An established preadipose cell line and its differentiation in culture. II. Factors affecting the adipose conversion

Coupling of proadipocyte growth arrest and differentiation. I. Induction by heparinized medium containing human plasma

The differentiation of proadipocytes in vitro typically required prolonged culture of cells as a high density in high concentrations of serum and added hormones. With such culture conditions it is difficult to design experiments to determine the mechanisms that control the differentiation process. We now describe the rapid and parasynchronous growth arrest and differentiation of low density murine proadipocytes in heparinized medium containing only human plasma. When low density cells are cultured under these conditions, growth arrest at a distinct state in the G1 phase of the cell cycle occurs within 2 d and the differentiation of 80-100% of the cell population occurs within 4 d thereafter. The factors in human plasma which promote growth arrest and differentiation are heat labile and can be separated by barium adsorption. In the following paper we have used these methods to show that there are five separate phases which regulate the coupling of proadipocyte growth arrest and differentiation. The data reported in this paper establish that: (a) high cell density and extensive cell-to-cell contact are not required for adipocyte differentiation, (b) prolonged culture is not required for adipocyte differentiation, and (c) high concentrations of serum and/or added hormones are not required for adipocyte differentiation.

Hormonal responsiveness of a preadipose cell line derived from newborn mouse calvaria

We established a clonal preadispose cell line from newborn mouse calvaria. Cells of this cell line, designated MC3T3-G2/PA6, had the capacity to convert to adipose cells, to accumulate triglycerides in their cytoplasm, and to mature to differentiated fat cells in a resting state. This adipose conversion was markedly accelerated by addition of dexamethasone, which was the most potent inducer among the steroid hormones tested. The presence of dexamethasone was needed during the steroid hormones tested. The presence of dexamethasone was needed during logarithmic growth phase for maximal conversion. The frequency of adipose conversion was dependent on exposure time to the hormone, but cells already committed to differentiation continued to accumulate lipid and developed into mature adipose cell even in its absence. This indicates that the hormone accelerates the initiation of the adipose conversion, but is not required for the ongoing conversion process. In fact, it was rather inhibitory for the process of fat accumulation. Insulin alone slightly inhibited the adipose conversion, but its combination with dexamethasone neutralized the above inhibitory effect of dexamethasone. The responsiveness of this cell line is consistent with that observed for mouse bone marrow preadipocytes in primary culture but differs from that for preadipose cell lines derived from extramedullary tissues. These results strongly suggest that the MC3T3-G2/PA6 cell line was derived from bone marrow.

Primary culture of adipoblasts from obese and lean Zucker rat adipose tissue

Adipocyte precursors derived from the epididymal fat pads of young adult lean (FaFa) and obese (fafa) Zucker rats were established in primary culture. The two types of culture were used to assess intrinsic cellular differences in proliferative capacity, lipoprotein lipase (LPL) activity and triglyceride (TG) accumulation related to genotype. Proliferative capacity was similar over seven days in vitro in lean- and obese-derived cultures. Heparin-releasable LPL activity was significantly greater in lean- than in obese-derived cultures grown in media supplemented with penicillin and streptomycin (pen-strep). However, when grown in media supplemented with cephalothin, heparin-releasable and total LPL activity increased significantly in obese-derived cultures and equalled LPL activity in lean-derived cultures. Substantial LPL activity was measurable in both types of culture at confluence, before exposure to media that promoted lipid-filling. TG accumulation was significantly greater in lean-than in obese-derived cultures in the presence of pen-strep but was similar in both culture types grown in cephalothin. These data support our hypothesis that the fa gene may affect mechanisms of protein turnover regulation, since pen-strep, but not cephalothin, has inhibitory effects on mammalian protein synthesis and degradation. The substantial LPL activity present in confluent, but unconverted cultures, suggests that some percentage of cells in the confluent monolayers are adipoblasts.

Lipoprotein lipase suppression in 3T3-L1 cells by an endotoxin-induced mediator from exudate cells

Conditioned medium from cultures of mouse peritoneal exudate cells incubated wih endotoxin contains a mediator that markedly suppresses (greater than 90%) lipoprotein lipase (triacylglycero-protein acylhydrolase, EC 3.1.1.34) activity in differentiating 3T3-L1 mouse preadipocytes. The effect is dependent upon the amount of mediator and is evident as early as 30 min after the addition of the mediator-containing medium to 3T3-L1 cell cultures. Neither endotoxin nor conditioned medium from cultures of exudate cells not exposed to endotoxin shows the presence of the mediator. Lysates of the exudate cells are also unable to suppress the lipase activity. Increasing the amount of insulin does not reverse this suppression, even at 1000 times the concentration used for standard experiments. The lipoprotein lipase suppression mediator present in the conditioned medium of endotoxin-treated exudate cells is heat labile and has an apparent molecular weight of at least 12,000. The mediator does not inhibit lipoprotein lipase activity directly nor does it affect the half-life of enzyme activity released in the medium. The present study demonstrates that endotoxin promotes the release of a mediator from exudate cells that suppresses the activity of lipoprotein lipase in 3T3-L1 preadipocytes.

Differentiation of cultured pre-adipose cells: a probability model

Some cells of the established preadipose cell line, 3T3-L1, synthesize triglyceride after becoming confluent and quiescent. An analysis of the distribution of clusters of lipid-containing cells was consistent with a commitment event during exponential growth followed by clonal growth of committed cells. Experiments were designed to determine if the final clonal pattern of fat among nonfat cells could be described by a probability model. Undifferentiated cells (fibroblastic cells with no detectable accumulation of triglycerides) were plated at various cell numbers so that the total number of cell divisions to confluence could be controlled. Cells were passaged by trypsinization and replating, or trypsinization followed by passage through a narrow-bore needle before replating. Passing cells through a 22G needle seems to eliminate already committed cells from the population. We determined the percentage of fat cells and the range of clone sizes in cultures in which clone sizes depended upon the number of allowed cell divisions. Patterns of clone sizes in experimental cultures were compared to expected patterns obtained by computer simulations of several programmed and stochastic commitment models. Both the observed range of clone sizes and pattern of clones can be approximated by a simple stochastic model, suggesting that commitment to fat production in 3T3-L1 cells is a random process occurring with a fixed probability in single cells in exponential growth, followed by division of both committed and uncommitted cells. The probability of commitment was essentially constant at each cell division. The number of cells committed during each passage is just large enough to replace "terminally differentiated" lipid-containing cells that have been lost, thereby maintaining a constant percentage of fat cells in any given culture of 3T3-L1.

Pre-adipocyte determination either by insulin or by 5-azacytidine

CHEF/18 is a diploid Chinese hamster cell line of embryonic origin, which is fibroblastic in structure, but behaves like a mesenchymal stem cell line in its ability to differentiate into adipocytes, myoblasts, and chondrocytes. With these cells, adipocyte formation has been divided experimentally into two stages: (i) determination of pre-adipocytes, which have lost the ability to form other cell types while retaining their fibroblast structure; and (ii) commitment or terminal differentiation, in which lipids accumulate, adipocyte structure develops, and cells lose the ability to divide. This paper reports that the first stage can be induced by exposure to 5-azacytidine or 2'-deoxy-5-azacytidine, drugs that also induce CHEF cells to form other mesenchymal cell types, or by growth with added insulin. Pre-adipocytes are distinguished from CHEF stem cells by (i) their inability to form other mesenchymal cell types; and (ii) their rapid accumulation of lipid in response to added insulin. The possibility is discussed that both insulin and the cytidine analogs promote differentiation by the same mechanism, namely changes in the pattern of DNA methylation.

Hemin enhances the differentiation of mouse 3T3 cells to adipocytes

Hemin enhances the adipose differentiation of mouse 3T3-F442A cells in a medium containing high levels of adipogenic factor that is present in fetal calf serum or calf serum. This enhancement is more prominent in the presence of insulin. Hemin also promotes adipose differentiation in a medium containing 10% cat serum, which has very little adipogenic factor; this effect is in contrast to that of insulin, which is relatively ineffective in promoting adipose differentiation in this medium. Adipose differentiation of 3T3-F442A cells is inhibited by the addition of aminotriazole, an inhibitor of heme biosynthesis. The inhibition of adipose differentiation by aminotriazole is prevented by the simultaneous addition of hemin and aminotriazole. These results indicate that treatment with hemin can promote the differentiation of fibroblasts to adipocytes in vitro, and they raise the possibility that endogenous heme may have a physiological role in the differentiation of fibroblasts to adipocytes in vivo.

Adipogenic and anti-adipogenic factors in the pituitary and other organs

Adipose conversion of cultured 3T3 cells is known to depend on an adipogenic factor present in serum. In the presence of this factor, extracts of different organs were found to inhibit the adipose conversion. The most active extracts were derived from brain, uterus, and pituitary, but other organs also possessed appreciable activity. Fibroblast growth factor partially purified from both brain and pituitary was much more active in suppressing adipose conversion than were crude extracts of the corresponding organs. Purified platelet-derived growth factor was also an effective inhibitor. Of all the tissue extracts tested, only pituitary possessed, in addition to the inhibitory activity, an adipogenic factor similar to that demonstrated previously in serum. This was revealed at concentrations of extract too low for the inhibitory factor to be effective. Under these conditions the pituitary extract had a specific adipogenic activity orders of magnitude higher than that of serum. We suggest that the adipogenic factor of serum may originate in the pituitary.

Regulation of 3T3-L1 fibroblast differentiation by prostacyclin (prostaglandin I2)

Prostaglandin biosynthesis and prostaglandin-stimulated cyclic AMP accumulation were studied in 3T3-L1 fibroblasts as they differentiated into adipocytes. Incubation of 3T3-L1 membranes with [1-14C]prostaglandin H2, and subsequent radio-TLC analysis, showed that prostacyclin (prostaglandin I2) is the principal enzymatically synthesized prostaglandin in this cell line. Confirmation of the radiochemical data was obtained by demonstrating the presence of 6-keto-prostaglandin F1 alpha, the stable hydrolysis product of prostaglandin I2, by gas chromatography-mass spectrometry. In support of previous work, indomethacin, the prostaglandin endoperoxide synthetase (EC 1.14.99.1) inhibitor, accelerated 3T3-L1 differentiation. More importantly, the incubation of 3T3-L1 cells with insulin and the prostaglandin I2 synthetase inhibitor 9,11-azoprosta-5,13-dienoic acid (azo analog I) also enhanced the rate of cellular differentiation, even though this compound does not inhibit the synthesis of other prostaglandins. The repeated addition of exogenous prostaglandin I2 to 3T3-L1 cells inhibited insulin- and indomethacin-mediated differentiation. When 3T3-L1 cells were exposed to various prostaglandins and the cyclic AMP levels were measured, prostaglandin I2 proved to be the most potent stimulator of cyclic AMP accumulation, followed by prostaglandin E1 greater than prostaglandin H2 much greater than prostaglandin E2, while prostaglandin D2 was inactive. As 3T3-L1 cells differentiate, the ability of prostaglandin I2 or prostaglandin H2 to stimulate cyclic AMP accumulation progressively diminishes. It is suggested that 3T3-L1 differentiation may be controlled by the rate of prostaglandin I2 synthesis and/or sensitivity of the adenylate cyclase to prostaglandin I2.

Hormone-sensitive lipase in differentiated 3T3-L1 cells and its activation by cyclic AMP-dependent protein kinase

Differentiation of 3T3-L1 fibroblasts to adipocyte-like cells was accompanied by a 19-fold increase in neutral triglyceride lipase activity, a 12-fold increase in diglyceride lipase activity, a 10-fold increase in monoglyceride lipase activity, and a 280-fold increase in cholesterol esterase activity. In contrast, acid acylhydrolase activities did not increase during differentiation. The rate of glycerol release from unstimulated intact cells increased by more than 1 order of magnitude upon differentiation. Isoproterenol (1 microM) and 1-methyl-3-isobutylxanthine (0.1 mM) further stimulated this rate of glycerol release 3-fold. The neutral triglyceride lipase activity in cell-free preparations of differentiated cells was activated 105% by cyclic AMP-dependent protein kinase. Neutral cholesterol esterase, diglyceride lipase, and monoglyceride lipase were also activated (117%, 10%, and 37+, respectively) by cyclic AMP-dependent protein kinase. In contrast, protein kinase had no effect on any of the four lysosomal acid acylhydrolase activities. Thus, hormone-sensitive lipase, the most characteristic and functionally important enzyme of adipose tissue, has been characterized in differentiated 3T3-L1 cells. The 3T3-L1 cell should be a valuable model system in which to study regulation of hormone-sensitive lipase, particularly its long-term regulation.

Effects of chronic exposure to insulin on lipid synthesis in 3T3-L1 fatty fibroblasts

The acute effects of insulin on 3H incorporation into lipid from glucose was measured in 3T3-L1 fatty fibroblasts cultured with and without insulin at 10 microgram/ml for 7 days. Basal lipid synthesis did not differ between control cells and cells treated chronically with insulin. There was no insulin stimulation in treated cells while 3H incorporation into lipid in control cells increased from a basal level of 1.39 to 3.85 nmol/dish/90 min with a maximally-stimulating concentration of insulin. This is the first study of 3T3-L1 fatty fibroblasts which describes a lack of acute insulin responsiveness in cells exposed chronically to insulin as compared to control cells.

Organelle relationships in cultured 3T3-L1 preadipocytes

In differentiating 3T3-L1 cells, lipid spheres, the endoplasmic reticulum (ER), microperoxisomes, and mitochondria form "constellations" that may reflect the interplay of lipid metabolizing enzymes in these organelles. ER cisternae are also situated very close to "rosettes,"plasmalemmal specializations found in mature adipocytes in vivo. As in hepatocytes and absorptive cells of the intestine, this spatial relationship of ER and plasmalemma suggests a role for rosettes in the uptake of exogenous lipid precursors. The morphological differentiation of 3T3-L1 preadipocytes includes the loss of "stress fibers" and the appearance of microfilament like structures that encase, in a complex manner, the cytosolic lipid spheres that appear during differentiation. Other features described for the first time in 3T3-L1 preadipocytes include: (a) the presence of an extensive acid phosphatase (AcPase) positive GERL from which coated vesicles apparently arise (these coated vesicles display AcPase activity and are much smaller and far more numerous than the coated vesicles that seem to arise from the plasmalemmal coated pits); (b) the abundance of AcPase-positive autophagic vacuoles; and (c) a high level of alpha-naphthyl-acetate-esterase activity which, by light microscopy cytochemistry, appears to be localized in the cytosol.

[Preadipocytes: a new model in obesity research (author's transl)]

The present knowledge about the differentiation of preadipocytes into adipocytes is reviewed. The adipose conversion is initiated by an as yet unknown serum factor and is enhanced by various hormones including insulin, prostaglandin F2 alpha, steroids, and prolactin; prostaglandin E1 and phorbol diesters are inhibitors of differentiation. Adipose conversion of fibroblasts (preadipocytes) is associated with the coordinate induction of key enzymes of the lipogenetic and lipolytic pathways and is accompanied by profound changes in hormone responsiveness. Pathophysiological studies with preadipocytes of genetically obese mice show differences between adipocyte precursors of obese animals and lean controls which may be casually related to obesity. Regional differences in the hormonal regulation of fibroblast conversion might be important for the sex differences of fat deposition in human beings.

Establishment of a clonal cell line that differentiates into adipose cells in vitro

From the fibroblastic cells of murine mammary tumor tissue we isolated a clonal cell line that could be induced to differentiate into fat-accumulating cells in vitro. Differentiation began after the cultures had reached confluence and was accompanied by (a) an increase in the incorporation of sodium acetate into the triglyceride (TG) fraction of cellular lipids, (b) a more than 50-fold increase in cellular TG content per milligram cell-layer protein basis and (c) the cessation of cellular DNA synthesis. The addition of insulin to the culture medium enhanced lipid accumulation and increased the fraction of cells that differentiated into adipocytes. When insulin (5 to 10 microgram/ml) was added exogenously, 80% or more of the cells were induced to differentiate into mature adipocytes within 2 weeks. This cell line can be used as a model for mammalian cell differentiation and also as a convenient material for the study of lipid metabolism in adipocytes.

Acquisition of an insulin-sensitive activity of adenosine-3',5'-monophosphate phosphodiesterase during adipose conversion of 3T3-L2 cells

Mouse 3T3-L2 fibroblasts differentiate when cultured and express a new phenotype which is characteristic of adipose cells. One aspect of this differentiation is the acquisition of hormone sensitivities typical of adipocytes. The sensitivity of adenosine-3',5'-monophosphate (cAMP) phosphodiesterase to brief insulin treatment in 3T3-L2 cells at various stages of adipose conversion was examined. The capacity to increase the activity of a low-Km form of the enzyme in response to acute insulin exposure is a property which is acquired as the cells begin to express the adipose phenotype. Enzyme activity in preadipocytes is not affected by inslin, nor is there an effect in the non-differentiating 3T3-C2 line under any of the conditions tested. Since insulin receptors and cAMP phosphodiesterase are both present in the preadipocytes, one result of this cytodifferentiation is to effectively couple receptors to the enzyme activity. Insulin-sensitive cAMP phosphodiesterase in 3T3-L2 adipocytes is characterized by a 40-50% increase in enzyme activity during insulin treatment when determined at 0.1 microM cAMP. This activation results from an increase in the apparent V of the enzyme and does not involve a change in enzyme-substrate affinity. Maximal stimulation is seen within 2-5 min and is sustained for at least 45 min when high levels of insulin (850 nM) are used. Lower concentrations of insulin (1.7 nM) also bring about rapid activation, although the activation is not completely sustained during longer incubations. Stimulated activity falls off to about 60% of peak values by 30 min. Re-addition of insulin after 30 min raises enzyme activity back to the maximal level. Further, the insulin response is completely reversible in that the insulin-sensitive cAMP phosphodiesterase activity disappears within 10 min after removal of insulin from the cultures.

Insulin-treated 3T3-L1 adipocytes and cell-free extracts derived from them incorporate 32P into ribosomal protein S6

Differentiated 3T3-L1 preadipocytes preincubated for 60 min with 32Pi incorporate 32P into ribosomal protein S6 within 5 min after exposure to 0.1-1.0 nM insulin. Undifferentiated 3T3-L1 cells, which possess only 3-5% of the high-affinity cell surface insulin receptors present on the differentiated cells, are less sensitive to this stimulation. Under the same conditions used for insulin, epidermal growth factor, antibody to the insulin receptor, and a combination of isoproterenol and 1-methyl-3-isobutylxanthine also promote 32P incorporation into S6 in the differentiated cells although less effectively than insulin. Cell-free extracts derived from cells treated for 5-10 min with either physiological concentrations of insulin or epidermal growth factor (0.1 microgram/ml) reflect intact cells and catalyze the incorporation of 32P from exogenous [gamma-32P]ATP into ribosomal protein S6.

Induction of pyruvate carboxylase apoenzyme and holoenzyme in 3T3-L1 cells during differentiation

The specific activity of pyruvate carboxylase [pyruvate:carbon-dioxide ligase (ADP-forming); EC 6.4.1.1] in 3T3-L1 cells increases approximately 20-fold when these cells differentiate to an adipocyte-like form [Mackall, J. C. & Lane, M. D. (1977) Biochem. Biophys. Res. Commun. 79, 720-725]. A specific antibody to the purified rat liver enzyme quantitatively precipitated pyruvate carboxylase from 3T3-L1 crude homogenates. Use of this immunological technique permitted us to demonstrate that the increase in pyruvate carboxylase activity is due to an increase in the intracellular concentration of the enzyme. The content of pyruvate carboxylase in differentiated 3T3-L1 cells is sufficiently high (1-2% of total protein) that the increase in this large protein (subunit M(r) = 130,000) can be visualized when 3T3-L1 crude extracts are subjected to electrophoresis on sodium dodecyl sulfate/polyacrylamide gels. When 3T3-L1 cells differentiated in the presence of avidin, they contained less than 5% of the pyruvate carboxylase activity of cells that differentiated in the absence of avidin. However, the immunoprecipitable pyruvate carboxylase content of the avidin-treated cells was essentially the same as that of cells that differentiated without avidin. Full activity of the enzyme was rapidly restored in the avidin-treated cells upon the addition of excess biotin. The recovery of activity was closely correlated with the incorporation of [(14)C]biotin into immunoprecipitable pyruvate carboxylase. The rapidity with which the activity was restored and the insensitivity of the process to inhibitors of protein synthesis strongly suggest that the apoenzyme of pyruvate carboxylase accumulates during differentiation in the presence of avidin.

Inhibition of adipose conversion in 3T3-L2 cells by retinoic acid

The role of retinoic acid in modulating the differentiation of 3T3-L2 fibroblasts into adipocytes has been examined. Results indicate that the retinoid is capable of effectively inhibiting the degree of adipose conversion which is brought about by treatment of preadipocytes with 1-methyl-3-isobutylxanthine plus dexamethasone. MOrphological and enzymatic (fatty acid synthetase activity) expression of the adipose phenotype are both inhibited more than 90% by 10(-6) M retinoic acid. The inhibition is concentration dependent with retinoic acid must be administered simultaneously with the triggering agents to be effective. Exposure of nongrowing preadipocytes to retinoic acid does not alter the ability o the cells to differentiate in response to a subsequent treatment with methylisobutylxanthine plus dexamethasone. Further, the inhibition is reversible. Cultures i which methylisobutylxanthine plus dexamethasone triggered differentiation has been blocked by addition of retinoic acid (10(-6) M) will readily undergo adipose conversion in response to a second treatment with methylisobutylxanthine plus dexamethasone in the absence of the retinoid. Similar inhibition of differentiation was found when cultures were treated with drugs in medium supplemented with either newborn calf serum or fetal calf serum. However, the extent to which methylisobutylxanthine plus dexamethasone are able to promote differentiation in these cells is considerably greater in medium containing fetal calf serum.

Insulin receptor synthesis and turnover in differentiating 3T3-L1 preadipocytes

A density-shift method is described for analyzing insulin receptor synthesis and turnover in cultured cells labeled with "heavy" amino acids (2H, 13C, and 15N). Solubilized newly synthesized heavy and old "light" receptors are separated by isopycnic banding on CsCl gradients and then quantitated. Insulin receptor synthesis and turnover were studied by this technique in 3T3-L1 preadipocytes which undergo an increase in insulin binding capacity during differentiation. The results indicate that the increase in insulin binding capacity is a consequence of new receptor synthesis, that the insulin receptor has a relatively short half-life (6.7 hr), and that an increased rate of receptor synthesis contributes to the increase of insulin receptor level during differentiation.