David K. Kreutter

Distribution of β3-adrenoceptor mRNA in human tissues

The beta 3-adrenoceptor is a G protein-coupled receptor which mediates metabolic functions of the endogenous catecholamines epinephrine and norepinephrine. Questions exist regarding distribution of the beta 3-adrenoceptor in human tissue. In order to examine the distribution of beta 3-adrenoceptor mRNA in human tissues, we used sensitive and specific RNase protection assays without previous PCR amplification in an extensive list of human tissues. We confirm the presence of beta 3-adrenoceptor mRNA in human white fat from several locations, gall bladder, and small intestine, as well as extend the distribution of beta 3-adrenoceptor mRNA to previously uncharacterized human tissues such as stomach and prostate. The presence of beta 3-adrenoceptor mRNA in human white adipose tissue has important implications regarding possible use of beta 3-adrenoceptor selective agonists as anti-obesity agents, and the demonstration of beta 3-adrenoceptor mRNA in a number of gastrointestinal tissues and prostate raises the question of the role of the beta 3-adrenoceptor in motility and secretory processes.

Treatment With Growth Hormone and Dexamethasone in Mice Transgenic for Human Islet Amyloid Polypeptide Causes Islet Amyloidosis and β-Cell Dysfunction

Islet amyloid derived from islet amyloid polypeptide (IAPP) is a well-recognized feature of type II diabetes. However, the mechanism of islet amyloidogenesis is unknown. In vitro studies suggest that amino acid residues 20–29 in human, but not mouse, IAPP confer amyloidogenicity consistent with the absence of spontaneous islet amyloidosis in mice. Several clinical and in vitro studies suggest that increased synthetic rates of IAPP predispose to IAPP-amyloidosis. In the present study, we sought to test the hypothesis that pharmacological induction of insulin resistance in a mouse transgenic (TG) for human IAPP would induce islet amyloid and β-cell dysfunction. TG and non-transgenic (N-TG) control mice were treated with both rat growth hormone (12 μg/day) and dexamethasone (0.24 mg/day) (dex/GH) or received no treatment for 4 weeks, after which animals were killed to examine islet morphology. Treatment with dex/GH caused hyperglycemia (7.3 ± 0.4 vs. 5.2 ± 0.1 mmol/l, TG vs. N-TG, P < 0.001) associated with a decreased plasma insulin concentration (595 ± 51 vs. 996 ± 100 pmol/1, TG vs. N-TG, P < 0.05) in TG versus control mice. Islet amyloid was induced in treated TG mice but not in control mice. Islet amyloid was identified in both intra- and extracellular deposits, the former being associated with evidence of β-cell degeneration. We conclude that dex/GH treatment in mice TG for human IAPP induces IAPP-derived islet amyloid, hyperglycemia, and islet dysfunction. The present model recapitulates the islet morphology and phenotype of type II diabetes.

Insulinlike Activity of New Antidiabetic Agent CP 68722 in 3T3-L1 Adipocytes

We examined the in vitro effects of CP 68722, a novel antidiabetic agent, in 3T3-L1 adipocytes. CP 68722 stimulated 2-deoxyglucose uptake in the absence of insulin. At least 30 min of incubation were required for stimulation of uptake. This effect increased over 5 h and was sustained up to 72 h. The stimulation of 2-deoxyglucose uptake by CP 68722 could be inhibited ∼60% by inhibition of protein synthesis with cycloheximide. Half-maximal and maximal responses to CP 68722 at 72 h of incubation were observed at 10 and 100 μM of drug, respectively, with a threefold stimulation of uptake at 100 μM approximating the maximal response of these cells to acute insulin stimulation. CP 68722 was able to overcome insulin resistance induced by dexamethasone in 3T3-L1 cells. The effect of drug, like that of insulin, was primarily to increase the Vmax of 2-deoxyglucose uptake. The stimulation of uptake by CP 68722 or insulin could be prevented by incubating the cells at 10°C, a temperature that impedes translocation of glucose transporters to the plasma membrane. Therefore, it appears that CP 68722, like insulin, stimulates glucose uptake by a mechanism that involves translocation of intracellular glucose transporters to the plasma membrane and de novo protein synthesis. We compared the effect of CP 68722 with the sulfonylureas, the primary drugs used in the treatment of non-insulin-dependent diabetes mellitus (NIDDM). CP 68722 was a more potent and effective stimulator of 2-deoxyglucose uptake in 3T3-L1 cells than either first- or second-generation sulfonylureas. Our results suggest that CP 68722 could be an effective therapeutic agent for the treatment of NIDDM.

Relationship of age and calcitonin gene-related peptide to postprandial hypotension

Falls following a meal occur commonly in older persons. These falls have been related to a decrease in postprandial blood pressure due to carbohydrates in the meal. The mechanism by which this occurs is not known. In this study, we examined the possible role of a vasodilatory peptide, calcitonin gene-related peptide (CGRP), which is released following carbohydrate loading in the pathophysiology of postprandial hypotension. Levels were assessed in 29 community-dwelling individuals aged 20-83 years during an oral glucose tolerance test, and heart rate and blood pressure were measured. Five subjects exhibited a postprandial reduction in systolic blood pressure (SBP) greater than 15 mmHg (mean reduction -30 +/- 4 mmHg). Four were aged over 60 (40% of the individuals in that group) and one was middle aged (11%). One individual in the older group was temporarily symptomatic, complaining of light-headedness. Linear regression analysis showed a significant association between the changes in CGRP and in blood pressure: SBP (r = 0.39, P = 0.037), and mean blood pressure (MBP) (r = 0.356, P = 0.06) in the oldest group. Individuals in this group with a greater than 15 mmHg drop in blood pressure, exhibited significantly greater changes in CGRP (SBP: P = 0.001, diastolic blood pressure (DBP): P = 0.05, MBP: P = 0.006). This association of log CGRP delta and BP change was not present in young or middle aged individuals. Thus, increases in CGRP levels were associated with blood pressure reduction, with older individuals more susceptible to these changes than younger individuals. CGRP may play a role in the pathogenesis of postprandial hypotension.

Age-related changes in amylin secretion

Amylin (islet amyloid polypeptide) is a recently identified pancreatic peptide. It has been shown to affect glucose metabolism both in vitro and in vivo. A cross sectional analysis of the effects of age on amylin secretion following a 75 g glucose tolerance test in a young (20-40 years), middle aged (41-60 years) and older (61-90 years) group was performed. Thirty lean (BMI less than 25) non-diabetic individuals were studied. Amylin secretion exhibited a U-shaped curve with greater secretion in young and old subjects than in middle aged persons. Baseline levels were 7.2 +/- 1, 4.7 +/- 1, and 5.3 +/- 0.75 pM respectively, at 60 min 9.5 +/- 0.9 (y), 5.5 +/- 1 (m), 8.6 +/- 1 (o) pM; and at 120 min 10.3 +/- 2 (y), 4.4 +/- 0.5 (m), 10.9 +/- 1.5 (o) pM. Amylin production (area under the curve) was 1102 +/- 131, 619.5 +/- 79 and 1043 +/- 120 pM per min respectively (P < 0.05). Amylin secretion was similar in both sexes. Variability in the insulin to amylin ratio for each of the age groups at different time points following a glucose load was found, suggesting that insulin and amylin are not co-secreted in a fixed ratio. A significant association was found between both maximum amylin and rise in amylin (delta) and a glucose greater than 120 mg/dl at 2 h. (P < 0.001, P < 0.001). This correlation of glucose and amylin may be interpreted as suggestive of a counterregulatory role for amylin. However, aging is also associated with changes in glucose metabolism and amylin may merely be acting as a marker of impaired glucose metabolism.

Possibility of Distinct Insulin-Signaling Pathways Beyond Phosphatidylinositol 3-Kinase–Mediating Glucose Transport and Lipogenesis

The aim of this study was to compare the effects of insulin and the insulinomimetic agent, englitazone, on functional end points and putative mediators of insulin action in 3T3-L1 adipocytes. Cells were incubated with englitazone for 48 h or with insulin for 10 or 30 min, or both, and 2-deoxy-D-[3H]glucose (2DG) uptake and lipogenesis (from [14C]glucose) were measured. Tyrosine phosphorylation of the insulin receptor (IR), insulin receptor substrates 1 and 2 (IRS-1 and IRS-2), and pp60, and phosphatidylinositol (PI) 3-kinase activity (using PI as substrate) and mitogen-activated protein kinase (MAPK) activity were assayed in cell lysates. Englitazone increased 2DG uptake in a concentrationdependent (10–100 μmol/l) manner by up to sixfold, and preincubation with englitazone significantly enhanced insulin-stimulated 2DG uptake. However, englitazone had a biphasic effect on lipogenesis (163 ± 13% basal at 10 μmol/l vs. 96 ± 14% at 100 μmol/l), but when acetate was used as substrate, only concentrationdependent inhibition of lipogenesis occurred. In addition, englitazone decreased insulin-stimulated lipogenesis in a concentration-dependent manner. Englitazone did not increase IR, IRS-1/IRS-2, pp60, or MAPK phosphorylation, nor did it enhance insulins stimulation of these parameters. Although englitazone alone did not activate PI 3-kinase, it did enhance the stimulation of the enzyme produced by a submaximally effective insulin concentration. Significant (63%) inhibition of insulin-stimulated lipogenesis occurred at a concentration of englitazone (30 μmol/l) that did not affect MAPK activation, which suggests that the drugs inhibitory effect on lipogenesis is not mediated by this pathway. Englitazone did not affect the expression of the peroxisome proliferator response element-containing fatty acyl CoA synthase gene, although it cannot be ruled out that expression of other lipogenic enzymes are altered by englitazone via peroxisome proliferator activated receptor-7 activation or by an alternate pathway. Thus englitazone stimulates 2DG uptake without affecting PI 3-kinase, but it can enhance both insulinstimulated 2DG uptake and PI 3-kinase activity. However, englitazone inhibits insulin-stimulated lipogenesis without inhibiting PI 3-kinase activity. Assuming activation of PI 3-kinase mediates insulin-stimulated 2-DG and lipogenesis, then the signaling pathways for each process diverge beyond PI 3-kinase.

Chapter 17. Recent Advancements in the Discovery and Development of Agents for the Treatment of Diabetes.

Publisher Summary A major portion of the diabetic patients have type II or noninsulin-dependent diabetes (NIDDM). NlDDM is characterized by abnormal insulin secretion from the pancreas, increased basal glucose output by the liver, and insulin resistance in peripheral tissues. The rest portion of diabetics has type I or insulin-dependent diabetes (IDDM). This form of the disease is caused by an autoimmune response, leading to the destruction of the endocrine pancreas and, therefore, the requirement for exogenous insulin arises to sustain life. The sulfonylurea class of agents, which stimulate the release of insulin, has served as the major oral therapy for NIDDM patients for a long time. It has been proposed that sulfonylureas inhibit the adenosine triphophate (ATP)-sensitive K + channels in the pancreatic B -cells leading to the efflux of potassium and a depolarization of the cell membrane. This in turn triggers voltage-dependent Ca 2+ channels, increasing intracellular calcium and inducing exocytosis of the insulin granules. Following the discovery of sulfonylureas, a number of other chemical entities have been identified that exhibit insulin secretatory activity. Amylin is of interest in terms of treatment of the NIDDM based on the inhibition of insulin action in muscle and liver by it. It is being investigated for the treatment of both IDDM and NIDDM. Phase II trials are in progress in patients with IDDM to determine whether amylin replacement reduces the risk of hypoglycemia associated with insulin therapy.