Retinol-binding protein 4 is associated with insulin resistance, but appears unsuited for metabolic screening in women with polycystic ovary syndrome

OBJECTIVE

Adiposity, insulin resistance (IR), and hyperandrogenism are features of polycystic ovary syndrome (PCOS). Retinol-binding protein 4 (RBP4) secreted from adipose and liver tissues has been linked to IR. The impact of RBP4 on IR in PCOS and its usability to identify women with metabolic syndrome (MS) or impaired glucose tolerance ((IGT) or diabetes) were investigated.

DESIGN

Plasma RBP4 was determined in 115 consecutive PCOS women. Associations with IR, body composition, and hyperandrogenemia were investigated by correlation and multiple linear regression analyses in 110 non-diabetics. Receiver operating characteristic curve analysis was used to evaluate RBP4 as a parameter for identifying MS and IGT or diabetes.

RESULTS

RBP4 increased over tertiles of IR (P=0.009). RBP4 correlated with HOMA %S (R=-0.286, P= 0.002), waist-to-hip ratio (WHR) (R=0.233, P=0.034), and dual energy X-ray absorptiometry (DEXA)-lean body mass (R=0.282, P=0.016) but not with body mass index (BMI), DEXA-total or -trunk fat mass, hsCRP, free testosterone, DHEAS, androstenedione, and 17beta-estradiol. Adjusted for age, BMI, smoking, and IGT, the association between RBP4 and HOMA %S remained significant (P=0.032). RBP4 explained 4.6% of the variation in HOMA %S. RBP4 was higher in MS and IGT or diabetes, but its ability to identify these women was low (area under the curve, AUC=0.631, P=0.041 or AUC=0.660, P=0.016).

CONCLUSIONS

In PCOS, RBP4 has a small independent impact on IR. It is not correlated with hyperandrogenemia, 17beta-estradiol, other adrenal steroids, or with markers of adiposity in general. Furthermore, RBP4 does not appear suitable for screening MS or impaired glucose metabolism (IGT or diabetes).

Euglycemic hyperinsulinemia differentially modulates circulating total and acylated-ghrelin in humans

Ghrelin is a powerful orexigenic gut hormone. Circulating concentrations of total ghrelin are downregulated by food intake in both acute and chronic hyperinsulinemic states. However, in blood des-acylated (des-acyl) ghrelin is the predominant form that has no orexigenic effects in humans. Circulating acyl-ghrelin has been shown to be suppressed post-prandially and by pharmacological hyperinsulinemia. However, up to now responses of circulating acyl-ghrelin to moderate hyperinsulinemic and hyperinsulinemic-hyperlipidemic clamp conditions have not been reported. Fourteen healthy subjects were investigated using two-stepped euglycemic-hyperinsulinemic clamps (40 mU insulin/ m2/min; mean 148+/-7 min till steady state, followed by 300 min lipid/heparin infusion). Responses of total ghrelin and acyl-ghrelin were measured at timed intervals throughout the clamps. Des-acyl-ghrelin concentrations were calculated by subtraction. Total ghrelin significantly decreased vs baseline concentrations (819+/-92 vs 564+/-58 pg/ml, p<0.001), thereby confirming previous observations. Des-acyl ghrelin closely followed total ghrelin concentrations and significantly decreased vs baseline (772+/-92 vs 517+/-56 pg/ml, p<0.001). In contrast, neither euglycemichyperinsulinemia nor euglycemic-hyperinsulinemic- hyperlipidemia suppressed acyl-ghrelin below baseline concentrations throughout the clamps (46+/-3 vs 47+/-8 pg/ml, p=0.90). In conclusion, moderate hyperinsulinemic and hyperinsulinemic- hyperlipidemic clamp conditions differentially modulated circulating total ghrelin and acylghrelin in humans. Factors other than changes in insulin and lipid concentrations are likely to contribute to the previously reported post-prandial reduction of circulating acyl-ghrelin.

Enhancement by lithium of cAMP-induced CRE/CREB-directed gene transcription conferred by TORC on the CREB basic leucine zipper domain

The molecular mechanism of the action of lithium salts in the treatment of bipolar disorder is not well understood. As their therapeutic action requires chronic treatment, adaptive neuronal processes are suggested to be involved. The molecular basis of this are changes in gene expression regulated by transcription factors such as CREB (cAMP-response-element-binding protein). CREB contains a transactivation domain, in which Ser119 is phosphorylated upon activation, and a bZip (basic leucine zipper domain). The bZip is involved in CREB dimerization and DNA-binding, but also contributes to CREB transactivation by recruiting the coactivator TORC (transducer of regulated CREB). In the present study, the effect of lithium on CRE (cAMP response element)/CREB-directed gene transcription was investigated. Electrically excitable cells were transfected with CRE/CREB-driven luciferase reporter genes. LiCl (6 mM or higher) induced an up to 4.7-fold increase in 8-bromo-cAMP-stimulated CRE/CREB-directed transcription. This increase was not due to enhanced Ser119 phosphorylation or DNA-binding of CREB. Also, the known targets inositol monophosphatase and GSK3beta (glycogen-synthase-kinase 3beta) were not involved as specific GSK3beta inhibitors and inositol replenishment did not mimic and abolish respectively the effect of lithium. However, lithium no longer enhanced CREB activity when the CREB-bZip was deleted or the TORC-binding site inside the CREB-bZip was specifically mutated (CREB-R300A). Otherwise, TORC overexpression conferred lithium responsiveness on CREB-bZip or the CRE-containing truncated rat somatostatin promoter. This indicates that lithium enhances cAMP-induced CRE/CREB-directed transcription, conferred by TORC on the CREB-bZip. We thus support the hypothesis that lithium salts modulate CRE/CREB-dependent gene transcription and suggest the CREB coactivator TORC as a new molecular target of lithium.

Making sense of muscle fatigue and liver lesions

Muscle weakness is a common complaint in clinical practice. If this symptom is combined with focal liver lesions there is a broad spectrum of differential diagnoses for the gastroenterologist to consider. Tumors of neuroendocrine origin such as small-cell lung carcinoma (SCLC) produce a wide array of peptide hormones and are common causes of paraneoplastic syndromes. We report on a 68-year-old woman who presented with progressing muscle fatigue and multiple liver lesions on ultrasonography. Hypertension, hyperglycemia, hypokalemia and metabolic alkalosis prompted consideration of underlying hypercortisolism. Further work-up demonstrated an acute ectopic ACTH syndrome as paraneoplastic manifestation of a small cell lung carcinoma. The woman deteriorated rapidly and finally died from intracranial tumor spread and septic complications. This case stresses the diagnostic and therapeutic difficulties of acute ectopic ACTH syndrome in the setting of SCLC.

Adipocyte fatty acid-binding protein is associated with markers of obesity, but is an unlikely link between obesity, insulin resistance, and hyperandrogenism in polycystic ovary syndrome women

OBJECTIVE

Many polycystic ovary syndrome (PCOS) women suffer from adiposity and insulin resistance (IR), which play an important role in the development and maintenance of PCOS. Adipocyte fatty acid-binding protein (A-FABP) is mainly expressed in adipocytes, and circulating A-FABP has been associated with markers of obesity and IR. Thus, as observed with other adipose tissue derived factors, secreted A-FABP might be involved in the pathogenesis of obesity-associated disorders such as PCOS.

DESIGN

Plasma A-FABP concentrations were measured in 102 non-diabetic PCOS women, and associations with markers of obesity, IR, inflammation, and hyperandrogenism were investigated by correlation and multiple linear regression analyses. The effect of lifestyle intervention on A-FABP was studied in a second cohort of 17 obese PCOS women.

RESULTS

A-FABP correlated with body mass index (BMI; R = 0.694, P < 0.001), dual-energy X-ray-absorptiometry (DEXA) fat mass (R = 0.729, P < 0.001), DEXA lean body mass (R = 0.399, P = 0.001), HOMA %S (R = -0.435, P < 0.001), hsCRP (R = 0.355, P = 0.001), and free testosterone (fT; R = 0.230, P = 0.02). Adjusted for age, smoking, and glucose metabolism the association of A-FABP with HOMA %S was still significant (P < 0.001), whereas the associations with fT (P = 0.09) and hsCRP (P = 0.25) were not. Inclusion of BMI into the model abolished the impact of A-FABP on HOMA %S. In BMI-matched PCOS women (n = 20 pairs), neither HOMA %S (P = 0.3) nor fT (P = 0.6) were different despite different A-FABP levels (P < 0.001), and in 17 obese PCOS women undergoing a lifestyle intervention, changes in IR were not paralleled by changes in A-FABP.

CONCLUSIONS

Circulating A-FABP was correlated with markers of obesity, but had no major impact on IR, inflammation, or hyperandrogenemia in PCOS women.

Free fatty acids increase cytosolic free calcium and stimulate insulin secretion from beta-cells through activation of GPR40

Free fatty acids (FFA) cause a rise in cytosolic free Ca2+ ([Ca2+]i) and stimulate insulin release from pancreatic beta-cells. The G-protein coupled receptor GPR40 can be activated by medium- and long-chain FFA. We investigated a potential role for GPR40 in the generation of the FFA-induced Ca2+ signal and insulin secretion. [Ca2+]i was measured in primary mouse beta-cells and in INS-1 cells, and insulin secretion was assessed from INS-1 cells. GPR40 expression was determined by RT-PCR and downregulation of GPR40 expression by siRNA transfection was carried out in INS-1 cells. A number of saturated, mono- and polyunsaturated medium- and long-chain FFA caused a rise in [Ca2+]i both in primary mouse beta-cells and in INS-1 cells. By contrast, the short-chain saturated caproic acid was ineffective at concentrations up to 300 microM. In INS-1 cells, the FFA-induced Ca2+ signal required mobilization of internal Ca2+ and Ca2+ influx through voltage-sensitive Ca2+ channels. RT-PCR analysis revealed that GPR40 is expressed in INS-1 cells. Downregulation of GPR40 by specific siRNA treatment lead to a significant inhibition of the FFA-induced [Ca2+]i response and insulin secretion, indicating that the FFA-stimulated Ca2+ signal and insulin secretion involve activation of GPR40 in pancreatic beta-cells.

Predicting impaired glucose metabolism in women with polycystic ovary syndrome by decision tree modelling

AIMS/HYPOTHESIS

Polycystic ovary syndrome (PCOS) is a risk factor of type 2 diabetes. Screening for impaired glucose metabolism (IGM) with an OGTT has been recommended, but this is relatively time-consuming and inconvenient. Thus, a strategy that could minimise the need for an OGTT would be beneficial.

MATERIALS AND METHODS

Consecutive PCOS patients (n=118) with fasting glucose <6.1 mmol/l were included in the study. Parameters derived from medical history, clinical examination and fasting blood samples were assessed by decision tree modelling for their ability to discriminate women with IGM (2-h OGTT value >/=7.8 mmol/l) from those with NGT.

RESULTS

According to the OGTT results, 93 PCOS women had NGT and 25 had IGM. The best decision tree consisted of HOMA-IR, the proinsulin:insulin ratio, proinsulin, 17-OH progesterone and the ratio of luteinising hormone:follicle-stimulating hormone. This tree identified 69 women with NGT. The remaining 49 women included all women with IGM (100% sensitivity, 74% specificity to detect IGM). Pruning this tree to three levels still identified 53 women with NGT (100% sensitivity, 57% specificity to detect IGM). Restricting the data matrix used for tree modelling to medical history and clinical parameters produced a tree using BMI, waist circumference and WHR. Pruning this tree to two levels separated 27 women with NGT (100% sensitivity, 29% specificity to detect IGM). The validity of both trees was tested by a leave-10%-out cross-validation.

CONCLUSIONS/INTERPRETATION

Decision trees are useful tools for separating PCOS women with NGT from those with IGM. They can be used for stratifying the metabolic screening of PCOS women, whereby the number of OGTTs can be markedly reduced.

Glucagon suppression of ghrelin secretion is exerted at hypothalamus-pituitary level

CONTEXT

The mechanisms underlying the well-known glucagon-induced satiety effect are unclear. Recently, we showed that glucagon induces a remarkable decrease in the orexigenic hormone ghrelin that might be responsible for this effect.

OBJECTIVE

The objective of this study was to evaluate the putative role of the hypothalamic pituitary axis in glucagon's suppressive effect on ghrelin secretion. DESIGN, SUBJECTS, AND METHODS: Prospectively, we studied the endocrine and metabolic responses to im glucagon administration in 22 patients (16 males; age, 21-68 yr; body mass index, 28.1 +/- 1.1 kg/m(2)) with a known hypothalamic-pituitary lesion and at least one pituitary hormone deficiency. Control experiments were performed in 27 healthy subjects (15 males; age, 19-65 yr; body mass index, 25.5 +/- 0.9 kg/m(2)).

RESULTS

The suppression of ghrelin by glucagon measured as area under the curve(240 min) was significantly greater in controls when compared with patients (P < 0.01). Although there was a significant decrease in ghrelin in controls (P < 0.001), ghrelin was almost unchanged in patients (P = 0.359). Changes in glucagon, glucose, and insulin levels were comparable between both groups.

CONCLUSIONS

We show that the hypothalamic-pituitary axis plays an essential role in the suppression of ghrelin induced by im glucagon administration. Glucagon significantly decreases ghrelin levels in healthy subjects. However, in the absence of an intact hypothalamic-pituitary axis, this effect was abolished. The mechanisms responsible for our observation are unlikely to include changes in glucose or insulin levels.

The androgen receptor CAG repeat modifies the impact of testosterone on insulin resistance in women with polycystic ovary syndrome

OBJECTIVE

Hyperandrogenism is a central feature of the polycystic ovary syndrome (PCOS) and might worsen insulin resistance (IR) often seen in PCOS. Androgens act through the androgen receptor (AR). A polymorphic CAG repeat sequence within the AR gene was reported to modulate its transactivation activity. Therefore, we investigated a putative interaction between testosterone and the CAG repeat length polymorphism with respect to IR.

DESIGN

In 63 PCOS women with normal glucose tolerance free testosterone, the biallelic CAG repeat length and a multiplicative interaction term were investigated by multiple linear regression analysis for an association with IR as indicated by the homeostasis model assessment of IR (HOMA-IR).

RESULTS

Free testosterone was correlated with HOMA-IR. The impact of testosterone on HOMA-IR was modified by the AR CAG length as indicated by an interaction term. This interaction remained significant after adjustment for smoking, age and body mass index. While there was a positive association of free testosterone with HOMA-IR, the interaction term was inversely associated. The model, which explained 42.5% of the variation of HOMA-IR predicted that in carriers of short CAG lengths, an increase in testosterone increased IR. This effect attenuated with rising biallelic CAG length until it turns into the opposite at a CAG length longer than 23. The results were confirmed by using CIGMA as another measure of IR.

CONCLUSIONS

The association between testosterone and IR is modified by the CAG repeat polymorphism within the AR. Therefore, the evaluation of testosterone effects on IR seems to require consideration of the AR CAG repeat polymorphism in PCOS women.

Cereal fiber improves whole-body insulin sensitivity in overweight and obese women

OBJECTIVE

Cereal fiber intake is linked to reduced risk of type 2 diabetes in epidemiological observations. The pathogenic background of this phenomenon is unknown. Based on recent findings, we hypothesized that intake of purified insoluble oat fiber may improve whole-body insulin sensitivity.

RESEARCH DESIGN AND METHODS

A randomized, controlled, single-blind, cross-over study was performed, and 17 overweight or obese subjects with normal glucose metabolism were analyzed. After consumption of nine macronutrient-matched portions of fiber-enriched bread (white bread enriched with 31.2 g insoluble fiber/day) or control (white bread) over a time period of 72 h, whole-body insulin sensitivity was assessed by euglycemic-hyperinsulinemic clamp. Energy intake was individually adjusted by providing standardized liquid meals. Hydrogen breath tests were performed to control for dietary adherence.

RESULTS

When analyzing the entire cohort, whole-body glucose disposal was improved after fiber consumption (M value 6.56 +/- 0.32 vs. 6.07 +/- 0.27 mg . min(-1) . kg(-1); P = 0.043). Thirteen subjects had increased hydrogen breath test concentrations after fiber consumption, indicating probable dietary adherence. Restricting analysis to these subjects, improvements in M value (6.85 +/- 0.34 vs. 6.06 +/- 0.32 mg . min(-1) . kg(-1); P = 0.003) and insulin sensitivity, expressed as M/I ratio (M value divided by mean serum insulin at steady state: 3.73 +/- 0.23 vs. 3.21 +/- 0.27; P = 0.02), after fiber consumption were more pronounced. Plasma lipids, serum magnesium, ghrelin, and adiponectin concentrations, as well as substrate utilization and body weight, were not significantly changed by fiber intake (P > 0.15).

CONCLUSIONS

Increased insoluble dietary fiber intake for 3 days significantly improved whole-body insulin sensitivity. These data suggest a potential mechanism linking cereal fiber intake and reduced risk of type 2 diabetes.

The insulin-like growth factor receptor 1 is a promising target for novel treatment approaches in neuroendocrine gastrointestinal tumours

Gastrointestinal neuroendocrine tumours (NET) represent a heterogeneous tumour entity. The anti-neoplastic therapy of advanced NET disease is still unsatisfactory and innovative therapeutic approaches are needed. As NET frequently express insulin-like growth factors (IGFs) and their receptors (IGFR), known to promote survival, oncogenic transformation, tumour growth and spreading, the inhibition of the IGF/IGF-receptor system may offer possibilities for novel targeted treatment strategies of NET. Here, we studied the anti-neoplastic effects of an inhibition of the IGF-I receptor (IGF-1R) signalling in NET cells by the novel IGF-1R tyrosine kinase (TK) inhibitor NVP-AEW541, whose anti-neoplastic potency has not yet been tested in NET disease. Using two human NET cell lines with different growth characteristics, we demonstrated that NVP-AEW541 dose-dependently inhibited the proliferation of NET cells by inducing apoptosis and cell cycle arrest. Anti-neoplastic effects of NVP-AEW541 were also detected in primary cultures of human neuroendocrine gastrointestinal tumours. Apoptosis was characterized by activation of the apoptotic key enzyme, caspase-3, as well as by detection of changes in the expression of the pro- and anti-apoptotic proteins, BAX and Bcl-2, after NVP-AEW541 treatment. Cell cycle was arrested at the G1/S checkpoint. The anti-neoplastic effects of NVP-AEW541 involved the inactivation of ERK1/2. Induction of immediate cytotoxicity did not account for the anti-neoplastic effects of NVP-AEW541, as shown by measurement of lactate dehydrogenase release. Moreover, additive anti-neoplastic effects were observed when NVP-AEW541 was combined with cytostatics such as doxorubicin or the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, fluvastatin. This is the first report on the induction of apoptosis and cell cycle arrest by the IGF-1R-TK inhibitor, NVP-AEW541, in NET cells. The inhibition of the IGF/IGFR system appears to be a promising novel approach for future treatment strategies of NET disease.