Protein kinase A regulatory subunits in human adipose tissue: decreased R2B expression and activity in adipocytes from obese subjects

Naringenin and β-carotene convert human white adipocytes to a beige phenotype and elevate hormone- stimulated lipolysis

Introduction

Naringenin, a peroxisome proliferator-activated receptor (PPAR) activator found in citrus fruits, upregulates markers of thermogenesis and insulin sensitivity in human adipose tissue. Our pharmacokinetics clinical trial demonstrated that naringenin is safe and bioavailable, and our case report showed that naringenin causes weight loss and improves insulin sensitivity. PPARs form heterodimers with retinoic-X-receptors (RXRs) at promoter elements of target genes. Retinoic acid is an RXR ligand metabolized from dietary carotenoids. The carotenoid β-carotene reduces adiposity and insulin resistance in clinical trials. Our goal was to examine if carotenoids strengthen the beneficial effects of naringenin on human adipocyte metabolism.

Methods

Human preadipocytes from donors with obesity were differentiated in culture and treated with 8µM naringenin + 2µM β-carotene (NRBC) for seven days. Candidate genes involved in thermogenesis and glucose metabolism were measured as well as hormone-stimulated lipolysis.

Results

We found that β-carotene acts synergistically with naringenin to boost UCP1 and glucose metabolism genes including GLUT4 and adiponectin, compared to naringenin alone. Protein levels of PPARα, PPARγ and PPARγ-coactivator-1α, key modulators of thermogenesis and insulin sensitivity, were also upregulated after treatment with NRBC. Transcriptome sequencing was conducted and the bioinformatics analyses of the data revealed that NRBC induced enzymes for several non-UCP1 pathways for energy expenditure including triglyceride cycling, creatine kinases, and Peptidase M20 Domain Containing 1 (PM20D1). A comprehensive analysis of changes in receptor expression showed that NRBC upregulated eight receptors that have been linked to lipolysis or thermogenesis including the β1-adrenergic receptor and the parathyroid hormone receptor. NRBC increased levels of triglyceride lipases and agonist-stimulated lipolysis in adipocytes. We observed that expression of RXRγ, an isoform of unknown function, was induced ten-fold after treatment with NRBC. We show that RXRγ is a coactivator bound to the immunoprecipitated PPARγ protein complex from white and beige human adipocytes.

Discussion

There is a need for obesity treatments that can be administered long-term without side effects. NRBC increases the abundance and lipolytic response of multiple receptors for hormones released after exercise and cold exposure. Lipolysis provides the fuel for thermogenesis, and these observations suggest that NRBC has therapeutic potential.

GPCR in Adipose Tissue Function-Focus on Lipolysis

Adipose tissue can be divided anatomically, histologically, and functionally into two major entities white and brown adipose tissues (WAT and BAT, respectively). WAT is the primary energy depot, storing most of the bioavailable triacylglycerol molecules of the body, whereas BAT is designed for dissipating energy in the form of heat, a process also known as non-shivering thermogenesis as a defense against a cold environment. Importantly, BAT-dependent energy dissipation directly correlates with cardiometabolic health and has been postulated as an intriguing target for anti-obesity therapies. In general, adipose tissue (AT) lipid content is defined by lipid uptake and lipogenesis on one side, and, on the other side, it is defined by the breakdown of lipids and the release of fatty acids by lipolysis. The equilibrium between lipogenesis and lipolysis is important for adipocyte and general metabolic homeostasis. Overloading adipocytes with lipids causes cell stress, leading to the recruitment of immune cells and adipose tissue inflammation, which can affect the whole organism (metaflammation). The most important consequence of energy and lipid overload is obesity and associated pathophysiologies, including insulin resistance, type 2 diabetes, and cardiovascular disease. The fate of lipolysis products (fatty acids and glycerol) largely differs between AT: WAT releases fatty acids into the blood to deliver energy to other tissues (e.g., muscle). Activation of BAT, instead, liberates fatty acids that are used within brown adipocyte mitochondria for thermogenesis. The enzymes involved in lipolysis are tightly regulated by the second messenger cyclic adenosine monophosphate (cAMP), which is activated or inhibited by G protein-coupled receptors (GPCRs) that interact with heterotrimeric G proteins (G proteins). Thus, GPCRs are the upstream regulators of the equilibrium between lipogenesis and lipolysis. Moreover, GPCRs are of special pharmacological interest because about one third of the approved drugs target GPCRs. Here, we will discuss the effects of some of most studied as well as "novel" GPCRs and their ligands. We will review different facets of in vitro, ex vivo, and in vivo studies, obtained with both pharmacological and genetic approaches. Finally, we will report some possible therapeutic strategies to treat obesity employing GPCRs as primary target.

Transcriptomics and Lipid Metabolomics Analysis of Subcutaneous, Visceral, and Abdominal Adipose Tissues of Beef Cattle

Fat deposition traits are influenced by genetics and environment, which affect meat quality, growth rate, and energy metabolism of domestic animals. However, at present, the molecular mechanism of fat deposition is not entirely understood in beef cattle. Therefore, the current study conducted transcriptomics and lipid metabolomics analysis of subcutaneous, visceral, and abdominal adipose tissue (SAT, VAT, and AAT) of Huaxi cattle to investigate the differences among these adipose tissues and systematically explore how candidate genes interact with metabolites to affect fat deposition. These results demonstrated that compared with SAT, the gene expression patterns and metabolite contents of VAT and AAT were more consistent. Particularly, SCD expression, monounsaturated fatty acid (MUFA) and triglyceride (TG) content were higher in SAT, whereas PCK1 expression and the contents of saturated fatty acid (SFA), diacylglycerol (DG), and lysoglycerophosphocholine (LPC) were higher in VAT. Notably, in contrast to PCK1, 10 candidates including SCD, ELOVL6, ACACA, and FABP7 were identified to affect fat deposition through positively regulating MUFA and TG, and negatively regulating SFA, DG, and LPC. These findings uncovered novel gene resources and offered a theoretical basis for future investigation of fat deposition in beef cattle.

The regulation of PKA signaling in obesity and in the maintenance of metabolic health

The cAMP-dependent protein kinase (PKA) system represents a primary cell-signaling pathway throughout systems and across species. PKA facilitates the actions of hormones, neurotransmitters and other signaling molecules that bind G-protein coupled receptors (GPCR) to modulate cAMP levels. Through its control of synaptic events, exocytosis, transcriptional regulation, and more, PKA signaling regulates cellular metabolism and emotional and stress responses making it integral in the maintenance and dysregulation of energy homeostasis. Neural PKA signaling is regulated by afferent and peripheral efferent signals that link specific neural cell populations to the regulation of metabolic processes in adipose tissue, liver, pancreas, adrenal, skeletal muscle, and gut. Mouse models have provided invaluable information on the roles for PKA subunits in brain and key metabolic organs. While limited, human studies infer differential regulation of the PKA system in obese compared to lean individuals. Variants identified in PKA subunit genes cause Cushing syndrome that is characterized by metabolic dysregulation associated with endogenous glucocorticoid excess. Under healthy physiologic conditions, the PKA system is exquisitely regulated by stimuli that activate GPCRs to alter intracellular cAMP concentrations, and by PKA cellular localization and holoenzyme stability. Adenylate cyclase activity generates cAMP while phosphodiesterase-mediated cAMP degradation to AMP decreases cAMP levels downstream of GPCRs. Chronic perturbations in PKA signaling appear to be capable of resetting PKA regulation at several levels; in addition, sex differences in PKA signaling regulation, while not well understood, impact the physiologic consequences of metabolic dysregulation and obesity. This review explores the roles for PKA signaling in the pathogenesis of metabolic diseases including obesity, type 2 diabetes mellitus and associated co-morbidities through neural-peripheral crosstalk and cAMP/PKA signaling pathway targets that hold therapeutic potential.

Activation of the adipocyte CREB/CRTC pathway in obesity

Obesity is a major risk factor for the development of type II diabetes. Increases in adipose tissue mass trigger insulin resistance via the release of pro-inflammatory cytokines from adipocytes and macrophages. CREB and the CRTC coactivators have been found to promote insulin resistance in obesity, although the mechanism is unclear. Here we show that high fat diet feeding activates the CREB/CRTC pathway in adipocytes by decreasing the expression of SIK2, a Ser/Thr kinase that phosphorylates and inhibits CRTCs. SIK2 levels are regulated by the adipogenic factor C/EBPα, whose expression is reduced in obesity. Exposure to PPARγ agonist rescues C/EBPα expression and restores SIK2 levels. CRTC2/3 promote insulin resistance via induction of the chemokines CXCL1/2. Knockout of CRTC2/3 in adipocytes reduces CXCL1/2 expression and improves insulin sensitivity. As administration of CXCL1/2 reverses salutary effects of CRTC2/3 depletion, our results demonstrate the importance of the CREB/CRTC pathway in modulating adipose tissue function.

The PRKAR1B p.R115K Variant is Associated with Lipoprotein Profile in African American Youth with Metabolic Challenges

Context

High childhood obesity rates coincide with increased incidence of nonalcoholic fatty liver disease (NAFLD) and other comorbidities. Understanding the genetics of susceptibility to obesity and its comorbidities could guide intervention. The cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) signaling pathway regulates energy balance, glucose homeostasis, and lipid metabolism.

Objective

We hypothesized that PKA-related gene variants may be associated with obesity or associated metabolic conditions.

Methods

We included 457 youths from the Yale Obesity Clinic into the Pathogenesis of Youth-Onset Diabetes cohort (NCT01967849); a variety of clinical tests were performed to characterize NAFLD. Exon sequencing of 54 PKA pathway genes was performed. Variants were confirmed by Sanger sequencing. Clinical data were analyzed, correcting for NAFLD status and body mass index z-score with adjustments for multiple comparisons. Fluorescence resonance energy transfer (FRET) and PKA enzymatic assays were performed in HEK293 cells transfected with the PRKAR1B p.R115K construct. In silico structural analysis for this variant was done.

Results

We identified the variant PRKAR1B p.R115K in 4 unrelated, African American patients. Analyses compared this variant group to other African American patients in the cohort. PRKAR1B p.R115K was associated with favorable circulating lipoprotein levels. Analysis of FRET and PKA enzymatic assay showed stronger interaction between the R1β mutant and PKA catalytic subunit Cα and decreased basal PKA activity compared with the wildtype (P < .0001). Structural analysis revealed that p.R115K may hinder conformational changes resulting from cAMP binding at cAMP binding domain A.

Conclusion

Data suggest PRKAR1B p.R115K affects cAMP signaling and may favorably modulate lipoprotein profile in African American youth, protecting them from some adverse metabolic outcomes.

Zyflamend, a unique herbal blend, induces cell death and inhibits adipogenesis through the coordinated regulation of PKA and JNK

The prevalence of obesity and its comorbidities has sparked a worldwide concern to address rates of adipose tissue accrual. Recent studies have demonstrated a novel role of Zyflamend, a blend of natural herbal extracts, in regulating lipid metabolism in several cancer cell lines through the activation of the AMPK signalling pathway. Yet, the role of Zyflamend in adipogenic differentiation and lipid metabolism remains largely unexplored. The objective of this study is to investigate the effects of Zyflamend on white 3T3-MBX pre-adipocyte differentiation and elucidate the molecular mechanisms. We demonstrate that Zyflamend treatment altered cell cycle progression, attenuated proliferation, and increased cell death of 3T3-MBX pre-adipocytes. In addition, treatment with Zyflamend inhibited lipid accumulation during the differentiation of 3T3-MBX cells, consistent with decreased expression of lipogenic genes and increased lipolysis. Mechanistically, Zyflamend-induced alterations in adipogenesis were mediated, at least in part, through the activation of AMPK, PKA, and JNK. Inhibition of AMPK partially reversed Zyflamend-induced inhibition of differentiation, whereas the inhibition of either JNK or PKA fully restored adipocyte differentiation and decreased lipolysis. Taken together, the present study demonstrates that Zyflamend, as a novel anti-adipogenic bioactive mix, inhibits adipocyte differentiation through the activation of the PKA and JNK pathways.

Abbreviation: 7-AAD: 7-amino-actinomycin D; ACC: acetyl-CoA carboxylase; AKT: protein kinase B; AMPK: AMP-activated protein kinase; ATGL: adipose triglyceride lipase; C/EBPα: CCAAT-enhancer binding protein alpha; DMEM: Dulbecco’s Modified Eagle Medium; DMSO: dimethyl sulphoxide; DTT: dithiothreitol; EGTA: ethylene glycol-bis-(2-aminoethyl)-N,N,N’,N’-tetraacetic acid; ERK: extracellular signal–regulated kinases; FASN: fatty acid synthase; FBS: foetal bovine serum; GLUT: glucose transporter; HSL: hormone-sensitive lipase; IR: insulin receptor; IRS: insulin receptor substrate; JNK: c-JUN N-terminal kinase; MGL: monoacylglycerol lipase; NaF: sodium fluoride; NF-κB: nuclear factor kappa-light-chain-enhancer of activated B cells; PBS: phosphate buffered- saline; PCB: pyruvate carboxylase; PDE: phosphodiesterase; PKA: protein kinase cAMP-dependent; PMSF: phenylmethylsulfonyl fluoride; PPARγ: perilipin peroxisome proliferator-activated receptor gamma; PREF-1: pre-adipocyte factor 1; PVDF: polyvinylidene fluoride; RIPA: radio-immunoprecipitation assay; SDS-PAGE: sodium dodecyl sulphate polyacrylamide gel electrophoresis; SEM: standard error of the mean; SOX9: suppressor of cytokine signalling 9; TGs: triacylglycerols.

Exercise Training-Enhanced Lipolytic Potency to Catecholamine Depends on the Time of the Day

Exercise training is well known to enhance adipocyte lipolysis in response to hormone challenge. However, the existence of a relationship between the timing of exercise training and its effect on adipocyte lipolysis is unknown. To clarify this issue, Wistar rats were run on a treadmill for 9 weeks in either the early part (E-EX) or late part of the active phase (L-EX). L-EX rats exhibited greater isoproterenol-stimulated lipolysis expressed as fold induction over basal lipolysis, with greater protein expression levels of hormone-sensitive lipase (HSL) phosphorylated at Ser 660 compared to E-EX rats. Furthermore, we discovered that Brain and muscle Arnt-like (BMAL)1 protein can associate directly with several protein kinase A (PKA) regulatory units (RIα, RIβ, and RIIβ) of protein kinase, its anchoring protein (AKAP)150, and HSL, and that the association of BMAL1 with the regulatory subunits of PKA, AKAP150, and HSL was greater in L-EX than in E-EX rats. In contrast, comparison between E-EX and their counterpart sedentary control rats showed a greater co-immunoprecipitation only between BMAL1 and ATGL. Thus, both E-EX and L-EX showed an enhanced lipolytic response to isoproterenol, but the mechanisms underlying exercise training-enhanced lipolytic response to isoproterenol were different in each group.

PKA functions in metabolism and resistance to obesity: lessons from mouse and human studies

Both direct and indirect evidence demonstrate a central role for the cAMP-dependent protein kinase (PKA) signaling pathway in the regulation of energy balance and metabolism across multiple systems. However, the ubiquitous pattern of PKA expression across cell types poses a challenge in pinpointing its tissue-specific regulatory functions and further characterizing its many downstream effects in certain organs or cells. Mouse models of PKA deficiency and over-expression and studies in living cells have helped clarify PKA function in adipose tissue (AT), liver, adrenal, pancreas, and specific brain nuclei, as they pertain to energy balance and metabolic dysregulation. Limited studies in humans suggest differential regulation of PKA in AT of obese compared to lean individuals and an overall dysregulation of PKA signaling in obesity. Despite its complexity, under normal physiologic conditions, the PKA system is tightly regulated by changes in cAMP concentrations upstream via adenylate cyclase and downstream by phosphodiesterase-mediated cAMP degradation to AMP and by changes in PKA holoenzyme stability. Adjustments in the PKA system appear to be important to the development and maintenance of the obese state and its associated metabolic perturbations. In this review we discuss the important role of PKA in obesity and its involvement in resistance to obesity, through studies in humans and in mouse models, with a focus on the regulation of PKA in energy expenditure, intake behavior, and lipid and glucose metabolism.

Levels of the neuropeptide phoenixin-14 and its receptor GRP173 in the hypothalamus, ovary and periovarian adipose tissue in rat model of polycystic ovary syndrome

Phoenixin (PNX) is a newly discovered peptide produced by proteolytic cleavage of a small integral membrane protein 20 (Smim20), which acts as an important regulator of energy homeostasis and reproduction. Since dysfunction of reproduction is characteristic in polycystic ovarian syndrome (PCOS), the role of PNX in pathogenesis of PCOS needs further investigation. The objective of this study was to determine expression of Smim20, PNX-14 and its receptor GRP173 in the hypothalamus, ovary and periovarian adipose tissue (PAT) of letrozole induced PCOS rats. Phosphorylation of extracellular signal-regulated kinase (ERK1/2), protein kinases A (PKA) and B (Akt) were also estimated. We observed that PCOS rats had high weight gain and a number of ovarian cyst, high levels of testosterone, luteinizing hormone and PNX-14, while low estradiol. Smim20 mRNA expression was higher in the ovary and PAT, while PNX-14 peptide production was higher only in the ovary of PCOS rat. Moreover, in PCOS rats Gpr173 level was lower in PAT but at the protein level increased only in the ovary. Depending on the tissues, kinases phosphorylation were significantly differ in PCOS rats. Our results showed higher levels of PNX-14 in PCOS rats and indicated some novel findings regarding the mechanisms of PCOS pathophysiology.

Nuclear Factor E2-Related Factor 2 Mediates Oxidative Stress-Induced Lipid Accumulation in Adipocytes by Increasing Adipogenesis and Decreasing Lipolysis

Aims: Nuclear factor E2-related factor 2 (Nrf2) is a regulator of cellular oxidative stress and is also involved in lipid metabolism in adipocytes. However, it remains unknown as to whether Nrf2 is the link between oxidative stress and the induction of lipid accumulation in adipocytes. Results: Here, we show that oxidative stress is markedly increased in white adipose tissue from mice with high-fat diet-induced or genetically (ob/ob)-induced obesity and from human subjects with obesity. Notably, in response to oxidative stress, Nrf2 expression and activity were induced, further promoting lipid accumulation in adipocytes and exacerbating the development of obesity. In contrast, Nrf2 ablation alleviated oxidative stress-induced lipid accumulation. Mechanistically, oxidative stress promoted Nrf2 recruitment to the sterol regulatory element binding protein 1 promoter, inducing target gene transcription and subsequent lipogenesis. In addition, Nrf2 mediated oxidative stress-inhibited lipolysis in adipocytes via the protein kinase A pathway. Innovation and Conclusion: Our data provide a novel insight that Nrf2, as a critical signaling node, links oxidative stress to the induction of fat accumulation in adipocytes.

Acute vs chronic exposure to high fat diet leads to distinct regulation of PKA

The cAMP-dependent protein kinase (PKA) is an essential regulator of lipid and glucose metabolism that plays a critical role in energy homeostasis. The impact of diet on PKA signaling has not been defined, although perturbations in individual PKA subunits are associated with changes in adiposity, physical activity and energy intake in mice and humans. We hypothesized that a high fat diet (HFD) would elicit peripheral and central alterations in the PKA system that would differ depending on length of exposure to HFD; these differences could protect against or promote diet-induced obesity (DIO). 12-week-old C57Bl/6J mice were randomly assigned to a regular diet or HFD and weighed weekly throughout the feeding studies (4 days, 14 weeks; respectively), and during killing. PKA activity and subunit expression were measured in liver, gonadal adipose tissue (AT) and brain. Acute HFD-feeding suppressed basal hepatic PKA activity. In contrast, hepatic and hypothalamic PKA activities were significantly increased after chronic HFD-feeding. Changes in AT were more subtle, and overall, altered PKA regulation in response to chronic HFD exposure was more profound in female mice. The suppression of hepatic PKA activity after 4 day HFD-feeding was indicative of a protective peripheral effect against obesity in the context of overnutrition. In response to chronic HFD-feeding, and with the development of DIO, dysregulated hepatic and hypothalamic PKA signaling was a signature of obesity that is likely to promote further metabolic dysfunction in mice.

Protein Kinase A Subunit Balance Regulates Lipid Metabolism in Caenorhabditis elegans and Mammalian Adipocytes

Protein kinase A (PKA) is a cyclic AMP (cAMP)-dependent protein kinase composed of catalytic and regulatory subunits and involved in various physiological phenomena, including lipid metabolism. Here we demonstrated that the stoichiometric balance between catalytic and regulatory subunits is crucial for maintaining basal PKA activity and lipid homeostasis. To uncover the potential roles of each PKA subunit, Caenorhabditis elegans was used to investigate the effects of PKA subunit deficiency. In worms, suppression of PKA via RNAi resulted in severe phenotypes, including shortened life span, decreased egg laying, reduced locomotion, and altered lipid distribution. Similarly, in mammalian adipocytes, suppression of PKA regulatory subunits RIα and RIIβ via siRNAs potently stimulated PKA activity, leading to potentiated lipolysis without increasing cAMP levels. Nevertheless, insulin exerted anti-lipolytic effects and restored lipid droplet integrity by antagonizing PKA action. Together, these data implicate the importance of subunit stoichiometry as another regulatory mechanism of PKA activity and lipid metabolism.

Targeting cAMP/PKA pathway for glycemic control and type 2 diabetes therapy

In mammals, cyclic adenosine monophosphate (cAMP) is an intracellular second messenger that is usually elicited by binding of hormones and neurotransmitters to G protein-coupled receptors (GPCRs). cAMP exerts many of its physiological effects by activating cAMP-dependent protein kinase (PKA), which in turn phosphorylates and regulates the functions of downstream protein targets including ion channels, enzymes, and transcription factors. cAMP/PKA signaling pathway regulates glucose homeostasis at multiple levels including insulin and glucagon secretion, glucose uptake, glycogen synthesis and breakdown, gluconeogenesis, and neural control of glucose homeostasis. This review summarizes recent genetic and pharmacological studies concerning the regulation of glucose homeostasis by cAMP/PKA in pancreas, liver, skeletal muscle, adipose tissues, and brain. We also discuss the strategies for targeting cAMP/PKA pathway for research and potential therapeutic treatment of type 2 diabetes mellitus (T2D).

Expression of protein kinase A regulatory subunits in benign and malignant human thyroid tissues: A systematic review

In this review, we discuss the molecular mechanisms and prognostic implications of the protein kinase A (PKA) signaling pathway in human tumors, with special emphasis on the malignant thyroid. The PKA signaling pathway is differentially activated by the expression of regulatory subunits 1 (R1) and 2 (R2), whose levels change during development, differentiation, and neoplastic transformation. Following the identification of gene mutations within the PKA regulatory subunit R1A (PRKAR1A) that cause Carney complex-associated neoplasms, several investigators have studied PRKAR1A expression in sporadic thyroid tumors. The PKA regulatory subunit R2B (PRKAR2B) is highly expressed in benign, as well as in malignant differentiated and undifferentiated lesions. PRKAR1A is highly expressed in follicular adenomas and malignant lesions with a statistically significant gradient between benign and malignant tumors; however, it is not expressed in hyperplastic nodules. Although the importance of PKA in human malignancy outcomes is not completely understood, PRKAR1A expression correlates with tumor dimension in malignant lesions. Additional studies are needed to determine whether a relationship exists between PKA subunit expression and clinical outcomes, particularly in undifferentiated tumors. In conclusion, the R1A subunit might be a good molecular candidate for the targeted treatment of malignant thyroid tumors.

Haploinsufficiency for either one of the type-II regulatory subunits of protein kinase A improves the bone phenotype of Prkar1a+/- mice

Carney Complex (CNC), a human genetic syndrome predisposing to multiple neoplasias, is associated with bone lesions such as osteochondromyxomas (OMX). The most frequent cause for CNC is PRKAR1A deficiency; PRKAR1A codes for type-I regulatory subunit of protein kinase A (PKA). Prkar1a(+/-) mice developed OMX, fibrous dysplasia-like lesions (FDL) and other tumors. Tumor tissues in these animals had increased PKA activity due to an unregulated PKA catalytic subunit and increased PKA type II (PKA-II) activity mediated by the PRKAR2A and PRKAR2B subunits. To better understand the effect of altered PKA activity on bone, we studied Prkar2a and Prkar2b knock out (KO) and heterozygous mice; none of these mice developed bone lesions. When Prkar2a(+/-) and Prkar2b(+/-) mice were used to generate Prkar1a(+/-)Prkar2a(+/-) and Prkar1a(+/-)Prkar2b(+/-) animals, bone lesions formed that looked like those of the Prkar1a(+/-) mice. However, better overall bone organization and mineralization and fewer FDL lesions were found in both double heterozygote groups, indicating a partial restoration of the immature bone structure observed in Prkar1a(+/-) mice. Further investigation indicated increased osteogenesis and higher new bone formation rates in both Prkar1a(+/-)Prkar2a(+/-) and Prkar1a(+/-)Prkar2b(+/-) mice with some minor differences between them. The observations were confirmed with a variety of markers and studies. PKA activity measurements showed the expected PKA-II decrease in both double heterozygote groups. Thus, haploinsufficiency for either of PKA-II regulatory subunits improved bone phenotype of mice haploinsufficient for Prkar1a, in support of the hypothesis that the PRKAR2A and PRKAR2B regulatory subunits were in part responsible for the bone phenotype of Prkar1a(+/-) mice.

Mitochondria: a possible nexus for the regulation of energy homeostasis by the endocannabinoid system?

The endocannabinoid system (ECS) regulates numerous cellular and physiological processes through the activation of receptors targeted by endogenously produced ligands called endocannabinoids. Importantly, this signaling system is known to play an important role in modulating energy balance and glucose homeostasis. For example, current evidence indicates that the ECS becomes overactive during obesity whereby its central and peripheral stimulation drives metabolic processes that mimic the metabolic syndrome. Herein, we examine the role of the ECS in modulating the function of mitochondria, which play a pivotal role in maintaining cellular and systemic energy homeostasis, in large part due to their ability to tightly coordinate glucose and lipid utilization. Because of this, mitochondrial dysfunction is often associated with peripheral insulin resistance and glucose intolerance as well as the manifestation of excess lipid accumulation in the obese state. This review aims to highlight the different ways through which the ECS may impact upon mitochondrial abundance and/or oxidative capacity and, where possible, relate these findings to obesity-induced perturbations in metabolic function. Furthermore, we explore the potential implications of these findings in terms of the pathogenesis of metabolic disorders and how these may be used to strategically develop therapies targeting the ECS.

PKA tightly bound to human placental mitochondria participates in steroidogenesis and is not modified by cAMP

INTRODUCTION

Protein phosphorylation plays an important role in the modulation of steroidogenesis and it depends on the activation of different signaling cascades. Previous data showed that PKA activity is related to steroidogenesis in mitochondria from syncytiotrophoblast of human placenta (HPM). PKA localization and contribution in progesterone synthesis and protein phosphorylation of HPM was assessed in this work.

METHODS

Placental mitochondria and submitochondrial fractions were used. Catalytic and regulatory PKA subunits were identified by Western blot. PKA activity was determined by the incorporation of (32)P into proteins in the presence or absence of specific inhibitors. The effect of PKA activators and inhibitors on steroidogenesis and protein phosphorylation in HPM was tested by radioimmunoassay and autoradiography.

RESULTS

The PKAα catalytic subunit was distributed in all the submitochondrial fractions whereas βII regulatory subunit was the main isoform observed in both the outer and inner membranes of HPM. PKA located in the inner membrane showed the highest activity. Progesterone synthesis and mitochondrial protein phosphorylation are modified by inhibitors of PKA catalytic subunit but are neither sensitive to inhibitors of the regulatory subunit nor to activators of the holoenzyme.

DISCUSSION

The lack of response in the presence of PKA activators and inhibitors of the regulatory subunit suggests that the activation of intramitochondrial PKA cannot be prevented or further activated.

CONCLUSIONS

The phosphorylating activity of PKA inside HPM could be an important component of the steroidogenesis transduction cascade, probably exerting its effects by direct phosphorylation of its substrates or by modulating other kinases and phosphatases.

Nitrite augments glucose uptake in adipocytes through the protein kinase A-dependent stimulation of mitochondrial fusion

Though it is well accepted that adipose tissue is central in the regulation of glycemic homeostasis, the molecular mechanisms governing adipocyte glucose uptake remain unclear. Recent studies demonstrate that mitochondrial dynamics (fission and fusion) regulate lipid accumulation and differentiation in adipocytes. However, the role of mitochondrial dynamics in glucose homeostasis has not been explored. The nitric oxide oxidation products nitrite and nitrate are endogenous signaling molecules and dietary constituents that have recently been shown to modulate glucose metabolism, prevent weight gain, and reverse the development of metabolic syndrome in mice. Although the mechanism of this protection is unclear, the mitochondrion is a known subcellular target for nitrite signaling. Thus, we hypothesize that nitrite modulates mitochondrial dynamics and function to regulate glucose uptake in adipocytes. Herein, we demonstrate that nitrite significantly increases glucose uptake in differentiated murine adipocytes through a mechanism dependent on mitochondrial fusion. Specifically, nitrite promotes mitochondrial fusion by increasing the profusion protein mitofusin 1 while concomitantly activating protein kinase A (PKA), which phosphorylates and inhibits the profission protein dynamin-related protein 1 (Drp1). Functionally, this signaling augments cellular respiration, fatty acid oxidation, mitochondrial oxidant production, and glucose uptake. Importantly, inhibition of PKA or Drp1 significantly attenuates nitrite-induced mitochondrial respiration and glucose uptake. These findings demonstrate that mitochondria play an essential metabolic role in adipocytes, show a novel role for both nitrite and mitochondrial fusion in regulating adipocyte glucose homeostasis, and have implications for the potential therapeutic use of nitrite and mitochondrial modulators in glycemic regulation.

Differences in adiposity in Cushing syndrome caused by PRKAR1A mutations: clues for the role of cyclic AMP signaling in obesity and diagnostic implications

CONTEXT

The cAMP signaling pathway is implicated in bilateral adrenocortical hyperplasias. Bilateral adrenocortical hyperplasia is often associated with ACTH-independent Cushing syndrome (CS) and may be caused by mutations in genes such as PRKAR1A, which is responsible for primary pigmented nodular adrenocortical disease (PPNAD). PRKAR1A regulates cAMP-dependent protein kinase (PKA), an essential enzyme in the regulation of adiposity. Although CS is invariably associated with obesity, its different forms, including those associated with PKA defects, have not been compared.

OBJECTIVE

The purpose of this study was to characterize the phenotypic and molecular differences in periadrenal adipose tissue (PAT) between patients with CS with and without PRKAR1A mutations.

DESIGN AND SETTING

Samples from adrenalectomies of 51 patients were studied: patients with CS with (n = 13) and without (n = 32) PRKAR1A mutations and a comparison group with aldosterone-producing adenomas (APAs) (n = 6). In addition, clinical data from a larger group of patients with Cushing disease (n = 89) and hyperaldosteronism (n = 26) were used for comparison.

METHODS

Body mass index (BMI), abdominal computed tomography scans, and cortisol data were collected preoperatively. PAT was assayed for PKA activity, cAMP levels, and PKA subunit expression.

RESULTS

BMI was lower in adult patients with CS with PRKAR1A mutations. cAMP and active PKA levels in PAT were elevated in patients with CS with PRKAR1A mutations.

CONCLUSIONS

Increased PKA signaling in PAT was associated with lower BMI in CS. Differences in fat distribution may contribute to phenotypic differences between patients with CS with and without PRKAR1A mutations. The observed differences are in agreement with the known roles of cAMP signaling in regulating adiposity, but this is the first time that germline defects of PKA are linked to variable obesity phenotypes in humans.

PKA regulatory subunit R2B is required for murine and human adipocyte differentiation

ADIPOGENESIS IS A COMPLEX PROCESS MODULATED BY SEVERAL FACTORS, INCLUDING CAMP SIGNALING. THE MAIN CAMP TARGET IS PROTEIN KINASE A (PKA), A TETRAMERIC ENZYME WITH FOUR REGULATORY SUBUNITS SHOWING TISSUE-SPECIFIC EXPRESSION AND FUNCTION: PRKAR2B is the main regulatory subunit in adipose tissue in mice and in adult humans. This study aimed to evaluate the expression of PKA regulatory subunits in human adipose tissue during fetal development and to investigate their role in the differentiation of 3T3-L1 and primary human preadipocytes. The expression of PKA regulatory subunits was evaluated in fetal adipose tissue (immunohistochemistry) and in cultured 3T3-L1 and primary human preadipocytes (western blot analysis). Cultured cells were transiently transfected with siRNA against PRKAR2B and induced to differentiate. Differentiation was evaluated by intracellular triglyceride staining (Oil Red O) and expression of molecular markers of adipocyte differentiation. In this study, we found that PRKAR2B is the main regulatory subunit in human adipose tissue during fetal development, from 12 weeks of gestation to the end of gestation, as well as in 3T3-L1 and primary human preadipocytes. The expression of PRKAR2B increases progressively during in vitro differentiation. The silencing of PRKAR2B abolishes the increase in the expression of peroxisome proliferator-activated receptor gamma (PPARγ (PPARG)), fatty acid synthase, aP2 (FABP4), and lipoprotein lipase, as well as intracellular triglyceride accumulation, resulting in impaired adipocyte differentiation in both mouse and human cell systems. In conclusion, PRKAR2B is the key PKA regulatory subunit involved in mouse and human adipose tissue development. The physiological increase in the expression of PRKAR2B is an essential event in adipogenesis in both mice and humans, and it might represent a possible target for future strategies for obesity treatment.

PTPIP51: a new interaction partner of the insulin receptor and PKA in adipose tissue

AIMS

Our previous experiments revealed an association of PTPIP51 (protein tyrosine phosphatase interacting protein 51) with the insulin signalling pathway through PTP1B and 14-3-3beta. We aimed to clarify the role of PTPIP51 in adipocyte metabolism.

METHODS

Four groups of ten C57Bl/6 mice each were used. Two groups were fed a standard diet; two groups were fed a high-fat diet. Two groups (one high-fat diet and one standard diet) were submitted to endurance training, while the remaining two groups served as untrained control groups. After ten weeks, we measured glucose tolerance of the mice. Adipose tissue samples were analyzed by immunofluorescence and Duolink proximity ligation assay to quantify interactions of PTPIP51 with either insulin receptor (IR) or PKA.

RESULTS

PTPIP51 and the IR and PTPIP51 and PKA, respectively, were colocalized in all groups. Standard diet animals that were submitted to endurance training showed low PTPIP51-IR and PTPIP51-PKA interactions. The interaction levels of both the IR and PKA differed between the feeding and training groups.

CONCLUSION

PTPIP51 might serve as a linking protein in adipocyte metabolism by connecting the IR-triggered lipogenesis with the PKA-dependent lipolysis. PTPIP51 interacts with both proteins, therefore being a potential gateway for the cooperation of both pathways.

Uncovering suitable reference proteins for expression studies in human adipose tissue with relevance to obesity

Background

Protein expression studies based on the two major intra-abdominal human fat depots, the subcutaneous and the omental fat, can shed light into the mechanisms involved in obesity and its co-morbidities. Here we address, for the first time, the identification and validation of reference proteins for data standardization, which are essential for accurate comparison of protein levels in expression studies based on fat from obese and non-obese individuals.

Methodology and Findings

To uncover adipose tissue proteins equally expressed either in omental and subcutaneous fat depots (study 1) or in omental fat from non-obese and obese individuals (study 2), we have reanalyzed our previously published data based on two-dimensional fluorescence difference gel electrophoresis. Twenty-four proteins (12 in study 1 and 12 in study 2) with similar expression levels in all conditions tested were selected and identified by mass spectrometry. Immunoblotting analysis was used to confirm in adipose tissue the expression pattern of the potential reference proteins and three proteins were validated: PARK7, ENOA and FAA. Western Blot analysis was also used to test customary loading control proteins. ENOA, PARK7 and the customary loading control protein Beta-actin showed steady expression profiles in fat from non-obese and obese individuals, whilst FAA maintained steady expression levels across paired omental and subcutaneous fat samples.

Conclusions

ENOA, PARK7 and Beta-actin are proper reference standards in obesity studies based on omental fat, whilst FAA is the best loading control for the comparative analysis of omental and subcutaneous adipose tissues either in obese and non-obese subjects. Neither customary loading control proteins GAPDH and TBB5 nor CALX are adequate standards in differential expression studies on adipose tissue. The use of the proposed reference proteins will facilitate the adequate analysis of proteins differentially expressed in the context of obesity, an aim difficult to achieve before this study.

PTPIP51 interaction with PTP1B and 14-3-3β in adipose tissue of insulin-resistant mice

OBJECTIVE

We investigated the expression of protein tyrosine phosphatase-interacting protein 51 (PTPIP51) and its interaction with protein tyrosine phosphatase 1B (PTP1B) and 14-3-3β in mice exhibiting insulin resistance and obesity.

DESIGN

A total of 20 mice were included in the study. Eight control animals were fed a normal standard diet, six animals were fed a high-fat diet and six animals were submitted to a treadmill training parallel to the feeding of a high-fat diet. After 10 weeks, a glucose tolerance test was performed and abdominal adipose tissue samples of the animals were collected.

RESULTS

PTPIP51 protein was identified in the adipocytes of all samples. PTPIP51 interacted with PTP1B and with 14-3-3β protein. Compared with untrained mice fed a standard diet, the interaction of PTPIP51 with PTP1B was reduced in high-fat diet-fed animals. The highest interaction of PTPIP51 with 14-3-3β was seen in trained animals on high-fat diet, whereas untrained animals on high-fat diet displayed lowest values.

CONCLUSION

PTPIP51 is expressed in adipose tissue of humans, rats and mice. Obesity with enhanced insulin resistance resulted in a reduction of PTPIP51 levels in adipocytes and influenced the interactions with PTP1B and 14-3-3β. The interaction of PTPIP51 with PTP1B suggests a regulatory function of PTPIP51 in insulin receptor signal transduction. The interaction of PTPIP51 with 14-3-3β, especially in trained individuals, hints to an involvement of PTPIP51 in the downstream regulation of insulin action.

Mice lacking the Cβ subunit of PKA are resistant to angiotensin II-induced cardiac hypertrophy and dysfunction

BACKGROUND

PKA is a ubiquitous, multi-subunit cellular kinase that regulates a number of different physiological responses in response to cAMP, including metabolism, cell division, and cardiac function. Numerous studies have implicated altered PKA signaling in cardiac dysfunction. Recently, it has been shown that mice lacking the catalytic β subunit of PKA (PKA Cβ) are protected from age-related problems such as weight gain and enlarged livers, and we hypothesized that these mice might also be resistant to cardiomyopathy.

FINDINGS

Angiotensin II (ang II) induced hypertension in both PKA Cβ null mice and their WT littermates. However, PKA Cβ null mice were resistant to a number of ang II-induced, cardiopathological effects observed in the WT mice, including hypertrophy, decreased diastolic performance, and enlarged left atria.

CONCLUSION

The Cβ subunit of PKA plays an important role in angiotensin-induced cardiac dysfunction. The Cβ null mouse highlights the potential of the PKA Cβ subunit as a pharmaceutical target for hypertrophic cardiac disease.

Adipose tissue lipolysis

PURPOSE OF REVIEW

Adipose tissue lipolysis is a critical pathway for the maintenance of energy homeostasis through the degradation of triglycerides and the release of fatty acids into the circulation. The understanding of the cellular factors regulating triglyceride hydrolysis and the metabolic function of lipases has considerably expanded in the last few years, revealing an unexpected complexity. This review aims at describing recent discoveries related to the lipolytic pathway and its regulatory mechanisms.

RECENT FINDINGS

Considerable progress has been made in understanding the role and the mechanisms of activation of the lipolytic enzymes. Recent discoveries have dramatically altered the view of adipose tissue lipolysis and highlighted the importance of additional molecular actors in regulating this process. Catecholamines, natriuretic peptides, and insulin are considered to be the major regulators of lipolysis in humans. However, autocrine/paracrine factors such as metabolites and prostaglandins may also participate in its regulation.

SUMMARY

The manipulation of lipolysis has therapeutic potential in the metabolic disorders frequently associated with obesity. Unraveling the molecular events occurring during regulation of lipolysis may lead to novel therapeutic targets.

Pseudohypoparathyroidism and GNAS epigenetic defects: clinical evaluation of albright hereditary osteodystrophy and molecular analysis in 40 patients

CONTEXT

The two main subtypes of pseudohypoparathyroidism (PHP), PHP-Ia and -Ib, are caused by mutations in GNAS exons 1-13 and methylation defects in the imprinted GNAS cluster, respectively. PHP-Ia patients show Albright hereditary osteodystrophy (AHO) and resistance toward PTH and additional hormones, whereas PHP-Ib patients do not have AHO, and hormone resistance appears to be limited to PTH and TSH. Recently, methylation defects have been detected in few patients with PHP and mild AHO, indicating a molecular overlap between the two forms.

OBJECTIVES

The aim of the study was to screen patients with clinically diagnosed PHP-Ia for methylation defects and to investigate the presence of correlations between the molecular findings and AHO severity.

PATIENTS AND METHODS

We investigated differential methylation of GNAS regions and STX16 microdeletions in genomic DNA from 40 patients with sporadic AHO and multihormone resistance, with no mutations in Gsalpha-coding GNAS exons.

RESULTS

Molecular analysis showed GNAS cluster imprinting defects in 24 of the 40 patients analyzed. No STX16 deletion was detected. The presence of imprinting defects was not associated with the severity of AHO or with specific AHO signs.

CONCLUSIONS

We report the largest series of the literature of patients with clinical AHO and multihormone resistance and no mutation in the Gsalpha gene. Our findings of frequent GNAS imprinting defects further confirm the existence of an overlap between molecular and clinical features of PHP-Ia and PHP-Ib and highlight the necessity of a new clinical classification of the disease that takes into account the recent knowledge on the molecular basis underlying these defects.

Characterization of human adult stem-cell populations isolated from visceral and subcutaneous adipose tissue

Adipose tissue is a dynamic endocrine organ with a central role in metabolism regulation. Functional differences in adipose tissue seem associated with the regional distribution of fat depots, in particular in subcutaneous and visceral omental pads. Here, we report for the first time the isolation of human adipose-derived adult stem cells from visceral omental and subcutaneous fat (V-ASCs and S-ASCs, respectively) from the same subject. Immunophenotyping shows that plastic culturing selects homogeneous cell populations of V-ASCs and S-ASCs from the corresponding stromal vascular fractions (SVFs), sharing typical markers of mesenchymal stem cells. Electron microscopy and electrophysiological and real-time RT-PCR analyses confirm the mesenchymal stem nature of both V-ASCs and S-ASCs, while no significant differences in a limited pattern of cytokine/chemokine expression can be detected. Similar to S-ASCs, V-ASCs can differentiate in vitro toward adipogenic, osteogenic, chondrogenic, muscular, and neuronal lineages, as demonstrated by histochemical, immunofluorescence, real-time RT-PCR, and electrophysiological analyses, suggesting the multipotency of such adult stem cells. Our data demonstrate that both visceral and subcutaneous adipose tissues are a source of pluripotent stem cells with multigermline potential. However, the visceral rather than the subcutaneous ASC could represent a more appropriate in vitro cell model for investigating the molecular mechanisms implicated in the pathophysiology of metabolic disorders such as obesity.

Lipolysis and lipid mobilization in human adipose tissue

Triacylglycerol (TAG) stored in adipose tissue (AT) can be rapidly mobilized by the hydrolytic action of the three main lipases of the adipocyte. The non-esterified fatty acids (NEFA) released are used by other tissues during times of energy deprivation. Until recently hormone-sensitive lipase (HSL) was considered to be the key rate-limiting enzyme responsible for regulating TAG mobilization. A novel lipase named adipose triglyceride lipase/desnutrin (ATGL) has been identified as playing an important role in the control of fat cell lipolysis. Additionally perilipin and other proteins of the surface of the lipid droplets protecting or exposing the TAG core of the droplets to lipases are also potent regulators of lipolysis. Considerable progress has been made in understanding the mechanisms of activation of the various lipases. Lipolysis is under tight hormonal regulation. The best understood hormonal effects on AT lipolysis concern the opposing regulation by insulin and catecholamines. Heart-derived natriuretic peptides (i.e., stored in granules in the atrial and ventricle cardiomyocytes and exerting stimulating effects on diuresis and natriuresis) and numerous autocrine/paracrine factors originating from adipocytes and other cells of the stroma-vascular fraction may also participate in the regulation of lipolysis. Endocrine and autocrine/paracrine factors cooperate and lead to a fine regulation of lipolysis in adipocytes. Age, anatomical site, sex, genotype and species differences all play a part in the regulation of lipolysis. The manipulation of lipolysis has therapeutic potential in the metabolic disorders frequently associated with obesity and probably in several inborn errors of metabolism.