Recent advances in our understanding of insulin action and insulin resistance

Dominant negative alpha-subunit of FTase inhibits effects of insulin and IGF-I in MCF-7 cells

We recently designed a dominant negative (DN) farnesyltransferase (FTase)/geranyl-gerahyltransferase I (GGTase I) alpha-subunit that when expressed in vascular smooth muscle cells decreased insulin-stimulated phosphorylation of FTase, FTase activity, amounts of farnesylated p21Ras, DNA synthesis, and cell migration. Currently, we explored the inhibitory effects of DN FTase/GGTase I alpha-subunit in MCF-7 cells on IGF-1- and insulin-stimulated DNA synthesis and cell proliferation. Expression of the DN FTase/GGTase I alpha-subunit completely blocked IGF-1- and insulin-stimulated BrdU incorporation and cell count. DN FTase/GGTase I alpha-subunit inhibited insulin-stimulated phosphorylation of FTase/GGTase I alpha-subunit, FTase and GGTase I activity, and prenylation of p21Ras and RhoA. Expression of DN FTase/GGTase I alpha-subunit diminished IGF-1- and insulin-stimulated phosphorylation of ERK (extracellular signal-regulated kinase), but had no effect on IGF-1- and insulin-stimulated phosphorylation of Akt. Taken together, these data suggest that DN FTase/GGTase I alpha-subunit can assuage the mitogenic effects of IGF-1 and insulin on MCF-7 breast cancer cells.

Human placental growth hormone causes severe insulin resistance in transgenic mice

OBJECTIVE

The insulin resistance of pregnancy is considered to be mediated by human placental lactogen, but the metabolic effects of human placental growth hormone have not been well defined. Our aim was to evaluate the effect of placental growth hormone on insulin sensitivity in vivo using transgenic mice that overexpress the human placental growth hormone gene.

STUDY DESIGN

Glucose and insulin tolerance tests were performed on 5 transgenic mice that overexpressed the human placental growth hormone variant gene and 6 normal littermate controls. The body composition of the mice was assessed by dual-energy radiograph absorptiometry, and free fatty acid levels were measured as a marker of lipolysis.

RESULTS

The human placental growth hormone levels in the transgenic mice were comparable to those attained in the third trimester of pregnancy. These mice were nearly twice as heavy as the control mice, and their body composition differed by a significant increase in bone density and a small decrease in percentage of body fat. Fasting insulin levels in the transgenic mice that overexpressed placental growth hormone were approximately 4-fold higher than the control mice (1.57 +/- 0.22 ng/mL vs 0.38 +/- 0.07 ng/mL; P <.001) and 7 times higher 30 minutes after glucose stimulation (4.17 +/- 0.54 ng/mL vs 0.62 +/- 0.10 ng/mL; P <.0001) with no significant difference in either fasting or postchallenge glucose levels. Insulin sensitivity was markedly decreased in the transgenic mice, as demonstrated by an insignificant decline in glucose levels after insulin injection compared with the control mice, which demonstrated more than a 65% reduction in glucose levels (P <.001).

CONCLUSION

Human placental growth hormone causes insulin resistance as manifested by fasting and postprandial hyperinsulinemia and minimal glucose lowering in response to insulin injection. Human placental growth hormone is a highly likely candidate to mediate the insulin resistance of pregnancy.

Genetics of insulin resistance

Insulin resistance, defined as the decreased ability of insulin to perform its biological functions, is likely to represent the primary physiologic defect underlying the insulin resistance syndrome (IRS), which includes insulin resistance/hyperinsulinemia, glucose intolerance and/or type 2 diabetes mellitus, visceral obesity, hypertension, and dyslipidemia. This constellation of traits is a leading cause of cardiovascular mortality and morbidity. Insulin sensitivity varies widely among individuals. Although environmental provocations including physical inactivity and caloric excess play an important role in the development of obesity and thus insulin resistance, epidemiologic and family studies show that there are also moderate genetic influences on the development of insulin resistance. Extreme forms of insulin resistance may be caused rarely by mutations in the genes for the insulin receptor and peroxisome proliferator-activated receptor gamma. However, the genetic basis for common more moderate forms of insulin resistance is likely to be polygenic and heterogeneous. Evidence further suggests that gene variants may have phenotypic influences on more than one IRS trait (so-called pleiotrophy), which may explain, in part, the clustering of these traits. This article reviews the evidence that insulin resistance has a genetic basis. Progress to date toward identifying specific gene variants are reviewed. Ultimately, the identification of specific gene variants that influence insulin resistance and other IRS traits will have profound influences on our understanding of the molecular and pathophysiologic basis of these disorders, from which new and more effective preventive and therapeutic interventions will be possible.

The insulin gene in diabetes

Lack of insulin production or abnormalities affecting insulin secretion are key to the development of almost all forms of diabetes, including the common type 1 (insulin-dependent) and type 2 (non-insulin-dependent) diabetes and the more rare forms of maturity-onset diabetes of the young (MODY). Because insulin has such a central role in the pathogenesis of both forms of diabetes, the insulin gene (INS) has always been considered a candidate susceptibility gene. A number of studies have shown that the allelic variation and parent-of-origin effects affect the transmission and expression of the insulin gene in pancreatic beta-cells and extra-pancreatic tissues. These observations have led to the formulation of new hypotheses to explain the biological mechanisms by which functional differences in the expression of the insulin gene may contribute to diabetes susceptibility.

Effects of fluvastatin treatment on insulin sensitivity in patients with hyperlipidaemia

This study aimed to determine the effects of fluvastatin treatment on insulin sensitivity in patients with hyperlipidaemia. Non-obese, normoglycaemic, normotensive patients with hyperlipidaemia (n = 20) and a reference group of healthy subjects of similar age, sex, and body mass index (n = 20) were evaluated. Patients with other causes of peripheral insulin resistance were excluded. All participants underwent a diagnostic protocol, which included measurements of insulin sensitivity index and other metabolic parameters. Insulin sensitivity was assessed by Homeostasis Model Assessment (HOMA). Serum insulin levels were tested by radioimmunoassay. Patients were treated with fluvastatin 40 mg once daily for 3 months. Before fluvastatin treatment, fasting serum insulin levels were significantly raised in patients with hyperlipidaemia compared with subjects from the reference group (19.1 +/- 13.4 versus 8.1 +/- 3.4 microIU/ml). The fasting serum insulin levels and HOMA-estimated insulin sensitivity were correlated in the whole group. Correlation analysis showed a significant relationship between HOMA-estimated insulin resistance and plasma cholesterol and triglyceride concentrations. Patients with hyperlipidaemia had reduced insulin sensitivity that was reflected by high serum fasting insulin levels. Anti-hyperlipidaemic treatment with fluvastatin increases insulin sensitivity.

Insulin resistance: a phosphorylation-based uncoupling of insulin signaling

Insulin resistance refers to a decreased capacity of circulating insulin to regulate nutrient metabolism. It is associated with the development of type 2 diabetes--an ever-increasing epidemic of the 21st century. Recent studies reveal that agents that induce insulin resistance exploit phosphorylation-based negative-feedback control mechanisms, otherwise utilized by insulin itself, to uncouple the insulin receptor from its downstream effectors and thereby terminate insulin signal transduction. This article describes recent findings that present novel viewpoints of the molecular basis of insulin resistance, focusing on the cardinal role of Ser/Thr protein kinases as emerging key players in this arena.

Serine phosphorylation of insulin receptor substrate-1: a novel target for the reversal of insulin resistance

Insulin resistance, the failure to respond to normal circulating concentrations of insulin, is a common state associated with obesity, aging, and a sedentary lifestyle. Compelling evidence implicates TNFalpha as the cause and link between obesity and insulin resistance. Serine phosphorylation of insulin receptor substrate-1 seems prominent among the mechanisms of TNFalpha-induced insulin resistance. Recent advances indicate that serine kinases may phosphorylate and thus inhibit the tyrosine phosphorylation of insulin receptor substrate-1, revealing an integration point of TNFalpha and insulin signaling pathways. Selective targeting of the molecular scenery whereby this key phosphorylation occurs/operates represents a rich area for the development of rationally designed new antidiabetic drugs. In relation to efficacy and side effects, this prospect should permit a more precise and perhaps individualized approach to therapeutic intervention, allowing clinicians to focus the attack where the problem lies.

The role of carbohydrates in insulin resistance

Insulin resistance is a metabolic disorder that is increasing worldwide and is associated with some of the most common diseases affecting modern societies including diabetes, hypertension, obesity and coronary heart disease. Although pharmacologic approaches to managing insulin resistance are being advocated by some, public health approaches involving changes in diet and physical activity are attractive because of their lower cost and risk. We briefly summarize some new information on the mechanisms that mediate insulin's many biological actions and examine the effects of dietary carbohydrates on insulin sensitivity. Specifically, we summarize some of the information available on the effects of simple sugars, complex carbohydrates including fiber, slowly digested starch and the general concept of glycemic index. The available data support the idea that consumption of diets high in total carbohydrate does not adversely affect insulin sensitivity compared with high fat diets. Animal data suggest that simple sugars, in particular fructose, have adverse effects on insulin action, but adverse effects have not been shown conclusively in humans. Increased intake of dietary fiber appears to improve insulin action and may protect against the development of diabetes. The effects of diets with high or low glycemic index on insulin action are controversial at this time. For firm conclusions to be reached, future studies must be of reasonable duration, be in defined populations and compare the effects of relevant doses of nutrients on specific endpoints of insulin action.

Human obesity: an evolutionary approach to understanding our bulging waistline

The unique worldwide spread of the human species and the remarkably long post-reproductive survival show that our genome permits excellent adaptation to vastly different environments. Moreover, the main scourges of later age, namely malignant growths and atherosclerosis, appear in humans later than in shorter-living animals. In recent years, excess weight and obesity have become mass phenomena with a pronounced upward trend in all developed countries. However, despite the detrimental effects of being overweight, these populations live longer than ever, which in part may be explained by the availability of better medical treatment. The prevalence and predicted further spread of obesity can be understood in the light of evolution. In all animal species energy metabolism is asymmetric with energy accumulation ('thrifty genotype') being the necessary condition of survival during hard times. For humans, which are no different to other animals in this respect, this genetic programming was necessary for survival because during the course of history, including the recorded history in the more developed Middle East, Europe or China, there was never a long period of uninterrupted food abundance, whereas famines were regular and frequent. Therefore fat accumulation, when food was available, meant survival at times of shortage, while the possible detrimental effects of overindulgence in food and being overweight expressed in unrealistically old age were irrelevant. It is the central, mostly intra-abdominal fat (in both humans and animals) that is more medically important than the subcutaneous truncal fat, and the accumulation of both types of fat is conditioned by high food consumption; therefore it is a historic novelty for human populations. In contrast, lower-body fat in human females is unique in the animal kingdom: it is much less metabolically active, it is of much lower pathologic significance than central fat, and it is programmed to be mobilized mostly during pregnancy and lactation. In view of all this, norms of desired weight should be based on hard mortality and morbidity statistics and not on theoretical, esthetic or fashion considerations. By this criterion, the upper limit of desirable weight is likely to be body mass index (BMI) 27 or 28, but specified for different populations (sex, race, ethnic origin); moreover, with aging, the detrimental effects of obesity diminish and finally disappear. Risks of other pathologies related to obesity (e.g. diabetes, hypertension and coronary disease) are also population-specific. However, total fatness, measured by BMI, is insufficiently sensitive as a risk factor, and fat distribution (upper-body versus low-body type, as reflected by waist circumference and waist:hip ratio) plays at least as prominent a role. Therefore the detailed norms, not yet available, should take into account both general obesity and fat distribution and be specific for different populations. Since long-term weight loss in adults is rarely achievable, public health measures should be aggressively directed at the prevention of obesity from childhood.