Neuroendocrine control of brown adipocyte function by prolactin and growth hormone.

Growth hormone (GH) is fundamental for growth and glucose homeostasis, and prolactin for optimal pregnancy and lactation outcome, but additionally, both hormones have multiple functions that include a strong impact on energetic metabolism. In this respect, prolactin and GH receptors have been found in brown, and white adipocytes, as well as in hypothalamic centers regulating thermogenesis. This review describes the neuroendocrine control of the function and plasticity of brown and beige adipocytes, with a special focus on prolactin and GH actions. Most evidence points to a negative association between high prolactin levels and the thermogenic capacity of BAT, except in early development. During lactation and pregnancy, prolactin may be a contributing factor that limits unneeded thermogenesis, downregulating BAT UCP1. Furthermore, animal models of high serum prolactin have low BAT UCP1 levels and whitening of the tissue, while lack of Prlr induces beiging in WAT depots. These actions may involve hypothalamic nuclei, particularly the DMN, POA and ARN, brain centers that participate in thermogenesis. Studies on GH regulation of BAT function present some controversies. Most mouse models with GH excess or deficiency point to an inhibitory role of GH on BAT function. Even so, a stimulatory role of GH on WAT beiging has also been described, in accordance with whole-genome microarrays that demonstrate divergent response signatures of BAT and WAT genes to the loss of GH signaling. Understanding the physiology of BAT and WAT beiging may contribute to the ongoing efforts to curtail obesity.

Severe Hyperprolactinemia Promotes Brown Adipose Tissue Whitening and Aggravates High Fat Diet Induced Metabolic Imbalance.

Background: The association of high serum prolactin and increased body weight is positive but controversial, therefore we hypothesized that additional factors such as diets and the impact of prolactin on brown adipose tissue may condition its metabolic effects. Methods: We used LacDrd2KO females with lifelong severe hyperprolactinemia due dopamine-D2 receptor deletion from lactotropes, and slow onset of metabolic disturbances, and compared them to their respective controls (Drd2 loxP/loxP ). Food intake, and binge eating was evaluated. We then challenged mice with a High Fat (HFD) or a Control Diet (CD) for 8 weeks, beginning at 3 months of age, when no differences in body weight are found between genotypes. At the end of the protocol brown and white adipose tissues were weighed, and thermogenic and lipogenic markers studied, using real time PCR (Ucp1, Cidea, Pgc1a, Lpl, adiponectin, Prlr) or immunohistochemistry (UCP1). Histochemical analysis of brown adipose tissue, and glucose tolerance tests were performed. Results: Hyperprolactinemic mice had increased food intake and binge eating behavior. Metabolic effects induced by a HFD were exacerbated in lacDrd2KO mice. Hyperprolactinemia aggravated HFD-induced body weight gain and glucose intolerance. In brown adipose tissue pronounced cellular whitening as well as decreased expression of the thermogenic markers Ucp1 and Pgc1a were observed in response to high prolactin levels, regardless of the diet, and furthermore, hyperprolactinemia potentiated the decrease in Cidea mRNA expression induced by HFD. In subcutaneous white adipose tissue hyperprolactinemia synergistically increased tissue weight, while decreasing Prlr, Adiponectin and Lpl mRNA levels regardless of the diet. Conclusions: Pathological hyperprolactinemia has a strong impact in brown adipose tissue, lowering thermogenic markers and evoking tissue whitening. Furthermore, it modifies lipogenic markers in subcutaneous white adipose, and aggravates HFD-induced glucose intolerance and Cidea decrease. Therefore, severe high prolactin levels may target BAT function, and furthermore represent an adjuvant player in the development of obesity induced by high fat diets.

Chronic high prolactin levels impact on gene expression at discrete hypothalamic nuclei involved in food intake.

To study the pathological effects of continuous hyperprolactinemia on food intake mechanisms we used female mice that lack dopamine D2 receptors in lactotropes (lacDrd2KO). These mice had lifelong hyperprolactinemia, increased food intake, and gradual development of obesity from 5 to 10 months of age. Ongoing endogenous prolactin signaling in lacDrd2KO mice was evidenced by increased basal phosphorylation of STAT5b in hypothalamic areas related to food intake, such as the arcuate (ARN), dorsomedial (DMN), and ventromedial nuclei. In the ARN of young lacDrd2KO mice there were higher Prlr mRNA levels and in obese 10-month-old lacDrd2KO mice increased expression of the orexigenic genes Neuropeptide Y (Npy) and Agouti-related peptide, compared to controls. Furthermore, Npy expression was increased in the DMN, probably contributing to increased food intake and decreased expression of Uncoupling protein-1 in brown adipose tissue, both events favoring weight gain. Leptin resistance in obese lacD2RKO mice was evidenced by its failure to lower food intake and a dampened response of STAT3 phosphorylation, specifically in the mediobasal hypothalamus. Our results suggest that pathological chronically high prolactin levels, as found in psychiatric treatments or patients with prolactinomas, may impact on specific hypothalamic nuclei altering gene expression, leptin response, and food intake.

Metabolic functions of prolactin: Physiological and pathological aspects.

Prolactin is named after its vital role of promoting milk production during lactation, although it has been implicated in multiple functions within the body, including metabolism and energy homeostasis. Prolactin has been hypothesised to play a key role in driving many of the adaptations of the maternal body to allow the mother to meet the physiological demands of both pregnancy and lactation, including the high energetic demands of the growing foetus followed by milk production to support the offspring after birth. Prolactin receptors are found in many tissues involved in metabolism and food intake, such as the pancreas, liver, hypothalamus, small intestine and adipose tissue. We review the literature examining the effects of prolactin in these various tissues and how they relate to changes in function in physiological states of high prolactin, such as pregnancy and lactation, and in pathological states of hyperprolactinaemia in the adult. In many cases, whether prolactin promotes healthy metabolism or leads to dysregulation of metabolic functions is highly dependent on the situation. Overall, although prolactin may not play a major role in regulating metabolism and body weight outside of pregnancy and lactation, it definitely has the ability to contribute to metabolic function.

Brain Control of Sexually Dimorphic Liver Function and Disease: The Endocrine Connection.

A multistep signaling cascade originates in brain centers that regulate hypothalamic growth hormone-releasing hormone (Ghrh) and somatostatin expression levels and release to control the pattern of GH secretion. This process is sexually fine-tuned, and relays important information to the liver where GH receptors can be found. The temporal pattern of pituitary GH secretion, which is sex-specific in many species (episodic in males and more stable in females), represents a major component in establishing and maintaining the sexual dimorphism of hepatic gene transcription. The liver is sexually dimorphic exhibiting major differences in the profile of more than 1000 liver genes related to steroid, lipid, and foreign compound metabolism. Approximately, 90% of these sex-specific liver genes were shown to be primarily dependent on sexually dimorphic GH secretory patterns. This proposes an interesting scenario in which the central nervous system, indirectly setting GH profiles through GHRH and somatostatin control, regulates sexual dimorphism of liver activity in accordance with the need for sex-specific steroid metabolism and performance. We describe the influence of the loss of sexual dimorphism in liver gene expression due to altered brain function. Among other many factors, abnormal brain sexual differentiation, xenoestrogen exposure and D2R ablation from neurons dysregulate the GHRH-GH axis, and ultimately modify the liver capacity for adaptive mechanisms. We, therefore, propose that an inefficient brain control of the endocrine growth axis may underlie alterations in several metabolic processes through an indirect influence of sexual dimorphism of liver genes.

Chronic hyperprolactinemia evoked by disruption of lactotrope dopamine D2 receptors impacts on liver and adipocyte genes related to glucose and insulin balance.

We studied the impact of high prolactin titers on liver and adipocyte gene expression related to glucose and insulin homeostasis in correlation with obesity onset. To that end we used mutant female mice that selectively lack dopamine type 2 receptors (D2Rs) from pituitary lactotropes (lacDrd2KO), which have chronic high prolactin levels associated with increased body weight, marked increments in fat depots, adipocyte size, and serum lipids, and a metabolic phenotype that intensifies with age. LacDrd2KO mice of two developmental ages, 5 and 10 mo, were used. In the first time point, obesity and increased body weight are marginal, although mice are hyperprolactinemic, whereas at 10 mo there is marked adiposity with a 136% increase in gonadal fat and a 36% increase in liver weight due to lipid accumulation. LacDrd2KO mice had glucose intolerance, hyperinsulinemia, and impaired insulin response to glucose already in the early stages of obesity, but changes in liver and adipose tissue transcription factors were time and tissue dependent. In chronic hyperprolactinemic mice liver Prlr were upregulated, there was liver steatosis, altered expression of the lipogenic transcription factor Chrebp, and blunted response of Srebp-1c to refeeding at 5 mo of age, whereas no effect was observed in the glycogenesis pathway. On the other hand, in adipose tissue a marked decrease in lipogenic transcription factor expression was observed when morbid obesity was already settled. These adaptive changes underscore the role of prolactin signaling in different tissues to promote energy storage.

Dopaminergic drugs in type 2 diabetes and glucose homeostasis.

The importance of dopamine in central nervous system function is well known, but its effects on glucose homeostasis and pancreatic ß cell function are beginning to be unraveled. Mutant mice lacking dopamine type 2 receptors (D2R) are glucose intolerant and have abnormal insulin secretion. In humans, administration of neuroleptic drugs, which block dopamine receptors, may cause hyperinsulinemia, increased weight gain and glucose intolerance. Conversely, treatment with the dopamine precursor l-DOPA in patients with Parkinson's disease reduces insulin secretion upon oral glucose tolerance test, and bromocriptine improves glycemic control and glucose tolerance in obese type 2 diabetic patients as well as in non diabetic obese animals and humans. The actions of dopamine on glucose homeostasis and food intake impact both the autonomic nervous system and the endocrine system. Different central actions of the dopamine system may mediate its metabolic effects such as: (i) regulation of hypothalamic noradrenaline output, (ii) participation in appetite control, and (iii) maintenance of the biological clock in the suprachiasmatic nucleus. On the other hand, dopamine inhibits prolactin, which has metabolic functions; and, at the pancreatic beta cell dopamine D2 receptors inhibit insulin secretion. We review the evidence obtained in animal models and clinical studies that posited dopamine receptors as key elements in glucose homeostasis and ultimately led to the FDA approval of bromocriptine in adults with type 2 diabetes to improve glycemic control. Furthermore, we discuss the metabolic consequences of treatment with neuroleptics which target the D2R, that should be monitored in psychiatric patients to prevent the development in diabetes, weight gain, and hypertriglyceridemia.

Angiogenesis in pituitary adenomas: human studies and new mutant mouse models.

The role of angiogenesis in pituitary tumor development has been questioned, as pituitary tumors have been usually found to be less vascularized than the normal pituitary tissue. Nevertheless, a significantly higher degree of vasculature has been shown in invasive or macropituitary prolactinomas when compared to noninvasive and microprolactinomas. Many growth factors and their receptors are involved in pituitary tumor development. For example, VEGF, FGF-2, FGFR1, and PTTG, which give a particular vascular phenotype, are modified in human and experimental pituitary adenomas of different histotypes. In particular, vascular endothelial growth factor, VEGF, the central mediator of angiogenesis in endocrine glands, was encountered in experimental and human pituitary tumors at different levels of expression and, in particular, was higher in dopamine agonist resistant prolactinomas. Furthermore, several anti-VEGF techniques lowered tumor burden in human and experimental pituitary adenomas. Therefore, even though the role of angiogenesis in pituitary adenomas is contentious, VEGF, making permeable pituitary endothelia, might contribute to adequate temporal vascular supply and mechanisms other than endothelial cell proliferation. The study of angiogenic factor expression in aggressive prolactinomas with resistance to dopamine agonists will yield important data in the search of therapeutical alternatives.