Requirement and role of arachidonic acid in the differentiation of pre-adipose cells

The peroxisome proliferator activated receptors (PPARS) and their effects on lipid metabolism and adipocyte differentiation

The three types of peroxisome proliferator activated receptor (PPAR), alpha, beta (or delta), and gamma, each with a specific tissue distribution, compose a subfamily of the nuclear hormone receptor gene family. Although peroxisome proliferators, including fibrates and fatty acids, activate the transcriptional activity of these receptors, only prostaglandin J2 derivatives have been identified as natural ligands of the PPAR gamma subtype, which also binds thiazolidinedione antidiabetic agents with high affinity. Activated PPARs heterodimerize with RXR and alter the transcription of target genes after binding to specific response elements or PPREs, consisting of a direct repeat of the nuclear receptor hexameric DNA core recognition motif spaced by one nucleotide. The different PPARs can be considered key messengers responsible for the translation of nutritional, pharmacological and metabolic stimuli into changes in the expression of genes, more specifically those genes involved in lipid metabolism. PPAR alpha is involved in stimulating beta-oxidation of fatty acids. In rodents, a PPAR alpha-mediated change in the expression of genes involved in fatty acid metabolism lies at the basis of the phenomenon of peroxisome proliferation, a pleiotropic cellular response, mainly limited to liver and kidney and which can lead to hepatocarcinogenesis. In addition to their role in peroxisome proliferation in rodents, PPAR is also involved in the control of HDL cholesterol levels by fibrates and fatty acids in rodents and humans. This effect is, at least partially, based on a PPAR-mediated transcriptional regulation of the major HDL apolipoproteins, apo A-I and apo A-II. The hypotriglyceridemic action of fibrates and fatty acids also involves PPARs and can be summarized as follows: (1) an increased lipolysis and clearance of remnant particles, due to changes in LPL and apo C-III levels, (2) a stimulation of cellular fatty acid uptake and their conversion to acyl-CoA derivatives by the induction of FAT, FATP and ACS activity, (3) an induction of fatty acid beta-oxidation pathways, (4) a reduction in fatty acid and triglyceride synthesis, and finally (5) a decrease in VLDL production. Hence, both enhanced catabolism of triglyceride-rich particles as well as reduced secretion of VLDL particles are mechanisms that contribute to the hypolipidemic effect of fibrates and FFAs. Whereas for PPAR beta no function so far has been identified, PPAR gamma triggers adipocyte differentiation by inducing the expression of several genes critical for adipogenesis.

Transcription, adipocyte differentiation, and obesity

Differentiation of adipogenic precursor cells into mature adipocytes is a complex phenomenon, characterized by an ordered expression of adipocyte-specific genes, triggered by a set of interacting transcription factors. The most important transcription factors involved in this process are the gamma form of peroxisome proliferator activated receptors (PPAR gamma) and the various members of the CCAAT enhancer binding proteins (alpha, beta, and delta). In addition to PPAR gamma and these enhancer binding proteins, several other transcription factors, including ADD-1 (SRE-BP), HMGI-C, are involved in regulating this process. Altered activity and/or expression of these transcription factors, will induce the expression of target genes in the differentiating cells, ultimately resulting in the phenotypical characteristics of the adipocytes. It is speculated that modulation of these transcription factors by either pharmacological or dietary manipulations might influence adipocyte differentiation and prove beneficial in the prevention and treatment of obesity.

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

Two adenosine receptors A1 and A2 are associated with either stimulation (A2) or inhibition (A1) of adenylate cyclase. Using the clonal cell line Ob1771, we have studied the expression of the two receptors during the process of adipose conversion accelerated by exposure to dexamethasone and 3-isobutyl-l-methylxanthine (IBMX) during the first 3 days post-confluence. The effects mediated by the two receptors on preadipocyte differentiation and adipocyte metabolism were also investigated. The two adenosine agonists NECA and PIA were used as preferential agonists of the A2- and A1-receptor, respectively. In preadipose cells (just confluent), both of the mouse clonal line and human primary culture, NECA dose-dependently stimulated cAMP production with a significant higher potency (P < 0.01) than did PIA. In adipose cells (16-day post-confluent) NECA was found to exert a biphasic effect on forskolin-stimulated cAMP production: i.e., NECA was clearly inhibitory in the femto- to picomolar concentration range whereas this effect gradually diminished at higher concentrations. The effect of PIA in 16-day post-confluent adipose cells however, was purely inhibitory on both cAMP production (IC50: 33.52 +/- 0.44 fM) and lipolysis (64% +/- 7%; P < 0.01). These findings were corroborated by Northern blot analysis which revealed A1-receptor mRNA to be exclusively expressed in the mature adipocytes, whereas A2-receptor mRNA gradually declined during the differentiation process except in 16-day post-confluent cells. In addition, NECA significantly enhanced the effect of corticosterone-induced differentiation by 46.8% (P < 0.05) but failed to have any adipogenic potency acting either alone or in concert with carbaprostacyclin (cPGI2). Thus, endogenous adenosine may have a bimodal action on adipose tissue metabolism mediated through stimulatory A2- and inhibitory A1-receptors, respectively, as a function of adipose conversion.

Developmental expression and functional activity of beta 1- and beta 3-adrenoceptors in murine 3T3-F442A differentiating adipocytes

Beta 1- and beta 3-adrenoceptor mRNA and protein expression, and contribution of each subtype to the catecholamine-sensitive adenylyl cyclase system were studied during the adipose conversion of the murine 3T3-F442A cell line. Northern and reverse transcriptase-polymerase chain reaction analyses indicated that emergence of beta 3-adrenoceptor transcripts was concomittant with that of the gene encoding adipsin, a very late marker of adipose differentiation. Conversely, the induction of the beta 1-adrenoceptor mRNA occurred early after cell commitment towards adipose conversion. Changes in beta-subtype gene expression were accompanied by parallel modifications in receptor expression and function. 125I-cyanopindolol saturation and competition binding experiments showed a 3-fold increase in beta 1-adrenoceptor density in day 3 post-confluent cells. The beta 3-subtype population became detectable later and represented approximately 95% of total beta-adrenoceptors in day 8 and day 12 post-confluent cells. Adenylyl cyclase activity in response to the beta 3-adrenoceptor-selective agonists CGP12177 (4-(3-t-butylamino-2-hydroxypropoxy)-benzimidazol-2-one), ICI201651 ([(R)-4-(2 hydroxy-3-phenoxypropylamino-ethoxy)-N-(2- methoxyethyl)phenoxy-acetamide]) and cyanopindolol was virtually absent in young adipocytes, but dramatically increased in mature cells. The respective contributions of the beta 1- and the beta 3-subtypes to the production of cAMP were resolved by an Eadie-Hofstee computer analysis of isoproterenol and norepinephrine concentration-response curve of adenylyl cyclase activity. Agonist response curves in the presence of beta 1- and beta 2-adrenoceptor antagonist indicated that the beta 1-subtype accounted for the totality of beta-adrenoceptor-mediated adenylyl cyclase activation in young adipocytes. In mature adipose cells approximately 90% of this response was due to an activation of the beta 3-adrenoceptor. In addition, approximately 84% of the maximal norepinephrine-stimulated lipolysis was mediated by the beta 3-adrenoceptor in fully differentiated adipocytes. The differentiation-dependent expression of beta-subtypes in adipocytes is a biphasic process involving an initial and moderate induction of beta 1-adrenoceptors followed by the emergence of a prominent beta 3-adrenoceptor population. Compared analysis of both receptor occupancy and cAMP production shows that the beta 3-subtype is more efficiently coupled to the adenylyl cyclase system than the beta 1-adrenoceptor. Thus in mature adipose cells this receptor subtype represents the core of cAMP-dependent regulation of the lipolytic, antilipogenic and thermogenic effects of catecholamines.

Increased hepatic delta 6-desaturase activity with growth hormone expression in the MG101 transgenic mouse

Growth hormone (GH) has many metabolic effects, but its mechanism(s) of action are not fully understood. We studied the short-term effects of endogenously produced GH on liver delta 6-desaturase activity and adipose and liver lipid fraction fatty acid composition in transgenic mice. MG101 transgenic mice ages 73-114 d received zinc to activate the ovine GH transgene for 7 d. Nontransgenic littermates, used as controls, also received zinc. Liver lipids were fractionated into phospholipids (PL), cholesteryl esters, and triglycerides (TG), and retroperitoneal adipose fractionated into PL and TG for fatty acid analysis. Liver microsomes were assayed for delta 6-desaturase activity. Animals expressing the ovine growth hormone transgene had a 2.5-fold higher liver delta 6-desaturase activity than controls. Arachidonate and docosahexaenoate were significantly higher in liver PL of GH transgenic animals compared to controls, but both were decreased in adipose PL in the GH animals. We conclude that increased production of GH affects both production and organ distribution of highly unsaturated fatty acids. The changes in arachidonate in various lipid pools following transgene expression may mediate the systemic actions of GH.

Regulation of triglyceride metabolism by PPARs: fibrates and thiazolidinediones have distinct effects

The molecular mechanism by which hypolipidemic fibrates and antidiabetic thiazolidinediones exert their hypotriglyceridemic action are discussed. Increased activity of lipoprotein lipase (LPL), a key lipolytic enzyme, and decreased levels of apolipoprotein C-III (apo C-III) seem to explain the hypotriglyceridemic effects of compounds. Both fibrates and thiazolidinediones exert their action by activating transcription factors of the peroxisome proliferator activated receptor (PPAR) family, thereby modulating the expression of the LPL and apo C-II genes. First, treatment of rats with PPAR alpha activators, such as fibrates induced LPL mRNA and activity selectively in the liver. In contrast, the thiazolidinediones, which are high affinity ligands for PPAR gamma, have no effect on liver, but induce LPL mRNA and activity levels in adipose tissue. In hepatocytes, fibrates, unlike the thiazolidinediones, induce LPL mRNA levels, whereas in preadipocyte cell lines the PPAR gamma ligand induces LPL mRNA levels much quicker and to a higher extent than fibrates. Second, apo C-III mRNA and protein production strongly decrease in livers of fibrate but not thiazolidinedione-treated animals. Fibrates also reduced apo C-III production in primary cultures of rat and human hepatocytes. The modulation of the expression of the LPL and apo C-III genes by either PPAR alpha or gamma activators, correlates with the tissue-specific distribution of the respective PPARs: PPAR gamma expression is restricted to adipose tissues, whereas PPAR alpha is expressed predominantly in liver. In both the LPL and apo C-III genes, sequence elements responsible for the modulation of their expression by activated PPARs have been identified which supports that the transcriptional regulation of these genes by fibrates and thiazolidinediones contributes significantly to their hypotriglyceridemic effects in vivo. Whereas thiazolidinediones predominantly affect adipocyte LPL production through activation of PPAR gamma, fibrates exert their effects mainly in the liver via a PPAR alpha-mediated reduction in apo C-III production. This tissue specific transcriptional regulation of genes involved in lipid metabolism by PPAR activators and/or ligands might have important therapeutic implications.

Control of adipocyte differentiation

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Evidence that dietary arachidonic acid increases circulating triglycerides

Male Syrian hamsters and male CD-1 mice were fed diets supplemented with ethyl esters of oleic, linoleic, arachidonic, and eicosapentaenoic acids (1.1-5%, w/w) for 3-4 wk. Plasma and serum triglycerides were significantly higher in the arachidonic acid-supplemented animals compared to those in the other supplementation groups. Changes in serum insulin and glucose levels did not appear to be related to the changes in circulating triglycerides observed in the arachidonic acid-supplemented group. These data indicate that dietary arachidonic acid elevates circulating triglyceride levels compared to other unsaturated fatty acids in hamsters and mice by unknown mechanisms.

Arachidonic acid and free fatty acids as second messengers and the role of protein kinase C

In addition to serving as the precursor to a plethora of eicosanoids and other bioactive molecules, arachidonate may function as a bona fide second messenger. A number of studies have documented the ability of arachidonate to regulate the function of multiple targets in vitro systems. This has been particularly well established and studied with the activation of protein kinase C by arachidonate in a mechanism distinct from activation by diacylglycerol. In cells, arachidonate induces a number of activities, many of which may be independent of further metabolism to eicosanoids; suggesting possible direct action of arachidonate. This review summarizes the current state of knowledge on the possible second messenger function of arachidonate with specific emphasis on the regulation of protein kinase C.

Proliferation and differentiation of rat adipose precursor cells in chemically defined medium: differential action of anti-adipogenic agents

Primary rat adipose precursor cells, maintained in the minimal chemically defined medium (ITT medium) able to promote differentiation, have been used to investigate the ability of several agents to modulate their proliferation and their differentiation. Fetuin and fibroblast growth factor (FGF), which exhibited a strong and a weak mitogenic activity, respectively, do not significantly affect the proportion of differentiated cells as indicated by glycerol-3-phosphate dehydrogenase (GPDH) activity values. In contrast, carbaprostacyclin (cPGI2), a stable analogue of prostacyclin, behaves as a true adipogenic factor leading to a 4 to 5-fold increase in GPDH-specific activities with no significant effect on cell growth. Submaxillary gland kallikrein (SMGK), transforming growth factor-beta (TGF-beta) and tumor necrosis factor-alpha (TNF-alpha) behave as growth-promoting agents but at the same time elicit a dose-dependent inhibition of differentiation. Epidermal growth factor (EGF) and prostaglandin F2 alpha (PGF2 alpha) do not show any effect on cell proliferation at concentrations which exert a maximal inhibitory action on differentiation. Upon removal of EGF from the culture medium, complete resumption of differentiation occurs, whereas upon removal of PGF2 alpha or SMGK, complete resumption only takes place when differentiation is triggered by cPGI2. Upon removal of TNF-alpha, a partial resumption of differentiation is observed, whereas no subsequent differentiation is observed upon TGF-beta removal. These results emphasize the adipogenic, nonmitogenic role of cPGI2 and also allow the distinction between the various adipogenic/mitogenic factors which affect adipose cell differentiation.

Retinoids are positive effectors of adipose cell differentiation

Retinoids, especially all-trans retinoic acid (t-RA), have been reported in the last decade to inhibit the differentiation of preadipose cells. In those studies, however, the concentrations of t-RA were supraphysiological (0.1-10 microM range). In contrast we show that, when present at concentrations below or close to the Kd values of retinoic acid receptors, retinoids behave as potent adipogenic hormones (1 pM to 10 nM range). As shown by the use of specific ligands for each RAR subtype, these positive effects on adipose differentiation involve in particular the RAR alpha subtype, and have been observed in Ob17 cells exposed to serum-supplemented or serum-free medium, and in rat preadipocytes exposed to serum-free medium. Among the two classes of retinoid acid receptors (RARs) and retinoid X receptors (RXRs), RAR alpha, RAR gamma, RXR alpha and RXR beta mRNAs could be detected in growing adipoblasts and were found to be increased in committed preadipocytes and differentiated cells upon retinoid treatment. Like other adipogenic hormones, retinoids were only effective in the terminal differentiation process leading from preadipocytes to adipocytes.

Molecular events in adipocyte development

Adipocyte hyperplasia occurs by the proliferation and differentiation of adipocyte precursor cells or preadipocytes. Although the process of commitment to the adipocyte lineage is poorly understood, a great deal of information has accumulated about the processes and regulatory mechanisms involved in preadipocyte differentiation. The differentiation of preadipocytes is known to be characterized by increased transcription of a number of specific genes. AP-1 and C/EBP binding sites within these genes have been identified as important regulatory sequences. In addition, a specific enhancer sequence has been shown to confer adipose tissue specificity. This article will review the changes in gene transcription that occur during preadipocyte differentiation and how these are regulated. The potential role of autocrine/paracrine acting factors in the proliferation and differentiation of the preadipocyte is also discussed.

Terminal differentiation of mouse preadipocyte cells: adipogenic and antimitogenic role of triiodothyronine

The role of triiodothyronine (T3) in the differentiation process of Ob1771 mouse preadipocyte cells has been studied under serum-free and hormone supplemented culture conditions which were previously shown to lead to terminal differentiation. In the absence of T3, a dramatic decrease in the adipogenic activity of the culture medium (EC50 = 0.1 nM) could be observed, as indicated 12 days after confluence by the low levels of late markers of differentiation such as adipsin, lipid-binding protein aP2 and glycerol-3-phosphate dehydrogenase as well as the sharp reduction of the number of triacyglycerol-containing cells. This decrease in adipogenic activity was accompanied by a parallel increase of the mitogenic potency of the culture medium. Therefore, T3 appears to be a hormone capable of modulating both proliferation and differentiation of preadipocytes. T3 ceased to be necessary provided the culture medium was supplemented with high concentrations of inducers of differentiation, such as 8-bromo-cAMP or carbaprostacyclin.

Inhibition of prostacyclin-induced Ca2+ mobilization by phorbol esters in Ob1771 preadipocytes

In addition to cAMP production, a transient elevation of intracellular free Ca2+ has been shown to take place in preadipose cells upon stimulation by carbaprostacyclin (cPGI2), both messengers acting in synergy to initiate adipose cell differentiation (Vassaux, G., Gaillard, D., Ailhaud, G., and Négrel, R. (1992) J. Biol. Chem.267, 11092-11097). Further studies reported herein show that this Ca2+ transient is i) elicited by the natural prostaglandin PGI2, ii) independent of the presence of extracellular Ca2+, suggesting a mobilization of Ca2+ from intracellular pools and ii) unaffected by cAMP elevating agents. Moreover, and in contrast to the InsP3-dependent Ca2+ signal evoked by PGF2 alpha, that induced by PGI2 is fully abolished by pretreatment with phorbol esters (EC50: 1-5 nM). Furthermore, experiments designed to empty the Ca2+ pools, using PGI2 or PGF2 alpha as Ca2+ mobilizing agents as well as pretreatments with drugs, allow to conclude that PGI2 mobilizes Ca2+ from an InsP3 sensitive, ryanodine insensitive intracellular pool. Altogether, these results strongly suggest that PGI2 mobilizes Ca2+ from an intracellular store common to that affected by InsP3, by means of a mechanism which remains to be elucidated.

Purification and characterization of fatty acid-binding proteins from brown adipose tissue of the rat

Fatty acid-binding proteins (FABPs) have been identified and purified from interscapular brown adipose tissue of the rat. The proteins were characterized and their properties compared with the FABP present in white adipose tissue. FABP was purified to electrophoretic homogeneity from brown adipose tissue by a procedure involving precipitation with 70% ammonium sulphate, followed sequentially by ion-exchange chromatography and gel filtration chromatography. The purified fraction migrated as a single band on SDS-PAGE with an apparent molecular mass of 14,200. Scatchard analysis of [14C]oleate-binding to purified FABP gave a Kd value of approx. 0.80 +/- 0.02 microM and a maximal binding of 0.65 +/- 0.03 mol per mol of protein; these values were similar to that found with the FABP purified from white fat. The FABP concentration in brown adipose tissue was almost twice that of FABP in white adipose tissue. Fatty acid analysis of FABP from brown adipose tissue revealed that the intrinsic arachidonic acid content was proportionately higher than that present in FABP of white adipose tissue. Isoelectric focusing of delipidated FABP indicated that it existed with two charge isoforms (pI 6.85 and 7.35). The purified FABP additionally emerged in two peaks (FABP-I and FABP-II) from a reverse phase HPLC column. Amino acid analysis showed that Gly, Thr, and Ser residues in FABP-I were almost twice as high as in FABP-II. The N-terminals of both FABP-I and -II were not blocked. These components have been partially sequenced and showed a sequence homology only between 25-31 residues from the N-terminal. Further studies are required to elucidate the precise function of the two different isoforms of FABP in brown adipose tissue.

Essential role of collagens for terminal differentiation of preadipocytes

In order to study the role of collagens in the differentiation of TA1 preadipose cells in vitro, ethyl-3,4-dihydroxybenzoate (EDHB) was used as a specific inhibitor of collagen synthesis. The secretion of collagenous proteins only was severely decreased after exposure to EDHB, and this was accompanied by a decrease of differentiation as indicated by low activity levels of glycerophosphate dehydrogenase. The effect of EDHB was dose-dependent and also dependent upon the stage of cell differentiation. Northern-blot analysis show that EDHB addition to undifferentiated cells did not prevent the induction of A2COL6 gene, a marker of the preadipose state, but prevented the induction of the gene encoding for the adipocyte lipid binding protein and the modulation of the expression of the lipoprotein lipase gene which are both indicators of the adipose state. These results demonstrate that differentiation of preadipose cells into adipose cells requires active synthesis of collagens during the preadipose state.

Multiple growth factor mRNAs are expressed in chicken adipocyte precursor cells

We have examined the expression of growth factor genes in primary cultures of chicken adipocyte precursors. RNA was extracted from proliferating and differentiated cells, reversed transcribed and amplified by PCR using gene specific primers. The identity of the PCR products was confirmed by restriction mapping. We show, for the first time, constitutive expression of TGF-beta 2, TGF-beta 3, TGF-beta 4 and bFGF genes in chicken adipocyte precursors. We also detect GH-independent, but differentiation-dependent IGF-I gene expression. The synthesis and action of these growth factors supports the hypothesis that they act as autocrine and/or paracrine regulators of adipocyte precursor cell proliferation and differentiation.

Terminal differentiation of mouse preadipocyte cells: the mitogenic-adipogenic role of growth hormone is mediated by the protein kinase C signalling pathway

The role of growth hormone (GH) in the differentiation process of Ob1771 mouse preadipocyte cells has been studied under culture conditions that were serum-free and hormone-supplemented and which were previously shown to lead to terminal differentiation. In the absence of GH, a dramatic decrease in the adipogenic activity of the culture medium could be observed, as indicated 12 days after confluence by the low levels of glycerol-3-phosphate dehydrogenase activity and the sharp reduction of the number of triacylglycerol-containing cells. This decrease in adipogenic activity was accompanied by a parallel loss of the mitogenic potency of the culture medium. Determination of the half-maximal and maximal concentrations of GH required for the restoration of growth and differentiation were identical, 0.5 and 2 nM, respectively. Despite the presence of insulin-like growth factor-I (IGF-I) to substitute for supraphysiological concentrations of insulin and to saturate IGF-I receptor, GH was still required to induce terminal differentiation of a maximal number of cells. However, protein kinase C activators such as prostaglandin F2 alpha, phorbol esters and diacylglycerol were able to mimic GH in promoting a maximal mitogenic-adipogenic response, indicating that the ability of GH to induce diacylglycerol production (Doglio et al., 1989; Catalioto et al., 1990) plays a prominent role in this process. Furthermore, in agreement with the fact that the mitoses which precede terminal differentiation of Ob1771 preadipocytes are strictly controlled by cAMP and only modulated by protein kinase C, terminal differentiation of Ob1771 preadipocytes occurred in the absence of GH upon supplementation with high concentrations of carbaprostacyclin, added as a cAMP-elevating agent or with 8-Br-cAMP, added as a cAMP analogue. It is concluded that the control exerted by GH on terminal differentiation of mouse preadipocytes corresponds to a modulating mitogenic effect mediated through protein kinase C activation and leading to a potentiation of the cAMP and IGF-I mitogenic signalling pathways.

Beta-adrenergic-cyclic AMP signalling pathway modulates cell function at the transcriptional level in 3T3-F442A adipocytes

We studied the role of cAMP in the regulation of the expression of the adipsin gene and of some other adipose-specific genes including lipoprotein lipase (LPL), glycerophosphate dehydrogenase (G3PDH), and adipocyte P2 (aP2) in 3T3-F442A adipocytes. Northern blot analysis of isoproterenol (10(-6) M)-, forskolin (10(-5) M)- or 8-bromo-cAMP (10(-3) M)-treated adipocytes showed that the steady-state levels of adipsin mRNA were strongly reduced in a time-dependent and reversible manner. The concentration of isoproterenol giving a half-maximal effect in the down-regulation of the adipsin message was approximately 5 x 10(-8) M. Similarly, cell treatment by forskolin elicited a down-regulation of LPL and G3PDH mRNA levels but did not alter aP2 mRNA level. As determined by nuclear run-on assays, the rate of transcription of adipsin, LPL and G3PDH in isoproterenol-treated adipocytes was respectively 3, 3, and 2 times lower than in control adipocytes. These results indicate (1) that cAMP plays a dominant antilipogenic role in the fat cell through the transcriptional down-regulation of the expression of two major genes involved in triglyceride biosynthesis; (2) that cAMP does not reverse the adipocyte character; (3) hence, that cAMP suppresses adipsin expression at the transcriptional level, providing additional support for the role of adipsin protein in adipocyte metabolism.

Autocrine control of adipose cell differentiation by prostacyclin and PGF2 alpha

The mitogenic-adipogenic effect exerted by arachidonic acid, which leads to terminal differentiation of Ob1771 mouse preadipocytes, has been shown to be (i) blocked by cyclooxygenase inhibitors, (ii) mimicked by a stable analogue of prostacyclin (carbaprostacyclin) and (iii) potentiated by PGF2 alpha. Since these prostanoids are known to be synthesized and secreted by preadipocytes, we have proposed that both prostacyclin as the key mediator and PGF2 alpha as a modulator control the expression of terminal events of adipose conversion by means of an autocrine mechanism (Gaillard, D. et al. and Negrel, R. et al. Biochem. J. (1989) 257, 389-397 and 399-405). In order to test this hypothesis, the release of prostacyclin, characterized under the form of its stable degradation product 6-keto-PGF1 alpha, and that of PGF2 alpha have been studied in the culture medium of Ob1771 cells. A striking increase in the release of 6-keto-PGF1 alpha and to a minor degree of PGF2 alpha was observed when cells were exposed to arachidonic acid as shown by using [3H]arachidonic acid prelabelled cells or by radio-immunoassays. Since antagonists of PGF2 alpha and PGI2 receptors were not available, specific antibodies directed against PGF2 alpha and 6 beta-PGI1, another stable analogue of prostacyclin, were added as neutralizing agents in the culture medium. These antibodies were able to counteract the mitogenic-adipogenic effect of arachidonic acid. Prostacyclin and PGF2 alpha thus appear as autocrine mediators in the process of adipose conversion.

Cholesterol efflux from adipose cells is coupled to diacylglycerol production and protein kinase C activation

Apolipoprotein A-I (apo A-I)*/DMPC complexes have been previously shown to promote cholesterol efflux from cholesterol-preloaded adipose cells whereas apo A-II/DMPC complexes, which bind to the same cell surface binding sites, were ineffective. Addition of apo A-I/DMPC complexes led to a rapid and transient formation of diacylglycerol. However, in contrast to PGF2 alpha (Doglio et al., Proc. Natl. Acad. Sci. USA, 1989, 86, 1148), no accumulation of inositol phosphates was observed. Apo A-II/DMPC complexes had no effect on diacylglycerol formation. Stimulation by apo A-I/DMPC complexes or native HDL3 of cells prelabelled with (2-palmitoyl 9,10[3H])phosphatidylcholine induced also the formation of labelled diacylglycerol whereas apo A-II/DMPC complexes and HDL3 treated with tetranitromethane showed no effect. Direct activation of protein kinase C(s) by PMA promoted cholesterol efflux providing that DMPC liposomes were present as cholesterol acceptor. It is proposed that lipoprotein particles have two separate effects, i.e. a ligand-induced effect leading to cholesterol translocation from intracellular stores to the cell surface and a bilayer-induced effect allowing cholesterol efflux from the cell surface to the acceptor.

The induction of adipose conversion by bezafibrate in 3T3-L1 cells. Synergism with dibutyryl-cAMP

The induction of adipose conversion in 3T3-L1 cells by bezafibrate (Brandes, R., Hertz, R. Arad R., Naishtat S., Weil, S. and Bar-Tana, J. (1987) Life Sci., 40, 935-941) was enhanced by dibutyryl-cAMP as well as forskolin, theophylline or isobutylmethylxanthine added to the incubation medium together with the bezafibrate inducer. The synergistic effect of bezafibrate and dibutyryl-cAMP resulted in enhancing the expression of late markers of adipose conversion, e.g., lipid accumulation or glycerol-3-phosphate dehydrogenase activity and its mRNA. This enhanced expression of late markers was reflected in shortening the time period required for their first appearance as well as increasing their yield during the course of adipose conversion. By following the accumulation of glutamine synthetase mRNA serving as an early marker for adipose conversion, the synergistic effect of bezafibrate and dibutyryl-cAMP was already evident as early as 5 h following their addition to confluent 3T3-L1 cells. Hence, the induction of adipose conversion by bezafibrate in 3T3-L1 cells appears to involve an early event which is rate-limited by the availability of intracellular cAMP.

Inhibition by serum components of the expression of lipoprotein lipase gene upon stimulation by growth hormone

Growth hormone regulates in a positive way the expression of the lipoprotein lipase gene at a transcriptional level in preadipocyte Ob1771 cells. Inhibition by serum components of this expression was investigated upon stimulation by growth hormone. Low-molecular weight, lipid-soluble components (a serum lipid extract, corticosteroids and oleic acid) and high-molecular weight, hydrophilic components (TGF-beta and those present in delipidated serum) were inhibitory. Inhibition of the expression of LPL mRNAs and that of LPL activity were parallel. It is concluded that the regulation of the expression of LPL gene occurs likely at a transcriptional level and that a balance between multiple effectors present in serum are active in an opposite manner.

Promoting effect of glucocorticoids on the differentiation of human adipocyte precursor cells cultured in a chemically defined medium

Stromal-vascular cells obtained from adult human subcutaneous adipose tissue were cultured in a chemically defined serum-free medium. In the presence of 0.2 nM triiodothyronine and 0.5 microM insulin, up to 25% of the cells were able to undergo terminal adipose differentiation within 18 d, as assessed by lipid accumulation and the expression of lipoprotein lipase (LPL) and glycerol-3-phosphate dehydrogenase (GPDH) activities. Addition of cortisol resulted in a potent dose-dependent stimulation of the adipose differentiation process. Cortisol could be replaced by dexamethasone and partly by aldosterone, but not by sex steroids. The proportion of differentiated cells was dependent upon the age of the donor; when isolated from young adults, up to 70% of the stromal-vascular cells expressed the adipocyte phenotype as compared with 5-10% when the cells were isolated from the oldest subjects. An inverse relationship was observed between the age of the 27 normal-weight donors and the extent of GPDH expression after maintenance of the cells for 18 d in chemically defined medium supplemented with insulin, triiodothyronine, and cortisol (r = -0.787, P less than 0.001). It is concluded that adult human adipose tissue still contains precursor cells that are able to undergo adipose differentiation in vitro. This improved culture system may offer the opportunity to characterize other adipogenic factors as well as antiadipogenic factors involved in the control of adipose tissue growth.

Prostacyclin as a potent effector of adipose-cell differentiation

The terminal differentiation of Ob1771 pre-adipose cells induced by arachidonic acid in serum-free hormone-supplemented medium containing insulin, transferrin, growth hormone, tri-iodothyronine and fetuin (5F medium) was strongly diminished in the presence of inhibitors of prostaglandin synthesis, namely aspirin or indomethacin. Carbaprostacyclin, a stable analogue of prostacyclin (prostaglandin I2) known to be synthesized by pre-adipocytes and adipocytes, behaved as an efficient activator of cyclic AMP production and was able, when added to 5F medium, to mimic the adipogenic effect of arachidonic acid. Prostaglandins E2, F2 alpha and D2, unable to affect the cyclic AMP production, failed to substitute for carbaprostacyclin. However, prostaglandin F2 alpha, which is another metabolite of arachidonic acid in pre-adipose and adipose cells, able to promote inositol phospholipid breakdown and protein kinase C activation, potentiated the adipogenic effect of carbaprostacyclin. In addition, carbaprostacyclin enhanced both a limited proliferation and terminal differentiation of adipose precursor cells isolated from rodent and human adipose tissues maintained in primary culture. These results demonstrate the critical role of prostacyclin and prostaglandin F2 alpha on adipose conversion in vitro and suggest a paracrine/autocrine role of both prostanoids in the development of adipose tissue in vivo.