Characterization of monoamine oxidase isoforms in human islets of Langerhans

6-[18F]fluoro-L-DOPA uptake in the rat pancreas is dependent on the tracer metabolism

PURPOSE

6-[(18)F]fluoro-L-3,4-dihydroxyphenyl alanine ([(18)F]FDOPA) positron emission tomography (PET) is a diagnostic tool which can detect malignancies of the pancreas. We aimed to study whether the manipulation of the [(18)F]FDOPA metabolic pathway would change the (18)F-behavior to provide a biochemical foundation for PET imaging of rat pancreas with [(18)F]FDOPA.

PROCEDURES

Inhibitors of aromatic amino acid decarboxylase, catechol-O-methyltransferase, monoamine oxidases A and B, or their combinations on [(18)F]FDOPA uptake, metabolism, and the regional distribution in the rat pancreas was evaluated using in vivo PET/computed tomography imaging, chromatographic metabolite analyses, and autoradiography.

RESULTS

Enzyme inhibition generally increased the uptake of [(18)F]FDOPA derived (18)F-radioactivity in rat pancreas. Dependent on which enzymatic pathway is blocked (or a combination of pathways), different radiolabeled metabolites in pancreas are responsible for this increase in uptake.

CONCLUSIONS

Altering the metabolism of [(18)F]FDOPA by using various enzymatic inhibitors increased the radioactivity uptake and changed the radiometabolic profile in the pancreas allowing better discrimination between pancreas and surrounding tissues of rat. However, these manipulations did not separate islets from the exocrine pancreas. Elucidating the metabolic behavior of [(18)F]FDOPA provides a biochemical foundation of PET imaging of the rat pancreas.

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.

MAO-A-induced mitogenic signaling is mediated by reactive oxygen species, MMP-2, and the sphingolipid pathway

The degradation of biogenic amines by monoamine oxidase A (MAO-A) generates reactive oxygen species (ROS) which participate in serotonin and tyramine signaling. This study aimed to investigate the role of ROS in the mitogenic signaling activated during tyramine and serotonin oxidation by MAO-A in smooth muscle cells (SMC). Incubation of SMC with serotonin or tyramine induced intracellular ROS generation, and a signaling cascade involving metalloproteases and the neutral sphingomyelinase-2 (nSMase2, the initial step of the sphingolipid pathway), ERK1/2 phosphorylation, and DNA synthesis. Silencing MAO-A by siRNA, pharmacological MAO-A inhibitors (pargyline and Ro41-1049), and the antioxidant/ROS scavenger butylated hydroxytoluene (BHT) inhibited the signaling cascade, suggesting that ROS generated during tyramine oxidation by MAO-A are required. The MMP inhibitor Batimastat, MMP2-specific siRNA, and MMP2 deletion (MMP2(-/-) fibroblasts) blocked nSMase activation and SMC proliferation, suggesting a role for MMP2 in this signaling pathway. Silencing nSMase2 by siRNA did not inhibit ROS generation and MMP2 activation, but blocked SMC proliferation induced by tyramine, suggesting that nSMase2 is downstream MMP2. These findings demonstrate that H(2)O(2)-generated during tyramine oxidation by MAO-A triggers a stress-induced mitogenic signaling via the MMP2/sphingolipid pathway, which could participate in excessive remodeling and alteration of the vascular wall.

The effects of cold acclimation and nitric oxide on antioxidative enzymes in rat pancreas

Alterations of pancreatic antioxidative defense (AD) and possible nitric oxide (NO) role in AD organization of adult rats receiving l-arginine.HCl (2.25%) or N(omega)-nitro-l-arginine methyl ester (L-NAME.HCl, 0.01%) as drinking liquids and maintained at room (22+/-1 degrees C) or low (4+/-1 degrees C) temperature for 45 days were studied. For that purpose, copper, zinc- and manganese superoxide dismutase (CuZnSOD, MnSOD), catalase (CAT), glutathione peroxidase (GSH-Px), glutathione S-transferase (GST) and glutathione reductase (GR) activities were determined. Cold-induced decrease of CuZnSOD was inhibited with L-NAME, while l-arginine produced the same effect as cold in both supplemented groups. Cold acclimation elevated GSH-Px activity. l-Arginine and L-NAME expressed no effect on GSH-Px in rats kept at room temperature. L-NAME additionally elevated cold-induced GSH-Px activity, l-arginine expressing a similar trend. Cold-induced increase in GST activity was inhibited by L-NAME, while l-arginine inhibited this enzyme in both supplemented groups. Cold acclimation increased GR activity in control and L-NAME-treated group and l-arginine expressed a similar trend. Neither of the treatments affected MnSOD and CAT activities. Cold-induced changes of pancreatic AD were additionally affected by the alterations in l-arginine-NO-producing pathway. Some AD changes in the same direction with l-arginine or L-NAME point to the complexity of nitrogen compounds metabolism and function, accompanied by tissue-specific response.

Glucose handling in streptozotocin-induced diabetic rats is improved by tyramine but not by the amine oxidase inhibitor semicarbazide

A soluble form of semicarbazide-sensitive amine oxidase (SSAO) circulating in plasma is known to increase in type 1 and 2 diabetes. This cuproenzyme generates hydrogen peroxide, ammonia, and aldehydes when oxidizing circulating biogenic or exogenous amines. Based on the angiotoxicity of these products, inhibition of SSAO has been proposed to prevent vascular complications of diabetes. However, substrates of SSAO and monoamine oxidase (MAO) have been recently evidenced to activate glucose utilisation in insulin-sensitive tissues and to exhibit antihyperglycemic actions. To determine whether amine oxidase blockade or activation could be beneficial for diabetes, we aimed at comparing the influence of prolonged treatments with semicarbazide (SSAO-inhibitor), pargyline (MAO-inhibitor), or tyramine (amine oxidase substrate) on amine oxidase activities and glycemic control in streptozotocin-induced diabetic rats. The increase in plasma SSAO was confirmed in diabetic rats, while MAO and SSAO were decreased in subcutaneous adipose tissue when compared with normoglycemic controls. Among the diabetic rats, only those receiving tyramine exhibited slightly decreased hyperglycemia and improved glucose tolerance. Adipocytes from untreated or treated diabetic rats shared similar sensitivity to insulin. However glucose uptake activation and lipolysis inhibition in response to amine oxidase substrates combined with vanadate were impaired in rats treated with amine oxidase inhibitors. Thus, amine oxidase inhibition does not improve metabolic control while prolonged administration of tyramine slightly improves glucose disposal. It is therefore concluded that amine oxidase activation by increased substrate supply elicits insulin-like actions that may be more beneficial in diabetes than SSAO inhibition formerly proposed to prevent vascular complications.

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.

Inhibition of MAO A and B by some plant-derived alkaloids, phenols and anthraquinones

A total of seventeen phytochemicals including seven alkaloids (piperine, strychnine, brucine, stachydrine, tetrandrine, frangchinoline and sinomenine), four phenols (paeonol, honokiol, magnolol and eugenol) and six anthraquinones (emodin, rhein, chrysorphanol, aloe-emodin, physcion and 1,8-dihydroxyanthraquinone) was examined for inhibitory activity of monoamine oxidase (MAO) A and B from rat brain mitochondrial. Among these compounds, piperine and paeonol were found to be inhibitory against MAO A in a dose-dependent manner with IC(50) values of 49.3 and 54.6 microM, respectively. Piperine, paeonol and emodin were shown to inhibit MAO B in a dose-dependent manner with the IC(50) data of 91.3, 42.5 and 35.4 microM, respectively. Lineweaver-Burk transformation of the inhibition data indicated that the inhibitory action of piperine on MAO A was of mixed type, and that of paeonol on the same type of the enzyme was of non-competitive type. For piperine, the K(i) and K(I) were determined to be 35.8 and 25.7microM, respectively. For paeonol, the K(i) was estimated to be 51.1 microM. The inhibition of piperine and paeonol on MAO B was of competitive type with K(i) values of 79.9 and 38.2 microM, respectively. The inhibition of emodin on MAO B was of mixed type with the K(i) and K(I) data of 15.1 and 22.9 microM, respectively. The present investigation showed that the phytochemicals piperine, paeonol and emodin are potent MAO inhibitors whereas other compounds were inactive against any type of MAO at 100 microM in the present assay.

Tissue activity and cellular localization of human semicarbazide-sensitive amine oxidase

Semicarbazide-sensitive amine oxidase (SSAO), widely distributed in highly vascularized mammalian tissues, metabolizes endogenous and xenobiotic aromatic and aliphatic monoamines. To assess whether its physiological role in humans is restricted to oxidation, we used an immunohistochemical approach to examine the cellular localization of SSAO in human peripheral tissues (adrenal gland, duodenum, heart, kidney, lung, liver, pancreas, spleen, thyroid gland, and blood vessels) and also analyzed its subcellular localization. The results are in agreement with the specific activities also determined in the same samples and are discussed with reference to the tissue distribution of monoamine oxidase A and B. Together with the oxidative deamination of monoamines, SSAO cellular localization indicates that, in most human peripheral tissues, it might participate in the regulation of physiological processes via H(2)O(2) generation. (J Histochem Cytochem 49:209-217, 2001)