Insulin stimulates the serine phosphorylation of the signal transducer and activator of transcription (STAT3) isoform

STAT4 expression and activation is increased during mitosis in vitro and in vivo in skin- and mucosa-derived cell types: implications in neoplastic and inflammatory skin diseases

BACKGROUND

The signal transducer and activator of transcription-4 (STAT4/Stat4) is a transcription factor known to convey signals from interleukin-12, interleukin-23, and interferon-alpha/beta to the nucleus, resulting in activation of dendritic cells, T-helper cell differentiation and production of interferon-gamma.

OBJECTIVE

To demonstrate a novel role for STAT4 in cell mitosis.

RESULTS

Phosphoserine STAT4 (pSerSTAT4) is increased in cells undergoing mitosis and is distributed throughout the cytoplasm during this stage of the cell cycle, whilst phosphotyrosine STAT4 (pTyrSTAT4) is confined to the chromosomal compartment. This distinct pattern of pSerSTAT4 during mitosis is seen in vitro in human keratinocytes and in other cell types. This is also present in vivo in cells undergoing mitosis in normal skin, psoriasis and squamous cell carcinoma. Inhibition of STAT4 phosphorylation by lisofylline and depletion of STAT4 by RNA interference results in a delay in progression of mitosis and leads to a reduction in cells completing cytokinesis.

CONCLUSION

Our data demonstrate that STAT4 plays a role in enabling the normal and timely division of cells undergoing mitosis.

The effectiveness of an anti-human IL-6 receptor monoclonal antibody combined with chemotherapy to target colon cancer stem-like cells

Recent studies have demonstrated that cancer stem cells (CSCs) can initiate and sustain tumor growth and exhibit resistance to clinical cytotoxic therapies. Therefore, CSCs represent the main target of anticancer therapy. Interleukin-6 (IL-6) promotes cellular proliferation and drug resistance in colorectal cancer, and its serum levels correlate with patient survival. Therefore, IL-6 and its downstream signaling molecule the signal transducer and activator of transcription-3 (STAT3) represent potential molecular targets. In the present study, we investigated the effects of IL-6 and its downstream signaling components on stem cell biology, particularly the chemoresistance of CSCs, to explore potential molecular targets for cancer therapy. The colon cancer cell line WiDr was cultured in serum-free, non-adherent, and three-dimensional spheroid-forming conditions to enrich the stem cell-like population. Spheroid-forming cells slowly proliferated and expressed high levels of Oct-4, Klf4, Bmi-1, Lgr5, IL-6, and Notch 3 compared with adherent cells. Treatment with an anti-human IL-6 receptor monoclonal antibody reduced spheroid formation, stem cell-related gene expression, and 5-fluorouracil (5-FU) resistance. In addition, IL-6 treatment enhanced the levels of p-STAT3 (Tyr705), the expression of Oct-4, Klf4, Lgr5, and Notch 3, and chemoresistance to 5-FU. siRNA targeting Notch 3 suppressed spheroid formation, Oct-4 and Lgr5 expression, and 5-FU chemoresistance, whereas STAT3 inhibition enhanced Oct-4, Klf4, Lgr5, and Notch 3 expression and 5-FU chemoresistance along with reduced spheroid growth. Taken together, these results indicate that IL-6 functions in dichotomous pathways involving Notch 3 induction and STAT3 activation. The former pathway is involved in cancer stem-like cell biology and enhanced chemoresistance, and the latter pathway leads to accelerated proliferation and reduced chemoresistance. Thus, an anti-human IL-6 receptor monoclonal antibody or Notch 3 inhibition may be superior to STAT3 inhibition for CSC-targeting therapies concomitant with anticancer drugs.

Evaluation of the prognostic role of pSTAT3 expression in temporal bone squamous cell carcinoma

OBJECTIVE

Temporal bone squamous cell carcinoma (SCC) accounts for less than 0.2% of all head and neck tumors. Although some progress has been made in treating this aggressive tumor, the prognosis in advanced cases remains poor. More effective therapeutic strategies need to be considered, including receptor-mediated carcinoma-targeted therapy. Phosphorylated STAT3 (pSTAT3) regulates many genes that are necessarily expressed in cancer initiation, development, and progression, being involved in proliferation, anti-apoptosis, invasion, angiogenesis, and immune surveillance evasion. The aim of the present study was to preliminarily investigate the potential prognostic role of pSTAT3 expression in temporal bone SCC.

STUDY DESIGN

Retrospective clinicopathologic investigation.

SETTING

Tertiary referral centers.

PATIENTS

Twenty-five consecutively operated patients with primary temporal bone SCC.

INTERVENTION

pSTAT3 immunohistochemical expression in primary temporal bone SCCs was assessed with the aid of computer-based image analysis.

MAIN OUTCOME MEASURES

Conventional clinicopathologic parameters and pSTAT3 expression were correlated with SCC prognosis.

RESULTS

pT, stage, and surgical margin status were significantly related with recurrence rate (p = 0.002, p = 0.01, and p = 0.047, respectively) and disease-free survival (DFS) (p = 0.0049, p = 0.031, and p = 0.035, respectively). pT classification was also related with disease-specific survival (DSS) (p = 0.035). The SCC recurrence rate did not correlate with pSTAT3 expression. Statistical analyses ruled out any significant difference in DFS or DSS when patients were stratified by pSTAT3 expression (>80.0% or ≤80.0%).

CONCLUSION

Despite our preliminary results, the role of pSTAT3 in temporal bone SCC warrants further investigation in larger series because there is increasing evidence in preclinical models to indicate that inhibiting STAT3 phosphorylation can be a useful addition to different anticancer strategies.

STAT3 and ERK1/2 cross-talk in leukaemia inhibitory factor mediated trophoblastic JEG-3 cell invasion and expression of mucin 1 and Fos

PROBLEM

The aim of this study was to investigate the relative importance of STAT3 and ERK1/2 activation in leukaemia inhibitory factor (LIF)-mediated invasion of JEG-3 cells.

METHOD OF STUDY

Matrigel matrix-based invasion assay; Western blot; cDNA microarray; quantitative RT-PCR; gene silencing by siRNA.

RESULTS

Leukaemia inhibitory factor treatment led to the activation of STAT3 and ERK1/2 signaling pathways which was followed by changes in the expression of several invasion-associated molecules such as mucin1 (MUC1), Fos, Jun, etc. Abrogation of either STAT3 or ERK1/2 signaling reduced (P < 0.05) the LIF-mediated invasion of JEG-3 cells. It was associated with a significant reduction in the expression of both MUC1 and Fos, suggesting a common denominator in LIF-STAT3-ERK1/2 signaling. To this effect, we observed a decrease in LIF-mediated p-STAT3 (Ser727) upon blocking STAT3 or ERK1/2 signaling.

CONCLUSIONS

ERK1/2 as well as JAK-STAT-mediated STAT3 (Ser727) phosphorylation play an important role in LIF-mediated JEG-3 trophoblastic cell invasion and gene expression.

Neuronal activity-dependent STAT3 localization to nucleus is dependent on Tyr-705 and Ser-727 phosphorylation in rat hippocampal neurons

Signal transducer and activator of transcription 3 (STAT3) dramatically increases during the first post-natal week, and supports the survival of mature hippocampal neurons. Recently, we reported that chronic elevation of excitability leads to a loss of STAT3 signal, inducing vulnerability in neurons. The loss of STAT3 signal was due to impaired Erk1/2 activation. While overnight elevation of activity attenuated STAT3 signal, brief low-frequency stimuli, which induce long-term depression, have been shown to activate STAT3. Here we investigated how STAT3 responds to depolarization in mature neurons. A brief depolarization results in the transient activation of STAT3: it induces calcium influx through L-type voltage-gated calcium channels, which triggers activation of Src family kinases. Src family kinases are required for phosphorylation of STAT3 at Tyr-705 and Ser-727. PTyr-705 is Janus kinase (JAK)-dependent, while PSer-727 is dependent on Akt, the Ser/Thr kinase. Both PTyr-705 and PSer-727 are necessary for nuclear translocation of STAT3 in these neurons. Chronic elevation of spontaneous activity by an A-type potassium blocker, 4-aminopyridine (4-AP), also induced the transient phosphorylation of STAT3, which after 4 h fell to basal levels despite the presence of 4-AP. These results suggest that phasic and chronic neuronal activation induce distinct molecular pathways, resulting in opposing regulation of STAT3 signal.

LIF-STAT signaling and trophoblast biology

Leukemia inhibitory factor (LIF) is a pleiotropic growth factor that regulates several biological functions. This review focuses on the LIF-dependent STAT activation and its impact on modulation of trophoblast functions during embryo implantation. LIF is mainly produced by the maternal endometrium at the time of implantation while its receptors are present both on the endometrium and trophoblasts. It might influence blastocyst attachment through STAT3 activation and expression of integrins. After attachment of the blastocyst, trophoblasts undergo proliferation and differentiation into invasive EVTs and non-invasive STBs. Under in vitro conditions, LIF regulates all these processes through activation of STAT- and MAPK-dependent signaling pathways. The observations that LIF and STAT3 knockout mice are infertile further strengthen the notion about the critical involvement of LIF-mediated signaling during embryo implantation. Hence, a better understanding of LIF-STAT signaling would help in improving fertility as use of LIF in in vitro blastocyst culture improves the implanting ability of blastocyst after IVF.

Loss of signal transducer and activator of transcription 3 (STAT3) signaling during elevated activity causes vulnerability in hippocampal neurons

Chronically altered levels of network activity lead to changes in the morphology and functions of neurons. However, little is known of how changes in neuronal activity alter the intracellular signaling pathways mediating neuronal survival. Here, we use primary cultures of rat hippocampal neurons to show that elevated neuronal activity impairs phosphorylation of the serine/threonine kinase, Erk1/2, and the activation of signal transducer and activator of transcription 3 (STAT3) by phosphorylation of serine 727. Chronically stimulated neurons go through apoptosis when they fail to activate another serine/threonine kinase, Akt. Gain- and loss-of-function experiments show that STAT3 plays the key role directly downstream from Erk1/2 as the alternative survival pathway. Elevated neuronal activity resulted in increased expression of a tumor suppressor, p53, and its target gene, Bax. These changes are observed in Kv4.2 knock-out mouse hippocampal neurons, which are also sensitive to the blockade of TrkB signaling, confirming that the alteration occurs in vivo. Thus, this study provides new insight into a mechanism by which chronic elevation of activity may cause neurodegeneration.

Serine 727 phosphorylation of STAT3: an early change in mouse hepatocarcinogenesis induced by neonatal treatment with diethylnitrosamine

STAT3 activation is involved in development and progression of hepatocellular carcinoma (HCC). We investigated STAT3 activation during hepatocarcinogenesis induced by neonatal diethylnitrosamine (DEN) treatment in mice. Nuclear accumulation and phosphorylation of STAT3 were detected in altered hepatocyte foci in the early stages as well as adenomas and HCCs in the late stages. Although total STAT3 levels were the same between the hepatic lesions and normal livers, S727-phosphorylated STAT3 was enhanced in adenomas and HCCs, whereas Y705-phosphorylated STAT3 was detected mainly in HCCs. In mouse HCC cell lines, although both S727 and Y705 remained un- or hypophosphorylated under serum-free conditions, fetal bovine serum (FBS) induced strong S727/weak Y705 phosphorylation, STAT3 nuclear accumulation and cell proliferation, whereas IL-6 treatment without FBS caused Y705 phosphorylation without S727 phosphorylation, STAT3 nuclear accumulation or cell proliferation. When HCCs were simultaneously treated with FBS/IL-6, selective suppression of S727 phosphorylation by an MEK inhibitor prevented STAT3 nuclear accumulation and cell proliferation. Furthermore, an S727 phosphorylation-deficient STAT3 mutant (S727A) had a diminished capacity to accumulate in the nucleus when compared with wild-type (WT) or the phosphorylation-mimic mutant (S727D) following treatment with FBS/IL-6. After treatment with FBS/IL-6, the cells expressing the S727A mutant proliferated more slowly than those expressing WT or S727D mutant. In contrast, suppression of Y705 phosphorylation by a JAK inhibitor in the FBS/IL-6 treated cells did not affect STAT3 nuclear accumulation or cell proliferation. Taken together, these data demonstrate that STAT3 activation, mainly through S727 phosphorylation, contributes to the DEN-induced hepatocarcinogenesis at the earliest stages.

Signal transducers and activators of transcription-from cytokine signalling to cancer biology

Signal transducers and activators of transcription (STATs) are, as the name indicates, both signal transducers and transcription factors. STATs are activated by cytokines and some growth factors and thus control important biological processes. These include cell growth, cell differentiation, apoptosis and immune responses. Dysregulation of STATs, either due to constitutive activation or function impairment, can have, therefore, deleterious biological consequences. This review places particular emphasis on their structural organization, biological activities and regulatory mechanisms most commonly utilized by cells to control STAT-mediated signalling. STATs also play important roles in cancer and immune deficiencies and are thus being exploited as therapeutic targets.

Negative regulation of STAT3 protein-mediated cellular respiration by SIRT1 protein

In mammals, the transcriptional activity of signal transducer and activator of transcription 3 (STAT3) is regulated by the deacetylase SIRT1. However, whether the newly described nongenomic actions of STAT3 toward mitochondrial oxidative phosphorylation are dependent on SIRT1 is unclear. In this study, Sirt1 gene knock-out murine embryonic fibroblast (MEF) cells were used to delineate the role of SIRT1 in the regulation of STAT3 mitochondrial function. Here, we show that STAT3 mRNA and protein levels and the accumulation of serine-phosphorylated STAT3 in mitochondria were increased significantly in Sirt1-KO cells as compared with wild-type MEFs. Various mitochondrial bioenergetic parameters, such as the oxygen consumption rate in cell cultures, enzyme activities of the electron transport chain complexes in isolated mitochondria, and production of ATP and lactate, indicated that Sirt1-KO cells exhibited higher mitochondrial respiration as compared with wild-type MEFs. Two independent approaches, including ectopic expression of SIRT1 and siRNA-mediated knockdown of STAT3, led to reduction in intracellular ATP levels and increased lactate production in Sirt1-KO cells that were approaching those of wild-type controls. Comparison of profiles of phospho-antibody array data indicated that the deletion of SirT1 was accompanied by constitutive activation of the pro-inflammatory NF-κB pathway, which is key for STAT3 induction and increased cellular respiration in Sirt1-KO cells. Thus, SIRT1 appears to be a functional regulator of NF-κB-dependent STAT3 expression that induces mitochondrial biogenesis. These results have implications for understanding the interplay between STAT3 and SIRT1 in pro-inflammatory conditions.

Interleukin-6 inhibits receptor activator of nuclear factor kappaB ligand-induced osteoclastogenesis by diverting cells into the macrophage lineage: key role of Serine727 phosphorylation of signal transducer and activator of transcription 3

Osteoclasts are bone-resorptive cells that differentiate from hematopoietic precursors upon receptor activator of nuclear factor kappaB ligand (RANKL) activation. Previous studies demonstrated that IL-6 indirectly stimulates osteoclastogenesis through the production of RANKL by osteoblasts. However, few data described the direct effect of IL-6 on osteoclasts. To investigate this effect, we used several models: murine RAW264.7 cells, mouse bone marrow, and human blood monocytes. In the three models used, the addition of IL-6 inhibited RANKL-induced osteoclastogenesis. Furthermore, IL-6 decreased the expression of osteoclast markers and up-modulated macrophage markers. To elucidate this inhibition, signal transducer and activator of transcription (STAT) 3, the main signaling molecule activated by IL-6, was analyzed. Addition of two STAT3 inhibitors completely abolished RANKL-induced osteoclastogenesis, revealing a key role of STAT3. We demonstrated that a basal level of phosphorylated-STAT3 on Serine(727) associated with an absence of phosphorylation on Tyrosine(705) is essential for osteoclastogenesis. Furthermore, a decrease of Serine(727) phosphorylation led to an inhibition of osteoclast differentiation, whereas an increase of Tyrosine(705) phosphorylation upon IL-6 stimulation led to the formation of macrophages instead of osteoclasts. In conclusion, we showed for the first time that IL-6 inhibits RANKL-induced osteoclastogenesis by diverting cells into the macrophage lineage, and demonstrated the functional role of activated-STAT3 and its form of phosphorylation in the control of osteoclastogenesis.

Neuropoietin attenuates adipogenesis and induces insulin resistance in adipocytes

Recent findings have implicated gp130 receptor ligands, particularly ciliary neurotrophic factor (CNTF), as potential anti-obesity therapeutics. Neuropoietin (NP) is a recently discovered cytokine in the gp130 family that shares functional and structural features with CNTF and signals via the CNTF receptor tripartite complex comprised of CNTFRalpha, LIF receptor, and gp130. NP plays a role in the development of the nervous system, but the effects of NP on adipocytes have not been previously examined. Because CNTF exerts anti-obesogenic effects in adipocytes and NP shares the same receptor complex, we investigated the effects of NP on adipocyte development and insulin action. Using cultured 3T3-L1 adipocytes, we observed that NP has the ability to block adipogenesis in a dose- and time-dependent manner. We also observed that cultured adipocytes, as well as murine adipose tissue, are highly responsive to acute NP treatment. Rodents injected with NP had a substantial increase in STAT3 tyrosine phosphorylation and ERK 1 and 2 activation. We also observed the induction of SOCS-3 mRNA in 3T3-L1 adipocytes following NP treatment. Unlike CNTF, our studies have revealed that NP also substantially attenuates insulin-stimulated glucose uptake in 3T3-L1 adipocytes. In addition, NP blocks insulin action in adipose tissue in vivo. These observations are supported by data demonstrating that NP impairs insulin signaling via decreased activation of both IRS-1 and Akt. In summary, we have observed that both adipocytes in vitro and in vivo are highly responsive to NP, and this cytokine has the ability to affect insulin signaling in fat cells. These novel observations suggest that NP, unlike CNTF, may not be a viable obesity therapeutic.

LIGHT, a member of the TNF superfamily, activates Stat3 mediated by NIK pathway

Stat3, a member of the signal transducers and activators of transcription (STAT) family, is a key signal transduction protein activated by numerous cytokines, growth factors, and oncoproteins that controls cell proliferation, differentiation, development, survival, and inflammation. Constitutive activation of Stat3 has been found frequently in a wide variety of human tumors and induces cellular transformation and tumor formation. In this study, we demonstrated that LIGHT, a member of tumor necrosis factor superfamily, activates Stat3 in cancer cells. LIGHT induces dose-dependent activation of Stat3 by phosphorylation at both the tyrosine 705 and serine 727 residues. The activation of Stat3 by LIGHT appears to be mediated by NIK phosphorylation. Expression of a kinase-inactive NIK mutant abolished LIGHT induced Stat3 activation. Overexpression of an active NIK induces Stat3 activation by phosphorylation at the both tyrosine 705 and serine 727 residues. Activation of Stat3 by NIK requires NIK kinase activity as showed by kinase assays. In addition, LIGHT increases the expression of Stat3 target genes including cyclin D1, survivin, and Bcl-xL, and stimulates human LNCaP prostate cancer cell growth in vitro which can be blocked by expression of a dominant-negative Stat3 mutant. Taken together, these results indicate that in addition to activating NF-kappaB/p52, LIGHT also activates Stat3. Activation of Stat3 together with activating non-canonical NF-kappaB/p52 signaling by LIGHT may maximize its effects on cellular proliferation, survival, and inflammation.

Insulin Antagonizes Interleukin-6 Signaling and Is Anti-inflammatory in 3T3-L1 Adipocytes

Adipose tissue secretes different adipokines, including interleukin-6 (IL-6), that have been implicated in the insulin resistance and inflammatory state characterizing obesity. We examined the putative cross-talk between insulin and IL-6 in adipose cells and found that insulin exerts an inhibitory effect on the IL-6 signaling pathway by altering the post-translational modifications of the signal transducer and activator of transcription 3 (STAT3). Insulin reduces the tyrosine phosphorylation and increases the serine phosphorylation of STAT3, thereby reducing its nuclear localization and transcriptional activity. Signaling through the MEK/MAPK pathway plays an important role as treatment with the MEK inhibitor PD98059 reduces the effects of insulin on IL-6 signaling. We also show that the protein tyrosine phosphatase SHP2 is activated upon insulin signaling and is required for the dephosphorylation of STAT3 and that insulin exerts a synergistic effect with IL-6 on suppressor of cytokine signaling 3 expression. As a consequence, the IL-6-induced expression of the inflammatory markers serum amyloid A 3 and haptoglobin are significantly decreased in cells incubated with both IL-6 and insulin. Thus, insulin exerts an important anti-inflammatory effect in adipose cells by impairing the IL-6 signal at several levels.

Structure, function, and regulation of STAT proteins

The Signal Transducer and Activator of Transcription (STAT) family of proteins was first discovered in the 1990's as key proteins in cytokine signaling. Since then, the field has greatly advanced in the past 15 years, providing significant insight into the structure, function, and regulation of STATs. STATs are latent cytoplasmic transcription factors consisting of seven mammalian members. They are Tyr phosphorylated upon activation, a post-translational modification critical for dimerization, nuclear import, DNA binding, and transcriptional activation. In recent years, unphosphorylated STATs have also been observed to dimerize and drive transcription, albeit by yet an obscure mechanism. In addition, the function of cytoplasmic STATs is beginning to emerge. Here, we describe the structure, function, and regulation of both unphosphorylated and phosphorylated STATs. STAT isoforms from alternative splicing or proteolytic processing, and post-translational modifications affecting STAT activities are also discussed.

STAT3 ser727 phosphorylation and its association with negative estrogen receptor status in breast infiltrating ductal carcinoma

Although it is known that STAT3 transcriptional activity is modulated by phosphorylation at serine residue 727, the role of STAT3 serine phosphorylation in breast cancer remains mostly unexplored. In this study, we examined the expression patterns of serine residue 727-phosphorylated STAT3 (p-ser727-STAT3) in breast infiltrating ductal carcinoma tissues and nearby noncancer tissues by using immunoblotting techniques, and correlated the expression profiles with clinicopathological characteristics. A significantly elevated p-ser727-STAT3 expression was observed in 61.8% (42/68) of breast cancer tissues as compared to corresponding noncancer tissues (p < 0.001). Further, immunohistochemical analysis also showed an increased nuclear p-ser727-STAT3 staining in cancer lesions. The increased p-ser727-STAT3 expression in breast infiltrating ductal carcinoma tissues correlated significantly with negative estrogen receptor (ER) status, increased stage of cancer and increased tumor size (p = 0.001, 0.024 and 0.014, individually). Intriguingly, we noticed that the expression levels of p-ser727-STAT3 in ER-negative breast cancer cell lines were higher than those in ER-positive breast cancer cell lines. In ER-positive MCF7 cells, treatment with ERalpha-specific siRNA increased, whereas treatment with anticancer drug tamoxifen decreased the expression of p-ser727-STAT3, phenomena not observed in ER-negative MDA-MB-231 cells. In conclusion, our results suggest that p-ser727-STAT3 may be involved in the pathogenesis of breast cancer in an ER-dependent manner.

STAT3 as a downstream mediator of Trk signaling and functions

Signal transducer and activator of transcription 3 (STAT3) has long been shown to regulate gene transcription in response to cytokines and growth factors. Recent evidence suggests that STAT3 activation may also occur downstream of receptor-tyrosine kinase activation. In the current study we have identified STAT3 as a novel signal transducer for TrkA, the receptor-tyrosine kinase that mediates the functions of nerve growth factor (NGF). Activation of TrkA by NGF triggered STAT3 phosphorylation at Ser-727, and enhanced the DNA binding and transcriptional activities of STAT3. More importantly, neurotrophin-induced increase in STAT3 activation was observed to underlie several downstream functions of neurotrophin signaling. First of all, knockdown of STAT3 expression using the RNA interference approach attenuated NGF-induced transcription of immediate early genes in PC12 cells. Furthermore, reduced STAT3 expression in PC12 cells suppressed NGF-induced cyclin D1 expression, thereby inhibiting growth arrest normally triggered by NGF treatment. Finally, inhibition of STAT3 expression decreased brain-derived neurotrophic factor-promoted neurite outgrowth in primary hippocampal neurons. Together, our findings have identified STAT3 as an essential component of neurotrophin signaling and functions.

The role of protein kinase C δ activation and STAT3 Ser727 phosphorylation in insulin-induced keratinocyte proliferation

Activation of the STAT family of transcription factors is regulated by cytokines and growth factors. STAT tyrosine and serine phosphorylation are linked to the transcriptional activation and function of STAT. We have previously described a unique pathway inducing keratinocyte proliferation, which is mediated by insulin stimulation and depends on protein kinase C δ (PKCδ). In this study, we assessed STAT3 activation downstream of this pathway and characterized the role of PKCδ activation in STAT3 tyrosine and serine phosphorylation and keratinocyte proliferation. Following insulin stimulation, STAT3 interacted with PKCδ but not with any other PKC isoform expressed in skin. Activated forms of PKCδ and STAT3 were essential for insulin-induced PKCδ-STAT3 activation in keratinocyte proliferation. Abrogation of PKCδ activity inhibited insulin-induced STAT3 phosphorylation, PKCδ-STAT3 association and nuclear translocation. In addition, overexpression of STAT3 tyrosine mutant eliminated insulin-induced PKCδ activation and keratinocyte proliferation. Finally, overexpression of a STAT3 serine mutant abrogated insulin-induced STAT3 serine phosphorylation and STAT3-induced keratinocyte proliferation, whereas STAT3 tyrosine phosphorylation was induced and nuclear localization remained intact. This study indicates that PKCδ activation is a primary regulator of STAT3 serine phosphorylation and that PKCδ is essential in directing insulin-induced signaling in keratinocyte proliferation.

Cross down-regulation of leptin and insulin receptor expression and signalling in a human neuronal cell line

Leptin and insulin are major signals to the hypothalamus to regulate energy homoeostasis and body adiposity. IR (insulin receptors) and leptin receptors (long isoform, ObRb) share a number of signalling cascades, such as JAK2/STAT-3 (Janus kinase 2/signal transduction and activator of transcription 3) and PI3K (phosphoinositide 3-kinase); the cross-talk between IR and ObRb have been described previously in non-neuronal cells. Differentiated human neuroblastoma (SH-SY5Y) cells express endogenous ObR and IR, and respond to leptin and insulin with stimulation of STAT-3 and MAPK (mitogen-activated protein kinase) phosphorylation, and PI3K activity. Insulin or leptin pre-treatment of SH-SY5Y cells increased basal STAT-3 phosphorylation, but abolished the acute effect of these hormones, and, interestingly, leptin pre-treatment abolished insulin effect and vice versa. Similar results were obtained for MAPK phosphorylation, but leptin or insulin pre-treatment did not completely abolish the acute effect of insulin or leptin. We have also showed that insulin and leptin are able to activate PI3K through IRS-1 (insulin receptor substrate 1) and IRS-2 respectively. Furthermore, leptin or insulin pre-treatment increased basal PI3K activity and IRS-1 or IRS-2 association with p85 and abolished acute insulin or leptin effect, in addition to the down-regulation of IRS-1 and IRS-2. Finally, insulin pre-treatment reduced leptin binding by approx. 60%, and leptin pre-treatment reduced the expression of insulin receptor by 40% in SH-SY5Y cells, which most likely accounts for the cross down-regulation of leptin and insulin receptors. These results provide evidence to suggest cross down-regulation of leptin and insulin receptors at both receptor and downstream signalling levels. This finding may contribute to the understanding of the complex relationship between leptin resistance and insulin resistance at the neuronal level.

Herpes simplex virus 1 gene products occlude the interferon signaling pathway at multiple sites

Earlier studies have shown that herpes simplex virus 1 (HSV-1) blocks the interferon response pathways, at least at two sites, by circumventing the effects of activation of protein kinase R by double-stranded RNA and interferon and through the degradation of promyelocytic leukemia protein (PML) since interferon has no antiviral effects in PML(-/-) cells. Here we report on two effects of viral genes on other sites of the interferon signaling pathway. (i) In infected cells, Jak1 kinase associated with interferon receptors and Stat2 associated with the interferon signaling pathway rapidly disappear from infected cells. The level of interferon alpha receptor is also reduced, albeit less drastically at times after 4 h postinfection. Other members of the Stat family of proteins were either decreased in amount or posttranslationally processed in a manner different from those of mock-infected cells. The decrease in the levels of Jak1 and Stat2 may account for the decrease in the formation of complexes consisting of Stat1 or ISGF3 and DNA sequences containing the interferon-stimulated response elements after exposure to interferon. (ii) The disappearance of Jak1 and Stat2 was related at least in part to the function of the virion host shutoff protein, the product of the viral U(L)41 gene. Consistent with this observation, a mutant lacking the U(L)41 gene and treated with interferon produced lesser amounts of a late protein (U(L)38) than the wild-type parent. We conclude that HSV-1 blocks the interferon signaling pathways at several sites.

Serine phosphorylation of STAT3 is essential for Mcl-1 expression and macrophage survival

The Bcl-2 family member Mcl-1 is essential for macrophage survival. However, the mechanisms that contribute to the expression of Mcl-1 in these cells have not been fully characterized. The present study focused on the role of signal transducer and activator of transcription 3 (STAT3) in regulation of Mcl-1 in macrophages. Sodium salicylate (NaSal) treatment induced apoptotic cell death in primary human macrophages in a dose- and time-dependent fashion. Incubation with NaSal resulted in the loss of mitochondrial transmembrane potential, the release of cytochrome c and second mitochondria-derived activator of caspase/direct IAP binding protein with low pH of isoelectric point (pI) from the mitochondria, and the activation of caspases 9 and 3. Western blot analysis and reverse transcription-polymerase chain reaction demonstrated that NaSal down-regulated the expression of Mcl-1. Electrophoretic mobility shift assay and Western blot analysis for phosphorylated STAT3 demonstrated that STAT3 was constitutively activated in macrophages and that this STAT3 activation was suppressed by NaSal. The activation of STAT3 in macrophages was dependent on Ser727 phosphorylation, in the absence of detectable Tyr705 phosphorylation. Ectopic expression of STAT3 in murine RAW264.7 macrophages rescued the inhibition of Mcl-1 promoter-reporter gene activation and the cell death induced by NaSal treatment, while a dominant-negative STAT3 resulted in cell death. To confirm its role in primary macrophages, STAT3 antisense (AS) oligodeoxynucleotides (ODNs) were employed. STAT3 AS, but not control, ODNs decreased STAT3 and Mcl-1 expression and resulted in macrophage apoptosis. These observations demonstrate that the STAT3-mediated expression of Mcl-1 is essential for the survival of primary human in vitro differentiated macrophages.

Ataxia telangiectasia mutated proteins, MAPKs, and RSK2 are involved in the phosphorylation of STAT3

Phosphorylation at Ser(727) is known to be required for complete activation of STAT3 by diverse stimuli including UV irradiation, but the kinase(s) responsible for phosphorylating STAT3 (Ser(727)) is still not well discerned. In the present study, we observed that activation of ATM is required for a UVA-stimulated increase in Ser(727) phosphorylation of STAT3 as well as in activation and phosphorylation of p90 ribosomal protein S6 kinases (RSKs). Moreover, UVA-stimulated activation of upstream kinases, such as c-Jun N-terminal kinases (JNKs) and ERKs, involved in mediating phosphorylation of RSKs and STAT3 was defective or delayed in ATM-deficient cells. Furthermore, we provide evidence that RSK2-deficient cells were defective for UV-induced Ser(727) phosphorylation of STAT3, and the defect was restored after ectopic expression of transfected full-length RSK2. In vitro experiments showed that active RSK2 and JNK1 induce the phosphorylation of STAT3 precipitates from immunoprecipitation but not from glutathione S-transferase (GST) pull-down. Interestingly, the GST fusion STAT3 proteins mixed together with STAT3 immunoprecipitates can be phosphorylated by JNK. However, the in vitro phosphorylation of STAT3 was reduced by the GST-STAT3 beta protein, a dominant negative form of STAT3. Taken together, our results demonstrate that the STAT3 phosphorylation at Ser(727) is triggered by active RSK2 or JNK1 in the presence of a downstream kinase or a cofactor, and thereby the intracellular phosphorylation process is stimulated through a signaling pathway involving ATM, MAPKs, RSK2, and an as yet unidentified kinase or cofactor. Additionally, RSK2-mediated phosphorylation of STAT3 (Ser(727)) was further determined to be required for basal and UVA-stimulated STAT3 transcriptional activities.

Growth hormone, but not insulin, activates STAT5 proteins in adipocytes in vitro and in vivo

STAT 5 proteins are latent transcription factors which have been shown to be activated by growth hormone (GH) in many cell types. However, some recent studies also suggest that STAT 5B is a physiological substrate of the insulin receptor. In our studies, we have shown that physiological levels of insulin do not induce STAT 5 tyrosine phosphorylation or affect the nuclear distribution of STATs 5A or 5B in 3T3-L1 adipocytes. Moreover, we did not observe the activation of STAT 5 in the adipose tissue or skeletal muscle of mice following an acute intraperitoneal injection of insulin. However, acute GH administration, both in vitro and in vivo, resulted in the activation of STAT 5 proteins. In summary, our results indicate that STAT 5 proteins are not activated by physiological levels of insulin in adipose tissue.

Interaction between leptin and insulin signaling pathways differentially affects JAK-STAT and PI 3-kinase-mediated signaling in rat liver

Chronic leptin treatment markedly enhances the effect of insulin on hepatic glucose production unproportionally with respect to body weight loss and increased insulin sensitivity. In the present study the cross-talk between insulin and leptin was evaluated in rat liver. Upon stimulation of JAK2 tyrosine phosphorylation, leptin induced JAK2 co-immunoprecipitation with STAT3, STAT5b, IRS-1 and IRS-2. This phenomenon parallels the leptin-induced tyrosine phosphorylation of STAT3, STAT5b, IRS-1 and IRS-2. Acutely injected insulin stimulated a mild increase in tyrosine phosphorylation of JAK2, STAT3 and STAT5b. Leptin was less effective than insulin in stimulating IRS phosphorylation and their association with PI 3-kinase. Simultaneous treatment with both hormones yielded no change in maximal phosphorylation of STAT3, IRS-1, IRS-2 and Akt, but led to a marked increase in tyrosine phosphorylation of JAK2 and STAT5b when compared with isolated administration of insulin or leptin. This indicates that there is a positive cross-talk between insulin and leptin signaling pathways at the level of JAK2 and STAT5b in rat liver.

MSK1 and JNKs mediate phosphorylation of STAT3 in UVA-irradiated mouse epidermal JB6 cells

Phosphorylation of Tyr(705) and Ser(727) of signal transducer and activator of transcription 3 (STAT3) are known to be required for maximal activation by diverse stimuli. Tyr(705) phosphorylation is generally accepted to be mediated by the Janus kinase family. But the mechanism for STAT3 (Ser(727)) phosphorylation is not well understood. Here, we provide evidence that UVA-induced phosphorylation of STAT3 at Ser(727) is inhibited by pretreatment of JB6 cells with PD98059 or SB202190. Phosphorylation of STAT3 (Ser(727)) is also markedly prevented by a dominant negative mutant of ERK2, c-Jun N-terminal kinase 1 (JNK1), or p38 kinase and in knockout Jnk1(-/-) or Jnk2(-/-) cells. Furthermore, STAT3 (Ser(727)) phosphorylation is suppressed by C- or N-terminal "kinase-dead" mutants of mitogen- and stress-activated protein kinase 1 (MSK1), a downstream kinase of ERKs and p38 kinase, and H89, a potential MSK1 inhibitor. In vitro experiments showed that active MSK1 and JNKs, but not ERKs or p38 kinase, phosphorylate STAT3 (Ser(727)). Additionally, the role of MAPKs in mediating UVA-stimulated DNA binding activity of STAT3 was investigated. Overall, these results suggest that UVA-induced Ser(727) phosphorylation of STAT3 may occur through MSK1 and JNKs.

Specific DNA binding and transactivation potential of recombinant, purified Stat5

The signal transducers and activators of transcriptions (Stats) are central mediators of cytokine responses especially in hematopoietic cells. The detailed molecular mechanisms of Stat activation, particularly the role of post-translational modifications and co-operation with cellular transcription factors are subject to intense investigation. The phosphorylation of a tyrosine residue in the carboxyl terminal domain is a common characteristic for the biologically active state of all known Stats. We studied the biological potential of purified recombinant murine Stat5a and Stat5b. These proteins were expressed in Sf9 insect cells upon infection with Stat5 encoding baculoviruses. We also obtained the tyrosine phosphorylated, activated forms of the Stat5 proteins by expressing the tyrosine kinase Janus kinase2 (Jak) in the same cells through co-infection with a kinase encoding virus. After purification, only the tyrosine phosphorylated form was able to bind specifically in vitro to the Stat5 DNA response element. This activated form of Stat5 is also able to support specific cell free in vitro transcription of a gene with a Stat5 response element in its promoter region. The recombinant purified Stat5 proteins were treated with the tyrosine specific protein phosphatase or with potato acidic phosphatase, which removes phosphate groups from serine and tyrosine residues. Phosphatase treatment resulted in the loss of specific DNA binding ability. This property could be restored by an in vitro reaction with recombinant, purified EGF or PDGF receptor kinases. Tyrosine rephosphorylation in vitro also restored the transactivation potential of Stat5. This modification is, therefore, a sufficient prerequisite for transcriptional induction by Stat5.

Simultaneous tyrosine and serine phosphorylation of STAT3 transcription factor is involved in Rho A GTPase oncogenic transformation

Stats (signal transducers and activators of transcription) are latent cytoplasmic transcription factors that on a specific stimulus migrate to the nucleus and exert their transcriptional activity. Here we report a novel signaling pathway whereby RhoA can efficiently modulate Stat3 transcriptional activity by inducing its simultaneous tyrosine and serine phosphorylation. Tyrosine phosphorylation is exerted via a member of the Src family of kinases (SrcFK) and JAK2, whereas the JNK pathway mediates serine phosphorylation. Furthermore, cooperation of both tyrosine as well as serine phosphorylation is necessary for full activation of Stat3. Induction of Stat3 activity depends on the effector domain of RhoA and correlates with induction of both Src Kinase-related and JNK activities. Activation of Stat3 has biological implications. Coexpression of an oncogenic version of RhoA along with the wild-type, nontransforming Stat3 gene, significantly enhances its oncogenic activity on human HEK cells, suggesting that Stat3 is an essential component of RhoA-mediated transformation. In keeping with this, dominant negative Stat3 mutants or inhibition of its tyrosine or serine phosphorylation completely abrogate RhoA oncogenic potential. Taken together, these results indicate that Stat3 is an important player in RhoA-mediated oncogenic transformation, which requires simultaneous phosphorylation at both tyrosine and serine residues by specific signaling events triggered by RhoA effectors.

Insulin modulates leptin-induced STAT3 activation in rat hypothalamus

Insulin and leptin have overlapping effects in the control of energy homeostasis, but the molecular basis of this synergism is unknown. Insulin signals through a receptor tyrosine kinase that phosphorylates and activates the docking proteins IRSs (insulin receptor substrates), whereas the leptin receptor and its associated protein tyrosine kinase JAK2 (Janus kinase 2) mediate phosphorylation and activation of the transcription factor STAT3 (signal transducer and activator of transcription). Here, we present evidence for the integration of leptin and insulin signals in the hypothalamus. Insulin induced JAK2 tyrosine phosphorylation, leptin receptor phosphorylation which, in the presence of leptin, augmented the interaction between STAT3 and this receptor. Insulin also increased the leptin-induced phosphorylation of STAT3 and its activation. These results indicate that insulin modulates the leptin signal transduction pathway, and may provide a molecular basis for the coordinated effects of insulin and leptin in feeding behavior and weight control.

Mechanisms of regulation of cell adhesion and motility by insulin receptor substrate-1 in prostate cancer cells

LNCaP cells are human prostatic cancer cells that have a frame-shift mutation of the tumor suppressor gene PTEN and do not express the insulin receptor substrate-1 (IRS-1), a major substrate of the type 1 insulin-like growth factor receptor (IGF-IR). Ectopic expression of IRS-1 in LNCaP cells increases cell adhesion and decreases cell motility by an IGF-I-independent mechanism. We show now that these effects of IRS-1 are accompanied by serine phosphorylation of IRS-1 and are inhibited by inhibitors of phosphatidylinositol 3-kinase (PI3K). We have confirmed the requirement for PI3K activity and serine phosphorylation by the use of IRS-1 mutants, expressed in LNCaP cells. Serine phosphorylation inhibits IGF-I-induced tyrosyl phosphorylation of IRS-1, which is restored by the expression of wild-type PTEN or by inhibition of PI3K activity. Finally, IRS-1 in LNCaP cells co-immunoprecipitates with integrin alpha 5 beta 1, and the association is again IGF-I-independent. We conclude that in LNCaP cells, IRS-1 is serine phosphorylated by PI3K, generating effects that are different, and even opposite, from those generated by IGF-I.

The Rac1/p38 mitogen-activated protein kinase pathway is required for interferon alpha-dependent transcriptional activation but not serine phosphorylation of Stat proteins

The p38 mitogen-activated protein (MAP) kinase is activated during engagement of the type I interferon (IFN) receptor and mediates signals essential for IFNalpha-dependent transcriptional activation via interferon-stimulated response elements without affecting formation of the ISGF3 complex. In the present study, we provide evidence that the small GTPase Rac1 is activated in a type I IFN-dependent manner and that its function is required for downstream engagement of the p38 MAP kinase pathway. We also demonstrate that p38 is required for IFNalpha-dependent gene transcription via GAS elements and regulates activation of the promoter of the PML gene that mediates growth inhibitory responses. In studies to determine whether the regulatory effects of p38 are mediated by serine phosphorylation of Stat1 or Stat3, we found that the p38 kinase inhibitors SB203580 or SB202190 or overexpression of a dominant negative p38 mutant do not inhibit phosphorylation of Stat1 or Stat3 on Ser-727 in several IFNalpha-sensitive cell lines. Altogether these data demonstrate that the Rac1/p38 MAP kinase signaling cascade plays a critical role in type I IFN signaling, functioning in cooperation with the Stat-pathway, to regulate transcriptional regulation of IFNalpha-sensitive genes and generation of growth inhibitory responses.

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.

In vivo administration of leptin activates signal transduction directly in insulin-sensitive tissues: overlapping but distinct pathways from insulin

To determine whether leptin signal transduction is exerted directly upon insulin-sensitive tissues in vivo, we examined the ability of iv leptin to acutely stimulate phosphorylation of STAT3, STAT1, and MAPK, and activities of PI 3-kinase and Akt, in insulin-sensitive tissues of normal rats. Both leptin (1 mg/kg iv x 3 min) and insulin (10 U/kg iv x 3 min) stimulated tyrosine phosphorylation of STAT3 5.6- to 6.0-fold and of STAT1 4.0-fold in adipose tissue. Leptin tended to increase STAT3 phosphorylation in liver and muscle. Both hormones also increased MAPK phosphorylation: leptin increased it 3.2- to 3.8-fold in adipose tissue and liver, whereas insulin stimulated MAPK phosphorylation 5.0-fold in adipose tissue, 6.8-fold in liver, and 2.5-fold in muscle. Leptin was much less effective than insulin at stimulating IRS pathways. Leptin increased IRS-1-associated PI 3-kinase activity in adipose tissue only 2.0-fold (P < 0.01) compared with the 10-fold effect of insulin. IRS-2-associated PI 3-kinase activity was increased 1.7-fold (P < 0.01) by leptin in liver and 6-fold by insulin. Akt phosphorylation and activity were not changed by leptin but increased with insulin. Lower concentrations of leptin (10 and 50 microg/kg) also stimulated STAT3 phosphorylation in fat. These effects appear to be direct because 3 min after leptin intracerebroventricular injection, phosphorylation of STAT3, STAT1, and MAPK were not stimulated in hypothalamus or adipose tissue. Furthermore, leptin activated STAT3 and MAPK in adipose tissue explants ex vivo and in 3T3-L1 adipocytes. Leptin did not activate STAT3 or MAPK in adipose tissue of db/db mice. Thus, leptin rapidly activates signaling pathways directly at the level of insulin sensitive tissues through the long-form leptin receptor, and these pathways overlap with, but are distinct from, those engaged by insulin.

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).

The role of STAT proteins in growth hormone signaling

Growth hormone (GH) has long been known to be the body's primary regulator of body growth and a regulator of metabolism, yet the mechanisms by which GH regulates the transcription of specific genes required for these processes are just now being delineated. GH binding to its receptor recruits and activates the receptor-associated JAK2 that in turn phosphorylates tyrosines within itself and the GH receptor. These tyrosines form binding sites for a number of signaling proteins, including members of the family of signal transducers and activators of transcription (STAT). Among the known signaling molecules for GH, STAT proteins play a particularly prominent role in the regulation of gene transcription. This paper will review what is currently understood about which STAT proteins are regulated by GH, how they are regulated by GH, the GH-dependent genes they regulate, and discuss current theories about how GH-activated STAT signaling is regulated. Particular attention will be given to the novel role that STAT5 plays in sexually dimorphic gene expression in the liver as determined by the secretory pattern of GH and the role of STAT5 in body growth. Oncogene (2000).

Stat5a serine phosphorylation. Serine 779 is constitutively phosphorylated in the mammary gland, and serine 725 phosphorylation influences prolactin-stimulated in vitro DNA binding activity

The activity of transcription factors of the Stat family is controlled by phosphorylation of a conserved, carboxyl-terminal tyrosine residue. Tyrosine phosphorylation is essential for Stat dimerization, nuclear translocation, DNA binding, and transcriptional activation. Phosphorylation of Stats on specific serine residues has also been described. We have previously shown that in HC11 mammary epithelial cells Stat5a is phosphorylated on Tyr(694) in a prolactin-sensitive manner, whereas serine phosphorylation is constitutive (Wartmann, M., Cella, N., Hofer, P., Groner, B., Xiuwen, L., Hennighausen, L., and Hynes, N. E. (1996) J. Biol. Chem. 271, 31863-31868). By using mass spectrometry and site-directed mutagenesis, we have now identified Ser(779), located in a unique Stat5a SP motif, as the site of serine phosphorylation. By using phospho-Ser(779)-specific antiserum, we have determined that Ser(779) is constitutively phosphorylated in mammary glands taken from different developmental stages. Stat5a isolated from spleen, heart, brain, and lung was also found to be phosphorylated on Ser(779). Ser(725) in Stat5a has also been identified as a phosphorylation site (Yamashita, H., Xu, J., Erwin, R. A., Farrar, W. L., Kirken, R. A., and Rui, H. (1998) J. Biol. Chem. 273, 30218-30224). Here we show that mutagenesis of Ser(725), Ser(779), or a combination of Ser(725/779) to an Ala had no effect on prolactin-induced transcriptional activation of a beta-casein reporter construct. However, following prolactin induction the Ser(725) mutant displayed sustained DNA binding activity compared with that of wild type Stat5a. The results suggest that Ser(725) phosphorylation has an impact on signal duration.

Regulation of the fatty acid synthase promoter by insulin

Expression of critical enzymes in fatty acid and fat biosynthesis is tightly controlled by nutritional and hormonal stimuli. The expression of fatty acid synthase, which catalyzes all reactions for synthesis of palmitate from acetyl-CoA and malonyl-CoA, and of mitochondrial glycerol-3-phosphate acyltransferase, which catalyzes the first acylation step in glycerophospholipid synthesis, is decreased to an undetectable level during fasting. Food intake, especially a high carbohydrate, fat-free diet after fasting, causes a dramatic increase in the transcription of these genes. Insulin secretion is increased during feeding and has a positive effect on expression. By using adipocytes in culture and transgenic mice that express the reporter gene driven by the fatty acid synthase promoter, the cis-acting sequence that mediates insulin regulation of the fatty acid synthase promoter was defined. Upstream stimulatory factors (USF) that bind to the -65 E-box are required for insulin-mediated transcriptional activation of the fatty acid symthase gene. Sterol regulatory element binding protein (SREBP)-1 may be also involved in induction of these genes during feeding. Using specific inhibitors and expressing various signaling molecules, we found that insulin regulation of the fatty acid synthase promoter is mediated by the phosphatidylinositol (PI)3-kinase signaling pathway and that protein kinase B/akt is a downstream effector.

Characterization of insulin-like growth factor-1-induced activation of the JAK/STAT pathway in rat cardiomyocytes

This study was designed to investigate whether insulin-like growth factor-1 (IGF-1) transduces signaling through the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway in cardiomyocytes and to assess the upstream signals of serine and tyrosine phosphorylation of STAT family proteins. Primary cultured neonatal rat cardiomyocytes were stimulated with IGF-1 (10(-8) mol/L). JAK1, but not JAK2 or Tyk2, was phosphorylated by IGF-1 as early as 2 minutes and peaked at 5 minutes. IGF-1 induced both tyrosine and serine phosphorylation of STAT1 and STAT3. Tyrosine phosphorylation of STAT1 peaked at 15 minutes and correlated with that of JAK1, whereas that of STAT3 was sustained up to 120 minutes and was dissociated from the activation of JAK1. Tyrosine phosphorylation of STAT3 was unaffected by the preincubation with CV11974 (AT(1) blocker), TAK044 (endothelin-1 receptor blocker), RX435 (anti-gp130 blocking antibody), PD98058, wortmannin, EDTA, or KN62 but was significantly attenuated by BAPTA-AM and chelerythrine. The time course of a gel mobility shift of SIE (sis-inducing element) coincided with the phosphorylation of STAT3. Serine phosphorylation of STAT1 peaked at 30 minutes and that of STAT3 was observed from 5 to 60 minutes. These results indicated that (1) IGF-1 activated JAK1 but not JAK2 or Tyk2 in rat cardiomyocytes; (2) IGF-1 induced both tyrosine and serine phosphorylation of STAT1 and STAT3; and (3) the tyrosine phosphorylation of STAT3 was not caused by JAK1 alone, and protein kinase C and intracellular Ca(2+) were required for phosphorylation.

Serine phosphorylation and negative regulation of Stat3 by JNK

STATs are activated by various cytokines and growth factors via tyrosine phosphorylation, which leads to sequential dimer formation, nuclear translocation, binding to specific DNA sequences, and regulation of gene expression. Recently, serine phosphorylation of Stat3 on Ser-727 by ERK has been identified in response to epidermal growth factor (EGF). Here, we report that Ser-727 phosphorylation of Stat3 can also be induced by JNK and activated either by stress or by its upstream kinase and that various stress treatments induce serine phosphorylation of Stat3 in the absence of tyrosine phosphorylation. Inhibitors of ERK and p38 did not inhibit UV-induced Stat3 serine phosphorylation, suggesting that neither of them is involved. We further demonstrate that JNK1, activated by its upstream kinase MKK7, negatively regulated the tyrosine phosphorylation and DNA binding and transcriptional activities of Stat3 stimulated by EGF. Correspondingly, pretreatment of cells with UV reduced the EGF-stimulated tyrosine phosphorylation and phosphotyrosine-dependent activities of Stat3. The inhibitory effect was not observed for Stat1. Our results suggest that Stat3 is a target of JNK that may regulate Stat3 activity via both Ser-727 phosphorylation-dependent and -independent mechanisms.

Extracellular signal-regulated kinase (ERK) interacts with signal transducer and activator of transcription (STAT) 5a

Serine phosphorylation of signal transducers and activators of transcription (STAT) 1 and 3 modulates their DNA-binding capacity and/or transcriptional activity. Earlier we suggested that STAT5a functional capacity could be influenced by the mitogen-activated protein kinase (MAPK) pathway. In the present study, we have analyzed the interactions between STAT5a and the MAPKs, extracellular signal-regulated kinases ERK1 and ERK2. GH treatment of Chinese hamster ovary cells stably transfected with the GH receptor (CHOA cells) led to rapid and transient activation of both STAT5a and ERK1 and ERK2. Pretreatment of cells with colchicine, which inhibits tubulin polymerization, did not inhibit STAT5a translocation to the nucleus and ERK1/2 activation. In vitro precipitation with a glutathione-S-transferase-fusion protein containing the C-terminal transactivation domain of STAT5a showed GH-regulated association of ERK1/2 with the fusion protein, while this was not seen when serine 780 in STAT5a was changed to alanine. In vitro phosphorylation of the glutathione-S-transferase-fusion proteins using active ERK only worked when the fusion protein contained wild-type STAT5a sequence. The same experiment, performed with full-length wild-type STAT5a and the corresponding S780A mutant, showed that serine 780 is the only substrate in full-length STAT5a for active ERK. In coimmunoprecipitation experiments, larger amounts of STAT5a-ERK1/2 complexes were detected in cytosol from untreated CHOA cells than in cytosol from GH-treated cells, suggesting the presence of preformed STAT5a-ERK1/2 complexes in unstimulated cells. Transfection experiments with COS cells showed that kinase-inactive ERK1 decreased GH stimulation of STAT5-regulated reporter gene expression. These observations show, for the first time, direct physical interaction between ERK and STAT5a and also clearly identify serine 780 as a target for ERK. Furthermore, it is also established that serine phosphorylation of STAT5a transactivation domain, via the MAPK pathway, is a means of modifying GH-induced transcriptional activation.

The designer cytokine hyper-interleukin-6 is a potent activator of STAT3-dependent gene transcription in vivo and in vitro

Interleukin-6 (IL-6) triggers pivotal pathways in vivo. The designer protein hyper-IL-6 (H-IL-6) fuses the soluble IL-6 receptor (sIL-6R) through an intermediate linker with IL-6. The intracellular pathways that are triggered by H-IL-6 are not defined yet. Therefore, we studied the molecular mechanisms leading to H-IL-6-dependent gene activation. H-IL-6 stimulates haptoglobin mRNA expression in HepG2 cells, which is transcriptionally mediated as assessed by run-off experiments. The increase in haptoglobin gene transcription correlates with higher nuclear translocation of tyrosine-phosphorylated STAT3 and its DNA binding. As H-IL-6 stimulates STAT3-dependent gene transcription, we compared the molecular mechanism between IL-6 and H-IL-6. Transfection experiments were performed with a STAT3-dependent luciferase construct. The same amount of H-IL-6 stimulated luciferase activity faster, stronger, and for a longer period of time. Dose response experiments showed that a 10-fold lower dose of H-IL-6 stimulated STAT3-dependent gene transcription comparable with the higher amount of IL-6. Cotransfection with the gp80 and/or gp130 receptor revealed that the effect of H-IL-6 on STAT3-dependent gene transcription is restricted to the gp80/gp130 receptor ratio. High amounts of gp130 increased and high amounts of gp80 decreased the effect on H-IL-6-dependent gene transcription. To investigate the in vivo effect of H-IL-6 on gene transcription in the liver, H-IL-6 and IL-6 were injected into C3H mice. H-IL-6 was at least 10-fold more effective in stimulating the DNA binding and nuclear translocation of STAT3, which enhances haptoglobin mRNA and protein expression. Thus H-IL-6 stimulates STAT3-dependent gene transcription in liver cells in vitro and in vivo at least 10-fold more effectively than IL-6. Our results provide evidence that H-IL-6 is a promising designer protein for therapeutic intervention during different pathophysiological conditions also in humans.

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

We have recently demonstrated that signal transducers and activators of transcription (STATs) 1, 3, 5A, 5B, and 6 are expressed in both cultured and native adipocytes. Our current studies have focused on the activation of STATs 1 and 3 by leukemia inhibitory factor (LIF), oncostatin-M (OSM), and interferon-gamma (IFNgamma) in 3T3-L1 adipocytes. IFNgamma is shown to be a potent activator of STAT 1 as indicated by both tyrosine phosphorylation and nuclear translocation. However, LIF and OSM, which are potent inducers of STAT 3, are less potent activators of STAT 1 as measured by both tyrosine phosphorylation and nuclear translocation. Both STATs 1 and 3 were translocated to the nucleus in a time-dependent fashion following LIF treatment. In addition, IFNgamma resulted in a time- and dose-dependent effect on STATs 1 and 3 nuclear translocation. Growth hormone, a potent activator of STATs 5A and 5B, had a minimal effect on STAT 1 and STAT 3 tyrosine phosphorylation. Preincubation with either insulin or growth hormone had no detectable effects on the tyrosine phosphorylation or nuclear translocation of STATs 1 and 3 induced by LIF, OSM, or IFNgamma. The effects of LIF and IFNgamma on STAT 1 and 3 tyrosine phosphorylation and nuclear translocation were confirmed in native rat adipocytes. In 3T3-L1 adipocytes, a low level of serine phosphorylation of STAT 3 on residue 727 was observed and was markedly enhanced by insulin, LIF, or OSM. This increase in STAT 3 Ser727 phosphorylation was dependent upon the activation of MAPK, since the MAPK kinase inhibitor (PD98059) reduced STAT 3 Ser727 phosphorylation to basal levels. The inhibition of MAPK had no effect on the ability of STATs 1 and 3 to be tyrosine-phosphorylated or translocate to the nucleus. These studies demonstrate the highly specific and quantitative activation of STATs 1 and 3 by LIF, OSM, and IFNgamma in adipocytes and indicate that STAT 3 is a substrate for MAPK in adipocytes.

Growth hormone stimulates phosphorylation and activation of elk-1 and expression of c-fos, egr-1, and junB through activation of extracellular signal-regulated kinases 1 and 2

Growth hormone (GH), a major regulator of normal body growth and metabolism, regulates cellular gene expression. The transcription factors Elk-1 and Serum Response Factor are necessary for GH-stimulated transcription of c-fos through the Serum Response Element (SRE). GH stimulates the serine phosphorylation of Elk-1, thereby enabling Elk-1 to mediate transcriptional activation. The contribution of the Ras/mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway to Elk-1-mediated transcriptional activation of the c-fos SRE in response to GH was examined. The MEK inhibitor PD098059 attenuated GH-induced expression of the endogenous SRE-regulated genes c-fos, egr-1, and junB as well as transcriptional activation mediated by the c-fos promoter. The MEK inhibitor blocked GH-stimulated activation of MEK, phosphorylation of ERK1/ERK2, and MAP kinase activity in 3T3-F442A cells. Blocking MEK activation prevented GH-induced phosphorylation of Elk-1, as well as the ability of Elk-1 to mediate transcriptional activation in response to GH. Overexpression of dominant-negative Ras or the ERK-specific phosphatase, mitogen-activated protein kinase phosphatase-1, blocked the Ras/MEK/ERK pathway and abrogated GH-induced phosphorylation of Elk-1. GH failed to stimulate phosphorylation or activation of Jun N-terminal kinase under the conditions used. GH slightly increased p38-mediated mitogen-activated protein kinase-activated protein (MAPKAP) kinase-2 activity, but the p38 inhibitor SB203580 did not attenuate GH-promoted Elk-1 phosphorylation. Wortmannin, which inhibited GH-induced ERK phosphorylation, also attenuated transcriptional activation of c-fos by GH. Taken together, these data suggest that GH-dependent activation of the Ras/MEK/ERK pathway and subsequent serine phosphorylation of Elk-1 contribute to GH-stimulated c-fos expression through the SRE.

Insulin-like growth factor-I augments erythropoietin-induced proliferation through enhanced tyrosine phosphorylation of STAT5

Insulin-like growth factor (IGF-I) is known to synergistically stimulate the proliferation of hematopoietic cells in combination with other hematopoietic growth factors. However, the precise mechanism underlying the cooperative effects of IGF-I is unknown. In a human interleukin-3 or erythropoietin (EPO)-dependent cell line, F-36P, IGF-I alone failed to stimulate DNA synthesis but did augment the EPO-dependent DNA synthesis of F-36P cells. The treatment of F-36P cells with a combination of EPO and IGF-I (EPO/IGF-I) was found to enhance EPO-induced tyrosine phosphorylation of STAT5, whereas IGF-I alone did not. Furthermore, c-CIS mRNA expression, one of the target molecules of STAT5, was more effectively induced by EPO/IGF-I than by EPO alone. To examine the mechanisms of the EPO- and EPO/IGF-I-induced proliferation of F-36P cells, we expressed dominant negative (dn) mutants of STAT5 and Ras in an inducible system. The EPO-induced DNA synthesis and the cooperative effect of EPO/IGF-I were significantly inhibited by the inducible expression of dn-STAT5 or dn-Ras. In addition, the inducible expression of dn-Ras abolished the IGF-I-enhanced tyrosine phosphorylation of STAT5. These results suggest that IGF-I may augment EPO-induced proliferation by enhancing tyrosine phosphorylation of STAT5 and raise the possibility that Ras may be involved in the augmentation of STAT5 tyrosyl phosphorylation.

Interleukin-6-type cytokine signalling through the gp130/Jak/STAT pathway

The family of cytokines signalling through the common receptor subunit gp130 comprises interleukin (IL)-6, IL-11, leukaemia inhibitory factor, oncostatin M, ciliary neurotrophic factor and cardiotrophin-1. These so-called IL-6-type cytokines play an important role in the regulation of complex cellular processes such as gene activation, proliferation and differentiation. The current knowledge on the signal-transduction mechanisms of these cytokines from the plasma membrane to the nucleus is reviewed. In particular, we focus on the assembly of receptor complexes after ligand binding, the activation of receptor-associated kinases of the Janus family, and the recruitment and phosphorylation of transcription factors of the STAT family, which dimerize, translocate to the nucleus, and bind to enhancer elements of respective target genes leading to transcriptional activation. The important players in the signalling pathway, namely the cytokines and the receptor components, the Janus kinases Jak1, Jak2 and Tyk2, the signal transducers and activators of transcription STAT1 and STAT3 and the tyrosine phosphatase SHP2 [SH2 (Src homology 2) domain-containing tyrosine phosphatase] are introduced and their structural/functional properties are discussed. Furthermore, we review various mechanisms involved in the termination of the IL-6-type cytokine signalling, namely the action of tyrosine phosphatases, proteasome, Jak kinase inhibitors SOCS (suppressor of cytokine signalling), protein inhibitors of activated STATs (PIAS), and internalization of the cytokine receptors via gp130. Although all IL-6-type cytokines signal through the gp130/Jak/STAT pathway, the comparison of their physiological properties shows that they elicit not only similar, but also distinct, biological responses. This is reflected in the different phenotypes of IL-6-type-cytokine knock-out animals.

Nutritional and hormonal regulation of enzymes in fat synthesis: studies of fatty acid synthase and mitochondrial glycerol-3-phosphate acyltransferase gene transcription

The activities of critical enzymes in fatty acid and triacylglycerol biosynthesis are tightly controlled by different nutritional, hormonal, and developmental conditions. Feeding previously fasted animals high-carbohydrate, low-fat diets causes a dramatic induction of enzymes-such as fatty acid synthase (FAS) and mitochondrial glycerol-3-phosphate acyltransferase (GPAT)-involved in fatty acid and triacylglycerol synthesis. During fasting and refeeding, transcription of these two enzymes is coordinately regulated by nutrients and hormones, such as glucose, insulin, glucagon, glucocorticoids, and thyroid hormone. Insulin stimulates transcription of the FAS and mitochondrial GPAT genes, and glucagon antagonizes the insulin effect through the cis-acting elements within the promoters and their bound trans-acting factors. This review discusses advances made in the understanding of the transcriptional regulation of FAS and mitochondrial GPAT genes, with emphasis on elucidation of the mechanisms by which multiple nutrients and hormones achieve their effects.

Regulation of fat synthesis and adipose differentiation

Adipocytes have highly specialized function of accumulating fat as stored energy that can be used during periods of food deprivation. The process of fat synthesis and development of adipose tissue are under hormonal and nutritional control. This review first describes transcription of the two critical enzymes involved in fat synthesis, fatty acid synthase and mitochondrial glycerol-3-phosphate acyltransferase, is decreased to an undetectable level during fasting. Food intake, especially a high carbohydrate, fat-free diet, subsequent to fasting causes dramatic increase in transcription of these genes. Insulin secretion is increased during feeding, having a positive effect, whereas cAMP, which mediates the effect of glucagon which increases during fasting, has a negative effect on transcription of these genes. Using adipocytes in culture and in transgenic mice that express liciferase driven by the fatty acid synthase promoter, cis-acting and trans-acting factors that may mediate the transcriptional regulation were examined. Upstream stimulatory factors (USFs) that bind to -65 E-box are required for insulin-mediated transcriptional activation of the fatty acid synthase gene. This review next describes how pref-1 is a novel inhibitor of adipose differentiation and is a plasma membrane protein containing six EGF-repeats in the extracellular domain. Pref-1 is highly expressed in 3T3-L1 preadipocytes, but is not detectable in mature fat cells. Down regulation of pref-1 is required for adipose differentiation, and constitutive expression of pref-1 inhibits adipogenesis. Moreover, the ectodomain of pref-1 is cleaved to generate a biologically active 50 kDa soluble form. There are four major forms of membrane pref-1 resulting from alternate splicing, but two of the forms with a larger deletion do not produce biologically active soluble form, indicating that alternate splicing determines the range of action, juxtacrine or paracrine, of the pref-1.

Ethanol potentiates the stimulatory effects of insulin and phosphocholine on mitogenesis by a zinc-dependent and rapamycin-sensitive mechanism in fibroblasts and JB6 cells

In most cellular systems ethanol inhibits growth factor-induced cell growth. Here we examined the effects of ethanol on DNA synthesis and cell proliferation induced by insulin and phosphocholine (PCho) in NIH3T3 fibroblasts, Swiss 3T3 fibroblasts and mouse epidermal JB6 cells. In serum-starved low (12-18) passage NIH3T3 fibroblasts, 60 mM ethanol enhanced the mitogenic effect of insulin in the absence or presence of 25 microM zinc about 2- or 12-fold, respectively. In contrast, in serum-starved high (30-47) passage NIH3T3 cells 60 mM ethanol had large (20-40-fold) potentiating effects on insulin-induced DNA synthesis even in the absence of zinc. Furthermore, ethanol also enhanced the effects of PCho on DNA synthesis in both the absence and presence of insulin. The potentiating effects of ethanol on insulin- and PCho-induced DNA synthesis were associated with 1.2-1.3-fold stimulation of cell proliferation. Rapamycin, an inhibitor of p70 S6 kinase action, strongly inhibited the potentiating effects of ethanol on insulin- and PCho-induced mitogenesis. Unexpectedly, ethanol inhibited synergistic activation of p42/p44 mitogen-activated protein kinases by insulin and PCho. In both Swiss 3T3 and JB6 cells, ethanol potentiated insulin-induced DNA synthesis only in the presence of zinc. In these cells, ethanol also increased the effects of PCho on both DNA synthesis and cell proliferation in the co-presence of either insulin or ATP. The results indicate that in various cell lines physiologically relevant concentrations of ethanol can increase the ability of insulin and PCho to induce DNA synthesis and, to smaller extents, cell proliferation. In low passage NIH3T3 cells as well as in Swiss 3T3 and JB6 cells potentiation of insulin-induced DNA synthesis by ethanol requires the presence of zinc.

The JAK-STAT pathway: signal transduction involved in proliferation, differentiation and transformation

STAT proteins become activated upon tyrosine and serine phosphorylation, are subsequently translocated from the cytosol to the nucleus where they exert DNA-binding activity. Several STAT binding consensus motifs have been identified in the promoters of distinct genes. These consensus elements mediate STAT recruitment and influence the kind of STAT proteins that are bound at a specific promoter site. Recent structure function analyses have revealed conserved amino terminal sequences to be crucial for phosphatase dependent deactivation of the STAT proteins. To date an increasing amount of data is available concerning the on- and off-regulation of STAT activity. Considerable convergence as well as crosstalk has been shown between the JAK-STAT pathway and the MAPK, RAS, PI3K, PKC, and PKA involving pathways. Moreover, the nature of the genes that are regulated by STAT proteins as well as the cell functions that result from STAT activation are of great current interest. Understanding the critical functional role of STAT mediated signalling events as well as their regulation by interfering pathways provides new insights into the mechanisms involved in malignant cell proliferation.

Identification of Stat 5B as a substrate of the insulin receptor

We have screened a human placenta library using the yeast two-hybrid system to identify proteins that interact with the cytoplasmic domain of the insulin receptor. Doing so, we trapped a cDNA clone which encodes the Stat 5B region comprising amino acids 469 to 786. We show that interaction between Stat 5B and the receptor requires a functional insulin-receptor kinase, Tyr960 of insulin receptor is implicated in the interaction with Stat 5B, whereas asparagine and proline forming the NPEY960-motif are not, and Stat 5B mutated at Thr684, a potential phosphorylation site of mitogen-activated protein kinase, loses its ability to interact with the insulin receptor. Further, we found that insulin promotes rapid tyrosine phosphorylation of endogenous Stat 5B in 293 EBNA cells overexpressing insulin receptor and in NHIR cells. Taken together, our findings suggest that Stat 5B corresponds to a substrate for the insulin-receptor kinase, and this widens the repertoire of insulin-signaling pathways.