Monoamine oxidase in pancreatic islets, exocrine pancreas, and liver from rats. Characterization with clorgyline, deprenyl, pargyline, tranylcypromine, and amezinium

Monoamines' role in islet cell function and type 2 diabetes risk

The two monoamines serotonin and melatonin have recently been highlighted as potent regulators of islet hormone secretion and overall glucose homeostasis in the body. In fact, dysregulated signaling of both amines are implicated in β-cell dysfunction and development of type 2 diabetes mellitus (T2DM). Serotonin is a key player in β-cell physiology and plays a role in expansion of β-cell mass. Melatonin regulates circadian rhythm and nutrient metabolism and reduces insulin release in human and rodent islets in vitro. Herein, we focus on the role of serotonin and melatonin in islet physiology and the pathophysiology of T2DM. This includes effects on hormone secretion, receptor expression, genetic variants influencing β-cell function, melatonin treatment, and compounds that alter serotonin availability and signaling.

Serotonergic regulation of insulin secretion

The exact physiological role for the monoamine serotonin (5-HT) in modulation of insulin secretion is yet to be fully understood. Although the presence of this monoamine in islets of Langerhans is well established, it is only with recent advances that the complex signalling network in islets involving 5-HT is being unravelled. With more than fourteen different 5-HT receptors expressed in human islets and receptor-independent mechanisms in insulin-producing β-cells, our understanding of 5-HT's regulation of insulin secretion is increasing. It is now widely accepted that failure of the pancreatic β-cell to release sufficient amounts of insulin is the main cause of type 2 diabetes (T2D), an ongoing global epidemic. In this context, 5-HT signalling may be of importance. In fact, 5-HT may serve an essential role in regulating the release of insulin and glucagon, the two main hormones that control glucose and lipid homoeostasis. In this review, we will discuss past and current understanding of 5-HT's role in the endocrine pancreas.

Progesterone induces apoptosis of insulin-secreting cells: insights into the molecular mechanism

Progesterone has been associated with the development of gestational diabetes (GD) due to the enhancement of insulin resistance. As β-cell apoptosis participates in type 1 and type 2 diabetes pathophysiology, we proposed the hypothesis that progesterone might contribute to the development of GD through a mechanism that also involves β-cell death. To address this question, RINm5F insulin-producing cells were incubated with progesterone (25-100 μM), in the presence or absence of α-tocopherol (40 μM). After 24 or 48 h, membrane integrity and DNA fragmentation were analyzed by flow cytometry. Caspase activity was used to identify the mode of cell death. The involvement of endoplasmic reticulum stress in the action of progesterone was investigated by western blotting. Oxidative stress was measured by 2',7'-dichlorofluorescein diacetate (DCFDA) oxidation. Isolated rat islets were used in similar experiments in order to confirm the effect of progesterone in primary β-cells. Incubation of RINm5F cells with progesterone increased the number of cells with loss of membrane integrity and DNA fragmentation. Progesterone induced generation of reactive species. Pre-incubation with α-tocopherol attenuated progesterone-induced apoptosis. Western blot analyses revealed increased expression of CREB2 and CHOP in progesterone-treated cells. Progesterone caused apoptotic death of rat islet cells and enhanced generation of reactive species. Our results show that progesterone can be toxic to pancreatic β-cells through an oxidative-stress-dependent mechanism that induces apoptosis. This effect may contribute to the development of GD during pregnancy, particularly under conditions that require administration of pharmacological doses of this hormone.

VMAT2 gene expression and function as it applies to imaging beta-cell mass

Diabetes mellitus is a metabolic disorder characterized by hyperglycemia. The two main forms of the disease are distinguished by different pathogenesis, natural histories, and population distributions and indicated as either type 1 (T1DM) or type 2 diabetes mellitus (T2DM). It is well established that T1DM is an autoimmune disease whereby beta-cells of pancreatic islets are destroyed leading to loss of endogenous insulin production. Albeit less dramatic, beta-cell mass (BCM) also drops in T2DM. Therefore, it is realistic to expect that noninvasive measures of BCM might provide useful information in the diabetes-care field. Preclinical studies have demonstrated that BCM measurements by positron emission tomography scanning, using the vesicular monoamine transporter type 2 (VMAT2) as a tissue-specific surrogate marker of insulin production and [11C] Dihydrotetrabenazine (DTBZ) as the radioligand specific for this molecule, is feasible in animal models. Unfortunately, the mechanisms underlying beta-cell-specific expression of VMAT2 are still largely unexplored, and a much better understanding of the regulation of VMAT2 gene expression and of its function in beta-cells will be required before the full utility of this technique in the prediction and treatment of individuals with diabetes can be understood. In this review, we summarize much of what is understood about the regulation of VMAT2 and identify questions whose answers may help in understanding what measurements of VMAT2 density mean in the context of diabetes.

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.

Tyrosine hydroxylase activity in the endocrine pancreas: changes induced by short-term dietary manipulation

BACKGROUND: Tyrosine hydroxylase (TH) activity and its possible participation in the control of insulin secretion were studied in pancreatic islets of adult Wistar rats fed a standard commercial diet (SD) or carbohydrates alone (CHD) for one week. TH activity, norepinephrine (NE) content, and glucose-induced insulin secretion were assessed. Blood glucose and insulin levels were measured at the time of sacrifice. RESULTS: CHD rats had significantly higher blood glucose and lower insulin levels than SD rats (114.5 PlusMinus; 6.7 vs 80.7 PlusMinus; 7.25 mg/dl, p < 0.001; 20.25 PlusMinus; 2.45 vs 42.5 PlusMinus; 4.99 &mgr;U/ml, p < 0.01, respectively). Whereas TH activity was significantly higher in CHD isolated islets (600 PlusMinus; 60 vs 330 PlusMinus; 40 pmol/mg protein/h; p < 0.001), NE content was significantly lower (18 PlusMinus; 1 vs 31 PlusMinus; 5 pmol/mg protein), suggesting that TH activity would be inhibited by the end-products of catecholamines (CAs) biosynthetic pathway. A similar TH activity was found in control and solarectomized rats (330 PlusMinus; 40 vs 300 PlusMinus; 80 pmol/mg protein/h), suggesting an endogenous rather than a neural origin of TH activity. CHD islets released significantly less insulin in response to glucose than SD islets (7.4 PlusMinus; 0.9 vs 11.4 PlusMinus; 1.1 ng/islet/h; p < 0.02). CONCLUSIONS: TH activity is present in islet cells; dietary manipulation simultaneously induces an increase in this activity together with a decrease in glucose-induced insulin secretion in rat islets. TH activity - and the consequent endogenous CAs turnover - would participate in the paracrine control of insulin secretion.

Characterization of monoamine oxidase isoforms in human islets of Langerhans

In this paper, we describe the characterization of the expression of monoamine oxidase (MAO) in whole pancreas and in isolated islets of Langerhans from human. Classical monamine oxidase activity assays reveal that both isoforms A & B are present in human pancreas. Two complementary approaches indicated that both MAO A and B are expressed in isolated islet: RT-PCR using specific primers revealed amplification products with the expected size for MAO-A and MAO-B: two peptides corresponding to MAO A (approximately 61 kDa) and B (approximately 55 kDa) were detected using a polyclonal anti MAO-A/MAO-B antiserum. Western blotting and subsequent densitometric analysis indicate that whole and endocrine pancreas express the two isoforms with different relative proportions. Islets appear to express almost twice as much MAO protein as whole pancreas, in near equal proportions of the two isoforms, whereas whole pancreas expresses more MAO-A than the B isoform. The expression of MAO A and B in islets could be the first step toward the characterization of the functional properties of these enzymes in the endocrine pancreas.

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

Islet beta-cell monoamines are known to influence the insulin-releasing mechanisms. These amines are localized in the insulin-secretory granules and are inactivated by the enzyme monoamine oxidase (MAO), a hydrogen peroxide (H2O2)-generating enzyme. The activity of islet MAO may consequently be of importance for insulin secretion. In the present investigation, we studied the relation between islet MAO activity and plasma levels of insulin and glucose in obese (ob/ob) hyperglycemic mice and their lean littermates. In addition, the effect of glucose on the MAO activity of in vitro-cultured islets was studied. MAO activity was assayed with serotonin, dopamine (DA), and beta-phenylethylamine (PEA) as substrates. After an overnight fast in adult (age, 6 months) lean mice, islet MAO activity was increased by 35% to 70%. Plasma levels of glucose and insulin were markedly decreased as expected. However, fasting in adult obese mice either did not affect islet MAO activity (PEA and DA) or induced a slight decrease (serotonin) of approximately 25% (P < .05). Plasma glucose levels in adult obese mice were not significantly affected by the overnight fast. However, a correlation analysis based on individual adult obese mice (fed and fasted) showed a negative correlation between plasma glucose concentration and islet MAO activity with PEA (r = -.65, P < .02) and DA (r = -.66, P < .02), respectively. Further, a positive correlation (r = +.58, P < .05) was found between glucose level and islet MAO activity when using serotonin as substrate. There was no difference in islet MAO activity with PEA and DA as substrates in fed obese versus fed lean mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization, localization and pharmacological profile of a high-affinity [3H]lidocaine binding site

Electrophysiological findings support the existence of voltage-dependent, sodium channel-associated receptors for class I antiarrhythmics. We have tried to identify such receptors with tritiated lidocaine. High-affinity binding sites were discovered in heart and brain membranes, but liver and kidney particulate fractions had the highest density of sites. The dissociation constants were 75 nM in bovine heart and 29 nM in guinea-pig liver membranes. Binding was reversible (t 1/2: 102 s at 2 degrees C), optimal at pH 9-10 and was only partly destroyed by heat treatment. Subcellular fractionation experiments excluded a plasmalemmal association of the lidocaine site in heart. The competition profile of 16 antiarrhythmics indicated chemical comparability of the sites in heart and liver. These data greatly challenge the applicability of labeled lidocaine as sodium channel probe. The pharmacological significance of the site described here remains to be clarified.

A novel 1,4-dihydropyridine-binding site on mitochondrial membranes from guinea-pig heart, liver and kidney

The 1,4-dihydropyridine (+/-)-[3H]nitrendipine reversibly binds to mitochondrial preparations from guinea-pig heart with a dissociation constant (Kd) of 593 +/- 77 nM and a maximum density of binding sites (Bmax.) of 1.75 +/- 0.27 nmol/mg of protein. This low-affinity high-capacity 1,4-dihydropyridine-binding site does not discriminate between the enantiomers of nitrendipine and is also found in mitochondrial membranes from guinea-pig liver (Kd 586 +/- 91 nM; Bmax. 0.36 +/- 0.04 nmol/mg of protein) and kidney (Kd 657 +/- 149 nM; Bmax. 0.56 +/- 0.12 nmol/mg of protein). Phenylalkylamines (e.g. verapamil) inhibit ( +/- )-[3H]nitrendipine binding with micromolar inhibition constants, but the benzothiazepine D-cis-diltiazem, a potent Ca2+-channel blocker, is without effect. The binding is heat-stable, shows a V-shaped pH-dependence with a minimum around pH 7.0, and is strongly dependent on ionic strength in the incubation medium. The cations La3+ greater than Cd2+ much greater than Co2+ greater than Ca2+ much greater than Ba2+ greater than Mg2+ greater than Li+ greater than Na+ and the anions NO3- greater than C1- greater than or equal to F- stimulate the binding, whereas PO4(3-) greater than SO4(2-) slightly inhibit it. The low-affinity ( +/- )-[3H]nitrendipine-binding site located on the mitochondrial inner membrane is biochemically and pharmacologically different from the 1,4-dihydropyridine-receptor domain of the L-type Ca2+ channel. Furthermore, it is not identical with any of the low-affinity 1,4-dihydropyridine-binding sites described so far.

Monoamine oxidase in rat and bovine endocrine tissues

Monoamine oxidase (MAO) was characterized in tissue homogenates from rat pancreatic islets, rat neurohypophysis and adenohypophysis, and rat and bovine adrenal medulla and adrenal cortex. Phenylethylamine was preferentially deaminated by rat pancreatic islet and bovine adrenal medulla MAO and with slight preference by rat neurohypophysis MAO, whereas 5-hydroxytryptamine was preferentially deaminated by MAO from all other endocrine tissues. Tyramine was a good substrate for all tissues. Clorgyline, a selective inhibitor of MAO-A, preferentially inhibited deamination of 5-hydroxytryptamine by all tissue homogenates, whereas deprenyl, a selective inhibitor of MAO-B, preferentially inhibited deamination of phenylethylamine. Km values for 5-hydroxytryptamine and tyramine were higher by one to two decimal powers than for phenylethylamine in homogenates from all endocrine tissues. Km values were significantly lower for 5-hydroxytryptamine and significantly higher for phenylethylamine in rat and bovine adrenal cortex than in adrenal medulla. According to these results, the contributions of MAO-B to total enzyme activity were 70% for rat pancreatic islets, 45% for rat neurohypophysis, 15% for rat adenohypophysis, 20% for rat adrenal medulla, 10% for rat adrenal cortex, 60% for bovine adrenal medulla, and 20% for bovine adrenal cortex. PC 12 cells also contained predominantly MAO-A (90%); however, an increased Km for phenylethylamine and a sensitivity of deamination of this MAO-B substrate to inhibition by clorgyline are indicators of abnormal behavior of MAO in this clonal rat pheochromocytoma cell line.