Properties of growth hormone receptors in relation to the adipose conversion of 3T3 cells

Cultured preadipose 3T3 cells undergo a process of differentiation in which they convert to adipose cells. Growth hormone promotes this conversion. Since 3T3 sublines vary in their susceptibility to adipose conversion, it was of interest to examine the properties of the growth hormone receptors in relation to that susceptibility. It was found that preadipose 3T3-F442A cells, which are able to convert to adipose cells with high frequency, are able to bind about 10(4) growth hormone molecules per cell with Kd approximately 10(-9) M. After adipose conversion, no appreciable change in hormone binding was detected. The binding of growth hormone to 3T3-C2 cells (a line virtually insusceptible to adipose conversion) was indistinguishable from that to 3T3-F442A cells. Internalization and degradation of the hormone were also similar in the two cell lines. Susceptibility to adipose conversion is therefore not determined by the relative ability of the cells to bind or degrade the hormone, but must instead depend on some response, as yet unidentified, that follows binding of the hormone.     

Sensitivity of preadipose 3T3 cells to growth hormone

Cultured preadipose 3T3 cells are able to undergo a process of differentiation through which they are converted into adipose cells. Growth hormone induces this conversion in resting cultures but not in growing cultures. It was of interest to determine the period of cell sensitivity to the hormone and the timing of the induction of glycerophosphate dehydrogenase, a key enzyme in lipogenesis. It was found that 3T3-F442A cells became highly sensitive to rat growth hormone at confluence but that high sensitivity remained for only 3 days; thereafter, the responsiveness to the rat growth hormone declined rapidly. Refeeding of the cells with fresh medium did not lead to the recovery of the hormone sensitivity, indicating that the decrease in sensitivity was not due to exhaustion of medium components but that it seemed to be a specific property of F442A cells. As glycerophosphate dehydrogenase activity was detected at nearly the same time as its mRNA was measurable, it is likely that the mRNA is translated immediately after its synthesis.     

Growth hormone-induced alteration in ErbB-2 phosphorylation status in 3T3-F442A fibroblasts

The growth hormone receptor (GHR), a cytokine receptor superfamily member, requires the JAK2 tyrosine kinase for signaling. We now examine functional interactions between growth hormone (GH) and epidermal growth factor (EGF) in 3T3-F442A fibroblasts. Although EGF enhanced ErbB-2 tyrosine phosphorylation, GH, while causing retardation of its migration on SDS-polyacrylamide gel electrophoresis, decreased ErbB-2's tyrosine phosphorylation. GH-induced retardation was reversed by treatment of anti-ErbB-2 precipitates with both alkaline phosphatase and protein phosphatase 2A, suggesting that GH induced serine/threonine phosphorylation of ErbB-2. Both GH-induced shift in ErbB-2 migration and GH-induced MAP kinase activation were unaffected by a protein kinase C inhibitor but were blocked by the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1 (MEK1) inhibitor, PD98059. Notably, leukemia inhibitory factor, but not interferon-gamma, also promoted ErbB-2 shift and mitogen-activated protein kinase activation. Cotreatment with EGF and GH versus EGF alone resulted in a 35% decline in acute ErbB-2 tyrosine 1248 autophosphorylation, a marked decline (approximately 50%) in DNA synthesis, and substantially decreased cyclin D1 expression. We conclude that in 3T3-F442A cells, 1) the GH-induced decrease in ErbB-2 tyrosine phosphorylation correlates with MEK1/mitogen-activated protein kinase activity and 2) GH antagonizes EGF-induced DNA synthesis and cyclin D1 expression in a pattern consistent with its alteration in ErbB-2 phosphorylation status.     

Characterization of syndecan-4 expression in 3T3-F442A mouse adipocytes: link between syndecan-4 induction and cell proliferation.

Heparan sulfate proteoglycans are found on the surface of most cells. Syndecan-4 is a widely expressed transmembrane heparan sulfate proteoglycan. Using quantitative RNase protection assays and immunoblotting, syndecan-4 expression was characterized in 3T3-F442A mouse adipoblasts. These cells exhibit dramatic changes in their biological and morphological characteristics during differentiation to adipocytes. During this process, the levels of syndecan-4 protein and mRNA expression changed dramatically. They peaked at the time when quiescent cells reentered the cell cycle before differentiation. Serum depletion-repletion also replicated the syndecan-4 mRNA induction when the cells were released back into proliferation, and a cycloheximide treatment abolished the peak of induction. In addition, inhibiting syndecan-4 induction with antisense oligonucleotides inhibited the proliferation of 3T3-F442A cells. In the terminally differentiated adipocytes characterized by the loss of proliferation capability, the serum inducibility of syndecan-4 is repressed, emphasizing the link between syndecan-4 induction in 3T3-F442A cells and cell proliferation.     

Efficient adenoviral transduction of 3T3-F442A preadipocytes without affecting adipocyte differentiation

The preadipocyte cell lines 3T3-L1 and 3T3-F442A are widely used to study the cellular mechanisms of preadipocyte differentiation and mature adipocyte functions. However, transfection with naked DNA is inefficient in these cell lines. Adenoviral gene transfer is a powerful technique to induce high levels of transgene expression. After failing to obtain 3T3-F442A stable transfectants, we studied different techniques designed to enhance the efficiency of adenoviral transduction in fat cells. First, we compared the effects of two agents known to significantly enhance adenoviral transgene transduction, namely the cationic lipid lipofectamine and the cationic polymer polylysine. We show here that lipofectamine-assisted adenoviral transduction was more efficient in 3T3-F442A than in 3T3-L1 preadipocytes at all tested multiplicity of infection. Lipofectamine, and more efficiently polylysine, yielded high and sustained levels of adenoviral transgene expression in 3T3-F442A preadipocytes. Adenoviral transgene expression was maintained throughout the differentiation process. Furthermore, the two agents also efficiently enhanced adenoviral transduction in mature 3T3-F442A adipocytes. Interestingly, neither protocol affected the differentiation process, morphological features or protein expression of mature adipocytes. These approaches could be of interest to study fat cell differentiation and the functions of mature adipocytes.     

Growth-related expression of a double-stranded RNA-dependent protein kinase in 3T3 cells

Cultured mouse 3T3-F442A and 3T3-C2 fibroblasts exhibit a transient double-stranded RNA (dsRNA)-dependent phosphorylation of a 67,000-dalton protein (67K) without prior treatment with interferon (IFN). This phosphoprotein is similar but not identical to the dsRNA-dependent eukaryotic initiation factor-2 (eIF-2) alpha protein kinase (dsI), which regulates protein synthesis in rabbit reticulocytes. We have studied the relationship between cell growth and phosphorylation of the 67K protein (designated 3T3-dsRNA-dependent eIF-2 alpha kinase). A low level of dsRNA-dependent phosphorylation of 3T3-dsI was detectable in extracts prepared from cells not treated with IFN and grown at a low cell density. The phosphorylation of dsI and the phosphorylation of a 38K protein identified as the alpha-subunit (38K) of 3T3-eIF-2 (eIF-2 alpha) occurred concomitantly; the levels of these phosphorylations confluent and thereafter decreased markedly. Treatment of cells with IFN at all stages of growth resulted in an increase in phosphorylation of dsI. 3T3-F442A and 3T3-C2 fibroblasts were found to produce and secrete IFN at levels sufficient to induce an elevated dsI activity.     

11β-hydroxysteroid Dehydrogenase 1 in Adipocytes: Expression is Differentiation-dependent and Hormonally Regulated

11β-hydroxysteroid dehydrogenase type 1 (11β-HSD-1) catalyses the reversible metabolism of physiological glucocorticoids (cortisol, corticosterone) to inactive metabolites (cortisone, 11-dehydrocorticosterone), thus regulating glucocorticoid access to receptors. 11β-HSD-1 expression is regulated during development and by hormones in a tissue specific manner. The enzyme is highly expressed in liver, where it may influence glucocorticoid action on fuel metabolism, processes also important in adipose tissue. Here we show that 11β-HSD-1 is expressed in white adipose tissue, in both the adipocyte and stromal/vascular compartments, and in the adipocyte cell lines 3T3-F442A and 3T3-L1. In these cells, 11β-HSD-1 expression is induced upon differentiation into adipocytes and is characteristic of a ‘late differentiation’ gene, with maximal expression 6-8 days after confluence is reached. In intact 3T3-F442A adipocytes the enzyme direction is predominantly 11β-reduction, activating inert glucocorticoids. The expression of 11β-HSD-1 mRNA is altered in fully differentiated 3T3-F442A adipocytes treated with insulin, dexamethasone or a combination of the hormones, in an identical manner to glycerol-3-phosphate dehydrogenase (GPDH) mRNA (encoding a key enzyme in triglyceride synthesis and a well-characterised marker of adipocyte differentiation). The demonstration of 11β-HSD-1 expression in adipocytes and its predominant reductase activity in intact 3T3-F442A adipocytes suggests that 11β-HSD-1 may play an important role in potentiating glucocorticoid action in these cells. 3T3-F442A and 3T3-L1 represent useful model systems in which to examine the factors which regulate 11β-HSD-1 gene expression and the role of 11β-HSD-1 in modulating glucocorticoid action in adipose tissue.     

Growth hormone promoted tyrosyl phosphorylation of growth hormone receptors in murine 3T3-F442A fibroblasts and adipocytes

Because many growth factor receptors are ligand-activated tyrosine protein kinases, the possibility that growth hormone (GH), a hormone implicated in human growth, promotes tyrosyl phosphorylation of its receptor was investigated. 125I-Labeled human GH was covalently cross-linked to receptors in intact 3T3-F442A fibroblasts, a cell line which differentiates into adipocytes in response to GH. The cross-linked cells were solubilized and passed over a column of phosphotyrosyl binding antibody immobilized on protein A-Sepharose. Immunoadsorbed proteins were eluted with a hapten (p-nitrophenyl phosphate) and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The eluate from the antibody column contained an Mr 134,000 125I-GH-receptor complex. A similar result was obtained when the adipocyte form of 3T3-F442A cells was used in place of the fibroblast form. O-Phosphotyrosine prevented 125I-GH-receptor complexes from binding to the antibody column, whereas O-phosphoserine and O-phosphothreonine did not. In studies of GH-promoted phosphorylation in 3T3-F442A fibroblasts labeled metabolically with [32P]Pi, GH was shown to stimulate formation of a 32P-labeled protein which bound to immobilized phosphotyrosyl binding antibodies. The molecular weight of 114,000 obtained for this protein is similar to that expected for non-cross-linked GH receptor. The Mr 114,000 phosphorylated protein could be immunoprecipitated with anti-GH antibody, indicating that GH remained noncovalently bound to this protein during absorption to and elution from the immobilized phosphotyrosyl binding antibody. Phosphoamino acid analysis after both limited acid hydrolysis and extensive base hydrolysis of the Mr 114,000 phosphoprotein confirmed the presence of phosphotyrosyl residues.(ABSTRACT TRUNCATED AT 250 WORDS)     

Aortic carboxypeptidase-like protein is regulated by transforming growth factor beta in 3T3-L1 preadipocytes

Adipogenesis is characterized by early remodeling of the extracellular matrix, allowing preadipocytes to adopt a more spherical shape and optimize lipid accumulation as they mature. Aortic carboxypeptidase-like protein (ACLP), found in collagen-rich tissues including adipose tissue, is expressed in 3T3-L1 and 3T3-F442A preadipocytes, and is downregulated during adipogenesis. We now report that ACLP is found in medium conditioned by 3T3-L1 preadipocytes. Transforming growth factor (TGF) beta, a known modulator of fibrillar matrix protein production, increased ACLP expression by 2.4+/-0.4-fold (mean+/-SE; n=3) in 3T3-L1 preadipocytes, through a mechanism that requires p42/44 MAPK activity. Addition of TGFbeta to differentiation medium, which inhibits adipogenesis, raised ACLP levels in 3T3-L1 cells. However, sustained expression of ACLP in stable clones of 3T3-L1 or 3T3-F442A preadipocytes did not interfere with adipogenesis.     

The effect of various substrates on cell attachment and differentiation of 3T3-F442A preadipocytes

The influence of extracellular matrix (Matrigel), collagen, and polylysine substrates on cell attachment and differentiation in 3T3-F442A preadipocytes was investigated. In comparison to an uncoated-polystyrene substrate, a concentrated Matrigel substrate (100 microg/cm2) markedly increased intracellular lipid level by about 30%, whereas a lower density Matrigel (10 microg/cm2) accelerated the differentiation rate but did not increase the amount of lipid 21 days after addition of adipogenic factors. Preadipocytes on the collagen surface differentiated less extensively than cells on the polystyrene. Polylysine did not effectively support attachment for either differentiated or undifferentiated cells. These results suggest that Matrigel provides the most suitable environment for both cell adhesion and differentiation for 3T3-F442A cells. This is in contrast to a previous report that extracellular matrix (from corneal endothelial cells) was detrimental to differentiation of 3T3-F442A cells.     

Commitment of 3T3-F442A cells to adipocyte differentiation takes place during the first 24-36 h after adipogenic stimulation: TNF-alpha inhibits commitment

We studied the commitment of 3T3-F442A cells during stimulation with adipogenic serum or growth hormone. Confluent 3T3-F442A preadipocytes were incubated with adipogenic medium for increasing times; the number of adipose clusters, GPDH activity, and lipid accumulation were evaluated. Results show that cell commitment took place during the first 24-36 h after stimulation under adipogenic conditions. Then, cultures underwent a 2-fold increase in total cell number through selective multiplication of committed cells, followed by a dramatic decrease in colony-forming ability and 300- to 1000-fold raise in GPDH activity. Cell commitment was not modulated by insulin, but this hormone stimulated clonal expansion of committed cells and lipogenesis. Commitment was inhibited by TNF-alpha at concentrations as low as 5 ng/ml, and by retinoic acid. The results show that TNF-alpha inhibits adipose conversion at two different levels: at concentrations as low as 5 ng/ml, it blocks commitment, and at concentrations of 100 ng/ml or higher the cytokine seems to block mitotic expansion and other steps of differentiation after cell commitment. The identification of a specific time for cell commitment would allow the study of the early genes that might regulate cell reprogramming into adipocytes.     

The CCAAT/enhancer binding protein and its role in adipocyte differentiation: evidence for direct involvement in terminal adipocyte development.

During the course of differentiation of preadipocytes into adipocytes, several differentiation-linked genes are activated synchronously with morphological changes. To follow this process we have used 3T3-F442A cells, known to undergo adipocyte conversion with high frequency. Accumulation of lipid droplets in the cytoplasm constitutes an easily visualized sign of the terminally differentiated phenotype. In this report we demonstrate that expression of the CCAAT/enhancer binding protein (C/EBP) is an important factor in determining the ability to accumulate lipid droplets in terminally differentiated adipocytes. In one experiment we can suppress C/EBP expression through administration of hydrocortisone to differentiating 3T3-F442A cells, which is accompanied by an inability of the cells to accumulate lipid. In another experiment a C/EBP antisense expression vector has been stably introduced into 3T3-F442A cells and as compared with control cells, a 62% decrease of C/EBP mRNA (p less than 0.01) is demonstrated. This decrease of C/EBP mRNA is accompanied by a change in cellular morphology characterized by a reduced ability to form lipid droplets. We can also demonstrate a correlation between the degree of reduction of C/EBP mRNA and the amount of lipid present in the cells. These findings strongly support the view that C/EBP is a necessary component of terminal adipocyte differentiation.     

Partial characterization of a cellular factor that regulates the double-stranded RNA-dependent eIF-2 alpha kinase in 3T3-F442A fibroblasts.

The interferon-induced double-stranded RNA-dependent eIF-2 alpha kinase (dsI) has an established role in mediating part of interferon's antiviral effects. Numerous studies have suggested that dsI also has regulatory functions in cells not infected with virus. Our previous results have indicated that the activation of this kinase may be an important regulatory signal in controlling growth arrest of mouse 3T3-F442A fibroblasts prior to their subsequent differentiation to adipocytes. Here, we report that extracts from 3T3-F442A cells cultured under conditions nonpermissive for differentiation exhibit significantly reduced dsI activity and that this reduction is due, at least in part, to the presence of elevated levels of a novel inhibitor of dsI activation (dRF). This inhibitor is also detected in reduced amounts in extracts from cells cultured under conditions which are permissive for differentiation. We have achieved a 1,000-fold purification of dRF activity, and highly purified dRF preparations were found to be greatly enriched for a 15-kDa protein that was greater than 90% pure. Our results indicate that dRF is not a protein phosphatase or protease but a reversible inhibitor of dsI autophosphorylation. In addition, our results imply that dRF is a physiologic regulator of dsI, since dRF activity correlates with the ability of 3T3-F442A cells to undergo adipose conversion.     

Differential regulation of two glucose transporters by chronic growth hormone treatment of cultured 3T3-F442A adipose cells

New methods for the analysis of glucose transporters were used to analyze the molecular mechanisms involved in the insulin-antagonistic effects of growth hormone (GH), which is known as a diabetogenic hormone. The ability of GH to alter the number and mRNA levels of two different glucose transporters in cultured 3T3-F442A adipocytes was investigated using specific antibodies and cDNA probes. At concentrations of GH as low as 0.5 and 5 ng/ml and at incubation times as short as 4 h, GH decreased rates of 2-deoxyglucose uptake in 3T3-F442A adipocytes. 3-O-Methyl-D-glucose uptake was inhibited to an extent similar to that of 2-deoxyglucose uptake (60-80%) after a 24-h incubation with GH (500 ng/ml), indicating that GH inhibits glucose metabolism specifically at the step of glucose transport. To determine whether reduced rates of glucose transport might result from reduced numbers of glucose transporters, whole cell lysates were prepared from GH-treated cells and subjected to immunoblotting using antibodies that identify Glut 1 (HepG2/rat brain) and Glut 4 (muscle/adipose) transporters. GH caused a time- and dose-dependent decrease in the number of Glut 1 transporters in the cell. Northern and slot-blot analyses showed a GH-induced dose-dependent decrease in levels of Glut 1 mRNA. In contrast, levels of Glut 4 transporter and mRNA were unchanged by GH. These data suggest that GH regulates Glut 1 and Glut 4 transporters differentially and that it exerts its inhibitory effect on glucose uptake at least in part by decreasing the synthesis of Glut 1 transporters. These studies provide the first evidence that GH regulates a key gene in metabolic regulation and can interfere with gene expression.     

A central role for hepatocyte growth factor in adipose tissue angiogenesis

Hepatocyte growth factor (HGF) is a potent mitogenic and angiogenic factor produced in human adipose tissue. In this study, we use 3T3-F442A preadipocytes to study the contribution of HGF to angiogenesis in an in vivo fat pad development model. As observed for human adipocytes, HGF is synthesized and secreted by 3T3-F442A preadipocytes and mature adipocytes. HGF knockdown with small-interfering RNA reduced HGF mRNA expression 82.3 +/- 4.2% and protein secretion 82.9 +/- 1.4% from 3T3-F442A preadipocytes. Silencing of HGF resulted in a 70.5 +/- 19.0% reduction in endothelial progenitor cell migration to 3T3-F442A-conditioned medium in vitro. 3T3-F442A preadipocytes injected under the skin of mice form a fat pad containing mature, lipid-filled adipocytes and a functional vasculature. At 72 h postinjection, expression of the endothelial cell genes TIE-1 and platelet endothelial cell adhesion molecule (PECAM)-1 was decreased 94.4 +/- 2.2 and 91.5 +/- 2.5%, respectively, in 3T3-F442A fat pads with HGF silencing. Knockdown of HGF had no effect on differentiation of 3T3-F442A preadipocytes to mature adipocytes in vitro or in vivo. In developing fat pads under the skin of HGF overexpressing transgenic mice, TIE-1 and PECAM-1 mRNA was increased 16.5- and 21.4-fold, respectively, at 72 h postinjection. The increase in gene expression correlated with immunohistochemical evidence of endothelial cell migration in the developing fat pad. These data suggest that HGF has a central role in regulating angiogenesis in adipose tissue.     

Thyroid hormone stimulates adipocyte differentiation of 3T3 cells

Triiodothyronine added at 0.1 nM to 3T3-F442A cells cultured in adipogenic medium having endogenous hormone concentrations similar to those of hypothyroid serum stimulated adipose conversion; activities of both lipogenic enzymes, glycerophosphate dehydrogenase and malic enzyme, increased with hormone treatment. The number of adipocytes was also augmented by L-T3 addition but the number of fat cell clusters remained the same as compared to non-treated cultures, suggesting that thyroid hormone increased the number of adipocytes probably through stimulating selective multiplication of precursor adipose cells. Hormone addition to cells cultured with non-adipogenic medium did not promote conversion showing that L-T3 is not an adipogenic factor by itself. Triiodothyronine added at concentrations similar to those found in hyperthyroidism, from 10 nM up to 10 µM, also increased the proportion of adipocytes without changing the number of fat cell clusters, but they decreased the activity of both lipogenic enzymes and lipid accumulation in mature adipocytes. It can be concluded that during 3T3-F442A differentiation into adipocytes L-T3 increases the number of differentiated adipocytes and, at low concentrations, also enhances lipogenic enzyme activities, whereas at the hyperthyroid hormone levels these enzyme activities are significantly reduced, remaining at levels similar to those of cells cultured with hypothyroid medium. This cloned cell line seems to be a useful model to study thyroid hormone action at both molecular and cellular level.