Prolactin (PRL) receptor gene expression in mouse adipose tissue: increases during lactation and in PRL-transgenic mice

The prolactin receptor: A cross-species comparison of gene structure, transcriptional regulation, tissue-specificity, and genetic variation

The conserved and multifaceted functions of prolactin (PRL) are coordinated through varied distribution and expression of its cell-surface receptor (PRLR) across a range of tissues and physiological states. The resultant heterogeneous expression of PRLR mRNA and protein across different organs and cell types supports a wide range of PRL-regulated processes including reproduction, lactation, development, and homeostasis. Genetic variation within the PRLR gene also accounts for several phenotypes impacting agricultural production and human pathology. The goal of this review is to highlight the many elements that control differential expression of the PRLR across tissues, and the various phenotypes that exist across species due to variation in the PRLR gene.

Obesity-derived alterations in the lactating mammary gland: Focus on prolactin

Obesity is a modern pandemic with negative consequences in women's reproductive health. Women with overweight and obesity can develop mammary gland alterations that unable exclusive breastfeeding. Obesity associates with a disturbed lactating mammary gland endocrine environment including a decreased action of the hormone prolactin (PRL), the master regulator of lactation. The PRL receptor and the action of PRL are reduced in the mammary gland of lactating rodents fed an obesogenic diet and are contributing factors to impaired lactation in obesity. Also, treatment with PRL improves milk yield in women with lactation insufficiency. This review focuses on the impact of diet-induced obesity in the lactating mammary gland and how obesity impairs the lactogenic action of PRL. Although obesity alters lactation performance in humans and rodents, the responsible mechanisms have been mainly addressed in rodents.

Prolactin and Maternal Metabolism in Women With a Recent GDM Pregnancy and Links to Future T2D: The SWIFT Study

CONTEXT

Prolactin is a multifaceted hormone known to regulate lactation. In women with gestational diabetes mellitus (GDM) history, intensive lactation has been associated with lower relative risk of future type 2 diabetes (T2D). However, the role of prolactin in T2D development and maternal metabolism in women with a recent GDM pregnancy has not been ascertained.

OBJECTIVE

We examined the relationships among prolactin, future T2D risk, and key clinical and metabolic parameters.

METHODS

We utilized a prospective GDM research cohort (the SWIFT study) and followed T2D onset by performing 2-hour 75-g research oral glucose tolerance test (OGTT) at study baseline (6-9 weeks postpartum) and again annually for 2 years, and also by retrieving clinical diagnoses of T2D from 2 years through 10 years of follow up from electronic medical records. Targeted metabolomics and lipidomics were applied on fasting plasma samples collected at study baseline from 2-hour 75-g research OGTTs in a nested case-control study (100 future incident T2D cases vs 100 no T2D controls).

RESULTS

Decreasing prolactin quartiles were associated with increased future T2D risk (adjusted odds ratio 2.48; 95% CI, 0.81-7.58; P = 0.05). In women who maintained normoglycemia during the 10-year follow-up period, higher prolactin at baseline was associated with higher insulin sensitivity (P = 0.038) and HDL-cholesterol (P = 0.01), but lower BMI (P = 0.001) and leptin (P = 0.002). Remarkably, among women who developed future T2D, prolactin was not correlated with a favorable metabolic status (all P > 0.05). Metabolomics and lipidomics showed that lower circulating prolactin strongly correlated with a T2D-high risk lipid profile, with elevated circulating neutral lipids and lower concentrations of specific phospholipids/sphingolipids.

CONCLUSION

In women with recent GDM pregnancy, low circulating prolactin is associated with specific clinical and metabolic parameters and lipid metabolites linked to a high risk of developing T2D.

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.

Glucose-Lipid Metabolism in Obesity with Elevated Prolactin Levels and Alteration of Prolactin Levels After Laparoscopic Sleeve Gastrectomy

PURPOSE

We investigated the differences in metabolism between obesity with or without increased prolactin (PRL) and the change in PRL after laparoscopic sleeve gastrectomy (LSG).

METHODS

Patients were divided into two groups: obesity with normal PRL (NP, n = 123) and high PRL (HP, n = 108). Glucose-lipid metabolism and inflammation were measured. A total of 115 patients with obesity (NP, n = 64; HP, n = 51) underwent LSG were recruited, and PRL was measured at 12 months after LSG.

RESULTS

(1) Blood glucose (BG), total cholesterol (TCH), LDL, triglyceride, and TNF-α were lower in the HP than in the NP group in the cross-sectional study (all P < 0.05). (2) PRL was negatively associated with neck circumference, waist-to-hip ratio, systolic blood pressure, heart rate, basal metabolism rate (BMR), ALP, TCH, and LDL in all subjects. PRL levels were positively associated with weight, HC, and BMR in males but were negatively associated with ALT, AST, ALP, BG 30 min, BG 60 min, FFA, and TCH in females (all P < 0.05). (3) Regression analysis showed that PRL negatively correlated with ALP and LDL-C in the whole baseline (β = - 0.051, P = 0.002; β = - 1.372, P = 0.033). PRL was a negative factor for ALP in females and a positive factor for BMR2 in males (β = - 0.099, P = 0.041; β = 0.005, P = 0.006). (4) PRL decreased in the HP group and increased in the NP group at 12 months post-operation (all P < 0.05). Increased PRL was associated with a change in TCH in the NP group (P < 0.05).

CONCLUSION

Increased PRL resulted in improved glucose-lipid metabolism and chronic low-grade inflammation. LSG led to increased PRL in NP and decreased PRL in HP. Improved lipid was associated with increased PRL in NP after surgery.

CLINICAL TRIAL REGISTRATION NUMBER

ChiCTR-OCS-12002381.

Prolactin - a pleiotropic factor in health and disease

The principal role of prolactin in mammals is the regulation of lactation. Prolactin is a hormone that is mainly synthesized and secreted by lactotroph cells in the anterior pituitary gland. Prolactin signalling occurs via a unique transmembrane prolactin receptor (PRL-R). The structure of the PRL-R has now been elucidated and is similar to that of many biologically fundamental receptors of the class 1 haematopoietic cytokine receptor family such as the growth hormone receptor. The PRL-R is expressed in a wide array of tissues, and a growing number of biological processes continue to be attributed to prolactin. In this Review, we focus on the newly discovered roles of prolactin in human health and disease, particularly its involvement in metabolic homeostasis including body weight control, adipose tissue, skin and hair follicles, pancreas, bone, the adrenal response to stress, the control of lactotroph cell homeostasis and maternal behaviour. New data concerning the pathological states of hypoprolactinaemia and hyperprolactinaemia will also be presented and discussed.

Extensive phenotypic characterization of a new transgenic mouse reveals pleiotropic perturbations in physiology due to mesenchymal hGH minigene expression

The human growth hormone (hGH) minigene used for transgene stabilization in mice has been recently identified to be locally expressed in the tissues where transgenes are active and associated with phenotypic alterations. Here we extend these findings by analyzing the effect of the hGH minigene in TgC6hp55 transgenic mice which express the human TNFR1 under the control of the mesenchymal cell-specific CollagenVI promoter. These mice displayed a fully penetrant phenotype characterized by growth enhancement accompanied by perturbations in metabolic, skeletal, histological and other physiological parameters. Notably, this phenotype was independent of TNF-TNFR1 signaling since the genetic ablation of either Tnf or Tradd did not rescue the phenotype. Further analyses showed that the hGH minigene was expressed in several tissues, also leading to increased hGH protein levels in the serum. Pharmacological blockade of GH signaling prevented the development of the phenotype. Our results indicate that the unplanned expression of the hGH minigene in CollagenVI expressing mesenchymal cells can lead through local and/or systemic mechanisms to enhanced somatic growth followed by a plethora of primary and/or secondary effects such as hyperphagia, hypermetabolism, disturbed glucose homeostasis, altered hematological parameters, increased bone formation and lipid accumulation in metabolically critical tissues.

Daytime decrease of prolactin levels is associated with PCOS regardless to nutritional status and other hormones levels

OBJECTIVE

The aim of the study is to analyze daytime changes of prolactin level depending on nutritional status and polycystic ovary syndrome (PCOS).

STUDY DESIGN

One hundred and fifteen (69 normal weight, 21 overweight and 25 obese) diagnosed with PCOS and 77 (37 normal weight, 18 overweight and 22 obese) women - Non-PCOS without concomitant diseases were enrolled. Body mass and height were measured and BMI was calculated. Serum concentrations of FSH, LH, E2, testosterone, TSH and PRL were determined morning 6.00 a.m. after wake. Second measurement of PRL was performed at 4 p.m.

RESULTS

The daytime decrease of prolactin level was higher in PCOS than in Non-PCOS group regardless of nutritional status (normal weight 35.8 ± 26.0 vs. 24.3 ± 15.3 ng/mL; overweight 28.5 ± 25.4 vs. 17.5 ± 8.8 ng/mL and obese 23.2 ± 21.1 vs. 18.4 ± 11.6 ng/ml, respectively). However, in both PCOS and Non-PCOS daytime changes of prolactin level were higher in normal weight than overweight and obese women (35.8 ± 26.0 vs. 28.5 ± 25.4 vs. 23.2 ± 21.1 ng/mL and 24.3 ± 15.3 vs. 17.5 ± 8.8 vs. 18.4 ± 11.6 ng/mL, respectively). The multivariate regression analysis revealed that the daytime changes of prolactin level are proportional to TSH concentration and coexistence of PCOS as well as inversely relative to BMI.

CONCLUSIONS

In conclusions, our results suggest that overweight and obesity decreased morning PRL level and impaired its daytime decrease, but coexistence of PCOS enlarged its.

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.

Effects of High-Fat Diet on Stress Response in Male and Female Wildtype and Prolactin Knockout Mice

Prolactin (PRL) is well characterized for its roles in initiation and maintenance of lactation, and it also suppresses stress-induced responses. Feeding a high-fat diet (HFD) disrupts activity of the hypothalamic-pituitary-adrenal (HPA) axis. Whether PRL regulates HPA axis activation under HFD feeding is not clear. Male and female wildtype (WT) and PRL knockout (KO) mice were fed either a standard low-fat diet (LFD) or HFD for 12 weeks. Circulating corticosterone (CORT) levels were measured before, during, and after mice were subjected to an acute restraint stress or remained in their home cages as no stress controls. HFD feeding increased leptin levels, but the increase was lower in KO than in WT mice. All stressed female groups and only LFD-fed stressed males had elevated CORT levels compared to their no stress same-sex counterparts regardless of genotype. These results indicated that HFD consumption blunted the HPA axis response to acute stress in males but not females. Additionally, basal hypothalamic CRH content was lower in HFD than LFD males, but was similar among female groups. Furthermore, although basal CORT levels were similar among KO and WT groups, CORT levels were higher in KO mice than their WT counterparts during stress, suggesting that loss of PRL led to greater HPA axis activation. Basal PRL receptor mRNA levels in the choroid plexus were higher in HFD than LFD same-sex counterparts, suggesting activation of central PRL’s action by HFD feeding in both males and females. Current results confirmed PRL’s roles in suppression of the stress-induced HPA axis activation. Although HFD feeding activated central PRL’s action in both sexes, only the male HPA axis was dampened by HFD feeding.

Adipocyte Versus Somatotrope Leptin: Regulation of Metabolic Functions in the Mouse

Leptin regulates food intake and energy expenditure (EE) and is produced in adipocytes, the pituitary, and several other tissues. Animals that are leptin or leptin receptor deficient have major metabolic complications, including obesity. This study tests the hypothesis that the pituitary somatotrope may contribute a source of leptin that maintains some of these metabolic functions. We created 2 different tissue-specific leptin knockout animals: a Somatotrope-Lep-null model and an Adipocyte-Lep-null model. Metabolic analysis of both models, along with a global deletion model, was performed. The Somatotrope-Lep-null animals had fewer somatotropes, and females had a 76% decrease in serum prolactin. During the dark (feeding) phase, females had a 35% increase in ambulation coupled with a 4% increase in EE. Mutants showed no change in food intake or weight gain and EE was unchanged in males. During the light (sleep) phase, Somatotrope-Lep-null mutant males had lower EE and females continued to have higher EE. The respiratory quotients (RQs) of mutants and littermate controls were decreased in males and increased in females; all were within the range that indicates predominant carbohydrate burning. The massively obese Adipocyte-Lep-null animals, however, had significant increases in food intake, sleep, and increased EE, with decreased activity. Changes in RQ were sexually dimorphic, with female mutants having higher RQ and males having decreased RQ. We conclude that both adipocyte and somatotrope leptin contribute to the metabolic homeostasis of the mouse, and that extraadipocyte sources of leptin cannot overcome the major metabolic challenges seen in these animals.

The modulation of adiponectin by STAT5-activating hormones

Adiponectin is a hormone secreted from adipocytes that plays an important role in insulin sensitivity and protects against metabolic syndrome. Growth hormone (GH) and prolactin (PRL) are potent STAT5 activators that regulate the expression of several genes in adipocytes. Studies have shown that the secretion of adiponectin from adipose tissue is decreased by treatment with PRL and GH. In this study, we demonstrate that 3T3-L1 adipocytes treated with GH or PRL exhibit a reduction in adiponectin protein levels. Furthermore, we identified three putative STAT5 binding sites in the murine adiponectin promoter and show that only one of these, located at -3,809, binds nuclear protein in a GH- or PRL-dependent manner. Mutation of the STAT5 binding site reduced PRL-dependent protein binding, and supershift analysis revealed that STAT5A and -5B, but not STAT1 and -3, bind to this site in response to PRL. Chromatin immunoprecipitation (IP) analysis demonstrated that only STAT5A, and not STAT1 and -3, bind to the murine adiponectin promoter in a GH-dependent manner in vivo. Adiponectin promoter/reporter constructs were responsive to GH, and chromatin IP analysis reveals that STAT5 binds the adiponectin promoter in vivo following GH stimulation. Overall, these data strongly suggest that STAT5 activators regulate adiponectin transcription through the binding of STAT5 to the -3,809 site that leads to decreased adiponectin expression and secretion. These mechanistic observations are highly consistent with studies in mice and humans that have high GH or PRL levels that are accompanied by lower circulating levels of adiponectin.

Phenomenon of leptin resistance in seasonal animals: the failure of leptin action in the brain

The core of the leptin resistance hypothesis promulgated several years ago to explain obesity as a result of environmental causes consists of 2 tenets: the extinction of leptin-induced intracellular signaling downstream of leptin binding to the long form of the neuronal receptor LTRb in the hypothalamus and the impedance to leptin entry imposed at the blood-brain barrier (BBB). A recent comprehensive investigation concluded that a central leptin insufficiency associated with obesity can be attributed to a decreased efficiency of BBB leptin transport and not to leptin insensitivity within the hypothalamus. Interestingly, anorectic leptin's effects are counteracted in some individuals by a natural resistance associated with hyperleptinemia, which is related to changes in hypothalamic sensitivity to leptin (eg, due to malnutrition, obesity, or seasonal variations due to day-length-dependent reproduction changes). In sheep, it has been observed that the hypothalamus is resistant to leptin in some periods, which is related to the adaptation of these animals to annual changes in energy supply and demand. However, a broad range of ambiguities exists regarding the implications that the intracellular signaling of signal transducer and activator of transcription-2/suppressor of cytokine signaling 3 (STAT2/SOCS3) imparts central leptin resistance. Furthermore, several plausible alternative possibilities have been proposed, such as compensatory functional and anatomic reorganizations in the appetite regulating network, rearrangements in the afferent hormonal feedback signaling involved in weight homeostasis, and modifications in leptin transport to the hypothalamus across the BBB. Taken together, these observations suggest that the contention that impaired intracellular signaling downstream of leptin entry into the appetite regulating network expedites environmentally induced obesity remains unsubstantiated and requires further evidence. Furthermore, pregnancy decreases hypothalamic sensitivity to leptin (or other unknown mechanisms), and lactation can also alter the appetite-suppressing central activity of leptin. The objective of this review was to offer an approach to understanding (1) how information regarding nutritional status is transmitted to and interpreted within the hypothalamus in animals, with special attention on seasonally breeding animals and (2) whether central leptin resistance and/or leptin insufficiency in the hypothalamus favors the development of obesity.

Prolactin and the dietary protein/carbohydrate ratio regulate the expression of SNAT2 amino acid transporter in the mammary gland during lactation

The sodium coupled neutral amino acid transporter 2 (SNAT2/SAT2/ATA2) is expressed in the mammary gland (MG) and plays an important role in the uptake of alanine and glutamine which are the most abundant amino acids transported into this tissue during lactation. Thus, the aim of this study was to assess the amount and localization of SNAT2 before delivery and during lactation in rat MG, and to evaluate whether prolactin and the dietary protein/carbohydrate ratio might influence SNAT2 expression in the MG, liver and adipose tissue during lactation. Our results showed that SNAT2 protein abundance in the MG increased during lactation and this increase was maintained along this period, while 24 h after weaning it tended to decrease. To study the effect of prolactin on SNAT2 expression, we incubated MG explants or T47D cells transfected with the SNAT2 promoter with prolactin, and we observed in both studies an increase in the SNAT2 expression or promoter activity. Consumption of a high-protein/low carbohydrate diet increased prolactin concentration, with a concomitant increase in SNAT2 expression not only in the MG during lactation, but also in the liver and adipose tissue. There was a correlation between SNAT2 expression and serum prolactin levels depending on the amount of dietary protein/carbohydrate ratio consumed. These findings suggest that prolactin actively supports lactation providing amino acids to the gland through SNAT2 for the synthesis of milk proteins.

Rorα deficiency and decreased adiposity are associated with induction of thermogenic gene expression in subcutaneous white adipose and brown adipose tissue

The Rar-related orphan receptor-α (Rorα) is a nuclear receptor that regulates adiposity and is a potential regulator of energy homeostasis. We have demonstrated that the Rorα-deficient staggerer (sg/sg) mice display a lean and obesity-resistant phenotype. Adaptive Ucp1-dependent thermogenesis in beige/brite and brown adipose tissue serves as a mechanism to increase energy expenditure and resist obesity. DEXA and MRI analysis demonstrated significantly decreased total fat mass and fat/lean mass tissue ratio in male chow-fed sg/sg mice relative to wt mice. In addition, we observed increased Ucp1 expression in brown adipose and subcutaneous white adipose tissue but not in visceral adipose tissue from Rorα-deficient mice. Moreover, this was associated with significant increases in the expression of the mRNAs encoding the thermogenic genes (i.e., markers of brown and beige adipose) Pparα, Errα, Dio2, Acot11/Bfit, Cpt1β, and Cidea in the subcutaneous adipose in the sg/sg relative to WT mice. These changes in thermogenic gene expression involved the significantly increased expression of the (cell-fate controlling) histone-lysine N-methyltransferase 1 (Ehmt1), which stabilizes the Prdm16 transcriptional complex. Moreover, primary brown adipocytes from sg/sg mice displayed a higher metabolic rate, and further analysis was consistent with increased uncoupling. Finally, core body temperature analysis and infrared thermography demonstrated that the sg/sg mice maintained greater thermal control and cold tolerance relative to the WT littermates. We suggest that enhanced Ucp1 and thermogenic gene expression/activity may be an important contributor to the lean, obesity-resistant phenotype in Rorα-deficient mice.

Effects of high affinity leptin antagonist on prolactin receptor deficient male mouse

Hyperprolactinemia occurs during gestation and lactation with marked hyperphagia associated with leptin resistance. Prolactin (PRL) induces the expression of orexigenic neuropeptide Y (NPY) in hypothalamic dorsomedial nucleus (DMH) leading to hyperphagia. Along this line prolactin receptor deficient (PRLR^−/−) mice are resistant to obesity under high fat diet due to increased energy expenditure. As these mice have an altered food intake, our objective was to test whether leptin is responsible for these characteristics. PRLR^−/− male mice and control littermates were injected subcutaneously every other day with 12 mg/kg pegylated superactive mouse leptin antagonist (PEG-SMLA) for 3 weeks. We tested the effect of PEG-SMLA on body weight, food intake and metabolic parameters. The antagonist led to a rapid increase in body weight (20%) but increased adipose mass in PEG-SMLA treated mice was less pronounced in PRLR^−/− than in WT mice. Food intake of PEG-SMLA-injected animals increased during the first week period of the experiment but then declined to a similar level of the control animals during the second week. Interestingly, PRLR^−/− mice were found to have the same bone volume than those of control mice although PEG-SMLA increased bone mass by 7% in both strains. In addition, PEG-SMLA led to insulin resistance and glucose intolerance as well as an altered lipid profile in treated mice. Altogether, these results suggest that PRLR^−/− mice respond to leptin antagonist similarly to the control mice, indicating no interaction between the actions of the two hormones.

Completely humanizing prolactin rescues infertility in prolactin knockout mice and leads to human prolactin expression in extrapituitary mouse tissues

A variety of fundamental differences have evolved in the physiology of the human and rodent prolactin (PRL) systems. The PRL gene in humans and other primates contains an alternative promoter, 5.8 kbp upstream of the pituitary transcription start site, which drives expression of PRL in "extrapituitary" tissues, where PRL is believed to exert local, or paracrine, actions. Several of these extrapituitary PRL tissues serve a reproductive function (eg, mammary gland, decidua, prostate, etc), consistent with the hypothesis that local PRL production may be involved in, and required for, normal reproductive physiology in primates. Rodent research models have generated significant findings regarding the role of PRL in reproduction. Specifically, disruption (knockout) of either the PRL gene or its receptor causes profound female reproductive defects at several levels (ovaries, preimplantation endometrium, mammary glands). However, the rodent PRL gene differs significantly from the human, most notably lacking the alternative promoter. Understanding of the physiological regulation and function of extrapituitary PRL has been limited by the absence of a readily accessible experimental model, because the rodent PRL gene does not contain the alternative promoter. To overcome these limitations, we have generated mice that have been "humanized" with regard to the structural gene and tissue expression of PRL. Here, we present the characterization of these animals, demonstrating that the human PRL transgene is responsive to known physiological regulators both in vitro and in vivo. More importantly, the expression of the human PRL transgene is able to rescue the reproductive defects observed in mouse PRL knockout (mPRL(-)) females, validating their usefulness in studying the function or regulation of this hormone in a manner that is relevant to human physiology.

Prolactin and adipose tissue

The pituitary lactogenic hormone prolactin (PRL) exerts various physiological actions in humans and rodents via its binding to a membrane receptor. Beside its role in lactation and reproduction, accumulating evidence suggests that PRL has a crucial impact on energy balance by acting on two key players, the pancreas and the adipose tissue. Adipose tissue is now recognized as an endocrine organ and its metabolic activity appears to play an important role in pathophysiology such as obesity and diabetes. White adipocytes store excess of energy in the form of triglycerides for future need while brown adipocytes metabolize lipids and glucose to produce heat, highlighting their different metabolic functionality. The plasticity of white adipose tissue, by the emergence of beige adipocytes, appears to be essential in energy homeostasis. PRL receptor deficient mice provided direct evidence that PRL signaling is involved in the regulation of adipogenesis affecting energy balance and metabolic adaptation most notably during development. Moreover, it was demonstrated that PRL signaling participates to brown adipose tissue differentiation and function, opening novel understanding of hormonal regulation of energy balance. This review summarizes our current knowledge about PRL signaling and its role on adipose tissue.

Prevalence, mass, and glucose-uptake activity of ¹⁸F-FDG-detected brown adipose tissue in humans living in a temperate zone of Italy

Background

The ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG)-detected brown adipose tissue (BAT), is enhanced by cold stimulus and modulated by other factors that still have to be disentangled. We investigated the prevalence, mass, and glucose-uptake activity of ¹⁸F-FDG-detected BAT in a population of adults living in the temperate climatic zone of the Rome area.

Methods and Findings

We retrospectively analyzed 6454 patients who underwent ¹⁸F-FDG positron emission tomography/computed tomography (PET/CT) examinations. We found ¹⁸F-FDG BAT in 217 of the 6454 patients (3.36%). Some of them underwent more than one scan and the positive scans were 278 among 8004 (3.47%). The prevalence of patients with at least one positive scan was lower in men (1.77%; 56 of 3161) compared with women (4.88%; 161 of 3293). The BAT positive patients were most frequently younger, thinner and with lower plasma glucose levels compared with BAT negative patients. The amount of BAT in the defined region of interest, the activity of BAT and the number of positive sites of active BAT were similar in both sexes. The prevalence of patients with ¹⁸F-FDG positive PET/CT was highest in December-February, lower in March-May and September-November, and lowest in June-August and was positively correlated with night length and negatively correlated with ambient temperature. Changes in day length and variations of temperature, associated with the prevalence of positive BAT patients. Among the patients who had multiple scans, outdoor temperature was significantly lower and day length was shorter on the occasion when BAT was detected.

Conclusions

This study identifies day length, outdoor temperature, age, sex, BMI, and plasma glucose levels as major determinants of the prevalence, mass, and activity of ¹⁸F-FDG-detected BAT.

The role of adipocyte XBP1 in metabolic regulation during lactation

The adipocyte is central to organismal metabolism and exhibits significant functional and morphological plasticity during its formation and lifespan. Remarkable transformations of this cell occur during obesity and lactation, and thus it is essential to gain a better understanding of adipocyte function in these two metabolic processes. Considering the critical importance of the cellular organelle endoplasmic reticulum (ER) in adapting to fluctuations in synthetic processes, we explored the role of XBP1, a central regulator of ER adaptive responses, in adipocyte formation and function. Unexpectedly, deletion of adipocyte-XBP1 in vivo in mice (XBP1ΔAd) had no effect on adipocyte formation or on systemic homeostatic metabolism in mice fed a a regular or high-fat diet. However, during lactation, XBP1ΔAd dams displayed increased adiposity, decreased milk production, and decreased litter growth as compared with control dams. Moreover, we demonstrate that XBP1 is regulated during lactation and responds to prolactin to alter lipogenic gene expression. These results demonstrate a role for adipocyte-XBP1 in the regulation of lactational metabolism.

Prolactin activation of the long form of its cognate receptor causes increased visceral fat and obesity in males as shown in transgenic mice expressing only this receptor subtype

To date the best defined function of prolactin (PRL) is its action on the ovary and mammary gland, although it has also been shown to have an effect on lipid metabolism. Using mice engineered to express only the long form of the prolactin receptor (PRL-RL), we demonstrate that PRL acting through PRL-RL alone causes severe adipose accumulation in visceral fat of males at 6 months of age. The increase in visceral fat accumulation is attributed to loss of adipose-derived leptin, which results in diminished lipolysis. The reduction in leptin also corresponds to decreased activation of AMP-activated protein kinase (AMPK), which further results in diminished fatty acid oxidation and increased fatty acid synthesis. Interestingly, the blunted AMPK response was only observed in adipose tissue and not in liver suggesting that this PRL mediated effect is tissue specific. A glucose tolerance study inferred that PRL-RL mice may suffer from insulin resistance or a reduction in insulin production that is not due to aberrant expression of glucose transporter 4 (Glut4). Collectively, our findings demonstrate that PRL signaling through the long form receptor causes reduced fatty acid oxidation, increased lipid storage, glucose intolerance, and obesity. These findings are of great importance towards understanding the etiology of obesity associated with hyperprolactinemia in humans as well as the role of PRL as a metabolic regulator in adipose tissue.

The obesity epidemic: from the environment to epigenetics - not simply a response to dietary manipulation in a thermoneutral environment

The prevalence of obesity continues to increase particularly in developed countries. To establish the primary mechanisms involved, relevant animal models which track the developmental pathway to obesity are required. This need is emphasized by the substantial rise in the number of overweight and obese children, of which a majority will remain obese through adulthood. The past half century has been accompanied with unprecedented transitions in our lifestyle. Each of these changes substantially contributes to enhancing our capacity to store energy into adipose tissues. The complex etiology of adiposity is critical as a majority of models investigating obesity utilize a simplistic high-fat/low-carbohydrate diet, fed over a short time period to comparatively young inbred animals maintained in fixed environment. The natural history of obesity is much more complex involving many other mechanisms and this type of challenge may not be the optimal experimental intervention. Such processes include changes in adipose tissue composition with time and the transition from brown to white adipose tissue. Brown adipose tissue, due its unique ability to rapidly produce large amounts of heat could have a pivotal role in energy balance and is under epigenetic regulation mediated by the histone H3k9-specific demethylase Jhdma2a. Furthermore, day length has a potential role in determining endocrine and metabolic responses in brown fat. The potential to utilize novel models and interventions across a range of animal species in adipose tissue development may finally start to yield sustainable strategies by which excess fat mass can, at last, be avoided in humans.

A common variant near the PRL gene is associated with increased adiposity in males

A common variant (rs4712652) adjacent to the prolactin gene was recently associated with obesity using a genome-wide association study. The aim of this study was to replicate the association between rs4712652 and obesity and further examine if rs4712652 is associated with fat percentage and adiponectin levels in a population based Scandinavian cohort. rs4712652 was genotyped in 4879 participants (mean BMI 26.5±4.5 kg/m(2)) from the population-based PPP-Botnia Study and related to BMI, fat percentage and adiponectin levels. We found that the risk A allele of rs4712652 is associated with increased BMI and fat percentage in males (P=0.0047 and P=0.025, respectively), but not in females (P=0.98, P=0.45). Male A allele carriers have a higher risk of being overweight with an OR of 1.16 (P=0.025). While there was a significant negative correlation between adiponectin levels and fat percentage (r=-0.36; P=0.039) in male carriers of the protective GG genotype, this correlation was lost in male carriers of the risk rs4712652 A allele (P=0.33). Thus, the common SNP rs4712652 near the PRL gene seems to affect body fat and adiposity in a sex-specific fashion. It remains to be shown whether this is mediated by different prolactin concentrations or differences in tissue sensitivity to prolactin.

Paternal recognition of adult offspring mediated by newly generated CNS neurons

In mammals, olfaction is often used to distinguish individuals on the basis of their unique odor types (genetically programmed body odors). Parental-offspring recognition behavior is mediated, in part, by learning and processing of different odor types and is crucial for reproductive success. Maternal recognition behavior and associated brain plasticity has been well characterized, but paternal recognition behavior and brain plasticity is poorly understood. We found that paternal-adult offspring recognition behavior in mice was dependent on postnatal offspring interaction and was associated with increased neurogenesis in the paternal olfactory bulb and hippocampus. Newly generated paternal olfactory interneurons were preferentially activated by adult offspring odors. Disrupting prolactin signaling abolished increased paternal neurogenesis and adult offspring recognition. Rescuing this neurogenesis restored recognition behavior. Thus, neurogenesis in the paternal brain may be involved in offspring recognition.

Transdifferentiation properties of adipocytes in the adipose organ

Mammals have two types of adipocytes, white and brown, but their anatomy and physiology is different. White adipocytes store lipids, and brown adipocytes burn them to produce heat. Previous descriptions implied their localization in distinct sites, but we demonstrated that they are mixed in many depots, raising the concept of adipose organ. We explain the reason for their cohabitation with the hypothesis of reversible physiological transdifferentiation; they are able to convert one into each other. If needed, the brown component of the organ could increase at the expense of the white component and vice versa. This plasticity is important because the brown phenotype of the organ associates with resistance to obesity and related disorders. Another example of physiological transdifferetiation of adipocytes is offered by the mammary gland; the pregnancy hormonal stimuli seems to trigger a reversible transdifferentiation of adipocytes into milk-secreting epithelial glands. The obese adipose organ is infiltrated by macrophages inducing chronic inflamation that is widely considered as a causative factor for insulin resistance. We showed that the vast majority of macrophages infiltrating the obese organ are arranged around dead adipocytes, forming characteristic crown-like structures. We recently found that visceral fat is more infiltrated than the subcutaneous fat despite a smaller size of visceral adipocytes. This suggests a different susceptibility of visceral and subcutaneous adipocytes to death, raising the concept of smaller critical death size that could be important to explain the key role of visceral fat for the metabolic disorders associated with obesity.

Effects of metoclopramide-induced hyperprolactinemia on the prolactin receptor of murine endometrium

OBJECTIVE

To evaluate the effects of metoclopramide-induced hyperprolactinemia on the prolactin receptor of murine endometrium.

DESIGN

Experimental study using the RNA extraction to detect tissue prolactin receptor isoforms by reverse-transcriptase polymerase chain reaction (RT-PCR).

SETTING

University-based laboratory.

ANIMAL(S)

Seventy-two female swiss albino mice (Mus musculus), approximately 100 days old, were divided into six 12-animal groups: (GI) nonoophorectomized mice given vehicle; (GII) nonoophorectomized mice treated with metoclopramide; (GIII) oophorectomized mice treated with metoclopramide; (GIV) oophorectomized mice treated with metoclopramide and 17beta-estradiol; (GV) oophorectomized mice treated with metoclopramide and micronized progesterone; (GVI) oophorectomized mice treated with metoclopramide and a solution of 17beta-estradiol and micronized progesterone.

INTERVENTION(S)

Drugs were administered for 50 days. Following euthanasia, the middle portions of the uterine horns were removed, sectioned, and immediately frozen for RT-PCR procedures. Blood was collected for the dosage of prolactin and serum estrogen and progesterone using radioimmune assay.

MAIN OUTCOME MEASURE(S)

Identification of uterine prolactin receptor isoforms.

RESULT(S)

The PRL receptor and its isoform L were identified only in GI (control group) and GII (metoclopramide), the two groups with nonoophorectomized animals. The amount of PRL receptor mRNA and that of its isoform L from GII were the largest. No other isoforms of the prolactin receptor were identified in any of the groups.

CONCLUSION(S)

Our results suggest that replacement of estrogen and progestin may not increase the mRNA of endometrial PRL receptor in metoclopromide-induced hyperprolactinemia in rats after castration.

Prolactin release by adipose explants, primary adipocytes, and LS14 adipocytes

BACKGROUND

Prolactin (PRL) is a multifunctional hormone produced in humans by both pituitary and extrapituitary sites, including adipose tissue.

OBJECTIVES

Our objectives were to: 1) compare PRL secretion by sc and visceral adipose explants and mature adipocytes from obese and nonobese patients; and 2) examine the effects of insulin and selected cytokines on PRL gene expression and release from primary adipocytes and LS14 adipocytes.

DESIGN AND SUBJECTS

Adipose tissue was obtained from morbidly obese [body mass index (BMI) > 40 kg/m(2)] and nonobese (BMI <30 kg/m(2)) patients. Explants and isolated mature adipocytes were incubated for 10 d. Primary adipocytes or LS14 cells were used before or after differentiation and incubated with the test compounds for 24 h. PRL release was analyzed by a bioassay, and PRL expression was determined by real-time PCR.

RESULTS

PRL release from explants and mature adipocytes increased in a time-dependent manner indicating removal from inhibition. Visceral explants from obese patients showed higher PRL release than that from sc explants; both types of explants from nonobese patients released similar amounts of PRL. Analysis of data from 50 patients revealed an inverse relationship between PRL release from sc depots and BMI. Insulin suppressed PRL expression and release from differentiated adipocytes but moderately stimulated PRL release from nondifferentiated cells. The cAMP elevating compound forskolin increased PRL release in both cell types.

CONCLUSIONS

PRL should be recognized as an important adipokine whose release is regulated by insulin and is affected by obesity in a depot-specific manner.

Prolactin and insulin ultradian secretion and adipose tissue lipoprotein lipase expression in severely obese women after bariatric surgery

BACKGROUND

Hyperprolactinemia is associated with obesity. Furthermore, in human adipose tissue cultured in vitro, prolactin (PRL) inhibited lipoprotein lipase (LPL) activity via functional PRL receptors.

OBJECTIVE

To study PRL and insulin ultradian rhythm and subcutaneous adipose tissue LPL mRNA and protein expressions in severely obese women before and after malabsorptive bariatric surgery.

METHODS AND PROCEDURES

Seven severely obese, fertile women were studied twice, once before and the second time 1 year after bilio-pancreatic diversion (BPD), when the weight was stable for at least 3 months. Metabolizable energy intake and 24-h energy expenditure (EE) were measured. Fourier and PULSEFIT analyses were applied to 24-h hormonal time-series to study daily fluctuations and hormonal clearance. Insulin sensitivity was assessed by euglycemic-hyperinsulinemic clamp. Quantitative-competitive reverse transcriptase-PCR and western blot analysis were used to measure LPL gene expression.

RESULTS

Spontaneous 24-h PRL secretion was significantly reduced after BPD (mean-daily release, 128.4 +/- 28.1 microg/l vs. 67.2 +/- 9.2 microg/l distribution volume (Vd/l.24 h), P = 0.02); insulin secretion also was significantly reduced (499.9 +/- 204.0 microg/Vd/l.24 h vs. 85.6 +/- 21.0 microg/Vd/l.24 h, P = 0.0001). Metabolizable energy/kg(FFM) did not change significantly after BPD. Twenty-four-hour EE, but not 24-h EE/FFM, was significantly decreased after BPD (P < 0.05). Insulin sensitivity significantly (P < 0.0001) increased after BPD from 21.41 +/- 1.92 to 68.62 +/- 5.03 micromol/kg(FFM)/min. LPL mRNA concentration (from 42.63 +/- 4.21% to 19.00 +/- 2.74% of cyclophilin mRNA, P = 0.001) as well as LPL protein level (from 8.94 +/- 2.73 to 3.16 +/- 1.05 as ratios of protein of interest vs. housekeeping protein, P = 0.038) significantly decreased after BPD. The major determinant of PRL secretion was insulin secretion, whereas the best predictors of LPL expression were insulin and PRL secretion rates.

DISCUSSION

The restriction of lipid metabolizable energy rather than weight loss seems to be responsible for both reduction in PRL circulating levels and normalization of its secretion rhythm after bariatric surgery. Furthermore, the reduced adipose tissue LPL expression, being significantly correlated with the decrease in insulin and PRL, suggests a role of hyperinsulinemia and hyperprolactinemia in inducing and sustaining obesity.

Prolactin receptor signaling is essential for perinatal brown adipocyte function: a role for insulin-like growth factor-2

BACKGROUND

The lactogenic hormones prolactin (PRL) and placental lactogens (PL) play central roles in reproduction and mammary development. Their actions are mediated via binding to PRL receptor (PRLR), highly expressed in brown adipose tissue (BAT), yet their impact on adipocyte function and metabolism remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

PRLR knockout (KO) newborn mice were phenotypically characterized in terms of thermoregulation and their BAT differentiation assayed for gene expression studies. Derived brown preadipocyte cell lines were established to evaluate the molecular mechanisms involved in PRL signaling on BAT function. Here, we report that newborn mice lacking PRLR have hypotrophic BAT depots that express low levels of adipocyte nuclear receptor PPARgamma2, its coactivator PGC-1alpha, uncoupling protein 1 (UCP1) and the beta3 adrenoceptor, reducing mouse viability during cold challenge. Immortalized PRLR KO preadipocytes fail to undergo differentiation into mature adipocytes, a defect reversed by reintroduction of PRLR. That the effects of the lactogens in BAT are at least partly mediated by Insulin-like Growth Factor-2 (IGF-2) is supported by: i) a striking reduction in BAT IGF-2 expression in PRLR KO mice and in PRLR-deficient preadipocytes; ii) induction of cellular IGF-2 expression by PRL through JAK2/STAT5 pathway activation; and iii) reversal of defective differentiation in PRLR KO cells by exogenous IGF-2.

CONCLUSIONS

Our findings demonstrate that the lactogens act in concert with IGF-2 to control brown adipocyte differentiation and growth. Given the prominent role of brown adipose tissue during the perinatal period, our results identified prolactin receptor signaling as a major player and a potential therapeutic target in protecting newborn mammals against hypothermia.

What can we learn from rodents about prolactin in humans?

Prolactin (PRL) is a 23-kDa protein hormone that binds to a single-span membrane receptor, a member of the cytokine receptor superfamily, and exerts its action via several interacting signaling pathways. PRL is a multifunctional hormone that affects multiple reproductive and metabolic functions and is also involved in tumorigenicity. In addition to being a classical pituitary hormone, PRL in humans is produced by many tissues throughout the body where it acts as a cytokine. The objective of this review is to compare and contrast multiple aspects of PRL, from structure to regulation, and from physiology to pathology in rats, mice, and humans. At each juncture, questions are raised whether, or to what extent, data from rodents are relevant to PRL homeostasis in humans. Most current knowledge on PRL has been obtained from studies with rats and, more recently, from the use of transgenic mice. Although this information is indispensable for understanding PRL in human health and disease, there is sufficient disparity in the control of the production, distribution, and physiological functions of PRL among these species to warrant careful and judicial extrapolation to humans.

Effects of lactogen resistance and GH deficiency on mouse metabolism: pancreatic hormones, adipocytokines, and expression of adiponectin and insulin receptors

We recently described a novel mouse model that combines resistance to lactogenic hormones with GH deficiency (GHD). The GHD/lactogen-resistant males develop obesity and insulin resistance with age. We hypothesized that altered production of pancreatic hormones and dysregulation of adipocytokine secretion and action contribute to the pathogenesis of their insulin resistance. Double-mutant males (age 12-16 months) had fasting hyperinsulinemia, hyperamylinemia, hyperleptinemia, and a decreased ratio of adiponectin to leptin. Adiponectin receptor 1 and 2 (AdipoR1 and R2) mRNA levels in liver and skeletal muscle were normal but hepatic insulin receptor mRNA was increased. Relative to double-mutant males, GHD males had lower levels of insulin, amylin, and leptin, higher levels of adiponectin, and higher expression of hepatic AdipoR1 and insulin receptor mRNAs. Lactogen-resistant mice had reduced hepatic adipoR2 mRNA. In response to stress the plasma concentrations of MCP-1 and IL-6 increased in double-mutant males but not GHD or lactogen-resistant males. Our findings suggest that the insulin resistance of GHD/lactogen-resistant males is accompanied by dysregulation of pancreatic hormone and adipocytokine secretion and receptor expression. Phenotypic differences between double-mutant and GHD males suggest that lactogens and GH exert differential but overlapping effects on fat deposition and adipocytokine secretion and action.

Male pheromone-stimulated neurogenesis in the adult female brain: possible role in mating behavior

The regulation of female reproductive behaviors may involve memories of male pheromone signatures, formed in part by neural circuitry involving the olfactory bulb and hippocampus. These neural structures are the principal sites of adult neurogenesis; however, previous studies point to their independent regulation by sensory and physiological stimuli. Here we report that the pheromones of dominant (but not subordinate) males stimulate neuronal production in both the olfactory bulb and hippocampus of female mice, which are independently mediated by prolactin and luteinizing hormone, respectively. Neurogenesis induced by dominant-male pheromones correlates with a female preference for dominant males over subordinate males, whereas blocking neurogenesis with the mitotic inhibitor cytosine arabinoside eliminated this preference. These results suggest that male pheromones are involved in regulating neurogenesis in both the olfactory bulb and hippocampus, which may be important for female reproductive success.

The STAT5A-mediated induction of pyruvate dehydrogenase kinase 4 expression by prolactin or growth hormone in adipocytes

The purpose of this study was to determine whether pyruvate dehydrogenase kinase (PDK)4 was expressed in adipocytes and whether PDK4 expression was hormonally regulated in fat cells. Both Northern blot and Western blot analyses were conducted on samples isolated from 3T3-L1 adipocytes after various treatments with prolactin (PRL), growth hormone (GH), and/or insulin. Transfection of PDK4 promoter reporter constructs was performed. In addition, glucose uptake measurements were conducted. Our studies demonstrate that PRL and porcine GH can induce the expression of PDK4 in 3T3-L1 adipocytes. Our studies also show that insulin pretreatment can attenuate the ability of these hormones to induce PDK4 mRNA expression. In addition, we identified a hormone-responsive region in the murine PDK4 promoter and characterized a STAT5 binding site in this region that mediates the PRL (sheep) and GH (porcine) induction in PDK4 expression in 3T3-L1 adipocytes. PDK4 is a STAT5A target gene. PRL is a potent inducer of PDK4 protein levels, results in an inhibition of insulin-stimulated glucose transport in fat cells, and likely contributes to PRL-induced insulin resistance.

White matter plasticity and enhanced remyelination in the maternal CNS

Myelination, the process in which oligodendrocytes coat CNS axons with a myelin sheath, represents an important but poorly understood form of neural plasticity that may be sexually dimorphic in the adult CNS. Remission of multiple sclerosis during pregnancy led us to hypothesize that remyelination is enhanced in the maternal brain. Here we report an increase in the generation of myelin-forming oligodendrocytes and in the number of myelinated axons in the maternal murine CNS. Remarkably, pregnant mice have an enhanced ability to remyelinate white matter lesions. The hormone prolactin regulates oligodendrocyte precursor proliferation and mimics the regenerative effects of pregnancy. This suggests that maternal white matter plasticity imparts a striking ability to repair demyelination and identifies prolactin as a potential therapeutic agent.

The prolactin-deficient mouse has an unaltered metabolic phenotype

Prolactin (PRL), best recognized for its lactogenic activity, is also involved in the regulation of metabolic homeostasis in both mammalian and nonmammalian species. Although several mouse models have been used to study the metabolic functions of PRL, a clear-cut consensus has not emerged given the limited and often conflicting data. To clarify the role of PRL in metabolic homeostasis in males and nonlactating females, we used the PRL-deficient mouse. Our objectives were to compare: 1) weight gain, 2) body composition, 3) serum lipid profile, 4) circulating leptin and adiponectin levels, and 5) glucose tolerance in PRL knockout, heterozygous, and wild-type mice maintained on standard chow, high-fat, or low-fat diets. In addition, we compared the lipolytic actions of PRL using adipose tissue explants from mice, rats, and humans. We are reporting that PRL deficiency does not affect the rate of weight gain, body composition, serum lipids, or adiponectin levels in either sex on any diet. Glucose tolerance was slightly impaired in very young PRL knockout male pups but not in adults or in females at any age. Leptin was elevated in male, but not female, PRL knockout mice maintained on a low-fat diet. PRL did not affect lipolysis in adipose tissue explants from mice but significantly inhibited glycerol release from both rat and human adipose explants in a dose-dependent manner. We conclude that PRL deficiency has negligible gross metabolic effects in mice.

Hypoadiponectinemia in lean lactating women: Prolactin inhibits adiponectin secretion from human adipocytes

Adiponectin is an adipocyte-derived hormone involved in glucose, lipid and energy metabolism. A low plasma adiponectin concentration is associated with insulin resistance, obesity and atherosclerosis. In women, energy homeostasis is remarkably changed during gestation and lactation in order to supply sufficient nutrition for a fetus or newborn. In this study we aimed to elucidate the physiological impact of gestation and lactation on the plasma adiponectin levels and the influence of reproduction-related hormones on adiponectin secretion. We studied the longitudinal changes in plasma adiponectin concentration during pregnancy (1st, 2nd and 3rd trimester) and lactation (3 days and 1 month after the delivery) in lean healthy women (n = 22). The plasma adiponectin level declined slightly as the pregnancy advanced and reached its lowest level during lactation (12.25 +/- 0.182 microg/ml at early pregnancy vs. 6.88 +/- 0.375 microg/ml at 3 days postpartum, p < 0.001). In order to investigate the role of the lactogenic hormone prolactin in the decrease of plasma adiponectin levels during lactation, we further performed in vitro experiments using human primary cultured adipocytes. Western blotting of the adipocyte lysate and ELISA of the culture medium revealed that exogenous prolactin inhibited both production and secretion of adiponectin in a dose-dependent manner. Our results thus suggests that prolactin affects the regulation of maternal metabolism through suppression of adiponectin.

Loss of mammary epithelial prolactin receptor delays tumor formation by reducing cell proliferation in low-grade preinvasive lesions

Top quartile serum prolactin levels confer a twofold increase in the relative risk of developing breast cancer. Prolactin exerts this effect at an ill defined point in the carcinogenic process, via mechanisms involving direct action via prolactin receptors within mammary epithelium and/or indirect action through regulation of other hormones such as estrogen and progesterone. We have addressed these questions by examining mammary carcinogenesis in transplants of mouse mammary epithelium expressing the SV40T oncogene, with or without the prolactin receptor, using host animals with a normal endocrine system. In prolactin receptor knockout transplants the area of neoplasia was significantly smaller (7 versus 17%; P < 0.001 at 22 weeks and 7 versus 14%; P = 0.009 at 32 weeks). Low-grade neoplastic lesions displayed reduced BrdU incorporation rate (11.3 versus 17% P = 0.003) but no change in apoptosis rate. Tumor latency increased (289 days versus 236 days, P < 0.001). Tumor frequency, growth rate, morphology, cell proliferation and apoptosis were not altered. Thus, prolactin acts directly on the mammary epithelial cells to increase cell proliferation in preinvasive lesions, resulting in more neoplasia and acceleration of the transition to invasive carcinoma. Targeting of mammary prolactin signaling thus provides a strategy to prevent the early progression of neoplasia to invasive carcinoma.

STAT5 activators modulate acyl CoA oxidase (AOX) expression in adipocytes and STAT5A binds to the AOX promoter in vitro

Growth hormone (GH) diminishes adipose tissue mass in vivo and prolactin (PRL) can also modulate adipocyte metabolism. Both GH and PRL are potent activators of STAT5 and exert a variety of effects on adipocyte gene expression. In this study, we have demonstrated that GH and PRL increase the mRNA of acyl CoA oxidase in 3T3-L1 adipocytes. We also identified seven putative STAT elements in the murine AOX promoter. We observed that GH modulates protein binding to the majority of these promoter elements. However, GH induced very potent binding to -1841 to -1825 of the murine AOX promoter. EMSA supershift analysis revealed that this site was specifically bound by STAT5A, but not by STAT1 or STAT3. Taken together, these data strongly suggest that GH directly induces the expression of AOX in adipocytes through STAT5A binding to the -1841 to -1825 site within the AOX promoter. Our observations are consistent with other studies that demonstrate that STAT5 activators modulate fatty acid oxidation.

Medullary thyroid carcinoma arises in the absence of prolactin signaling

Prolactin, a pituitary hormone, exerts pleiotropic effects in various cells. These effects are mediated by a membrane receptor highly expressed in many tissues. To analyze prolactin effects on the thyroid gland, we first identified prolactin receptor (PRLR) mRNAs by in situ hybridization. To further evaluate the physiologic relevance of PRLR actions in the thyroid in vivo, we used PRLR knockout mice. Whereas the histologic structure of thyroid of PRLR-null mice was not disturbed, we show that T4 levels are lower in null animals (13.63 +/- 2.98 versus 10.78 +/- 2.25 pmol/L in null mice), confirming that prolactin participates in the control of thyroid metabolism. To further investigate thyroid effects in mice, we measured body temperature and thyroid-stimulating hormone in young and adult male and/or female PRLR-null mice and their normal siblings. Surprisingly, in null animals, we saw medullary thyroid carcinoma (MTC) arising from parafollicular C cells producing calcitonin. The incidence of these carcinomas attained 41% in PRLR-null mice, whereas this malignant tumor occurs sporadically or as a component of the familial cancer syndrome in humans. This finding suggests that PRLR-null mice could represent a valuable animal model for MTC, which could be compared with existing MTC models. These observations suggest a possible link between the appearance of this carcinoma and the absence of prolactin signaling.

The regulation of fatty acid synthase by STAT5A

Growth hormone (GH) diminishes adipose tissue mass in vivo and decreases expression and activity of fatty acid synthase (FAS) in adipocytes. GH and prolactin (PRL) are potent activators of STAT5 and exert adipogenic and antiadipogenic effects in adipocytes. In this study, we demonstrate that GH and PRL decrease the mRNA and protein levels of FAS in 3T3-L1 adipocytes. We present evidence that indicates that FAS is repressed at the level of transcription. In addition, PRL responsiveness was shown to exist between -1,594 and -700 of the rat FAS promoter. Moreover, responsiveness to PRL was abolished with mutation of a site at position -908 to -893, which we have shown to bind STAT5A in a PRL-dependent manner. Taken together, these data strongly suggest that PRL directly represses expression of FAS in adipocytes through STAT5A binding to the -908 to -893 site. Furthermore, our results indicate that STAT5A has an antilipogenic function in adipocytes and may contribute to the regulation of energy balance.

Prolactin and growth hormone regulate adiponectin secretion and receptor expression in adipose tissue

Adiponectin is a hormone secreted from adipose tissue, and serum levels are decreased with obesity and insulin resistance. Because prolactin (PRL) and growth hormone (GH) can affect insulin sensitivity, we investigated the effects of these hormones on the regulation of adiponectin in human adipose tissue in vitro and in rodents in vivo. Adiponectin secretion was significantly suppressed by PRL and GH in in vitro cultured human adipose tissue. Furthermore, PRL increased adiponectin receptor 1 (AdipoR1) mRNA expression and GH decreased AdipoR2 expression in the cultured human adipose tissue. In transgenic mice expressing GH, and female mice expressing PRL, serum levels of adiponectin were decreased. In contrast, GH receptor deficient mice had elevated adiponectin levels, while PRL receptor deficient mice were unaffected. In conclusion, we demonstrate gene expression of AdipoR1 and AdipoR2 in human adipose tissue for the first time, and show that these are differentially regulated by PRL and GH. Both PRL and GH reduced adiponectin secretion in human adipose tissue in vitro and in mice in vivo. Decreased serum adiponectin levels have been associated with insulin resistance, and our data in human tissue and in transgenic mice suggest a role for adiponectin in PRL and GH induced insulin resistance.

Roles of the lactogens and somatogens in perinatal and postnatal metabolism and growth: studies of a novel mouse model combining lactogen resistance and growth hormone deficiency

To delineate the roles of the lactogens and GH in the control of perinatal and postnatal growth, fat deposition, insulin production, and insulin action, we generated a novel mouse model that combines resistance to all lactogenic hormones with a severe deficiency of pituitary GH. The model was created by breeding PRL receptor (PRLR)-deficient (knockout) males with GH-deficient (little) females. In contrast to mice with isolated GH or PRLR deficiencies, double-mutant (lactogen-resistant and GH-deficient) mice on d 7 of life had growth failure and hypoglycemia. These findings suggest that lactogens and GH act in concert to facilitate weight gain and glucose homeostasis during the perinatal period. Plasma insulin and IGF-I and IGF-II concentrations were decreased in both GH-deficient and double-mutant neonates but were normal in PRLR-deficient mice. Body weights of the double mutants were reduced markedly during the first 3-4 months of age, and adults had striking reductions in femur length, plasma IGF-I and IGF binding protein-3 concentrations, and femoral bone mineral density. By age 6-12 months, however, the double-mutant mice developed obesity, hyperleptinemia, fasting hyperglycemia, relative hypoinsulinemia, insulin resistance, and glucose intolerance; males were affected to a greater degree than females. The combination of perinatal growth failure and late-onset obesity and insulin resistance suggests that the lactogen-resistant/GH-deficient mouse may serve as a model for the development of the metabolic syndrome.

Prostate hyperplasia in a transgenic mouse with prostate-specific expression of prolactin

Prolactin (PRL) is one of several polypeptide factors known to exert trophic effects on the prostate. We have previously reported a dramatic prostate enlargement with concurrent chronic hyperprolactinemia and elevated serum androgen levels in a PRL transgenic mouse (Mt-PRL) with ubiquitous expression of the transgene. To address the role of local PRL action in the prostate, a new transgenic mouse model (Pb-PRL) was generated using the prostate-specific rat probasin (Pb) minimal promoter to drive expression of the rat PRL gene. Pb-PRL transgenic males developed a significant enlargement of both the dorsolateral and ventral prostate lobes evident from 10 wk of age and increasing with age. Expression of the transgene was restricted to the prostate and detected from 4 wk of age. Low levels of transgenic rat PRL were detectable in the serum of adult Pb-PRL animals. Serum androgen levels were normal. The Pb-PRL prostate displayed significant stromal hyperplasia, ductal dilation, and focal areas of epithelial dysplasia. Quantitative analysis of prostatic tissue cellularity demonstrated a marked increase in the stromal to epithelial ratio in all lobes of Mt-PRL and Pb-PRL transgenic prostates compared with controls. Microdissections demonstrated an increased ductal morphogenesis in dorsolateral and ventral prostate lobes of Mt-PRL prostate vs. Pb-PRL and controls. In conclusion, this study indicates the ability of PRL to promote, directly or indirectly, ductal morphogenesis in the developing prostate and further to induce abnormal growth primarily of the stroma in the adult gland in a setting of normal androgen levels.

Prolactin overexpression by MDA-MB-435 human breast cancer cells accelerates tumor growth

Prolactin (PRL) is an important hormone in mammary tumorigenesis in rodents but its involvement in human breast cancer has been controversial. A role for locally produced PRL in breast carcinogenesis is suggested by its mitogenic action on breast cancer cells and the expression of both PRL and its receptor (PRL-R) in breast carcinomas. Our objective was to examine whether PRL, overexpressed by breast cancer cells, forms an autocrine/paracrine loop that confers a growth advantage for tumors. MDA-MB-435 breast cancer cells overexpressing 23K human PRL were generated, and PRL production and secretion by the clones were confirmed by RT-PCR, western blotting, and the Nb2 bioassay; control clones contain vector only. In vitro the 23K PRL clones proliferated faster and expressed higher levels of the PRL-R protein than controls only when incubated in charcoal-stripped serum (CSS) devoid of lactogenic hormones. When injected into the mammary fatpad of female nude mice or subcutaneously into males, the PRL-overexpressing clones formed tumors that grew 2-4-fold faster than tumors derived from control clones or wild type MDA-MB-435 cells. Western analysis demonstrated significantly higher PRL, PRL-R, and bcl-2 levels in the tumors overexpressing PRL compared to control tumors. These data support a role for breast PRL as a growth/anti-apoptotic factor and suggest that it may serve as a novel therapeutic target for the treatment of breast cancer.

Body composition of prolactin-, growth hormone, and thyrotropin-deficient Ames dwarf mice

Ames dwarf mice have primary deficiency of prolactin (PRL), growth hormone (GH), and thyroid-stimulating hormone (TSH), and live considerably longer than normal animals from the same line. In view of the documented effects of GH, PRL, and thyroid hormones on lean and fat body mass and skeletal growth, and the suspected relationship of body size and composition to life expectancy, it was of interest to examine age-related changes in body composition of Ames dwarf mice. Lean mass, fat mass, bone area, and bone mineral content (BMC) were determined in dwarf and normal mice at the ages of 2, 4.5 6, and 18 mo using dual X-ray absorptiometry. In addition to the expected significant declines in lean mass, bone area, and BMC, dwarf mice exhibited attenuation of the age-related increase in bone mineral density and delayed or attenuated increase in percentage of body fat. Percentage of body fat was lower in adult dwarfs than in the corresponding normal controls. Patterns of age-related changes in body composition in Ames dwarf mice are consistent with the recent report of age-related changes in body composition in PRL receptor knockout mice. We suspect that reduction in relative adiposity may contribute to the previously reported increase in insulin sensitivity of Ames dwarf mice and thus may be a factor in delayed aging and increased longevity of these animals.

The ribonuclease protection assay: a powerful tool for the veterinary pathologist

Veterinary pathologists engaged in basic research use a variety of methods to study disease pathogenesis at the light microscopic and submicroscopic (protein and mRNA) levels. The ribonuclease protection assay is a sensitive and accurate method to measure mRNA expression. The major advantages of the assay are that multiple mRNA species can be measured simultaneously in a single total RNA sample and that the assay has relatively high throughput. The major disadvantage is that the assay requires moderate technical skill.