The cyclic-ADP-ribose signaling pathway in human myometrium

The CD38 glycohydrolase and the NAD sink: implications for pathological conditions

Nicotinamide adenine dinucleotide (NAD) acts as a cofactor in several oxidation-reduction (redox) reactions and is a substrate for a number of nonredox enzymes. NAD is fundamental to a variety of cellular processes including energy metabolism, cell signaling, and epigenetics. NAD homeostasis appears to be of paramount importance to health span and longevity, and its dysregulation is associated with multiple diseases. NAD metabolism is dynamic and maintained by synthesis and degradation. The enzyme CD38, one of the main NAD-consuming enzymes, is a key component of NAD homeostasis. The majority of CD38 is localized in the plasma membrane with its catalytic domain facing the extracellular environment, likely for the purpose of controlling systemic levels of NAD. Several cell types express CD38, but its expression predominates on endothelial cells and immune cells capable of infiltrating organs and tissues. Here we review potential roles of CD38 in health and disease and postulate ways in which CD38 dysregulation causes changes in NAD homeostasis and contributes to the pathophysiology of multiple conditions. Indeed, in animal models the development of infectious diseases, autoimmune disorders, fibrosis, metabolic diseases, and age-associated diseases including cancer, heart disease, and neurodegeneration are associated with altered CD38 enzymatic activity. Many of these conditions are modified in CD38-deficient mice or by blocking CD38 NADase activity. In diseases in which CD38 appears to play a role, CD38-dependent NAD decline is often a common denominator of pathophysiology. Thus, understanding dysregulation of NAD homeostasis by CD38 may open new avenues for the treatment of human diseases.

Recent advances in physiological and pathological significance of NAD+ metabolites: roles of poly(ADP-ribose) and cyclic ADP-ribose in insulin secretion and diabetogenesis

Poly(ADP-ribose) synthetase/polymerase (PARP) activation causes NAD+ depletion in pancreatic beta-cells, which results in necrotic cell death. On the other hand, ADP-ribosyl cyclase/cyclic ADP-ribose hydrolase (CD38) synthesizes cyclic ADP-ribose from NAD+, which acts as a second messenger, mobilizing intracellular Ca2+ for insulin secretion in response to glucose in beta-cells. PARP also acts as a regenerating gene (Reg) transcription factor to induce beta-cell regeneration. This provides the new concept that NAD+ metabolism can control the cellular function through gene expression. Clinically, PARP could be one of the most important therapeutic targets; PARP inhibitors prevent cell death, maintain the formation of a second messenger, cyclic ADP-ribose, to achieve cell function, and keep PARP functional as a transcription factor for cell regeneration.

NAADP as a second messenger: neither CD38 nor base-exchange reaction are necessary for in vivo generation of NAADP in myometrial cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) has recently been shown to act as a second messenger controlling intracellular Ca(2+) responses in mammalian cells. Many questions remain regarding this signaling pathway, including the role of the ryanodine receptor (RyR) in NAADP-induced Ca(2+) transients. Furthermore, the exact metabolic pathway responsible for the synthesis of NAADP in vivo has not been determined. Here, we demonstrate that the NAADP mediated Ca(2+) release system is present in human myometrial cells. We also demonstrate that human myometrial cells use the NAADP second messenger system to generate intracellular Ca(2+) transients in response to histamine. It has been proposed in the past that the NAADP system in mammalian cells is dependent on the presence of functional RyRs. Here, we observed that the histamine-induced Ca(2+) transients are dependent on both the NAADP and inositol 1,4,5-trisphosphate signaling pathways but are independent of RyRs. The enzyme CD38 has been shown to catalyze the synthesis of NAADP in vitro by the base-exchange reaction. Furthermore, it has been proposed that this enzyme is responsible for the intracellular generation of NAADP in vivo. Using CD38 knockout mice, we observed that both the basal and histamine stimulated levels of NAADP are independent of CD38 and the base-exchange reaction. Our group is the first to demonstrate that NAADP is a second messenger for histamine-elicited Ca(2+) transients in human myometrial cells. Furthermore, the NAADP mediated mechanism in mammalian cells can be independent of RyRs and CD38. Our data provides novel insights into the understanding of the mechanism of action and metabolism of this new second messenger system.

The CD38/CD157 mammalian gene family: An evolutionary paradigm for other leukocyte surface enzymes

Human CD38 is the mammalian prototype of a family of phylogenetically conserved proteins which share structural similarities and enzymatic activities involved in the production of an intracellular second messenger with calcium mobilizing effects. Engagement of CD38 by agonistic monoclonal antibodies and the CD31 ligand initiates a cytoplasmic signaling cascade involving tyrosine phosphorylation of the proto-oncogene c-cbl and of the extracellular regulated kinase 1 of 2 complex. Further requirements for signal transduction include a privileged localization within the cholesterol-rich areas of the plasma membrane and physical association with specialized surface receptors. CD38-mediated signals are crucial in heterotypic cell adhesion and migration as well as in the activation of proliferation/survival programs by normal and neoplastic cells. Here we review the most recent literature on this complex topic and attempt to formulate a single model reconciling the enzymatic and receptorial activities of CD38.

Ryanodine receptor subtype 2 encodes Ca2+ oscillations activated by acetylcholine via the M2 muscarinic receptor/cADP-ribose signalling pathway in duodenum myocytes

In this study, we characterized the signalling pathway activated by acetylcholine that encodes Ca2+ oscillations in rat duodenum myocytes. These oscillations were observed in intact myocytes after removal of external Ca2+, in permeabilized cells after abolition of the membrane potential and in the presence of heparin (an inhibitor of inositol 1,4,5-trisphosphate receptors) but were inhibited by ryanodine, indicating that they are dependent on Ca2+ release from intracellular stores through ryanodine receptors. Ca2+ oscillations were selectively inhibited by methoctramine (a M2 muscarinic receptor antagonist). The M2 muscarinic receptor-activated Ca2+ oscillations were inhibited by 8-bromo cyclic adenosine diphosphoribose and inhibitors of adenosine diphosphoribosyl cyclase (ZnCl2 and anti-CD38 antibody). Stimulation of ADP-ribosyl cyclase activity by acetylcholine was evaluated in permeabilized cells by measuring the production of cyclic guanosine diphosphoribose (a fluorescent compound), which resulted from the cyclization of nicotinamide guanine dinucleotide. As duodenum myocytes expressed the three subtypes of ryanodine receptors, an antisense strategy revealed that the ryanodine receptor subtype 2 alone was required to initiate the Ca2+ oscillations induced by acetylcholine and also by cyclic adenosine diphosphoribose and rapamycin (a compound that induced uncoupling between 12/12.6 kDa FK506-binding proteins and ryanodine receptors). Inhibition of cyclic adenosine diphosphoribose-induced Ca2+ oscillations, after rapamycin treatment, confirmed that both compounds interacted with the ryanodine receptor subtype 2. Our findings show for the first time that the M2 muscarinic receptor activation triggered Ca2+ oscillations in duodenum myocytes by activation of the cyclic adenosine diphosphoribose/FK506-binding protein/ryanodine receptor subtype 2 signalling pathway.

Ciglitizone inhibits cell proliferation in human uterine leiomyoma via activation of store-operated Ca2+ channels

This study investigated the acute effects of a peroxisome proliferator-activated receptor (PPAR)-γ ligand, ciglitizone, on cell proliferation and intracellular Ca2+ signaling in human normal myometrium and uterine leiomyoma. Changes in intracellular Ca2+ concentration ([Ca2+]i) were measured with fura-2 AM, and cellular viabilities were determined by viable cell count and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide reduction assay. Ciglitizone (100 μM) induced greater inhibition of cell proliferation in uterine leiomyoma than in myometrium. Ciglitizone also dose-dependently increased [Ca2+]i in both myometrium and uterine leiomyoma; these [Ca2+]i increases were inhibited by PPAR-γ antagonists and raloxifene. Ciglitizone-induced [Ca2+]i increase showed only an initial peak in normal myometrial cells, whereas in uterine leiomyoma there was a second sustained [Ca2+]i increase as well. The initial [Ca2+]i increase in both myometrium and uterine leiomyoma resulted from the release of Ca2+ by the sarcoplasmic reticulum via activation of ryanodine receptors. The second [Ca2+]i increase was observed only in uterine leiomyoma because of a Ca2+ influx via an activation of store-operated Ca2+ channels (SOCCs). Cell proliferation was inhibited and secondary [Ca2+]i increase in uterine leiomyoma was attenuated by cotreatment of ciglitizone with a SOCC blocker, lanthanum. The results suggest that ciglitizone inhibits cell proliferation and increases [Ca2+]i through the activation of SOCCs, especially in human uterine leiomyoma.

Emerging role of cyclic ADP-ribose (cADPR) in smooth muscle

Cyclic adenosine diphosphate ribose (cADPR) is a naturally occurring cyclic nucleotide and represents a novel class of endogenous Ca(2+) messengers implicated in the regulation of the gating properties of ryanodine receptors (RyRs). This action of cADPR occurs independently from the inositol-1,4,5-trisphosphate (IP(3)) receptor. The regulation of intracellular Ca(2+) release is a fundamental element of cellular Ca(2+) homeostasis since a number of smooth muscle functions (tone, proliferation, apoptosis, and gene expression) are modulated by intracellular Ca(2+) concentration ([Ca(2+)](i)). There has been a surge in the efforts aimed at understanding the mechanisms of cADPR-mediated Ca(2+) mobilization and its impact on smooth muscle function. This review summarizes the proposed roles of cADPR in the regulation of smooth muscle tone.

Role of CD38 in myometrial Ca2+ transients: modulation by progesterone

Oxytocin-induced Ca(2+) transients play an important role in myometrial contractions. Here, using a knockout model, we found that the enzyme CD38, responsible for the synthesis of the second messenger cyclic ADP-ribose (cADPR), plays an important role in the oxytocin-induced Ca(2+) transients and contraction. We also observed that CD38 is necessary for TNF-alpha-increased agonist-stimulated Ca(2+) transients in human myometrial cells. We provide experimental evidence that the TNF-alpha effect is mediated by increased expression of the enzyme CD38. First, we observed that TNF-alpha increased oxytocin-induced Ca(2+) transients and CD38 expression in human myometrial cells. Moreover, using small interference RNA technology, we observed that TNF-alpha stimulation of agonist-induced Ca(2+) transients was abolished by blocking the expression of CD38. In control experiments, we observed that activation of the component of the TNF-alpha signaling pathway, NF-kappaB, was not affected by the treatments. Finally, we observed that the effects of TNF-alpha on CD38 cyclase and oxytocin-induced Ca(2+) transients are abolished by progesterone. In conclusion, we provide the first experimental evidence that CD38 is important for myometrial Ca(2+) transients and contraction. Moreover, CD38 is necessary for the TNF-alpha-mediated augmentation of agonist-induced Ca(2+) transients in myometrial cells. We propose that the balance between cytokines and placental steroids regulates the expression of CD38 in vivo and cell responsiveness to oxytocin.

The role of cyclic-ADP-ribose-signaling pathway in oxytocin-induced Ca2+ transients in human myometrium cells

Human myometrial contraction plays a fundamental role in labor. Dysfunction of uterine contraction is an important cause of labor progression failure. Although the mechanisms controlling uterine contraction are not completely understood, intracellular Ca2+ mobilization plays an important role during uterine contraction. Several mechanisms of intracellular Ca2+ mobilization are present in smooth muscle, but in the human uterus, only 1,4,5-trisphosphate-induced Ca2+ release has been studied extensively. Ryanodine receptor channels are present in myometrium. We determined the role of the cyclic ADP-ribose (cADPR)-signaling pathway in oxytocin-induced intracellular Ca2+ [(Ca2+)i] transients in human myometrial cells. We found that oxytocin-induced Ca2+ transient is dependent on several sources of Ca2+, including extracellular Ca2+ and intracellular Ca2+ stores. In addition, we found that both the 1,4,5-trisphosphate- and the cADPR-induced Ca2+ releasing systems are important for the induction of [Ca2+]i transients by oxytocin in human myometrial cells. Furthermore, we investigated TNFalpha regulation of oxytocin-induced [Ca2+]i transients, CD38 cyclase activity, and CD38 expression in human myometrial cells. We found that oxytocin-induced [Ca2+]i transients were significantly increased by 50 ng/ml TNF. Similarly, CD38 mRNA levels, CD38 expression, and cyclase activity were increased by TNFalpha, thus increasing cADPR levels. We propose that a complex interaction between multiple signaling pathways is important for the development of intracellular Ca2+ transients induced by oxytocin and that TNFalpha may contribute for the myometrium preparation for labor by regulating the cADPR-signaling pathway. The observation that the cADPR-signaling pathway is important for the development of intracellular Ca2+ transients in human myometrial cells raises the possibility that this signaling pathway could serve as a target for the development of new therapeutic strategies for abnormal myometrial contraction observed during pregnancy.