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Insulin stimulates glucose uptake through the membrane translocation of GLUT4 and GLUT1. Peroxisome proliferator-activated receptor gamma (PPAR gamma) enhances insulin sensitivity. Here, we demonstrate that insulin stimulates GLUT4 and GLUT1 translocation, and glucose uptake, by activating the signaling pathway involving nicotinic acid adenine dinucleotide phosphate (NAADP), a calcium mobilizer, in adipocytes. We also demonstrate that PPAR gamma mediates insulin sensitization by enhancing NAADP production through upregulation of CD38, the only enzyme identified for NAADP synthesis. Insulin produced NAADP by both CD38-dependent and -independent pathways, whereas PPAR gamma produced NAADP by CD38-dependent pathway. Blocking the NAADP signaling pathway abrogated both insulin-stimulated and PPAR gamma-induced GLUT4 and GLUT1 translocation, thereby inhibiting glucose uptake. CD38 knockout partially inhibited insulin-stimulated glucose uptake. However, CD38 knockout completely blocked PPAR gamma-induced glucose uptake in adipocytes and PPAR gamma-mediated amelioration of glucose tolerance in diabetic mice. These results demonstrated that the NAADP signaling pathway is a critical molecular target for PPAR gamma-mediated insulin sensitization.

Lipopolysaccharide (LPS), an endotoxin derived from gram-negative bacteria, promotes the secretion of proinflammatory cytokines and mediates endotoxemia through activation of mitogen activated protein kinases, NF-κB and interferon regulatory factor-3. Silent information regulator transcript-1 (SIRT1), an NAD-dependent deacetylase, mediates NF-κB deacetylation and inhibits its function. SIRT1 may affect LPS-mediated signaling pathways and endotoxemia. Here we demonstrate that SIRT1 blocks LPS-induced secretion of interleukin 6 and tumor necrosis factor α in murine macrophages and protects against lethal endotoxic and septic shock in mice. We also demonstrate that interferon β increases SIRT1 expression by activating the Janus kinase – signal transducer and activator of transcription (JAK-STAT) pathway in mouse bone marrow derived macrophages. In vivo treatment of interferon β protects against lethal endotoxic and septic shock, which is abrogated by infection with dominant negative SIRT1-expressing adenovirus. Our work suggests that both SIRT1 and SIRT1-inducing cytokines are useful targets for treating patients with sepsis.

Human dental pulp cells (hDPCs) are a valuable source for the generation of patient-specific human induced pluripotent stem cells (hiPSCs). An advanced strategy for the safe and efficient reprogramming of hDPCs and subsequent lineage-specific differentiation is a critical step toward clinical application. In present research, we successfully generated hDPC-iPSCs using only two non-oncogenic factors: Oct4 and Sox2 (2F hDPC-hiPSCs) and evaluated the feasibility of hDPC-iPSCs as substrates for endothelial progenitor cells (EPCs), contributing to EPC-based therapies. Under conventional differentiation conditions, 2F hDPC-hiPSCs showed higher differentiation efficiency, compared to hiPSCs from other cell types, into multipotent CD34+ EPCs (2F-hEPCs) capable to differentiate into functional endothelial and smooth muscle cells. The angiogenic and neovasculogenic activities of 2F-hEPCs were confirmed using a Matrigel plug assay in mice. In addition, the therapeutic effects of 2F-hEPC transplantation were confirmed in mouse models of hind-limb ischemia and myocardial infarction. Importantly, 2F-EPCs effectively integrated into newly formed vascular structures and enhanced neovascularization via likely both direct and indirect paracrine mechanisms. 2F hDPC-hiPSCs have a robust capability for the generation of angiogenic and vasculogenic EPCs, representing a strategy for patient-specific EPC therapies and disease modeling, particularly for ischemic vascular diseases.

CD38, an ADP ribosyl cyclase, is a 45 kDa type II transmembrane protein having a short N-terminal cytoplasmic domain and a long C-terminal extracellular domain, expressed on the surface of various cells including macrophages, lymphocytes, and pancreatic β cells. It is known to be involved in cell adhesion, signal transduction and calcium signaling. In addition to its transmembrane form, CD38 is detectable in biological fluids in soluble forms. The mechanism by which CD38 is solubilized from the plasma membrane is not yet clarified. In this study, we found that lipopolysaccharide (LPS) induced CD38 upregulation and its extracellular release in J774 macrophage cells. Furthermore, it also increased CD38 expression at the mRNA level by activating the Janus kinase-signal transducers and activators of transcription (JAK-STAT) pathway. However, LPS decreased the levels of CD38 in the plasma membrane by releasing CD38 into the culture supernatant. LPS-induced CD38 release was blocked by the metalloproteinase-9 inhibitor indicating that MMP-9 solubilizes CD38. In conclusion, the present findings demonstrate a potential mechanism by which C38 is solubilized from the plasma membrane.

Crosstalk between intracellular signaling pathways has been extensively studied to understand the pluripotency of human pluripotent stem cells (hPSCs), including human embryonic stem cells and human induced pluripotent stem cells (hiPSCs); however, the contribution of NAD+-dependent pathways remains largely unknown. Here, we show that NAD+ depletion by FK866 (a potent inhibitor of NAD+ biosynthesis) was fatal in hPSCs, particularly when deriving pluripotent cells from somatic cells and maintaining pluripotency. NAD and its precursors (nicotinamide [NAM] and nicotinic acid) fully replenished the NAD+ depletion by FK866 in hPSCs. However, only NAM effectively enhanced the reprogramming efficiency and kinetics of hiPSC generation and was also significantly advantageous for the maintenance of undifferentiated hPSCs. Our molecular and functional studies reveal that NAM lowers the barriers to reprogramming by accelerating cell proliferation and protecting cells from apoptosis and senescence by alleviating oxidative stress, reactive oxygen species accumulation, and subsequent mitochondrial membrane potential collapse. We provide evidence that the positive effects of NAM (occurring at concentrations well above the physiological range) on pluripotency control are molecularly associated with the repression of p53, p21, and p16. Our findings establish that adequate intracellular NAD+ content is crucial for pluripotency; the distinct effects of NAM on pluripotency may be dependent not only on its metabolic advantage as a NAD+ precursor but also on the ability of NAM to enhance resistance to cellular stress.

The ADP-ribosyl cyclase CD38 whose catalytic domain resides in outside of the cell surface produces the second messenger cyclic ADP-ribose (cADPR) from NAD(+). cADPR increases intracellular Ca(2+) through the intracellular ryanodine receptor/Ca(2+) release channel (RyR). It has been known that intracellular NAD(+) approaches ecto-CD38 via its export by connexin (Cx43) hemichannels, a component of gap junctions. However, it is unclear how cADPR extracellularly generated by ecto-CD38 approaches intracellular RyR although CD38 itself or nucleoside transporter has been proposed to import cADPR. Moreover, it has been unknown what physiological stimulation can trigger Cx43-mediated export of NAD(+). Here we demonstrate that Cx43 hemichannels, but not CD38, import cADPR to increase intracellular calcium through RyR. We also demonstrate that physiological stimulation such as Fcγ receptor (FcγR) ligation induces calcium mobilization through three sequential steps, Cx43-mediated NAD(+) export, CD38-mediated generation of cADPR and Cx43-mediated cADPR import in J774 cells. Protein kinase A (PKA) activation also induced calcium mobilization in the same way as FcγR stimulation. FcγR stimulation-induced calcium mobilization was blocked by PKA inhibition, indicating that PKA is a linker between FcγR stimulation and NAD(+)/cADPR transport. Cx43 knockdown blocked extracellular cADPR import and extracellular cADPR-induced calcium mobilization in J774 cells. Cx43 overexpression in Cx43-negative cells conferred extracellular cADPR-induced calcium mobilization by the mediation of cADPR import. Our data suggest that Cx43 has a dual function exporting NAD(+) and importing cADPR into the cell to activate intracellular calcium mobilization.