Activation of signal transducers and activators of transcription 1 and 3 by leukemia inhibitory factor, oncostatin-M, and interferon-gamma in adipocytes

Interferon-gamma-mediated activation and ubiquitin-proteasome-dependent degradation of PPARgamma in adipocytes

Interferon-gamma (IFNgamma) treatment of adipocytes results in a down-regulation of the peroxisome proliferator-activated receptor gamma (PPARgamma). The decrease in PPARgamma expression is mediated by inhibition of PPARgamma synthesis and increased degradation of PPARgamma. In this study, we demonstrate that both PPARgamma1 and PPARgamma2 are targeted to the proteasome under basal conditions and that PPARgamma1 is more labile than PPARgamma2. The IFNgamma-induced increase in PPARgamma turnover is blocked by proteasome inhibition and is accompanied by an increase in PPARgamma-polyubiquitin conjugates. In addition, IFNgamma treatment results in the transcriptional activation of PPARgamma. Similar to ligand-dependent activation of PPARgamma, IFNgamma-induced activation was greater in the phosphorylation-deficient S112A form of PPARgamma when compared with wild-type PPARgamma. Moreover, the inhibition of ERKs 1 and 2 with a MEK inhibitor, U1026, lead to an inhibition in the decay of PPARgamma proteins, indicating that serine phosphorylation influences the degradation of PPARgamma in fat cells. Our results also demonstrate that the proteasome-dependent degradation of PPARgamma does not require nuclear export. Taken together, these results indicate that PPARgamma is targeted to the ubiquitin-proteasome pathway for degradation under basal conditions and that IFNgamma leads to an increased targeting of PPARgamma to the ubiquitin-proteasome system in a process that is affected by ERK-regulated serine phosphorylation of PPARgamma proteins.

The ribosomal S6 kinases, cAMP-responsive element-binding, and STAT3 proteins are regulated by different leukemia inhibitory factor signaling pathways in mouse embryonic stem cells

Mouse embryonic stem (ES) cells remain "pluripotent" in vitro in the continuous presence of leukemia inhibitory factor (LIF). In the absence of LIF, ES cells are irreversibly committed to differentiate into various lineages. In this study we have set up an in vitro assay based on the anti-apoptotic activity of LIF to distinguish pluripotent from "differentiation-committed" ES cells. We have examined the phosphorylation profiles of known (STAT3 and ERKs) and identified new (ribosomal S6 kinases (RSKs) and cAMP-responsive element-binding protein (CREB)) LIF-regulated targets in ES and in ES-derived neuronal cells. We have demonstrated that although STAT3, a crucial player in the maintenance of ES cell pluripotency, is induced by LIF in all cell types tested, the LIF-dependent activation of RSKs is restricted to ES cells. We have shown that LIF-induced phosphorylation of RSKs in ES cells is dependent on ERKs, whereas STAT3 phosphorylation is not mediated by any known MAPK activities. Our results also demonstrate that the LIF-dependent phosphorylation of CREB is partially under the control of the RSK2 kinase.

The identification and characterization of a STAT 1 binding site in the PPARgamma2 promoter

Interferon-gamma (IFNgamma) has been shown to decrease the expression of peroxisome proliferator activated receptor-gamma (PPARgamma) in fat cells by blocking the synthesis and increasing the degradation of this transcription factor. Since IFNgamma is a potent activator of STAT 1, we searched for IFNgamma-sensitive binding sites in the PPARgamma promotors. A region of the murine PPARgamma2 promoter was identified that bound nuclear protein from adipocyte nuclei that had been acutely treated with IFNgamma. Supershift analysis revealed that STAT 1, and no other STATs present in the adipocyte nucleus, was capable of binding to this site within the PPARgamma2 promoter. NIH 3T3 and 3T3-L1 cells were transiently transfected with a PPARgamma2 promoter reporter construct, which contained the STAT 1 binding site. Treatment of these cells with IFNgamma resulted in a decrease in reporter activity, demonstrating the modulation of the PPARgamma2 promoter by IFNgamma. We also examined the ability of leukemia inhibitory factor (LIF) to regulate binding at this site. LIF, a potent activator of STAT3 and a weak activator of STAT 1 in these cells, resulted in some binding to the IFNgamma responsive element in the PPARgamma2 promoter that was mediated by STAT 1. Therefore, we examined the ability of LIF to regulate PPARgamma mRNA and observed that LIF, unlike IFNgamma, had little effect on PPARgamma expression. These results and our previous work suggest that cytokine induced STAT 1 homodimers modulate the transcriptional repression of PPARgamma2 in adipocytes.

Mediators of interferon gamma-initiated signaling in bovine luteal cells

Interferon gamma (IFNgamma) has been implicated as a mediator of luteal steroidogenesis and cell fate. IFNgamma-initiated signaling events, although implied by studies in cell lines, have yet to be described in primary luteal cells. The objective of these studies was to begin to characterize IFNgamma-initiated signaling within luteal cells. Dispersed bovine luteal cell cultures were challenged with increasing levels of bovine recombinant IFNgamma (0-1000 U) or IFNgamma (200 U) in the presence or absence of tumor necrosis factor alpha (TNFalpha, 10 ng/ml) over time (short term, 0-60 min; long term, 0, 24, 48 h). Fractionated or total cell lysates were evaluated by the Western blotting technique to determine the changes in the levels of signal transducers and activators of transcription (STAT), interferon regulatory factor 1 (IRF-1), and I kappa B alpha (IkappaB-alpha). Utilizing antibodies that recognize the nonphosphorylated forms of STAT-1 and STAT-3, it was determined that levels of STAT-1 and STAT-3 in total cell lysates were constitutively expressed and did not change in response to treatment with IFNgamma or TNFalpha. In contrast, nuclear levels of STAT-1 and phosphorylated STAT-3 were elevated in a time-dependent manner in response to IFNgamma treatment. Furthermore, IFNgamma and TNFalpha treatment elevated levels of IRF-1 within 2 h. TNFalpha-induced increases in the levels of IRF-1 were transient, whereas the levels of IRF-1 in response to IFNgamma treatment remained elevated at 48 h. These data suggest that IFNgamma treatment can activate members of the STAT pathway, resulting in increased levels of IRF-1. TNFalpha treatment induced a rapid decrease in the levels of IkappaB-alpha. IFNgamma treatment did not alter the levels of IkappaB-alpha and failed to inhibit the TNFalpha-initiated decrease in the levels of IkappaB-alpha. The present experiment demonstrates that the steroidogenic cells of the corpus luteum have the capacity to respond to IFNgamma via activation of STAT and IRF-1, providing further evidence that IFNgamma may be involved in the luteolytic process. These data also suggest that IFNgamma does not signal through the nuclear factor kappa B cell survival signaling pathway.

Differentiation therapy of human cancer: basic science and clinical applications

Current cancer therapies are highly toxic and often nonspecific. A potentially less toxic approach to treating this prevalent disease employs agents that modify cancer cell differentiation, termed 'differentiation therapy.' This approach is based on the tacit assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment, results in tumor reprogramming and a concomitant loss in proliferative capacity and induction of terminal differentiation or apoptosis (programmed cell death). Laboratory studies that focus on elucidating mechanisms of action are demonstrating the effectiveness of 'differentiation therapy,' which is now beginning to show translational promise in the clinical setting.

Agouti regulates adipocyte transcription factors

Agouti is a secreted paracrine factor that regulates pigmentation in hair follicle melanocytes. Several dominant mutations cause ectopic expression of agouti, resulting in a phenotype characterized by yellow fur, adult-onset obesity and diabetes, increased linear growth and skeletal mass, and increased susceptibility to tumors. Humans also produce agouti protein, but the highest levels of agouti in humans are found in adipose tissue. To mimic the human agouti expression pattern in mice, transgenic mice (aP2-agouti) that express agouti in adipose tissue were generated. The transgenic mice develop a mild form of obesity, and they are sensitized to the action of insulin. We correlated the levels of specific regulators of insulin signaling and adipocyte differentiation with these phenotypic changes in adipose tissue. Signal transducers and activators of transcription (STAT)1, STAT3, and peroxisome proliferator-activated receptor (PPAR)-gamma protein levels were elevated in the transgenic mice. Treatment of mature 3T3-L1 adipocytes recapitulated these effects. These data demonstrate that agouti has potent effects on adipose tissue. We hypothesize that agouti increases adiposity and promotes insulin sensitivity by acting directly on adipocytes via PPAR-gamma.

Differential expression of signal transducers and activators of transcription during human adipogenesis

Signal Transducers and Activators of Transcription (STATs) display unique expression patterns upon induction of differentiation of murine 3T3-L1 preadipocytes into adipocytes. During differentiation, expression of STAT1 and STAT5 increase, while STAT3 and STAT6 remain relatively unchanged. Here, we determined whether human subcutaneous preadipocytes expressed STATs and if the pattern of expression changed during adipogenesis. We found by Western blot analysis that freshly isolated preadipocytes expressed STAT1, STAT3, STAT5, and STAT6, but not STAT2 and STAT4. Induction of preadipocyte differentiation with 1-methyl-3-isobutylxanthine, dexamethasone, insulin, and BRL49653 decreased expression of STAT1, and increased expression of STAT3 and STAT5. STAT6 expression did not change during adipogenesis. Changes in expression of CCAAT/enhancer binding protein beta (C/EBPbeta), C/EBPdelta, C/EBPalpha, and peroxisome proliferator-activated receptor gamma were similar to murine cell lines. These results suggest that unlike the traditional adipogenic transcription factors, unique differences exist in STAT expression patterns between murine and human adipose cells.

Interferon-gamma-induced regulation of peroxisome proliferator-activated receptor gamma and STATs in adipocytes

Interferon-gamma (IFN-gamma) is known primarily for its roles in immunological responses but also has been shown to affect fat metabolism and adipocyte gene expression. To further investigate the effects of IFN-gamma on fat cells, we examined the effects of this cytokine on the expression of adipocyte transcription factors in 3T3-L1 adipocytes. Although IFN-gamma regulated the expression of several adipocyte transcription factors, IFN-gamma treatment resulted in a rapid reduction of both peroxisome proliferator-activated receptor (PPAR) protein and mRNA. A 48-h exposure to IFN-gamma also resulted in a decrease of both CCAAT/enhancer-binding alpha and sterol regulatory element binding protein (SREBP-1) expression. The short half-life of both the PPARgamma mRNA and protein likely contributed to the rapid decline of both cytosolic and nuclear PPARgamma in the presence of IFN-gamma. Our studies clearly demonstrated that the IFN-gamma-induced loss of PPARgamma protein is partially inhibited in the presence of two distinct proteasome inhibitors. Moreover, IFN-gamma also inhibited the transcription of PPARgamma, which was accompanied by a decrease in PPARgamma mRNA accumulation. In addition, exposure to IFN-gamma resulted in a substantial increase in STAT 1 expression and a small increase in STAT 3 expression. IFN-gamma treatment of 3T3-L1 adipocytes (48-96 h) resulted in a substantial inhibition of insulin-sensitive glucose uptake. These data clearly demonstrate that IFN-gamma treatment results in the development of insulin resistance, which is accompanied by the regulation of various adipocyte transcription factors, in particular the synthesis and degradation of PPARgamma.

Specificity of signaling by STAT1 depends on SH2 and C-terminal domains that regulate Ser727 phosphorylation, differentially affecting specific target gene expression

Complete activation of signal transducer and activator of transcription 1 (STAT1) requires phosphorylation at both Y701 and a conserved PMS(727)P sequence. S727 phosphorylation of STAT1 in interferon-gamma (IFN-gamma)-treated mouse fibroblasts occurred without a need for p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinases 1 and 2 or c-Jun kinases, and required both an intact SH2 domain and phosphorylation of Y701. In contrast, UV irradiation-induced STAT1 phosphorylation on S727 required p38MAPK, but no SH2 domain- phosphotyrosine interactions. Mutation of S727 differentially affected IFN-gamma target genes, at the level of both basal and induced expression. Particularly strong effects were noted for the GBP1 and TAP1 genes. The PMS(727)P motif of STAT3 was phosphorylated by stimuli and signaling pathways different from those for STAT1 S727. Transfer of the STAT3 C-terminus to STAT1 changed the stimulus and pathway specificity of STAT1 S727 phosphorylation to that of STAT3. Our data suggest that STAT C-termini contribute to the specificity of cellular responses by linking individual STATs to different serine kinase pathways and through an intrinsically different requirement for serine phosphorylation at different target gene promoters.

Glycogen synthase kinase-3 enhances nuclear export of a Dictyostelium STAT protein

Extracellular cAMP stimulates the rapid tyrosine phosphorylation and nuclear translocation of the DICTYOSTELIUM: STAT protein Dd-STATa. Here we show that it also induces serine phosphorylation by GskA, a homologue of glycogen synthase kinase-3 (GSK-3). Tyrosine phosphorylation occurs within 10 s of stimulation, whereas serine phosphorylation takes 5 min, matching the kinetics observed for the cAMP regulation of GskA. Phosphorylation by GskA enhances nuclear export of Dd-STATa. The phosphorylated region, however, is not itself a nuclear export signal and we identify a region elsewhere in the protein that mediates nuclear export. These results suggest a biphasic regulation of Dd-STATa, in which extracellular cAMP initially directs nuclear import and then, via GskA, promotes its subsequent export. It also raises the possibility of an analogous regulation of STAT nuclear export in higher eukaryotes.

Interferons activate the p42/44 mitogen-activated protein kinase and JAK-STAT (Janus kinase-signal transducer and activator transcription factor) signalling pathways in hepatocytes: differential regulation by acute ethanol via a protein kinase C-dependent mechanism

Interferons (IFNs) have been used in the treatment of viral hepatitis. However, their effectiveness is much reduced (<10%) in alcoholics. The mechanism underlying this resistance remains unknown. Here, we report that IFN-alpha/beta and IFN-gamma rapidly activate the JAK-STAT1 (Janus kinase-signal transducer and activator transcription factor 1) and p42/44 mitogen-activated protein kinase (p42/44 MAPK) in freshly isolated rat hepatocytes. Treatment of hepatocytes with 25-100 mM ethanol for 30 min inhibited IFN-beta- or IFN-gamma-induced STAT1 activation and tyrosine phosphorylation. The inhibitory effect of ethanol was not reversed by pretreatment with either sodium vanadate, a non-selective tyrosine phosphatase inhibitor, or with MG132, a specific proteasome inhibitor. This suggests that protein tyrosine phosphatases or the ubiquitin-proteasome pathway are not involved in the inhibitory action of ethanol. In contrast with the JAK-STAT signalling pathway, acute ethanol exposure significantly potentiated IFN-beta or IFN-gamma-induced activation of p42/44 MAPK, and caused marked activation of protein kinase C (PKC). Inhibition of PKC partially antagonized ethanol attenuation of IFN-induced STAT1 activation, suggesting that PKC may be involved. Taken together, these findings suggest that the ability of biologically relevant concentrations of ethanol (less than 100 mM) to markedly inhibit IFN-activated STAT1 is one of the cellular mechanisms responsible for the observed resistance of IFN therapy in alcoholics.

Activation of signal transducer and activator of transcription-3 during proliferative phases of 3T3-L1 adipogenesis

Signal transducer and activator of transcription-3 (STAT3) is abundantly expressed in preadipocytes and adipocytes, but little is known about its activation status or functional role during adipogenesis. In this report we investigate STAT3 activation in 3T3-L1 preadipocytes before and after differentiation into adipocytes. STAT3 was highly tyrosine phosphorylated and bound to DNA in proliferating preadipocytes, but not in growth-arrested preadipocytes or adipocytes. In growth-arrested confluent preadipocytes, induction of differentiation with methylisobutylxanthine, dexamethasone, and high dose insulin led to a delayed, but prolonged (3-day), increase in STAT3 tyrosine phosphorylation. This increase in STAT3 phosphorylation coincided temporally with postconfluent preadipocyte mitotic clonal expansion. Insulin and methylisobutylxanthine alone, but not dexamethasone, induced STAT3 tyrosine phosphorylation in postconfluent cells. Diminution of endogenous STAT3 expression by antisense morpholino oligonucleotides significantly decreased preconfluent preadipocyte proliferation. Collectively, these findings suggest a regulatory role for STAT3 during the proliferative phases of adipogenesis.

Leukemia-inhibitory factor-neuroimmune modulator of endocrine function

Leukemia-inhibitory factor (LIF) is a pleiotropic cytokine expressed by multiple tissue types. The LIF receptor shares a common gp130 receptor subunit with the IL-6 cytokine superfamily. LIF signaling is mediated mainly by JAK-STAT (janus-kinase-signal transducer and activator of transcription) pathways and is abrogated by the SOCS (suppressor-of cytokine signaling) and PIAS (protein inhibitors of activated STAT) proteins. In addition to classic hematopoietic and neuronal actions, LIF plays a critical role in several endocrine functions including the utero-placental unit, the hypothalamo-pituitary-adrenal axis, bone cell metabolism, energy homeostasis, and hormonally responsive tumors. This paper reviews recent advances in our understanding of molecular mechanisms regulating LIF expression and action and also provides a systemic overview of LIF-mediated endocrine regulation. Local and systemic LIF serve to integrate multiple developmental and functional cell signals, culminating in maintaining appropriate hormonal and metabolic homeostasis. LIF thus functions as a critical molecular interface between the neuroimmune and endocrine systems.

Serine phosphorylation of STATs

Tyrosine phosphorylation regulates the dimerization of STATs as an essential prerequisite for the establishment of a classical JAK-STAT signaling path. However, most vertebrate STATs contain a second phosphorylation site within their C-termini. The phosphorylated residue in this case is a serine contained within a P(M)SP motif, and in the majority of situations its mutation to alanine alters transcription factor activity. This review addresses recent advances in understanding the regulation of STAT serine phosphorylation, as well as the kinases and other signal transducers implied in this process. The biochemical and biological consequences of STAT serine phosphorylation are discussed. Oncogene (2000).

Inhibition of ornithine decarboxylase induces STAT3 tyrosine phosphorylation and DNA binding in IEC-6 cells

Polyamines are required for the proliferation of the rat intestinal mucosal IEC-6 cell line. Ornithine decarboxylase (ODC) is the enzyme that catalyzes the first step in polyamine synthesis. ODC inhibition not only leads to polyamine depletion but also leads to inhibition of cell proliferation and regulates the expression of the immediate-early genes c-fos, c-myc, and c-jun. Members of the signal transducers and activators of transcription (STAT) transcription factor family bind to the sis-inducible element (SIE) present in the promoters to regulate the expression of a variety of important genes. In the present study, we tested the hypothesis that the STAT3 transcription factor, which is responsible for activation of the acute phase response genes, is activated after inhibition of ODC. We found that inhibition of ODC rapidly induces STAT3 activation as determined by STAT3 tyrosine phosphorylation, translocation of STAT3 from the cytoplasm into the nucleus, and the presence of STAT3 in SIE-dependent DNA-protein complexes. STAT3 activation upon inhibition of ODC was accompanied by the activation of a STAT3-dependent reporter construct. Moreover, prolonged polyamine depletion resulted in downregulation of cellular STAT3 levels.

Interleukin-4 mediates STAT6 activation in 3T3-L1 preadipocytes but not adipocytes

STAT6 is abundantly expressed in 3T3-L1 preadipocytes and adipocytes but activating ligands are not well defined. In this report, we provide evidence that interleukin 4 (IL-4) induced JAK2-mediated STAT6 tyrosine phosphorylation and DNA binding in 3T3-L1 preadipocytes but not in 3T3-L1 adipocytes. Loss of IL-4-mediated STAT6 tyrosine phosphorylation occurred 2 days after preadipocytes were induced to differentiate into adipocytes but when cells remained phenotypically preadipocytes. 3T3-L1 adipocytes were still responsive to IL-4 through tyrosine phosphorylation of other cellular proteins. We conclude that IL-4 signals through STAT6 in 3T3-L1 preadipocytes but not in 3T3-L1 adipocytes. This differentiation-dependent loss of STAT6 activation may be critical for distinct biological effects of IL-4 in 3T3-L1 preadipocytes and adipocytes.

Oncostatin M: signal transduction and biological activity

Oncostatin M (OSM) is a multifunctional cytokine produced by activated T lymphocytes and monocytes that is structurally and functionally related to the subfamily of cytokines known as the IL-6-type cytokine family. OSM shares properties with all members of this family of cytokines, but is most closely related structurally and functionally to LIE OSM acts on a wide variety of cells and elicits diversified biological responses in vivo and in vitro which suggest potential roles in the regulation of gene activation, cell survival, proliferation and differentiation. OSM and LIF can bind to the same functional receptor complex (LIF-receptor beta and gp130 heteromultidimers) and thus mediate overlapping spectra of biological activities. There is a second specific beta receptor that binds OSM with high affinity and also involves the subunit gp130. The two receptors for OSM can be functionally different and be coupled to different signal transduction pathways. OSM-specific receptors are expressed in a wide variety of cell types and do not possess an intrinsic tyrosine kinase domain, but the JAK/STAT tyrosine kinase pathway mediates signal transduction.

Regulation of signal transducers and activators of transcription (STATs) by effectors of adipogenesis: coordinate regulation of STATs 1, 5A, and 5B with peroxisome proliferator-activated receptor-gamma and C/AAAT enhancer binding protein-alpha

We have recently demonstrated that three signal transducers and activators of transcription (STAT) family members are induced during adipocyte differentiation (Stephens et al., J. Biol. Chem. 271 (1996) 10441-10444). Since STATs 1, 5A, and 5B are induced during adipocyte differentiation, we have examined the ability of these proteins to be regulated by components of the differentiation cocktail. In addition, we have examined the effects of potent effectors of differentiation on STAT protein expression during adipogenesis. A negative effector, tumor necrosis factor-alpha (TNFalpha), and a positive effector, a thiazolidinedione, were used in these experiments. Our results demonstrate that the expression of STATs 1, 5A, and 5B is not dramatically influenced by individual components of the differentiation cocktail. However, the expression of these three STAT family members tightly correlates with lipid accumulation. Moreover, the expression of STATs 1, 5A, and 5B, but not STATs 3 and 6, are regulated in an identical fashion to both C/AAAT enhancer binding proteins alpha and peroxisome proliferator-activated receptor-gamma by TNFalpha and a thiazolidinedione. Furthermore, the expression of adipocyte-expressed JAK kinases are unaffected by effectors of differentiation. These findings suggest that three STAT family members may play a role in the regulation of adipocyte gene expression.

Signaling pathways activated by interferons

Interferons are pleiotropic cytokines that exhibit negative regulatory effects on the growth of normal and malignant hematopoietic cells in vitro and in vivo. There are two different classes of interferons, Type I (alpha, beta, and omega) and Type II (gamma) interferons. Although the precise mechanisms by which these cytokines exhibit their potent effects on hematopoiesis remain unknown, there has been considerable progress in our understanding of the cellular changes that occur upon engagement of interferon receptors. It is now well established that Type I interferons activate multiple signaling pathways in hematopoietic cells, a finding consistent with their pleiotropic biological effects. One major pathway in Type I IFN signaling involves activation of Stat- proteins and formation of complexes that translocate to the nucleus and bind to specific elements to regulate gene transcription. The activation of this pathway (Jak-Stat pathway) is apparently regulated by members of the Jak-family of kinases, which are constitutively associated with the Type I IFN receptor. In addition to the Jak-Stat pathway, multiple other Jak-kinase-dependent signaling cascades are activated, including the IRS-PI 3'-kinase pathway, a pathway involving the vav proto-oncogene product, and a pathway involving adaptor proteins of the Crk-family (CrkL and CrkII). The only Type II interferon, IFNgamma, also activates multiple Jak-kinase-dependent signaling cascades, including the Stat and Crk pathways. Recent evidence suggests that non-Stat pathways play a critical role in the generation of signals for both Type I and Type II interferons and may be the primary mediators of their growth inhibitory effects on hematopoietic cells.

Leukemia inhibitory factor and its receptor promote adipocyte differentiation via the mitogen-activated protein kinase cascade

Extracellular factors and intracellular signaling pathways involved in early events of adipocyte differentiation are poorly defined. It is shown herein that expression of leukemia inhibitory factor (LIF) and LIF receptor is developmentally regulated during adipocyte differentiation. Preadipocytes secrete bioactive LIF, and an antagonist of LIF receptor inhibits adipogenesis. Genetically modified embryonic stem (ES) cells combined with culture conditions to commit stem cells into the adipocyte lineage were used to examine the requirement of LIF receptor during in vitro development of adipose cells. The capacity of embryoid bodies derived from lifr(-/-) ES cells to undergo adipocyte differentiation is dramatically reduced. LIF addition stimulates adipocyte differentiation of Ob1771 and 3T3-F442A preadipocytes and that of peroxisome proliferator-activated receptor gamma2 ligand-treated mouse embryonic fibroblasts. Expression of the early adipogenic transcription factors C/EBPbeta and C/EBPdelta is rapidly stimulated following exposure of preadipose cells to LIF. The selective inhibitors of mitogen-activated protein kinase kinase, i.e. PD98059 and U0126, inhibit LIF-induced C/EBP gene expression and prevent adipocyte differentiation induced by LIF. These results are in favor of a model that implicates stimulation of LIF receptor in the commitment of preadipocytes to undergo terminal differentiation by controlling the early expression of C/EBPbeta and C/EBPdelta genes via the mitogen-activated protein kinase cascade.