Changes in adenosine A1- and A2-receptor expression during adipose cell differentiation

Energy Regulation in Inflammatory Sarcopenia by the Purinergic System

The purinergic system has a dual role: the maintenance of energy balance and signaling within cells. Adenosine and adenosine triphosphate (ATP) are essential for maintaining these functions. Sarcopenia is characterized by alterations in the control of energy and signaling in favor of catabolic pathways. This review details the association between the purinergic system and muscle and adipose tissue homeostasis, discussing recent findings in the involvement of purinergic receptors in muscle wasting and advances in the use of the purinergic system as a novel therapeutic target in the management of sarcopenia.

RNA-seq reveals that anti-obesity irisin and triiodothyronine (T3) hormones differentially affect the purinergic signaling transcriptomics in differentiated human adipocytes

The anti-obesity thyroid hormone, triiodothyronine (T3), and irisin, an exercise- and/or cold-induced myokine, stimulate thermogenesis and energy consumption while decreasing lipid accumulation. The involvement of ATP signaling in adipocyte cell function and obesity has attracted increasing attention, but the crosstalk between the purinergic signaling cascade and anti-obesity hormones lacks experimental evidence. In this study, we investigated the effects of T3 and irisin in the transcriptomics of membrane-bound purinoceptors, ectonucleotidase enzymes and nucleoside transporters participating in the purinergic signaling in cultured human adipocytes. The RNA-seq analysis revealed that differentiated adipocytes express high amounts of ADORA1, P2RY11, P2RY12, and P2RX6 gene transcripts, along with abundant levels of transcriptional products encoding to purine metabolizing enzymes (ENPP2, ENPP1, NT5E, ADA and ADK) and transporters (SLC29A1, SCL29A2). The transcriptomics of purinergic signaling markers changed in parallel to the upsurge of "browning" adipocyte markers, like UCP1 and P2RX5, after treatment with T3 and irisin. Upregulation of ADORA1, ADORA2A and P2RX4 gene transcription was obtained with irisin, whereas T3 preferentially upregulated NT5E, SLC29A2 and P2RY11 genes. Irisin was more powerful than T3 towards inhibition of the leptin gene transcription, the SCL29A1 gene encoding for the ENT1 transporter, the E-NPP2 (autotaxin) gene, and genes that encode for two ADP-sensitive P2Y receptors, P2RY1 and P2RY12. These findings indicate that anti-obesity irisin and T3 hormones differentially affect the purinergic signaling transcriptomics, which might point towards new directions for the treatment of obesity and related metabolic disorders that are worth to be pursued in future functional studies.

Multipotent Stromal Cells from Subcutaneous Adipose Tissue of Normal Weight and Obese Subjects: Modulation of Their Adipogenic Differentiation by Adenosine A1 Receptor Ligands

Adenosine A₁ receptor (A₁R) activation, stimulating lipogenesis and decreasing insulin resistance, could be useful for metabolic syndrome management in obese subjects. Since full A₁R agonists induce harmful side-effects, while partial agonists show a better pharmacological profile, we investigated the influence of two derivatives of the full A₁R agonist 2-chloro-N⁶-cyclopentyladenosine (CCPA), C1 and C2 behaving as A₁R partial agonists in animal models, on the adipogenic differentiation of stromal/stem cells (ASCs) from human subcutaneous adipose tissue, which mainly contribute to increase fat mass in obesity. The ASCs from normal-weight subjects showed increased proliferation and A₁R expression but reduced adipogenic differentiation compared to obese individual-derived ASCs. Cell exposure to CCPA, C1, C2 or DPCPX, an A₁R antagonist, did not affect ASC proliferation, while mainly C2 and DPCPX significantly decreased adipogenic differentiation of both ASC types, reducing the activity of glycerol-3-phosphate dehydrogenase and the expression of PPARγ and FABP-4, all adipogenic markers, and phosphorylation of Akt in the phosphatidylinositol-3-kinase pathway, which plays a key-role in adipogenesis. While requiring confirmation in in vivo models, our results suggest that A₁R partial agonists or antagonists, by limiting ASC differentiation into adipocytes and, thereby, fat mass expansion, could favor development/worsening of metabolic syndrome in obese subjects without a dietary control.

The adenosinergic pathway in mesenchymal stem cell fate and functions

Mesenchymal stem cells (MSCs) play an important role in tissue homeostasis and damage repair through their ability to differentiate into cells of different tissues, trophic support, and immunomodulation. These properties made them attractive for clinical applications in regenerative medicine, immune disorders, and cell transplantation. However, despite multiple preclinical and clinical studies demonstrating beneficial effects of MSCs, their native identity and mechanisms of action remain inconclusive. Since its discovery, the CD73/ecto-5'-nucleotidase is known as a classic marker for MSCs, but its role goes far beyond a phenotypic characterization antigen. CD73 contributes to adenosine production, therefore, is an essential component of purinergic signaling, a pathway composed of different nucleotides and nucleosides, which concentrations are finely regulated by the ectoenzymes and receptors. Thus, purinergic signaling controls pathophysiological functions such as proliferation, migration, cell fate, and immune responses. Despite the remarkable progress already achieved in considering adenosinergic pathway as a therapeutic target in different pathologies, its role is not fully explored in the context of the therapeutic functions of MSCs. Therefore, in this review, we provide an overview of the role of CD73 and adenosine-mediated signaling in the functions ascribed to MSCs, such as homing and proliferation, cell differentiation, and immunomodulation. Additionally, we will discuss the pathophysiological role of MSCs, via CD73 and adenosine, in different diseases, as well as in tumor development and progression. A better understanding of the adenosinergic pathway in the regulation of MSCs functions will help to provide improved therapeutic strategies applicable in regenerative medicine.

Bone Marrow and Adipose Tissue Adenosine Receptors Effect on Osteogenesis and Adipogenesis

Adenosine is an extracellular signaling molecule that is particularly relevant in times of cellular stress, inflammation and metabolic disturbances when the levels of the purine increase. Adenosine acts on two G-protein-coupled stimulatory and on two G-protein-coupled inhibitory receptors, which have varying expression profiles in different tissues and conditions, and have different affinities for the endogenous ligand. Studies point to significant roles of adenosine and its receptors in metabolic disease and bone health, implicating the receptors as potential therapeutic targets. This review will highlight our current understanding of the dichotomous effects of adenosine and its receptors on adipogenesis versus osteogenesis within the bone marrow to maintain bone health, as well as its relationship to obesity. Therapeutic implications will also be reviewed.

Identification and characterization of adipose surface epitopes

Adipose tissue is a central regulator of metabolism and an important pharmacological target to treat the metabolic consequences of obesity, such as insulin resistance and dyslipidemia. Among the various cellular compartments, the adipocyte cell surface is especially appealing as a drug target as it contains various proteins that when activated or inhibited promote adipocyte health, change its endocrine function and eventually maintain or restore whole-body insulin sensitivity. In addition, cell surface proteins are readily accessible by various drug classes. However, targeting individual cell surface proteins in adipocytes has been difficult due to important functions of these proteins outside adipose tissue, raising various safety concerns. Thus, one of the biggest challenges is the lack of adipose selective surface proteins and/or targeting reagents. Here, we discuss several receptor families with an important function in adipogenesis and mature adipocytes to highlight the complexity at the cell surface and illustrate the problems with identifying adipose selective proteins. We then discuss that, while no unique adipocyte surface protein might exist, how splicing, posttranslational modifications as well as protein/protein interactions can create enormous diversity at the cell surface that vastly expands the space of potentially unique epitopes and how these selective epitopes can be identified and targeted.

Polypharmacology of N6-(3-Iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA) and Related A3 Adenosine Receptor Ligands: Peroxisome Proliferator Activated Receptor (PPAR) γ Partial Agonist and PPARδ Antagonist Activity Suggests Their Antidiabetic Potential

A3 adenosine receptor (AR) ligands including A3 AR agonist, N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (1a, IB-MECA) were examined for adiponectin production in human bone marrow mesenchymal stem cells (hBM-MSCs). In this model, 1a significantly increased adiponectin production, which is associated with improved insulin sensitivity. However, A3 AR antagonists also promoted adiponectin production in hBM-MSCs, indicating that the A3 AR pathway may not be directly involved in the adiponectin promoting activity. In a target deconvolution study, their adiponectin-promoting activity was significantly correlated to their binding activity to both peroxisome proliferator activated receptor (PPAR) γ and PPARδ. They functioned as both PPARγ partial agonists and PPARδ antagonists. In the diabetic mouse model, 1a and its structural analogues A3 AR antagonists significantly decreased the serum levels of glucose and triglyceride, supporting their antidiabetic potential. These findings indicate that the polypharmacophore of these compounds may provide therapeutic insight into their multipotent efficacy against various human diseases.

DbcAMP regulates adipogenesis in sheep inguinal preadipocytes

Background

The cyclic adenosine-monophosphate (cAMP) pathway is generally recognized as one of the essential pathways for the adipose conversion of rodent preadipocytes in vitro. However, divergent effects of cAMP on adipocyte differentiation have also been reported. Since there is very little data on non-rodent preadipose cells, the aim of the present work was to analyze the effects of one classic activator (dbcAMP) of the cAMP pathway on the proliferation and differentiation of sheep preadipocytes grown.

Method

We retrospectively analyzed the regulation of dbcAMP on the proliferation and differentiation of sheep preadipocytes through observation on cell dynamic morphology, drawing on the growth curve, Oil Red O staining and induction of cell differentiation.

Results

1) During first 5 days of treatment, at lower levels of dbcAMP (1 nmol/L to 1 × 10⁴ nmol/L), sheep cells were not increased, but at higher levels (1 × 10⁵ nmol/l to 1 × 10⁶ nmol/l), they were significantly increased (P < 0.05); 2) dbcAMP had the trendency to promote cell differentition, but it was not significant (P > 0.05); 3) treated for 4 days, dbcAMP at the levels of 1 nmol/L, 1 × 10⁴ nmol/L and 1 × 10⁶ nmol/L increased C20:0 abundance (P < 0.05), but other fatty acids had no significant changes; 4) treated for 4 days, expression of SCD mRNA had no significant change (P > 0.05), but expression of HSL mRNA increased at the level of 1 × 10⁶ nmol/L dbcAMP (P < 0.05).

Conclusion

This study demonstrated that the mechanisms by which of the cAMP pathway affects on preadipocytes between sheep and rodent animals was different.

An adenosine receptor-Krüppel-like factor 4 protein axis inhibits adipogenesis

Adipogenesis represents a key process in adipose tissue development and remodeling, including during obesity. Exploring the regulation of adipogenesis by extracellular ligands is fundamental to our understanding of this process. Adenosine, an extracellular nucleoside signaling molecule found in adipose tissue depots, acts on adenosine receptors. Here we report that, among these receptors, the A2b adenosine receptor (A2bAR) is highly expressed in adipocyte progenitors. Activation of the A2bAR potently inhibits differentiation of mouse stromal vascular cells into adipocytes, whereas A2bAR knockdown stimulates adipogenesis. The A2bAR inhibits differentiation through a novel signaling cascade involving sustained expression of Krüppel-like factor 4 (KLF4), a regulator of stem cell maintenance. Knockdown of KLF4 ablates the ability of the A2bAR to inhibit differentiation. A2bAR activation also inhibits adipogenesis in a human primary preadipocyte culture system. We analyzed the A2bARKLF4 axis in adipose tissue of obese subjects and, intriguingly, found a strong correlation between A2bAR and KLF4 expression in both subcutaneous and visceral human fat. Hence, our study implicates the A2bAR as a regulator of adipocyte differentiation and the A2bAR-KLF4 axis as a potentially significant modulator of adipose biology.

G protein-coupled receptors and adipogenesis: a focus on adenosine receptors

G-protein coupled receptors (GPCRs) are a large family of proteins that coordinate extracellular signals to produce physiologic outcomes. Adenosine receptors (AR) are one class of GPCRs that have been shown to regulate functions as diverse as inflammation, blood flow, and cellular differentiation. Adenosine signals through four GPCRs that either inhibit (A1AR and A3AR) or activate (A2aAR and A2bAR) adenylyl cyclase. This review will focus on the role of GPCRs, and in particular, adenosine receptors, in adipogenesis. Preadipocytes differentiate to mature adipocytes as the adipose tissue expands to compensate for the consumption of excess nutrients. These newly generated adipocytes contribute to maintaining metabolic homeostasis. Understanding the key drivers of this differentiation process can aid the development of therapeutics to combat the growing obesity epidemic and associated metabolic consequences. Although much literature has covered the transcriptional events that culminate in the formation of an adipocyte, less focus has been on receptor-mediated extracellular signals that direct this process. This review will highlight GPCRs and their downstream messengers as significant players controlling adipocyte differentiation.

Extracellular purines promote the differentiation of human bone marrow-derived mesenchymal stem cells to the osteogenic and adipogenic lineages

Extracellular nucleotides are potent signaling molecules mediating cell-specific biological functions, mostly within the processes of tissue damage and repair and flogosis. We previously demonstrated that adenosine 5'-triphosphate (ATP) inhibits the proliferation of human bone marrow-derived mesenchymal stem cells (BM-hMSCs), while stimulating, in vitro and in vivo, their migration. Here, we investigated the effects of ATP on BM-hMSC differentiation capacity. Molecular analysis showed that ATP treatment modulated the expression of several genes governing adipogenic and osteoblastic (i.e., WNT-pathway-related genes) differentiation of MSCs. Functional studies demonstrated that ATP, under specific culture conditions, stimulated adipogenesis by significantly increasing the lipid accumulation and the expression levels of the adipogenic master gene PPARγ (peroxisome proliferator-activated receptor-gamma). In addition, ATP stimulated osteogenic differentiation by promoting mineralization and expression of the osteoblast-related gene RUNX2 (runt-related transcription factor 2). Furthermore, we demonstrated that ATP stimulated adipogenesis via its triphosphate form, while osteogenic differentiation was induced by the nucleoside adenosine, resulting from ATP degradation induced by CD39 and CD73 ectonucleotidases expressed on the MSC membrane. The pharmacological profile of P2 purinergic receptors (P2Rs) suggests that adipogenic differentiation is mainly mediated by the engagement of P2Y1 and P2Y4 receptors, while stimulation of the P1R adenosine-specific subtype A2B is involved in adenosine-induced osteogenic differentiation. Thus, we provide new insights into molecular regulation of MSC differentiation.

Adenosine receptor subtype expression and activation influence the differentiation of mesenchymal stem cells to osteoblasts and adipocytes

Osteoblasts and adipocytes differentiate from a common precursor cell, the mesenchymal stem cell (MSC). Adenosine is known to signal via four adenosine receptor subtypes, and significantly, recent findings indicate that these may play a role in MSC differentiation. We therefore investigated adenosine receptor expression and activation during the differentiation of MSCs to osteoblasts and adipocytes. The A(2B) R was dominant in MSCs, and its expression and activity were transiently upregulated at early stages of osteoblastic differentiation. Both activation and overexpression of A(2B) R induced the expression of osteoblast-related genes [Runx2 and alkaline phosphatase (ALP)], as well as ALP activity, and stimulation increased osteoblast mineralization. The expression of A(2A) R was upregulated during later stages of osteoblastic differentiation, when its activation stimulated ALP activity. Differentiation of MSCs to adipocytes was accompanied by significant increases in A(1) R and A(2A) R expression, and their activation was associated with increased adipogenesis. Enhanced A(2A) R expression was sufficient to promote expression of adipocyte-related genes (PPARγ and C/EBPα), and its activation resulted in increased adipocytic differentiation and lipid accumulation. In contrast, the A(1) R was involved mainly in lipogenic activity of adipocytes rather than in their differentiation. These results show that adenosine receptors are differentially expressed and involved in lineage-specific differentiation of MSCs. We conclude, therefore, that fruitful strategies for treating diseases associated with an imbalance in the differentiation and function of these lineages should include targeting adenosine receptor signal pathways. Specifically, these research avenues will be useful in preventing or treating conditions with insufficient bone or excessive adipocyte formation.

Contrasting effects of A1 and A2b adenosine receptors on adipogenesis

BACKGROUND

Adenosine mediates its actions through four G protein-coupled receptors, A1, A2a, A2b and A3. The A1 receptor (A1R) is dominant in adipocytes where it mediates many actions that include inhibition of lipolysis, stimulation of leptin secretion and protection against obesity-related insulin resistance.

OBJECTIVE

The objective of this study is to investigate whether induced expression of A1Rs stimulates adipogenesis, or whether A1R expression is a consequence of cells having an adipocyte phenotype.

METHODOLOGY

Human A1R and A2b receptors (A2bRs) were stably transfected into a murine osteoblast precursor cell line, 7F2. Adipogenesis was determined by lipid accumulation and expression of adipocyte and osteoblast marker molecules. Adenosine receptor expression and activation of associated signal molecules were also evaluated as 7F2 cells were induced to differentiate to adipocytes.

RESULTS

7F2 cells transfected with the A1R showed increased adipocyte marker mRNA expression; lipoprotein lipase and glycerol-3-phosphate dehydrogenase were both upregulated, whereas the osteoblast marker alkaline phosphatase (ALP) was downregulated. When cultured in adipocyte differentiating media, such cells also showed increased adipogenesis as judged by lipid accumulation. Conversely, A2bR transfection stimulated osteocalcin and ALP expression, and in addition, adipogenesis was inhibited in the presence of adipocyte differentiation media. Adipogenic differentiation of naive 7F2 cells also resulted in increased expression of the A1R and reduced or modified expression of the A2a and A2bR. The loss of A2 receptors after adipogenic differentiation was accompanied by a loss of cyclic adenosine monophosphate and ERK1/2 signalling.

CONCLUSION

These data show that expression of A1Rs induced adipocyte differentiation, whereas A2bR expression inhibited adipogenesis and stimulated an osteoblastic phenotype. These data suggest that targeting A1 and A2bR could be considered in the management of obesity and diabetes. Targeting adenosine signal pathways may be useful in treatment strategies for diseases in which there is an imbalance between osteoblasts and adipocytes.

Sequential synthesis and methylation of phosphatidylethanolamine promote lipid droplet biosynthesis and stability in tissue culture and in vivo

Triacylglycerols are stored in eukaryotic cells within lipid droplets (LD). The LD core is enwrapped by a phospholipid monolayer with phosphatidylcholine (PC), the major phospholipid, and phosphatidylethanolamine (PE), a minor component. We demonstrate that the onset of LD formation is characterized by a change in cellular PC, PE, and phosphatidylserine (PS). With induction of differentiation of 3T3-L1 fibroblasts into adipocytes, the cellular PC/PE ratio decreased concomitant with LD formation, with the most pronounced decline between confluency and day 5. The mRNA for PS synthase-1 (forms PS from PC) and PS decarboxylase (forms PE from PS) increased after day 5. Activity and protein of PE N-methyltransferase (PEMT), which produces PC by methylation of PE, are absent in 3T3-L1 fibroblasts but were induced at day 5. High fat challenge induced PEMT expression in mouse adipose tissue. PE, produced via PS decarboxylase, was the preferred substrate for methylation to PC. A PEMT-GFP fusion protein decorated the periphery of LD. PEMT knockdown in 3T3-L1 adipocytes correlated with increased basal triacylglycerol hydrolysis. Pemt(-/-) mice developed desensitization against adenosine-mediated inhibition of basal hydrolysis in adipose tissue, and adipocyte hypotrophy was observed in Pemt(-/-) animals on a high fat diet. Knock-out of PEMT in adipose tissue down-regulated PS synthase-1 mRNA, suggesting coordination between PE supply and converting pathways during LD biosynthesis. We conclude that two consecutive processes not previously related to LD biogenesis, (i) PE production via PS and (ii) PE conversion via PEMT, are implicated in LD formation and stability.

A1 adenosine receptor: role in diabetes and obesity

Adenosine mediates its diverse effects via four subtypes (A(1), A(2A), A(2B) and A(3)) of G-protein-coupled receptors. The A(1) adenosine receptor (A(1)AR) subtype is the most extensively studied and is well characterized in various organ systems. The A(1)ARs are highly expressed in adipose tissue, and endogenous adenosine has been shown to tonically activate adipose tissue A(1)ARs. Activation of the A(1)ARs in adipocytes reduces adenylate cyclase and cAMP content and causes inhibition of lipolysis. The role of A(1)ARs in lipolysis has been well characterized by using several selective A(1)AR agonists as well as A(1)AR knockout mice. However, the contribution of A(1)ARs to the regulation of lipolysis in pathological conditions like insulin resistance, diabetes and dyslipidemia, where free fatty acids (FFA) play an important role, has not been well characterized. Pharmacological agents that reduce the release of FFA from adipose tissue and thus the availability of circulating FFA have the potential to be useful for insulin resistance and hyperlipidemia. Toward this goal, several selective and efficacious agonists of the A(1)ARs are now available, and some have entered early-phase clinical trials; however, none have received regulatory approval yet. Here we review the existing knowledge on the role of A(1)ARs in insulin resistance, diabetes and obesity, and the progress made in the development of A(1)AR agonists as antilipolytic agents, including the challenges associated with this approach.

Regulation of lipolysis in adipocytes

Lipolysis of white adipose tissue triacylglycerol stores results in the liberation of glycerol and nonesterified fatty acids that are released into the vasculature for use by other organs as energy substrates. In response to changes in nutritional state, lipolysis rates are precisely regulated through hormonal and biochemical signals. These signals modulate the activity of lipolytic enzymes and accessory proteins, allowing for maximal responsiveness of adipose tissue to changes in energy requirements and availability. Recently, a number of novel adipocyte triacylglyceride lipases have been identified, including desnutrin/ATGL, greatly expanding our understanding of adipocyte lipolysis. We have also begun to better appreciate the role of a number of nonenzymatic proteins that are critical to triacylglyceride breakdown. This review provides an overview of key mediators of lipolysis and the regulation of this process by changes in nutritional status and nutrient intakes.

Eliminating the antilipolytic adenosine A1 receptor does not lead to compensatory changes in the antilipolytic actions of PGE2 and nicotinic acid

AIM

We examined whether compensatory changes after adenosine A(1) receptor knockout [A(1)R (-/-)] eliminate the antilipolytic actions mediated by this receptor.

METHODS

Lipolysis experiments were performed on adipocytes prepared from the wild type A(1)R (+/+), A(1)R (-/-) and heterozygous mice. Gene expression was assayed with cDNA microarray technique and real-time PCR; protein expression with immunoblotting.

RESULTS

The A(1)R was the only adenosine receptor involved in lipolysis. The effects of adenosine deaminase and 2-chloroadenosine were abolished in A(1)R (-/-) mice. The IC(50) value of 2-chloroadenosine doubled from 16.6 to 33.6 nm when half of the A(1)Rs were eliminated. Adrenergic alpha(2) agonists had no effects on lipolysis. Prostaglandin E(2) (PGE(2)) inhibited lipolysis with an IC(50) value of 5.8 nm (4.7-7.2 nm) in the A(1)R (+/+) mice and 10.6 nm (9.0-12.6 nm) in the A(1)R (-/-) mice. Nicotinic acid inhibited lipolysis with an IC(50) value of 0.30 microm (0.19-0.46 microm) in the A(1)R (+/+) mice and 0.24 microm (0.16-0.37 microm) in the A(1)R (-/-) mice. G(i)alpha(1) mRNA was significantly up-regulated in adipose tissue from A(1)R (-/-) mice. However, immunoblotting showed that G(ialpha1) was not up-regulated at the protein level.

CONCLUSION

The A(1)R mediates the antilipolytic actions of adenosine. Deletion of the A(1)R in mice does not result in compensatory increases in G-protein-mediated antilipolytic actions of PGE(2) or nicotinic acid.

Differential control of lipogenesis and lipolysis during development of ovine preadipocytesin vitro

The stromovascular fraction of adipose tissue from sheep, like that of other species, contains preadipocytes which can be induced to differentiate in culture, providing a potentially useful system for studying adipocyte development. Differentiation of ruminant preadipocytes has only been partly characterized previously so we have investigated the factors regulating the development of lipogenesis and lipolysis in sheep cells. Insulin, rosiglitazone (a peroxisome proliferation activated receptor-γ agonist) and either dexamethasone or a lipid suplement are required during differentiation for maximum rates of lipogenesis, whereas all four components are required to achieve maximum rates of catecholamine-stimulated lipolysis. Tri-iodothyronine had no effect on the development of lipogenesis but resulted in a reduced rate of catecholamine-stimulated lipolysis. Lipogenesis and lipolysis also differed in that the rate of lipogenesis increased to a maximum at about 10 days of differentiation and then fell, whereas the rate of lipolysis reached a plateau at about 10 days. By contrast to catecholamine-stimulated lipolysis, there is little or no evidence for development of the adenosine-based antilipolytic system; this may be because response to adenosine develops very late during preadipocyte differentiation or additional, unidentified factors are required to induce this antilipolytic system. Lipogenesis in differentiated preadipocytes responded to both insulin and growth hormone. These studies show that the development of lipogenesis and lipolysis are under distinct control systems. Furthermore, while preadipocytes differentiatedin vitroshow many of the characteristics of adipocytes differentiatedin vivo, there are still significant differences.

Human adipose cells have voltage-dependent potassium currents

The whole-cell patch-clamp method was used to study the membrane electrical properties of human adipocyte cells obtained by differentiating from precursors of human abdominal and mammary tissues. All differentiated cells exhibited outward currents with sigmoidal activation kinetics. The outward currents showed activation thresholds between -20 to -30 mV and slow inactivation. The ionic channels underlying the macroscopic current were highly selective for K(+). Their selectivity was for typical K(+) channels with relative permeabilities of K(+)>NH4+>Cs(+)>Na(+). No evidence of any other type of voltage-gated channel was found. The potassium currents ( I(KV)) were blocked reversibly by tetraethylammonium and barium. The IC(50) value and Hill coefficient of tetraethylammonium inhibition of I(KV) were 0.56 m M and 1.17 respectively. These results demonstrate that human adipose cells have voltage-dependent potassium currents.

Initiation of a process of differentiation by stable transfection of ob17 preadipocytes with the cDNA of human A1 adenosine receptor

A process of differentiation was observed when ob17 preadipocyte cells were stably transfected with a vector containing the cDNA of the human A1 adenosine receptor of adipose tissue. Growth of the cell lines continued but was slowed relative to untransfected cells and cells transfected with vector alone, never attaining confluence. During this process, cells were observed to differentiate morphologically and to accumulate lipid droplets in their cytoplasm, droplets that stained with Oil red-O. During that same period of time, cells transfected with vector alone multiplied rapidly, attained confluence, and showed no signs of differentiation. We conclude that expression of the A1 adenosine receptor initiated a differentiation process that resembled aspects of the normal differentiation of preadipocytes. If so, a role for the A1 receptor in normal preadipocyte differentiation should be considered, perhaps after cell-to-cell contact occurs at confluence.

Various stimulators of the cyclic AMP pathway fail to promote adipose conversion of porcine preadipocytes in primary culture

The cyclic adenosine-monophosphate (cAMP) pathway is generally recognized as one of the essential pathways for the adipose conversion of rodent preadipocytes in vitro. However, divergent effects of cAMP on adipocyte differentiation have also been reported. Since there is very little data on non-rodent preadipose cells, the aim of the present work was to analyze the effects of classic activators of the cAMP pathway on the proliferation and differentiation of porcine preadipocytes grown either in serum-free or in serum-containing medium. In both media, the addition of 10 microM forskolin from day 1 after cell plating to day 3 or 7 did not affect cell proliferation. Such stimulations also failed to enhance preadipocyte differentiation, as assessed by the measurement of lipoprotein lipase (LPL) and glycerol 3-phosphate dehydrogenase (GPDH) activities, two markers of adipose conversion. Similar results were obtained when various concentrations of forskolin (0.1 nM-100 microM) were added for 2 days either during the growth phase (days 1-3) or after confluence (days 5-7). Addition of methylisobutylxanthine (MIX) or 8-bromo-cAMP was also found inefficient to stimulate porcine preadipocytes differentiation clearly. By contrast, post-confluence treatment of the murine 3T3-L1 cell line with either forskolin or MIX markedly enhanced lipid accumulation and led to a dramatic increase in GPDH activity (up to 120 times). This indicates that similar culture conditions are adipogenic for the murine 3T3-L1 preadipocytes but not for porcine preadipose cells. In summary, this work clearly highlights the finding that porcine preadipocytes do not respond to classic activators of the cAMP pathway like rodent cells do. This calls in question again the general model proposed for the action of this pathway in adipose conversion and suggests that the mechanisms regulating adipocyte differentiation may differ among species.

Adenylyl cyclase P-site ligands accelerate differentiation in Ob1771 preadipocytes

Differentiation of Ob1771 preadipocytes to adipocytes was characterized by morphological changes and elevated expression of the specific marker enzyme, glycerol-3-phosphate dehydrogenase. A differentiation response substantially more complete and rapid than that obtained with insulin and 3,5,3'-triiodothyronine was observed with established inhibitors of adenylyl cyclases: 2', 5'-dideoxyadenosine (2',5'-dd-Ado), 9-(cyclopentyl)adenine (9-CP-Ade), and 9-(arabinofuranosyl)adenine (9-Ara-Ade), coincident with decreased cellular cAMP levels. These ligands inhibit adenylyl cyclases noncompetitively, via a domain referred to as the P-site because of its requirement for an intact purine moiety. Differentiation was not induced by inosine, a nucleoside known not to act at the P-site, or by N6-(2-phenylisopropyl)adenosine or 1, 3-diethyl-8-phenylxanthine, agonist and antagonist, respectively, for adenosine A1 receptors. Also ineffective were IBMX or forskolin, agents that can raise intracellular cAMP levels. Potency of the differentiation response followed the order 2',5'-dd-Ado (1-20 microM) > 9-CP-Ade (10-100 microM) = 9-Ara-Ade (10-100 microM) >> inosine, consistent with their potencies to inhibit adenylyl cyclases. The data suggest that inhibition of adenylyl cyclase via the P-site and the consequent reduction in cell cAMP levels facilitate the induction of differentiation in Ob1771 cells. The findings raise the question whether the known endogenous P-site ligands participate in the differentiation response induced by hormones.

Mechanisms regulating adipocyte lipolysis

Mechanisms regulating adipocyte lipolysis are reviewed in three stages. The first stage examines plasma membrane hormone receptors and G-proteins. The primary regulators of adipose tissue lipolysis, the catecholamines, bind to the alpha 2, beta 1, beta 2, and beta 3 adrenergic receptors. The alpha 2 receptor couples with Gi-proteins to inhibit cyclic AMP formation and lipolysis, while the beta receptors couple with Gs-proteins to stimulate cyclic AMP formation and lipolysis. The beta 1 receptor may mediate low level catecholamine stimulation, while the beta 3 receptor, which is activated by higher levels of catecholamines, may deliver a more sustained signal. The second stage examines the regulation of cyclic AMP, the intracellular messenger that activates protein kinase A. Adenylyl cyclase synthesizes cyclic AMP from ATP and is regulated by the G-proteins. Phosphodiesterase 3B hydrolyzes cyclic AMP to AMP and is activated and phosphorylated by both insulin and the catecholamines norepinephrine and epinephrine. The third stage focuses on the rate-limiting enzyme of lipolysis, hormone-sensitive lipase (HSL). This 82 to 88 kDa protein is regulated by reversible phosphorylation. Protein kinase A activates and phosphorylates the enzyme at 2 sites, and 3 phosphatases have been implicated in HSL dephosphorylation. The translocation of HSL from the cytosol to the lipid droplet in response to lipolytic stimulation may be facilitated by a family of lipid-associated droplets called perilipins that are heavily phosphorylated by protein kinase A and dephosphorylated by insulin. As the mechanisms regulating adipocyte lipolysis continue to be uncovered, we look forward to the challenges of integrating these findings with research at the in situ and in vivo levels.