Novel Mechanism for Nicotinamide Phosphoribosyltransferase Inhibition of TNF-α-mediated Apoptosis in Human Lung Endothelial Cells

Nicotinamide phosphoribosyltransferase (NAMPT) exists as both intracellular NAMPT and extracellular NAMPT (eNAMPT) proteins. eNAMPT is secreted into the blood and functions as a cytokine/enzyme (cytozyme) that activates NF-κB signaling via ligation of Toll-like receptor 4 (TLR4), further serving as a biomarker for inflammatory lung disorders such as acute respiratory distress syndrome. In contrast, intracellular NAMPT is involved in nicotinamide mononucleotide synthesis and has been implicated in the regulation of cellular apoptosis, although the exact mechanisms for this regulation are poorly understood. We examined the role of NAMPT in TNF-α-induced human lung endothelial cell (EC) apoptosis and demonstrated that reduced NAMPT expression (siRNA) increases EC susceptibility to TNF-α-induced apoptosis as reflected by PARP-1 cleavage and caspase-3 activation. In contrast, overexpression of NAMPT served to reduce degrees of TNF-α-induced EC apoptosis. Inhibition of nicotinamide mononucleotide synthesis by FK866 (a selective NAMPT enzymatic inhibitor) failed to alter TNF-α-induced human lung EC apoptosis, suggesting that NAMPT-dependent NAD⁺ generation is unlikely to be involved in regulation of TNF-α-induced EC apoptosis. We next confirmed that TNF-α-induced EC apoptosis is attributable to NAMPT secretion into the EC culture media and subsequent eNAMPT ligation of TLR4 on the EC membrane surface. Silencing of NAMPT expression, direct neutralization of secreted eNAMPT by an NAMPT-specific polyclonal antibody (preventing TLR4 ligation), or direct TLR4 antagonism all served to significantly increase EC susceptibility to TNF-α-induced EC apoptosis. Together, these studies provide novel insights into NAMPT contributions to lung inflammatory events and to novel mechanisms of EC apoptosis regulation.

Synthetic analogs of FTY720 [2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol] differentially regulate pulmonary vascular permeability in vivo and in vitro

Novel therapies are needed to address the vascular endothelial cell (EC) barrier disruption that occurs in inflammatory diseases such as acute lung injury (ALI). We previously demonstrated the potent barrier-enhancing effects of both sphingosine 1-phosphate (S1P) and the structurally similar compound FTY720 [2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol] in inflammatory lung injury. In this study, we examined the therapeutic potential of several novel FTY720 analogs to reduce vascular leak. Similar to S1P and FTY720, the (R)- and (S)-enantiomers of FTY720 phosphonate and enephosphonate analogs produce sustained EC barrier enhancement in vitro, as seen by increases in transendothelial electrical resistance (TER). In contrast, the (R)- and (S)-enantiomers of FTY720-regioisomeric analogs disrupt EC barrier integrity in a dose-dependent manner. Barrier-enhancing FTY720 analogs demonstrate a wider protective concentration range in vitro (1-50 microM) and greater potency than either S1P or FTY720. In contrast to FTY720-induced EC barrier enhancement, S1P and the FTY720 analogs dramatically increase TER within minutes in association with cortical actin ring formation. Unlike S1P, these FTY720 analogs exhibit differential phosphorylation effects without altering the intracellular calcium level. Inhibitor studies indicate that barrier enhancement by these analogs involves signaling via G(i)-coupled receptors, tyrosine kinases, and lipid rafts. Consistent with these in vitro responses, the (S)-phosphonate analog of FTY720 significantly reduces multiple indices of alveolar and vascular permeability in a lipopolysaccharide-mediated murine model of ALI (without significant alterations in leukocyte counts). These results demonstrate the capacity for FTY720 analogs to significantly decrease pulmonary vascular leakage and inflammation in vitro and in vivo.

Cell physiology of cAMP sensor Epac

Epac is an acronym for the exchange proteins activated directly by cyclic AMP, a family of cAMP-regulated guanine nucleotide exchange factors (cAMPGEFs) that mediate protein kinase A (PKA)-independent signal transduction properties of the second messenger cAMP. Two variants of Epac exist (Epac1 and Epac2), both of which couple cAMP production to the activation of Rap, a small molecular weight GTPase of the Ras family. By activating Rap in an Epac-mediated manner, cAMP influences diverse cellular processes that include integrin-mediated cell adhesion, vascular endothelial cell barrier formation, and cardiac myocyte gap junction formation. Recently, the identification of previously unrecognized physiological processes regulated by Epac has been made possible by the development of Epac-selective cyclic AMP analogues (ESCAs). These cell-permeant analogues of cAMP activate both Epac1 and Epac2, whereas they fail to activate PKA when used at low concentrations. ESCAs such as 8-pCPT-2'-O-Me-cAMP and 8-pMeOPT-2'-O-Me-cAMP are reported to alter Na(+), K(+), Ca(2+) and Cl(-) channel function, intracellular [Ca(2+)], and Na(+)-H(+) transporter activity in multiple cell types. Moreover, new studies examining the actions of ESCAs on neurons, pancreatic beta cells, pituitary cells and sperm demonstrate a major role for Epac in the stimulation of exocytosis by cAMP. This topical review provides an update concerning novel PKA-independent features of cAMP signal transduction that are likely to be Epac-mediated. Emphasized is the emerging role of Epac in the cAMP-dependent regulation of ion channel function, intracellular Ca(2+) signalling, ion transporter activity and exocytosis.

A cAMP and Ca2+ coincidence detector in support of Ca2+-induced Ca2+ release in mouse pancreatic beta cells

The blood glucose-lowering hormone glucagon-like peptide-1 (GLP-1) stimulates cAMP production, promotes Ca2+ influx, and mobilizes an intracellular source of Ca2+ in pancreatic beta cells. Here we provide evidence that these actions of GLP-1 are functionally related: they reflect a process of Ca2+-induced Ca2+ release (CICR) that requires activation of protein kinase A (PKA) and the Epac family of cAMP-regulated guanine nucleotide exchange factors (cAMPGEFs). In rat insulin-secreting INS-1 cells or mouse beta cells loaded with caged Ca2+ (NP-EGTA), a GLP-1 receptor agonist (exendin-4) is demonstrated to sensitize intracellular Ca2+ release channels to stimulatory effects of cytosolic Ca2+, thereby allowing CICR to be generated by the uncaging of Ca2+ (UV flash photolysis). This sensitizing action of exendin-4 is diminished by an inhibitor of PKA (H-89) or by overexpression of dominant negative Epac. It is reproduced by cell-permeant cAMP analogues that activate PKA (6-Bnz-cAMP) or Epac (8-pCPT-2'-O-Me-cAMP) selectively. Depletion of Ca2+ stores with thapsigargin abolishes CICR, while inhibitors of Ca2+ release channels (ryanodine and heparin) attenuate CICR in an additive manner. Because the uncaging of Ca2+ fails to stimulate CICR in the absence of cAMP-elevating agents, it is concluded that there exists in beta cells a process of second messenger coincidence detection, whereby intracellular Ca2+ release channels (ryanodine receptors, inositol 1,4,5-trisphosphate (IP3) receptors) monitor a simultaneous increase of cAMP and Ca2+ concentrations. We propose that second messenger coincidence detection of this type may explain how GLP-1 interacts with beta cell glucose metabolism to stimulate insulin secretion.

Interplay of Ca2+ and cAMP signaling in the insulin-secreting MIN6 beta-cell line

Ca2+ and cAMP are important second messengers that regulate multiple cellular processes. Although previous studies have suggested direct interactions between Ca2+ and cAMP signaling pathways, the underlying mechanisms remain unresolved. In particular, direct evidence for Ca2+-regulated cAMP production in living cells is incomplete. Genetically encoded fluorescence resonance energy transfer-based biosensors have made possible real-time imaging of spatial and temporal gradients of intracellular cAMP concentration in single living cells. Here, we used confocal microscopy, fluorescence resonance energy transfer, and insulin-secreting MIN6 cells expressing Epac1-camps, a biosynthetic unimolecular cAMP indicator, to better understand the role of intracellular Ca2+ in cAMP production. We report that depolarization with high external K+, tolbutamide, or glucose caused a rapid increase in cAMP that was dependent on extracellular Ca2+ and inhibited by nitrendipine, a Ca2+ channel blocker, or 2',5'-dideoxyadenosine, a P-site antagonist of transmembrane adenylate cyclases. Stimulation of MIN6 cells with glucose in the presence of tetraethylammonium chloride generated concomitant Ca2+ and cAMP oscillations that were abolished in the absence of extracellular Ca2+ and blocked by 2',5'-dideoxyadenosine or 3-isobutyl-1-methylxanthine, an inhibitor of phosphodiesterase. Simultaneous measurements of Ca2+ and cAMP concentrations with Fura-2 and Epac1-camps, respectively, revealed a close temporal and causal interrelationship between the increases in cytoplasmic Ca2+ and cAMP levels following membrane depolarization. These findings indicate highly coordinated interplay between Ca2+ and cAMP signaling in electrically excitable endocrine cells and suggest that Ca2+-dependent cAMP oscillations are derived from an increase in adenylate cyclase activity and periodic activation and inactivation of cAMP-hydrolyzing phosphodiesterase.

Imaging endoplasmic reticulum calcium with a fluorescent biosensor in transgenic mice

The use of biosynthetic fluorescent sensors is an important new approach for imaging Ca2+ in cells. Genetically encoded indicators based on green fluorescent protein, calmodulin, and fluorescence resonance energy transfer (FRET) have been utilized to measure Ca2+ in nonmammalian transgenic organisms and provide information about the organization and regulation of Ca2+ signaling events in vivo. However, expression of biosynthetic FRET-based Ca2+ indicators in transgenic mammals has proven to be problematic. Here, we report transgenic expression of an endoplasmic reticulum (ER) Ca2+ biosensor in mouse pancreas. We targeted expression of a yellow cameleon3.3er (YC3.3er) transgene with mouse insulin I promoter. YC3.3er protein expression was limited to pancreatic β-cells within islets of Langerhans and absent in the exocrine pancreas and other tissues. Animals developed and matured normally; sensor expression was unaffected by age. Glucose tolerance in transgenic mice was also unaffected, indicating the transgenic biosensor did not impair endocrine pancreas function. ER Ca2+ responses after administration of thapsigargin, carbachol, and glucose were measured in individual β-cells of intact islets using confocal microscopy and confirmed the function of the biosensor. We conclude that controlling transgene transcription with a cell-specific promoter permits transgenic expression of FRET-based Ca2+ sensors in mammals and that this approach will facilitate real-time optical imaging of signal transduction events in living tissues.

Tyrosine dephosphorylation and deactivation of insulin receptor substrate-1 by protein-tyrosine phosphatase 1B. Possible facilitation by the formation of a ternary complex with the Grb2 adaptor protein

Regulation of the steady-state tyrosine phosphorylation of the insulin receptor and its postreceptor substrates are essential determinants of insulin signal transduction. However, little is known regarding the molecular interactions that influence the balance of these processes, especially the phosphorylation state of postinsulin receptor substrates, such as insulin receptor substrate-1 (IRS-1). The specific activity of four candidate protein-tyrosine phosphatases (protein-tyrosine phosphatase 1B (PTP1B), SH2 domain-containing PTPase-2 (SHP-2), leukocyte common antigen-related (LAR), and leukocyte antigen-related phosphatase) (LRP) toward IRS-1 dephosphorylation was studied using recombinant proteins in vitro. PTP1B exhibited the highest specific activity (percentage dephosphorylated per microg per min), and the enzyme activities varied over a range of 5.5 x 10(3). When evaluated as a ratio of activity versus IRS-1 to that versus p-nitrophenyl phosphate, PTP1B remained significantly more active by 3.1-293-fold, respectively. Overlay blots with recombinant Src homology 2 domains of IRS-1 adaptor proteins showed that the loss of IRS-1 binding of Crk, GRB2, SHP-2, and the p85 subunit of phosphatidylinositol 3'-kinase paralleled the rate of overall IRS-1 dephosphorylation. Further studies revealed that the adaptor protein GRB2 strongly promoted the formation of a stable protein complex between tyrosine-phosphorylated IRS-1 and catalytically inactive PTP1B, increasing their co-immunoprecipitation from an equimolar solution by 13.5 +/- 3.3-fold (n = 7; p < 0.01). Inclusion of GRB2 in a reaction mixture of IRS-1 and active PTP1B also increased the overall rate of IRS-1 tyrosine dephosphorylation by 2.7-3.9-fold (p < 0.01). These results provide new insight into novel molecular interactions involving PTP1B and GRB2 that may influence the steady-state capacity of IRS-1 to function as a phosphotyrosine scaffold and possibly affect the balance of postreceptor insulin signaling.

Purification and identification of bovine liver gamma-carboxylase

The microsomal gamma-carboxylase catalyzes modification of a limited set of glutamyl residues to gamma-carboxyglutamyl residues in a vitamin K-dependent reaction that also utilizes O2 and CO2. We report the purification to apparent homogeneity of the bovine liver microsomal carboxylase. Affinity chromatography exploiting the association of the carboxylase with prothrombin precursor and carboxylase binding to the propeptide sequence were combined with ion-exchange chromatography and fractionation using immobilized lectins. A 3.5 x 10(5)-fold purification was obtained, which is the highest purification, by a factor of 35, yet reported for this enzyme. A single 98-kDa protein is obtained from this isolation. Carboxylase activity is associated with this protein by two different criteria. Antibodies prepared against the carboxylase detected the 98-kDa protein when used in Western analysis. In addition, the single 98-kDa protein was shown to comigrate with activity when electrophoresed in a nondenaturing gel system. The availability of purified preparations of carboxylase will facilitate an increased understanding of the complex biochemical reaction carried out by this protein.