Glucose modulation of islet monoamine oxidase activity in lean and obese hyperglycemic mice

Regulation of monoamine oxidase A (MAO-A) expression, activity, and function in IL-13-stimulated monocytes and A549 lung carcinoma cells

Monoamine oxidase A (MAO-A) is a mitochondrial flavoenzyme implicated in the pathogenesis of atherosclerosis and inflammation and also in many neurological disorders. MAO-A also has been reported as a potential therapeutic target in prostate cancer. However, the regulatory mechanisms controlling cytokine-induced MAO-A expression in immune or cancer cells remain to be identified. Here, we show that MAO-A expression is co-induced with 15-lipoxygenase (15-LO) in interleukin 13 (IL-13)-activated primary human monocytes and A549 non-small cell lung carcinoma cells. We present evidence that MAO-A gene expression and activity are regulated by signal transducer and activator of transcription 1, 3, and 6 (STAT1, STAT3, and STAT6), early growth response 1 (EGR1), and cAMP-responsive element-binding protein (CREB), the same transcription factors that control IL-13-dependent 15-LO expression. We further established that in both primary monocytes and in A549 cells, IL-13-stimulated MAO-A expression, activity, and function are directly governed by 15-LO. In contrast, IL-13-driven expression and activity of MAO-A was 15-LO-independent in U937 promonocytic cells. Furthermore, we demonstrate that the 15-LO-dependent transcriptional regulation of MAO-A in response to IL-13 stimulation in monocytes and in A549 cells is mediated by peroxisome proliferator-activated receptor γ (PPARγ) and that signal transducer and activator of transcription 6 (STAT6) plays a crucial role in facilitating the transcriptional activity of PPARγ. We further report that the IL-13-STAT6-15-LO-PPARγ axis is critical for MAO-A expression, activity, and function, including migration and reactive oxygen species generation. Altogether, these results have major implications for the resolution of inflammation and indicate that MAO-A may promote metastatic potential in lung cancer cells.

Altered serotonin (5-HT) 1D and 2A receptor expression may contribute to defective insulin and glucagon secretion in human type 2 diabetes

Islet produced 5-hydroxy tryptamine (5-HT) is suggested to regulate islet hormone secretion in a paracrine and autocrine manner in rodents. Hitherto, no studies demonstrate a role for this amine in human islet function, nor is it known if 5-HT signaling is involved in the development of beta cell dysfunction in type 2 diabetes (T2D). To clarify this, we performed a complete transcriptional mapping of 5-HT receptors and processing enzymes in human islets and investigated differential expression of these genes in non-diabetic and T2D human islet donors. We show the expression of fourteen 5-HT receptors as well as processing enzymes involved in the biosynthesis of 5-HT at the mRNA level in human islets. Two 5-HT receptors (HTR1D and HTR2A) were over-expressed in T2D islet donors. Both receptors (5-HT1d and 5-HT2a) were localized to human alpha, beta and delta cells. 5-HT inhibited both insulin and glucagon secretion in non-diabetic islet donors. In islets isolated from T2D donors the amine significantly increased release of insulin in response to glucose. Our results suggest that 5-HT signaling participates in regulation of overall islet hormone secretion in non- diabetic individuals and over-expression of HTR1D and HTR2A may either contribute to islet dysfunction in T2D or arise as a consequence of an already dysfunctional islet.

Monoamine oxidase A (MAO-A): a signature marker of alternatively activated monocytes/macrophages

Monocytes/macrophages are versatile cells centrally involved in host defense and immunity. Th1 cytokines induce a classical activation program in monocytes/macrophages leading to a proinflammatory M1 macrophage phenotype while Th2 cytokines IL-4 and IL-13 promote monocyte differentiation into an alternatively activated, anti-inflammatory M2 macrophage phenotype. Although monoamine oxidase A (MAO-A) is primarily known for its action in the nervous system, several recent studies have identified MAO-A as a signature marker of alternative activation of monocytes/macrophages. In this brief review we explore the signaling pathways/molecules that regulate MAO-A expression in alternatively activated monocytes/macrophages. We further discuss the contribution of MAO-A to the resolution of inflammation and identify potential therapeutic targets for controlling inflammation. Altogether this review provides deeper insight into the role of MAO-A in alternative activation of monocytes/macrophages and their participation in the inflammatory response.

MAOA promoter methylation and susceptibility to carotid atherosclerosis: role of familial factors in a monozygotic twin sample

BACKGROUND

Atherosclerosis is a complex process involving both genetic and epigenetic factors. The monoamine oxidase A (MAOA) gene regulates the metabolism of key neurotransmitters and has been associated with cardiovascular risk factors. This study investigates whether MAOA promoter methylation is associated with atherosclerosis, and whether this association is confounded by familial factors in a monozygotic (MZ) twin sample.

METHODS

We studied 84 monozygotic (MZ) twin pairs drawn from the Vietnam Era Twin Registry. Carotid intima-media thickness (IMT) was measured by ultrasound. DNA methylation in the MAOA promoter region was quantified by bisulfite pyrosequencing using genomic DNA isolated from peripheral blood leukocytes. The association between DNA methylation and IMT was first examined by generalized estimating equation, followed by matched pair analyses to determine whether the association was confounded by familial factors.

RESULTS

When twins were analyzed as individuals, increased methylation level was associated with decreased IMT at four of the seven studied CpG sites. However, this association substantially reduced in the matched pair analyses. Further adjustment for MAOA genotype also considerably attenuated this association.

CONCLUSIONS

The association between MAOA promoter methylation and carotid IMT is largely explained by familial factors shared by the twins. Because twins reared together share early life experience, which may leave a long-lasting epigenetic mark, aberrant MAOA methylation may represent an early biomarker for unhealthy familial environment. Clarification of familial factors associated with DNA methylation and early atherosclerosis will provide important information to uncover clinical correlates of disease.

Immunohistochemical localization of monoamine oxidase type B in pancreatic islets of the rat

Monoamine oxidase (MAO) is regarded as a mitochondrial enzyme. This enzyme localizes on the outer membrane of mitochondria. There are two kinds of MAO isozymes, MAO type A (MAOA) and type B (MAOB). Previous studies have shown that MAOB activity is found in the pancreatic islets. This activity in the islets is increased by the fasting-induced decrease of plasma glucose level. Islet B cells contain monoamines in their secretory granules. These monoamines inhibit the secretion of insulin from the B cells. MAOB is active in degrading monoamines. Therefore, MAOB may influence the insulin-secretory process by regulating the stores of monoamines in the B cells. However, it has not been determined whether MAOB is localized on B cells or other cell types of the islets. In the present study, we used both double-labeling immunofluorescence histochemical and electron microscopic immunohistochemical methods to examine the subcellular localization of MAOB in rat pancreatic islets. MAOB was found in the mitochondrial outer membranes of glucagon-secreting cells (A cells), insulin-secreting cells (B cells), and some pancreatic polypeptide (PP)-secreting cells (PP cells), but no MAOB was found in somatostatin-secreting cells (D cells), nor in certain other PP cells. There were two kinds of mitochondria in pancreatic islet B cells: one contains MAOB on their outer membranes, but a substantial proportion of them lack this enzyme. Our findings indicate that pancreatic islet B cells contain MAOB on their mitochondrial outer membranes, and this enzyme may be involved in the regulation of monoamine levels and insulin secretion in the B cells.

Nitric oxide and hydroperoxide affect islet hormone release and Ca(2+) efflux

We have investigated the influence of the intracellular free radical donors hydroxylamine (giving nitric oxide [NO]) and tert-butylhydroperoxide (giving hydroperoxide ["H2O2"]) on glucose- and cyclic adenosine monophosphate (cAMP)-induced transduction signaling in islet hormone release. Both donors dose dependently inhibited glucose-stimulated insulin release and induced modest (hydroxylamine) or profound (tertbutylhydroperoxide) suppression of 45Ca2+-efflux from perifused islets. By contrast, both donors stimulated glucagon release. Similar effects on hormone release were displayed after K+-depolarization. Insulin and glucagon release stimulated by activation of the cAMP system through isobutylmethylxanthine (IBMX) at basal glucose was modestly potentiated by low concentrations of both donors. These effects were still observed, although less pronounced, in K+-depolarized islets. In vitro as well as in vivo, the NO-synthase inhibitor N(G)-nitro-L-arginine methyl ester inhibited IBMX-induced glucagon release, but did not affect insulin release. The results suggest that NO and hydroperoxide inhibit glucose-stimulated insulin release by perturbing Ca2+ fluxes and probably acting through S-nitrosylation (NO) or oxidation (hydroperoxide) of thiol groups critical to the secretory process. These effects are largely independent of depolarization events. By contrast, both NO and hydroperoxide can potentiate cAMP-stimulated hormone release presumably at a distal site in the stimulus-secretion coupling.

Interaction of the islet nitric oxide system with L-arginine-induced secretion of insulin and glucagon in mice

1. Several recent in vitro studies have suggested that production of nitric oxide (NO) from the islet NO system may have an important regulatory influence on the secretion of insulin and glucagon. In the present paper we have investigated, mainly with an in vivo approach, the influence and specificity of the NO synthase (NOS) blocker NG-nitro-L-arginine methyl ester (L-NAME) on L-arginine-induced secretion of insulin and glucagon. 2. In freely fed mice, L-NAME pretreatment (1.2 mmol kg-1) influenced the dynamics of insulin and glucagon release following an equimolar dose of L-arginine, the specific substrate for NOS activity, in that the NOS inhibitor enhanced the insulin response but suppressed the glucagon responses. This was reflected in a large decrease in the plasma glucose levels of the L-NAME pretreated animals. 3. L-NAME pretreatment did not influence the insulin and glucagon secretory responses to the L-arginine-enantiomer D-arginine, which cannot serve as a substrate for NOS activity. 4. Replacing L-NAME pretreatment by pretreatment with D-arginine or L-arginine itself, which both carry the same cationic change and are devoid of NOS inhibitory properties, did not mimic the effects of L-NAME on L-arginine-induced hormone release. 5. Fasting the animals for 24 h totally abolished the L-NAME-induced potentiation of L-arginine stimulated insulin release suggesting that the sensitivity of the beta-cell secretory machinery to NO-production is greatly changed in the fasting state. However, the L-NAME-induced suppression of L-arginine stimulated glucagon release was unaffected by starvation. 6. In isolated islets from freely fed mice, L-arginine (5 mM) stimulated insulin release was greatly enhanced and glucagon release markedly suppressed by the presence of the NOS inhibitor L-NAME in the incubation medium. These effects were abolished in isolated islets taken from 24 h fasted mice. 7. Our present results, which showed that the NOS inhibitor L-NAME markedly enhances insulin release but suppresses glucagon release induced by L-arginine in the intact animal, give strong support to our previous hypothesis that the islet NO system is a negative modulator of insulin secretion and a positive modulator of glucagon secretion. Additionally, we observed that the importance of the beta-cell NO-production for secretory mechanisms, as evaluated by the effect of L-NAME on L-arginine-induced insulin release, was greatly changed after starvation, an effect less prominent with regard to glucagon release.

Influence of nitric oxide synthase inhibition, nitric oxide and hydroperoxide on insulin release induced by various secretagogues

1. Recent studies have suggested that the generation of nitric oxide (NO) and hydrogen peroxide (H2O2) by islet NO synthase and monoamine oxidase, respectively, may have a regulatory influence on insulin secretory processes. We have investigated the pattern of insulin release from isolated islets of Langerhans in the presence of various pharmacological agents known to perturb the intracellular levels of NO and the oxidation state of SH-groups. 2. The NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) dose-dependently increased L-arginine-induced insulin release. D-Arginine did not influence L-arginine-induced insulin secretion. However, D-NAME which reportedly has no inhibitory action on NO synthase, modestly increased L-arginine-induced insulin release, but was less effective than L-NAME. High concentrations (10 mM) of D-arginine as well as L-NAME and D-NAME could enhance basal insulin release. 3. The intracellular NO donor, hydroxylamine, dose-dependently inhibited insulin secretion induced by L-arginine and L-arginine+L-NAME. 4. Glucose-induced insulin release was increased by NO synthase inhibition (L-NAME) and inhibited by the intracellular NO donor, hydroxylamine. Sydnonimine-1 (SIN-1), an extracellular donor of NO and superoxide, induced a modest suppression of glucose-stimulated insulin release. SIN-1 did not influence insulin secretion induced by L-arginine or the adenylate cyclase activator, forskolin. 5. The intracellular 'hydroperoxide donor' tert-butylhydroperoxide in the concentration range of 0.03-3 mM inhibited insulin release stimulated by the nutrient secretagogues glucose and L-arginine. Low concentrations (0.03-30 microM) of tert-butylhydroperoxide, however enhanced insulin secretion induced by the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX). 6. Islet guanosine 3':5'-cyclic monophosphate (cyclic GMP) content was not influenced by 10 mML-arginine or tert-butylhydroperoxide at 3 or 300 micro M but was markedly increased (14 fold) by a high hydroxylamine concentration (300 micro M). In contrast, islet adenosine 3':5'-cyclic monophosphate (cyclicAMP) content was increased (3 fold) by L-arginine (10 mM) and (2 fold) by tert-butylhydroperoxide(300 micro M).7. Our results strongly suggest that NO is a negative modulator of insulin release induced by the nutrient secretagogues L-arginine and glucose. This effect is probably not mediated to any major extent by the guanylate cyclase-cyclic GMP system but may rather be exerted by the S-nitrosylation of critical thiol groups involved in the secretory process. Similarly the inhibitory effect of tert-butylhydroperoxide is likely to be elicited through affecting critical thiol groups. The mechanism underlying the secretion promoting action of tert-butylhydroperoxide on IBMX-induced insulin release is probably linked to intracellular Ca2+-perturbations affecting exocytosis.8. Taken together with previous data the present results suggest that islet production of low physiological levels of free radicals such as NO and H202 may serve as important modulators of insulin secretory processes.