Study On Regulation Of ROS On Somatostatin Secretion And Effects On Metabolic Syndrome

Reactive oxygen species (ROS) play important roles in mediating diverse physiological responses such as cell proliferation, differentiation, immunity, and apoptosis. In addition, it has been demonstrated that increased intracellular generation of ROS is involved in the pathogenesis of insulin resistance, metabolic syndrome, diabetes, and cardiovascular disease. Importantly, ingestion of macronutrients including carbohydrate, fat, and protein can bring about increased ROS production. Somatostatin (SS) has been recognized as a peptide that exerts a negative action on a variety of physiologic functions, so the inhibitory effects of SS may be related to controlling the levels of ROS produced by absorption and metabolism of nutrients in the digestive systems. The aim of our experiment was to study the regulation mechanisms of ROS production and SS secretion, and effects on function of digestive systems, which may in turn play a part in high fat diet (HFD)-induced metabolic derangement and pathological status.1. Study on the mechanism of glucose-stimulated somatostatin secretion from cultured rat gastric primary D-cellsThe mechanism of glucose-stimulated somatostatin secretion from cultured rat gastric primary D-cells was investigated using mitochondrial blocker and uncoupler, and antioxidant. Glucose-stimulated somatostatin secretion (GSSS) was accompanied by increases in cellular ROS. LA (0.5 mM, 2h), an antioxidant reduced ROS production accompanied by a complete blunting of GSSS. In contrast, the catalase inhibitor 3AT (2 mM, 2h) increased endogenous ROS and resulted in a pronounced enhancement of GSSS. Moreover, in these conditions, a significant positive correlation between ROS production and SS secretion was found (R2 = 0.9673, P < 0.001). Preincubation with antioxidants LA (0.5 mM, 2h) or NAC (10 mM, 2h) reduced the H2O2-stimulated SS secretion. GSSS is dependent on the mitochondrial ROS and independently of the ATP production linked to glucose metabolism. Elevated glucose concentration caused an increase of [Ca2+]i that could be abolished by pretreatment with antioxidant LA and that could be further elevated by pretreatment with 3-Aminotriazole, the catalase inhibitor. These results suggested that ROS production may serve as a signal modulating the necessary Ca2+ recruitment for GSSS.2. Regulation of somatostatin on function of digestive systems and ROS productionThe effects of SS and ROS production on physiologic functions of digestive systems were studied with SS antagonist cyclosomatostatin and the SS analog octreotide. Ingestion of fat increased endogenous ROS and resulted in a pronounced enhancement of SS secretion. In 60-120min after ingestion, a significant positive correlation between ROS production and SS secretion was found (R2 = 0.933, P = 0.002). Treatment with the catalase inhibitor 3AT significantly increased fat ingestion-induced ROS production and SS secretion. There were significant (P<0.05) decreases in gastric acidity (71.8%), bile acid (70.9%), lipase activity of duodenum (79.1%), levels of total cholesterol (76.9%) triacylglycerol (81.4%), and ROS production (39.0%) in SS analog octreotide treatment mice compared with control mice. The SS antagonist cyclosomatostatin induced increases in gastric acidity, bile acid, and ROS production and a decrease in antioxidant capability of mice. Treatment with octreotide significantly increased fecal fat excretion (151.8%, P<0.05), and apparent digestibility of crude fat of HFD-fed mice. These results suggested that SS-regulated function of digestive systems affected fat digestion, which in turn regulated ROS production; Change in levels of SS secretion resulted in imbalance of antioxidant system.3. Effect of somatostatin on high fat diet-induced metabolic syndrome of miceThe role of SS and ROS production in the progression of HFD-induced derangement and pathological status was investigated with mouse obese and obese-resistant model. Following HFD intake for four weeks, there was hyperlipidemia, glycometabolism disorders, and an increase in body weight, adipose tissue weight. Intragastric administration of lard emulsion bring about increased ROS production of plasma (122.2%, P<0.05), stomach (151.0%, P<0.05), duodenum (158.6%, P<0.05), liver (159.2%, P<0.05), and pancreas (146.9%, P<0.05), while a decrease in plasma SS (52.5%, P<0.05) and antioxidant capability was observed. Single injection of SS analog octreotide to HFD fed mice significantly reduced plasma gastrin and glucagon, and ROS production of digestive systems. Simultaneous administration of SS analog octreotide to HFD-induced obese mice (DO group) throughout the 10-days significantly reduced body weight, levels of gastric acidity, bile acid, ROS production, lipid peroxidation, plasma glucose (87.9%, P<0.05), and low-density lipoprotein cholesterol (78.9%, P<0.05) and resulted in the improvement of SS secretion, hyperlipidemia and glycometabolism disorders. The SS antagonist cyclosomatostatin induced increases in body weight, low-density lipoprotein cholesterol, ROS production, and a decrease in antioxidant capability of control mice. The plasma SS (43% of control mice, P<0.05) and the expression of SS genes of stomach and duodenum in HFD-induced obese mice were significantly decreased.The results of immunohistochemistry showed that SS positive cells of stomach and pancreas were not significantly changed. The decrease in antioxidant capability may play an important role in decrease of SS secretion of HFD-induced obese mice. Feeding of HFD did not decrease the expression of SS receptors genes of stomach and pancreas. These results suggested that SS-regulated ROS production and function of digestive systems appeared to be involved in HFD-induced obesity and metabolic syndrome.4. Reduced serum somatostatin levels and antioxidant capability in patients with metabolic syndromeSerum SS concentrations, glucose, lipid level and certain antioxidant indexes were measured in the serum of 41 normal controls (NC), 60 metabolic syndrome high risk subjects (MSR), and 39 patients with metabolic syndrome (MS). The associations of SS with atherosclerotic index (AI), HOMA-IR and malondialdehyde (MDA) were analyzed. The AI, HOMA-IR significantly increased in MSR compared with NC, and was even higher in patients with MS (P<0.05). There were significant decreases in the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), whereas higherer MDA concentrations were observed in subjects with MS or MSR compared with NC (P<0.05). Median SS levels were lower in subjects with MS or MSR than in control subjects. A significant inverse relationships between SS level and AI (r =-0.24, P<0.01), HOMA-IR (r =-0.23, P<0.05), and MDA (r=-0.25, P<0.01) were observed. The severity of serum glucose and lipid disorders and oxidative stress was accompanied with decreased SS levels. These results suggest a possible role of decreased SS on MS generation.