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Serum Leptin Concentrations in Women with Polycystic Ovary Syndrome

Department of Pharmacology and Clinical Pharmacology (J.R., R.H., M.K.), University of Turku; Department of Obstetrics and Gynecology (L.A., T.P.), Turku University Central Hospital; Department of Clinical Chemistry (P.K., K.I.), Turku University Central Hospital, Turku, Finland

Address all correspondence and requests for reprints to: Dr. Juha Rouru, M.D., Department of Pharmacology and Clinical Pharmacology, University of Turku, Kiinamyllynkatu 10, FIN-20520 Turku, Finland. E-mail: juha.rouru{at}utu.fi

	Abstract

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The role of gonadotropins, androgens, and insulin in the regulation of circulating leptin levels is obscure. In order to clarify the relationships of these parameters we studied serum leptin levels in 19 healthy control subjects and in 35 hyperandrogenic and hyperinsulinemic patients with polycystic ovary syndrome (PCOS).

Serum leptin concentrations did not differ significantly between PCOS patients and control subjects. When PCOS and control groups were analyzed together by univariate analysis, serum leptin was positively correlated with body mass index (BMI), body weight, serum insulin, serum triglyceride, and serum free testosterone concentrations. Serum leptin was inversely correlated with serum sex hormone binding globulin (SHBG) concentrations. There were no significant correlations between serum leptin and testosterone, androstenedione, or gonadotropin concentrations. Serum insulin, triglyceride, and free testosterone concentrations were positively correlated, and serum SHBG was negatively correlated with BMI. However, when BMI on one hand and serum insulin, triglyceride, free testosterone, or SHBG on other hand were used as independent variables in the partial correlation analysis with leptin, BMI turned out to be the variable primarily responsible for all of the correlations with leptin.

In conclusion, the concept that circulating leptin levels would be different in PCOS patients than in regularly menstruating control subjects is not supported by our data.

	Introduction

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RECENT IDENTIFICATION of leptin, the protein product of the obese gene, has rapidly increased our understanding of the regulation of body energy balance (1, 2, 3, 4, 5). Leptin appears to be released to circulation from adipose tissue, and it is believed to mediate a signal of the size of body fat stores to hypothalamic centers regulating appetite and energy expenditure (5). Besides that, leptin has been suggested to serve as a permissive signal to reproductive functions (6). Plasma leptin levels correlate well to body weight, body mass index (BMI), and especially to body fat content (7). Fasting plasma insulin levels correlate with circulating leptin levels, which are elevated in insulin resistant subjects (8). However, the role of insulin in the regulation of plasma leptin is controversial. There is also very little data on the role of androgens and gonadotropins in the regulation of plasma leptin levels.

Pre- and postmenopausal women exhibit higher serum leptin levels than men (9). This phenomenon is thought to be explained by higher androgen levels in men. Polycystic ovary syndrome (PCOS) is the most common endocrine disorder causing anovulatory infertility (10, 11, 12). Hypersecretion of androgens is a typical biochemical feature of PCOS. Furthermore, PCOS patients frequently have increased secretion of LH and insulin resistance (10, 11, 12). Thus, PCOS patients are good models for studying androgen-leptin interactions in humans. The aims of the present study were 1) to evaluate the effects of BMI and PCOS on the levels of leptin in serum; 2) to evaluate correlation of serum leptin with markers of hyperandrogenism and insulin resistance in PCOS.

	Materials and Methods

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Subjects and study design

Subjects for the analysis of serum leptin concentrations were collected from a study published earlier (13). The characteristics of the study groups are given in Table 1. The PCOS study group consisted of 35 oligo/amenorrheic women with PCOS diagnosed by ovarian morphology assessed by vaginal ultrasonography (14). The criteria for a polycystic ovary were: an enlarged or normal sized ovary with multiple (10 or more) small subcortical follicles (2–10 mm in diameter). Biochemical hyperandrogenism (i.e. elevated serum testosterone and/or androstenedione concentrations and/or increased testosterone/sex hormone binding globulin (SHBG) ratio) was found in all the women with PCOS. The control group consisted of 19 healthy women with regular menstrual cycles (26–30 days) and with no signs of hyperandrogenism. Normal ovaries were verified by vaginal ultrasonography. The PCOS patients were examined during their oligo/amenorrheic periods, and the controls during the early follicular phase of the cycle (period days 3–7). All subjects attending the study were caucasians (Finns), and all were both euthyroid and normoprolactinemic. They had not used any hormonal medication for at least 2 months before the study. Blood samples were obtained at 0800, after an overnight fast. The sera were stored at -20 C until analyzed.

Serum leptin was measured by commercial radioimmunoassay kit with human leptin as a standard (Linco Research Inc, St. Charles, MO). The intraassay coefficient of variation was 3.9%, and interassay coefficient of variation was 4.7% at the mean serum leptin concentration of 10.4 µg/L (values given by the manufacturer). Serum insulin, triglyceride, testosterone, free testosterone, androstenedione, SHBG, LH, and FSH were determined as described earlier (13).

Statistical analysis

Statistical analysis of the data was carried out by analysis of covariance (ANCOVA) or by Student’s t-test. BMI was calculated by dividing body weight by height², and this value was used as a covariate in ANCOVA. In correlation analyses Pearson correlation coefficients were used. If a parameter was correlated with both leptin and BMI, partial correlations with BMI as a second independent variable were calculated. If necessary, log transformation of the data was performed before analysis. The calculations were performed by Statistica software (version 4.5 win, StatSoft Inc., Tulsa, OK). A P-value less than 0.05 was considered statistically significant.

	Results

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Serum leptin levels were 12.5 ± 10.4 and 19.7 ± 17.6 µg/L in control subjects and PCOS patients, respectively (means ± SD), and the statistical difference was not significant (ANCOVA: P = 0.38). Because PCOS patients were slightly heavier than control subjects (Table 1.), the data was analyzed separately for the subjects whose BMI was lower than 30. In these subgroups no statistically significant difference was observed in serum leptin values (P = 0.26, Student’s t-test for unpaired data). The BMIs were 23.0 ± 2.6 and 23.0 ± 3.5 kg/m², and serum leptin concentrations were 10.4 ± 5.1 and 13.4 ± 9.8 µg/L (means ± SD) in these control (n = 18) and PCOS (n = 24) subgroups, respectively.

In simple linear regression analysis serum leptin was positively correlated with BMI (r = 0.79, P < 0.001, Fig. 1), body weight (r = 0.73, P < 0.001), serum insulin (r = 0.55, P < 0.001, Fig. 1), serum triglyceride (r = 0.49, P < 0.001), and serum free testosterone (r = 0.40, P = 0.004, Fig. 2) concentrations. Serum leptin was inversely correlated with serum SHBG concentrations (r = -0.49, P = 0.001, Fig. 2). When PCOS and control groups were analyzed separately the correlations for serum leptin with BMI, body weight, and serum insulin were significant in both groups (Table 2). In contrast, the correlations of leptin with serum triglyceride, SHBG, and free testosterone existed only in PCOS group (Table 2). There were no significant correlations between serum leptin and testosterone (r = -0.13), androstenedione (r = 0.15), LH (r = -0.013), or FSH (r = -0.058) concentrations.

View larger version (7K): [in this window] [in a new window]   Figure 1. The relationship between serum leptin concentrations and BMI (r = 0.79, P < 0.001) or fasting serum insulin (r = 0.55, P < 0.001) in PCOS patients and control subjects

View larger version (7K): [in this window] [in a new window]   Figure 2. The relationship between serum leptin and SHBG (r = -0.49, P = 0.001) or free testosterone (r = 0.40, P = 0.004) concentrations in PCOS patients and control subjects

	Discussion

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The present study confirms the finding that circulating leptin levels correlate with body weight and BMI in humans (7, 15). This appears to be the case both in PCOS patients and control subjects. The main finding of the present study was that serum leptin concentrations were not significantly different in PCOS and control subjects. This is in disagreement with the paper published during the preparation of our manuscript (16), where PCOS patients were found to have about two times higher circulating leptin levels than control subjects. When analyzing correlations of serum leptin with BMI in that study, there seemed to be a subgroup of PCOS patients in whom circulating leptin levels were higher than predicted by the BMIs. We failed to find this kind of subgroup in our Finnish PCOS population. Other than ethnic difference, no obvious explanations can be provided at the moment.

In our study serum leptin concentrations correlated with fasting serum insulin, triglyceride, free testosterone, and SHBG concentrations, which is consistent with the results presented by Brzechffa et al. (16). However, by using partial correlation analysis, these correlations were accounted by the correlations of these parameters with BMI. This kind of analysis was missing from the paper of Brzechffa et al.

In humans, insulin does not acutely affect plasma leptin levels (17, 18, 19, 20). Even so, leptin may be regulated by insulin in the long term as long-lasting hyperglycaemic clamp causes elevation in circulating leptin levels (17, 19, 20). In insulin resistant, hyperinsulinemic men, plasma leptin levels are higher than in men matched for body fat mass and percentage of body fat (8). In contrast, in weight-matched noninsulin-dependent diabetes melitus patients and in control subjects serum leptin levels were similar (21). As expected, fasting plasma insulin levels were clearly higher in PCOS patients than in control subjects. This was also clearly evident even if BMI was used as a covariate. These results suggest that insulin resistance is not solely dependent on obesity in these patients, but that PCOS also contributes to insulin resistance, as reported earlier (10). However, as serum leptin levels were similar in PCOS patients and in control subjects, the concept that circulating leptin levels would be elevated in insulin resistant states such as PCOS independent of obesity, is not supported by our findings.

In addition to its role in the regulation of body energy balance, leptin has been recently suggested to serve as a permissive signal to the reproductive system (6). Thus, the interaction of leptin with gonadotropins and sex steroids is of particular interest. In obese ob/ob mice, which do not have circulating leptin, treatment with leptin restores fertility (22) and elevates circulating LH and FSH levels (6). Leptin treatment also increases the weights of gonads both in male and female ob/ob mice, which suggests increased sex steroid production in these animals (6). In a recent report plasma leptin levels were higher in females than in males after correction with body fat mass, suggesting that androgens could have a suppressive effect on plasma leptin levels (8). In the present study serum leptin concentrations were not different in PCOS patients and control subjects, although each group had clearly different circulating androgen and gonadotrophin levels. Furthermore, serum leptin was not correlated to serum FSH, LH, testosterone, or androstenedione concentrations, and the correlation of leptin with serum free testosterone was explained by correlation with BMI.

In conclusion, the concept that circulating leptin levels in PCOS patients would differ from that in regularly menstruating control subjects is not supported by our data.

	Acknowledgments

 
Ms. Taina Lehti is acknowledged for excellent technical assistance.

Received December 5, 1996.

Revised January 27, 1997.

Accepted February 28, 1997.

	References

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This Article Abstract Full Text (PDF) Submit a related Letter to the Editor Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Rouru, J. Articles by Koulu, M. Search for Related Content PubMed PubMed Citation Articles by Rouru, J. Articles by Koulu, M.