Mathias Z. Strowski

Somatostatin Inhibits Insulin and Glucagon Secretion via Two Receptor Subtypes: An in Vitro Study of Pancreatic Islets from Somatostatin Receptor 2 Knockout Mice1

Somatostatin (SST) potently inhibits insulin and glucagon release from pancreatic islets. Five distinct membrane receptors (SSTR1-5) for SST are known, and at least two (SSTR2 and SSTR5) have been proposed to regulate pancreatic endocrine function. Our current understanding of SST physiology is limited by the receptor subtype selectivity of peptidyl SST analogs, making it difficult to assign a physiological function to an identified SST receptor subtype. To better understand the physiology of SSTRs we studied the in vitro effects of potent subtype-selective nonpeptidyl SST analogs on the regulation of pancreatic glucagon and insulin secretion in wild-type (WT) and in somatostatin receptor 2 knockout (SSTR2KO) mice. There was no difference in basal glucagon and insulin secretion between islets isolated from SSTR2KO and WT mice; however, potassium/arginine-stimulated glucagon secretion was approximately 2-fold higher in islets isolated from SSTR2KO mice. Neither SST nor any SSTR-selective agonist inhibited basal glucagon or insulin release. SST-14 potently inhibited stimulated glucagon secretion in islets from WT mice and much less effectively in islets from SSTR2KO mice. The SSTR2 selective analog L-779,976 inhibited glucagon secretion in islets from WT, but was inactive in islets from SSTR2KO mice. L-817,818, an SSTR5 selective analog, slightly reduced glucagon release in both animal groups, whereas SSTR1, -3, and -4 selective analogs were inactive. SST and L-817,818 inhibited glucose stimulated insulin release in islets from WT and SSTR2KO mice. L-779,976 much less potently reduced insulin secretion from WT islets. In conclusion, our data demonstrate that SST inhibition of glucagon release in mouse islets is primarily mediated via SSTR2, whereas insulin secretion is regulated primarily via SSTR5.

Small molecule insulin mimetics reduce food intake and body weight and prevent development of obesity

Obesity and insulin resistance are major risk factors for a number of metabolic disorders, such as type 2 diabetes mellitus. Insulin has been suggested to function as one of the adiposity signals to the brain for modulation of energy balance. Administration of insulin into the brain reduces food intake and body weight, and mice with a genetic deletion of neuronal insulin receptors are hyperphagic and obese. However, insulin is also an anabolic factor; when administered systemically, pharmacological levels of insulin are associated with body weight gain in patients. In this study, we investigated the efficacy and feasibility of small molecule insulin mimetic compounds to regulate key parameters of energy homeostasis. Central intracerebroventricular (i.c.v.) administration of an insulin mimetic resulted in a dose-dependent reduction of food intake and body weight in rats, and altered the expression of hypothalamic genes known to regulate food intake and body weight. Oral administration of a mimetic in a mouse model of high-fat diet-induced obesity reduced body weight gain, adiposity and insulin resistance. Thus, insulin mimetics have a unique advantage over insulin in the control of body weight and hold potential as a novel anti-obesity treatment.

Somatostatin Receptor Subtypes 2 and 5 Inhibit Corticotropin-Releasing Hormone-Stimulated Adrenocorticotropin Secretion from AtT-20 Cells

Somatostatin (SRIH) regulates pituitary adrenocorticotropin (ACTH) secretion by interacting with a family of homologous G protein-coupled membrane receptors. The SRIH receptor subtypes (sst₁–sst₅) that control ACTH release remain unknown. Using novel, subtype-selective SRIH analogs, we have identified the SRIH receptor subtypes involved in regulating ACTH release from AtT-20 cells, a model for cell line pituitary corticotropes. Radioligand-binding studies with ¹²⁵I-SRIH-14 and ¹²⁵I-SRIH-28 showed that SRIH-14 and SRIH-28 recognized specific, high-affinity and saturable membrane-binding sites. Nonpeptidyl agonists with selectivity for the sst₂ (L-779,976; compound 2) or sst₁/sst₅) (L-817,818; compound 5) receptor subtypes potently displaced ¹²⁵I-SRIH-28 from AtT-20 cell membranes, while agonists selective for the sst₁ (L-779,591; compound 1), sst₃ (L-796,778; compound 3) or sst₄ (L-803,087; compound 4) subtypes were inactive. Tyr¹¹-SRIH-14, compound 2 (sst₂) or compound 5 (sst₅) inhibited forskolin and corticotropin-releasing hormone (CRH)-induced increases in intracellular cAMP. Furthermore, the sst₂ and sst₅ agonists potently inhibited CRH-induced ACTH release from AtT-20 cells. These results provide the first evidence that sst₂ and sst₅ receptor subtypes, but not sst₁, sst₃ or sst₄, inhibit cAMP accumulation and regulate ACTH secretion in the AtT-20 cell model of the rodent corticotrope.

Transient Receptor Potential Channel TRPM8 Agonists Stimulate Calcium Influx and Neurotensin Secretion in Neuroendocrine Tumor Cells

TRPM8 is a member of the melastatin-type transient receptor potential ion channel family. Activation by cold or by agonists (menthol, icilin) induces a transient rise in intracellular free calcium concentration ([Ca2+]i). Our previous study demonstrated that Ca2+-permeable cation channels play a role in IGF-1-induced secretion of chromogranin A in human neuroendocrine tumor (NET) cell line BON [Mergler et al.: Neuroendocrinology 2006;82:87–102]. Here, we extend our earlier study by investigating the expression of TRPM8 and characterizing its impact on [Ca2+]i and the secretion of neurotensin (NT). We identified TRPM8 expression in NET BON cells by RT-PCR, Western blotting and immunofluorescence staining. Icilin increased [Ca2+]i in TRPM8-transfected human embryonic kidney cells (HEK293) but not in mock-transfected cells. Icilin and menthol induced Ca2+ transients in BON cells as well as in primary NET cell cultures of two different pancreatic NETs as detected by single cell fluorescence imaging. Icilin increased non-selective cation channel currents in BON cells as detected by patch-clamp recordings. This activation was associated with increased NT secretion. Taken together, this study demonstrates for the first time the expression TRPM8 in NET cells and its role in regulating [Ca2+]i and NT secretion. The regulation of NT secretion in NETs by TRPM8 may have a potential clinical implication in diagnosis or therapy.

Probiotic carbohydrates reduce intestinal permeability and inflammation in metabolic diseases

The epidemic of obesity and type 2 diabetes mellitus is dramatically increasing. Environmental factors, such as sedentary life-style, hypercaloric, fat-rich diet and genetic susceptibility are considered major determinants of type 2 diabetes mellitus. Obesity and peripheral insulin resistance are hallmarks and major risk factors for development of type 2 diabetes mellitus. Cardiovascular complications (eg, atherosclerosis, coronary heart disease) of both metabolic disorders are associated with chronic subclinical inflammation.1 2 The low degree of inflammation in obesity and type 2 diabetes mellitus results from increased expression and production of cytokines and acute phase reactants such as C-reactive proteins, interleukins (ILs), tumour necrosis factor (TNF)α, or lipopolysaccharides (LPS).1 2 Inflammatory cytokines induce peripheral insulin resistance by impairing the insulin receptor (IR)-dependent signalling.1 2 For instance, experimental application of TNFα leads to the phosphorylation of the serine residues of the insulin receptor substrate-1 (IRS-1) with the consecutive inhibition of insulin-stimulated tyrosine autophosphorylation of the IR. The consequence of the impaired IR signalling is a blunted glucose uptake by peripheral tissues. The resulting chronic hyperglycaemia causes deleterious effects on pancreatic beta-cell function and morphology. Inflammatory cytokines reduce insulin secretion, induce apoptosis of pancreatic beta-cells in type 2 diabetes mellitus.3 Administration of anti-inflammatory agents (eg, high doses of aspirin) or experimental deletion of receptors for inflammatory cytokines results in increased insulin sensitivity and improved pancreatic beta-cell function, which is paralleled by a reduction of hyperglycaemia and loss of body weight.4 5 Despite numerous studies describing the activation …