The role of the opioid peptides in the development of hyperinsulinemia in obese women with abdominal body fat distribution

The role of the endogenous opioid system in polycystic ovary syndrome

OBJECTIVE

To review the complex role of the opioid system in reproduction and carbohydrate metabolism, abnormalities in the opioid system in women with polycystic ovary syndrome (PCOS), and the role of opioid antagonists in the management of PCOS-related infertility.

DESIGN

Pertinent articles were identified through a computer PubMed search. References of selected articles were hand searched for additional citations.

CONCLUSION(S)

Endogenous opioids are generally considered inhibitory central neurotransmitters. Peripherally, opioids are involved in the regulation of pancreatic islet function, hepatic insulin clearance, and glucose metabolism, potentially contributing to the pathogenesis of hyperinsulinemia and insulin resistance in PCOS. The presence of sex steroids is required for normal function of the opioid system in both GnRH secretion and carbohydrate metabolism. In women with PCOS, growing evidence suggests dysregulation of the opioid system both centrally and peripherally, with complex interactions. The opioid system effects on carbohydrate metabolism appear to be modulated by obesity. Finally, naltrexone has been demonstrated to successfully augment traditional ovulation induction regimens, but has limited support as a single ovulation induction agent for PCOS.

Use of naltrexone in postmenopausal women with exaggerated insulin secretion: a pilot study

OBJECTIVE

To determine the effect of naltrexone (an opiate receptor blocker) on insulin metabolism in postmenopausal women with different insulinemic patterns.

DESIGN

Randomized placebo-controlled study.

SETTING

Academic research environment.

PATIENT(S)

Forty-one healthy normoinsulinemic or hyperinsulinemic postmenopausal women.

INTERVENTION(S)

Oral glucose tolerance test (OGTT) before and after 5 weeks of the opioid antagonist (naltrexone, 50 mg/d orally) or the placebo administration; euglycemic-hyperinsulinemic glucose clamp.

MAIN OUTCOME MEASURE(S)

Glucose, insulin, and C-peptide plasma levels assessed in fasting condition and during the OGTT. Insulin sensitivity was calculated as total body glucose utilization.

RESULT(S)

Naltrexone reduced fasting and stimulated insulin response to the glucose load while inducing a significant improvement of the hepatic extraction, only in the hyperinsulinemic patients. No differences were found in the C-peptide pancreatic secretion and in the peripheral insulin sensitivity. No net change in the glycoinsulinemic metabolism was observed in normoinsulinemic patients or in placebo-controlled normoinsulinemic and hyperinsulinemic subjects.

CONCLUSION(S)

Similar to that reported in premenopausal women, endogenous opioid peptides are involved in the modulation of glycoinsulinemic metabolism in postmenopause. Through a prevalent action on liver insulin metabolism, without any clear improvement of insulin resistance and pancreatic beta-cell function, the chronic administration of naltrexone appears to reduce the hyperinsulinemia in those women with an exaggerated insulin response to the OGTT.

Insulin resistance and the sympathetic nervous system

The obesity epidemic is driving metabolic (insulin resistance) syndrome-related health problems including an approximately threefold increased coronary heart disease risk. Sympathetic hyperfunction may participate in the pathogenesis and complications of the metabolic syndrome including higher blood pressure, a more active renin-angiotensin system, insulin resistance, faster heart rates, and excess cardiovascular disease including sudden death. Possible factors augmenting sympathetic activation in the metabolic syndrome include alterations of insulin, leptin, nonesterified fatty acids (NEFAs), cytokines, tri-iodothyronine, eicosanoids, sleep apnea, nitric oxide, endorphins, and neuropeptide Y. Of note, high plasma NEFAs are a risk factor for hypertension and sudden death. In short-term human studies, NEFAs can raise blood pressure, heart rate, and a(1)-adrenoceptor vasoreactivity, while reducing baroreflex sensitivity, endothelium-dependent vasodilatation, and vascular compliance. Efforts to further identify the mechanisms and consequences of sympathetic dysfunction in the metabolic syndrome may provide insights for therapeutic advances to ameliorate the excess cardiovascular risk and adverse outcomes.

Naloxone decreases insulin secretion in hyperinsulinemic postmenopausal women and may positively affect hormone replacement therapy

OBJECTIVE

To evaluate the influence of the opioid system on glyco-regulation in postmenopausal women before and after hormone replacement therapy (HRT).

DESIGN

Prospective nonrandomized clinical study.

SETTING

Academic research environment.

PATIENT(S)

Twenty-one healthy normo- or hyperinsulinemic postmenopausal women.

INTERVENTION(S)

Oral glucose tolerance test (OGTT) (saline study), OGTT with IV injection of naloxone (naloxone study), and hyperinsulinemic euglycemic clamp performed before treatment, after 12 weeks of estrogen replacement therapy (ERT), and after 12 additional weeks of estro-progestin combined therapy (i.e., HRT).

MAIN OUTCOME MEASURE(S)

Glucose, insulin, and c-peptide plasma levels assessed in fasting condition and during the two OGTTs (area under the curve [AUC]). Evaluation of fractional hepatic insulin extraction (FHIE) and peripheral sensitivity to insulin.

RESULT(S)

At baseline, there is a greater increase of the FHIE and a more significant reduction of the insulin AUC in the hyperinsulinemic patients during the naloxone study compared with the saline study. In these women, ERT enhanced the c-peptide AUC and improved the FHIE; naloxone infusion mainly increased these two parameters. HRT did not induce any further change.

CONCLUSION(S)

Endogenous opioid peptides are involved in the modulation of carbohydrate metabolism in menopause in hyperinsulinemic patients more than in other patients. The favorable changes of the glyco-insulinemic metabolism induced by HRT may be partially due to the induction of the opioidergic activity.

Obesity and the polycystic ovary syndrome

The polycystic ovary syndrome (PCOS) is a condition characterized by hyperandrogenism and chronic oligo-anovulation. However, many features of the metabolic syndrome are inconsistently present in the majority of women with PCOS. Approximately 50% of PCOS women are overweight or obese and most of them have the abdominal phenotype. Obesity may play a pathogenetic role in the development of the syndrome in susceptible individuals. In fact, insulin possesses true gonadotrophic function and an increased insulin availability at the level of ovarian tissue may favour excess androgen synthesis. Obesity, particularly the abdominal phenotype, may be partly responsible for insulin resistance and associated hyperinsulinemia in women with PCOS. Therefore, obesity-related hyperinsulinemia may play a key role in favouring hyperandrogenism in these women. Other factors such as increased estrogen production rate, increased activity of the opioid system and of the hypothalamic-pituitary-adrenal axis, decreased sex hormone binding globulin synthesis and, possibly, high dietary lipid intake, may be additional mechanisms by which obesity favours the development of hyperandrogenism in PCOS. Irrespective of the pathogenetic mechanism involved, obese PCOS women have more severe hyperandrogenism and related clinical features (such as hirsutism, menstrual abnormalities and anovulation) than normal-weight PCOS women. This picture tends to be more pronounced in obese PCOS women with the abdominal phenotype. Body weight loss is associated with beneficial effects on hormones, metabolism and clinical features. A further clinical and endocrinological improvement can also be achieved by adding insulin-sensitizing agents and/or antiandrogens to weight reduction programmes. These obviously emphasize the role of obesity in the pathophysiology of PCOS.

Effect of long-term naltrexone treatment on endocrine profile, clinical features, and insulin sensitivity in obese women with polycystic ovary syndrome

OBJECTIVE

Evaluation of clinical and endocrine effects of naltrexone administration in obese women with PCOS.

DESIGN

Open, controlled, clinical study.

SETTING

Department of Reproductive Medicine and Child Development, Section of Gynecology and Obstetrics, University of Pisa, Pisa, Italy.

PATIENT(S)

Ten PCOS women were studied.

INTERVENTION(S)

Women were treated with naltrexone (50 mg/day) for 6 months.

MAIN OUTCOME MEASURE(S)

Body mass index and the menstrual cyclicity during naltrexone treatment were assessed. Basal levels of LH, FSH, 17beta-estradiol (E(2)), 17-hydroxyprogesterone, total and free T, androstenedione, dehydroepiandrosterone sulfate, cortisol, sex hormone-binding globulin were evaluated before treatment and every 3 months. Progesterone levels were measured in the luteal phase during the sixth month. Gonadotropin response to GnRH administration (10 microg) and a 75-g oral glucose tolerance test were performed before and every 3 months.

RESULT(S)

Body mass index significantly decreased from 29.94 +/- 1.04 to 26.07 +/- 0.81 during treatment. The menstrual cyclicity improved in 80% of PCOS women: the mean cycle length was 40-360 days before treatment and ranged between 25 and 120 days and 28-120 days after 3 and 6 months of treatment. Plasma levels of free T, androstenedione, dehydroepiandrosterone sulfate, and cortisol significantly decreased. Fasting glucose-to-insulin ratio improved in women with insulin resistance.

CONCLUSION(S)

Naltrexone may have a beneficial effect on the clinical and endocrine-metabolic disturbances of obese PCOS women. Whether these effects are the consequences of weight loss or are due to changes in opioidergic tone is debatable.

The influence of obesity on hyperandrogenism and infertility in the female

Although a critical mass of adipose tissue is essential for the normal development of female reproductive function, obesity has been shown to produce menstrual disturbances and subfertility. The severity of obesity and the distribution of fat tissue are important factors that influence the female reproductive system. The pathogenetic mechanistic links between them aren't clearly elucidated. Obese women, especially those with upper body obesity, have insulin resistance and hyperinsulinaemia, hyperandrogenaemia, increased peripheral aromatization of androgens to oestrogens, altered gonadotrophin secretion, decreased sex hormone binding globulin, decreased growth hormone (GH) and insulin like growth factor binding proteins (IGFBPs), increased leptin levels and altered neuroregulation of the hypothalamic-pituitary-gonadal axis. These have been considered as some of the links in the sequence of events of the disrupted ovulatory process. The mechanisms of these actions and their influence on female reproductive function are discussed below.

Testosterone therapy in men: clinical and pharmacological perspectives

There is increasing evidence that androgen therapy in men may be effectively applied in several conditions to improve well being and health. Classical indications for androgen therapy in males are represented by primary or secondary hypogonadism, delayed puberty, aplastic anemia and that secondary to chronic renal failure, protein wasting diseases such as trauma, burns, tumors and infectious diseases. Androgen innovating applications in men are represented by aging and visceral obesity associated with the metabolic syndrome. In addition, it is clear that appropriate testosterone treatment can be adequately used in male contraception, provided spermatogenesis is abolished and tolerability is adequate. Due to unphysiological hormone levels achieved by currently available testosterone preparations, new delivery systems have been produced to achieve more physiological and sustained hormone levels and improve tolerability and action at the levels of target tissues. Some of them are now available in several countries and new formulas are under development.

Influence of body mass on the hypothalamic-pituitary-adrenal-axis response to naloxone in patients with polycystic ovary syndrome

OBJECTIVE

To evaluate the influence of body mass on the hypothalamic-pituitary-adrenal (HPA)-axis response to naloxone in polycystic ovary syndrome (PCOS).

DESIGN

Controlled clinical study.

SETTING

Academic research environment.

PATIENT(S)

Ten lean and 10 obese women with PCOS compared with 7 lean and 8 obese control subjects matched for body mass index.

INTERVENTION(S)

Each patient received an IV bolus of naloxone at a dosage of 125 microg/kg.

MAIN OUTCOME MEASURE(S)

Samples were collected 30 minutes before and 0, 15, 30, 60, 90, and 120 minutes after injection: ACTH and cortisol levels were measured in all plasma samples.

RESULT(S)

No significant differences were found in the ACTH and cortisol responses to opioid blockade between lean women with PCOS and lean as well as obese control subjects; conversely, obese patients with PCOS showed a higher ACTH and cortisol responses to naloxone compared with all other groups.

CONCLUSION(S)

Hypothalamic-pituitary-adrenal-axis abnormalities of PCOS may be central in origin and abdominal obesity seems to play a key role in the HPA-axis hyperactivity of women with PCOS when naloxone is administered.

The effect of vertical banded gastroplasty on glucose-induced beta-endorphin response

BACKGROUND

beta-Endorphin is an endogenous opioid involved in the regulation of food intake and obesity as well as in insulin metabolism. In this study, we investigated glucose-induced beta-endorphin, insulin, and glucose responsiveness in morbidly obese patients and the effect of surgically induced weight loss.

METHODS

Thirty-two healthy, nondiabetic, morbidly obese patients (body mass index over 40 kg/m2) and 32 normal-weight controls were studied. Serum levels of beta-endorphin, insulin, and glucose were measured under basal conditions and during an oral glucose tolerance test (OGTT) before and 12 months following vertical banded gastroplasty.

RESULTS

Preoperative basal levels of beta-endorphin, insulin, and glucose and their responses during OGTT in obese patients were significantly higher compared with those of controls. After surgery, basal beta-endorphin, insulin, and glucose levels decreased significantly compared with preoperative values. Postoperative basal insulin and glucose levels were similar to those in controls, while beta-endorphin levels remained significantly higher than those of controls. A significant reduction in total responses of beta-endorphin, insulin, and glucose during OGTT was also observed; however, postoperative beta-endorphin and insulin responses remained significantly higher than in controls.

CONCLUSION

Morbidly obese patients have an increased glucose-stimulated response of beta-endorphin, insulin, and glucose which is partially corrected with weight loss following vertical banded gastroplasty.

Pulsatile secretion of ACTH and cortisol in premenopausal women: effect of obesity and body fat distribution

OBJECTIVE

There is emerging evidence that women with visceral obesity may have hyper-responsiveness of the hypothalamic-pituitary-adrenal axis. There are no studies on basal daily secretory pattern of ACTH and cortisol in subjects with different obesity phenotypes.

DESIGN AND PATIENTS

In this study we examined daytime pulsatile secretion of ACTH and cortisol in two groups of premenopausal obese women with visceral (V-BFD) (BMI 37.1 +/- 1.7) and subcutaneous (S-BFD) (BMI 38.8 +/- 1.5) body fat distribution (measured by CT scan) and in a group of normal weight healthy controls (BMI 21.1 +/- 0.5). After an overnight fast, blood samples were taken at 15-minute intervals for 12 h (49 samples, from 0800 h until 2000 h). All women avoided breakfast but had a normal lunch and dinner, both containing similar food, energy and nutrient composition. ACTH and cortisol responses to mixed meals at noon and in the evening were also investigated.

RESULTS

Mean values of ACTH and cortisol did not differ between the groups. However, ACTH pulse frequency was significantly higher in V-BFD (P < 0.06) and S-BFD (P < 0.02) obese women than in controls, without any significant differences between the two obese subgroups. Mean ACTH pulse amplitude was lower in the V-BFD than in S-BFD obese (P < 0.02) and control (P < 0.05) groups. Cortisol episodic characteristics did not differ between V-BFD and S-BFD obese and controls. All differences in ACTH pulsatile parameters between obese and controls and between the two obese subgroups were evident only in the morning, with no further significant differences during the early and late afternoon. There were no significant differences in cortisol parameters during the three periods of the day between the various groups, apart from late afternoon cortisol pulse frequencies, which were significantly lower in V-BFD than in controls. After lunch, ACTH and cortisol levels significantly increased in all groups, but the cortisol increase tended to be more rapid in V-BFD than in the other two groups. After dinner, ACTH significantly increased in V-BFD and controls but not in the S-BFD group, whereas cortisol rose significantly in all groups, but significantly less in S-BFD than in V-BFD and controls. CortisolAUC (but not ACTHAUC) after lunch was significantly higher than after dinner in all groups. ACTH response after each meal was similar in all groups, but cortisolAUC after dinner was significantly lower in S-BFD than in V-BFD women.

CONCLUSION

This study demonstrates that in premenopausal women, obesity, particularly the visceral phenotype, is associated with several abnormalities of ACTH pulsatile secretion, particularly in the morning. On the contrary, no major differences were present in either blood concentrations, diurnal rhythm or secretory pattern of cortisol between obese and controls. The responses to meals seem to indicate a much more rapid cortisol response after lunch in women with visceral obesity and a reduced activation of the hypothalamic-pituitary-adrenal axis after dinner in women with subcutaneous obesity.

Hormonal regulation of appetite and food intake

Several clinical disorders are strongly influenced by hormones involved in appetite and weight regulation. Obesity and eating disorders are of major importance, because they are associated with severe morbidity and considered to be among the greatest health problems in the Western world today. This review describes recent findings in hormonal regulation of food intake by substances acting both centrally, such as corticotropin-releasing factor, neuropeptide Y and leptin, and peripherally, such as cholecystokinin and somatostatin. Sex hormones and glucocorticoids play an important role in long-term regulation of metabolism. The role of these hormones in appetite and weight changes during life as well as during pregnancy and lactation is discussed. Furthermore, the development of obesity and eating disorders is influenced, in particular, by steroid hormones. Treatment with sex hormones, as in hormone replacement therapy, affects appetite and weight and may have beneficial effects in preventing android obesity. Currently, there is great effort in developing endogenous neurohumoral substances into effective drugs for the treatment of obesity and eating disorders. Leptin and neuropeptide Y analogues are of interest as potential antiobesity agents.

Impact of long-term naltrexone treatment on growth hormone and insulin secretion in hyperandrogenic and normal obese patients

The growth hormone (GH) response to stimulation tests is impaired in obesity. Moreover, obese patients exhibit a "paradoxical" increase of GH to GH-releasing hormone (GHRH) stimulation after food ingestion; this paradoxical response is reversed by naloxone infusion. On the other hand, beta-endorphin seems to exert profound effects on insulin release. Recent studies also demonstrated an impairment of GH response to several stimuli in polycystic ovary syndrome (PCOS), a condition associated with obesity, hyperinsulinism, and insulin resistance. Chronic inhibition of opioid tone by the opioid antagonist naltrexone (NTX) is able to reduce the insulin response to an oral glucose tolerance test (OGTT) in hyperinsulinemic PCOS patients. Since insulin and GH may reciprocally influence their secretion and the opioid system may have a role in the pathogenesis of hyperinsulinemia and reduced GH secretion, we have explored the involvement of these neuroendocrine mechanisms in essential obesity and in obesity associated with hyperandrogenism by a long-term treatment with an opiate antagonist. We tested seven obese patients affected by PCOS, seven matched women with essential obesity (EO), and five non-obese control subjects. All patients, in the follicular phase, underwent an OGTT (75 g) and basal hormone assay. Two days later, patients were subjected to a GHRH test. The patients then had 4 weeks of treatment with NTX 50 mg/d. Following continuation of the treatment, OGTT and GHRH tests were repeated. Insulin-like growth factor-I (IGF-I) and IGF-binding protein-3 (IGFBP-3) plasma concentrations were also determined in the basal condition before and after NTX treatment. NTX treatment reduced fasting insulin levels in patients with EO (P < .05) and restored a normal GH response to GHRH without affecting IGF-1 and IGFBP-3 levels. In PCOS subjects, NTX reduced the insulin response to a glucose load and failed to modify the blunted GH response to GHRH. Our data suggest a significant difference in opioid system function in PCOS and EO subjects, indicating a particular form of obesity in PCOS. The opiate antagonist treatment in EO may act through the reduction of negative insulin feedback on GH secretion. In PCOS patients, the failure to improve GH secretion in obese hyperandrogenized patients may be related to a high opioidergic tone or to the inhibitory predominance of other neurotransmitters.

The hypothalamic-pituitary-adrenal axis in obesity

The hypothalamic-pituitary-adrenal (HPA) axis normally maintains the concentration of cortisol within a narrow range with a diurnal variation characterized by higher cortisol concentrations in the morning and reduced levels in the evening. Excessive or deficient secretion of cortisol is associated with pathologic changes. Obesity has been linked with age, sex and racial alterations in the functioning of the HPA axis which are reviewed. The possible relationship of altered HPA axis activity with the long-term complications of obesity are considered.

Proopiomelanocorticotropin (POMC) peptides and lipoprotein lipase activity in vitro

Effects of alpha-melanocyte-stimulating hormone (alpha-MSH), beta-melanocyte-stimulating hormone (beta-MSH), beta-lipotropin (beta-LPH), and beta-endorphin (beta-EPH) at concentrations from 10(-9) M up to 10(-6) M on human adipose tissue lipoprotein lipase (LPL) were studied in a cell-free system. alpha-MSH and beta-MSH did not exert any effect on LPL; no degradation of these peptides in the incubation medium could be detected by HPLC analysis. beta-LPH and beta-EPH failed to alter enzyme activity. However, HPLC analysis revealed an unspecific rapid degradation of the peptides due to the activity of tissue proteases. Therefore, the protease inhibitors amastatin, antipain, APMSF, and TPCK were tested at concentrations of 10(-5), 10(-4), and 10(-3) M for their efficacy to inhibit degradation. None of the inhibitors was able to substantially reduce proteolysis of beta-LPH, as was the case with amastatin, APMSF, and TPCK for beta-EPH. However, antipain at 10(-4) M preserved at least 20% of the initial peptide concentration from proteolysis up to 150 min. Antipain caused a decrease in lipoprotein lipase activity (LPLA), which was dependent on concentration. The adverse effect of antipain at concentrations of 10(-4) M on LPL was completely abolished by beta-EPH at a concentration of 10(-6) M.