Differentiation of ob 17 preadipocytes to adipocytes. Effects of prostaglandin F2alpha and relationship to prostaglandin synthesis

Control of adipogenesis by oxylipins, GPCRs and PPARs

Oxylipins are bioactive metabolites derived from the oxygenation of ω3 and ω6 polyunsaturated fatty acids, triggered essentially by cyclooxygenase and lipoxygenase activities. Oxylipins are involved in the development and function of adipose tissue and their productions are strictly related to diet quality and quantity. Oxylipins signal via cell surface membrane (G Protein-coupled receptors) and nuclear receptors (peroxisome proliferator-activated receptors), two pathways playing a pivotal role in adipocyte biology. In this review, we made an attempt to cover the available knowledge about synthesis and molecular function of oxylipins known to modulate adipogenesis, adipocyte function and phenotype conversion, with a focus on their interaction with peroxisome proliferator-activated nuclear receptor family.

The adipose renin-angiotensin system: role in cardiovascular disease

Several reviews have highlighted the importance of local tissue production of components of the renin-angiotensin system (RAS) [Bader, M., Ganten, D., 2008. Update on tissue renin-angiotensin systems. J. Mol. Med. 86, 615-621; Krop, M., Danser, A.H., 2008. Circulating versus tissue renin-angiotensin system: on the origin of (pro)renin. Curr. Hypertens. Rep. 10, 112-118; Paul, M., Poyan Mehr, A., Kreutz, R., 2006. Physiology of local renin-angiotensin systems. Physiol. Rev. 86, 747-803]. While the concept of tissue RAS is gaining more widespread acceptance, the concept of local angiotensin II (AngII) production, acting in coordinate or independently of the endocrine RAS, continues to be debated. The primary reasons that local AngII production has been studied by many investigators are that components of the RAS are expressed by multiple cell types, and that the endocrine RAS cannot fully explain all effects of AngII. Moreover, through the development and study of genetically altered models for over-expression or knockdown of individual RAS components within specific cell types, it is becoming increasingly more evident that local RAS contribute to effects of AngII in normal physiology and disease. The purpose of this review is to define the presence and physiological significance of a local RAS in adipose tissue in relation to cardiovascular disease.

Board-invited review: the biology and regulation of preadipocytes and adipocytes in meat animals

The quality and value of the carcass in domestic meat animals are reflected in its protein and fat content. Preadipocytes and adipocytes are important in establishing the overall fatness of a carcass, as well as being the main contributors to the marbling component needed for consumer preference of meat products. Although some fat accumulation is essential, any excess fat that is deposited into adipose depots other than the marbling fraction is energetically unfavorable and reduces efficiency of production. Hence, this review is focused on current knowledge about the biology and regulation of the important cells of adipose tissue: preadipocytes and adipocytes.

Sodium arsenite inhibits terminal differentiation of murine C3H 10T1/2 preadipocytes

Cancer represents an imbalance between cell proliferation and differentiation, two processes that are coordinately and antagonistically regulated. Aberrant cell proliferation is considered to be an important etiological factor in the development of arsenic-induced cancer, suggesting that arsenic also dysregulates differentiation. Based on evidence that arsenic modulates mitogenic events that antagonize the process of differentiation, this study addresses the hypothesis that sodium arsenite inhibits insulin/dexamethasone-induced differentiation of C3H 10T1/2 preadipocytes; it was further postulated that arsenic-treated cells retain mitogenic responsiveness under differentiating conditions. To test this hypothesis, the differentiation capacity of C3H 10T1/2 preadipocytes was examined in control cells and cells treated with sodium arsenite. Differentiation was assessed morphologically and quantified by Oil Red-O staining of accumulated lipids. The effect of long-term arsenic exposure on mitogenic competence was quantified by flow cytometry, [(3)H]thymidine incorporation, and cell counting under conditions favorable for adipocyte differentiation. Results indicate that arsenic inhibits morphological differentiation of wild-type C3H 10T1/2 preadipocytes. Short-term arsenic exposure inhibits differentiation in a dose-dependent manner, with arsenic concentrations > or = 3 microM producing a significant inhibition of dexamethasone/insulin-induced lipid accumulation. Furthermore, arsenic-treated cells exhibit an accentuated response to mitogenic stimulation under differentiating conditions. These data suggest that arsenic exposure results in the inhibition of cellular programming required for terminal differentiation of C3H 10T1/2 preadipocytes and that cells acquire mitogenic hyperresponsiveness. The ability of arsenic to dysregulate the balance between proliferation and differentiation is proposed to be one mechanism by which this metalloid causes cancer in humans.

Understanding adipocyte differentiation

The adipocyte plays a critical role in energy balance. Adipose tissue growth involves an increase in adipocyte size and the formation of new adipocytes from precursor cells. For the last 20 years, the cellular and molecular mechanisms of adipocyte differentiation have been extensively studied using preadipocyte culture systems. Committed preadipocytes undergo growth arrest and subsequent terminal differentiation into adipocytes. This is accompanied by a dramatic increase in expression of adipocyte genes including adipocyte fatty acid binding protein and lipid-metabolizing enzymes. Characterization of regulatory regions of adipose-specific genes has led to the identification of the transcription factors peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and CCAAT/enhancer binding protein (C/EBP), which play a key role in the complex transcriptional cascade during adipocyte differentiation. Growth and differentiation of preadipocytes is controlled by communication between individual cells or between cells and the extracellular environment. Various hormones and growth factors that affect adipocyte differentiation in a positive or negative manner have been identified. In addition, components involved in cell-cell or cell-matrix interactions such as preadipocyte factor-1 and extracellular matrix proteins are also pivotal in regulating the differentiation process. Identification of these molecules has yielded clues to the biochemical pathways that ultimately result in transcriptional activation via PPAR-gamma and C/EBP. Studies on the regulation of the these transcription factors and the mode of action of various agents that influence adipocyte differentiation will reveal the physiological and pathophysiological mechanisms underlying adipose tissue development.

Expression of the two isoforms of prostaglandin endoperoxide synthase (PGHS-1 and PGHS-2) during adipose cell differentiation

Expression of mRNAs encoding the two prostaglandin endoperoxide synthase (PGHS) isoenzymes (PGHS-1 and -2) was investigated in differentiating clonal Ob1771 mouse preadipocytes and in mouse adipose tissues. Northern analysis revealed that the expression level of PGHS-1 mRNA was reduced by 98+/-0.2% (P <0.01) during differentiation of Ob1771 cells, whereas PGHS-2 mRNA was not detected. By reverse transcriptase-polymerase chain reaction analysis, however, both PGHS-1 and -2 mRNA was detected in Ob1771 preadipose cells. In addition. mRNAs encoding both isoforms were markedly expressed in primary adipose precursor cells with considerably lower expression levels in mature adipocytes (56 75% reduction, P<0.01). Furthermore, exposure to dexamethasone (10 nM) for both 24 h (explants of adipose tissue) and 48 h (Ob1771 adipose cells) resulted in enhanced expression of PGHS-1 mRNA. whereas expression of PGHS-2 mRNA in explants of adipose tissue (24 h incubation) was reduced by 83 +/- 9% (P<0.05). In contrast, exposure to angiotensin II (100 nM) enhanced expression of PGHS-1 mRNA both in mature adipocytes (4 h incubation) and explants of adipose tissue (24 h incubation), and elevated PGHS-2 mRNA expression in mature adipocytes (4 h incubation). In conclusion, this report suggests a differential expression of PGHS mRNAs during adipose cell differentiation, and further suggests that the machinery for prostaglandin synthesis in mature adipocytes may be induced by various hormones.

The pulmonary lipofibroblast (lipid interstitial cell) and its contributions to alveolar development

The pulmonary lipofibroblast is located in the alveolar interstitium and is recognizable by its characteristic lipid droplets. During alveolar development it participates in the synthesis of extracellular matrix structural proteins, such as collagen and elastin, and as an accessory cell to the type II pneumocyte, in the synthesis of surfactant. The lipofibroblast contains cortical contractile filaments and is thereby related to the contractile interstitial cells that are normally found at the alveolar septal tips and after lung injury. The morphologic, immunologic, and biochemical characteristics of the lipofibroblast and its probable physiologic functions are reviewed. The retinoid and lipid metabolism of the lipofibroblast is compared with that of the hepatic lipocyte and the adipocyte. Although the functions of the lipofibroblast remain incompletely characterized, this cell type is emerging as an important contributor to pulmonary alveolar septal development.

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.

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.

Prostaglandin F2 alpha inhibits the differentiation of adipocyte precursors in primary culture

Influence of arachidonate metabolite pathway on adipose differentiation was investigated using primary culture of adipocyte precursors in defined medium. Treatment of the cells with cyclooxygenase inhibitors stimulates adipose differentiation by at least 2-fold. Among the various arachidonate metabolites tested, only prostaglandin F2 alpha (PGF2 alpha) was found to inhibit the differentiation of adipocyte precursors in a dose dependent fashion. Other eicosanoids tested did not have any effect. A 50% inhibition of adipose differentiation was observed with a dose of PGF2 alpha of 3 x 10(-9)M to 7 x 10(-9)M according to the strain of rats used. Maximal inhibition occurred at PGF2 alpha concentrations equal or higher than 10(-8)M. PGF2 alpha inhibited not only the expression of late markers of adipose differentiation such as G3PDH and triglycerides accumulation but also the mRNA expression of early markers of adipose differentiation such as clone 154, lipoprotein lipase and ap2 gene. These results indicate that PGF2 alpha represents a physiological negative modulator of adipose differentiation.

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.

Effect of exogenous prostaglandins and nonsteroidal anti-inflammatory agents on prostaglandin secretion and proliferation of mouse embryo fibroblasts in culture

During wound healing, the positive and negative modulation of fibroblast proliferation may be due, in part, to the high prostaglandin concentration of the inflammatory exudates. In vitro, PGF2 alpha has been shown to stimulate, whereas PGE2 inhibits, the growth of different fibroblast cell lines. Therefore, we have investigated the effect of exogenous prostaglandins (PGs) and of various nonsteroidal anti-inflammatory drugs (NSAIDs) on the proliferation and the prostaglandin (PG) synthesis of normal mouse embryo fibroblasts. PGF2 alpha, 6-keto PGF1 alpha and PGE2 increase fibroblast proliferation. On the other hand, PGF2 alpha increases the synthesis of PGE2 and 6-keto PGF1 alpha while 6-keto PGF1 alpha solely inhibits PGF2 alpha release, PGE2 being inactive. The mouse embryo fibroblasts partially transform the prodrug sulindac sulfoxide in the sulfide form, which completely inhibits PG synthesis, as does indomethacin. In contrast, ibuprofen exerts a differential action, according to the type of PG measured. Among the NSAIDs tested, only sulindac (sulfoxide or sulfide) stimulates fibroblast proliferation and this effect appears independent of an alteration of PG synthesis. Therefore, in this model of normal mouse embryo fibroblasts, while endogenous prostaglandins are not involved in the control of cell proliferation, exogenous PGs have the ability to alter fibroblast growth and PG synthesis.

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

The terminal adipose differentiation of Ob1771 cells, characterized by glycerol-3-phosphate dehydrogenase activity and triacylglycerol accumulation, was studied in serum-free hormone-supplemented medium containing growth hormone, tri-iodothyronine, insulin, transferrin and fetuin. Arachidonic acid was able to substitute for a crude adipogenic fraction isolated from fetal bovine serum but not for growth hormone or tri-iodothyronine. Arachidonic acid was also able to increase in a rapid and dramatic manner cyclic AMP production; moreover it was able to amplify the adipose conversion promoted by other agents elevating cyclic AMP concentrations and to induce inositol phospholipid breakdown. Both phorbol 12-myristate 13-acetate, a protein kinase C activator and ionomycin, a Ca2+-mobilizing agent, showed potent synergy with agents elevating cyclic AMP concentrations for the promotion of adipose conversion, whereas 8-bromo cyclic GMP and 4 alpha-phorbol 12,13-dibutyrate were ineffective. The triggering of both the cyclic AMP and inositol phospholipid pathways was accompanied by a single round of cell division, and within a few days all the cells became differentiated. Similar results were obtained, after exposure to arachidonic acid, with preadipose 3T3-F442A cells and with rat adipose precursor cells in primary culture. The availability of arachidonic acid from intracellular stores and/or of exogenous origin should play a major role for the onset of critical mitoses leading to terminal differentiation in pre-adipose cells.

Development of a chemically defined serum-free medium for differentiation of rat adipose precursor cells

Stromal-vascular cells from the epididymal fat pad of 4-week-old rats, when cultured in a medium containing insulin or insulin-like growth factor, IGF-I, triiodothyronine and transferrin, were able to undergo adipose conversion. Over ninety percent of the cells accumulated lipid droplets and this proportion was reduced in serum-supplemented medium. The adipose conversion was assessed by the development of lipoprotein lipase (LPL) and glycerol-3-phosphate dehydrogenase (GPDH) activities, [14C]glucose incorporation into polar and neutral lipids, triacylglycerol accumulation and lipolysis in response to isoproterenol. Similar results were obtained with stromal-vascular cells from rat subcutaneous and retroperitoneal adipose tissues. Stromal-vascular cells required no adipogenic factors in addition to the components of the serum-free medium. Insulin was required within a physiological range of concentrations for the emergence of LPL and at higher concentrations for that of GPDH. When present at concentrations ranging from 2 to 50 nM, IGF-I was able to replace insulin for the expression of both LPL and GPDH. The development of a serum-free, chemically defined medium for the differentiation of diploid adipose precursor cells opens up the possibility of characterizing inhibitors or activators of the adipose conversion process.

Effect of alpha-fetoprotein on arachidonic acid metabolism in the preadipocyte cell line OB 17

Previous work has shown that alpha-fetoprotein (AFP) is able to bind specifically polyunsaturated fatty acids, one of the major ligands being arachidonic acid (C20:4). In the present study, we demonstrate that AFP is able to reduce the metabolism of exogenous C20:4 by ob17 cells. Both prostaglandins and lipoxygenase products formation were reduced when cells were maintained in the presence of AFP. This decrease was counterbalanced by a higher release of C20:4 into the culture medium by the cells. The amount of C20:4 incorporated into cellular lipids was decreased but the distribution of C20:4 in the different lipid classes remained unchanged. The modification of C20:4 metabolism by AFP might be of primordial importance in developmental biology and may shed a new light on the physiological actions of AFP which have been described in the past years such as ovarian inhibition, cell growth control and immuno-suppressive activity.

Prostaglandin and hydroxyeicosatetraenoic acid synthesis by human mesenchymal tumors

The metabolism of arachidonic acid was investigated by radioimmunoassay and chromatographic techniques in 5 sarcomas and one embryonal carcinoma of human origin maintained as transplantable tumors in nude mice. The results obtained indicate that: the absolute quantities of arachidonic acid metabolites produced by a given tumor varied between experiments but the overall distribution pattern of these products, in general, remained constant from passage to passage; each tumor showed a different arachidonic acid metabolite profile in quality and quantity; 2 sarcomas of the same histological type could be clearly distinguished by their arachidonic acid metabolites; the predominant product in all tumors was 12-HETE or 15-HETE, whereas thromboxane A2 was synthesized in low quantities by all tumors; PGF2 alpha was synthesized at the highest rate by an alveolar rhabdomyosarcoma; PGE2 synthesis was highest in a malignant fibrous histiocytoma; and total prostaglandin synthesis was low in the chondrosarcoma and synovial-cell sarcomas. All results reported in this study are for the complete tumor which includes both neoplastic and stromal cells. The role that these products play in the biological behavior of mesenchymal tumor cells and normal tissues of the host remains to be determined.

Fetuin modulates growth and differentiation of Ob17 preadipose cells in serum-free hormone-supplemented medium

A serum-free hormone-supplemented medium able to support the growth of rodent adipose precursor cells has been used to characterize additional components from serum required for the differentiation of preadipose Ob17 cells into adipose-like cells. Fetuin is shown to behave as a growth-promoting agent for these cells. In addition to growth hormone, triiodothyronine and a low-molecular weight component(s) also purified from serum, fetuin is required for the full expression of the differentiation program. Other serum proteins as well as other mitogenic factors are unable to substitute for fetuin. A possible role of fetuin in the development of adipose tissue is discussed.

ob17 cells transformed by the middle-T-only gene of polyoma virus differentiate in vitro and in vivo into adipose cells

Cell lines were derived from ob17 preadipocyte cells by focus formation after transfer of the complete early region of polyoma virus (ob17PY) or of a modified genome encoding only the middle T protein (ob17MT). Both ob17PY and ob17MT cell lines exhibited a high cloning efficiency in agarose medium containing 10% fetal bovine serum. Fully transformed ob17PY cells grew to high saturation densities and did not differentiate in vitro and in vivo. ob17MT cells and derived subclones did not grow in the absence of serum and were able to differentiate in vitro and to give rise in vivo to adipose tumors. Among these different clones an inverse relationship was observed in culture between their potentiality to overproliferate at low serum and their potentiality to convert into adipose cells. The expression of enzyme markers of adipose conversion was strictly dependent upon the presence of growth hormone. In addition, the hormonal requirements for differentiation were simpler than those of the original ob17 cells and the adipose conversion could take place in serum-free hormone-supplemented medium.

Growth of preadipocyte cell lines and cell strains from rodents in serum-free hormone-supplemented medium

Ob17 is a clonal cell line isolated from the epididymal fat pad of C57 BL/6J ob/ob mouse that differentiates into adiposelike cells in serum-supplemented medium. In serum-free medium, this cell line shows increased growth under the addition of insulin, transferrin, fibroblast growth factor (FGF), and a factor present in extract of rat submaxillary gland (SMGE). This medium is referred to as 4F. Epidermal growth factor or nerve growth factor cannot replace SMGE, whereas partially purified platelet extract can substitute for FGF but only partially for SMGE. 4F Medium is able to support the proliferation of cells from other established preadipocyte clonal lines, HGFu and 3T3-F442A, and also of preadipocyte cells isolated from the stromal-vascular fraction of rat and mouse adipose tissues. In each case 4F medium is insufficient to support the differentiation of these cells into adipocytes. Ob17 cells grown and maintained in serum-free hormone-supplemented medium retain the ability to convert to adiposelike cells after serum addition. This serum requirement for differentiation cannot be substituted by the addition of growth hormone or of other putative adipogenic factors, or both. The results are discussed with respect to the requirements for growth and differentiation of the 3T3-L1 and 1246 preadipocyte cell lines previously described.