Oxytocin stimulates glucose uptake in skeletal muscle cells through the calcium-CaMKK-AMPK pathway

Glycaemic patterns during breastfeeding with postpartum use of closed-loop insulin delivery in women with type 1 diabetes

AIMS/HYPOTHESIS

This study aimed to describe the relationship between breastfeeding episodes and maternal glucose levels, and to assess whether this differs with closed-loop vs open-loop (sensor-augmented pump) insulin therapy.

METHODS

Infant-feeding diaries were collected at 6 weeks, 12 weeks and 24 weeks postpartum in a trial of postpartum closed-loop use in 18 women with type 1 diabetes. Continuous glucose monitoring (CGM) data were used to identify maternal glucose patterns within the 3 h of breastfeeding episodes. Generalised mixed models adjusted for breastfeeding episodes in the same woman, repeat breastfeeding episodes, carbohydrate intake, infant age at time of feeding and early pregnancy HbA1c. This was a secondary analysis of data collected during a randomised trial (ClinicalTrials.gov registration no. NCT04420728).

RESULTS

CGM glucose remained above 3.9 mmol/l in the 3 h post-breastfeeding for 93% (397/427) of breastfeeding episodes. There was an overall decrease in glucose at nighttime within 3 h of breastfeeding (1.1 mmol l-1 h-1 decrease on average; p=0.009). A decrease in nighttime glucose was observed with open-loop therapy (1.2 ± 0.5 mmol/l) but was blunted with closed-loop therapy (0.4 ± 0.3 mmol/l; p<0.01, open-loop vs closed-loop).

CONCLUSIONS/INTERPRETATION

There is a small decrease in glucose after nighttime breastfeeding that usually does not result in maternal hypoglycaemia; this appears to be blunted with the use of closed-loop therapy.

Does activation of oxytocinergic reward circuits postpone the decline of the aging brain?

Oxytocin supports reproduction by promoting sexual- and nursing behavior. Moreover, it stimulates reproductive organs by different avenues. Oxytocin is released to the blood from terminals of oxytocinergic neurons which project from the hypothalamus to the pituitary gland. Concomitantly, the dendrites of these neurons discharge oxytocin into neighboring areas of the hypothalamus. At this location it affects other neuroendocrine systems by autocrine and paracrine mechanisms. Moreover, sensory processing, affective functions, and reward circuits are influenced by oxytocinergic neurons that reach different sites in the brain. In addition to its facilitating impact on various aspects of reproduction, oxytocin is revealed to possess significant anti-inflammatory, restoring, and tranquilizing properties. This has been demonstrated both in many in-vivo and in-vitro studies. The oxytocin system may therefore have the capacity to alleviate detrimental physiological- and mental stress reactions. Thus, high levels of endogenous oxytocin may counteract inadequate inflammation and malfunctioning of neurons and supportive cells in the brain. A persistent low-grade inflammation increasing with age-referred to as inflammaging-may lead to a cognitive decline but may also predispose to neurodegenerative diseases such as Alzheimer's and Parkinson. Interestingly, animal studies indicate that age-related destructive processes in the body can be postponed by techniques that preserve immune- and stem cell functions in the hypothalamus. It is argued in this article that sexual activity-by its stimulating impact on the oxytocinergic activity in many regions of the brain-has the capacity to delay the onset of age-related cerebral decay. This may also postpone frailty and age-associated diseases in the body. Finally, oxytocin possesses neuroplastic properties that may be applied to expand sexual reward. The release of oxytocin may therefore be further potentiated by learning processes that involves oxytocin itself. It may therefore be profitable to raise the consciousness about the potential health benefits of sexual activity particularly among the seniors.

Genomic analysis of Tibetan ground tits identifies molecular adaptations associated with cooperative breeding

Cooperative breeding is a sophisticated altruistic social behavior that helps social animals to adapt to harsh environments. The Tibetan ground tit, Pseudopodoces humilis, is a high-altitude bird endemic to the Tibetan plateau. Recently, it has become an exciting system for studying the evolution of facultative cooperative breeding. To test for molecular adaptations associated with cooperative breeding, we resequenced the whole genome of ground tits from 6 wild populations that display remarkable variation in the frequency of cooperative breeding. Population structure analyses showed that the 6 populations were divided into 4 lineages, which is congruent with the major geographical distribution of the sampling sites. Using genome-wide selective sweep analysis, we identified putative positively selected genes (PSGs) in groups of tits that displayed high and low cooperative breeding rates. The total number of PSGs varied from 146 to 722 in high cooperative breeding rate populations, and from 272 to 752 in low cooperative breeding rate populations. Functional enrichment analysis of these PSGs identified several significantly enriched ontologies related to oxytocin signaling, estrogen signaling, and insulin secretion. PSGs involved in these functional ontologies suggest that molecular adaptations in hormonal regulation may have played important roles in shaping the evolution of cooperative breeding in the ground tit. Taken together, our study provides candidate genes and functional ontologies involved in molecular adaptations associated with cooperative breeding in Tibetan ground tits, and calls for a better understanding of the genetic roles in the evolution of cooperative breeding.

AMPK and the Endocrine Control of Metabolism

Complex multicellular organisms require a coordinated response from multiple tissues to maintain whole-body homeostasis in the face of energetic stressors such as fasting, cold, and exercise. It is also essential that energy is stored efficiently with feeding and the chronic nutrient surplus that occurs with obesity. Mammals have adapted several endocrine signals that regulate metabolism in response to changes in nutrient availability and energy demand. These include hormones altered by fasting and refeeding including insulin, glucagon, glucagon-like peptide-1, catecholamines, ghrelin, and fibroblast growth factor 21; adipokines such as leptin and adiponectin; cell stress-induced cytokines like tumor necrosis factor alpha and growth differentiating factor 15, and lastly exerkines such as interleukin-6 and irisin. Over the last 2 decades, it has become apparent that many of these endocrine factors control metabolism by regulating the activity of the AMPK (adenosine monophosphate-activated protein kinase). AMPK is a master regulator of nutrient homeostasis, phosphorylating over 100 distinct substrates that are critical for controlling autophagy, carbohydrate, fatty acid, cholesterol, and protein metabolism. In this review, we discuss how AMPK integrates endocrine signals to maintain energy balance in response to diverse homeostatic challenges. We also present some considerations with respect to experimental design which should enhance reproducibility and the fidelity of the conclusions.

Prolactin does not seem to mediate the improvement on insulin resistance markers and blood glucose levels related to breastfeeding

Introduction

The prevalence of type 2 diabetes mellitus (T2DM) is increasing worldwide. Strategies to decrease this risk should be strongly encouraged. Lactation has been associated, for the mother, with reduction in future T2DM risk in several studies. The mechanisms behind this phenomenon, however, are poorly understood. The aims of this study were, first, to compare blood glucose levels and markers of insulin resistance (MIR) in early postpartum women with overweight/obesity according to their breastfeeding status and, second, to evaluate whether prolactin (PRL) levels could mediate improvements in these parameters.

Methods

The prospective study followed 95 women older than 18 years from early pregnancy for up to 60 to 180 days postpartum. All participants had a BMI > 25 kg/m2 and a singleton pregnancy. At each visit, questionnaires and clinical and biochemical evaluations were performed. Participants were divided into two groups according to the breastfeeding status as "yes" for exclusive or predominant breastfeeding, and "no" for not breastfeeding.

Results

Breastfeeding women (n = 44) had significantly higher PRL levels [47.8 (29.6-88.2) vs. 20.0 (12.0-33.8), p< 0.001]. They also had significantly lower fasting blood glucose levels [89.0 (8.0) vs. 93.9 (12.6) mg/dl, p = 0.04], triglycerides (TG) [92.2 (37.9) vs. 122.4 (64.4) mg/dl, p = 0.01], TG/HDL ratio [1.8 (0.8) vs. 2.4 (1.6) mg/dl, p = 0.02], TyG index [8.24 (0.4) vs. 8.52 (0.53), p = 0.005], fasting serum insulin [8.9 (6.3-11.6) vs. 11.4 (7.7-17.0), p = 0.048], and HOMA-IR [2.0 (1.3-2.7) vs. 2.6 (1.6-3.9), p = 0.025] in the postpartum period compared to the non-breastfeeding group. Groups were homogeneous in relation to prevalence of GDM, pre-gestational BMI, as well as daily caloric intake, physical activity, and weight loss at postpartum. Linear regression analysis with adjustments for confounders showed a statistically significant association of breastfeeding with fasting blood glucose [-6.37 (-10.91 to -1.83), p = 0.006], HOMA-IR [-0.27 (-0.51 to -0.04), p = 0.024], TyG index [-0.04 (-0.06 to -0.02), p = 0.001], and TG/HDL ratio [-0.25 (-0.48 to -0.01), p = 0.038]. Mediation analysis showed that PRL did not mediate these effects. Sensitivity analyses considering different cutoffs for PRL levels also did not show modification effect in the mediation analyses.

Conclusion

Breastfeeding was associated with improvement in glucose metabolism and MIR 60 to 180 days after birth in overweight and obese women, even when adjusted for confounders. PRL levels were not found to mediate the association between breastfeeding and improvement in MIR.

Novel Risk Variants in the Oxytocin Receptor Gene (OXTR) Possibly Linked to and Associated with Familial Type 2 Diabetes

The oxytocin system is well-known for its role in social bonding and reproduction. Recently, the oxytocin system was found to play other metabolic roles such as regulation of food intake, peripheral glucose uptake, and insulin sensitivity. Variants in OXTR gene have been associated with overeating, increased cardiovascular risk, and type 2 diabetes (T2D). We tested 20 microarray-derived single nucleotide polymorphisms in the OXTR gene in 212 Italian families with rich family history for T2D and found four novel and one previously reported variant suggestively significant for linkage and association with the risk of T2D. Our study has shed some light into the genetics of susceptibility to T2D at least in Italian families.

Oxytocin Release: A Remedy for Cerebral Inflammaging

Oxytocin facilitates reproduction both by physiological and behavioral mechanisms. Oxytocinergic neurons emerging from the hypothalamus release oxytocin from the pituitary gland to the blood by axonal discharge to regulate reproductive organs. However, at the same time, oxytocin is secreted into neighboring areas of the hypothalamus from the dendrites of these neurons. Here, the peptide acts by autocrine and paracrine mechanisms to influence other neuroendocrine systems. Furthermore, oxytocinergic neurons project to many different locations in the brain, where they affect sensory processing, affective functions, and reward. Additional to its regulatory role, significant anti-inflammatory and restoring effects of oxytocin have been reported from many invivo and in-vitro studies. The pervasive property of the oxytocin system may enable it generally to dampen stress reactions both peripherally and centrally, and protect neurons and supportive cells from inadequate inflammation and malfunctioning. Animal experiments have documented the importance of preserving immune- and stem cell functions in the hypothalamus to impede age-related destructive processes of the body. Sexual reward has a profound stimulating impact on the oxytocinergic activity, and the present article therefore presents the hypothesis that frequent sexual activity and gratigying social experiance may postpone the onset of frailty and age-associated diseases by neural protection from the bursts of oxytocin. Furthermore, suggestions are given how the neuroplastic properties of oxytocin may be utilized to enhance sexual reward by learning processes in order to further reinforce the release of this peptide.

The Importance of Experimental Investigation of the Peripheral Oxytocin System

Oxytocin and oxytocin receptors are synthesized in the periphery where paracrine/autocrine actions have been described alongside endocrine actions effected by central release of oxytocin from the posterior pituitary. In the female reproductive system, classical actions of uterine contraction and milk ejection from mammary glands are accompanied by actions in the ovaries where roles in steroidogenesis, follicle recruitment and ovulation have been described. Steroidogenesis, contractile activity, and gamete health are similarly affected by oxytocin in the male reproductive tract. In the cardiovascular system, a local oxytocinergic system appears to play an important cardio-protective role. This role is likely associated with emerging evidence that peripheral oxytocin is an important hormone in the endocrinology of glucose homeostasis due to its actions in adipose, the pancreas, and the largely ignored oxytocinergic systems of the adrenal glands and liver. Gene polymorphisms are shown to be associated with a number of reported traits, not least factors associated with metabolic syndrome.

Oxytocin as an Anti-obesity Treatment

Obesity is a growing health concern, as it increases risk for heart disease, hypertension, type 2 diabetes, cancer, COVID-19 related hospitalizations and mortality. However, current weight loss therapies are often associated with psychiatric or cardiovascular side effects or poor tolerability that limit their long-term use. The hypothalamic neuropeptide, oxytocin (OT), mediates a wide range of physiologic actions, which include reproductive behavior, formation of prosocial behaviors and control of body weight. We and others have shown that OT circumvents leptin resistance and elicits weight loss in diet-induced obese rodents and non-human primates by reducing both food intake and increasing energy expenditure (EE). Chronic intranasal OT also elicits promising effects on weight loss in obese humans. This review evaluates the potential use of OT as a therapeutic strategy to treat obesity in rodents, non-human primates, and humans, and identifies potential mechanisms that mediate this effect.

Intranasal Oxytocin Improves Lean Muscle Mass and Lowers LDL Cholesterol in Older Adults with Sarcopenic Obesity: A Pilot Randomized Controlled Trial

OBJECTIVES

Obesity is associated with sarcopenia in older adults, and weight loss can lead to further muscle mass loss. Oxytocin decreases with age, and animal studies suggest that oxytocin administration has trophic effects on skeletal muscle cells and reduces adiposity. We conducted a clinical trial to examine the safety and preliminary efficacy of intranasal oxytocin for older adults with sarcopenic obesity.

DESIGN

A double-blind, placebo-controlled randomized controlled trial of intranasal oxytocin (24 IU 4 times per day) for 8 weeks.

SETTING AND PARTICIPANTS

Twenty-one older (67.5 ± 5.4 years), obese (30-43 kg/m2), sedentary (<2 strenuous exercise per week) adults with slow gait speed (<1 m/s, proxy measure of sarcopenia) were recruited.

MEASURES

Generalized estimating equations were used to evaluate the effect of oxytocin on safety/tolerability of oxytocin administration and whole body muscle and fat mass.

RESULTS

At baseline, body mass index (BMI) was 36.8 ± 3.6 kg/m2, fat mass 46.09 ± 6.99 kg, lean mass 50.98 ± 11.77 kg, fasting plasma glucose (FPG) 92.0 ± 8.9 mg/dL, hemoglobin A1c (HbA1c) 5.7% ± 0.4%, low density lipoprotein (LDL) 111.3 ± 41.5 mg/dL, high-density lipoprotein (HDL) 47.85 ± 10.96 mg/dL, and triglycerides 140.55 ± 83.50 mg/dL. Oxytocin administration was well tolerated without any significant adverse events. Oxytocin led to a significant increase of 2.25 kg in whole body lean mass compared with placebo (P < .01) with a trend toward decreasing fat mass, and a significantly reduced plasma LDL cholesterol by -19.3 mg/dL (P = .023) compared against placebo. There were no significant changes in BMI, appetite scores, glycemia, plasma HDL, triglycerides, or depressive symptoms.

CONCLUSIONS AND IMPLICATIONS

This proof-of-concept study indicates that oxytocin may be useful for the treatment of sarcopenic obesity in older adults. Oxytocin administration may also provide additional cardiovascular benefits.

Sensory Circumventricular Organs, Neuroendocrine Control, and Metabolic Regulation

The central nervous system is critical in metabolic regulation, and accumulating evidence points to a distributed network of brain regions involved in energy homeostasis. This is accomplished, in part, by integrating peripheral and central metabolic information and subsequently modulating neuroendocrine outputs through the paraventricular and supraoptic nucleus of the hypothalamus. However, these hypothalamic nuclei are generally protected by a blood-brain-barrier limiting their ability to directly sense circulating metabolic signals—pointing to possible involvement of upstream brain nuclei. In this regard, sensory circumventricular organs (CVOs), brain sites traditionally recognized in thirst/fluid and cardiovascular regulation, are emerging as potential sites through which circulating metabolic substances influence neuroendocrine control. The sensory CVOs, including the subfornical organ, organum vasculosum of the lamina terminalis, and area postrema, are located outside the blood-brain-barrier, possess cellular machinery to sense the metabolic interior milieu, and establish complex neural networks to hypothalamic neuroendocrine nuclei. Here, evidence for a potential role of sensory CVO-hypothalamic neuroendocrine networks in energy homeostasis is presented.

Oxytocin Involvement in Body Composition Unveils the True Identity of Oxytocin

The origin of the Oxytocin/Vasopressin system dates back about 600 million years. Oxytocin (Oxt) together with Vasopressin (VP) regulate a diversity of physiological functions that are important for osmoregulation, reproduction, metabolism, and social behavior. Oxt/VP-like peptides have been identified in several invertebrate species and they are functionally related across the entire animal kingdom. Functional conservation enables future exploitation of invertebrate models to study Oxt’s functions not related to pregnancy and the basic mechanisms of central Oxt/VP signaling. Specifically, Oxt is well known for its effects on uteri contractility and milk ejection as well as on metabolism and energy homeostasis. Moreover, the striking evidence that Oxt is linked to energy regulation is that Oxt- and Oxytocin receptor (Oxtr)-deficient mice show late onset obesity. Interestingly Oxt^−/− or Oxtr^−/− mice develop weight gain without increasing food intake, suggesting that a lack of Oxt reduce metabolic rate. Oxt is expressed in a diversity of skeletal muscle phenotypes and regulates thermogenesis and bone mass. Oxt may increases skeletal muscle tonicity and/or increases body temperature. In this review, the author compared the three most recent theories on the effects of Oxt on body composition.

Oxytocin induces anti-catabolic and anabolic effects on protein metabolism in the female rat oxidative skeletal muscle

AIMS

Although it is well established that skeletal muscle contains oxytocin (OT) receptors and OT-knockout mice show premature development of sarcopenia, the role of OT in controlling skeletal muscle mass is still unknown. Therefore, the present work aimed to determine OT's effects on skeletal muscle protein metabolism.

MAIN METHODS

Total proteolysis, proteolytic system activities and protein synthesis were assessed in isolated soleus muscle from prepubertal female rats. Through in vivo experiments, rats received 3-day OT treatment (3UI.kg^-1.day^-1, i.p.) or saline, and muscles were harvested for mass-gain assessment.

KEY FINDINGS

In vitro OT receptor stimulation reduced total proteolysis, specifically through attenuation of the lysosomal and proteasomal proteolytic systems, and in parallel activated the Akt/FoxO1 signaling and suppressed atrogenes (e.g., MuRF-1 and atrogin-1) expression induced by motor denervation. On the other hand, the protein synthesis was not altered by in vitro treatment with the OT receptor-selective agonist. Although short-term OT treatment did not change the atrogene mRNA levels, the protein synthesis was stimulated, resulting in soleus mass gain, probably through an indirect effect.

SIGNIFICANCE

Taken together, these data show for the first time that OT directly inhibits the proteolytic activities of the lysosomal and proteasomal systems in rat oxidative skeletal muscle by suppressing atrogene expression via stimulation of Akt/FoxO signaling. Moreover, the data obtained from in vivo experiments suggest OT's ability to control rat oxidative skeletal muscle mass.

Exercise training rescues the electrical activity of liver-projecting DMNV neurones in response to oxytocin in spontaneously hypertensive rats

A neural circuit between the paraventricular nucleus of the hypothalamus (PVN) and the dorsal motor nucleus of the vagus (DMNV) constitutes part of an important parasympathetic autonomic pathway that controls hepatic glucose production. Intracerebroventricular injection of insulin activates oxytocinergic neurones in the PVN and elicits the release of oxytocin into the circulation, which plays an important role in the metabolism of glucose. Moreover, the central action of insulin can reduce the concentration of glucose in blood taken from the hepatic vein of Wistar rats via activation of vagal efferent nerves to the liver. This mechanism is impaired in sedentary spontaneously hypertensive rats (SHR). Because aerobic exercise increases vagal tone, partly mediated by increasing the oxytocinergic connections between the PVN and DMNV, we hypothesised that oxytocin (OT) might alter the excitability of liver-projecting DMNV neurones. Thus, we investigated the effects of OT on electrical properties of the liver-projecting DMNV neurones from Wistar, SHR subjected to 4 weeks of exercise training, as well sedentary controls, using whole cell patch-clamping. The results show that OT increased the resting membrane potential of DMNV neurones in Wistar rats, as well as the firing frequency of these cells, but not in sedentary SHR. However, in SHR subjected to 4 weeks of exercise training, the effects of OT on liver-projecting DMNV neurones of were similar to those seen in Wistar rats. These findings show that OT elicits similar changes in the electrophysiological properties of liver-projecting DMNV neurones of Wistar and exercise-trained but not sedentary SHR. These results indicate that exercise training can restore the sensitivity of liver-projecting DMNV neurones of exercise-trained SHR to OT.

Genomic Mechanisms of Physiological and Morphological Adaptations of Limestone Langurs to Karst Habitats

Knowledge of the physiological and morphological evolution and adaptation of nonhuman primates is critical to understand hominin origins, physiological ecology, morphological evolution, and applications in biomedicine. Particularly, limestone langurs represent a direct example of adaptations to the challenges of exploiting a high calcium and harsh environment. Here, we report a de novo genome assembly (Tfra_2.0) of a male François's langur (Trachypithecus francoisi) with contig N50 of 16.3 Mb and resequencing data of 23 individuals representing five limestone and four forest langur species. Comparative genomics reveals evidence for functional evolution in genes and gene families related to calcium signaling in the limestone langur genome, probably as an adaptation to naturally occurring high calcium levels present in water and plant resources in karst habitats. The genomic and functional analyses suggest that a single point mutation (Lys1905Arg) in the α1c subunit of the L-type voltage-gated calcium channel Cav1.2 (CACNA1C) attenuates the inward calcium current into the cells in vitro. Population genomic analyses and RNA-sequencing indicate that EDNRB is less expressed in white tail hair follicles of the white-headed langur (T. leucocephalus) compared with the black-colored François's langur and hence might be responsible for species-specific differences in body coloration. Our findings contribute to a new understanding of gene-environment interactions and physiomorphological adaptative mechanisms in ecologically specialized primate taxa.

A complete map of the Calcium/calmodulin-dependent protein kinase kinase 2 (CAMKK2) signaling pathway

Calcium/calmodulin-dependent protein kinase kinase 2 (CAMKK2) is a serine/threonine-protein kinase belonging to the Ca²⁺/calmodulin-dependent protein kinase subfamily. CAMKK2 has an autocatalytic site, which gets exposed when Ca²⁺/calmodulin (CAM) binds to it. This results in autophosphorylation and complete activation of CAMKK2. The three major known downstream targets of CAMKK2 are 5'-adenosine monophosphate (AMP)-activated protein kinase (AMPKα), calcium/calmodulin-dependent protein kinase 1 (CAMK1) and calcium/calmodulin-dependent protein kinase 4 (CAMK4). Activation of these targets by CAMKK2 is important for the maintenance of different cellular and physiological processes within the cell. CAMKK2 is found to be important in neuronal development, bone remodeling, adipogenesis, and systemic glucose homeostasis, osteoclastgensis and postnatal myogensis. CAMKK2 is reported to be involved in pathologies like Duchenne muscular dystrophy, inflammation, osteoporosis and bone remodeling and is also reported to be overexpressed in prostate cancer, hepatic cancer, ovarian and gastric cancer. CAMKK2 is involved in increased cell proliferation and migration through CAMKK2/AMPK pathway in prostate cancer and activation of AKT in ovarian cancer. Although CAMKK2 is a molecule of great importance, a public resource of the CAMKK2 signaling pathway is currently lacking. Therefore, we carried out detailed data mining and documentation of the signaling events associated with CAMKK2 from published literature and developed an integrated reaction map of CAMKK2 signaling. This resulted in the cataloging of 285 reactions belonging to the CAMKK2 signaling pathway, which includes 33 protein-protein interactions, 74 post-translational modifications, 7 protein translocation events, and 22 activation/inhibition events. Besides, 124 gene regulation events and 25 activator/inhibitors involved in CAMKK2 activation were also cataloged. The CAMKK2 signaling pathway map data is made freely accessible through WikiPathway database ( https://www.wikipathways.org/index.php/Pathway:WP4874 ). We expect that data on a signaling map of CAMKK2 will provide the scientific community with an improved platform to facilitate further molecular as well as biomedical investigations on CAMKK2 and its utility in the development of biomarkers and therapeutic targets.

Oxytocin Receptor Signaling in Vascular Function and Stroke

The oxytocin receptor (OXTR) is a G protein-coupled receptor with a diverse repertoire of intracellular signaling pathways, which are activated in response to binding oxytocin (OXT) and a similar nonapeptide, vasopressin. This review summarizes the cell and molecular biology of the OXTR and its downstream signaling cascades, particularly focusing on the vasoactive functions of OXTR signaling in humans and animal models, as well as the clinical applications of OXTR targeting cerebrovascular accidents.

The New Frontier in Oxytocin Physiology: The Oxytonic Contraction

Oxytocin (Oxt) is a nine amino acid peptide important in energy regulation and is essential to stress-related disorders. Specifically, low Oxt levels are associated with obesity in human subjects and diet-induced or genetically modified animal models. The striking evidence that Oxt is linked to energy regulation is that Oxt- and oxytocin receptor (Oxtr)-deficient mice show a phenotype characterized by late onset obesity. Oxt-/- or Oxtr-/- develop weight gain without increasing food intake, suggesting that a lack of Oxt reduce metabolic rate. Oxt is differentially expressed in skeletal muscle exerting a protective effect toward the slow-twitch muscle after cold stress challenge in mice. We hypothesized that Oxt potentiates the slow-twitch muscle as it does with the uterus, triggering "the oxytonic contractions". Physiologically, this is important to augment muscle strength in fight/flight response and is consistent with the augmented energetic need at time of labor and for the protection of the offspring when Oxt secretion spikes. The normophagic obesity of Oxt-/- or Oxtr-/- mice could have been caused by decreased skeletal muscle tonicity which drove the metabolic phenotype. In this review, we summarized our findings together with the recent literature on this fascinating subjects in a "new oxytonic perspective" over the physicology of Oxt.

Oxytocin may have a therapeutical potential against cardiovascular disease. Possible pharmaceutical and behavioral approaches

Based on the ancient role of oxytocin and its homologues as amplifiers of reproduction we argue for an evolutionary coupling of oxytocin to signaling pathway which support restorative mechanisms of cells and tissue. In particular, the survival and function of different categories of stem cells and primordial cells are enhanced by mitogen-activated protein kinase (MAPK) pathways. Furthermore, oxytocin stimulates the AMP-activated protein kinase pathway (AMPK) in numerous of cell types which promotes the maintenance of different cell structures. This involves autophagic processes and, in particular, may support the renewal of mitochondria. Mitochondrial fitness may protect against oxidative and inflammatory stress - a well-documented effect of oxytocin. The combined specific trophic and protective effects oxytocin may delay several degenerative phenomena including sarcopenia, type-2 diabetes and atherosclerosis. These effects may be exerted both on a central level supporting the function and integrity of the hypothalamus and peripherally acting directly on blood vessels, pancreas, heart, skeletal muscles and adipose tissue etc. Furthermore, in the capacity of being both a hormone and neuromodulator, oxytocin interacts with numerous of regulatory mechanisms particularly the autonomic nervous system and HPA-axis which may reduce blood pressure and affect the immune function. The potential of the oxytocin system as a behavioral and molecular target for the prevention and treatment of cardiovascular disease is discussed. Focus is put on the affiliative and sexual significance and the different options and limitations associated with a pharmaceutical approach. MeSH: Aging, Atherosclerosis, Heart, Hypothalamus, Inflammation, Love, Orgasm, Oxytocin.

Oxytocin and Vasopressin Systems in Obesity and Metabolic Health: Mechanisms and Perspectives

PURPOSE OF REVIEW

The neurohypophysial endocrine system is identified here as a potential target for therapeutic interventions toward improving obesity-related metabolic dysfunction, given its coinciding pleiotropic effects on psychological, neurological and metabolic systems that are disrupted in obesity.

RECENT FINDINGS

Copeptin, the C-terminal portion of the precursor of arginine-vasopressin, is positively associated with body mass index and risk of type 2 diabetes. Plasma oxytocin is decreased in obesity and several other conditions of abnormal glucose homeostasis. Recent data also show non-classical tissues, such as myocytes, hepatocytes and β-cells, exhibit responses to oxytocin and vasopressin receptor binding that may contribute to alterations in metabolic function. The modulation of anorexigenic and orexigenic pathways appears to be the dominant mechanism underlying the effects of oxytocin and vasopressin on body weight regulation; however, there are apparent limitations associated with their use in direct pharmacological applications. A clearer picture of their wider physiological effects is needed before either system can be considered for therapeutic use.

Oxytocin: Potential to mitigate cardiovascular risk

Cardiovascular disease (CVD) remains the leading cause of death worldwide, despite multiple treatment options. In addition to elevated lipid levels, oxidative stress and inflammation are key factors driving atherogenesis and CVD. New strategies are required to mitigate risk and most urgently for statin-intolerant patients. The neuropeptide hormone oxytocin, synthesized in the brain hypothalamus, is worthy of consideration as a CVD ancillary treatment because it moderates factors directly linked to atherosclerotic CVD such as inflammation, weight gain, food intake and insulin resistance. Though initially studied for its contribution to parturition and lactation, oxytocin participates in social attachment and bonding, associative learning, memory and stress responses. Oxytocin has shown promise in animal models of atherosclerosis and in some human studies as well. A number of properties of oxytocin make it a candidate CVD treatment. Oxytocin not only lowers fat mass and cytokine levels, but also improves glucose tolerance, lowers blood pressure and relieves anxiety. Further, it has an important role in communication in the gut-brain axis that makes it a promising treatment for obesity and type 2 diabetes. Oxytocin acts through its receptor which is a class I G-protein-coupled receptor present in cells of the vascular system including the heart and arteries. While oxytocin is not used for heart disease at present, residual CVD risk remains in a substantial portion of patients despite multidrug regimens, leaving open the possibility of using the endogenous nonapeptide as an adjunct therapy. This review discusses the possible role for oxytocin in human CVD prevention and treatment.

Therapeutic Potential of Oxytocin in Atherosclerotic Cardiovascular Disease: Mechanisms and Signaling Pathways

Coronary artery disease (CAD) is a major cardiovascular disease responsible for high morbidity and mortality worldwide. The major pathophysiological basis of CAD is atherosclerosis in association with varieties of immunometabolic disorders that can suppress oxytocin (OT) receptor (OTR) signaling in the cardiovascular system (CVS). By contrast, OT not only maintains cardiovascular integrity but also has the potential to suppress and even reverse atherosclerotic alterations and CAD. These protective effects of OT are associated with its protection of the heart and blood vessels from immunometabolic injuries and the resultant inflammation and apoptosis through both peripheral and central approaches. As a result, OT can decelerate the progression of atherosclerosis and facilitate the recovery of CVS from these injuries. At the cellular level, the protective effect of OT on CVS involves a broad array of OTR signaling events. These signals mainly belong to the reperfusion injury salvage kinase pathway that is composed of phosphatidylinositol 3-kinase-Akt-endothelial nitric oxide synthase cascades and extracellular signal-regulated protein kinase 1/2. Additionally, AMP-activated protein kinase, Ca²⁺/calmodulin-dependent protein kinase signaling and many others are also implicated in OTR signaling in the CVS protection. These signaling events interact coordinately at many levels to suppress the production of inflammatory cytokines and the activation of apoptotic pathways. A particular target of these signaling events is endoplasmic reticulum (ER) stress and mitochondrial oxidative stress that interact through mitochondria-associated ER membrane. In contrast to these protective effects and machineries, rare but serious cardiovascular disturbances were also reported in labor induction and animal studies including hypotension, reflexive tachycardia, coronary spasm or thrombosis and allergy. Here, we review our current understanding of the protective effect of OT against varieties of atherosclerotic etiologies as well as the approaches and underlying mechanisms of these effects. Moreover, potential cardiovascular disturbances following OT application are also discussed to avoid unwanted effects in clinical trials of OT usages.

Oxytocin in metabolic homeostasis: implications for obesity and diabetes management

Oxytocin was once understood solely as a neuropeptide with a central role in social bonding, reproduction, parturition, lactation and appetite regulation. Recent evidence indicates that oxytocin enhances glucose uptake and lipid utilization in adipose tissue and skeletal muscle, suggesting that dysfunction of the oxytocin system could underlie the pathogenesis of insulin resistance and dyslipidaemia. Murine studies revealed that deficiencies in oxytocin signalling and oxytocin receptor expression lead to obesity despite normal food intake, motor activity and increased leptin levels. In addition, plasma oxytocin concentration is notably lower in obese individuals with diabetes, which may suggest an involvement of the oxytocin system in the pathogenesis of cardiometabolic disease. More recently, small scale studies demonstrated that intranasal administration of oxytocin was associated with significant weight loss as well as improvements in insulin sensitivity and pancreatic β-cell responsivity in human subjects. The multi-pronged effects of oxytocin signalling on improving peripheral insulin sensitivity, pancreatic function and lipid homeostasis strongly suggest a role for this system as a therapeutic target in obesity and diabetes management. The complexity of obesity aetiology and the pathogenesis of obesity-related metabolic complications underscore the need for a systems approach to better understand the role of oxytocin in metabolic function.

AMP-activated protein kinase activation in mediating phenylalanine-induced neurotoxicity in experimental models of phenylketonuria

Phenylketonuria (PKU), one of the most prevalent autosomal recessive disorders of amino acid metabolism, is characterized by abnormal accumulation of phenylalanine, which can lead to intellectual disability. The main pathologic changes in the central nervous system of untreated phenylketonuric patients are reductions in the number of axons, dendrites, and synapses in the brain. Such alterations are thought to be mainly associated with the toxic effects caused by phenylalanine. However, the underlying molecular mechanisms have not been fully elucidated. The present study shows that a high concentration of phenylalanine remarkably inhibited neuronal neurite formation in vitro. Interestingly, AMP-activated protein kinase (AMPK), the energy status sensor, was activated in cultured cerebral cortical neurons upon phenylalanine treatment. Pretreatment with an AMPK inhibitor ameliorated the reduction of neurite formation caused by phenylalanine. In addition, the levels of the phosphorylated AMPK, the active form of AMPK, were significantly higher in the cerebral cortices of PKU mice with elevated phenylalanine levels in this brain region compared to those in wild-type control mice, whereas the density of dendritic spines on basal secondary dendrites of pyramidal neurons in prefrontal cortices of PKU mice was significantly decreased. Collectively, these findings indicate that AMPK activation is a key event in impaired neuronal dendritic development in PKU and consequently, a potential therapeutic target for developing neuroprotective strategies against phenylalanine-evoked brain injury in PKU.

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

Revisiting amino acids and peptides as anti-glycation agents

The importance of controlling or preventing protein glycation cannot be overstated and is of prime importance in the treatment of diabetes and associated complications including Alzheimer's disease, cataracts, atherosclerosis, kidney aliments among others. In this respect, simple molecules such as amino acids and peptides hold much promise both in terms of ease and scale-up of synthesis as well as in relation to negligible/low associated toxicity. In view of this, a comprehensive account of literature reports is presented, that documents the anti-glycation activity of natural and non-natural amino acids and peptides. This review also discusses the chemical reactions involved in glycation and the formation of advanced glycation end-products (AGEs) and possible/probable intervention sites and mechanism of action of the reported amino acids/peptides. This aspect of amino acids/peptides adds to their growing importance in medicinal and therapeutic applications.

The physiological and glycaemic changes in breastfeeding women with type 1 diabetes mellitus

The World Health Organisation recommends exclusive breastfeeding for the first six months of life (Australian institute of health and welfare, 2011). Breastfeeding confers many short- and long-term benefits for infants and mothers, including reduced childhood obesity and lower maternal body weight (Infant feeding survey, 2010; CDC National immunization surveys, 2012 and 2013; Sorkio et al., 2010; Hummel et al., 2014; Finkelstein et al., 2013). Exclusive breastfeeding is also recommended in women with type 1 diabetes mellitus (T1DM), for at least four months (Nucci et al., 2017). However, the impact of breastfeeding on mothers with T1DM, and, conversely, the impact of maternal T1DM on breastfeeding, is not clear. This review summarizes current knowledge regarding the epidemiology and physiology of breastfeeding in women with T1DM. In particular, it highlights the relationship between breastfeeding and glycaemia. Potential areas for future research are also identified.

Rosmarinic Acid, a Rosemary Extract Polyphenol, Increases Skeletal Muscle Cell Glucose Uptake and Activates AMPK

Skeletal muscle is a major insulin-target tissue and plays an important role in glucose homeostasis. Impaired insulin action in muscles leads to insulin resistance and type 2 diabetes mellitus. 5' AMP-activated kinase (AMPK) is an energy sensor, its activation increases glucose uptake in skeletal muscle and AMPK activators have been viewed as a targeted approach in combating insulin resistance. We previously reported AMPK activation and increased muscle glucose uptake by rosemary extract (RE). In the present study, we examined the effects and the mechanism of action of rosmarinic acid (RA), a major RE constituent, in L6 rat muscle cells. RA (5.0 µM) increased glucose uptake (186 ± 4.17% of control, p < 0.001) to levels comparable to maximum insulin (204 ± 10.73% of control, p < 0.001) and metformin (202 ± 14.37% of control, p < 0.001). Akt phosphorylation was not affected by RA, while AMPK phosphorylation was increased. The RA-stimulated glucose uptake was inhibited by the AMPK inhibitor compound C and was not affected by wortmannin, an inhibitor of phosphoinositide 3-kinase (PI3K). The current study shows an effect of RA to increase muscle glucose uptake and AMPK phosphorylation. RA deserves further study as it shows potential to be used as an agent to regulate glucose homeostasis.

17β-estradiol upregulates oxytocin and the oxytocin receptor in C2C12 myotubes

Background

The endocrinology of skeletal muscle is highly complex and many issues about hormone action in skeletal muscle are still unresolved. Aim of the work is to improve our knowledge on the relationship between skeletal muscle and 17β-estradiol.

Methods

The skeletal muscle cell line C2C12 was treated with 17β-estradiol, the oxytocin peptide and a combination of the two hormones. The mRNA levels of myogenic regulatory factors, myosin heavy chain, oxytocin, oxytocin receptor and adipogenic factors were analysed in C2C12 myotubes.

Results

It was demonstrated that C2C12 myoblasts and myotubes express oxytocin and its receptor, in particular the receptor levels physiologically increase in differentiated myotubes. Myotubes treated with 17β-estradiol overexpressed oxytocin and oxytocin receptor genes by approximately 3- and 29-fold, respectively. A decrease in the expression of fatty acid binding protein 4 (0.62-fold), a fat metabolism-associated gene, was observed in oxytocin-treated myotubes. On the contrary, fatty acid binding protein 4 was upregulated (2.66-fold) after the administration of the combination of 17β-estradiol and oxytocin. 17β-estradiol regulates oxytocin and its receptor in skeletal muscle cells and they act in a synergic way on fatty acid metabolism.

Discussion

Oxytocin and its receptor are physiologically regulated along differentiation. 17β-estradiol regulates oxytocin and its receptor in skeletal muscle cells. 17β-estradiol and oxytocin act in a synergic way on fatty acid metabolism. A better understanding of the regulation of skeletal muscle homeostasis by estrogens and oxytocin peptide could contribute to increase our knowledge of muscle and its metabolism.

Oxytocin Improves β-Cell Responsivity and Glucose Tolerance in Healthy Men

In addition to its pivotal role in psychosocial behavior, the hypothalamic neuropeptide oxytocin contributes to metabolic control by suppressing eating behavior. Its involvement in glucose homeostasis is less clear, although pilot experiments suggest that oxytocin improves glucose homeostasis. We assessed the effect of intranasal oxytocin (24 IU) administered to 29 healthy, fasted male subjects on glucose homeostasis measured by means of an oral glucose tolerance test. Parameters of glucose metabolism were analyzed according to the oral minimal model. Oxytocin attenuated the peak excursion of plasma glucose and augmented the early increases in insulin and C-peptide concentrations in response to the glucose challenge, while slightly blunting insulin and C-peptide peaks. Oral minimal model analyses revealed that oxytocin compared with placebo induced a pronounced increase in β-cell responsivity (PHI_total) that was largely due to an enhanced dynamic response (PHI_d), and a more than twofold improvement in glucose tolerance (disposition index). Adrenocorticotropic hormone (ACTH), cortisol, glucagon, and nonesterified fatty acid (NEFA) concentrations were not or were only marginally affected. These results indicate that oxytocin plays a significant role in the acute regulation of glucose metabolism in healthy humans and render the oxytocin system a potential target of antidiabetic treatment.

Glycaemic behaviour during breastfeeding in women with Type 1 diabetes

AIM

To describe glycaemia in both breastfeeding women and artificially feeding women with Type 1 diabetes, and the changes in glycaemia induced by suckling.

METHODS

A blinded continuous glucose monitor was applied for up to 6 days in eight breastfeeding and eight artificially feeding women with Type 1 diabetes 2-4 months postpartum. Women recorded glucose levels, insulin dosages, oral intake and breastfeeding episodes. A standardized breakfast was consumed on 2 days. A third group (clinic controls) were identified from a historical database.

RESULTS

Carbohydrate intake tended to be higher in breastfeeding than artificially feeding women (P = 0.09) despite similar insulin requirements. Compared with breastfeeding women, the high blood glucose index and standard deviation of glucose were higher in artificially feeding women (P = 0.02 and 0.06, respectively) and in the clinical control group (P = 0.02 and 0.05, respectively). The low blood glucose index and hypoglycaemia were similar. After suckling, the low blood glucose index increased compared with before (P < 0.01) and during (P < 0.01) suckling. Hypoglycaemia (blood glucose < 4.0 mmol/l) occurred within 3 h of suckling in 14% of suckling episodes, and was associated with time from last oral intake (P = 0.04) and last rapid-acting insulin (P = 0.03). After a standardized breakfast, the area under the glucose curve was positive. In breastfeeding women the area under the glucose curve was positive if suckling was avoided for 1 h after eating and negative if suckling occurred within 30 min of eating.

CONCLUSIONS

Breastfeeding women with Type 1 diabetes had similar hypoglycaemia but lower glucose variability than artificially feeding women. Suckling reduced maternal glucose levels but did not cause hypoglycaemia in most episodes.

Oxytocin Regulates Stress-Induced Crf Gene Transcription through CREB-Regulated Transcription Coactivator 3

The major regulator of the neuroendocrine stress response in the brain is corticotropin releasing factor (CRF), whose transcription is controlled by CREB and its cofactors CRTC2/3 (TORC2/3). Phosphorylated CRTCs are sequestered in the cytoplasm, but rapidly dephosphorylated and translocated into the nucleus following a stressful stimulus. As the stress response is attenuated by oxytocin (OT), we tested whether OT interferes with CRTC translocation and, thereby, Crf expression. OT (1 nmol, i.c.v.) delayed the stress-induced increase of nuclear CRTC3 and Crf hnRNA levels in the paraventricular nucleus of male rats and mice, but did not affect either parameter in the absence of the stressor. The increase in Crf hnRNA levels at later time points was parallel to elevated nuclear CRTC2/3 levels. A direct effect of Thr(4) Gly(7)-OT (TGOT) on CRTC3 translocation and Crf expression was found in rat primary hypothalamic neurons, amygdaloid (Ar-5), hypothalamic (H32), and human neuroblastoma (Be(2)M17) cell lines. CRTC3, but not CRCT2, knockdown using siRNA in Be(2)M17 cells prevented the effect of TGOT on Crf hnRNA levels. Chromatin-immunoprecipitation demonstrated that TGOT reduced CRTC3, but not CRTC2, binding to the Crf promoter after 10 min of forskolin stimulation. Together, the results indicate that OT modulates CRTC3 translocation, the binding of CRTC3 to the Crf promoter and, ultimately, transcription of the Crf gene.

SIGNIFICANCE STATEMENT

The neuropeptide oxytocin has been proposed to reduce hypothalamic-pituitary-adrenal (HPA) axis activation during stress. The underlying mechanisms are, however, elusive. In this study we show that activation of the oxytocin receptor in the paraventricular nucleus delays transcription of the gene encoding corticotropin releasing factor (Crf), the main regulator of the stress response. It does so by sequestering the coactivator of the transcription factor CREB, CRTC3, in the cytosol, resulting in reduced binding of CRTC3 to the Crf gene promoter and subsequent Crf gene expression. This novel oxytocin receptor-mediated intracellular mechanism might provide a basis for the treatment of exaggerated stress responses in the future.

Two Birds with One Stone: Possible Dual-Role of Oxytocin in the Treatment of Diabetes and Osteoporosis

Oxytocin (OT), a hormone most commonly associated with labor and lactation, may have a wide variety of physiological and pathological functions, which makes OT and its receptor potential targets for drug therapy. In this review, we highlight the newly discovered metabolic role of OT in diabetes and its complication, such as diabetic osteopathy. OT may have positive metabolic effects; this is based on the change in glucose metabolism, lipid profile, and insulin sensitivity. It may modify glucose uptake and insulin sensitivity both through direct and indirect effects. It may also cause regenerative changes in diabetic pancreatic islet cells. Moreover, it has an anabolic effect on the bone biology. So, the activation of the OT receptor pathway by infusion of OT, OT analogs, or OT agonists may represent a promising approach for the treatment of diabetes and some of its complications, including diabetic osteopathy.

Neurohypophyseal Hormones: Novel Actors of Striated Muscle Development and Homeostasis

Since the 1980’s, novel functional roles of the neurohypophyseal hormones vasopressin and oxytocin have emerged. Several studies have investigated the effects of these two neurohormones on striated muscle tissues, both in vitro and in vivo. The effects of vasopressin on skeletal myogenic cells, developing muscle and muscle homeostasis have been documented. Oxytocin appears to have a greater influence on cardiomyocite differentiation and heart homeostasis. This review summarizes the studies on these novel roles of the two neurohypophyseal hormones, and open the possibility of new therapeutic approaches for diseases affecting striated muscle.

Protective effect of oxytocin on ovarian ischemia-reperfusion injury in rats

Oxytocin (OT), a neurohypophysial nonapeptide, plays dual role as a neurotransmitter/neuromodulator and a hormone. It has also well known protective properties against ischemia/reperfusion organ damage. This study investigated the effect of OT on experimentally induced ovarian torsion/de-torsion ischemia/reperfusion (I/R) injury in rats. Sprague-Dawley rats were assigned to five treatment groups (n=7/group): Group 1, sham-operated; Group 2, torsion; Group 3, 80 IU/kg of OT administration 30 min prior to torsion; Group 4, torsion/de-torsion; and Group 5, torsion followed by 80 IU/kg of OT administration 30 min prior to de-torsion. OT administration significantly decreased the tissue malondialdehyde (MDA) levels in both the torsion and OT group (Group 3), and torsion/de-torsion OT group (Group 5) in comparison with the torsion-only group (Group 2) and torsion/de-torsion group (Group 4). Histopathological finding scores including follicular degeneration, edema, hemorrhage, vascular congestion, and infiltration by inflammatory cells were found to be significantly decreased in the torsion and OT group (Group 3), and torsion/de-torsion OT group (Group 5) when compared with the torsion-only group (Group 2) and torsion/de-torsion group (Group 4). In conclusion, these results, verified with histopathologic evaluation and biochemical assays, suggest a probable protective role for OT in ischemia and I/R injury in rat ovaries.

Hypooxytocinaemia in obese Zucker rats relates to oxytocin degradation in liver and adipose tissue

The metabolic action of oxytocin has recently been intensively studied to assess the ability of the peptide to regulate energy homeostasis. Despite the obvious weight-reducing effect of oxytocin observed in experimental studies, plasma oxytocin levels were found to be unchanged or even elevated in human obesity. The aim of our study was to evaluate the changes in the oxytocin system in Zucker rats, an animal model closely mirroring morbid obesity in humans. Plasma oxytocin levels were measured in obese Zucker rats and lean controls by enzyme immunoassay after plasma extraction. The expression of oxytocin and oxytocin receptor (OXTR) was assessed at the mRNA and protein levels by quantitative real-time PCR and immunoblotting respectively. Plasma and tissue activity of oxytocinase, the main enzyme involved in oxytocin degradation, were measured by fluorometric assay using an arylamide derivate as the substrate. Obese Zucker rats displayed a marked reduction in plasma oxytocin levels. Elevated liver and adipose tissue oxytocinase activity was noticed in obese Zucker rats. Hypothalamic oxytocin gene expression was not altered by the obese phenotype. OXTR mRNA and protein levels were upregulated in the adipose tissue of obese animals in contrast to the reduced OXTR protein levels in skeletal muscle. Our results show that obesity is associated with reduced plasma oxytocin due to increased peptide degradation by liver and adipose tissue rather than changes in hormone synthesis. This study highlights the importance of the oxytocin system in the pathogenesis of obesity and suggests oxytocinase inhibition as a candidate approach in the therapy of obesity.

Is oxytocin a therapeutic factor for ischemic heart disease?

Ischemic heart disease (IHD) is among the most important and top ranked causes of death in the world, and its preventive and interventional mechanisms are actively being investigated. Preconditioning may still be beneficial in some situations such as IHD. Development of cardioprotective agents to improve myocardial function, to decrease the incidence of arrhythmias, to delay the onset of necrosis, and to limit the total extent of infarction during IHD is of great clinical importance. In order to reduce morbidity, a new treatment modality must be developed, and oxytocin may indeed be one of the candidates. There is increasing experimental evidence indicating that oxytocin may have cardioprotective effects either by decreasing the extent of reperfusion injury or by pharmacologic preconditioning activity. This review shows that in the presence of oxytocin, the cardioprotective effects may be increased to some extent. The presented board of evidence focuses on the valuable effects of oxytocin on myocardial function and candidates it for future clinical studies in the realm of ischemic heart diseases.

Oxytocin revisited: its role in cardiovascular regulation

Traditionally associated with female reproduction, oxytocin (OT) was revisited recently and was revealed to have several new roles in the cardiovascular system. Functional OT receptors have been discovered in the rat and human heart, as well as in vascular beds. The cardiovascular activities of OT include: (i) lowering blood pressure; (ii) negative cardiac inotropy and chronotropy; (iii) parasympathetic neuromodulation; (iv) vasodilatation; (v) anti-inflammatory; (vi) antioxidative; and (vii) metabolic effects. These outcomes are mediated, at least in part, by stimulating cardioprotective mediators, such as nitric oxide and atrial natriuretic peptide. OT and its extended form OT-Gly-Lys-Arg have been shown to be abundant in the foetal mouse heart. OT has the capacity to generate cardiomyocytes from various types of stem cells, including the cardiac side population. Mesenchymal cells transfected with OT-Gly-Lys-Arg, or preconditioned with OT, are resistant to apoptosis and express endothelial cell markers. OT increases glucose uptake in cultured cardiomyocytes from newborn and adult rats, in normal, hypoxic and even insulin resistance conditions. In rats with experimentally-induced myocardial infarction, continuous in vivo OT delivery improves the cardiac healing process, as well as cardiac work, reduces inflammation and stimulates angiogenesis. Therefore, in pathological conditions, OT exerts anti-inflammatory and cardioprotective properties, and improves vascular and metabolic functions. Thus, OT has potential for therapeutic use.

Downregulation in GATA4 and Downstream Structural and Contractile Genes in the db/db Mouse Heart

Reduced expression of GATA4, a transcriptional factor for structural and cardioprotective genes, has been proposed as a factor contributing to the development of cardiomyopathy. We investigated whether the reduction of cardiac GATA4 expression reported in diabetes alters the expression of downstream genes, namely, atrial natriuretic peptide (ANP), B-type natriuretic, peptide (BNP), and α- and β-myosin heavy chain (MHC). db/db mice, a model of type 2 diabetes, with lean littermates serving as controls, were studied. db/db mice exhibited obesity, hyperglycemia, and reduced protein expression of cardiac GLUT4 and IRAP (insulin-regulated aminopeptidase), the structural protein cosecreted with GLUT4. Hearts from db/db mice had reduced protein expression of GATA4 (~35%) with accompanying reductions in mRNA expression of ANP (~40%), BNP (~85%), and α-MHC mRNA (~50%) whereas expression of β-MHC mRNA was increased by ~60%. Low GATA4 was not explained by an increased ligase or atrogin1 expression. CHIP protein content was modestly downregulated (27%) in db/db mice whereas mRNA and protein expression of the CHIP cochaperone HSP70 was significantly decreased in db/db hearts. Our results indicate that low GATA4 in db/db mouse heart is accompanied by reduced expression of GATA4-regulated cardioprotective and structural genes, which may explain the development of cardiomyopathy in diabetes.

Subchronic treatment of rats with oxytocin results in improved adipocyte differentiation and increased gene expression of factors involved in adipogenesis

BACKGROUND AND PURPOSE

Treatment with thiazolidinediones, insulin-sensitizing drugs, enhances adipogenesis, which may result in unwanted increase in adiposity. Based on the suggested metabolic effects of oxytocin, the aims of the present study were to: (i) determine whether chronic treatment with oxytocin exerts positive effects on white adipose tissue growth without increasing adiposity; (ii) investigate possible mechanisms of action of oxytocin by measuring the level of gene expression of adipogenic factors; and (iii) test the hypothesis that oxytocin's effect on adipose tissue involves specific activation of eukaryotic elongation factor 2 (eEF2).

EXPERIMENTAL APPROACH

Adult rats were subcutaneously treated with oxytocin (3.6 µg·100 g⁻¹ body weight day⁻¹) via osmotic minipumps for 2 weeks. Adipocytes from epididymal adipose tissue were isolated and their size evaluated by light microscopy. Gene expression of adipogenic and angiogenic factors was determined by real-time PCR and dephosphorylation of eEF2 by immunoblotting.

KEY RESULTS

Oxytocin treatment decreased the diameter of adipocytes and increased the epididymal adipose tissue protein content without changing the adipose tissue mass. Increases in fatty acid binding protein, peroxisome proliferator-activated receptor γ, insulin-sensitive glucose transporter 4, leptin and CD31 mRNA levels were noted in the epididymal and/or retroperitoneal fat tissue of oxytocin-treated rats. Oxytocin enhanced the dephosphorylation of eEF2 in the epididymal adipose tissue.

CONCLUSIONS AND IMPLICATIONS

The present results demonstrate that subchronic treatment with oxytocin induces adipogenic and angiogenic effects and that the eEF2 signalling pathway is involved in these effects of oxytocin on adipose tissue in vivo. These findings are likely to motivate further research and indicate new approaches for modulating adipose tissue morphology and metabolism.

Cardiac oxytocin receptor blockade stimulates adverse cardiac remodeling in ovariectomized spontaneously hypertensive rats

An increasing amount of evidence demonstrates the beneficial role of oxytocin (OT) in the cardiovascular system. Similar actions are attributed to genistein, an isoflavonic phytoestrogen. The treatment with genistein activates the OT system in the aorta of ovariectomized (OVX) Sprague-Dawley (SD) rats. The objective of this study was to determine the effects of low doses of genistein on the OT-induced effects in rat hypertension. The hypothesis tested was that treatment of OVX spontaneously hypertensive rats (SHRs) with genistein improves heart structure and heart work through a mechanism involving the specific OT receptor (OTR). OVX SHRs or SD rats were treated with genistein (in microg/g body wt sc, 10 days) in the presence or absence of an OT antagonist (OTA) [d(CH(2))(5), Tyr(Me)(2), Orn(8)]-vasotocin or a nonspecific estrogen receptor antagonist (ICI-182780). Vehicle-treated OVX rats served as controls. RT-PCR and Western blot analysis demonstrated that left ventricular (LV) OTR, downregulated by ovariectomy, increased in response to genistein. In SHRs or SD rats, this effect was blocked by OTA or ICI-182780 administration. The OTR was mainly localized in microvessels expressing the CD31 marker and colocalized with endothelial nitric oxide synthase. In SHRs, the genistein-stimulated OTR increases were associated with improved fractional shortening, decreased blood pressure (12 mmHg), decreased heart weight-to-body weight ratio, decreased fibrosis, and lowered brain natriuretic peptide in the LV. The prominent finding of the study is the detrimental effect of OTA treatment on the LV of SHRs. OTA treatment of OVX SHRs resulted in a dramatic worsening of ejection fractions and an augmented fibrosis. In conclusion, these results demonstrate that cardiac OTRs are involved in the regulation of cardiac function of OVX SHRs. The decreases of OTRs may contribute to cardiac pathology following menopause.

Unacylated ghrelin rapidly modulates lipogenic and insulin signaling pathway gene expression in metabolically active tissues of GHSR deleted mice

Background

There is increasing evidence that unacylated ghrelin (UAG) improves insulin sensitivity and glucose homeostasis; however, the mechanism for this activity is not fully understood since a UAG receptor has not been discovered.

Methodology/Principal Findings

To assess potential mechanisms of UAG action in vivo, we examined rapid effects of UAG on genome-wide expression patterns in fat, muscle and liver of growth hormone secretagogue receptor (GHSR)-ablated mice using microarrays. Expression data were analyzed using Ingenuity Pathways Analysis and Gene Set Enrichment Analysis. Regulation of subsets of these genes was verified by quantitative PCR in an independent experiment. UAG acutely regulated clusters of genes involved in glucose and lipid metabolism in all three tissues, consistent with enhancement of insulin sensitivity.

Conclusions/Significance

Fat, muscle and liver are central to the control of lipid and glucose homeostasis. UAG rapidly modulates the expression of metabolically important genes in these tissues in GHSR-deleted mice indicating a direct, GHSR-independent, action of UAG to improve insulin sensitivity and metabolic profile.

Targeting intermediary metabolism in the hypothalamus as a mechanism to regulate appetite

The central nervous system mediates energy balance (energy intake and energy expenditure) in the body; the hypothalamus has a key role in this process. Recent evidence has demonstrated an important role for hypothalamic malonyl CoA in mediating energy balance. Malonyl CoA is generated by the carboxylation of acetyl CoA by acetyl CoA carboxylase and is then either incorporated into long-chain fatty acids by fatty acid synthase, or converted back to acetyl-CoA by malonyl CoA decarboxylase. Increased hypothalamic malonyl CoA is an indicator of energy surplus, resulting in a decrease in food intake and an increase in energy expenditure. In contrast, a decrease in hypothalamic malonyl CoA signals an energy deficit, resulting in an increased appetite and a decrease in body energy expenditure. A number of hormonal and neural orexigenic and anorexigenic signaling pathways have now been shown to be associated with changes in malonyl CoA levels in the arcuate nucleus (ARC) of the hypothalamus. Despite compelling evidence that malonyl CoA is an important mediator in the hypothalamic ARC control of food intake and regulation of energy balance, the mechanism(s) by which this occurs has not been established. Malonyl CoA inhibits carnitine palmitoyltransferase-1 (CPT-1), and it has been proposed that the substrate of CPT-1, long-chain acyl CoA(s), may act as a mediator(s) of appetite and energy balance. However, recent evidence has challenged the role of long-chain acyl CoA(s) in this process, as well as the involvement of CPT-1 in hypothalamic malonyl CoA signaling. A better understanding of how malonyl CoA regulates energy balance should provide novel approaches to targeting intermediary metabolism in the hypothalamus as a mechanism to control appetite and body weight. Here, we review the data supporting an important role for malonyl CoA in mediating hypothalamic control of energy balance, and recent evidence suggesting that targeting malonyl CoA synthesis or degradation may be a novel approach to favorably modify appetite and weight gain.

Oxytocin increases glucose uptake in neonatal rat cardiomyocytes

We have recently shown that an entire oxytocin (OT) system, a peptide and its cognate receptors, is synthesized in the heart. In fetal and newborn hearts, OT exists in its extended three-amino acid form, OT-Gly-Lys-Arg (OT-GKR). OT translocates glucose transporter type 4 to the plasma membrane in human endothelial cells. Therefore, we hypothesized that the cardiac OT/OT-GKR system may be involved in the regulation of myocardial glucose uptake in physiological conditions and during metabolic stress such as hypoxia. Primary cultures of neonatal rat cardiomyocytes (CM) and cardiac progenitor cells expressing ATP-binding cassette efflux transporter G2 transporter (stem cell marker) were studied. OT (10 nm) increased basal glucose uptake in CM to 4.0 +/- 0.2 fmol/mg protein, with OT-GKR (10 nm) elevating it to 5.3 +/- 0.4 fmol/mg protein (P < 0.001) in comparison with 2.2 fmol/mg in control cells. OT had a moderate synergistic effect with 0.1 mm 2,4-dinitrophenol, augmenting basal glucose uptake to 5.5 +/- 0.5 fmol/mg. OT-GKR (10 nm) was even more potent in combination with 2,4-dinitrophenol, increasing glucose uptake to 9.0 +/- 1.0 fmol/mg. Wortmannin (0.1 microm), an inhibitor of phosphatidylinositol-3-kinase, significantly suppressed the effect of OT and insulin (10 nm) (P < 0.001), indicating common pathways. Our data suggest that OT and OT-GKR influence glucose uptake in neonatal rat CM and may thus play a role in the maintenance of cardiac function and cell survival during metabolic stress.