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Metformin treatment does not affect total leptin levels and free leptin index in obese patients with polycystic ovary syndrome

2007; Elsevier BV; Volume: 89; Issue: 5 Linguagem: Inglês

10.1016/j.fertnstert.2007.05.004

1556-5653

Daniela Romualdi, Giuseppe Campagna, Luigi Selvaggi, R. M. Cento, C. Proto, Antonio Lanzone, Maurizio Guido,

Adipokines, Inflammation, and Metabolic Diseases

Leptin, particularly in its free form, is deeply involved in the regulation of energy balance. Insulin was suggested to influence plasmatic leptin levels and soluble leptin receptor in humans. However, this study indicates that metformin treatment, although improving insulin levels, does not exert a significant effect on either total leptin level or free leptin index in obese women with hyperinsulinemia and PCOS. Leptin, particularly in its free form, is deeply involved in the regulation of energy balance. Insulin was suggested to influence plasmatic leptin levels and soluble leptin receptor in humans. However, this study indicates that metformin treatment, although improving insulin levels, does not exert a significant effect on either total leptin level or free leptin index in obese women with hyperinsulinemia and PCOS. Beyond the reproductive abnormalities, polycystic ovary syndrome (PCOS) is characterized by a high prevalence of insulin resistance, hyperinsulinemia, and obesity (1Barber T.M. McCarthy M.I. Wass J.A. Franks S. Obesity and polycystic ovary syndrome.Clin Endocrinol (Oxf). 2006; 65: 135-145Crossref Scopus (302) Google Scholar). Previous studies have suggested that leptin, which is produced predominantly by the adipose tissue, could have a pathogenic role in the hormonal and metabolic imbalance of this syndrome (2Brzechffa P.R. Jakimiuk A.J. Agarwal S.K. Weitsman S.R. Buyalos R.P. Magoffin D.A. Serum immunoreactive leptin concentrations in women with polycystic ovary syndrome.J Clin Endocrinol Metab. 1996; 81: 4166-4169Crossref PubMed Scopus (191) Google Scholar). Leptin is able to interact with the brain centers, where it promotes satiety and energy expenditure (3Ahima R.S. Prabakaran D. Mantzoros C. Qu D. Lowell B. Maratos-Flier J.S. Role of leptin in the neuroendocrine response to fasting.Nature. 1996; 382: 250-252Crossref PubMed Scopus (2653) Google Scholar) and may influence the hypothalamic-pituitary gonadal axis. Conversely, plasma leptin concentrations are affected by either hormonal status or metabolic parameters. In particular, insulin has been proved to stimulate leptin gene expression and leptin secretion in animal models, and some evidence exists also in humans, though with less consistent data (4Cusin I. Sainsbury A. Doyle P. Rohner-Jeanrenaud F. Jeanrenaud B. The ob gene and insulin. A relationship leading to clues to the understanding of obesity.Diabetes. 1995; 44: 1467-1470Crossref PubMed Google Scholar, 5Kolaczynski J.W. Nyce M.R. Considine R.V. Boden G. Nolan J.J. Henry R. et al.Acute and chronic effect of insulin on leptin production in humans.Diabetes. 1996; 45: 699-701Crossref PubMed Scopus (757) Google Scholar). Serum leptin circulates in two fractions, a protein-bound and a free form, which is the biologically active amount (6Sinha M.K. Opentanova I. Ohannesian J.P. Kolacynski J.W. Heiman M.L. Hale J. et al.Evidence of free and bound leptin in human circulation.J Clin Invest. 1996; 98: 1277-1282Crossref PubMed Scopus (512) Google Scholar). The soluble leptin receptor represents the main leptin-binding activity in human blood (7Lammert A. Kiess W. Bottner A. Glasow A. Kratzsch J. Soluble leptin receptor (SLR) represents the main leptin binding activity in human blood.Biochem Biophys Res Commun. 2001; 283: 982-988Crossref PubMed Scopus (242) Google Scholar), and its concentrations are modulated by both hormonal and metabolic parameters: E2 and T levels were found to be, respectively, inversely and positively correlated with the soluble leptin receptor amount; the percentage of body fat is negatively correlated with the soluble leptin receptor; accordingly, insulin and total leptin seem to be negatively correlated with the soluble leptin receptor (8Chan J.L. Bluher S. Yiannakouris N. Suchard M.A. Kratzsch J. Mantzoros C.S. Regulation of circulating soluble leptin receptor levels by gender, adiposity, sex steroids, and leptin: observational and interventional studies in humans.Diabetes. 2002; 51: 2105-2112Crossref PubMed Scopus (207) Google Scholar). The ratio between total leptin concentrations and soluble leptin receptor indicates the free leptin index. Metformin is an insulin-sensitizing agent with an ascertained efficacy in improving the metabolic and hormonal disturbances of PCOS (9Nestler J.E. Stovall D. Akhter N. Iuorno M.J. Jakubowicz D.J. Strategies for the use of insulin-sensitizing drugs to treat infertility in women with polycystic ovary syndrome.Fertil Steril. 2002; 77: 209-215Abstract Full Text Full Text PDF PubMed Scopus (203) Google Scholar). Because insulin is recognized as one of the major determinants of leptin levels, we sought to investigate the effect of a 4-month treatment with metformin on total and free leptin in a group of obese women with hyperinsulinemia and PCOS. We enrolled seven obese patients with PCOS. Obesity was defined as a body mass index (BMI) >27 kg/m2. The mean of age was 23.57 ± 6.97 years (range 19 to 38 years). The diagnostic criteria of PCOS were defined according to the new consensus criteria (10The Rotterdam ESHRE/ASRM-Sponsored PCOS Consensus Workshop Group Revised 2003 consensus on diagnostic criteria and long-term health risks related to polycystic ovary syndrome.Fertil Steril. 2004; 81: 19-25Abstract Full Text Full Text PDF Scopus (4508) Google Scholar, 11Belosi C. Selvaggi L. Apa R. Guido M. Romualdi D. Fulghesu A.M. et al.Is the PCOS diagnosis solved by ESHRE/ASRM 2003 consensus or could it include ultrasound examination of the ovarian stroma?.Hum Reprod. 2006; 21: 3108-3115Crossref PubMed Scopus (75) Google Scholar). Significant liver or renal impairment, neoplasms, cardiovascular disease, and unstable mental illness were considered exclusion criteria. Studies were conducted during the early follicular phase of spontaneous or induced (medroxyacetateprogesterone [MAP] 10 mg/d for 7 days) menstrual cycles (days 3–7). The study was approved by the ethical board of our institution, and a signed informed consent was obtained from each patient. On the first visit, the patients were hospitalized and underwent a gynecologic and medical examination and transvaginal ultrasonography. Blood samples were collected to perform the following laboratory assays: basal hormone assessment: FSH, LH, E2, T, androstenedione, sex hormone–binding protein, DHEAS, total leptin and soluble receptor leptin, and renal and hepatic chemical analysis. Then patients underwent an oral glucose tolerance test. Blood samples were collected basally and, after the ingestion of 75 g glucose in 150 mL water within 5 minutes, at 30, 60, 90, 120, 180, and 240 minutes. Insulin, glucose, and C-peptide were assayed in all samples. Then, therapy with metformin (Glucophage; Merck Pharma, Darmstadt, Germany) was started: 1,500 mg daily for 4 months. After metformin treatment, all patients had a second hospitalization during the early follicular phase of a spontaneous or induced (MAP 10 mg/d for 7 days) menstrual cycle (day 3–7) and repeated the protocol study. Normally distributed variables were compared by Student's paired t-test. Correlation between total leptin, soluble leptin receptor, and BMI, hormonal parameters, and insulin levels were determined by Pearson correlation coefficient. Data are presented as mean ± SD, and significance was set for a P value <.05. As shown in Table 1, no significant changes were observed in the anthropometric characteristics of the studied subjects. At baseline, mean values of insulin (area under the curve) were indicative for a state of hyperinsulinism. A statistically significant decrease (P<.01) in the insulinemic response to the glucose load occurred after treatment. A trend toward improvement, though devoid of statistical significance, was seen in the mean values of T, sex hormone–binding protein, and LH plasma levels. Total leptin basal values were statistically significantly correlated with BMI (P<.05; r = 0.76). We failed to observe any other significant correlation between total leptin or the free leptin index and other parameters under evaluation, both at baseline and at the end of the study. Despite the net amelioration in the glycoinsulinemic assessment, metformin treatment was not able to affect total and free leptin levels, as suggested by the comparison between the baseline values of these variables and those of end therapy.Table 1Effects of metformin treatment on the metabolic and hormonal parameters of obese patients with PCOS.Before metforminAfter metforminSignificanceAge (y)23.57 ± 6.97—BMI (kg/m2)32.64 ± 4.3531.64 ± 3.15NSWaist-to-hip ratio0.80 ± 0.110.81 ± 0.11NSFSH (mIU/mL)4.35 ± 1.474.89 ± 1.18NSLH (mIU/mL)7.3 ± 5.164.44 ± 1.27NSE2 (pg/mL)43.71 ± 16.0934.57 ± 7.41NST (ng/mL)0.62 ± 0.220.50 ± 0.14NSAndrostenedione (ng/mL)2.35 ± 0.952.27 ± 0.76NSDHEAS (ng/mL)2,038.29 ± 574.571,995.71 ± 497.11NSSex hormone–binding globulin (ng/mL)25.29 ± 8.5231.24 ± 7.53NSArea under the curve insulin (IU/L·240 min)21,969.79 ± 7,428.5118,622.93 ± 6,288.59P<.01Total leptin (ng/mL)22.23 ± 5.4122.74 ± 10.78NSSoluble leptin receptors (IU/mL)12.8 ± 3.5913.07 ± 3.52NSFree leptin index (ng/mL)1.81 ± 0.471.85 ± 0.83NSNote: Data are indicated as mean ± SD. NS = not significant. Open table in a new tab Note: Data are indicated as mean ± SD. NS = not significant. Literature studies on the role of leptin in PCOS, although numerous, are rather discordant: although the majority showed no differences in leptin levels between women with PCOS and controls, some others reported higher leptin levels in PCOS, identifying in the elevated rate of hyperinsulinemia the responsible factor for the stimulation of leptin production by the adipose tissue (12Remsberg K.E. Talbott E.O. Zborowski J.V. Evans R.W. McHugh-Pemu K. Evidence for competing effects of body mass, hyperinsulinemia, insulin resistance, and androgens on leptin levels among lean, overweight, and obese women with polycystic ovary syndrome.Fertil Steril. 2002; 78: 479-486Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar, 13Nader S. Riad-Gabriel M.G. Saad M.F. The effect of a desogestrel-containing oral contraceptive on glucose tolerance and leptin concentrations in hyperandrogenic women.J Clin Endocrinol Metab. 1997; 82: 3074-3077Crossref PubMed Scopus (96) Google Scholar, 14Gennarelli G. Holte J. Wide L. Berne C. Lithell H. Is there a role for leptin in the endocrine and metabolic aberrations of polycystic ovary syndrome?.Hum Reprod. 1998; 13: 535-541Crossref PubMed Scopus (50) Google Scholar). Data from the present study, which investigated for the first time the effect of metformin on both free and bound leptin, give evidence against this latter hypothesis. In fact, in spite of a significant decrease in insulin secretion, leptin values remained unvaried. Our results are in line with previous reports dealing with the effect on leptin levels exerted by insulin sensitizers belonging to the thiazolidinediones class (15Sepilian V.P. Crochet J.R. Nagamani M. Serum soluble leptin receptor levels and free leptin index in women with polycystic ovary syndrome: relationship to insulin resistance and androgens.Fertil Steril. 2006; 85: 1441-1447Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar, 16Mantzoros C.S. Dunaif A. Flier J.S. Leptin concentrations in the polycystic ovary syndrome.J Clin Endocrinol Metab. 1997; 82: 1687-1691Crossref PubMed Scopus (190) Google Scholar). Some discordances exist with a previous report on metformin treatment in PCOS, documenting an initial decrement in total leptin levels after the first 2 months of therapy, followed by a return to basal values after two more months (17Morin-Papunen L.C. Koivunen R.M. Tomas C. Ruokonen A. Martikainen H.K. Decreased serum leptin concentrations during metformin therapy in obese women with polycystic ovary syndrome.J Clin Endocrinol Metab. 1998; 83: 2566-2568Crossref PubMed Scopus (0) Google Scholar). Because our observation was conducted only at the fourth month of treatment, we were not able to detect a possible transitory effect of metformin on total leptin levels. The strong linear correlation between BMI and total leptin, observed in this and previous studies (14Gennarelli G. Holte J. Wide L. Berne C. Lithell H. Is there a role for leptin in the endocrine and metabolic aberrations of polycystic ovary syndrome?.Hum Reprod. 1998; 13: 535-541Crossref PubMed Scopus (50) Google Scholar, 15Sepilian V.P. Crochet J.R. Nagamani M. Serum soluble leptin receptor levels and free leptin index in women with polycystic ovary syndrome: relationship to insulin resistance and androgens.Fertil Steril. 2006; 85: 1441-1447Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar, 16Mantzoros C.S. Dunaif A. Flier J.S. Leptin concentrations in the polycystic ovary syndrome.J Clin Endocrinol Metab. 1997; 82: 1687-1691Crossref PubMed Scopus (190) Google Scholar), seems to indicate that adiposity may represent the main determinant of total leptin levels in women with PCOS, as in the general population. Hence it could be borne in mind that the higher incidence of obesity, rather than of hyperinsulinemia, could represent a confounding factor in the comparison of leptin levels between patients with PCOS and the nonaffected women. The novelty of the present study relies on the analysis of the impact of metformin treatment on the soluble leptin receptor. Actually, excepting rare genetic mutations leading to leptin deficiency, human obesity is characterized by high circulating leptin levels, which are unable to exert an adequate anorexigenic effect, leading to a condition known as leptin resistance (18Munzberg H. Bjornholm M. Bates S.H. Myers Jr., M.G. Leptin receptor action and mechanisms of leptin resistance.Cell Mol Life Sci. 2005; 62: 642-652Crossref PubMed Scopus (201) Google Scholar). Disturbances in the interaction between leptin and its own receptors could have a putative role in this abnormality (19Arch J.R. Central regulation of energy balance: inputs, outputs and leptin resistance.Proc Nutr Soc. 2005; 64: 39-46Crossref PubMed Scopus (50) Google Scholar). The shortest isoform of leptin receptor is responsible for the transport of leptin across the blood-brain barrier and the production of circulating leptin receptor extracellular domain (soluble leptin receptor) to complex with leptin (7Lammert A. Kiess W. Bottner A. Glasow A. Kratzsch J. Soluble leptin receptor (SLR) represents the main leptin binding activity in human blood.Biochem Biophys Res Commun. 2001; 283: 982-988Crossref PubMed Scopus (242) Google Scholar, 18Munzberg H. Bjornholm M. Bates S.H. Myers Jr., M.G. Leptin receptor action and mechanisms of leptin resistance.Cell Mol Life Sci. 2005; 62: 642-652Crossref PubMed Scopus (201) Google Scholar). In a recent study, soluble leptin receptor was found to be lower in patients with PCOS than in controls, thus suggesting that an increase in free leptin index could play a compensatory role against leptin resistance in such patients (20Hahn S. Haselhorst U. Quadbeck B. Tan S. Kimmig R. Mann K. et al.Decreased soluble leptin receptor levels in women with polycystic ovary syndrome.Eur J Endocrinol. 2006; 154: 287-294Crossref PubMed Scopus (27) Google Scholar). Polycystic ovary syndrome–related hyperinsulinism could account for this finding: in fact, insulin seems to be able to affect soluble leptin receptor secretion negatively, and insulin resistance indexes are considered by some authors as the main predictor of free leptin levels (20Hahn S. Haselhorst U. Quadbeck B. Tan S. Kimmig R. Mann K. et al.Decreased soluble leptin receptor levels in women with polycystic ovary syndrome.Eur J Endocrinol. 2006; 154: 287-294Crossref PubMed Scopus (27) Google Scholar, 21Ogawa T. Hiroshi H. Yamamoto Y. Nishikai K. Miyashita K. Nakamura H. et al.Relationships between serum soluble leptin receptor level and serum leptin and adiponectin levels, insulin resistance index, lipid profile, and leptin receptor gene polymorphisms in the Japanese population.Metabolism. 2004; 53: 879-885Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar). However, other investigators identified in the adiposity the major determinant of soluble leptin receptor concentrations (8Chan J.L. Bluher S. Yiannakouris N. Suchard M.A. Kratzsch J. Mantzoros C.S. Regulation of circulating soluble leptin receptor levels by gender, adiposity, sex steroids, and leptin: observational and interventional studies in humans.Diabetes. 2002; 51: 2105-2112Crossref PubMed Scopus (207) Google Scholar, 21Ogawa T. Hiroshi H. Yamamoto Y. Nishikai K. Miyashita K. Nakamura H. et al.Relationships between serum soluble leptin receptor level and serum leptin and adiponectin levels, insulin resistance index, lipid profile, and leptin receptor gene polymorphisms in the Japanese population.Metabolism. 2004; 53: 879-885Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar). In agreement with the latter hypothesis, our data indicate that a decrease in insulin secretion is not followed by evident changes in the free leptin index, as previously observed also after rosiglitazone treatment (15Sepilian V.P. Crochet J.R. Nagamani M. Serum soluble leptin receptor levels and free leptin index in women with polycystic ovary syndrome: relationship to insulin resistance and androgens.Fertil Steril. 2006; 85: 1441-1447Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar). Notwithstanding that insulin might take part in the regulation of leptin bioavailability, our data require some explanations: BMI, which remained unvaried in our patients, could be by far the most important regulator of total and free leptin levels; alternatively, the parallel pharmacologic amelioration in insulin levels and insulin sensitivity could lead to a balance in the insulin action on the adipocytes. We also evaluated the possible relationship between the leptin system and the hormonal milieu. Probably because of the relatively small sample under evaluation, we were not able to detect any significant change in the endocrine parameters after metformin treatment and, of consequence, we cannot draw any conclusion on a possible role of hormonal change on free and total leptin values. Nevertheless, when the regression analysis of baseline data was performed, neither total leptin nor soluble leptin receptor levels resulted to be correlated with gonadotropins, gonadal or adrenal hormones. This finding could contribute to the discordances regarding this issue: although a previous report documented a significant correlation between DHEAS levels and free leptin index (15Sepilian V.P. Crochet J.R. Nagamani M. Serum soluble leptin receptor levels and free leptin index in women with polycystic ovary syndrome: relationship to insulin resistance and androgens.Fertil Steril. 2006; 85: 1441-1447Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar), other authors failed to find any relation between the hormonal milieu and total (16Mantzoros C.S. Dunaif A. Flier J.S. Leptin concentrations in the polycystic ovary syndrome.J Clin Endocrinol Metab. 1997; 82: 1687-1691Crossref PubMed Scopus (190) Google Scholar) and free (20Hahn S. Haselhorst U. Quadbeck B. Tan S. Kimmig R. Mann K. et al.Decreased soluble leptin receptor levels in women with polycystic ovary syndrome.Eur J Endocrinol. 2006; 154: 287-294Crossref PubMed Scopus (27) Google Scholar) leptin concentrations. Because the presence of obesity has an important impact on the reproductive outcome and long-term health consequences in women with PCOS, further research efforts are needed to understand the role, if any, of the leptin system in this disorder.