Activation of protein kinase B and induction of adipogenesis by insulin in 3T3-L1 preadipocytes: contribution of phosphoinositide-3,4,5-trisphosphate versus phosphoinositide-3,4-bisphosphate

miR‑139‑5p affects cell proliferation, migration and adipogenesis by targeting insulin‑like growth factor 1 receptor in hemangioma stem cells

Infant hemangioma is the most common benign tumor in infancy. The pathological development process of this tumor is separated into the proliferation period, the involution period and the composite period in which a few residual capillary-like vessels grow through the loose fibrofatty tissue. Previous studies have confirmed that insulin-like growth factor 1 (IGF-1) is able to facilitate the cell proliferation of hemangioma stem cells (HemSCs) and the differentiation of HemSCs into adipocytes. Additionally, studies have confirmed that microRNAs (miRs) may serve a crucial function in regulating the IGF-1 receptor (IGF-1R). miR-139-5p often functions as a tumor suppressor. The present study was designed to investigate the mechanism of miR-139-5p in HemSCs. Dual luciferase reporter results verified that IGF-1R is the target gene of miR-139-5p. miR-139-5p overexpression reduced IGF-1R expression, and miR-139-5p inhibition increased IGF-1R expression. Cell Counting Kit-8 and Transwell migration assays demonstrated that miR-139-5p overexpression may target IGF-1R to inhibit the proliferation in addition to the migration of HemSCs. Reverse transcription-quantitative PCR, oil red o staining and western blot analysis confirmed that miR-139-5p overexpression was able to reduce adipogen-esis in HemSCs via the IGF-1/IGF-1R pathway. In contrary, miR-139-5p inhibition substantially enhanced the proliferation, migration and adipogenesis of HemSCs. Overall, miR-139-5p is able to affect the IGF-1/IGF-1R pathway by regulating IGF-1R expression, which ultimately affects the proliferation, migration and adipogenesis of HemSCs.

miR-148a-3p promotes rabbit preadipocyte differentiation by targeting PTEN

Although emerging data support crucial roles for microRNAs (miRNAs) during adipogenesis, the detailed mechanisms remain largely unknown. In this study, it was shown that in rabbits, levels of miR-148a-3p not only increased in white adipose tissue during early stages of growth but also during in vitro cultured preadipocyte differentiation. Furthermore, overexpression of miR-148a-3p significantly upregulated the mRNA levels of PPARγ, C/EBPα, and FABP4, as well as the protein levels of PPARγ, as indicated by qPCR and western blotting analyses. Overexpression of miR-148a-3p also promoted intracellular triglyceride accumulation. In contrast, downregulation of miR-148a-3p inhibited the differentiation of rabbit preadipocytes. Next, based on target gene prediction and a luciferase reporter assay, we further demonstrated that miR-148a-3p directly targeted one of the 3' untranslated regions of PTEN. Finally, it was observed inhibition of PTEN by siRNA promoted rabbit preadipocyte differentiation. Taken together, our results suggested that miR-148a-3p could be involved in regulating rabbit preadipocyte differentiation through inhibiting expression of PTEN, which further highlighted the importance of miRNAs during adipogenesis.

miR-26b Promotes 3T3-L1 Adipocyte Differentiation Through Targeting PTEN

microRNAs (miRNAs) play important roles in adipogenesis that is closely linked to obesity and energy homeostasis. Thus far, only a few miRNAs have been identified to regulate adipocyte development, arousing interest in the detailed function of miRNAs during adipogenesis. In this study, we found that the miR-26b expression showed an increasing trend during 3T3-L1 cells differentiation. To investigate the role of miR-26b in adipogenesis, the synthetic miR-26b agomirs and antagomirs were used to perform overexpression and knockdown experiment, respectively. Our data revealed that overexpression of miR-26b significantly accelerated the mRNA expression of the adipogenic markers, peroxisome proliferator-activated receptor gamma (PPARγ), fatty acid synthase (FAS), CCAAT/enhancer binding protein alpha (C/EBPα), and lipoprotein lipase, and the protein level of PPARγ and FAS. miR-26b overexpression also resulted in a significant increase in lipid accumulation. In contrast, inhibition of miR-26b expression decreased differentiation of 3T3-L1 cells. By target gene prediction and luciferase reporter assay, we demonstrated that miR-26b may directly bind to the 3' UTR of phosphatase and tensin homolog (PTEN). Taken together, these results demonstrate that miR-26b might participate in regulating adipogenic differentiation in 3T3-L1 cells by inhibiting the PTEN expression, further highlighting the importance of miRNA in adipogenesis.

Nuclear phosphoproteome analysis of 3T3-L1 preadipocyte differentiation reveals system-wide phosphorylation of transcriptional regulators

Adipocytes (fat cells) are important endocrine and metabolic cells critical for systemic insulin sensitivity. Both adipose excess and insufficiency are associated with adverse metabolic function. Adipogenesis is the process whereby preadipocyte precursor cells differentiate into lipid-laden mature adipocytes. This process is driven by a network of transcriptional regulators (TRs). We hypothesized that protein PTMs, in particular phosphorylation, play a major role in activating and propagating signals within TR networks upon induction of adipogenesis by extracellular stimulus. We applied MS-based quantitative proteomics and phosphoproteomics to monitor the alteration of nuclear proteins during the early stages (4 h) of preadipocyte differentiation. We identified a total of 4072 proteins including 2434 phosphorylated proteins, a majority of which were assigned as regulators of gene expression. Our results demonstrate that adipogenic stimuli increase the nuclear abundance and/or the phosphorylation levels of proteins involved in gene expression, cell organization, and oxidation-reduction pathways. Furthermore, proteins acting as negative modulators involved in negative regulation of gene expression, insulin stimulated glucose uptake, and cytoskeletal organization showed a decrease in their nuclear abundance and/or phosphorylation levels during the first 4 h of adipogenesis. Among 288 identified TRs, 49 were regulated within 4 h of adipogenic stimulation including several known and many novel potential adipogenic regulators. We created a kinase-substrate database for 3T3-L1 preadipocytes by investigating the relationship between protein kinases and protein phosphorylation sites identified in our dataset. A majority of the putative protein kinases belong to the cyclin-dependent kinase family and the mitogen-activated protein kinase family including P38 and c-Jun N-terminal kinases, suggesting that these kinases act as orchestrators of early adipogenesis.

Original Research: Orexins A and B stimulate proliferation and differentiation of porcine preadipocytes

Orexin A (OXA) and B (OXB) are neuropeptides which regulate appetite, energy expenditure, and arousal via G-protein coupled receptors termed as OXR1 and OXR2. The aim of this study was to characterize the effects of OXA and OXB on proliferation and differentiation of porcine preadipocytes. Porcine preadipocytes express both OXRs. OXA and OXB enhance porcine preadipocyte proliferation by 54.8% or 63.2 %, respectively. OXA and OXB potentiate differentiation of porcine preadipocytes, as judged by the increased lipid accumulation and expression of proadipogenic genes. Cellular lipid content after exposure of preadipocytes for six days to 100 nM OXA or OXB increased by 82.2% or 59.2%, respectively. OXA and OXB suppressed glycerol release by 23.9% or 24.9% in preadipocytes differentiated for six days. OXA (100 nM) increased peroxisome proliferator-activated receptor gamma (PPARγ) expression in cells differentiated for 24 h by 100.5%. PPARγ expression was also stimulated in preadipocytes differentiated in the presence of 10 nM (58.3%) or 100 nM OXA (50.6%) for three days. OXB potentiated PPARγ mRNA expression at 1 nM (59%), 10 nM (53.2%), and 100 nM (73.9%) in cells differentiated for three days. OXA increased CCAAT/enhancer binding protein alpha expression in preadipocytes differentiated for six days by 65%. OXB stimulated CCAAT/enhancer binding protein beta expression in preadipocytes differentiated for three days at 10 nM (149.5%) as well as 100 nM (207.2%). Lipoprotein lipase mRNA expression increased in cells treated with 10 nM OXA by 152.6% and 100 nM OXA by 162%. Lipoprotein lipase expression increased by 134% at 100 nM OXB. Furthermore, OXA (100 nM) and OXB (100 nM) increased leptin mRNA expression in preadipocytes differentiated for three days by 49.9% or 71.3%, respectively. These data indicate that orexin receptors may be relevant in the context of white adipose tissue formation.

Inpp5e suppresses polycystic kidney disease via inhibition of PI3K/Akt-dependent mTORC1 signaling

Polycystic kidney disease (PKD) is a common cause of renal failure with few effective treatments. INPP5E is an inositol polyphosphate 5-phosphatase that dephosphorylates phosphoinositide 3-kinase (PI3K)-generated PI(3,4,5)P₃ and is mutated in ciliopathy syndromes. Germline Inpp5e deletion is embryonically lethal, attributed to cilia stability defects, and is associated with polycystic kidneys. However, the molecular mechanisms responsible for PKD development upon Inpp5e loss remain unknown. Here, we show conditional inactivation of Inpp5e in mouse kidney epithelium results in severe PKD and renal failure, associated with a partial reduction in cilia number and hyperactivation of PI3K/Akt and downstream mammalian target of rapamycin complex 1 (mTORC1) signaling. Treatment with an mTORC1 inhibitor improved kidney morphology and function, but did not affect cilia number or length. Therefore, we identify Inpp5e as an essential inhibitor of the PI3K/Akt/mTORC1 signaling axis in renal epithelial cells, and demonstrate a critical role for Inpp5e-dependent mTORC1 regulation in PKD suppression.

Cellular mechanisms of MR regulation of adipose tissue physiology and pathophysiology

In addition to the well-documented expression and activity of the mineralocorticoid receptor (MR) in the kidney, in the last decade research on MR has also revealed its important role in regulating functions of extrarenal tissues, including adipose tissue, where MR is involved in adipocyte fundamental processes such as differentiation, autophagy and adipokine secretion. MR expression is increased in adipose tissue of murine models of obesity and in obese human subjects, suggesting that over-activation of the mineralocorticoid signaling leads to dysfunctional adipocyte and associated metabolic disorders. Notably, pharmacological blockade of MR prevents metabolic dysfunctions observed in obese mice and suggests a potential therapeutic use of MR antagonists in the treatment of obesity and metabolic syndrome. However, the molecular pathways affected by MR blockade have been poorly investigated. This review summarizes the functions of MR in the adipocyte, discusses potential signaling pathways mediating MR action, and describes post-translational modifications regulating its activity.

KCNK10, a tandem pore domain potassium channel, is a regulator of mitotic clonal expansion during the early stage of adipocyte differentiation

KCNK10, a member of tandem pore domain potassium channel family, gives rise to leak K⁺ currents. It plays important roles in stabilizing the negative resting membrane potential and in counterbalancing depolarization. We previously demonstrated that kcnk10 expression is quickly elevated during the early stage of adipogenesis of 3T3-L1 cells and that reduction of kcnk10 expression inhibits adipocyte differentiation. However, the molecular mechanism of KCNK10 in adipocyte differentiation remains unclear. Here we revealed that kcnk10 is induced by 3-isobutyl-1-methylxanthine, a cyclic nucleotide phosphodiesterase inhibitor and a potent inducer of adipogenesis, during the early stage of adipocyte differentiation. We also demonstrated that KCNK10 functions as a positive regulator of mitotic clonal expansion (MCE), a necessary process for terminal differentiation. The reduction of kcnk10 expression repressed the expression levels of CCAAT/enhancer-binding protein β (C/EBPβ) and C/EBPδ as well as the phosphorylation level of Akt during the early phase of adipogenesis. In addition, knockdown of kcnk10 expression suppressed insulin-induced Akt phosphorylation. These results indicate that KCNK10 contributes to the regulation of MCE through the control of C/EBPβ and C/EBPδ expression and insulin signaling.

Adipose tissue insulin sensitivity and macrophage recruitment: Does PI3K pick the pathway?

In the United States, obesity is a burgeoning health crisis, with over 30% of adults and nearly 20% of children classified as obese. Insulin resistance, a common metabolic complication associated with obesity, significantly increases the risk of developing metabolic diseases such as hypertension, coronary heart disease, stroke, type 2 diabetes, and certain cancers. With the seminal finding that obese adipose tissue harbors cytokine secreting immune cells, obesity-related research over the past decade has focused on understanding adipocyte–macrophage crosstalk and its impact on systemic insulin sensitivity. Indeed, adipose tissue has emerged as a central mediator of obesity- and diet-induced insulin resistance. In this mini-review, we focus on a potential role of adipose tissue phosphoinositide 3-kinase (PI3K) as a point of convergence of cellular signaling pathways that integrates nutrient sensing and inflammatory signaling to regulate tissue insulin sensitivity.

Effects of orexin A on proliferation, survival, apoptosis and differentiation of 3T3-L1 preadipocytes into mature adipocytes

Metabolic activities of orexin A (OXA) in mature adipocytes are mediated via PI3K/PKB and PPARγ. However, the effects of OXA on preadipocytes are largely unknown. We report here that OXA stimulates the proliferation and viability of 3T3-L1 preadipocytes and protects them from apoptosis via ERK1/2, but not through PKB. OXA reduces proapoptotic activity of caspase-3 via ERK1/2. Inhibition of ERK1/2 prevents the differentiation of preadipocytes into adipocytes. Unlike insulin, neither short-term nor prolonged exposure of 3T3-L1 preadipocytes to OXA induces preadipocyte differentiation to adipocytes, despite increased ERK1/2 phosphorylation. Unlike insulin, OXA fails to activate PKB, which explains its inability to induce the differentiation of preadipocytes.

Piceatannol, natural polyphenolic stilbene, inhibits adipogenesis via modulation of mitotic clonal expansion and insulin receptor-dependent insulin signaling in early phase of differentiation

Piceatannol, a natural stilbene, is an analog and a metabolite of resveratrol. Despite a well documented health benefit of resveratrol in intervention of the development of obesity, the role of piceatannol in the development of adipose tissue and related diseases is unknown. Here, we sought to determine the function of piceatannol in adipogenesis and elucidate the underlying mechanism. We show that piceatannol inhibits adipogenesis of 3T3-L1 preadipocytes in a dose-dependent manner at noncytotoxic concentrations. This anti-adipogenic property of piceatannol was largely limited to the early event of adipogenesis. In the early phase of adipogenesis, piceatannol-treated preadipocytes displayed a delayed cell cycle entry into G(2)/M phase at 24 h after initiation of adipogenesis. Furthermore, the piceatannol-suppressed mitotic clonal expansion was accompanied by reduced activation of the insulin-signaling pathway. Piceatannol dose-dependently inhibited differentiation mixture-induced phosphorylation of insulin receptor (IR)/insulin receptor substrate-1 (IRS-1)/Akt pathway in the early phase of adipogenesis. Moreover, we showed that piceatannol is an inhibitor of IR kinase activity and phosphatidylinositol 3-kinase (PI3K). Our kinetics study of IR further identified a K(m) value for ATP of 57.8 μm and a K(i) value for piceatannol of 28.9 μm. We also showed that piceatannol directly binds to IR and inhibits IR kinase activity in a mixed noncompetitive manner to ATP, through which piceatannol appears to inhibit adipogenesis. Taken together, our study reveals an anti-adipogenic function of piceatannol and highlights IR and its downstream insulin signaling as novel targets for piceatannol in the early phase of adipogenesis.

Estrogen sulfotransferase inhibits adipocyte differentiation

The estrogen sulfotransferase (EST) is a phase II drug-metabolizing enzyme known to catalyze the sulfoconjugation of estrogens. EST is highly expressed in the white adipose tissue of male mice, but the role of EST in the development and function of adipocytes remains largely unknown. In this report, we showed that EST played an important role in adipocyte differentiation. EST was highly expressed in 3T3-L1 preadipocytes and primary mouse preadipocytes. The expression of EST was dramatically reduced in differentiated 3T3-L1 cells and mature primary adipocytes. Overexpression of EST in 3T3-L1 cells prevented adipocyte differentiation. In contrast, preadipocytes isolated from EST knockout (EST-/-) mice exhibited enhanced differentiation. The inhibitory effect of EST on adipogenesis likely resulted from the sustained activation of ERK1/2 MAPK and inhibition of insulin signaling, leading to a failure of switch from clonal expansion to differentiation. The enzymatic activity of EST was required for the inhibitory effect of EST on adipogenesis, because an enzyme-dead EST mutant failed to inhibit adipocyte differentiation. In vivo, overexpression of EST in the adipose tissue of female transgenic mice resulted in smaller adipocyte size. Taken together, our results suggest that EST functions as a negative regulator of adipogenesis.

CNP/GC-B system: a new regulator of adipogenesis

Adipogenesis is regulated by a wide variety of compounds. An adipogenic cocktail containing insulin (INS), dexamethasone (DEX) and 3-isobutyl-1-methyl xanthine (IBMX) is routinely used to induce adipogenesis in 3T3-L1 preadipocytes, but the biochemical actions in adipogenesis of IBMX, a non-specific phosphodiesterase inhibitor, are not completely understood. In this study we show that C-type natriuretic peptide (CNP) is an endogenous adipogenesis regulator which can largely replace the function of IBMX. In 3T3-L1 preadipocytes, CNP potently elevated cGMP production through guanylyl cyclase-B (GC-B). Lipid droplets were evident in these cells upon stimulation with CNP for 12 days in the presence of INS and DEX, and their adiposity, evaluated by Oil Red O, was significantly higher than in cells stimulated with INS and DEX only. Membrane-permeable cGMP analogue also enhanced adiposity when cells were cultured together with INS and DEX, and KT5823, a non-specific cGMP-dependent kinase (cGK) inhibitor, suppressed the stimulatory effect of IBMX on adipogenesis, revealing that IBMX-stimulated adipogenesis is mediated through cGK. The enhancement of adiposity elicited by CNP was accompanied by increased mRNA levels of adipocyte-specific genes including those encoding peroxisome proliferator-activated receptor gamma and glucose transporter 4. Interestingly, the mRNA level of CNP itself was markedly enhanced in 3T3-L1 cells upon stimulation with INS, DEX and IBMX, reaching a maximum at 8h incubation with the cocktail. These observations suggest that the CNP/GC-B system participates in regulation of adipogenesis, particularly at an early stage in the process.

The antiadipogenic effect of macrophage-conditioned medium depends on ERK1/2 activation

The proatherogenic state of obesity is associated with hypertrophied adipocytes that may arise because of deficient adipogenesis. Macrophages infiltrate adipose tissue as a function of obesity and may release factors that attenuate adipogenesis. Macrophage-conditioned medium inhibits human and 3T3-L1 adipocyte differentiation in culture, but underlying molecular mechanisms have yet to be defined. Exposure of 3T3-L1 cells throughout the 8-day period of differentiation to medium conditioned by THP-1 macrophages (THP-1-MacCM) blocked adipogenesis. Triacylglycerol (TG) accumulation and induction of peroxisome proliferator-activated receptor gamma and fatty acid synthase protein levels were inhibited by 59% (n = 4, P < .001), 29% (n = 4, P < .01), and 47% (n = 4, P < .01), respectively. THP-1-MacCM had no effect when added after the first 2 days of differentiation, indicating that early exposure of its targets must be needed to inhibit 3T3-L1 adipogenesis. Cell enumeration revealed a 44% decrease in clonal expansion compared with standard differentiation (n = 3, P < .01). Addition of THP-1-MacCM to 3T3-L1 preadipocytes increased ERK1/2 phosphorylation by 6.5-fold (n = 3, P < .01). PD98059 (an inhibitor of the ERK1/2 pathway) impaired the negative effect of THP-1-MacCM on TG accumulation, indicated by an inhibition of 25% vs 69% (n = 3, P < .001), without altering fatty acid synthase or peroxisome proliferator-activated receptor gamma levels. Our data implicate ERK1/2 as an important signaling mediator for the inhibitory effect of THP-1-MacCM on TG accumulation during 3T3-L1 adipogenesis.

Hyperglycemia enhances adipogenic induction of lipid accumulation: involvement of extracellular signal-regulated protein kinase 1/2, phosphoinositide 3-kinase/Akt, and peroxisome proliferator-activated receptor gamma signaling

The molecular events of hyperglycemia-triggered increase in adipogenic induction of lipid accumulation remain unclear. We examined the effects of hyperglycemia on adipogenic induction of lipid accumulation and its involved signaling molecules, such as phosphoinositide 3-kinase (PI3K), ERKs, and peroxisome proliferator-activated receptor gamma (PPAR gamma). Bone marrow-derived mesenchymal stem cells (MSCs) isolated from FVB/N mice were capable of differentiating into adipocytes in adipogenic medium. The effects of high glucose (HG) (25.5 mm) were assessed in vitro by RT-PCR, ELISA, flow cytometry, immunostaining, and immunoblotting. The in vivo effect of hyperglycemia was further studied in streptozotocin (STZ)-induced diabetic FVB/N mice. Exposure of MSCs to HG enhanced adipogenic induction of lipid accumulation as compared with 5.5 mm glucose. HG increased PPAR gamma expression and PI3K activity and its downstream effector Akt phosphorylation during adipogenesis. Inhibition of PI3K/Akt activity with PI3K inhibitor LY294002 or by expressing the dominant negative p85 or Akt prevented the HG-enhanced PPAR gamma-dependent adipogenic induction of lipid accumulation. Moreover, HG increased the phosphorylation of ERK1/2 during adipogenesis. MAPK/ERK inhibitor PD98059 inhibited the PI3K activity, Akt phosphorylation, and lipid accumulation triggered by HG. PI3K inhibitor LY294002 did not affect the HG-increased ERK1/2 phosphorylation during adipogenesis. We next observed that adipogenic induction of lipid accumulation of MSCs isolated from STZ-induced diabetic mice is enhanced. Moreover, triglyceride, PPAR gamma expression, phosphorylated Akt and ERK1/2, and marrow fat in bones of STZ-diabetic mice were also increased. These results suggest that hyperglycemia enhances the adipogenic induction of lipid accumulation through an ERK1/2-activated PI3K/Akt-regulated PPAR gamma pathway.

TRB3 blocks adipocyte differentiation through the inhibition of C/EBPbeta transcriptional activity

TRB3 has been implicated in the regulation of several biological processes in mammalian cells through its ability to influence Akt and other signaling pathways. In this study, we investigated the role of TRB3 in regulating adipogenesis and the activity of adipogenic transcription factors. We find that TRB3 is expressed in 3T3-L1 preadipocytes, and this expression is transiently suppressed during the initial days of differentiation concomitant with induction of C/EBPbeta. This event appears to be a prerequisite for adipogenesis. Overexpression of TRB3 blocks differentiation of 3T3-L1 cells at a step downstream of C/EBPbeta. Ectopic expression of TRB3 in mouse fibroblasts also inhibits the C/EBPbeta-dependent induction of PPARgamma2 and blocks their differentiation into adipocytes. This inhibition of preadipocyte differentiation by TRB3 appears to be the result of two complementary effects. First, TRB3 inhibits extracellular signal-regulated kinase activity, which prevents the phosphorylation of regulatory sites on C/EBPbeta. Second, TRB3 directly interacts with the DR1 domain of C/EBPbeta in the nucleus, further inhibiting both its ability to bind its response element and its ability to transactivate the C/EBPalpha and a-FABP promoters. Thus, TRB3 is an important negative regulator of adipogenesis that acts at an early step in the differentiation cascade to block the C/EBPbeta proadipogenic function.

Role of epidermal growth factor and ErbB2 receptors in 3T3-L1 adipogenesis

OBJECTIVE

Epidermal growth factor (EGF) stimulates proliferation in 3T3-L1 preadipocytes, but EGF action in differentiation is less clear. EGF promotes differentiation at concentrations <1 nM but inhibits differentiation at higher concentrations, suggesting a dual role in adipogenesis. We hypothesized that differences in EGF receptor activation and downstream signaling mediate distinct biological effects of EGF at low vs. high abundance.

RESEARCH METHODS AND PROCEDURES

We compared the effects of low (0.1 nM) vs. high (10 nM) EGF on the activation of EGF receptors, proximal signaling molecules Src and Shc, and the downstream mitogen-activated protein kinase (MAPK) pathways extracellular regulated kinase (ERK) and p38 in proliferating and differentiated 3T3-L1 cells.

RESULTS

Both low and high EGF activated ERK and p38 in preadipocytes. Src inhibitors PP1 and PP2 blocked ERK and p38 activation by low but not high EGF, and only high EGF increased Shc phosphorylation. Selective inhibition of the EGF receptor (EGFR) with AG1478 blocked ERK and p38 activation at both concentrations; however, selective inhibition of the ErbB2 receptor (EB2R) with AG825 or small interfering RNA (siRNA) blocked low but not high EGF activation of ERK and p38. Coimmunoprecipitation of EGFR with EB2R and Src was observed with low EGF in preadipocytes but at both concentrations in adipocytes. EB2R inhibition during differentiation decreased p38 activity and peroxisome proliferator-activated receptor gamma (PPARgamma) abundance.

DISCUSSION

Our results show that EGFR homodimers mediate action of EGF at high abundance, but at low abundance, EGF promotes differentiation through EGFR/EB2R heterodimer activation of Src and p38. These results may partially explain the observations that high EGF concentrations inhibit, whereas low concentrations support, preadipocyte differentiation.

Regulation of PDGF-stimulated SHIP2 tyrosine phosphorylation and association with Shc in 3T3-L1 preadipocytes

In 3T3-L1 and human preadipocytes, insulin results in the isolated rise in phosphatidylinositol (PI)-3,4,5-P3, whereas PDGF produces PI(3,4)P2 in addition to PI(3,4,5)P3. SH2 domain-containing inositol 5-phosphatase 2 (SHIP2) converts PI(3,4,5)P3 into PI(3,4)P2. PDGF, but not insulin, stimulates SHIP2 tyrosine phosphorylation and its association with Shc in human and 3T3-L1 preadipocytes. We now demonstrate that SHIP2 tyrosine phosphorylation and association with Shc in PDGF-treated 3T3-L1 preadipocytes was reduced by bisindolylmaleimide I (BisI), an inhibitor of conventional/novel protein kinase C (PKC). However, the production of PI(3,4)P2 and PI(3,4,5)P3 by PDGF was unaffected by BisI. Activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) was not sufficient to induce SHIP2 tyrosine phosphorylation. Furthermore, we identified threonine 958 (T958) as a novel PDGF-responsive SHIP2 phosphorylation site. Mutation of T958 to alanine reduced PDGF-stimulated SHIP2 tyrosine phosphorylation and association with Shc, but did not alter its anti-proliferative effect on preadipocytes. This study demonstrates that SHIP2 tyrosine phosphorylation and Shc association can be regulated by serine/threonine signaling pathways, either indirectly (via PKC), or directly (via T958). Interestingly, the anti-proliferative effect of SHIP2 T958A, as well as another SHIP2 mutant (Y986F, Y987F) that also displays defective tyrosine phosphorylation and Shc association, does not depend on these molecular events.

Isoform-specific requirement for Akt1 in the developmental regulation of cellular metabolism during lactation

The metabolic demands and synthetic capacity of the lactating mammary gland exceed that of any other tissue, thereby providing a useful paradigm for understanding the developmental regulation of cellular metabolism. By evaluating mice bearing targeted deletions in Akt1 or Akt2, we demonstrate that Akt1 is specifically required for lactating mice to synthesize sufficient quantities of milk to support their offspring. Whereas cellular proliferation, differentiation, and apoptosis are unaffected, loss of Akt1 disrupts the coordinate regulation of metabolic pathways that normally occurs at the onset of lactation. This results in a failure to upregulate glucose uptake, Glut1 surface localization, lipid synthesis, and multiple lipogenic enzymes, as well as a failure to downregulate lipid catabolic enzymes. These findings demonstrate that Akt1 is required in an isoform-specific manner for orchestrating many of the developmental changes in cellular metabolism that occur at the onset of lactation and establish a role for Akt1 in glucose metabolism.

Depletion of cAMP-response Element-binding Protein/ATF1 Inhibits Adipogenic Conversion of 3T3-L1 Cells Ectopically Expressing CCAAT/Enhancer-binding Protein (C/EBP) α, C/EBP β, or PPARγ2

The differentiation of preadipocytes to adipocytes is orchestrated by the expression of the "master adipogenic regulators," CCAAT/enhancer-binding protein (C/EBP) beta, peroxisome proliferator-activated receptor gamma (PPARgamma), and C/EBP alpha. In addition, activation of the cAMP-response element-binding protein (CREB) is necessary and sufficient to promote adipogenic conversion and prevent apoptosis of mature adipocytes. In this report we used small interfering RNA to deplete CREB and the closely related factor ATF1 to explore the ability of the master adipogenic regulators to promote adipogenesis in the absence of CREB and probe the function of CREB in late stages of adipogenesis. Loss of CREB/ATF1 blocked adipogenic conversion of 3T3-L1 cells in culture or 3T3-F442A cells implanted into athymic mice. Loss of CREB/ATF1 prevented the expression of PPARgamma, C/EBP alpha, and adiponectin and inhibited the loss of Pref-1. Loss of CREB/ATF1 inhibited adipogenic conversion even in cells ectopically expressing C/EBP alpha, C/EBP beta, or PPARgamma2 individually. CREB/ATF1 depletion did not attenuate lipid accumulation in cells expressing both PPARgamma2 and C/EBP alpha, but adiponectin expression was severely diminished. Conversely ectopic expression of constitutively active CREB overcame the blockade of adipogenesis due to depletion of C/EBP beta but not due to loss of PPARgamma2 or C/EBP alpha. Depletion of CREB/ATF1 did not suppress the expression of C/EBP beta as we had previously observed using dominant negative forms of CREB. Finally results are presented showing that CREB promotes PPARgamma2 gene transcription. The results indicate that CREB and ATF1 play a central role in adipogenesis because expression of individual master adipogenic regulators is unable to compensate for their loss. The data also indicate that CREB not only functions during the initiation of adipogenic conversion but also at later stages.

Generation and characterization of monoclonal antibodies against FABP4

Fatty acid binding protein 4 (FABP4) is a key mediator of intracellular transport and metabolism of fatty acids in adipose tissues. FABP4 binds fatty acids with high affinity and transports them to various compartments in the cell. When in complex with fatty acids, FABP4 interacts with and modulates the activity of two important regulators of metabolism: hormone-sensitive lipase and peroxisome proliferator-activated receptor gamma. Genetic studies in mice clearly indicated that deregulation of FABP4 function may lead to the development of severe diseases such as diabetes II type and atherosclerosis. In this study, we report the production and detailed characterization of monoclonal antibodies (MAbs) against FABP4. Recombinant glutathione S-transferase (GST)-FABP4 or His-FABP4 was expressed in bacteria, affinity purified, and used for immunization of mice, enzyme-linked immunosorbent assay (ELISA) screening, and characterization of selected clones. We have isolated two hybridoma clones that produced antibodies specific for recombinant and native FABP4, as shown by Western blotting and immunoprecipitation. The specificity of generated antibodies was further tested in a cell-based model of adipogenesis. In this analysis, the accumulation of FABP4 during NIH 3T3-L1 differentiation into adipocytes was detected by generated antibodies, which correlates well with previously published data. Taken together, we produced MAbs that will be useful for the scientific community working on fatty acid-binding proteins and lipid metabolism.

Macrophage-conditioned medium inhibits the differentiation of 3T3-L1 and human abdominal preadipocytes

AIMS/HYPOTHESIS

In obesity, a limited adipogenic capacity may promote adipocyte hypertrophy and increase the risk of insulin resistance and type 2 diabetes. Recent data indicate that macrophages reside within adipose tissue in obese rodents and humans. We hypothesised that secreted macrophage factors may inhibit adipogenesis.

MATERIALS AND METHODS

Conditioned media from cultured murine J774 or human THP-1 macrophages were collected, and added to either murine 3T3-L1 preadipocytes or human abdominal stromal preadipocytes from subcutaneous or omental fat depots.

RESULTS

Macrophage-conditioned medium (MacCM) strongly inhibited 3T3-L1 adipogenesis. Dose-response studies with J774-MacCM revealed that 80 and 100% of J774-MacCM completely suppressed triacylglycerol accumulation as well as the induction of fatty acid synthase, peroxisome proliferator-activated receptor gamma, CCAAT/enhancer binding protein alpha, and adiponectin. Similar inhibitory effects on 3T3-L1 preadipocytes were observed with THP-1-MacCM. Differentiation of human abdominal subcutaneous stromal preadipocytes was moderately reduced (subcutaneous>omental) by J744-MacCM. In contrast, the differentiation of both subcutaneous and omental stromal preadipocytes was completely inhibited by THP-1-MacCM, as determined on the basis of morphology and triacylglycerol accumulation, as well as fatty acid synthase and adiponectin protein expression.

CONCLUSIONS/INTERPRETATION

Secreted macrophage products inhibit the differentiation of 3T3-L1 preadipocytes as well as human abdominal stromal preadipocytes.

Lipid products of phosphoinositide 3-kinase abrogate genistein-induced fusion inhibition in myoblasts

Genistein (4',5,7-trihydroxyisoflavone) is a tyrosine kinase inhibitor. Although the agent has shown to inhibit myoblast differentiation, neither intracellular target(s) as a tyrosine kinase inhibitor nor action mechanism of the agent is well known. Here we studied the effect of genistein on the differentiation of myoblasts. Genistein strongly but reversibly blocked both myoblast fusion and synthesis of the muscle-specific proteins. The agent also reversibly reduced the phosphorylation level of focal adhesion kinase (FAK), a cytoplasmic tyrosine kinase, and its interaction with p85, the regulatory subunit of phosphoinositide 3-kinase (PI3-kinase). In addition, genistein indirectly inhibited PI3-kinase activity and blocked calcium influx which is required for myoblast fusion. However, both genistein-induced inhibition of cell fusion and calcium influx were abrogated by the lipid products of PI3-kinase. These results demonstrate that genistein can exert their effect on the signaling pathway from FAK to calcium influx via PI3-kinase in the differentiation of myoblasts.

Anti-adipogenic effect of PDGF is reversed by PKC inhibition

Healthy adipose tissue function depends on adipogenesis. The capacity to form new adipocytes prevents the emergence of insulin-resistant hypertrophied adipocytes, as well as the deleterious lipid deposition in muscle, liver, and pancreas. It is therefore important to understand how adipogenesis is modulated. Platelet-derived growth factor (PDGF) is anti-adipogenic, but the stage of differentiation that it targets, and the signaling pathways that it triggers, are not defined. We have studied the inhibitory effect of PDGF on murine 3T3-L1 preadipocyte and human preadipocyte differentiation. There was a significant attenuation in the protein expression of the adipogenic transcription factors, PPARgamma and C/EBPalpha, as well as in the levels of later differentiation markers, including adiponectin, aP2, and fatty acid synthase. PDGF treatment resulted in the persistence of PDGF receptor and PKCalpha expression, in contrast to the expected downregulation of both proteins that occurs during differentiation. Inactivation of conventional PKC isoforms, by bisindolylmaleimide I or PKC pseudosubstrate M20-28, partially reversed the inhibition of 3T3-L1 and human preadipocyte differentiation by PDGF, as assessed by fatty acid synthase expression and morphological appearance.

Phosphoinositide 3-kinase is required for human adipocyte differentiation in culture

OBJECTIVE

Phosphoinositide 3-kinase (PI3K) is required for murine adipocyte differentiation. However, a recent report concluded that PI3K was not involved in the differentiation of human preadipocytes into adipocytes. We have re-examined the role of PI3K in human preadipocyte differentiation, enrolling more patients and using more adipogenic indices.

METHODS

Human preadipocytes, isolated from nine patients, were induced to differentiate in the presence or absence of 100 nM wortmannin. After 12-15 days, triacylglycerol accumulation and the expression of adipogenic markers (fatty acid synthase and adiponectin) were measured.

RESULTS

A significant inhibition in triacylglycerol accumulation and in the induction of fatty acid synthase protein expression was observed, but there was no effect on adiponectin protein expression.

CONCLUSION

Inhibition of PI3K reduces the differentiation of human preadipocytes into adipocytes, suggesting a role for this enzyme in the human adipogenic process.

Activation of the mammalian target of rapamycin pathway acutely inhibits insulin signaling to Akt and glucose transport in 3T3-L1 and human adipocytes

The mammalian target of rapamycin (mTOR) pathway has recently emerged as a chronic modulator of insulin-mediated glucose metabolism. In this study, we evaluated the involvement of this pathway in the acute regulation of insulin action in both 3T3-L1 and human adipocytes. Insulin rapidly (t(1/2) = 5 min) stimulated the mTOR pathway, as reflected by a 10-fold stimulation of 70-kDa ribosomal S6 kinase 1 (S6K1) activity in 3T3-L1 adipocytes. Inhibition of mTOR/S6K1 by rapamycin increased insulin-stimulated glucose transport by as much as 45% in 3T3-L1 adipocytes. Activation of mTOR/S6K1 by insulin was associated with a rapamycin-sensitive increase in Ser636/639 phosphorylation of insulin receptor substrate (IRS)-1 but, surprisingly, did not result in impaired IRS-1-associated phosphatidylinositol (PI) 3-kinase activity. However, insulin-induced activation of Akt was increased by rapamycin. Insulin also activated S6K1 and increased phosphorylation of IRS-1 on Ser636/639 in human adipocytes. As in murine cells, rapamycin treatment of human adipocytes inhibited S6K1, blunted Ser636/639 phosphorylation of IRS-1, leading to increased Akt activation and glucose uptake by insulin. Further studies in 3T3-L1 adipocytes revealed that rapamycin prevented the relocalization of IRS-1 from the low-density membranes to the cytosol in response to insulin. Furthermore, inhibition of mTOR markedly potentiated the ability of insulin to increase PI 3,4,5-triphosphate levels concomitantly with an increased phosphorylation of Akt at the plasma membrane, low-density membranes, and cytosol. However, neither GLUT4 nor GLUT1 translocation induced by insulin were increased by rapamycin treatment. Taken together, these results indicate that the mTOR pathway is an important modulator of the signals involved in the acute regulation of insulin-stimulated glucose transport in 3T3-L1 and human adipocytes.

Galectin-12 Is Required for Adipogenic Signaling and Adipocyte Differentiation

Galectin-12 is a member of the galectin family consisting of beta-galactoside-binding proteins with conserved carbohydrate recognition domains. This protein is preferentially expressed in peripheral blood leukocytes and adipocytes. We previously showed that galectin-12 is induced by cell cycle block at the G(1) phase and causes G(1) arrest when overexpressed (Yang, R.-Y., Hsu, D. K., Yu, L., Ni, J., and Liu, F.-T. (2001) J. Biol. Chem. 276, 20252-20260). Here, we show that the galectin-12 gene is expressed in mouse preadipocytes and is up-regulated when preadipocytes undergo cell cycle arrest, concomitant with acquisition of the competence to undergo differentiation in response to adipogenic hormone stimulation. Following a brief down-regulation 1 day after adipogenic treatment, its expression was once again markedly elevated when cells underwent terminal differentiation. Down-regulation of endogenous galectin-12 expression by RNA interference greatly reduced the expression of the adipogenic transcription factors CCAAT/enhancer-binding protein-beta and -alpha and peroxisome proliferator-activated receptor-gamma and severely suppressed adipocyte differentiation as a result of defective adipogenic signaling. We conclude that galectin-12 is required for signal transduction that conveys hormone stimulation to the induction of adipogenic factors essential for adipocyte differentiation. The findings suggest that galectin-12 is a major regulator of adipose tissue development.

Protein kinase B/AKT 1 plays a pivotal role in insulin-like growth factor-1 receptor signaling induced 3T3-L1 adipocyte differentiation

During 3T3-L1 preadipocyte differentiation induction, the insulin-stimulated insulin-like growth factor-1 (IGF-1) receptor signal is responsible for the induction of adipocyte differentiation. Treatment with inhibitors of phosphatidylinositol 3-kinase, LY294002 or wortmannin, leads to the complete blockade of adipocyte differentiation in 3T3-L1 preadipocytes. Of the three factors (1-methyl-3-isobutylxanthine, dexamethasone, and insulin) inducing 3T3-L1 preadipocyte differentiation, only insulin was able to activate the phosphatidylinositol 3-kinase-protein kinase B/Akt signal cascade. In 3T3-L1 preadipocytes, protein kinase B/Akt 1 RNA interference not only suppressed the expression of protein kinase B/Akt 1 but also blocked hormone-induced adipocyte differentiation. In these protein kinase B/Akt 1 RNA interference cells, the signal molecules upstream of protein kinase B/Akt 1, such as IGF-1 receptor and insulin receptor substrate-1, were normally activated by insulin stimulation, whereas insulin-stimulated phosphorylation of forkhead transcription factor (FKHR), which is a downstream molecule of PKB/Akt 1, was blocked. Thus, protein kinase B/Akt 1 is an important signal mediator in IGF-1 receptor signal cascade for inducing adipocyte differentiation.

Inhibition of insulin signaling and adipogenesis by rapamycin: effect on phosphorylation of p70 S6 kinase vs eIF4E-BP1

OBJECTIVE

Insulin-responsive adipogenic signaling molecules include insulin receptor substrates (IRS)-1 and -2, phosphoinositide 3-kinase (PI3K), and protein kinase B (PKB; also known as Akt). Mammalian target of rapamycin (mTOR) is a PKB substrate, and regulates p70 S6 kinase (p70 S6K). Since p70 S6K is an insulin-responsive kinase downstream of PI3K and PKB, its potential role in adipogenic insulin signaling was investigated.

DESIGN

We measured the effect of rapamycin, a specific inhibitor of mTOR, on insulin-induced 3T3-L1 adipogenesis and on insulin-stimulated p70 S6K activation.

RESULTS

Rapamycin partially reduced differentiation, measured by Oil Red O staining, triacylglycerol accumulation (by up to 46%), and peroxisome proliferator-activated receptor gamma protein expression (by 50%). In contrast, rapamycin completely inhibited insulin-stimulated p70 S6K activation, assessed by phosphorylation of p70 S6K and its substrate, S6. Expression of a constitutively activated form of p70 S6K did not promote 3T3-L1 adipogenesis. The considerable residual differentiation in the presence of rapamycin, despite the complete blockade of p70 S6K activation, prompted us to measure the phosphorylation of another rapamycin-sensitive protein, eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1). Insulin-stimulated 4E-BP1 phosphorylation in 3T3-L1 preadipocytes was only partially affected by rapamycin, consistent with the differentiation data. Phosphorylation of eIF4E itself, an expected consequence of 4E-BP1 phosphorylation, was also only partially inhibited.

CONCLUSION

Our data suggest that adipogenic mTOR signaling occurs via the 4E-BP1/eIF4E pathway, rather than through p70 S6K.

Differential roles of insulin receptor substrates in brown adipocyte differentiation

Insulin promotes adipocyte differentiation via a complex signaling network involving multiple insulin receptor substrates (IRSs). In cultured brown preadipocytes, expression of IRS-1 and IRS-2 mRNAs and proteins was at relatively high levels before and after differentiation into mature fat cells, while IRS-3 transcript was not detectable in preadipocytes but increased during the course of differentiation, and IRS-4 mRNA was barely detected in both states. To determine more precisely the roles of various IRS proteins in adipogenesis, we established and characterized brown preadipocyte cell lines from wild-type and IRS knockout (KO) animals. While wild-type, IRS-2 KO, and IRS-4 KO cells fully differentiated into mature adipocytes, IRS-3 KO cells showed a moderate defect in differentiation and IRS-1 KO cells exhibited a severe defect in the process. Cells lacking both IRS-1 and IRS-3 completely failed to differentiate. Expression of the adipogenic markers peroxisome proliferator-activated receptor gamma (PPARgamma), CCAAT/enhancer-binding protein alpha, fatty acid synthase, glucose transporter 4, and the transcription factor signal transducer and activator of transcription 5, as well as the brown-fat-specific markers PPARgamma coactivator 1 alpha and uncoupling protein 1, mirrored the differentiation pattern. Reconstitution of the IRS-1 KO cells with IRS-1 and IRS-4, but not IRS-2 or IRS-3, compensated for the lack of differentiation in IRS-1 KO cells. A chimeric molecule containing the N terminus of IRS-1 and the C terminus of IRS-2, but not one with the N terminus of IRS-2 and the C terminus of IRS-1, also rescued differentiation. Expression of Wnt 10a, a molecule known to inhibit adipogenesis, was dramatically increased in the IRS-1 KO cells, and this could be reduced by overexpression of IRS-1 or IRS-4, which was correlated with restoration of differentiation. These data indicate that both IRS-1 and -3 play important roles in the differentiation of brown adipocytes and that the N terminus of IRS-1 is more important for this function of the molecule. Although IRS-4 is not essential for the process, overexpression of IRS-4 can compensate for the deficiency in differentiation in IRS-1 KO cells.

Insulin regulates hepatic apolipoprotein B production independent of the mass or activity of Akt1/PKBalpha

Insulin is known to be a downregulator of apolipoprotein B (apoB) via the phosphatidylinositol 3-kinase (PI3K) pathway. Akt, also known as protein kinase B (PKB), is a serine/threonine kinase downstream target of PI3K. Recent studies in the fructose-fed hamster model of insulin resistance have shown that hepatic very-low-density lipoprotein (VLDL) secretion is associated with reduced phosphorylation of Akt, suggesting a potential link between Akt expression and/or activity and apoB production in hepatocytes. We hypothesized that overexpression of Akt1 downregulates apoB production. An expression vector with a constitutively active form of Akt1 was transfected in the rat hepatoma McArdle cells (McA RH-7777), McA cells stably expressing human apoB-15 and apoB-48 (15% and 48% of total apoB length), and human hepatoma HepG2. The overexpressed Akt1 was phosphorylated at Ser473 independent of acute insulin stimulation, suggesting that it was catalytically active. Despite dosage-dependent overexpression of Akt1 in both McA and HepG2 cells, neither intracellular nor secreted protein mass of intact apoB or transfected human apoB-15/apoB-48 was significantly affected by high intracellular levels of Akt1. Radiolabeling experiments also yielded no difference in the amount of newly synthesized apoB when comparing transfected and mock-transfected cells. Transfection in conjunction with high-dose insulin did not significantly decrease the secretion of either apoB-100 or apoB-48 in McA cells, or apoB-100 in HepG2 cells. HepG2 cells were more sensitive to the inhibitory effects of insulin on apoB secretion compared to McA cells, but neither model responded to Akt1. Overall, the data suggest that acute insulin-mediated inhibition of apoB may not be mediated by Akt1 and that insulin signaling molecules upstream of Akt1 may be more important in mediating control of apoB secretion.

Regulation of endogenous SH2 domain-containing inositol 5-phosphatase (SHIP2) in 3T3-L1 and human preadipocytes

The role of the inositol lipid 5-phosphatase (SHIP2) in preadipocyte signaling is not known. Although overexpression of SHIP2 inhibited proliferation and (3)H-thymidine incorporation in 3T3-L1 preadipocytes, there was no effect on insulin-induced adipogenesis. Insulin promoted SHIP2 tyrosine phosphorylation in differentiated 3T3-L1 adipocytes, but did not do so in preadipocytes. The absence of SHIP2 tyrosine phosphorylation suggests a potential explanation for the isolated rise in PI(3,4,5)P3, without any changes in PI(3,4)P2, previously observed following insulin treatment of these cells. Lack of SHIP2 tyrosine phosphorylation by insulin was also observed in primary cultures of human abdominal subcutaneous preadipocytes. These cells also produced PI(3,4,5)P3, but not PI(3,4)P2, in response to insulin. Comparison of insulin vs. PDGF treatment on SHIP2 tyrosine phosphorylation in 3T3-L1 and human preadipocytes revealed that only PDGF, which stimulates the accumulation of PI(3,4,5)P3 as well as PI(3,4)P2, was active in this regard, and only PDGF promoted the association of 52 kDa form of Shc with SHIP2. Nevertheless, both insulin and PDGF were equally effective in translocating SHIP2 to the plasma membrane in 3T3-L1 preadipocytes. Lack of SHIP2 tyrosine phosphorylation may account for the insulin-specific inositol phospholipid pattern of accumulation in preadipocytes.

Apoptosis of human abdominal preadipocytes before and after differentiation into adipocytes in culture

Differentiation of murine 3T3-L1 preadipocytes into adipocytes is associated with the acquisition of apoptotic resistance accompanied by the upregulation of cell survival genes. We have now examined the effect of adipogenesis on apoptotic susceptibility of human abdominal preadipocytes in primary culture. To induce apoptosis, human preadipocytes, or their differentiated counterparts, were serum-deprived for 24 or 48 hours. When indicated, ceramide was also used as an apoptotic trigger. Cell death was assessed by enumeration of adherent viable cells, and its apoptotic nature was verified by Hoechst staining and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL). After 48 hours of serum withdrawal, cell death was 26% +/- 4% in preadipocytes and was increased to 41% +/- 4% in differentiated adipocytes (mean +/- SE; n = 7 patients; P <.002). Under serum-free conditions for 24 hours, ceramide-induced cell death was 40% +/- 6% in preadipocytes and increased to 68% +/- 8% in adipocytes (mean +/- SE; P <.01; n = 8 patients). Neuronal apoptosis inhibitor protein (NAIP), an antiapoptotic protein cell survival that increases upon 3T3-L1 adipogenesis, was reduced in human preadipocytes undergoing differentiation (n = 6 patients). Preadipocytes derived from omental versus subcutaneous abdominal fat were more susceptible to apoptosis induced by serum deprivation, 16% +/- 4% versus 31% +/- 3% cell death, respectively (mean +/- SE; P <.02; n = 7 patients). Although the murine 3T3-L1 preadipocyte cell line is a useful model that approximates primary preadipocyte cell biology, our data derived from human preadipocyte studies suggest important differences with respect to the regulation of apoptosis.

Down-regulation of aortic carboxypeptidase-like protein during the early phase of 3T3-L1 adipogenesis

Aortic carboxypeptidase-like protein (ACLP) is a 175-kDa protein that is expressed in vascular smooth muscle cells and contains a signal peptide sequence, a lysine- and proline-rich repeating motif, a discoidin-like domain with 35% identity to discoidin I, and a carboxypeptidase-like domain that is 39% identical with carboxypeptidase E. It is secreted into the extracellular matrix and may play a role in abdominal wall development and dermal wound healing. ACLP is also expressed in adipose tissue, but at lower levels. In this study we demonstrate that ACLP protein and mRNA are severely down-regulated in the early phase of 3T3-L1 preadipocyte differentiation induced by insulin, dexamethasone, and isobutylmethylxanthine. Neither dexamethasone, isobutylmethylxanthine, nor insulin treatment alone reduced the level of ACLP protein, suggesting that ACLP down-regulation is a differentiation-associated event. ACLP down-regulation coincided with the onset of the postconfluent mitotic clonal expansion phase of adipogenesis. In contrast, subconfluent 3T3-L1 cell proliferation did not alter ACLP expression, suggesting a specific linkage between ACLP and differentiation-induced clonal expansion. Stable overexpression of ACLP had no effect on preadipocyte differentiation assessed by triacylglycerol accumulation and peroxisome proliferator-activated receptor-gamma levels. The role of ACLP and its marked reduction during adipogenesis merit further study.

Inhibition of cAMP-response element-binding protein activity decreases protein kinase B/Akt expression in 3T3-L1 adipocytes and induces apoptosis

White adipose tissue mass is governed by competing processes that control lipid synthesis and storage, the development of new adipocytes, and their survival. We have shown that the transcription factor cAMP-response element-binding protein (CREB) participates in adipogenesis, with constitutively active forms of CREB inducing adipocyte differentiation and dominant negative forms of CREB blocking this process. In other cell types, CREB and related factors have been shown to play important roles in survival and apoptosis. Here we demonstrate that reduction of CREB activity by ectopic expression of the dominant negative CREB, KCREB, induces apoptosis of mature 3T3-L1 adipocytes in culture. Death by apoptosis was confirmed by increased nuclear condensation, changes in membrane morphology, and increased DNA fragmentation. Gene microarray analysis indicated that KCREB expression increased expression of several pro-apoptotic genes like Interleukin Converting Enzyme and decreased the expression of the anti-apoptotic signaling molecule, Akt/protein kinase B. Finally, introduction of constitutively active CREB, CREB-DIEDML, blocked death of mature adipocytes treated with TNF-alpha. The data indicate that CREB plays a central role in adipocyte survival, perhaps by regulating the expression of certain pro- and anti-apoptotic genes. These results not only extend the role of CREB in adipocyte biology but also highlight the general developmental and survival role of this factor in numerous cell and tissue types.

Rapamycin-sensitive phase of 3T3-L1 preadipocyte differentiation after clonal expansion

Inhibition of insulin-induced 3T3-L1 preadipocyte differentiation by rapamycin has been attributed to a blockade of the early critical clonal expansion phase of the adipogenic program. Rapamycin binds to, and inhibits, mTOR (mammalian target of rapamycin), leading to diminution of p70 S6 kinase activity and eukaryotic initiation factor 4E binding protein 1 (eIF4E-BP1) function. Our objective was to determine if rapamycin-sensitive pathways exist subsequent to the clonal expansion phase. We determined that the mitotic clonal expansion was complete by day 4 of the differentiation protocol, based on the response to Ara-C (cytosine beta-D-arabinofuranoside), which only inhibits differentiation when administered during this phase. Treatment of differentiating 3T3-L1 cells with rapamycin, starting on day 4, exerted potent negative effects on glycerol phosphate dehydrogenase activity, and triacylglycerol accumulation, as well as on the protein expression of adipogenic transcription factors, C/EBPalpha and PPARgamma. Insulin-stimulated p70 S6 kinase activity, and its inhibition by rapamycin, were comparable in preadipocytes at day 0 vs. day 4 post-differentiation. We conclude that a component of the adipogenic program, operating after the completion of clonal expansion, is inhibited by rapamycin, suggesting an ongoing need for mTOR function in this process.

Insulin-induced adipocyte differentiation. Activation of CREB rescues adipogenesis from the arrest caused by inhibition of prenylation

Insulin is a potent adipogenic hormone that triggers an induction of a series of transcription factors governing differentiation of pre-adipocytes into mature adipocytes. However, the exact link between the insulin signaling cascade and the intrinsic cascade of adipogenesis remains incompletely understood. Herein we demonstrate that inhibition of prenylation of p21ras and Rho-A arrests insulin-stimulated adipogenesis. Inhibition of farnesylation of p21ras also blocked the ability of insulin to activate mitogen-activated protein (MAP) kinase and cyclic AMP response element-binding (CREB) protein. Expression of two structurally different inducible constitutively active CREB constructs rescued insulin-stimulated adipocyte differentiation from the inhibitory influence of prenylation inhibitors. Constitutively active CREB constructs induced expression of PPARgamma2, fatty acid synthase, GLUT-4, and leptin both in control and prenylation inhibitors-treated cells. It appears that insulin-stimulated prenylation of the Ras family GTPases assures normal phosphorylation and activation of CREB that, in turn, triggers the intrinsic cascade of adipogenesis.

Phosphoinositide 3-kinase facilitates antigen-stimulated Ca(2+) influx in RBL-2H3 mast cells via a phosphatidylinositol 3,4,5-trisphosphate-sensitive Ca(2+) entry mechanism

This study presents evidence that phosphoinositide 3-kinase (PI3K) plays a concerted role with phospholipase Cgamma in initiating antigen-mediated Ca(2+) signaling in mast cells via a phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3))-sensitive Ca(2+) entry pathway. Exogenous PI(3,4,5)P(3) at concentrations close to its physiological level induces instantaneous Ca(2+) influx into RBL-2H3 cells. This PI(3,4,5)P(3)-induced intracellular Ca(2+) increase is independent of phospholipase C activity or the depletion of internal stores. Moreover, inhibition of PI3K by LY294002 or by overexpression of the dominant negative inhibitor Deltap85 suppresses the Ca(2+) response to the cross-linking of the high affinity receptor for IgE (FcepsilonRI). Concomitant treatment of RBL-2H3 cells with LY294002 or Deltap85 and 2-aminoethyl diphenylborate, a cell-permeant antagonist of D-myo-inositol 1,4,5-trisphosphate receptors, abrogates antigen-induced Ca(2+) signals, whereas either treatment alone gives rise to partial inhibition. Conceivably, PI(3,4,5)P(3)-sensitive Ca(2+) entry and capacitative Ca(2+) entry represent major Ca(2+) influx pathways that sustain elevated [Ca(2+)]i to achieve optimal physiological responses. This study also refutes the second messenger role of D-myo-inositol 1,3,4,5-tetrakisphosphate in regulating FcepsilonRI-mediated Ca(2+) response. Considering the underlying mechanism, our data suggest that PI(3,4,5)P(3) directly stimulates a Ca(2+) transport system in plasma membranes. Together, these data provide a molecular basis to account for the role of PI3K in the regulation of FcepsilonRI-mediated degranulation in mast cells.

Phosphatidylinositol-3,4,5-trisphosphate is required for insulin-like growth factor 1-mediated survival of 3T3-L1 preadipocytes

Adipocyte number, a determinant of adipose tissue mass, reflects the balance between the rates of proliferation/differentiation vs. apoptosis of preadipocytes. The percentage of 3T3-L1 preadipocytes undergoing cell death following serum deprivation was reduced by 10 nM insulin-like growth factor (IGF)-1 (from 50.0 +/- 0.7% for control starved cells to 27.5 +/- 3.1%). TUNEL staining confirmed the apoptotic nature of the cell death. The protective effect of IGF-1 was blocked by phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin, and LY294002, but was unaffected by rapamycin, PD98059, or SB203580, which inhibit mammalian target of rapamycin (mTOR), ERK kinase (MEK1), and p38 MAPK respectively. Exogenous PI(3,4,5)P3 (10 microM), the principal product of IGF-1-stimulated PI3K in 3T3-L1 preadipocytes, had a modest survival effect on its own, reducing cell death from 47.9 +/- 3.4% to 35.6 +/- 3.5%. When added to the combination of IGF-1 and LY294002, PI(3,4,5)P3 reversed most of the inhibitory effect of LY294002 on IGF-1-dependent cell survival, protein kinase B/Akt phosphorylation, and caspase-3 activity. Taken together, these results implicate PI(3,4,5)P3 as a necessary signal for the anti-apoptotic action of IGF-1 on 3T3-L1 preadipocytes.

A phosphatidylinositol 3,4,5-trisphosphate analogue with low serum protein-binding affinity

Phosphatidylinositol 3,4,5-trisphosphate (PIP3) plays an important role in the regulation of diverse physiological functions. Recent evidence indicates that PIP3 is cell permeant, and can be added exogenously to modulate cellular responses. However, like many other phospholipids, PIP3 binds serum proteins with high affinity, resulting in rapid deactivation of this lipid second messenger. Our study indicates that bovine serum albumin (BSA) at concentrations as low as 10 microg/mL abrogated the biological activity of dipalmitoyl-PIP3. This nonspecific interaction with serum proteins hampers the use of PIP3 in biological studies where serum is needed. We report here an ether-linked PIP3 analogue, 1-O-(1-O-hexadecyl-2-O-methyl-sn-glycero-3-phosphoryl)-myo-inositol 3,4,5-trisphosphate (C16Me-PIP3). which displays low serum protein-binding affinity while retaining the biological function of PIP3. The affinity of C16Me-PIP3 with BSA was two orders of magnitude lower than that of its dipalmitoyl-counterpart. Biochemical data indicate that C16Me-PIP3 was able to stimulate Ca2+ influx in T cells in the presence of moderate levels (up to 1 mg/mL) of BSA. Thus. C16Me-PIP3 may provide a useful tool to study the physiological function of phosphoinositide (PI) 3-kinase in vivo.

Ritonavir increases the level of active ADD-1/SREBP-1 protein during adipogenesis

OBJECTIVE

A novel lipodystrophy syndrome characterized by truncal adiposity, peripheral fat atrophy, type 2 diabetes mellitus, and dyslipidemia occurs in HIV-infected individuals, and may be aggravated by HIV-1 protease inhibitors. The increase in truncal fat could be due to enhanced preadipocyte differentiation. Using the 3T3-L1 preadipocyte model, we reported that ritonavir enhances adipocyte differentiation in culture. The goal of this study was to characterize the molecular mechanism of ritonavir on preadipocyte differentiation.

DESIGNS AND METHODS

Time course studies of 3T3-L1 preadipocytes placed in standard differentiation medium (insulin, dexamethasone, and isobutylmethylxanthine) were performed. Glycerol phosphate dehydrogenase (GPDH) was assayed enzymatically, and triacylglycerol (TG) mass was quantified. The adipogenic transcription factors adipocyte determination and differentiation-dependent factor 1 (ADD-1)/sterol regulatory element binding protein 1 (SREBP-1), CCAAT/enhancer-binding protein-alpha (CEBPalpha), and peroxisome proliferator activated receptor-gamma (PPARgamma), were measured by Western analysis.

RESULTS

Ritonavir (10 microg/ml) enhanced 3T3-L1 preadipocyte differentiation (30% increase in TG mass; 50% increase in GPDH activity), and transiently raised levels of the 68 kDa active mature form of ADD-1/SREBP-1 during adipogenesis by threefold, compared with standard differentiation. In contrast, ritonavir attenuated the differentiation-induced increase in CEBPalpha and PPARgamma.

CONCLUSIONS

Our data suggest that ritonavir enhances 3T3-L1 adipogenesis by increasing the level of active mature ADD-1/SREBP-1. This effect may be due to reduced proteolysis of ADD-1/SREBP-1, as ritonavir inhibits an N-acetyl-leucyl-leucyl-norleucinal (ALLN)-sensitive proteosomal degradation pathway in lymphocytes, and ALLN itself inhibits the breakdown of mature ADD-1/SREBP-1. As mature ADD-1/SREBP-1 regulates several lipogenic enzymes, higher levels may explain the effect of ritonavir on TG accumulation and GPDH activity. Studying ADD-1/SREBP-1 may lead to better understanding and prevention of the lipodystrophy syndrome.

The critical role of Shc in insulin-like growth factor-I-mediated mitogenesis and differentiation in 3T3-L1 preadipocytes

Insulin-like growth factor-I (IGF-I) stimulates mitogenesis in proliferating preadipocytes, but when cells reach confluence and become growth arrested, IGF-I stimulates differentiation into adipocytes. IGF-I induces signaling pathways that involve IGF-I receptor-mediated tyrosine phosphorylation of Shc and insulin receptor substrate 1 (IRS-1). Either of these adaptor proteins can lead to activation of the three-kinase cascade ending in activation of the extracellular signal-regulated kinase 1 and -2 (ERK-1 and -2) mitogen-activated protein kinases (MAPKs). Several lines of evidence suggest that activation of MAPK inhibits 3T3-L1 preadipocyte differentiation. We have shown that IGF-I stimulation of MAPK activity is lost as 3T3-L1 preadipocytes begin to differentiate. This change in MAPK signaling coincides with loss of IGF-I-mediated Shc, but not IRS-1, tyrosine phosphorylation. We hypothesized that down-regulation of MAPK via loss of proximal signaling through Shc is an early component in the IGF-I switch from mitogenesis to differentiation in 3T3-L1 preadipocytes. Treatment of subconfluent cells with the MEK inhibitor PD098059 inhibited both IGF-I-activation of MAPK as well as 3H-thymidine incorporation. PD098059, in the presence of differentiation-inducing media, accelerated differentiation in subconfluent cells as measured by expression of adipocyte protein-2 (aP-2), peroxisome proliferator-activated receptor gamma (PPARgamma) and lipoprotein lipase (LPL). Transient transfection of subconfluent cells with Shc-Y317F, a dominant-negative mutant, attenuated IGF-I-mediated MAPK activation, inhibited DNA synthesis, and accelerated expression of differentiation markers aP-2, PPARgamma, and LPL. We conclude that signaling through Shc to MAPK plays a critical role in mediating IGF-I-stimulated 3T3-L1 mitogenesis. Our results suggest that loss of the ability of IGF-I to activate Shc signaling to MAPK may be an early component of adipogenesis in 3T3-L1 cells.