Genetics of insulin action

Bivariate genetic analysis of fasting insulin and glucose levels

The main aim of this study was to estimate the relative influence of genes and environment on fasting insulin levels, which were considered a proxy of insulin resistance. Possible sex differences in genetic and environmental influences, and the origin of the covariance between fasting insulin and glucose were investigated. Subjects were 209 pairs of middle-aged twins, divided into 5 sex-by-zygosity groups. A general bivariate model and a reciprocal causation model including fasting insulin and glucose were used in the analyses. For both quantitative genetic models, a model specifying additive genetic and unique environmental factors, which were the same in males and females, showed the best fit to the data. Heritability estimates were modest and highly similar in both models: 20-25% of the variance in fasting insulin, and around 50% of the variance in fasting glucose levels could be attributed to genetic factors. The two models could not be discriminated on the basis of their fit to the data. A submodel of the general bivariate model suggested that the covariance between glucose and insulin has a unique environmental basis, whereas for the reciprocal causation model both causal paths were needed to explain the phenotypic correlation between insulin and glucose and estimates of the reciprocal paths were of opposite sign, an indication for the expected negative feedback loop.

Mechanisms of insulin resistance in rat models of hypertension and their relationships with salt sensitivity

Several lines of evidence suggest that insulin resistance and the resultant hyperinsulinaemia are causally related to hypertension. Insulin actions are initiated by binding to a high-affinity transmembrane protein receptor which is present in all mammalian cells. These effects are predominant in skeletal muscle, liver, and fat and involve a number of tissue-specific and biochemically diverse events. Less well known are effects of insulin occurring in tissues not usually considered as insulin targets, which are hypothetical contributors to the pro-hypertensive action of the hormone. These effects include activation of renal sodium reabsorption, stimulation of the sympathetic nervous system, growth-promoting activity on vascular smooth muscle cells, and modulation of transmembrane cation transport. Epidemiological investigations have implicated sodium intake in the pathogenesis of hypertension. Because of the sodium-retaining effects of insulin, it has been postulated that insulin resistance with associated hyperinsulinaemia may be critical for the pathogenesis of salt-sensitivity in essential hypertensive subjects. Insulin resistance is present also in strains of rats with genetic hypertension that can be utilized as models to study the molecular mechanisms of this abnormality. In the present article, we summarize the current knowledge of the mechanisms of insulin resistance in rat models of arterial hypertension in which decreased sensitivity to insulin occurs and propose a rationale hypothesis that links insulin resistance with salt-sensitivity and hypertension.

Nutrition and breast cancer risk: can an effect via insulin resistance be demonstrated?

It is postulated that the metabolic/endocrine concomitants of insulin resistance resulting from high animal fat intake and weight gain after the age of 30 could be contributing to the increasing incidence of postmenopausal breast cancer and recent changes in its biologic characteristics. Case/control studies have shown that hyperinsulinaemia and abdominal obesity, which are recognized as markers of insulin resistance, are risk markers for postmenopausal breast cancer also. Excess weight gain linked to high dietary intake of saturated fat is thought to be a major cause of insulin resistance. The hypothesis is compatible with the "breast tissue age" model for breast cancer risk. Biological evidence suggests that the concomitants of insulin resistance may stimulate growth activity in existing breast cancer also. The hypothesis that nutritional factors which favour hyperinsulinaemia may also favour breast cancer growth can be tested. Restriction of dietary fat and high intake of fibre and complex carbohydrate have been shown to normalise insulin levels in a proportion of subjects with hyperinsulinaemia. Restriction of dietary fat intake has also been shown to reduce bioavailable oestrogen levels in healthy postmenopausal women. A randomised trial of a low fat, high fibre, high complex carbohydrate regimen is proposed as adjuvant treatment following primary surgery in postmenopausal women with early breast cancer. A cancer preventive or delaying ability can be assessed by comparing the incidence of contralateral second breast cancer and the metastasis rate in the diet and control groups. Insulin levels, abdominal obesity, and body mass should be monitored although normalisation of insulin levels need not necessarily involve decrease in body mass.

Generalized lipodystrophy, congenital and acquired (lipoatrophy)

This review is based on longitudinal studies on our seven patients with congenital generalized lipodystrophy, our patient with acquired generalized lipodystrophy, and published papers on these subjects. An inability to store energy in adipose tissue is of pathogenetic importance. In congenital lipodystrophy, insulin resistance is present from birth, resulting in hyperinsulinaemia, dyslipidaemia. and insulin-resistant diabetes with an anabolic syndrome worsened by a voracious appetite. Clinically, we observed increased height velocity in pre-school age children, and organomegaly with hypertrophic cardiomyopathy, which seems to be lethal in early adulthood: three of our patients died at the ages of 24, 32 and 37 years. The oldest alive, 39 years, suffers from stenocardia. Regarding treatment, it is most important to reduce energy consumption. The congenital form is recessively inherited. The aetiology may be related to insulin receptor or postreceptor mechanisms. Acquired generalized lipodystrophy seems to be an autoimmune disorder with secondary destruction of the adipose organ: the anabolic syndrome with insulin-resistant diabetes is secondary. Our patient died when 24 years old from pneumonia.