Macrophage gene expression in adipose tissue is associated with insulin sensitivity and serum lipid levels independent of obesity.

OBJECTIVE: Obesity is linked to both increased metabolic disturbances and increased adipose tissue macrophage infiltration. However, whether macrophage infiltration directly influences human metabolism is unclear. The aim of this study was to investigate if there are obesity-independent links between adipose tissue macrophages and metabolic disturbances. DESIGN AND METHODS: Expression of macrophage markers in adipose tissue was analyzed by DNA microarrays in the SOS Sib Pair study and in patients with type 2 diabetes and a BMI-matched healthy control group. RESULTS: The expression of macrophage markers in adipose tissue was increased in obesity and associated with several metabolic and anthropometric measurements. After adjustment for BMI, the expression remained associated with insulin sensitivity, serum levels of insulin, C-peptide, high density lipoprotein cholesterol (HDL-cholesterol) and triglycerides. In addition, the expression of most macrophage markers was significantly increased in patients with type 2 diabetes compared to the control group. CONCLUSION: Our study shows that infiltration of macrophages in human adipose tissue, estimated by the expression of macrophage markers, is increased in subjects with obesity and diabetes and associated with insulin sensitivity and serum lipid levels independent of BMI. This indicates that adipose tissue macrophages may contribute to the development of insulin resistance and dyslipidemia.

Variation in the resistin gene is associated with obesity and insulin-related phenotypes in Finnish subjects.

AIMS/HYPOTHESIS: Resistin is a peptide hormone produced by adipocytes that is present at high levels in sera of obese mice and may be involved in glucose homeostasis through regulation of insulin sensitivity. Several studies in humans have found associations between polymorphisms in the resistin gene and obesity, insulin sensitivity and blood pressure. An association between variation in the resistin gene and type 2 diabetes has been reported in some, but not all studies. The aim of this study was to analyse variants of the resistin gene for association with type 2 diabetes and related traits in a Finnish sample. METHODS: In 781 cases with type 2 diabetes, 187 spouse controls and 222 elderly controls of Finnish origin, we genotyped four previously identified non-coding single-nucleotide polymorphisms (SNPs): -420C>G from the promoter region, +156C>T and +298G>A from intron 2, and +1084G>A from the 3' untranslated region. We then tested whether these SNPs were associated with type 2 diabetes and related traits. RESULTS: The SNPs were not significantly associated with type 2 diabetes. However, SNPs -420C>G, +156C>T and +298G>A and the common haplotype for these three markers were associated with increased values of weight-related traits and diastolic blood pressure in cases, lower weight in elderly control subjects, and lower insulin sensitivity and greater acute insulin response in spouses. Furthermore, the +1084G allele was associated with lower HDL cholesterol in both cases and controls, higher systolic blood pressure and waist circumference in cases, and greater acute insulin response in spouse controls. CONCLUSIONS/INTERPRETATION: Our results add to growing evidence that resistin is associated with variation in weight, fat distribution and insulin resistance.

Insulin secretion, obesity, and potential behavioral influences: results from the Insulin Resistance Atherosclerosis Study (IRAS).

BACKGROUND: This work was conducted to evaluate associations of insulin secretion with overall and central obesity, dietary fats, physical activity, and alcohol. METHODS: A frequently sampled intravenous glucose tolerance test (FSIGT) was used to assess acute insulin response to glucose (AIR) and insulin sensitivity (S(I)) among adult participants (n=675 with normal, NGT; n=332 with impaired glucose tolerance, IGT) in the Insulin Resistance Atherosclerosis Study (IRAS). Disposition index (DI) was calculated as the sum of the log-transformed AIR and S(I) to reflect pancreatic compensation for insulin resistance. Obesity was measured as body mass index (kg/m(2), BMI) and central fat distribution by waist circumference (cm). Dietary fat intake (total, saturated, polyunsaturated, oleic acid), physical activity, and alcohol intake were assessed by standardized interview. RESULTS: In unadjusted analyses, BMI and waist were each positively correlated with AIR among NGTs (r=0.26 and 0.23, respectively; p<0.0001) but correlations were weaker among the IGTs (r=0.10, NS; r=0.13, p<0.05 for BMI and waist, respectively). BMI and waist were inversely correlated with DI among NGTs (r=-0.13 and -0.20, respectively; p<0.0001) and among IGTs (r=-0.20 and -0.19, respectively, p<0.0001). Dietary fat variables were positively related, and alcohol was inversely related, to AIR among NGTs (p<0.01) but not among IGTs. With all factors considered simultaneously in a pooled analysis of IGTs and NGTs, waist, but not BMI, was positively associated with AIR (p<0.001) and inversely associated with DI (p<0.01). None of the behavioral variables were independently related to either outcome. CONCLUSION: Among non-diabetic patients, central obesity appears to be related to higher insulin secretion, but to lower capacity of the pancreas to respond to the ambient insulin resistance.

Central role of the adipocyte in the metabolic syndrome.

Insulin resistance is associated with a plethora of chronic illnesses, including Type 2 diabetes, dyslipidemia, clotting dysfunction, and colon cancer. The relationship between obesity and insulin resistance is well established, and an increase in obesity in Western countries is implicated in increased incidence of diabetes and other diseases. Central, or visceral, adiposity has been particularly associated with insulin resistance; however, the mechanisms responsible for this association are unclear. Our laboratory has been studying the physiological mechanisms relating visceral adiposity and insulin resistance. Moderate fat feeding of the dog yields a model reminiscent of the metabolic syndrome, including visceral adiposity, hyperinsulinemia, and insulin resistance. We propose that insulin resistance of the liver derives from a relative increase in the delivery of free fatty acids (FFA) from the omental fat depot to the liver (via the portal vein). Increased delivery results from 1) more stored lipids in omental depot, 2) severe insulin resistance of the central fat depot, and 3) possible regulation of visceral lipolysis by the central nervous system. The significance of portal FFA delivery results from the importance of FFA in the control of liver glucose production. Insulin regulates liver glucose output primarily via control of adipocyte lipolysis. Thus, because FFA regulate the liver, it is expected that visceral adiposity will enhance delivery of FFA to the liver and make the liver relatively insulin resistant. It is of interest how the intact organism compensates for insulin resistance secondary to visceral fat deposition. While part of the compensation is enhanced B-cell sensitivity to glucose, an equally important component is reduced liver insulin clearance, which allows for a greater fraction of B-cell insulin secretion to bypass liver degradation, to enter the systemic circulation, and to result in hyperinsulinemic compensation. The signal(s) resulting in B-cell up-regulation and reduced liver insulin clearance with visceral adiposity is (are) unknown, but it appears that the glucagon-like peptide (GLP-1) hormone plays an important role. The integrated response of the organism to central adiposity is complex, involving several organs and tissue beds. An investigation into the integrated response may help to explain the features of the metabolic syndrome.

Inhibition of lipolysis causes suppression of endogenous glucose production independent of changes in insulin.

We have shown that insulin controls endogenous glucose production (EGP) indirectly, via suppression of adipocyte lipolysis. Free fatty acids (FFA) and EGP are suppressed proportionately, and when the decline in FFA is prevented during insulin infusion, suppression of EGP is also prevented. The present study tested the hypothesis that suppression of lipolysis under conditions of constant insulin would yield a suppression of EGP. N(6)-cyclohexyladenosine (CHA) was used to selectively suppress adipocyte lipolysis during euglycemic clamps in conscious male dogs. FFA suppression by CHA caused suppression of EGP. Liposyn control experiments, which maintained FFA levels above basal during CHA infusion, completely prevented the decline in EGP, whereas glycerol control experiments, which maintained glycerol levels close to basal, did not prevent a decline in EGP. These controls suggest that the EGP suppression was secondary to the suppression of FFA levels specifically. A difference in the sensitivity of FFA and EGP suppression (FFA were suppressed approximately 85% whereas EGP only declined approximately 40%) was possibly caused by confounding effects of CHA, including an increase in catecholamine and glucagons levels during CHA infusion. Thus suppression of lipolysis under constant insulin causes suppression of EGP, despite a significant rise in catecholamines.

Non-esterified fatty acids and the liver: why is insulin secreted into the portal vein?

AIMS/HYPOTHESIS: Insulin control of glucose output is a major mechanism by which appropriate amounts of glucose are produced to supply energy to the central nervous system, without causing long-term increases of the plasma glucose concentration. It is hypothesised that the primary route by which insulin maintains control over glucose production is indirect and is mediated by regulation of non-esterified fatty acid release from the adipocyte. The question arises as to why evolution has chosen insulin to be secreted into the portal vein, if control of the liver is partially or primarily indirect. It is suggested that alterations in hepatic insulin clearance which attend increases in central adiposity are an important part of the compensation for insulin resistance and limit the necessity for up-regulation of insulin secretion in insulin resistance secondary to central adiposity. METHODS: Review of research from author's group and other laboratories. RESULTS: Data over the previous decade indicate that suppression of glucose output by increased insulin is a relatively slow process, much slower than the rate of binding of insulin to hepatocytes. One explanation is that insulin acts on an extrahepatic tissue, which in turn alters a signal to the liver, reducing glucose output. Additional evidence for an extrahepatic primary effect of insulin emerges from experiments in which insulin was given portally or peripherally at half the portal dose. Endogenous glucose production was related to systemic, not portal insulin, supporting the concept that the primary step in insulin's action on liver is on some other tissue, altering signalling to the liver itself. Strong correlation between plasma non-esterified fatty acids (NEFA) and liver glucose output suggests that the primary effect is on the adipocyte. The primacy of the adipocyte locus for the insulin effect included data that insulin's action on liver is prevented when plasma NEFA are maintained, as well as data showing proportional decline in glucose production and fatty acids when antilipolysis is induced by an adenosine agonist. Why then, from an evolutionary point of view is insulin secreted into the portal vein? Institution of central adiposity in dogs with fat feeding causes hepatic insulin resistance, at least partially due to the provision of NEFA in portal blood. The initial response to resistance is enhanced beta-cell sensitivity to glucose; a secondary compensation is, however, a substantial reduction in liver clearance, allowing for a greater proportion of secreted insulin to reach muscle, where it can more efficiently stimulate glucose utilisation. CONCLUSION/INTERPRETATION: Non-esterified fatty acids act as a signal as well as a metabolic substrate. They can regulate glucose utilisation in muscle and apparently are important signals to the liver and the beta cells as well. The importance of portal vein NEFA concentrations to the function of the liver could explain insulin resistance of the liver with central pattern obesity.

New concepts in extracellular signaling for insulin action: the single gateway hypothesis.

Insulin resistance is a precursor to and primary cause of Type 2 diabetes mellitus. In addition, insulin resistance is associated with other chronic diseases, including gestational diabetes, cardiovascular disease, and cancer. Resistance to insulin's effects on carbohydrate metabolism include diminished actions of insulin to enhance glucose uptake and suppress endogenous glucose production. This chapter introduces new concepts related to the mechanism by which insulin stimulates glucose utilization in vivo and demonstrates that these processes are mechanistically linked to glucose production. Insulin acts rapidly in vitro to stimulate glucose uptake; in contrast, its effects in vivo are relatively slow in the conscious animal or human subject. The explanation for this difference between in vitro and in vivo dynamics is the delay associated with insulin transport across capillary endothelium of insulin-sensitive tissues (primarily muscle). Also, interstitial insulin is attenuated in concentration compared to plasma insulin at basal as well as under hyperinsulinemic conditions (plasma:interstitial ratio, 3:2). The sluggishness of insulin action and the attenuation in insulin concentration can be explained by a model in which transendothelial insulin transport is restricted and interstitial insulin binds to insulin-sensitive cells, where the hormone is internalized and degraded. Whether insulin transport occurs by a hormone-specific mechanism (i.e., via receptors on endothelial cells) was tested by comparing transport at physiological with pharmacological insulin concentrations-evidence supports a nonspecific mechanism of transport across endothelium (i.e., diffusion or transcytosis). Transendothelial transport alters the in vivo patterns of insulin signaling-biphasic plasma insulin after glucose injection is reflected in a simple, rapid increase in interstitial insulin to an elevated concentration. The time course of insulin's effect to suppress endogenous glucose output is a mirror image of its effect to enhance glucose uptake; however, there is no transendothelial barrier to insulin action at the liver. The similarity in action dynamics at periphery and liver was explained by a mechanism in which insulin crosses into peripheral tissue and alters a "second (blood-borne) signal" that, in turn, suppresses liver glucose production. Of various possible alternative candidates for the second signal, declining plasma free fatty acids appear to signal suppression of glucose production. We have proposed the "single gateway hypothesis" to explain insulin's action on carbohydrate metabolism in vivo: insulin crosses the endothelial boundary in skeletal muscle (to stimulate glucose disposal) and traverses the endothelial barrier in adipose tissue to suppress lipolysis. The declining free fatty acids are proposed to be a major factor in the insulin-mediated decline in glucose output. This mechanism can be contrasted with the classical concept that portal insulin controls the liver directly. Recent evidence supports the concept that, under normal levels of glucagonemia, less than 25% of the suppression of hepatic glucose output by insulin is due to a direct effect of insulin via the portal vein and that most of the effect (approximately 75%) is explained by the indirect single gateway mechanism. These results raise the question of whether hepatic insulin resistance in Type 2 diabetes can be explained by insulin resistance at the adipocyte, which causes a failure of reduction of FFA by insulin, leading to overproduction of glucose by the liver. The possible role of the single gateway mechanism in diabetes is under investigation.

The influence of luteinizing hormone and insulin on sex steroids and sex hormone-binding globulin in the polycystic ovarian syndrome.

OBJECTIVE: To examine the relationship between hyperinsulinemia, sex hormone-binding globulin (SHBG), and body mass index (BMI) on LH-induced hyperandrogenemia in patients with polycystic ovarian syndrome (PCOS). DESIGN: Insulin responses during an oral glucose tolerance test (OGTT) were assessed in 25 consecutive women with PCOS and 20 control women matched for BMI. Insulin responses and sensitivity (SI) were also determined using a frequently sampled intravenous glucose tolerance test (IVGTT). SETTING: The clinical research center at a university medical center. MAIN OUTCOME MEASURES: Serum LH, SI, and basal, peak, and area under the curve (AUC-insulin responses) were determined and correlated with SHBG, androstenedione (A), T, and free T concentrations. RESULTS: Compared with controls, the AUC-insulin response during OGTT was greater in PCOS, with an average increase of 44%. During IVGTT, AUC-insulin response was also significantly higher in PCOS versus controls, with an average increase of 53%. In addition, SI was reduced in PCOS versus controls with an average decrease of 53%. The average differences in oral- and intravenous-glucose-induced hyperinsulinemia and in insulin sensitivity between PCOS and controls were relatively constant across the entire physiological range of BMI. In PCOS, baseline LH showed strong positive correlations with baseline A and T. However, there were no significant correlations between either basal, peak, or AUC-insulin response during OGTT and IVGTT with basal T or A concentrations or between insulin and androgen levels measured at 30-minute intervals throughout the OGTT. However, basal, peak, and AUC-insulin responses during OGTT were strongly correlated with fasting SHBG binding capacity. CONCLUSIONS: These data are consistent with the hypothesis that hyperinsulinemia in PCOS influences the biologically active component of T by lowering SHBG concentrations while having little apparent impact on LH-induced secretion of androgens in vivo.

Insulin and insulin-like growth factor-I responsiveness in polycystic ovarian syndrome.

OBJECTIVE: To assess insulin and insulin-like growth factor I (IGF-I) action in women with polycystic ovarian syndrome (PCOS). DESIGN: Hyperinsulinemia was determined by measuring the insulin responses during a 2-hour oral glucose tolerance test (OGTT). Quantification of in vivo insulin action was determined by a frequently sampled intravenous (IV) OGTT with minimal modeling analysis. In vitro sensitivity to insulin at physiological and supraphysiological concentrations and to IGF-I was assessed by examining colony formation of two hematopoietic cell populations, burst-forming units of the erythroid line (BFU-E) and human leukemia virus immortalized T-cell lines. (The proliferative responses of BFU-E, a primary tissue explant, are presumably conditioned by factors in the immediate blood-borne environment, whereas proliferative responses of T-cell lines are presumed to reflect intrinsic target-cell hormone sensitivity.) SETTING: Tertiary care research institution. PATIENTS: Eight patients (4 obese and 4 nonobese) with PCOS and three healthy women for reference controls. RESULTS: Nonobese (P less than 0.04) and obese patients with PCOS (P less than 0.01) both demonstrated significant hyperinsulinemia after OGTT. In vivo insulin resistance was observed in both nonobese (P less than 0.03) and obese PCOS subjects (P less than 0.01) using frequently sampled IV OGTT. Both nonobese (P less than 0.03) and obese patients with PCOS (P less than 0.01) had blunted in vitro clonal responses of BFU-E, with normal T-cell line clonal responsiveness to physiological levels of insulin and normal BFU-E and T-cell line clonal responses to IGF-I. CONCLUSIONS: These findings demonstrate the following in both nonobese and obese patients with PCOS: (1) there is in vivo hyperinsulinemia and resistance to insulin action on glucose disposal; (2) with BFU-E, there is in vitro resistance to the mitogenic action of insulin but normal responsiveness to IGF-I; and (3) there is normal in vitro mitogenic responsiveness of T-cell lines to both insulin and IGF-I. The intrinsically normal mitogenic responsiveness to insulin and, especially to IGF-I, whether or not under the influence of the bloodborne milieu, provides a mechanism whereby hyperinsulinemia could directly contribute to the ovarian abnormalities that characterize PCOS.

Insulin action and metabolism in patients with head and neck cancer.

Cachexia is a phenomenon commonly observed in patients with cancer, but its etiology is poorly understood. Abnormalities of insulin action and metabolism that have been hypothesized to promote cancer cachexia were investigated in this study using a computerized euglycemic clamp and modified frequently sampled intravenous glucose tolerance test (FSIGT) in a group of malnourished patients with localized head and neck cancer, and in healthy, well-nourished, age- and, sex-matched controls. Glucose disposal rates, determined by the euglycemic clamp at three different rates of insulin infusion did not differ significantly between the two groups. However, mean plasma insulin concentrations at each level of insulin infused were significantly lower in the patients with cancer than in the control subjects resulting in unexpectedly higher calculated insulin metabolic clearance rate in the patients with cancer. Peripheral insulin sensitivity calculated from the slope of glucose disposal versus plasma insulin concentration did not differ between the two groups. Results from the FSIGT demonstrated no difference in insulin sensitivity or insulin-independent glucose disappearance between the two groups. However, whole body glucose appearance was significantly elevated in the patients with cancer. Thus, increased whole body glucose utilization in the absence of insulin resistance or increased insulin-dependent glucose disposal was observed. These data are consistent with the concept of a localized tumor acting as a glucose drain in which case increased glucose appearance and increased insulin clearance would defend against hypoglycemia.

Pathophysiology of malnutrition in the adult cancer patient.

A number of common metastatic cancers are associated with marked weight loss at the time of diagnosis. Cancer patients with weight loss at the time of diagnosis have decreased mean survival compared to similar cancer patients without weight loss. Provision of excess calories alone does not appear to change median survival in patients with advanced cancer and many patients either maintain body weight or lose weight while receiving calories which would be predicted to result in weight gain. The authors recently have extended their studies to head and neck cancer patients without detectable metastatic disease in order to detect systemic metabolic effects of a localized tumor. These patients failed to gain weight despite the administration of apparently adequate calories by continuous enteral alimentation. Abnormalities of carbohydrate metabolism with secondary effects on fat and protein metabolism have been identified in several populations of patients with common cancers. These abnormalities offer potential points of intervention which may enhance nutritional therapy as rehabilitation and as a potential biological modifier of the response of specific cancers to chemotherapy, radiation therapy, or surgery.

Diminished in vitro responsiveness of circulating erythroid progenitor cells to insulin as an indicator of insulin resistance.

While insulin resistance is considered characteristic of extreme obesity, it may be more difficult to demonstrate in less severe forms of obesity. We studied five moderately obese individuals [mean body mass index (MBMI), 34.1 +/- 1.85 (+/- SE) kg/m2], one massively obese patient (BMI, 50.2 kg/m2), and seven age-matched normal subjects (MBMI, 22.4 +/- 0.93 kg/m2). While two of the obese patients had normal glucose tolerance, all had fasting hyperinsulinemia (P less than 0.02 vs. normal subjects) and exaggerated insulin responses after oral glucose challenge, as defined by area under the 3-h insulin response curve (P less than 0.01 vs. normal subjects). That this hyperinsulinemia represented in vivo insulin resistance was supported by the glucose and insulin responses in four individuals to an iv glucose bolus analyzed by the minimal modeling technique. Study of monocyte insulin receptors revealed no reduction in total insulin binding in the four obese patients tested. Since physiological concentrations of insulin stimulate the in vitro growth of normal human erythroid progenitor cells (EPC), we reasoned that this response might be blunted in cells from individuals with endogenous insulin resistance. The mean peak EPC proliferative response (26.7 +/- 9.11% above baseline) in the obese hyperinsulinemic group was significantly less than the corresponding mean value in the control group (92.6 +/- 5.24% above baseline, P less than 0.001). These results suggest that the minimal modeling technique is a sensitive method for the in vivo demonstration of insulin resistance in moderately obese individuals and that EPC responsiveness to physiological concentrations of insulin reflects in vivo insulin sensitivity and may be used as an in vitro indicator of insulin resistance.

Extrapancreatic effect of somatostatin infusion to increase glucose clearance.

Constant intraportal insulin, coupled with variable intraportal glucagon, was used in the attempt to reestablish basal metabolic conditions in dogs during somatostatin (SRIF) infusion (0.8 micrograms X min-1 X kg-1). SRIF alone lowered glucose (G), insulin (I), and glucagon (GN) (G: 90 +/- 5 to 69 +/- 1 mg/dl; I: 18 +/- 4 to 4 +/- 1 microU/ml; GN: 257 +/- 52 to 168 +/- 40 pg/ml; P less than 0.05 or better). Hormone replacement. Hypoglycemia persisted (G at steady state, SS, 60-150 min): 12 +/- 3 mg/dl below basal; P = 0.006) despite intraportal insulin replacement (200 microU X min-1 X kg-1; insulin at basal: 14 +/- 1; at SS: 14 +/- 2 microU/ml; P greater than 0.9) and glucagon overreplacement (basal: 341 +/- 42; SS: 486 +/- 80 pg/ml; P less than 0.05). Glucose clearance was increased 65% above basal (P less than 0.0001). Insulin underreplacement. With a lower intraportal insulin infusion rate (50 microU X min-1 X kg-1), insulin fell from basal (10 +/- 2 microU/ml) to 4 +/- 1 microU/ml during steady state (P = 0.03). Glucose and glucose clearance were normalized to basal values (G: 85 +/- 3 mg/dl, P = 0.3; clearance: 5.7 +/- 0.5 ml X min-1 X kg-1; P = 0.2) with full glucagon replacement (basal: 281 +/- 120; SS: 264 +/- 80 pg/ml; P greater than 0.9). Thus, during constant SRIF infusion, normoglycemia was reattained when insulin was underreplaced via the portal vein. The failure to reattain euglycemia with normoinsulinemia was due to a SRIF-induced increase in extrahepatic glucose clearance. Insulin replacement and growth hormone (GH) infusion. GH (15 ng X min-1 X kg-1) partially reversed the hypoglycemia during SRIF, with full insulin replacement. The SRIF-induced increase in glucose clearance may be partially mediated by a decrease in GH.