Impaired insulin action in subcutaneous adipocytes from women with visceral obesity

Visceral obesity is associated with resistance to the antilipolytic effect of insulin in vivo. We investigated whether subcutaneous abdominal and gluteal adipocytes from viscerally obese women exhibit insulin resistance in vitro. Subjects were obese black and white premenopausal nondiabetic women matched for visceral adipose tissue (VAT), total adiposity, and age. Independently of race and adipocyte size, increased VAT was associated with decreased sensitivity to insulin's antilipolytic effect in subcutaneous abdominal and gluteal adipocytes. Absolute lipolytic rates at physiologically relevant concentrations of insulin or the adenosine receptor agonist N(6)-(phenylisopropyl)adenosine were higher in subjects with the highest vs. lowest VAT area. Independently of cell size, abdominal adipocytes were less sensitive to the antilipolytic effect of insulin than gluteal adipocytes, which may partly explain increased nonesterified fatty acid fluxes in upper vs. lower body obese women. Moreover, increased VAT was associated with decreased responsiveness, but not decreased sensitivity, to insulin's stimulatory effect on glucose transport in abdominal adipocytes. These data suggest that insulin resistance of subcutaneous abdominal and, to a lesser extent, gluteal adipocytes may contribute to increased systemic lipolysis in both black and white viscerally obese women.     

Clinical characteristics of primary hyperaldosteronism due to adrenal microadenoma

An increasing number of patients are being diagnosed with primary aldosteronism (PA) due to aldosterone-producing macroadenoma (APA). However, there are only limited data available on the clinical characteristics of PA that are associated with adrenal microadenoma. Of the 55 patients that were diagnosed with PA in our study, 22 patients showed a unilateral adrenal over-production of aldosterone. The histopathology of the surgically removed adrenal tissues led to six patients being diagnosed with microadenoma, and the clinical features of microadenoma, macroadenoma and idiopathic hyperaldosteronism (IHA) were studied. The expression levels of CYP11B2, CYP17, CYP21 and 3β-hydroxysteroid dehydrogenase 2 (HSD3B2) mRNA in the adrenal cortices (n = 5 and 6, respectively) that remained attached to the adrenal microadenomas or macroadenomas were examined by real time-PCR and then compared to the expression levels in the adrenal cortices (n = 5) of non-functioning adrenal adenomas (NF). The patients with microadenoma (n = 6) had significantly higher diastolic blood pressure than the patients with macroadenoma (n = 16) or IHA (n = 33) (p < 0.05). The systolic blood pressure, plasma aldosterone concentration, serum potassium level and renal function did not differ between the PA sub-groups. The levels of CYP11B2 and CYP17 mRNA were significantly increased in the adjacent tissues of microadenomas, as compared with macroadenomas or NF (p < 0.05), whereas no significant differences in the CYP21 and HSD3B2 mRNA levels were found between the PA sub-groups. The tumor size did not influence the clinical characteristics of APA. The non-tumor portions of the microadenomas showed marked and sustained CYP11B2 mRNA expression under the suppressed renin-angiotensin system. We suggest that an increased number of microadenomas should be sampled, and the immunohistochemistry for steoridogenic enzymes should be investigated to clarify the etiology of microadenoma.     

Value of localizing examinations used in the diagnosis of primary hyperaldosteronism

Localizing examinations were performed in 115 patients with the clinical and biochemical signs of the primary hyperaldosteronism between 1975 and 1978. Adenoma of the adrenal cortex was diagnosed in 52 examined patients out of whom 42 underwent surgery, and 12 were classified for the operation. The remaining patients, in whom a cause of hyperaldosteronism was not found, are treated conservatively and followed-up. It was shown that CT-scanning and scintigraphy of the suprarenal cortex are the most convenient techniques in the localization of tumours in patients with the primary hyperaldosteronism. Ultrasound is less valuable diagnostically but may help in tumour localization in about 50% of patients.     

Effect of T3 on insulin action, insulin binding, and insulin receptor kinase activity in primary cultures of rat hepatocytes

In vivo studies have demonstrated that the liver is the main site of insulin resistance in hyperthyroidism. To further investigate the effect of thyroid hormone in the liver, we have incubated primary cultures of rat hepatocytes in the presence and absence of triiodothyronine (T3) 1 ng/ml and 5 ng/ml for 20 hr. Without affecting basal activity, T3 5 ng/ml decreased insulin-stimulated (1 x 10(-7) M) lipid synthesis but not insulin-stimulated alpha-aminoisobutyric acid uptake. These changes occur in the absence of any abnormalities in 125I-insulin binding, degradation, internalization or insulin receptors structure as determined by affinity-labeling methods. However, basal insulin receptor kinase activity using Glu4: Tyrl as phospho-acceptor was decreased by T3 without altering its insulin responsiveness. These results demonstrate the heterogeneity of T3's effects at the postinsulin binding level in the liver.     

Chemical mediator of insulin action stimulates lipid synthesis and down regulates the insulin receptors in primary cultures of rat hepatocytes

Incubation of rat liver particulate fraction with insulin causes the release of a substance that stimulates lipid synthesis and down regulates the insulin receptor in primary cultures of isolated rat hepatocytes. This substance may be similar to putative mediator(s) of insulin action which has been shown to modulate the activity of key enzymes of lipid and carbohydrate metabolism in various cell free systems. Our data demonstrate that the mediator of insulin is also biologically active in an intact cell system. Down regulation of the insulin receptor by the mediator supports the concept that this phenomenon is a post binding event of insulin action.     

Fluidity of insulin action

Unlike the intensive research in pursuit of understanding the molecular mechanisms of insulin signaling and resistance to its biological action associated most significantly with obesity and type 2 diabetes, the influence of the plasma membrane on insulin sensitivity has been intermittently studied over the years—mainly because it was thought that mediators of insulin action, such as the insulin receptor and the insulin-responsive glucose transporter GLUT4, localize more or less uniformly in the lipids that form cell membranes. Recent insights into membrane physiology suggest that the plasma membrane impacts the function of membrane proteins mediating insulin action. Furthermore, membrane disturbances may be the basis of insulin resistance. Relevant insulin signal transduction data in terms of plasma membrane and insulin resistance are the focus of this review. The discussion visits the cell membrane hypothesis of insulin resistance that suggests insulin action could be related to changes in cell membrane properties.     

Early steps in insulin action.

The biological effects of insulin are initiated by the binding of insulin to the insulin receptor. Insulin binds to the extracellular domain of the insulin receptor and induces conformational changes in the receptor, leading to autophosphorylation of the receptor on intracellular tyrosine residues. These phosphorylated tyrosine residues act as binding sites for proteins which subsequently may be phosphorylated by the insulin receptor. As a result, yet other proteins can be recruited to form larger complexes and, in the case of enzymes, changes in their activity may take place. By a combination of these processes, the activated insulin receptor initiates cascades of biochemical events which are regulated mainly by specific phosphorylation or dephosphorylation reactions. Intermediates which are involved in the normal insulin signalling pathway are subjects of expanding research.     

Impaired insulin secretion in a mouse model of ataxia telangiectasia

Ataxia telangiectasia (A-T) is an autosomal recessive disease caused by mutations in the A-T mutated (ATM) gene. The gene encodes a serine/threonine kinase with important roles in the cellular response to DNA damage, including the activation of cell cycle checkpoints and induction of apoptosis. Although these functions might explain the cancer predisposition of A-T patients, the molecular mechanisms leading to glucose intolerance and diabetes mellitus (DM) are unknown. We have investigated the pathogenesis of DM in a mouse model of A-T. Here we show that young Atm-deficient mice show normal fasting glucose levels and normal insulin sensitivity. However, oral glucose tolerance testing revealed delayed insulin secretion and resulting transient hyperglycemia. Aged Atm-/- mice show a pronounced increase in blood glucose levels and a decrease in insulin and C-peptide levels. Our findings support a role for ATM in metabolic function and point toward impaired insulin secretion as the primary cause of DM in A-T.     

Impaired pancreatic polypeptide response to insulin hypoglycemia in obese subjects

We have studied the effect of insulin hypoglycemia on the secretion of pancreatic polypeptide (PP) in 14 obese subjects with normal glucose tolerance and in 6 normal controls. Infusion of insulin 0.1 U/kg/h in controls and 0.12 U/kg/h in the obese, for one hour, produced a progressive hypoglycemia, similar in both groups (nadir 2 mmol/l at 50 min). The secretion of PP was less in obese subjects than in controls (peak 116 mmol/l vs 184 pmol/l, P less than 0.01) (integrated secretion sigma delta PP 288 vs 472 pmol/l, P less than 0.01) and was also delayed in the obese subjects beginning at 50 min instead of 40 min. The secretion of glucagon and of C-peptide were not different in the two groups, but the integrated response of ACTH was higher in the obese (sigma delta ACTH 52 pmol/l vs 25 pmol/l, P less than 0.01). The secretory response of growth hormone (STH) was smaller in the obese group (peak 8.6 +/- 1.28 vs 21.4 +/- 6.4 ng/ml, P less than 0.01). The reduced secretion of PP in obese subjects could be due to impaired sensitivity to hypoglycemia of the central control mechanism for PP release. The similarity of the reductions in the secretion of both PP and STH support this hypothesis, although a reduction in the secretory capacity of pancreatic PP cells cannot be excluded.     

Familial hyperaldosteronism

Primary aldosteronism (PAL) may be as much as ten times more common than has been traditionally thought, with most patients normokalemic. The study of familial varieties has facilitated a fuller appreciation of the nature and diversity of its clinical, biochemical, morphological and molecular aspects. In familial hyperaldosteronism type I (FH-I), glucocorticoid-remediable PAL is caused by inheritance of an ACTH-regulated, hybrid CYP11B1/CYP11B2 gene. Genetic testing has greatly facilitated diagnosis. Hypertension severity varies widely, demonstrating relationships with gender, affected parent's gender, urinary kallikrein level, degree of biochemical disturbance and hybrid gene crossover point position. Analyses of aldosterone/PRA/cortisol 'day-curves' have revealed that (1) the hybrid gene dominates over wild type CYP11B2 in terms of aldosterone regulation and (2) correction of hypertension in FH-I requires only partial suppression of ACTH, and much smaller glucocorticoid doses than those previously recommended. Familial hyperaldosteronism type II is not glucocorticoid-remediable, and is clinically, biochemically and morphologically indistinguishable from apparently sporadic PAL. In one informative family available for linkage analysis, FH-II does not segregate with either the CYP11B2, AT1 or MEN1 genes, but a genome-wide search has revealed linkage with a locus in chromosome 7. As has already occurred in FH-I, elucidation of causative mutations is likely to facilitate earlier detection of PAL and other curable or specifically treatable forms of hypertension.