spleen and thymus. We obtained evidence that rapid phosphorylation of p38 mitogen-activated protein kinase and activating transcription factor 2 also takes place in IM-9 lymphoblastoid cells, which naturally express GPR55, after stimulation with LPI. These results suggest that GPR55 and its endogenous ligand LPI play essential roles in the homoeostatic responses to stress signals in several mammalian tissues and cells including certain types of immune cells.

  H Kubagawa , S Oka , Y Kubagawa , I Torii , E Takayama , D. W Kang , G. L Gartland , L. F Bertoli , H Mori , H Takatsu , T Kitamura , H Ohno and J. Y. Wang

Although Fc receptors (FcRs) for switched immunoglobulin (Ig) isotypes have been extensively characterized, FcR for IgM (FcµR) has defied identification. By retroviral expression and functional cloning, we have identified a complementary DNA (cDNA) encoding a bona fide FcµR in human B-lineage cDNA libraries. FcµR is defined as a transmembrane sialoglycoprotein of ~60 kD, which contains an extracellular Ig-like domain homologous to two other IgM-binding receptors (polymeric Ig receptor and Fc/µR) but exhibits an exclusive Fcµ-binding specificity. The cytoplasmic tail of FcµR contains conserved Ser and Tyr residues, but none of the Tyr residues match the immunoreceptor tyrosine-based activation, inhibitory, or switch motifs. Unlike other FcRs, the major cell types expressing FcµR are adaptive immune cells, including B and T lymphocytes. After antigen-receptor ligation or phorbol myristate acetate stimulation, FcµR expression was up-regulated on B cells but was down-modulated on T cells, suggesting differential regulation of FcµR expression during B and T cell activation. Although this receptor was initially designated as Fas apoptotic inhibitory molecule 3, or TOSO, our results indicate that FcµR per se has no inhibitory activity in Fas-mediated apoptosis and that such inhibition is only achieved when anti-Fas antibody of an IgM but not IgG isotype is used for inducing apoptosis.

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