Leptin, the ob gene product, in female health and disease

Leptin is a recently discovered hormone which is involved in the regulation of body weight. It provides a molecular basis for the lipostatic theory of the regulation of energy balance. White adipose tissue is the main site of leptin synthesis and there is some evidence of ob gene expression in brown fat. Leptin seems to play a key role in the control of body fat stores by coordinated regulation of feeding behaviour, metabolic rate, autonomic nervous system regulation and body energy balance in rodents, primates and humans. Apart from the function of leptin in the central nervous system on the regulation of energy balance, it may well be one of the hormonal factors that signal the body's readiness for sexual maturation and reproduction to the brain. During late pregnancy and at birth when maternal fat stores have been developed leptin levels are high. Leptin could then be a messenger molecule signaling the adequacy of the fat stores for reproduction and maintenance of pregnancy. At later stages of gestation leptin could signal the expansion of fat stores in order to prepare the expectant mother for the energy requirements of full term gestation, labour and lactation. This overview focuses on those topics of leptin research which are of particular interest in reproductive medicine and gynecology.     

The role of the leptin in reproduction

PURPOSE OF REVIEW: Since its discovery in 1994, leptin has appeared to be a pleiotrophic hormone, governing energy homeostasis and affecting many tissues in the body. Numerous pieces of evidence have accumulated showing that leptin potentially plays an important role in the control of the reproductive function. RECENT FINDINGS: This review presents the major concepts for the role of leptin in the modulation of reproductive function. As a marker of the nutritional status, leptin affects the hypothalamo-pituitary-gonadal axis, regulating gonadotrophin-releasing hormone and luteinising hormone secretion, and appears to be a permissive factor in the onset of the puberty. This protein and its receptor have been found in the reproductive tissues, indicating that this system could be also implicated in several processes such as embryo development, implantation and pregnancy. Moreover, disorders of the leptin system have been related to some reproductive pathologies such as pre-eclampsia and polycystic ovary syndrome. However, controversy surrounds several aspects of the action of leptin in reproduction that require a deeper investigation of this system. SUMMARY: Results to date suggest that this system could be implicated in important reproductive processes such as embryonic development and implantation. Moreover, understanding the role of leptin might be useful for new treatments in reproductive pathologies.     

Appetite control and reproduction: leptin and beyond

It is now recognized that appropriate regulation of reproduction, energy intake, and energy expenditure, and thus maintenance of body weight and fertility, relies on complex hypothalamic neurocircuitry. Feeding and reproductive function are closely linked. During times of undernourishment and falling body fat the reproductive axis is down-regulated. Circulating factors and hypothalamic circuits coordinate these responses. Leptin has been described to be an important peripheral signal that indicates body fat stores to the hypothalamus and thus links nutrition and reproduction. Leptin acts by altering neuropeptide circuits in the hypothalamus, which alter gonadotropin-releasing hormone release and food intake. The importance of key neuropeptide systems identified in rodents is now being established in man. Notably mutations in the melanocortin MC4 receptor are found in up to 4% of the morbidly obese, and in a proportion of patients with anorexia nervosa mutations have been identified in the agouti-related peptide gene (AgRP), which codes for an endogenous antagonist of this receptor. Intranasal administration of a melanocortin fragment known to activate the MC4 receptor decreases adiposity in humans. The melanocortin system has been shown to influence the reproductive axis in rodents. However, the role of the melanocortin system in the control of reproduction in humans remains to be established. Since the discovery of leptin, attention has also been focused on peripheral signals that regulate reproduction, food intake, and energy expenditure, either directly or via feedback on hypothalamic circuits. Notable new discoveries in this area include the gastric hormone ghrelin. Circulating ghrelin stimulates food intake in rodents and humans, although an influence on the reproductive axis is yet to be reported.     

Leptin

Leptin, a central hunger-regulating metabolic hormone discovered in 1994, is important in regulating energy homeostasis and exerts its regulatory role also on other endocrine functions at hypothalamic, pituitary and gonadal levels. The leptin signaling pathway is complex and needs to be further elucidated. Several conditions, including changes in body weight, are associated with altered serum levels of leptin. Obesity presents with leptin resistance, a decreased sensitivity for endogenous leptin, resulting in elevated leptin levels. During times of malnutrition, low leptin levels correlate with cycle abnormalities, especially with amenorrhea. In animal models, there is experimental evidence that leptin deficiency is associated with infertility. Therefore, leptin is thought to mediate between adipose tissue and reproductive function. In addition to its effects on menstrual cyclicity and fertility, leptin is linked to the onset of puberty and to pregnancy-specific pathologies that include fetal growth restriction. Although a variety of studies support the significant interactions between leptin and the reproductive system, clarification of its exact pathophysiological role needs further studies. Even so, the determination of leptin is not currently a routine parameter to assess metabolic or reproductive function.     

Leptin and its influence on the main gynecoobstetric diseases

Leptin is a protein hormone synthesized and secreted by adipose tissue and also probably in other organs and systems in human body. It has multiple functions such as to regulate feed intake and energy balance, gonadal regulation, action in the hypothalamo-pituitary-gonadal axis, regulates the metabolism of the fetal-placental unit in the pregnancy, fertility and reproductive systems, actions in the endometrium, mammary gland with corresponding influences on important physiologic processes such as menstruation, pregnancy and lactation. In the gynecologic surgery the serum leptin concentration is also modified. The knowledge of serum leptin concentration in the oncological diseases is going-up. Leptin is modified in the choriocarcinoma, Meigs' syndrome and other tumors. A better understanding of regulatory mechanisms will have direct clinical significance, as leptin has been proposed to impact on those causes of human perinatal morbidity and mortality that are associated with abnormalities of fetal maturity and development, general concept growth, trophoblast endocrinology, and placental sufficiency. Further investigations in this area will be necessary to improve new knowledge and a better understanding of the actions about this hormone.     

Leptin is not involved in the pathophysiology of endometriosis-related infertility

OBJECTIVE: Changes in peritoneal fluid (PF) composition may affect fertilization as well as early embryonic development. Leptin, an adipocyte hormone, has been shown to act as a link between adipose tissue and the reproductive system. Therefore, we decided to assess peritoneal and serum leptin levels in infertile endometriotic patients. PATIENTS: Seventy-two women were studied, including 30 fertile and 18 infertile women with ovarian endometriotic cysts and, as a reference group, 24 patients with unexplained infertility. RESULTS: No significant difference in the peritoneal and leptin levels was found between the studied groups. Significantly higher PF leptin concentration was observed in patients with stages III and IV of endometriosis as compared to those with minimal stage of the disease. In fertile patients with endometriosis a positive correlation has been found between PF and serum leptin concentrations. CONCLUSIONS: No differences in peritoneal or serum leptin levels between infertile and fertile women with endometriosis suggest that this cytokine is not involved in pathophysiology of endometriosis-related infertility.     

Possible interactions between leptin,gonadotrophin-releasing hormone (GnRH-I and II) and human chorionic gonadotrophin (hCG)

Leptin is a metabolic signal to the reproductive axis, where it increases the plasma levels of luteinising hormone (LH) and follicle stimulating hormone (FSH). Since the placental regulation of human chorionic gonadotrophin (hCG) mimics that of the pituitary LH, we undertook this study to see if leptin could be involved in the secretion and synthesis of hCG in first-trimester trophoblast. We incubated cytotrophoblastic cells (CTB) with GnRH-I or GnRH-II, for 4 or 48 h and collected the media at different times thereafter. GnRH-II was more potent than GnRH-I when incubated for 4 h with CTB. Leptin secretion, as measured at 4 h, was significantly stimulated by GnRH-II. When measured at 24 h leptin values were also increased as compared to controls. Neither GnRH-I, nor GnRH-II had any effect on leptin secretion when incubated for 48 h with CTB. Leptin was also added to perifused placental explants, and samples (in which hCG was measured) were collected every 3 min. Leptin significantly stimulated hCG secretion by explants and induced a pulse of hCG immediately (within 6 min) after its injection, increasing significantly the area under the curve (P=0.04) and the amplitude (P=0.02) of hCG pulses. We conclude that GnRH-II is more effective than GnRH-I in stimulating leptin secretion. This difference could be explained by the existence of two different types of placental GnRH receptors or two different pathways of GnRH degradation. Furthermore, we observe that leptin has a significant stimulatory effect on hCG pulsatility.     

Leptin and ovarian folliculogenesis: implications for ovulation induction and ART outcomes

Leptin participates in regulation of ovarian folliculogenesis indirectly via control of luteinizing hormone and follicle-stimulating hormone secretion. More recent evidence suggests that leptin also has direct regulatory actions on the developing follicle. The presence of leptin receptors on follicular cells, including oocytes, and early preimplantation embryos suggests that leptin may play a direct physiologic role in follicular maturation, oocyte development, and early cleavage. Because circulating leptin levels are directly related to body adiposity, elevated leptin concentrations associated with obesity may partly explain the negative impact of obesity on fertility. The influence of leptin on follicular development and oocyte maturation has important implications for ovulation induction and assisted reproductive technologies. Moreover, polycystic ovarian syndrome may be associated with altered leptin phsyiology.     

Leptin regulation of aromatase activity in adipose stromal cells from regularly cycling women.

OBJECTIVES AND DESIGN: Leptin, a product of adipocytes, is a cytokine with multiple effects on the reproductive axis. Leptin causes the activation of STAT proteins within target cells. The aromatase gene promoter in adipose stromal cells contains a functional STAT binding region, leading to the hypothesis that leptin may regulate aromatase activity in fat tissue. To test this hypothesis, adipose stromal cells were isolated from subcutaneous abdominal fat or breast fat then placed into tissue culture. MATERIALS AND METHODS: The cells were treated for three days with increasing concentrations of recombinant human leptin. Aromatase activity in the stromal cells was measured by the release of 3H2O from radiolabeled androstenedione precursor. RESULTS: Basal aromatase activity varied markedly between, but there were no differences between abdominal fat and breast fat. Leptin concentrations in the physiological range of normal weight or thin women (10 ng/ml) had no effect on aromatase activity. In 2 of 8 abdominal fat cultures and 1 of 2 breast fat cultures, a high obese concentration of leptin (100 ng/ml) stimulated a significant increase in aromatase activity. In the remaining subjects there was no effect of leptin, even at high concentrations. CONCLUSIONS: These data demonstrate that in approximately 30 percent of our subject population leptin was able to stimulate aromatase activity in adipose stromal cells at high concentrations. The elevated levels of aromatase activity may contribute to increase circulating estrogen levels in certain obese women and suggest that elevated leptin concentrations in obese women may cause locally elevated estrogen concentrations in the breast and thereby promote tumor formation.     

Peritoneal fluid and serum leptin concentrations in women with primary infertility

Aim  Leptin is proposed to participate in the reproductive system of women by acting on either ovaries or hypothalamic-pituitary axis. The objective of the present study is to investigate the leptin concentrations in peritoneal fluid and serum samples of women diagnosed with primary infertility. Methods  A prospective study was carried out in women who underwent laparoscopy within the diagnostic process of primary infertility between January 2005 and January 2007. Leptin concentrations were determined in blood samples obtained before surgery and in peritoneal fluid samples collected during laparoscopy. Results  Peritoneal fluid was obtained from 112 subjects; 21 with unexplained infertility 28 with polycystic ovary syndrome (PCOS), 30 with bilateral tubal occlusion, and 33 with endometriosis. Subjects with PCOS have significantly higher body weights, BMI values and plasma leptin levels when compared to other study groups. Peritoneal fluid levels of leptin were significantly higher in the endometriosis group compared to other three study groups. A positive correlation was found between peritoneal fluid leptin levels and the endometriosis stage (r = 0.51, P = 0.01). However, plasma leptin levels were unrelated to the disease extent. Discussion  It might be hypothesized that leptin may be an active factor in the pathogenesis of PCOS and endometriosis, which are two major causes of primary infertility. A mild leptin deficiency in peritoneal environment may interrupt follicular development and ultimately lead to PCOS. Leptin has angiogenic and mitogenic properties, which trigger inflammatory cytokines and eventually result in the development of endometriosis implants. Significantly, higher levels of leptin in peritoneal environments of endometriosis subjects strongly imply the important role of this common pathology.     

Metabolic Fuel and Clinical Implications for Female Reproduction

Reproduction is a physiologically costly process that consumes significant amounts of energy. The physiological mechanisms controlling energy balance are closely linked to fertility. This close relationship ensures that pregnancy and lactation occur only in favourable conditions with respect to energy. The primary metabolic cue that modulates reproduction is the availability of oxidizable fuel. An organism’s metabolic status is transmitted to the brain through metabolic fuel detectors. There are many of these detectors at both the peripheral (e.g., leptin, insulin, ghrelin) and central (e.g., neuropeptide Y, melanocortin, orexins) levels. When oxidizable fuel is scarce, the detectors function to inhibit the release of gonadotropin-releasing hormone and luteinizing hormone, thereby altering steroidogenesis, reproductive cyclicity, and sexual behaviour. Infertility can also result when resources are abundant but food intake fails to compensate for increased energy demands. Examples of these conditions in women include anorexia nervosa and exercise-induced amenorrhea. Infertility associated with obesity appears to be less related to an effect of oxidizable fuel on the hypothalamic-pituitary-ovarian axis.Impaired insulin sensitivity may play a role in the etiology of these conditions, but their specific etiology remains unresolved.Research into the metabolic regulation of reproductive function has implications for elucidating mechanisms of impaired pubertal development, nutritional amenorrhea, and obesity-related infertility. A better understanding of these etiologies has far-reaching implications for the prevention and management of reproductive dysfunction and its associated comorbidities.     

Food intake and leptin concentrations of lactating rats nursing various sized litters

Purpose:   Leptin, the obesity ( ob ) gene product, has been proposed as an adipose-related satiety factor. It might act as a signal, from peripheral adipose stores to the central nervous system, to decrease food intake and increase energy expenditure. Conversely, low leptin concentrations after fasting or energy restriction might act to increase food intake. The present study investigated the role of the suckling stimulus in the mechanisms regulating body weight, food consumption and leptin concentration in lactating rats nursing different sized litters.
Methods:   The day of parturition was designated as day 0 of lactation. The rats were allocated randomly to three groups and the number of pups per litter was adjusted within one day of birth to eight (Group A), four (Group B) and two (Group C), with equal numbers of both sexes. From day 1 to day 10 of lactation, the separate weights of the mothers and pups, and the mothers' food intake were measured daily. On day 10, the rats were decapitated and blood was collected from the mothers for measurement of serum concentrations of leptin by ELISA.
Results:   Food intake by the mothers in Groups B and C decreased by about 20% and 30%, respectively, by day 10 of lactation, compared with Group A, but the serum leptin concentrations of those in Group C increased by about 80%, compared with Groups A and B. There was a high positive correlation between leptin concentration and fat pad weight.
Conclusion:   The findings suggest that the smaller litter in Group C reduced the energy cost of lactation, which induced an increase in the serum leptin concentration. (Reprod Med Biol 2005; 4 : 203–206)     

Leptin in relation to growth and developmental processes in the fetus

Leptin, a 16-kilodalton protein secreted by the adipose tissue in proportion to the amount of energy stored in adipose tissue, conveys to the hypothalamus information on energy homeostasis and regulates reproductive function. In addition, there is accumulating evidence that leptin produced by placental or fetal tissues acts through specific leptin receptors to regulate fetal growth and development. Although leptin levels are correlated with insulin and insulin-like growth factor (IGF)-1 levels, observational studies in humans indicate that its effects on fetal growth are independent of these axes and of adiposity. The extent to which leptin per se mediates the fetal growth and developmental abnormalities associated with disease states such as diabetes, hypoxia, or preeclampsia remains to be fully clarified by future studies in humans. It is hoped that clarification of these mechanisms may provide novel therapeutic approaches.     

The role of leptin in fertility

The relationship between metabolism and reproduction remains a mystery in female endocrinology. Such substances as insulin, amino acids and IGFBP-I have been proposed as signals of body mass fat on the genital axis. Today this role is claimed by leptin, a protein hormone decoded from the obesity gene and is secreted exclusively from adipose tissue. This hormone acts on the central nervous system (CNS) to result in the suppression of food intake and increase in energy consumption. What is more, it also influences the capacity for reproduction. This paper reports findings with regard to the factors influencing the secretion of leptin and identification of the leptin's hormonal receptors. Particular emphasis was placed on the relationship between secretion of leptin and disturbances in menstruation, the anticipated role of this hormone in the pathogenesis of the polycystic ovarian syndrome (PCOS) and its effects on the reproductive capacity.     

Leptin Affects System A Amino Acid Transport Activity in the Human Placenta: Evidence for STAT3 Dependent Mechanisms

BackgroundAmino acids are important nutrients during fetal development, and the activity of placental amino acid transporters is crucial in the regulation of fetal growth. Leptin, an adipocyte- and placenta-derived hormone, has been proposed to act as a peripheral signal in reproduction in humans. Leptin is elevated during pregnancy and elevated further in pathologic pregnancies such as preeclampsia. However, the role of leptin in placental function has not been fully elucidated. We hypothesize that leptin plays a role in the regulation of placental amino acid transport by activation of the JAK–STAT pathway.MethodsPlacental amino acid transport, specifically system A transport was studied in placental villous fragments using the amino acid analog, methylaminoisobutyric acid (MeAIB). Specific inhibitors of the JAK–STAT signal transduction pathway were used to further elucidate their role in leptin-mediated effects on amino acid transport activity. Western blotting was performed to identify STAT3 phosphorylation as a measure of leptin receptor activation.ResultsLeptin significantly increased system A amino acid transporter activity by 22–42% after 1 h of incubation. Leptin activated JAK–STAT signaling pathway as evidenced by STAT3 phosphorylation, and inhibition of STAT3 or JAK2 resulted in 36–45% reduction in system A amino acid transporter activity. Furthermore, blocking endogenously produced leptin also decreased system A transport by 45% comparable to STAT3 inhibition.ConclusionsThese data demonstrate that leptin stimulates system A by JAK–STAT dependent pathway in placental villous fragments. Our findings support the autocrine/paracrine role of leptin in regulating amino acid transport in the human placenta.     

Effects of sports training in adolescence on growth,puberty and bone health

Athletic training in adolescent females is important for their well-being; indeed, it may have both positive and negative effects on some physiological processes, as growth, reproductive axis and bone health. Adequate physical activity likely exerts neither a positive nor a negative effect on growth. By contrast, intensive training and insufficient diet may have a negative influence on growth, probably due to energy deficiency and impairment of the growth hormone–insulin-like growth factor-I axis; net long term-effects of such alterations remain to be established. Adolescents who perform regular athletic training present with normal or slightly advanced sexual maturation, because increased strength and power associated with earlier maturation advantage them. However, intensive training and inadequate energy intake may induce delayed menarche and menstrual dysfunctions. The consequent hypoestrogenism, in association with the nutritional deficiencies, may affect bone health. On the contrary, regular physical activity increases the amount of bone mass gained during childhood and adolescence mainly at the bone sites which are trained. Since the number of adolescent females involved in strenuous sports from an early age is increasing, physicians must be aware of such effects, explain to girls and their parents the ‘right’ sports training and appropriate dietary regimens, and recognize problems due to excessive training as soon as possible. These issues should not be a cause of lesser involvement in athletic participation of young people.     

Leptin and pubertal development

Sexual development after birth in rodents, nonhuman primates, and humans is driven by the gonadotropin-releasing hormone (GnRH) pulse generator. During the neonatal period in primates, pulsatile GnRH discharge from the medial basal hypothalamus drives an active period of pituitary gonadotropin and gonadal hormone secretion. During the transition from the neonatal to the juvenile period, however, the activity of the GnRH pulse generator is restrained or arrested and gonadotropin and gonadal hormone secretion enters a quiescent period that continues until the onset of puberty. As puberty approaches the GnRH pulse generator is reactivated, resulting in enhanced gonadotropin secretion, accelerated growth, maturation of the gonads, and the achievement of sexual competence. Rodents do not appear to exhibit a developmental phase analogous to the quiescent juvenile period in primates when the GnRH pulse generator is held in check. Instead, progressive maturational changes in the pattern of GnRH pulsatility appear to drive sexual development in rodents. The role that leptin plays in sexual development has not been fully defined, but the balance of current evidence appears to support the idea that, in both rodents and primates, leptin plays a permissive rather than a causal role in timing this process. When body energy reserves rise above a critical level, blood leptin increases to a threshold concentration signaling to the central nervous system that the body can support sexual function. Puberty can apparently occur over a wide range of concentrations above this critical leptin threshold. Leptin does not appear to act as a trigger to time the initiation of puberty but, instead, once leptin reaches this threshold pubertal development may proceed if, and only if, other critical control mechanisms are operational.     

The relation of serum and follicular fluid leptin and ovarian steroid levels in response to induction of ovulation in in vitro fertilization cycles

OBJECTIVE: Leptin restores energy homeostasis and regulates appetite and body weight by communicating the energy status to the central nervous system. Although there is strong evidence that leptin affects reproduction, its role in the control of reproductive physiology is little understood. STUDY DESIGN: We studied leptin concentrations in the serum and follicular fluid of 65 women undergoing ovarian hyperstimulation for in vitro fertilization (IVF). Fasting serum samples were collected (1) on the 3rd day of the cycle before IVF and (2) at the time of oocyte retrieval. Serum concentrations of leptin, estradiol (E2), progesterone, FSH, LH, prolactin, total testosterone, DHEA-SO4, and TSH and follicular fluid concentrations of leptin, E2, and progesterone were measured. RESULTS: Serum leptin values increased on average by 66.4% over basal leptin levels on the day of oocyte pick-up (OPU). A positive correlation between leptin increase and body mass index was observed. The serum leptin level was similar to that in follicular fluid o the day of OPU. E2 levels increased 34.5-fold with controlled ovarian hyperstimulation. There was a negative correlation between the increase in leptin levels and in E2 levels (P <0.05) and in the number of oocytes harvested (P <0.05). CONCLUSION: The significant increase in serum leptin levels during controlled ovarian hyperstimulation indicates a possible role of leptin in reproductive function. The increase in leptin levels is negatively correlated with ovarian response evaluated by E2 production and number of oocytes retrieved. This might be due to the reduced ovarian response through negative feedback of leptin to the ovaries at high levels.     

Placental leptin and pregnancy pathologies

Leptin, the protein encoded by the Ob gene, is produced by the white adipose tissue and by the placenta during pregnancy. Placental leptin production makes a substantial contribution to maternal circulating levels during pregnancy which rapidly decrease and return to normal after delivery. Leptin has been detected in fetal plasma as early as week 18 of gestation, and umbilical leptin concentrations are closely related to birth weight. This has led to the hypothesis that fetal fat mass mainly determines fetal circulating leptin. Placental leptin production is increased in choriocarcinoma, preeclampsia and type 1 diabetes. Estrogens, hypoxia and insulin have been suggested as positive regulators of placental leptin production. Maternal leptinemia might act as a sensor of energy balance during pregnancy. The presence of both leptin and leptin receptors in the placenta suggests that leptin can act by autocrine or endocrine pathways in the human placenta. The roles of fetal leptin and consequences of increased placental leptin production in pathological pregnancies have yet to be elucidated.     

The known and unknown of leptin in pregnancy

Leptin, which was identified originally as an adipocyte-derived protein, was regarded for years as an exclusive regulator of satiety and energy homeostasis. A role for leptin in pregnancy was later suggested by the findings that plasma levels during gestation are greater than in nongravid individuals and that leptin is synthesized within the fetoplacental unit. Observational studies have established that leptin production is dysregulated in several pathologic stages of pregnancy in association with alterations of fetal growth. For example, an overproduction of leptin by the placenta in pregnancy with diabetes mellitus or hypertension is associated with maternal hyperleptinemia. Evidence is also accumulating that umbilical leptin levels can be viewed as a biomarker of fetal adiposity. Ten years after its discovery as a hormone, we review the known and unknowns of leptin in pregnancy with particular emphasis on its functions in health and disease. We aim to demonstrate that studies of leptin in pregnancy largely have contributed to insight into the mechanisms of leptin action, both as a hormone and as a cytokine.