Insulin-like growth factor-binding protein-1: a role in glucose counterregulation?

Salvianolic acid A attenuates non-alcoholic fatty liver disease by regulating the AMPK-IGFBP1 pathway

Non-alcoholic fatty liver disease (NAFLD) affects approximately a quarter of the population and, to date, there is no approved drug therapy for this condition. Individuals with type 2 diabetes mellitus (T2DM) are at a significantly elevated risk of developing NAFLD, underscoring the urgency of identifying effective NAFLD treatments for T2DM patients. Salvianolic acid A (SAA) is a naturally occurring phenolic acid that is an important component of the water-soluble constituents isolated from the roots of Salvia miltiorrhiza Bunge. SAA has been demonstrated to possess anti-inflammatory and antioxidant stress properties. Nevertheless, its potential in ameliorating diabetes-associated NAFLD has not yet been fully elucidated. In this study, diabetic ApoE-/- mice were employed to establish a NAFLD model via a Western diet. Following this, they were treated with different doses of SAA (10 mg/kg, 20 mg/kg) via gavage. The study demonstrated a marked improvement in liver injury, lipid accumulation, inflammation, and the pro-fibrotic phenotype after the administration of SAA. Additionally, RNA-seq analysis indicated that the primary pathway by which SAA alleviates diabetes-induced NAFLD involves the cascade pathways of lipid metabolism. Furthermore, SAA was found to be effective in the inhibition of lipid accumulation, mitochondrial dysfunction and ferroptosis. A functional enrichment analysis of RNA-seq data revealed that SAA treatment modulates the AMPK pathway and IGFBP-1. Further experimental results demonstrated that SAA is capable of inhibiting lipid accumulation through the activation of the AMPK pathway and IGFBP-1.

Adiponectin, IGFBP-1 and -2 are independent predictors in forecasting prediabetes and type 2 diabetes

Objective

Adiponectin and insulin-like growth factor (IGF) binding proteins IGFBP-1 and IGFBP-2 are biomarkers of insulin sensitivity. IGFBP-1 reflects insulin sensitivity in the liver, adiponectin in adipose tissue and IGFBP-2 in both tissues. Here, we study the power of the biomarkers adiponectin, IGFBP-1, IGFBP-2, and also included IGF-I and IGF-II, in predicting prediabetes and type 2 diabetes (T2D) in men and women with normal oral glucose tolerance (NGT).

Design

Subjects with NGT (35-56 years) recruited during 1992-1998 were re-investigated 8-10 years later. In a nested case control study, subjects progressing to prediabetes (133 women, 164 men) or to T2D (55 women, 98 men) were compared with age and sex matched NGT controls (200 women and 277 men).

Methods

The evaluation included questionnaires, health status, anthropometry, biochemistry and oral glucose tolerance test.

Results

After adjustment, the lowest quartile of adiponectin, IGFBP-1 and IGFBP-2 associated independently with future abnormal glucose tolerance (AGT) in both genders in multivariate analyses. High IGFs predicted weakly AGT in women. In women, low IGFBP-2 was the strongest predictor for prediabetes (OR:7.5), and low adiponectin for T2D (OR:29.4). In men, low IGFBP-1 was the strongest predictor for both prediabetes (OR:13.4) and T2D (OR:14.9). When adiponectin, IGFBP-1 and IGFBP-2 were combined, the ROC-AUC reached 0.87 for women and 0.79 for men, higher than for BMI alone.

Conclusion

Differences were observed comparing adipocyte- and hepatocyte-derived biomarkers in forecasting AGT in NGT subjects. In women the strongest predictor for T2D was adiponectin and in men IGFBP-1, and for prediabetes IGFBP-2 in women and IGFBP-1 in men.

Role of IGF-binding proteins in regulating IGF responses to changes in metabolism

The IGF-binding protein family contains six members that share significant structural homology. Their principal function is to regulate the actions of IGF1 and IGF2. These proteins are present in plasma and extracellular fluids and regulate access of both IGF1 and II to the type I IGF receptor. Additionally, they have functions that are independent of their ability to bind IGFs. Each protein is regulated independently of IGF1 and IGF2, and this provides an important mechanism by which other hormones and physiologic variables can regulate IGF actions indirectly. Several members of the family are sensitive to changes in intermediary metabolism. Specifically the presence of obesity/insulin resistance can significantly alter the expression of these proteins. Similarly changes in nutrition or catabolism can alter their synthesis and degradation. Multiple hormones such as glucocorticoids, androgens, estrogen and insulin regulate IGFBP synthesis and bioavailability. In addition to their ability to regulate IGF access to receptors these proteins can bind to distinct cell surface proteins or proteins in extracellular matrix and several cellular functions are influenced by these interactions. IGFBPs can be transported intracellularly and interact with nuclear proteins to alter cellular physiology. In pathophysiologic states, there is significant dysregulation between the changes in IGFBP synthesis and bioavailability and changes in IGF1 and IGF2. These discordant changes can lead to marked alterations in IGF action. Although binding protein physiology and pathophysiology are complex, experimental results have provided an important avenue for understanding how IGF actions are regulated in a variety of physiologic and pathophysiologic conditions.

Obesity and Cancer Metabolism: A Perspective on Interacting Tumor-Intrinsic and Extrinsic Factors

Obesity is associated with increased risk and poor prognosis of many types of cancers. Several obesity-related host factors involved in systemic metabolism can influence tumor initiation, progression, and/or response to therapy, and these have been implicated as key contributors to the complex effects of obesity on cancer incidence and outcomes. Such host factors include systemic metabolic regulators including insulin, insulin-like growth factor 1, adipokines, inflammation-related molecules, and steroid hormones, as well as the cellular and structural components of the tumor microenvironment, particularly adipose tissue. These secreted and structural host factors are extrinsic to, and interact with, the intrinsic metabolic characteristics of cancer cells to influence their growth and spread. This review will focus on the interplay of these tumor cell-intrinsic and extrinsic factors in the context of energy balance, with the objective of identifying new intervention targets for preventing obesity-associated cancer.

Vitamin A status and its metabolism contribute to the regulation of hepatic genes during the cycle of fasting and refeeding in rats

Vitamin A (VA) status and its metabolism affect hepatic metabolic homeostasis. We investigated if VA status and metabolism contribute to energy metabolism and expression of hepatic genes in the cycle of fasting and refeeding. Zucker lean rats with VA sufficient (VAS) or VA deficient (VAD) status were respectively grouped as: ad libitum (VAS-AD or VAD-AD), 48-h fasted (VAS-Fasted or VAD-Fasted), 48-h fasted and refed a VAS diet (VAS-Refed-VAS or VAD-Refed-VAS), or refed a VAD diet (VAS-Refed-VAD or VAD-Refed-VAD) for 6 h. Respiratory exchange ratio (RER) of rats fed the VAS or VAD diet was monitored for 6 weeks. From week four, rats fed the VAS diet had higher RER than those fed the VAD diet. VAS-Refed rats had higher plasma levels of glucose, triglyceride, insulin and leptin than VAD-Refed rats. The mRNA and protein levels of hepatic genes for fuel metabolism in the fasting and refeeding cycle were determined using real-time polymerase chain reaction and immunoblot, respectively. The mRNA levels of glucokinase (Gck), sterol regulatory element-binding protein 1c (Srebp-1c), and fatty acid synthase (Fas) were lowered in VAS-Fasted and VAD-Fasted rats, and increased in VAS-Refed-VAS, VAS-Refed-VAD and VAD-Refed-VAS, but not VAD-Refed-VAD, rats. The ACL and FAS protein levels only dropped in VAS-Fasted rats and increased in VAS-Refed-VAS rats. The GK protein level decreased only in VAS-Fasted rats, and increased in VAS-Refed-VAS, VAS-Refed-VAD and VAD-Refed-VAS (but not VAD-Refed-VAD) rats. We conclude that VA status and its metabolism in the fasting and refeeding cycle contribute to the regulation of hepatic gene expression in rats.

Metabolic, anabolic, and mitogenic insulin responses: A tissue-specific perspective for insulin receptor activators

Insulin acts as the major regulator of the fasting-to-fed metabolic transition by altering substrate metabolism, promoting energy storage, and helping activate protein synthesis. In addition to its glucoregulatory and other metabolic properties, insulin can also act as a growth factor. The metabolic and mitogenic responses to insulin are regulated by divergent post-receptor signaling mechanisms downstream from the activated insulin receptor (IR). However, the anabolic and growth-promoting properties of insulin require tissue-specific inter-relationships between the two pathways, and the nature and scope of insulin-regulated processes vary greatly across tissues. Understanding the nuances of this interplay between metabolic and growth-regulating properties of insulin would have important implications for development of novel insulin and IR modulator therapies that stimulate insulin receptor activation in both pathway- and tissue-specific manners. This review will provide a unique perspective focusing on the roles of "metabolic" and "mitogenic" actions of insulin signaling in various tissues, and how these networks should be considered when evaluating selective pharmacologic approaches to prevent or treat metabolic disease.

The Insulin-Like Growth Factor System in Obesity, Insulin Resistance and Type 2 Diabetes Mellitus

The insulin-like growth factor (IGF) system, acting in concert with other hormone axes, is important in normal metabolism. In obesity, the hyperinsulinaemia that accompanies peripheral insulin resistance leads to reduced growth hormone (GH) secretion, while total IGF-I levels are relatively unchanged due to increased hepatic GH sensitivity. IGF-binding protein (IGFBP)-1 levels are suppressed in relation to the increase in insulin levels in obesity and low levels predict the development of type 2 diabetes several years later. Visceral adiposity and hepatic steatosis, along with a chronic inflammation, contribute to the IGF system phenotype in individuals with metabolic syndrome and type 2 diabetes mellitus, including changes in the normal inverse relationship between IGFBP-1 and insulin, with IGFBP-1 concentrations that are inappropriately normal or elevated. The IGF system is implicated in the vascular and other complications of these disorders and is therefore a potential therapeutic target.

IGF binding proteins in cancer: mechanistic and clinical insights

The six members of the family of insulin-like growth factor (IGF) binding proteins (IGFBPs) were originally characterized as passive reservoirs of circulating IGFs, but they are now understood to have many actions beyond their endocrine role in IGF transport. IGFBPs also function in the pericellular and intracellular compartments to regulate cell growth and survival - they interact with many proteins, in addition to their canonical ligands IGF-I and IGF-II. Intranuclear roles of IGFBPs in transcriptional regulation, induction of apoptosis and DNA damage repair point to their intimate involvement in tumour development, progression and resistance to treatment. Tissue or circulating IGFBPs might also be useful as prognostic biomarkers.

Effect of Ghrelin on Hepatic IGF-Binding Protein-1 Production

Ghrelin plays key roles in energy homeostasis by central and peripheral actions that include effects on insulin signalling pathways in liver. Insulin is an important inhibitor of production by hepatocytes of insulin-like growth factor-binding protein-1 (IGFBP-1) which has an endocrine role to inhibit IGF availability. The effects of ghrelin, insulin, an AMPK activator, and an AMPK inhibitor on IGFBP-1 secretion were studied in H4-II-E rat liver cells. Ghrelin (100 nM) blocked the inhibitory effect of a maximally effective concentration of insulin (10 ng/mL) on IGFBP-1 secretion during a 5 h incubation period (P < 0.001) in the absence and presence of an AMPK inhibitor. Ghrelin, alone, had no effect on IGFBP-1 production, but enhanced secretion independently of insulin under conditions of AMPK activation (P < 0.001). In conclusion, IGFBP-1 is identified as a novel target of ghrelin action in liver that may contribute to its metabolic effects in obesity.

Nrf2 deficiency improves glucose tolerance in mice fed a high-fat diet

Nrf2, a master regulator of intracellular redox homeostasis, is indicated to participate in fatty acid metabolism in liver. However, its role in diet-induced obesity remains controversial. In the current study, genetically engineered Nrf2-null, wild-type (WT), and Nrf2-activated, Keap1-knockdown (K1-KD) mice were fed either a control or a high-fat Western diet (HFD) for 12 weeks. The results indicate that the absence or enhancement of Nrf2 activity did not prevent diet-induced obesity, had limited effects on lipid metabolism, but affected blood glucose homeostasis. Whereas the Nrf2-null mice were resistant to HFD-induced glucose intolerance, the Nrf2-activated K1-KD mice exhibited prolonged elevation of circulating glucose during a glucose tolerance test even on the control diet. Feeding a HFD did not activate the Nrf2 signaling pathway in mouse livers. Fibroblast growth factor 21 (Fgf21) is a liver-derived anti-diabetic hormone that exerts glucose- and lipid-lowering effects. Fgf21 mRNA and protein were both elevated in livers of Nrf2-null mice, and Fgf21 protein was lower in K1-KD mice than WT mice. The inverse correlation between Nrf2 activity and hepatic expression of Fgf21 might explain the improved glucose tolerance in Nrf2-null mice. Furthermore, a more oxidative cellular environment in Nrf2-null mice could affect insulin signaling in liver. For example, mRNA of insulin-like growth factor binding protein 1, a gene repressed by insulin in hepatocytes, was markedly elevated in livers of Nrf2-null mice. In conclusion, genetic alteration of Nrf2 does not prevent diet-induced obesity in mice, but deficiency of Nrf2 improves glucose homeostasis, possibly through its effects on Fgf21 and/or insulin signaling.

Vitamin A status affects obesity development and hepatic expression of key genes for fuel metabolism in Zucker fatty rats

We hypothesized that vitamin A (VA) status may affect obesity development. Male Zucker lean (ZL) and fatty (ZF) rats after weaning were fed a synthetic VA deficient (VAD) or VA sufficient (VAS) diet for 8 weeks before their plasma parameters and hepatic genes' expression were analyzed. The body mass (BM) of ZL or ZF rats fed the VAD diet was lower than that of their corresponding controls fed the VAS diet at 5 or 2 weeks, respectively. The VAD ZL and ZF rats had less food intake than the VAS rats after 5 weeks. The VAD ZL and ZF rats had lower plasma glucose, triglyceride, insulin, and leptin levels, as well as lower liver glycogen content, net mass of epididymal fat, and liver/BM and epididymal fat/BM ratios (ZL only) than their respective VAS controls. VAD rats had lower hepatic Cyp26a1, Srebp-1c, Fas, Scd1, Me1, Gck, and Pklr (ZL and ZF); and higher Igfbp1 (ZL and ZF), Pck1(ZF only), and G6pc (ZF only) mRNA levels than their respective VAS controls. We conclude that ZL and ZF rats responded differently to dietary VA deficiency. VA status affected obesity development and altered the expression of hepatic genes for fuel metabolism in ZF rats. The mechanisms will help us to combat metabolic diseases.

Short-term effects of the long-acting insulin analog detemir and human insulin on plasma levels of insulin-like growth factor-I and its binding proteins in humans

OBJECTIVE

The objective of the study was to compare responses of plasma levels of IGF-I and IGF binding proteins (IGFBP-1 and IGFBP-3) induced by human regular insulin (HI) and the long-acting insulin analog detemir (IDet) at doses equivalent with respect to the glucose-lowering effect.

EXPERIMENTAL DESIGN

Ten nondiabetic subjects (six males, four females; age, 36 +/- 7 yr; body mass index, 22.9 +/- 2.6 kg/m(2)) were studied on four randomized occasions with iv infusion of IDet (2 mU/kg . min for 4 h, followed by 4 mU/kg . min for 1 h) or HI (1 mU/kg . min for 4 h, followed by 2 mU/kg . min for 1 h) in euglycemia [plasma glucose (PG), 90 mg/dl] or during stepped hypoglycemia (PG, 90, 78, 66, 54, and 42 mg/dl).

RESULTS

PG was maintained at preselected plateaus, without any significant difference between IDet and HI (P > 0.2). Plasma insulin concentrations were on average approximately nine times greater with IDet than HI (749 +/- 52 vs. 83 +/- 19 muU/ml, respectively). Plasma IGF-I concentrations did not change from baseline during insulin infusion in euglycemia (IDet, 147 +/- 16 ng/ml; HI, 155 +/- 15 ng/ml) and hypoglycemia (IDet, 163 +/- 14 ng/ml; HI, 165 +/- 14 ng/ml) with no differences between the two insulins (P > 0.2). A similar pattern was observed for plasma IGFBP-3 levels. Insulin infusion resulted in a suppression of plasma IGFBP-1 concentrations with no differences between IDet (baseline, 16.6 +/- 3.8 ng/ml; endpoint, 2.0 +/- 0.6 ng/ml) and HI (baseline, 16.6 +/- 4.1 ng/ml; endpoint, 2.6 +/- 1.4 ng/ml) (P > 0.2) and study conditions (P > 0.2).

CONCLUSIONS

The greater plasma insulin concentrations obtained with IDet exert effects on plasma levels of IGF-I, IGFBP-1, and IGFBP-3 similar to those of HI. Additional studies are needed to confirm these short-term results in patients with diabetes mellitus on long-term treatment with IDet.

Insulin-like growth factor binding protein 1 as a novel specific marker of hepatic insulin sensitivity

BACKGROUND AND AIMS

The liver is the main source and insulin the main regulator of IGF binding protein 1 (IGFBP-1) in humans. Here we examined how serum IGFBP-1 concentrations are related to directly measured hepatic insulin sensitivity and liver fat content in humans.

METHODS

We measured fasting serum (fS) IGFBP-1 concentrations and liver fat content by proton magnetic resonance spectroscopy in 113 nondiabetic subjects. In addition, hepatic insulin sensitivity was measured using the euglycemic hyperinsulinemic clamp (insulin 0.3 mU/kg.min) technique in combination with the infusion of [3-(3)H]glucose in 78 subjects.

RESULTS

fS-IGFBP-1 concentrations were inversely related to liver fat content (r = -0.38, P < 0.0001). Of circulating parameters, fS-IGFBP-1 was better correlated to hepatic insulin sensitivity (r = 0.48, P < 0.0001) than fS-insulin (r = -0.42, P = 0.0001), fS-C-peptide (r = -0.41, P = 0.0002), fS-triglyceride (r = -0.33, P = 0.003), or fS-high-density lipoprotein cholesterol (r = 0.30, P = 0.007). In multiple linear regression analyses, body mass index (P < 0.0001) and fS-IGFBP-1 (P = 0.008), but neither age nor gender, were independently associated with hepatic insulin sensitivity (P < 0.0001 for ANOVA). Neither fS-insulin nor fS-C-peptide were independent determinants of hepatic insulin sensitivity after adjusting for age, gender, and body mass index.

CONCLUSIONS

fS-IGFBP-1 is inversely correlated with liver fat and is an obesity-independent and liver-specific circulating marker of hepatic insulin sensitivity.

Postprandial paradoxical IGFBP-1 response in obese patients with Type 2 diabetes

IGFs (insulin-like growth factors), which in an unbound form induce glucose and amino acid uptake, circulate bound to IGFBPs (IGF-binding proteins), which modulate their bioavailability and activity. The aim of the present study was to examine the effect of a standard meal [2301 kJ (550 kcal)] on the serum levels of IGFBP-1 in obese patients with T2DM (Type 2 diabetes mellitus), non-obese patients with T1DM (Type 1 diabetes mellitus) and healthy controls, using the artificial pancreas (Biostator®) to obtain a normal glycaemic response to the meal. IGFBP-1 levels decreased by 50% over 2 h following the meal at a similar clearance in both the healthy controls and patients with T1DM, but no significant decline was seen in the patients with T2DM, despite a several-fold increase in insulin levels. The patients with T2DM were also studied during Sandostatin® (somatostatin) infusion to decrease the inappropriate secretion of glucagon during the meal. During the 210 min of somatostatin infusion, the glucagon response was suppressed and IGFBP-1 levels were increased concomitantly with the peak in insulin levels, without any significant decrease after the meal. In conclusion, the impaired IGFBP-1 response to meal-related hyperinsulinaemia in obese patients with T2DM suggests a decreased availability of active IGF-1, leading to a decrease in glucose uptake during and after a meal in these patients. The stimulated meal response to glucagon, which contributes to postprandial hyperglycaemia, could not explain the increase in serum IGFBP-1 in these obese patients with T2DM.

Insulin-like growth factor-binding protein-1 in the prediction and development of type 2 diabetes in middle-aged Swedish men

AIMS/HYPOTHESIS

Insulin-like growth factor-binding protein-1 (IGFBP-1) production in the liver is inhibited by insulin, and low circulating levels are associated with the metabolic syndrome. The aim of this study was to evaluate the predictive role and change in IGFBP-1 concentrations during development of abnormal glucose regulation.

METHODS

IGFBP-1 levels were determined at baseline and at 10 years in an incident case-control prospective study of Swedish white men aged 35-56 years. Individuals with normal glucose tolerance at baseline who developed abnormal glucose tolerance during a 10 year period (n = 355) according to WHO criteria were pair-matched to controls for age and family history of diabetes.

RESULTS

Fasting IGFBP-1 concentrations were lower in individuals who later developed abnormal glucose regulation and correlated inversely with fasting proinsulin values (r = -0.48; p < 0.0001), and both were significant predictors. Individuals in the highest quartile at baseline for an algorithm incorporating fasting IGFBP-1, blood glucose, proinsulin and waist and height had a 40-fold increased risk of developing type 2 diabetes compared with the lowest quartile (95% CI 7.7-214). IGFBP-1 increased 32% (95% CI 17-49%) during the 10 years in those developing diabetes and was increased in relation to insulin levels, suggesting the emergence of hepatic insulin resistance. Moreover, elevated IGFBP-1 levels at follow-up were associated with higher 2 h glucose values during an OGTT.

CONCLUSIONS/INTERPRETATION

Low IGFBP-1 predicts the development of abnormal glucose regulation and, as an inhibitor of the insulin-like actions of insulin-like growth factors, elevated levels of IGFBP-1 after the development of diabetes may also play a pathophysiological role.

Altered response of insulin-like growth factor-binding protein 1 to nutritional deprivation in type 2 diabetes mellitus

Circulating insulin-like growth factor-binding protein 1 (IGFBP-1) normally has a close inverse relationship to insulin secretion, which results in a characteristic diurnal variation. However, in type 2 diabetes the correlation with insulin may be lost and IGFBP-1 concentrations relatively increased. The aim of this study was to determine the effect of nutritional deprivation on the diurnal patterns of IGFBP-1 regulation in type 2 diabetes mellitus. After a baseline assessment period, food intake was reduced over 48 hours to 627.6 kJ/d (150 kcal/d) for 72 hours and increased again over 24 hours to baseline (refeeding). Blood samples were taken at 2-hour intervals, for 24 hours in the baseline period, 48 hours during nutritional deprivation, and 24 hours during refeeding. Six individuals with type 2 diabetes were compared with 2 groups that were selected for normal fasting glucose and insulin levels and comprised 6 obese and 6 lean subjects. During energy (caloric) restriction, fasting insulin levels decreased to a similar extent in each study group. At baseline, IGFBP-1 concentrations were similar in each of the study groups and at the end of the period of energy (caloric) restriction the 6:00 AM fasting levels had increased by 144% in the obese control group and by 245% in the lean individuals (each P < .001). In the patients with type 2 diabetes there was a blunted increase in IGFBP-1 concentrations with nutritional deprivation by 33% compared with baseline. During refeeding after nutritional deprivation the IGFBP-1 response to insulin was restored in the individuals with diabetes. In conclusion, patients with type 2 diabetes mellitus have altered IGFBP-1 regulation, relating to impaired hepatic insulin sensitivity, which improves after a period of energy (caloric) restriction.

Insulin resistance and pre-eclampsia: a role for tumor necrosis factor-alpha?

Insulin resistance occurs in pre-eclampsia, but the cause is unknown. Furthermore, it is uncertain whether women destined to develop pre-eclampsia have a pre-existing insulin resistance or whether it is acquired with the development of the disease. We carried out this study to test the hypotheses that the increase in insulin resistance associated with pre-eclampsia is related to higher levels of tumor necrosis factor (TNF)-alpha, and that the increase in insulin resistance precedes the clinical onset of the disease. Fasting plasma samples were obtained from ten women who subsequently developed pre-eclampsia and ten normal pregnant controls at 16, 20, 24, 28, 32 and 36 weeks' gestation to measure circulating levels of insulin, glucose and TNF-alpha. Fasting insulin resistance index (FIRI) was calculated from insulin and glucose concentrations. In the normal controls, fasting insulin and TNF-alpha levels, and FIRI increased with gestation, and these were significantly greater than baseline values from 24, 28 and 28 weeks, respectively. In the group of women who developed pre-eclampsia, plasma levels of insulin and the FIRI were significantly higher than baseline from 20 and 24 weeks, respectively, but both were significantly higher than in the control group at 32 and 36 weeks. The increase in TNF-alpha in the pre-eclampsia group was significantly greater than in normal pregnant controls (p < 0.001). However, there was no significant association between TNF-alpha levels and FIRI in either normal pregnancy or pregnancies developing pre-eclampsia. These data suggest that insulin resistance in pre-eclampsia precedes the clinical onset of the disease, but that it is not related to elevated levels of TNF-alpha.

The relationship between insulin and insulin-like growth factor binding protein-1 is modified by pre-eclampsia

Insulin is the main negative regulator of insulin-like growth factor binding protein-1 (IGFBP-1) in the non-pregnant state. Although changes in insulin resistance and circulating level of IGFBP-1 occur in pre-eclampsia, little is known about the relationship between insulin and IGFBP-1 in pregnancies complicated by the disease. In this study, we have investigated whether the relationship between insulin and IGFBP-1 is modified by pre-eclampsia. Maternal levels of insulin and IGFBP-1 were measured, at 4-weekly intervals between 16 and 36 weeks' gestation, in plasma samples obtained from ten normal pregnant controls and ten women who developed pre-eclampsia. The controls were chosen to be similar in maternal age and booking body mass index to the pre-eclampsia group. Insulin levels increased in both the normal controls and the women who developed pre-eclampsia. The levels in pre-eclampsia were significantly greater than those in normal pregnancy at 32 and 36 weeks' gestation (p = 0.02 and 0.005, respectively). IGFBP-1 levels were unchanged in normal pregnancy and rose in pre-eclampsia. In normal pregnancy, insulin levels were inversely related to IGFBP-1 levels throughout. In women developing pre-eclampsia, the relationship between insulin and IGFBP-1 was negative at 16 weeks and positive from 24 weeks. These data suggest that whereas the inverse relationship between insulin and IGFBP-1 is maintained during normal pregnancy, this relationship is reversed in women who develop pre-eclampsia.

Leptin and insulin downregulate leptin receptor gene expression in chicken-derived leghorn male hepatoma cells

In chickens, leptin is expressed mainly in the liver, where its receptor gene expression has also been reported, and in adipose tissue. In view of the key role played by the liver in lipogenesis in avian species, the hepatic expression of leptin may have physiological significance. In this study, we showed that leptin is constitutively expressed and secreted in a chicken-derived hepatoma cell line (LMH). Although insulin regulates leptin expression in vivo, incubation of LMH cells in the presence of 100 nM insulin for 24 or 48 h had no effect on leptin expression or its secretion in the culture medium. In addition, we developed a specific chicken leptin receptor real-time reverse transcription (RT)-PCR, and downregulation of leptin receptor gene expression by homologous and heterologous signals was demonstrated, as relative leptin receptor mRNA levels were significantly decreased after exposure of LMH cells to recombinant chicken leptin or porcine insulin. In conclusion, our results indicate that leptin is probably able to desensitize its own response in the chicken liver. Finally, the ability of insulin and leptin to regulate chicken leptin receptor gene expression suggests a direct role of leptin in the control of hepatic metabolism.

Vascular endothelial function and blood pressure homeostasis in mice overexpressing IGF binding protein-1

IGFs and their binding proteins (IGFBPs) play a significant role in metabolic regulation, and there is growing evidence that they also exert important vascular effects. IGFBP-1 contributes to glucose counterregulation, and observational studies demonstrate an inverse association between circulating IGFBP-1 levels and cardiovascular risk factors. Furthermore, IGFBP-1 levels are lower in subjects with overt macrovascular disease. We therefore hypothesized that IGFBP-1 exerts potentially beneficial effects, either directly or indirectly, on blood pressure regulation and vascular function. We tested this hypothesis using a unique transgenic mouse, which overexpresses human IGFBP-1, and explored the effect of this protein on metabolic, blood pressure, and vascular homeostasis. IGFBP-1-overexpressing mice exhibited postprandial hyperinsulinemia with preservation of glucocompetence and insulin sensitivity. Blood pressure was unchanged in the fasting state but was significantly lower in transgenic mice after a carbohydrate load. Aortic rings from IGFBP-1-overexpressing mice were hypocontractile in response to vasoconstrictors, and relaxation responses were unimpaired. Basal nitric oxide production was increased and endothelial nitric oxide synthase mRNA expression upregulated in aortae of these mice. Our data suggest that IGFBP-1 plays an important and potentially beneficial role in regulating metabolic and vascular homeostasis.

A wholly nutritional 'multifocal angiostatic therapy' for control of disseminated cancer

A great deal of effort is now being devoted to the development of new drugs that hopefully will control the spread of inoperable cancer by safely inhibiting tumor-evoked angiogenesis. However, there is growing evidence that certain practical nutritional measures have the potential to slow tumor angiogenesis, and it is reasonable to anticipate that, by combining several measures that work in distinct but complementary ways to impede the angiogenic process, a clinically useful 'multifocal angiostatic therapy' (MAT) might be devised. Several measures which might reasonably be included in such a protocol are discussed below, and include: a low-fat, low-glycemic index vegan diet, which may down-regulate the systemic IGF-I activity that supports angiogenesis; supplemental omega-3-rich fish oil, which has been shown to inhibit endothelial expression of Flk-1, a functionally crucial receptor for VEGF, and also can suppress tumor production of pro-angiogenic eicosanoids; high-dose selenium, which has recently been shown to inhibit tumor production of VEGF; green tea polyphenols, which can suppress endothelial responsiveness to both VEGF and fibroblast growth factor; and high-dose glycine, whose recently reported angiostatic activity may reflect inhibition of endothelial cell mitosis, possibly mediated by activation of glycine-gated chloride channels. In light of evidence that tumor-evoked angiogenesis has a high requirement for copper, copper depletion may have exceptional potential as an angiostatic measure, and is most efficiently achieved with the copper-chelating drug tetrathiomolybdate. If logistical difficulties make it difficult to acquire this experimental drug, high-dose zinc supplementation can achieve a slower depletion of the body's copper pool, and in any case can be used as maintenance therapy to maintain an adequate level of copper depletion. A provisional protocol is offered for a nutritionally based MAT entailing a vegan diet and supplemental intakes of fish oil, selenium, green tea polyphenols, glycine, and zinc. Inasmuch as cox-2 is overexpressed in many cancers, and cAMP can boost tumor production of various angiogenic factors as well as autogenous growth factors, adjunctive use of cox-2-specific NSAIDS may be warranted in some cases.

A low-fat, whole-food vegan diet, as well as other strategies that down-regulate IGF-I activity, may slow the human aging process

A considerable amount of evidence is consistent with the proposition that systemic IGF-I activity acts as pacesetter in the aging process. A reduction in IGF-I activity is the common characteristic of rodents whose maximal lifespan has been increased by a wide range of genetic or dietary measures, including caloric restriction. The lifespans of breeds of dogs and strains of rats tend to be inversely proportional to their mature weight and IGF-I levels. The link between IGF-I and aging appears to be evolutionarily conserved; in worms and flies, lifespan is increased by reduction-of-function mutations in signaling intermediates homologous to those which mediate insulin/IGF-I activity in mammals. The fact that an increase in IGF-I activity plays a key role in the induction of sexual maturity, is consistent with a broader role for-IGF-I in aging regulation. If down-regulation of IGF-I activity could indeed slow aging in humans, a range of practical measures for achieving this may be at hand. These include a low-fat, whole-food, vegan diet, exercise training, soluble fiber, insulin sensitizers, appetite suppressants, and agents such as flax lignans, oral estrogen, or tamoxifen that decrease hepatic synthesis of IGF-I. Many of these measures would also be expected to decrease risk for common age-related diseases. Regimens combining several of these approaches might have a sufficient impact on IGF-I activity to achieve a useful retardation of the aging process. However, in light of the fact that IGF-I promotes endothelial production of nitric oxide and may be of especial importance to cerebrovascular health, additional measures for stroke prevention-most notably salt restriction-may be advisable when attempting to down-regulate IGF-I activity as a pro-longevity strategy.

Insulin's stimulation of endothelial superoxide generation may reflect up-regulation of isoprenyl transferase activity that promotes rac translocation

Recent research demonstrates that statin drugs exert a number of favorable effects on endothelial function, independent of lipid modulation, that appear to be mediated by a partial inhibition of prenylation reactions. Statin-induced suppression of PKC-evoked superoxide production may be attributable to an inhibition of rac prenylation and thus translocation that impedes activation of the membrane-bound NAD(P)H oxidase. Conversely, it is now known that hyperinsulinemia up-regulates prenylation reactions by boosting the activities of isoprenyl transferases. In light of new evidence that hyperinsulinemia stimulates endothelial superoxide production via NAD(P)H oxidase, it is tempting to conclude that up-regulation of rac prenylation is at least partially responsible for this phenomenon. In patients afflicted with insulin resistance syndrome, this adverse impact of hyperinsulinemia may be exacerbated by an excessive free fatty acid flux that activates endothelial PKC - another stimulant of the NAD(P)H oxidase - while impeding insulin-mediated activation of nitric oxide synthase. The resulting imbalance of endothelial nitric oxide and superoxide production may be responsible for much of the excess vascular risk associated with this syndrome.

Changes in the IGF-IGFBP axis in critical illness

In critical illness, serum concentrations of the growth hormone-dependent complexes containing insulin-like growth factors I and II are decreased. This is initially due to a transient growth hormone resistance, but in the longer term, the less pulsatile pattern of growth hormone secretion may be a major factor since only pulsatile growth hormone increases the levels of insulin-like growth factor-I and the acid-labile subunit. Other factors contributing to a low insulin-like growth factor level are nutritional deficiency and the direct effects of inflammatory cytokines. The growth hormone-independent proteins IGFBP-2, IGFBP-4 and IGFBP-6 increase in critical illness, suggesting a redistribution of insulin-like growth factors from growth hormone-dependent ternary complexes with IGFBP-3 and IGFBP-5 to binary complexes with these binding proteins, which might facilitate transport to the tissues. IGFBP-1, which is acutely regulated by metabolic status, is elevated on admission to intensive care but may fall in response to nutritional support. Initial evidence suggests that the level of IGFBP-1 may be predictive of outcome in critically ill patients, suggesting a possible prognostic role for this protein.

Modulation of adipocyte lipoprotein lipase expression as a strategy for preventing or treating visceral obesity

As compared to subcutaneous adipocytes, visceral adipocytes have high basal lipolysis, are highly sensitive to catecholamines, and are poorly sensitive to insulin; these traits are amplified when visceral adipocytes hypertrophy. As a result, enlarged visceral fat stores tend to flood the portal circulation with free fatty acids at metabolically inappropriate times when fatty acids are unlikely to be oxidized, thus exposing tissues to excessive free fatty acid levels and giving rise to the insulin resistance syndrome. A logical approach to preventing or correcting visceral obesity is to down-regulate the lipoprotein lipase (LPL) activity of visceral adipocytes relative to that expressed in subcutaneous adipocytes and skeletal muscle. IGF-I activity appears to be a primary determinant of visceral LPL activity in humans; systemic IGF-I activity is decreased when diurnal insulin secretion is low, when hepatocytes detect a relative paucity of certain essential amino acids, and when estrogens are administered orally. The ability of alpha-glucosidase inhibitor therapy to selectively reduce visceral adiposity suggests that down-regulation of diurnal insulin secretion and/or IGF-I activity may indeed have a greater impact on LPL activity in visceral fat than in subcutaneous fat. Thus, low-glycemic-index, vegan, high-protein, or hypocaloric diets can be expected to decrease visceral LPL activity, as can postmenopausal estrogen therapy. Furthermore, estrogen enhances the LPL activity of non-pathogenic gluteofemoral fat cells, whereas testosterone decreases visceral LPL activity in men; this may explain why sex hormone replacement in middle-aged people of both sexes has a favorable impact on visceral fat and insulin sensitivity. Beta-adrenergic activity suppresses transcription of LPL in adipocytes; this phenomenon may contribute to the favorable impact of exercise training on visceral obesity; conceivably, preadministration of safe drugs that boost catecholamine activity (caffeine, yohimbine) could potentiate this beneficial effect of exercise. Glucocorticoids selectively increase the LPL activity of visceral adipocytes; while there is currently no convincing evidence that psychological stress is a major determinant of visceral adiposity, or that stress management techniques can help to correct visceral obesity, reports that anxiolytic therapy can improve glycemic control in type 2 diabetes should encourage further research along these lines.

Mortality from Western cancers rose dramatically among African-Americans during the 20th century: are dietary animal products to blame?

Statistics compiled by the National Cancer Institute indicate that, between 1935 and 1974, age-adjusted mortality from most 'Western' cancers (those of the breast, colon, prostate, pancreas, ovary, and kidney) rose dramatically in African-Americans. This phenomenon is paralleled by marked increases in the incidence of these cancers in Asia and Southern Europe during the latter 20th century, in conjunction with increased intakes of dietary animal products. A credible case can be made that diets rich in animal products work in various complementary ways to up-regulate serum levels of insulin, free IGF-I, and free sex hormones: hormones that appear to have important promotional activity for Western cancers. It seems likely that dietary animal product intake by black Americans increased substantially during the 20th century, and that this fact is primarily responsible for their concurrent marked increase in mortality from Western cancers. A whole-food vegan diet rich in fruits and vegetables, especially if coupled with regular exercise and smoking avoidance, could be expected to have a remarkably positive impact on African-American cancer risk, reversing the increases in cancer risk incurred during the 20th century.

The role of IGF-binding proteins in mediating the effects of recombinant human IGF-I on insulin requirements in type 1 diabetes mellitus

To determine the role of IGF-binding proteins in mediating the direct effects of recombinant human IGF-I on insulin requirements in type 1(insulin-dependent) diabetes mellitus, overnight changes in IGF-I, IGF-II, and IGF-binding protein-1, -2, and -3, collected under euglycemic conditions, were compared in nine subjects after double blind, randomized, sc administration of recombinant human IGF-I (40 microg/kg) or placebo at 1800 h. On both nights a somatostatin analog infusion (300 ng/kg x h) suppressed endogenous GH production, and three timed discrete GH pulses (total, 0.029 IU/kg x night) ensured identical GH levels. After recombinant human IGF-I administration, IGF-I levels and the IGF-I/IGF-binding protein-3 ratio increased [mean +/- SEM:IGF-I, 401 +/- 22 ng/ml; placebo, 256 +/- 20 ng/ml (P = 0.0002); IGF-I, 0.108 +/- 0.006; placebo, 0.074 +/- 0.004 (P = 0.0003), respectively], and insulin requirements decreased (IGF-I, 0.12 +/- 0.03; placebo, 0.23 +/- 0.03 U/kg x min; P = 0.008). The normal within-individual inverse relationships between insulin and IGF-binding protein-1 levels were observed (lag time 2 h: r = -0.34; P < 0.01). Yet despite reduced free insulin levels (8.5 +/- 1.5; placebo, 12.2 +/- 1.2 mU/liter; P = 0.03), IGF-binding protein-1 levels were reduced after recombinant human IGF-I administration (53.7 +/- 6.8; placebo, 82.2 +/- 11.8 ng/ml; P = 0.008). The largest reductions in free insulin levels after recombinant human IGF-I and thus putative improvement in insulin sensitivity occurred in subjects with the smallest increase in the plasma IGF-I/IGF-binding protein-3 ratio (r = 0.7; P = 0.03). Taken together, these data are consistent with the hypothesis that transcapillary movement of IGF-I (perhaps mediated by IGF-binding protein-1), out of the circulation facilitates altered insulin sensitivity. These data have important implications for risk-benefit assessment of recombinant human IGF-I therapy in type 1 diabetes mellitus.

Up-regulation of hepatic IGFBP-1 production as a strategy for preventing benign prostatic hyperplasia

Many lines of evidence point to a prominent role for excess production and activity of stroma-derived, androgen-induced IGF-II in the stromal and epithelial hyperplasia characteristic of benign prostatic hyperplasia (BPH). Increased stromal expression of the type I IGF receptor, as well as altered local production of IGF binding proteins, appear to contribute to this increase in IGF activity. Systemic IGFBP-1, primarily of hepatic origin, is a functional antagonist of IGF-II; thus, boosting IGFBP-1 production might be expected to lessen risk for BPH. Minimizing diurnal insulin secretion, and possibly avoiding intake of animal proteins over-rich in essential amino acids, are practical strategies for increasing hepatic IGFBP-1 synthesis. This hypothesis may rationalize recent evidence that exercise and moderate alcohol consumption decrease risk for BPH, whereas heavy smoking increases this risk. A clinical impression that BPH is becoming more common in Japan, and evidence that Japanese making frequent use of meat and dairy products are at increased risk for this disorder, also appear consistent with this view.

Growth hormone: a promising treatment for the failing heart?

Many pathologic processes that accelerate the progression of heart failure, such as cardiac remodeling and impaired contractility, may be modulated by administration of recombinant growth hormone. The agent improves structural and functional aspects of the failing heart both in the short term and after several months of therapy. However, conflicting clinical results cast doubt on whether it has a clear benefit in all of these patients. In addition, growth hormone therapy may be associated with cardiac and noncardiac adverse effects. Many questions must be addressed before its place in heart failure therapy is established. Optimal patient population, dosing regimen, duration of therapy, and effect on patient survival are unknown. Until larger, blinded studies are completed, growth hormone therapy remains an investigational approach to managing refractory heart failure.

Effects of homologous pituitary hormone treatment on serum insulin-like growth-factor-binding proteins (IGFBPs) in hypophysectomized tilapia, Oreochromis mossambicus, with special reference to a novel 20-kDa IGFBP

In the circulation, insulin-like growth factors (IGFs) bind to high-affinity-binding proteins. Insulin-like growth-factor-binding proteins (IGFBPs) appear to be present in all vertebrates. To examine the hormonal regulation of serum IGFBPs in a fish, tilapia (Oreochromis mossambicus) were hypophysectomized (Hx) and then treated with homologous tilapia growth hormone (tGH) or either form of tilapia prolactin (tPRL177, tPRL188). Hormones were administered at three doses: 15, 150, and 500 ng/g of body weight. Serum IGFBP profiles were analyzed by SDS-PAGE and Western ligand blotting using 125I-rhIGF-I as a probe. A prominent IGFBP (ca 20 kDa), termed IGFBP-20K, appeared after hypophysectomy. Administration of tGH at all dose levels suppressed this BP and restored levels back to those seen in sham-operated control fish. tPRL177 and tPRL188 were also effective in lowering IGFBP-20K levels. Levels of the 29-kDa IGFBP (termed IGFBP-29K) increased after hypophysectomy; tGH at all doses and tPRL177 at the two lower doses further increased these levels. All doses of tGH, tPRL177, and tPRL188 significantly increased levels of the 32-kDa IGFBP (termed IGFBP-32K). Hypophysectomy significantly lowered levels of the 40-kDa IGFBP (termed IGFBP-40K) below levels seen in the sham-operated controls. tGH treatment significantly raised IGFBP-40K levels at all doses examined, but not to the levels seen in intact tilapia. The 42-kDa IGFBP (termed IGFBP-42K) was not affected by hypophysectomy or hormone replacement. Our data suggest that the novel 20-kDa IGFBP and the 40-kDa IGFBP species may be similar in function to mammalian IGFBP-1 and IGFBP-3, respectively.

Growth hormone suppression of insulin-like growth factor binding protein-1 promoter activity

Hepatic transcription of insulin-like growth factor binding protein-1 (IGFBP-1) is rapidly downregulated by growth hormone (GH) which is also known to induce expression of c-fos and c-jun. Co-expression of c-fos or c-jun in rat hepatocytes, individually or together, suppresses IGFBP-1 promoter activity by approximately 60%. When hepatic nuclear extracts from sham-operated, hypophysectomized (hypox) and GH-treated hypox rats were analyzed by DNase-1 footprinting, differences in the protection pattern were identified in three regions of the IGFBP-1 promoter. F1 corresponding to - 660 to - 640 bp showed acute changes in response to GH administration. In additional regions, F2 and F3, representing - 758 to - 748 bp and - 477 to - 447 bp, respectively, differences were apparent between nuclear extracts from the hypox and sham-operated rats. When F1 and F2 were removed by deletion of the region from - 824 to - 557 bp, the GH response was lost but suppression by co-expression of c-fos and c-jun was preserved. A putative AP-1 binding site was present in the F3 footprint region, however removal of F3 did not affect the GH responsiveness. These data indicate that several distinct sequences, other than the putative AP-1 site are involved in mediating the GH effects on IGFBP-1 transcription.

Insulin-like growth factor-I effect on chicken hepatoma cells (LMH) is inhibited by endogenous IGF-binding proteins

LMH chicken hepatoma cells show type 1 IGF receptors and a 28 kDa IGF-binding protein (IGFBP) on their membranes. They also secrete large amounts of the 28 kDa IGFBP. Following overnight incubation in serum-free medium, human IGF-I was markedly less effective than insulin in stimulating amino acid (AIB) uptake. Chicken and human IGF-I were equipotent, consistent with their equipotency in inhibiting [125I]IGF-I binding to wheat germ agglutinin-purified IGF receptors or membrane solubilized IGFBP. When cells were supplied with fresh medium, cell-associated IGFBP were unaffected, but the level of soluble IGFBP was largely reduced. This potentiated the effect of IGF-I on AIB uptake. The effect of chicken Long-[Arg3]-IGF-I, which exhibited low affinity for the IGFBP, was unchanged. In fresh or conditioned medium, this analog was more potent than IGF-I, suggesting that both soluble and membrane-bound 28 kDa IGFBP inhibited the effect of IGF-I.

Insulin-like growth factor binding protein-1 induces insulin release in the rat

Injections of human insulin-like growth factor binding protein (hIGFBP-1) are reported to induce hyperglycemia in the rat, suggesting that IGFBP-1 acutely regulates glucose homeostasis. We now report the effects on glucose and insulin levels of administering recombinant (r) hIGFBP-1. In a series of studies, normal and streptozotocin (STZ) diabetic male Wistar rats (180-210 g), fasted for 6 or 16 h, were injected with rhIGFBP-1 (i.v., 80-500 microg/rat). rhIGFBP-1 did not affect blood glucose acutely but did stimulate insulin release in normal rats (5 min post injection; PBS, 103.5 +/- 8.5; rhIGFBP-1 (500 microg), 166.8 +/- 15.7; rhIGFBP-1 (100 microg); 151.4 +/- 14.1% initial). rhIGFBP-1 pretreatment, in normal and diabetic rats, reduced the hypoglycemic response to rhIGF-I (diabetic rats after 20 min: PBS, 103.4 +/- 11.4; BP-1 (500 microg) +/- rhIGF-I (50 microg), 97.6 +/- 3.6; rhIGF-I, 48.2 +/- 4.3% initial) but did not affect the hypoglycemic response to des(1-3)IGF-I or insulin (0.5 U/kg). These studies show that rhIGFBP-1 causes insulin release, has a minimal effect on blood glucose, and inhibits the hypoglycemic effect of rhIGF-I. These data suggest that endogenous IGF-I tonically suppresses insulin secretion and imply that aberrant IGFBP levels or reduced IGF-I bioactivity may lead to chronic hyperinsulinemia.

Up-regulation of IGF binding protein-1 as an anticarcinogenic strategy: relevance to caloric restriction, exercise, and insulin sensitivity

The mitotic rate of stem cells is a major determinant of cancer risk. Insulin-like growth factors (IGFs) are virtually obligate stimulants of cell turnover in nearly every tissue. IGF activity is subject to rapid modulation by hepatic release of IGF binding protein-1 (IGFBP-1), a factor whose synthesis is suppressed by insulin and increased by glucagon. Up-regulation of IGFBP-1 production can be expected to decrease IGF activity and thereby diminish cancer risk. Measures that sensitize peripheral tissues to insulin, and thereby down-regulate insulin secretion, can be expected to increase IGFBP-1 synthesis, provided that they do not unduly sensitize hepatocytes as well. Prolonged aerobic exercise and caloric restriction also increase IGFBP-1 production. Since IGF-1 suppresses hepatic synthesis of sex hormone binding globulin (SHBG), down-regulation of IGF activity will increase SHBG levels and thus diminish the availability of free sex hormones--an effect that should further decrease cancer risk in sex hormone-responsive tissues. These considerations rationalize many findings in animal and epidemiologic studies, and suggest that non-diabetic insulin resistance may be a significant cancer risk factor. Increased IGF activity associated with insulin resistance may also promote benign hyperplasias-most notably atherosclerosis. Hyperinsulinemia stimulates intimal hyperplasia indirectly, via IGF.

Purification and characterization of the insulin-like growth factor-binding protein-1 phosphoform found in normal plasma

Our previous work has shown that, in the normal circulation, insulin-like growth factor-binding protein-1 (IGFBP-1) is present as a single highly phosphorylated species. In this study, we have purified this previously uncharacterized isoform of IGFBP-1 to determine its ligand-binding affinity and the potential significance of highly phosphorylated IGFBP-I. Immunoaffinity chromatography was used to isolate IGFBP-1 from normal human plasma and from human hepatoma (Hep G2) cell medium as an alternative source of the IGFBP-1 phosphoform in the circulation. The affinity of this highly phosphorylated IGFBP-1 was compared with that of nonphosphorylated IGFBP-1 and recombinant human (rh) IGFBP-3 by equilibrium binding to IGF-II and IGF-II. Anion exchange (IEX) HPLC, nondenaturing electrophoresis, alkaline phosphatase treatment, and ligand-binding studies indicated that the highly phosphorylated IGFBP-1 from HepG2 cells was comparable with IGFBP-1 from plasma. In binding to IGF-I, the plasma phosphoform of IGFBP-1 was found to have a higher affinity (2.3 +/- 1.1 x 10(10) M-1) than nonphosphorylated IGFBP-1 (2.5 +/- 1.7 x 10(9) M-1, P < 0.002). However, when binding to IGF-II, phosphorylation had no affect on the affinity of IGFBP-1 (3.6 +/- 2 x 10(9) M-1 vs. 1.8 +/- 3 x 10(9) M-1, P not significant). Therefore, in the circulation, IGF-I has a considerably higher affinity than IGF-II for IGFBP-1 (P < 0.02). The affinity of phosphorylated IGFBP-1 from plasma (2.3 +/- 1.1 x 10(10) M-1) also was significantly higher than the affinity of IGFBP-3 for IGF-I (5.6 +/- 4.2 x 10(9) M-1, P < 0.005). These data suggest that the highly phosphorylated IGFBP-1 in the normal circulation will preferentially bind IGF-I rather than IGF-II, whereas in pregnancy, the affinity of IGFBP-1 for IGF-I will be reduced because of the appearance of non- and lesser-phosphorylated forms. This lends support to the theory that changes in IGFBP-1 phosphorylation may influence the modulatory effects of IGFBP-1 on IGF bioavailability.

Impaired glucose homeostasis in insulin-like growth factor binding protein-1 transgenic mice

Transgenic mice that overexpressed IGFBP-1 are hyperinsulinemic in the first week of life and gradually develop fasting hyperglycemia. In adult transgenic mice, the hypoglycemic response to IGF-I but not insulin or des (1-3) IGF-I was attenuated (P < 0.05) compared with wild-type mice. Furthermore, in isolated adipocytes from transgenic mice, the stimulatory effect of IGF-I but not insulin on 2-deoxy-[3H]-glucose uptake was reduced (P < 0.02). In contrast, in isolated soleus muscle, the effects of both IGF-I and insulin on 2-deoxy-3H-glucose uptake and on [3H]-glucose incorporation into glycogen were significantly reduced compared to wild-type mice. The decline in specific activity of the 2-deoxy-3H-glucose, a measure of glucose appearance in the circulation, was more marked in transgenic animals (P < 0.05). In addition, tissue uptake of glucose was significantly higher in diaphragm, heart, intestine, liver, soleus muscle, and adipose tissue from fasting transgenic mice. Plasma concentrations of alanine, lysine, and methionine were also elevated in transgenic mice. These data suggest that overexpression of IGFBP-1 attenuates the hypoglycemic effect of endogenous IGF-I, which is initially compensated for by enhanced pancreatic insulin production. However, in adult mice pancreatic insulin content is reduced, insulin resistance is demonstrable in skeletal muscle and fasting hyperglycemia develops.

Insulin-like growth factor bioactivity and its modification in growth hormone resistant states

Acquired growth hormone (GH) resistance is an increasingly recognized feature of catabolic states. Low circulating levels of the insulin-like growth factors (IGF-I and II) have been shown to be associated with changes in the IGF binding proteins (IGFBP-1 to -6) that may significantly impact on IGF bioactivity. IGFBP-3 binds IGF and a third glycoprotein, the acid labile subunit (ALS), to form a stable 150 kDa ternary complex that serves as an intravascular store for IGFs and prolongs IGF half-life. IGFBP-1 is present at much lower concentration in serum but levels fluctuate acutely, suggesting regulation of IGF bioactivity in response to short-term metabolic changes. The function of IGFBP-2 remains unclear, but studies suggest that this protein may act as an alternative carrier for IGF when IGFBP-3 levels are low. Multiple regulatory influences on circulating IGFBP levels have been identified but three appear prominent. Nutritional influences, in particular substrate availability, appear to be a central regulatory influence on IGFBP levels in catabolic states. Low substrate availability increases IGFBP-1 levels acutely and decreases IGFBP-3 and IGFBP-2 levels in the intermediate term, with each of these changes likely to further limit IGF bioactivity. End organ failure, particularly of liver and kidney significantly affects production and clearance rates of the circulating IGFBPs and may contribute to the catabolism frequently seen in these states. Severe protein catabolism often accompanies malignancy and chronic sepsis and it is likely that additional ill-defined factors influence IGF bioactivity in this setting. Recent studies have identified post-translational modifications to the IGFBPs such as proteolysis and phosphorylation, which appear to further impact on IGF bioactivity. The relative contributions of these changes to the overall impairment of IGF bioactivity in GH-resistant states remains to be fully elucidated.

Interaction of insulin, glucocorticoids, and protein kinase C in the regulation of insulin-like growth factor-binding protein-1 production by H4IIE rat hepatoma cells

A sensitive RIA was used to examine regulation of IGFBP-1 in H4IIE rat hepatoma cells. IGFBP-1 was stimulated up to tenfold by dexamethasone and corticosterone, and this stimulation was abolished by RU486. The effect of dexamethasone increased with time in culture. Phorbol 12-myristate 13-acetate (PMA) stimulated IGFBP-1 up to fourfold with a maximal effect in short-term culture. Dexamethasone and PMA were additive in stimulating IGFBP-1. Under basal conditions IGFBP-1 production was linearly related to cell density: however, stimulation by dexamethasone was greatest in confluent cells, and PMA had a greater effect in sparse cultures. Insulin inhibited IGFBP-1 up to 80%, and this effect diminished with time in culture but was unaffected by cell density. Dexamethasone was stimulatory in the presence of a maximal inhibitory concentration of insulin, and insulin was inhibitory in the presence of maximal dexamethasone from 3-48 h in culture, regardless of cell density. PMA abolished the inhibitory action of insulin on IGFBP-1 secretion and mRNA expression during incubation periods of less than 4 h and not during longer incubations. PMA did not influence the stability of IGFBP-1 mRNA. We conclude that, in rat H4IIE cells, dexamethasone and PMA stimulate IGFBP-1 by independent mechanisms and speculate that when protein kinase C is activated the inhibitory action of insulin is blocked.

Insulin-like growth factor binding proteins as glucoregulators

Circulating insulin-like growth factors (IGFs) represent an important pool of potential hypoglycemic activity, which is largely inhibited by their sequestration in a heterotrimeric complex comprising growth factor, IGF-binding protein-3 (IGFBP-3), and acid-labile subunit (ALS). Less than 1% of total IGFs circulate in the free form, yet even this amount might contribute significantly to circulating insulin-like activity. The ternary binding protein complex appears to inhibit insulin-like activity of bound IGFs by preventing their egress from the circulation. Although the integrity of this complex might be affected by limited proteolysis of IGFBP-3 in pregnancy and catabolic conditions, the evidence that this increases IGF bioavailability, and thus hypoglycemic potential, is as yet unclear. However, in patients with IGF-II-secreting tumors, hypoglycemia may result from a failure of the ternary complex to adequately sequester the IGFs. Improvement in complex formation, by treatment with corticosteroids or growth hormone, alleviates the hypoglycemia, even if (as seen with growth hormone treatment) IGF-II hypersecretion persists. In these patients, blood glucose levels are inversely correlated with IGFBP-2 levels, suggesting that this protein might play a part in transporting IGFs to their target tissues. Conversely, ALS levels correlate positively with blood glucose, emphasizing the importance of the ternary complex in preventing hypoglycemia. Unlike the other IGF-binding proteins, IGFBP-1 is acutely regulated in the circulation, in a manner consistent with its acting as a glucose counterregulator. It might act in this way by inhibiting the activity of free IGFs in the circulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Diurnal variation of sex hormone binding globulin and insulin-like growth factor binding protein-1 in women with polycystic ovary syndrome

OBJECTIVES

The aim of this study was to examine (1) the diurnal variation in SHBG and (2) the inter-relationships of insulin, IGF-I, SHBG and IGFBP-1 over 24 hours in 10 women with anovulatory PCOS and compare them with weight-matched ovulatory controls.

PATIENTS AND METHODS

The two groups comprised 10 anovulatory women with PCOS (as defined by clinical, ultrasound and biochemical criteria) and 10 weight matched controls. Serum samples were taken at two-hourly intervals for 24 hours and stored for measurement of SHBG, IGFBP-1, insulin and IGF-I. Differences between the groups were compared using the Wilcoxon ranked paired tests of the individual peak and trough concentrations in each group. The variation in insulin, IGFBP-1 and SHBG concentrations over 24 hours was tested using two-way analysis of variance with the factors time and subject. Spearman's correlation coefficient was calculated from the subjects' median value over 24 hours.

RESULTS

The median (interquartile range) body mass index (BMI) was 25.2 (22.2-29.3) in the PCOS group and 24.3 (23.2-25.7) kg/m2 in the control group. Serum testosterone (T) and LH levels were significantly raised in the PCOS group compared to the control group; T 3.8 (2.9-5.6) vs 1.9 (1.9-2.5) nmol/l (P < 0.007) and LH 12 (10-15) vs 4.1 (3.6-4.5) IU/I (P < 0.005) respectively. There was no diurnal variation in SHBG. The median (interquartile ranges) of the peak SHBG concentrations was lower in the PCOS group: 29.4 (14.9-39.4) vs 52.1 (39.4-61) nmol/l in the control group (P < 0.01). The fasting levels of insulin at 0600 h (median (interquartile ranges)) were not significantly different between the groups; 6.6 (5.4-9.8) and 6.2 (1.9-7.6) mU/l, respectively, although the peak median concentrations were significantly different; PCOS 66.1 (50.9-129.2) vs 40 (36.1-74.2) mU/l (P < 0.05). Two-way analysis of variance showed a diurnal variation in insulin concentrations in the control group (P = 0.001) but not in the PCOS group (P = 0.1). The diurnal variation in IGFBP-1 was similar in the two groups but the peak median levels were lower in the women with PCOS 54.9 (22.3-79.2) vs 71.5 (60.5-99.3) micrograms/l (P < 0.03). The decline in IGFBP-1 concentrations correlated with the increase in insulin concentrations. The IGF-I concentrations were similar in the two groups. There was a significant negative correlation between SHBG and insulin (P < 0.05) and between insulin and IGFBP-1 (P < 0.01).

CONCLUSION

This study demonstrates that there is no diurnal variation in SHBG concentrations and confirms the finding of a marked diurnal variation in the concentration of IGFBP-1. Women with PCOS who are anovulatory have an abnormal pattern of insulin secretion with an absence of diurnal variation compared to weight matched controls. This provides further evidence of the relative insulin resistance which is independent of weight found in women with anovulatory PCOS. The inverse correlations of insulin concentrations with SHBG and IGFBP-1 support the role of insulin as a possible regulator of the circulating levels of these binding proteins although the difference in the time course of their response makes it unlikely that they are co-regulated.

Phenotypic manifestations of insulin-like growth factor binding protein-1 (IGFBP-1) and IGFBP-3 overexpression in transgenic mice

To provide further insight into the function of the IGFBPs, transgenic (Tg) mice which overexpressed IGFBP-1 and IGFBP-3 were generated. In this report we have compared the phenotypic manifestations observed in these Tg mice. The IGFBP-1 Tg mice were significantly smaller at birth, birth weight and gained less weight in the postnatal period. Organ weight was proportionately reduced relative to body weight in most organs. However the brain was markedly smaller in IGFBP-1 Tg mice. Mean plasma levels of Tg-derived IGFBP-1 ranged from 8 to 80 ng ml-1 in the different groups of IGFBP-1 Tg mice. In addition homozygous mice also demonstrated fasting hyperglycemia, impaired glucose tolerance and reduced fecundity. Two of the seven IGFBP-3 founders had measurable levels of hlGFBP-3 in the circulation and were bred to homozygosity. Maximal plasma levels of transgene-derived IGFBP-3 were 72-198 ng ml-1. Transgene expression was detected in the kidney, small intestine and colon by Northern blot analysis. The birth weight, litter size and body weight of IGFBP-3 Tg mice were not significantly different from wild-type mice. However, the spleen, liver and heart of IGFBP-3 Tg mice derived from both founders were significantly heavier compared with organs from wild-type mice. The relative weight of other organs such as the brain, kidney and lungs were similar to wild-type mice. From these data, we conclude that over expression of IGFBP-1 results in inhibition of IGF action and in profound impairment of brain development, modest inhibition of fetal and postnatal growth and inhibition of the metabolic effects of the IGFs. In contrast, modest over-expression of hlGFBP-3 has little effect other than some selective organomegaly.

Role of the insulin-like growth factors in the endocrine control of glucose homeostasis

There is a growing body of evidence that the insulin-like growth factors (IGF-I and IGF-II) are dynamically involved in the regulation of glucose homeostasis, with one of their binding proteins, IGFBP-1, playing a counterregulatory role. The IGFs are structurally and functionally related to insulin and in the circulation they represent a huge hypoglycemic potential which is buffered by their association with the IGFBPs. The predominant IGFBP in serum, IGFBP-3, is able to form a high molecular weight complex with the IGFs and this complex is retained in the circulation and appears to act as a reservoir of IGFs. The IGFs and IGFBP-3 are regulated in the long term by changes in nutritional status. In contrast, IGFBP-1 is acutely regulated in a manner similar to glucose counterregulatory hormones. IGFBP-1 is able to block the insulin-like actions of the circulating IGFs and when administered alone as a bolus infusion causes an increase in blood glucose levels. There is recent evidence that more IGFs are available for an endocrine glucoregulatory role than indicated by estimates of steady-state 'free' IGF levels. The IGF/IGFBP system may thus play a complementary role to insulin and the classical counterregulatory hormones in the control of blood glucose.

Response of sex hormone binding globulin and insulin-like growth factor binding protein-1 to an oral glucose tolerance test in obese women with polycystic ovary syndrome before and after calorie restriction

OBJECTIVE

We determined the relationship of short-term changes in circulating insulin concentrations, resulting from an oral glucose load, to those in both sex hormone binding globulin (SHBG) and insulin-like growth factor binding protein 1 (IGFBP-1) and assessed the effect of a short-term low calorie diet on the levels of SHBG and IGFBP-1 during an oral glucose tolerance test.

DESIGN

A within-group comparison of biochemical indices during an oral glucose tolerance test before and after calorie restriction.

PATIENTS AND METHODS

Six obese women with polycystic ovary syndrome with mean (SD) BMI 34.2 (3.4) kg/m2 were studied before and after one month on a very low calorie diet (350 kcal/day; Cambridge diet). Each subject was given a 75-g oral glucose load after an overnight fast and blood samples were taken every 30 minutes for 3 hours. These were analysed for glucose, insulin, SHBG, and IGFBP-1.

RESULTS

All the women lost weight (range 1.7-9.5 kg). The SHBG concentrations did not change significantly during the oral glucose tolerance test but there was a highly significant decline in IGFBP-1 levels both before (0 min, mean (SD) 27.3 (10.6); 180 min, 8.9 (4.2) micrograms/l) and after (0 min, 28.4 (12.1); 180 min, 6.2 (2.1) micrograms/l, P < 0.001) dieting. The sum of the SHBG concentrations during the test, however, was significantly lower prior (129.9 (40.5) nmol/l) to calorie restriction than after (164.3 (70.6) nmol/l), whereas there was no significant effect of dieting on the IGFBP-1 response to glucose.

CONCLUSIONS

The changes in insulin and SHBG concentrations found after dieting have been confirmed. SHBG levels, in contrast to IGFBP-1, do not change in response to a short-term increase in insulin or glucose concentrations. The difference in the response of the two binding proteins may be explained by differences in their half-lives in the circulation or the regulation of mRNA for the peptides by insulin. This study confirms that insulin regulates both SHBG and IGFBP-1 but that there is a difference in the time course of the response of the two proteins to insulin.

Thyroxine treatment increases circulating levels of insulin-like growth factor binding protein-1: a placebo-controlled study

OBJECTIVE

Thyroid hormones affect carbohydrate metabolism in the liver, and hepatic insulin-like growth factor binding protein-1 (IGFBP-1) participates in glucose counter-regulation, so we studied the effects of oral thyroxine on serum IGFBP-1.

DESIGN

The study was performed on a placebo-controlled cross-over basis covering 3 months' thyroxine and 3 months' placebo administration.

PATIENTS

Eight patients taking anticonvulsant medication and four patients with hypothalamic hypothyroidism were given thyroxine, 150-200 micrograms daily for 3 months, or placebo.

MEASUREMENTS

Serum IGFBP-1, sex hormone binding globulin, free T3 and free T4, TSH and IGF-I levels were measured after an overnight fast before treatment, and at the end of each 3-month period.

RESULTS

Anticonvulsant medication had no significant effect on serum IGFBP-1. After 3 months' thyroxine treatment the serum IGFBP-1 levels (69; 58-167 micrograms/l; median and interquartile range, respectively) were significantly higher than those after placebo treatment (44; 23-58 micrograms/l; P = 0.002), or the pretreatment levels (54; 19-81 micrograms/l, P = 0.005). The IGFBP-1 levels rose in all 12 patients after thyroxine treatment, the median rise being 2.1-fold compared to placebo levels. No change was found in serum IGF-I concentrations.

CONCLUSIONS

Oral thyroxine produces a rise in serum IGFBP-1 levels without a change in serum IGF-I concentration.