Critical role of STAT3 in leptin's metabolic actions

Leptin controls adipose tissue lipogenesis via central, STAT3-independent mechanisms

Leptin (encoded by Lep) controls body weight by regulating food intake and fuel partitioning. Obesity is characterized by leptin resistance and increased endocannabinoid tone. Here we show that leptin infused into the mediobasal hypothalamus (MBH) of rats inhibits white adipose tissue (WAT) lipogenesis, which occurs independently of signal transducer and activator of transcription-3 (STAT3) signaling. Correspondingly, transgenic inactivation of STAT3 signaling by mutation of the leptin receptor (s/s mice) leads to reduced adipose mass compared to db/db mice (complete abrogation of leptin receptor signaling). Conversely, the ability of hypothalamic leptin to suppress WAT lipogenesis in rats is lost when hypothalamic phosphoinositide 3-kinase signaling is prevented or when sympathetic denervation of adipose tissue is performed. MBH leptin suppresses the endocannabinoid anandamide in WAT, and, when this suppression of endocannabinoid tone is prevented by systemic CB1 receptor activation, MBH leptin fails to suppress WAT lipogenesis. These data suggest that the increased endocannabinoid tone observed in obesity is linked to a failure of central leptin signaling to restrain peripheral endocannabinoids.

Mechanisms of leptin action and leptin resistance

The adipose tissue-derived hormone leptin acts via its receptor (LRb) in the brain to regulate energy balance and neuroendocrine function. LRb signaling via STAT3 and a number of other pathways is required for the totality of leptin action. The failure of elevated leptin levels to suppress feeding and mediate weight loss in common forms of obesity defines a state of so-called leptin resistance. A number of mechanisms, including the leptin-stimulated phosphorylation of Tyr(985) on LRb and the suppressor of cytokine signaling 3, attenuate leptin signaling and promote a cellular leptin resistance in obesity. Several unique features of the arcuate nucleus of the hypothalamus may contribute to the severity of cellular leptin resistance in this region. Other mechanisms that govern feeding behavior and food reward may also underlie the inception of obesity.

Cross-talk between estrogen and leptin signaling in the hypothalamus

Obesity, characterized by enhanced food intake (hyperphagia) and reduced energy expenditure that results in the accumulation of body fat, is a major risk factor for various diseases, including diabetes, cardiovascular disease, and cancer. In the United States, more than half of adults are overweight, and this number continues to increase. The adipocyte-secreted hormone leptin and its downstream signaling mediators play crucial roles in the regulation of energy balance. Leptin decreases feeding while increasing energy expenditure and permitting energy-intensive neuroendocrine processes, such as reproduction. Thus, leptin also modulates the neuroendocrine reproductive axis. The gonadal steroid hormone estrogen plays a central role in the regulation of reproduction and also contributes to the regulation of energy balance. Estrogen deficiency promotes feeding and weight gain, and estrogen facilitates, and to some extent mimics, some actions of leptin. In this review, we examine the functions of estrogen and leptin in the brain, with a focus on mechanisms by which leptin and estrogen cooperate in the regulation of energy homeostasis.

Adipokines and the peripheral and neural control of energy balance

Adipokines are secreted by adipose tissue and control various physiological systems. Low leptin levels during fasting stimulate feeding, reduce energy expenditure, and modulate neuroendocrine and immune function to conserve energy stores. On the other hand, rising leptin levels in the overfed state prevent weight gain by inhibiting food intake and increasing energy expenditure. These actions are mediated by neuronal circuits in the hypothalamus and brainstem. Leptin also controls glucose and lipid metabolism by targeting enzymes such as AMP-activated protein kinase and stearoyl-coenzyme A desaturase-1 in liver and muscle. Likewise, adiponectin and resistin control energy balance and insulin sensitivity via central and peripheral targets. As highlighted in this review, there are distinct as well as common signaling pathways for adipokines. Understanding adipokine signaling in the brain and other organs will provide insights into the pathogenesis and treatment of obesity, diabetes and various metabolic disorders.

LRb signals act within a distributed network of leptin-responsive neurones to mediate leptin action

The adipose tissue-derived hormone, leptin, acts via its receptor (LRb) in the brain to regulate energy balance and neuroendocrine function. In order to understand leptin action we have explored the physiological function of LRb signalling pathways, defining important roles for signal transducer and activator of transcription-3 (STAT3) in positive signalling and for LRbTyr(985)-mediated feedback inhibition in leptin signal attenuation. As the cells on which leptin acts are not homogeneous, but rather represent a broadly distributed network of neurones with divergent projections and functions, it is also crucial to consider how each of these populations responds to LRb signals to contribute to leptin action. While well-known LRb-expressing neurones within the arcuate nucleus of the hypothalamus mediate crucial effects on satiety and energy expenditure, other populations of LRb-expressing neurones in the ventral tegmental area and elsewhere likely control the mesolimbic dopamine system. Additional populations of LRb-expressing neurones likely contribute to other aspects of neuroendocrine regulation. It will be important to define the molecular mechanisms by which leptin acts to regulate neurophysiology in each of these LRb-expressing neural populations in order to understand the totality of leptin action.

Specific physiological roles for signal transducer and activator of transcription 3 in leptin receptor-expressing neurons

Leptin is a fat-derived hormone that exerts pleiotropic effects on energy balance and neuroendocrine functions. Mice defective in leptin or its receptor [leptin receptor, isoform b (LepRb)] exhibit profound obesity, infertility, and reduced linear growth. Leptin binding to its receptor triggers multiple signaling pathways, including signal transducer and activator of transcription 3 (Stat 3), phosphatidylinositol-3-kinase, and ERK. A considerable amount of effort has been focused on how these signaling pathways mediate diverse leptin functions. Mice containing a mutant LepRb incapable of Stat3 signaling are obese but remain fertile with enhanced linear growth. In contrast, deletion of Stat3 in the whole brain with Nestin-Cre results in infertility and decreased linear growth, in addition to obesity. The additional phenotypes of the Nestin-mediated deletion could reflect Stat3 action in non-LepRb neurons or leptin-independent Stat3 actions in LepRb neurons. To resolve this discrepancy and to gain more insight into the metabolic actions of Stat3, we have generated mice in which Stat3 is disrupted specifically in LepRb neurons after the onset of leptin receptor expression. We show that mutant mice exhibit profound obesity with increased linear growth and normal fertility. In addition, impaired glycemic control in these animals correlates with their degree of obesity. These results demonstrate that Stat3 in LepRb neurons does not regulate linear growth or fertility. These results further suggest that leptin's effects on growth and reproduction are mediated by other signaling pathways, and that Stat3-mediated control of these functions is mediated independently of leptin and LepRb neurons.

Association of serum and follicular fluid leptin concentrations with granulosa cell phosphorylated signal transducer and activator of transcription 3 expression in fertile patients with polycystic ovarian syndrome

OBJECTIVE

Our objective was to evaluate whether polycystic ovarian syndrome (PCOS)-associated infertility is related to alterations of leptin, leptin receptor (Ob-R), and the phosphorylated signal transducer and activator of transcription 3 (p-STAT3)/suppressor of cytokine signal 3 (SOCS3) system in the ovary.

DESIGN AND SETTING

A case-control study was conducted in a university hospital.

PATIENTS

Thirty-one infertile PCOS women with oligoovulation plus polycystic ovarian morphology and 79 infertile women with tubal blockage (control) participated in the study. The subjects were stratified according to in vitro fertilization outcomes: successful and failed subgroups.

METHODS

Serum and follicular fluid (FF) leptin levels were measured with ELISA. RT-PCR and Western blotting were performed to assess expression of mRNA encoding leptin and Ob-R and proteins of p-STAT3 and SOCS3 in granulosa cells (GCs).

RESULTS

Leptin levels in serum and FF of PCOS women were significantly higher than those of control (P < 0.01). There were no significant differences in expression of leptin mRNA and short and long Ob-Rs between PCOS and control (P > 0.05). The p-STAT3 level was decreased in PCOS compared with control (P < 0.01), whereas SOCS3 remained significantly unchanged (P > 0.05). Further analysis showed that serum and FF leptin levels were significantly higher, whereas p-STAT3 in GCs was lower in the failed subgroup of PCOS than those in the successful subgroup of PCOS (P < 0.05).

CONCLUSION

Hyperleptinemia and high FF leptin are important pathologies of PCOS with infertility. Lower levels of p-STAT3 in GCs may be related to ovarian leptin resistance and fecundity in PCOS women. Relatively high serum and FF leptin and low p-STAT3 in GCs may account for decreased fertilization, implantation, and pregnancy rates of in vitro fertilization in PCOS women.

CD36-facilitated fatty acid uptake inhibits leptin production and signaling in adipose tissue

Leptin plays an important role in regulating energy expenditure in response to food intake, but nutrient regulation of leptin is incompletely understood. In this study using in vivo and in vitro approaches, we examined the role of fatty acid uptake in modulating leptin expression and production. Leptin levels are doubled in the CD36-null mouse, which has impaired cellular fatty acid uptake despite a 40% decrease in fat mass. The CD36-null mouse is protected from diet-induced weight gain but not from that consequent to leptin deficiency. Leptin secretion in the CD36-null mouse is strongly responsive to glucose intake, whereas a blunted response is observed in the wild-type mouse. This indicates that leptin regulation integrates opposing influences from glucose and fatty acid and loss of fatty acid inhibition allows unsuppressed stimulation by glucose/insulin. Fatty acid inhibition of basal and insulin-stimulated leptin release is linked to CD36-facilitated fatty acid flux, which is important for fatty acid activation of peroxisome proliferator-activated receptor gamma and likely contributes to the nutrient sensing function of adipocytes. Fatty acid uptake also may modulate adipocyte leptin signaling. The ratio of phosphorylated to unphosphorylated signal transducer and activator of transcription 3, an index of leptin activity, is increased in CD36-null fat tissue disproportionately to leptin levels. In addition, expression of leptin-sensitive fatty acid oxidative enzymes is enhanced. Targeting adipocyte CD36 may offer a way to uncouple leptin production and adiposity.

Appropriate inhibition of orexigenic hypothalamic arcuate nucleus neurons independently of leptin receptor/STAT3 signaling

Leptin directly suppresses the activity of orexigenic neurons in the hypothalamic arcuate nucleus (ARC). We examined c-Fos-like immunoreactivity (CFLIR) as a marker of ARC neuronal activity in db/db mice devoid of the signaling form of the leptin receptor (LRb) and s/s mice that express LRb(S1138) [which is defective for STAT3 (signal transducer and activator of transcription) signaling]. Both db/db and s/s animals are hyperphagic and obese. This analysis revealed that CFLIR in agouti related peptide-expressing orexigenic ARC neurons is basally elevated in db/db but not s/s mice. Consistent with these observations, electrophysiologic evaluation of a small number of neurons in s/s animals suggested that leptin appropriately suppresses the frequency of IPSCs on ARC proopiomelanocortin (POMC) neurons that are mediated by the release of GABA from orexigenic ARC neurons. CFLIR in POMC neurons of s/s mice was also increased compared with db/db animals. Thus, these data suggest that, although LRb-->STAT3 signaling is crucial for the regulation of feeding, it is not required for the acute or chronic regulation of orexigenic ARC neurons, and the activation of STAT3-mediated transcription by leptin is not required for the appropriate development of leptin responsiveness in these neurons.