Serine phosphorylation of GH-activated signal transducer and activator of transcription 5a (STAT5a) and STAT5b: impact on STAT5 transcriptional activity

Impact of STAT5A-CNVs on growth traits in Chinese beef cattle breeds

CNVs, which are a type of structural variation, make a substantial impact on diverse characteristics in multiple species. Q-PCR and data association analysis were used for STAT5A gene copy in this study. This study aimed to investigate the copy number variation (CNV) of the STAT5A gene in seven Chinese cattle breeds, namely Qinchuan cattle, Xianan cattle, Yunling cattle, Ji'an cattle, Jiaxian Red cattle, Qaidam cattle, and Guyuan yellow cattle. Blood samples were collected for CNV typing, and the correlation between CNV type and growth traits was analyzed using SPSS 23.0 software and ANOVA. The findings revealed variations in the distribution of different copy number types among the different cattle breeds. Furthermore, association analysis demonstrated a positive impact of CNV in the STAT5A gene on cattle growth: in the JX, individuals with duplication types exhibited superior performance in terms of rump length (P < 0.05). Conversely, normal GY cattle demonstrated better body height and abdomen circumference (P < 0.05), while QD cattle exhibited a significant correlation between weight and body length with normal individuals (P < 0.05). Moreover, QC bovine duplication individuals outperformed other types, with copy number variation significantly associated with chest depth, chest width, and body length (P < 0.05). The results validate the correlation between copy number variation (CNV) of the STAT5A gene and growth characteristics in five different cattle breeds, providing a reliable benchmark for the purpose of cattle breeding.

Dynamic Roles for IL-2-STAT5 Signaling in Effector and Regulatory CD4+ T Cell Populations

CD4⁺ Th cells are responsible for orchestrating diverse, pathogen-specific immune responses through their differentiation into a number of subsets, including T_H1, T_H2, T_H9, T follicular helper, T follicular regulatory, and regulatory T cells. The differentiation of each subset is guided by distinct regulatory requirements, including those derived from extracellular cytokine signals. IL-2 has emerged as a critical immunomodulatory cytokine that both positively and negatively affects the differentiation of individual Th cell subsets. IL-2 signals are propagated, in part, via activation of STAT5, which functions as a key regulator of CD4⁺ T cell gene programs. In this review, we discuss current understanding of the mechanisms that allow IL-2-STAT5 signaling to exert divergent effects across CD4⁺ T cell subsets and highlight specific roles for this pathway in the regulation of individual Th cell differentiation programs.

Conversion of antigen-specific effector/memory T cells into Foxp3-expressing Treg cells by inhibition of CDK8/19

A promising way to restrain hazardous immune responses, such as autoimmune disease and allergy, is to convert disease-mediating T cells into immunosuppressive regulatory T (T_reg) cells. Here, we show that chemical inhibition of the cyclin-dependent kinase 8 (CDK8) and CDK19, or knockdown/knockout of the CDK8 or CDK19 gene, is able to induce Foxp3, a key transcription factor controlling T_reg cell function, in antigen-stimulated effector/memory as well as naïve CD4⁺ and CD8⁺ T cells. The induction was associated with STAT5 activation, independent of TGF-β action, and not affected by inflammatory cytokines. Furthermore, in vivo administration of a newly developed CDK8/19 inhibitor along with antigen immunization generated functionally stable antigen-specific Foxp3⁺ T_reg cells, which effectively suppressed skin contact hypersensitivity and autoimmune disease in animal models. The results indicate that CDK8/19 is physiologically repressing Foxp3 expression in activated conventional T cells and that its pharmacological inhibition enables conversion of antigen-specific effector/memory T cells into Foxp3⁺ T_reg cells for the treatment of various immunological diseases.

You-Gui-Yin improved the reproductive dysfunction of male rats with chronic kidney disease via regulating the HIF1α-STAT5 pathway

ETHNOPHARMACOLOGICAL RELEVANCE

You-Gui-Yin (YGY) is a famous Chinese traditional medicine compound that has been used to treat renal function diseases for more than 300 years. It is recorded in Jing Yue Quanshu, which was written by a famous medical scientist named Jiebing Zhang in the Ming Dynasty.

AIM OF THE STUDY

Reproductive dysfunction is one of the most serious complications of chronic kidney disease (CKD). The aim of this study was to observe the effect of You-Gui-Yin (YGY) on reproductive dysfunction of male rats with adenine-induced CKD and to determine if any effects occurred via regulation of the HIF1α-STAT5 pathway.

MATERIALS AND METHODS

UPLC-Q-TOF-MS was used to detect the main medicinal components and conduct quality control of YGY. A total of 60 rats were randomly divided into 2 groups: the NC group (10 rats) and the CKD model group (50 rats). The CKD model rats was established by administration of adenine 150 mg kg^-1 orally for 14 days. After that, the CKD rats were randomly divided into 5 groups: the CKD group, YGY (10 g kg^-1 group, 20 g kg^-1 group, 40 g kg^-1 group) and the GUI-LU-ER-XIAN-JIAO (GL) 10 g kg^-1 group with 10 rats in each group. From the 15th day to the 45th day rats were given 150 mg kg^-1 adenine orally every other day to maintain the model (except in the NC group). The YGY groups and the GL group were orally administered the relevant drug once per day for 30 days. The NC group and the CKD group were orally administered an equal volume of normal saline for 30 days. On the 45th day, the rats' sexual behavior index was tested. On the 46th day, the rats were sacrificed. Biochemical indexes, histopathological changes of the kidneys and testes, sperm morphology, sperm abnormality rate, and key proteins in the HIF1α-STAT5 pathway in the kidney and testis were detected.

RESULTS

Thirteen components in the YGY extract were identified by UPLC-Q-TOF-MS for quality control of the YGY extract. The results of the biochemical and physiological tests validated the success of inducing CKD accompanied by reproductive dysfunction in rats. YGY significantly retarded the CKD progression and improved the hormone levels of male CKD rats. Sexual behavior tests showed YGY can significantly improve CKD rats' sexual function. In addition, the pathological changes of the kidney and testis, sperm abnormality rate and sperm morphological abnormalities of the CKD rats were reduced by YGY. Furthermore, decreased expression of HIF1α and EPO, and increased expression of p-EPOR (Tyr368), p-JAK2 (Tyr570) and p-STAT5 (Ser725) were observed in the kidney and the testis of the CKD rats. The YGY extract dramatically increased the expression of HIF1α and EPO, and decreased the expression of p-EPOR (Tyr368), p-JAK2 (Tyr570) and p-STAT5 (Ser725) to regulate key proteins in the HIF1α-STAT5 pathway of the kidney and testis.

CONCLUSIONS

YGY has obvious reversal effects on the abnormal symptoms of adenine-induced CKD and the abnormal symptoms of rats with hypothyroidism and male reproductive hypotension. Its mechanism is related to its ability to regulate the HIF1α-STAT5 pathway.

Signal Transducer and Activator of Transcription (STATs) Proteins in Cancer and Inflammation: Functions and Therapeutic Implication

Signal Transducer and Activator of Transcription (STAT) pathway is connected upstream with Janus kinases (JAK) family protein and capable of integrating inputs from different signaling pathways. Each family member plays unique functions in signal transduction and crucial in mediating cellular responses to different kind of cytokines. STAT family members notably STAT3 and STAT5 have been involved in cancer progression whereas STAT1 plays opposite role by suppressing tumor growth. Persistent STAT3/5 activation is known to promote chronic inflammation, which increases susceptibility of healthy cells to carcinogenesis. Here, we review the role of STATs in cancers and inflammation while discussing current therapeutic implications in different cancers and test models, especially the delivery of STAT3/5 targeting siRNA using nanoparticulate delivery system.

Epilepsy Associates with Decreased HIF-1α/STAT5b Signaling in Glioblastoma

Epilepsy at presentation is an independent favorable prognostic factor in glioblastoma (GBM). In this study, we analyze the oncologic signaling pathways that associate with epilepsy in human GBMs, and that can underlie this prognostic effect. Following ethical approval and patient consent, fresh frozen GBM tissue was obtained from 76 patient surgeries. Hospital records were screened for the presence of seizures at presentation of the disease. mRNA and miRNA expression-based and gene set enrichment analyses were performed on these tissues, to uncover candidate oncologic pathways that associate with epilepsy. We performed qPCR experiments and immunohistochemistry on tissue microarrays containing 286 GBMs to further explore the association of these candidate pathways and of markers of mesenchymal transformation (NF-κB, CEBP-β, STAT3, STAT5b, VEGFA, SRF) with epilepsy. Gene sets involved in hypoxia/HIF-1α, STAT5, CEBP-β and epithelial-mesenchymal transformation signaling were significantly downregulated in epileptogenic GBMs. On confirmatory protein expression analyses, epileptogenic tumors were characterized by a significant downregulation of phospho-STAT5b, a target of HIF-1α. Epilepsy status did not associate with molecular subclassification or miRNA expression patterns of the tumors. Epileptogenic GBMs correlate with decreased hypoxia/ HIF-1α/STAT5b signaling compared to glioblastomas that do not present with epilepsy.

STAT5-Interacting Proteins: A Synopsis of Proteins that Regulate STAT5 Activity

Signal Transducers and Activators of Transcription (STATs) are key components of the JAK/STAT pathway. Of the seven STATs, STAT5A and STAT5B are of particular interest for their critical roles in cellular differentiation, adipogenesis, oncogenesis, and immune function. The interactions of STAT5A and STAT5B with cytokine/hormone receptors, nuclear receptors, transcriptional regulators, proto-oncogenes, kinases, and phosphatases all contribute to modulating STAT5 activity. Among these STAT5 interacting proteins, some serve as coactivators or corepressors to regulate STAT5 transcriptional activity and some proteins can interact with STAT5 to enhance or repress STAT5 signaling. In addition, a few STAT5 interacting proteins have been identified as positive regulators of STAT5 that alter serine and tyrosine phosphorylation of STAT5 while other proteins have been identified as negative regulators of STAT5 via dephosphorylation. This review article will discuss how STAT5 activity is modulated by proteins that physically interact with STAT5.

Simvastatin impairs growth hormone-activated signal transducer and activator of transcription (STAT) signaling pathway in UMR-106 osteosarcoma cells

Recent studies have demonstrated that statins reduce cell viability and induce apoptosis in various types of cancer cells. The molecular mechanisms underlying these effects are poorly understood. The JAK/STAT pathway plays an important role in the regulation of proliferation and apoptosis in many tissues, and its deregulation is believed to be involved in tumorigenesis and cancer. The physiological activation of STAT proteins by GH is rapid but transient in nature and its inactivation is regulated mainly by the expression of SOCS proteins. UMR-106 osteosarcoma cells express a GH-responsive JAK2/STAT5 signaling pathway, providing an experimental model to study the influence of statins on this system. In this study we investigated the actions of simvastatin on cell proliferation, migration, and invasion on UMR-106 cells and examined whether alterations in GH-stimulated JAK/STAT/SOCS signaling may be observed. Results showed that treatment of osteosarcoma cells with simvastatin at 3 to 10 µM doses decreases cell proliferation, migration, and invasion in a time- and dose-dependent manner. At the molecular level, although the mechanisms used by simvastatin are not entirely clear, the effect of the statin on the reduction of JAK2 and STAT5 phosphorylation levels may partially explain the decrease in the GH-stimulated STAT5 transcriptional activity. This effect correlated with a time- and dose-dependent increase of SOCS-3 expression levels in cells treated with simvastatin, a regulatory role that has not been previously described. Furthermore, the finding that simvastatin is capable of inducing SOCS-3 and CIS genes expression shows the potential of the JAK/STAT pathway as a therapeutic target, reinforcing the efficacy of simvastatin as chemotherapeutic drug for the treatment of osteosarcoma.

STAT2 phosphorylation and signaling

STAT2 is an essential transcription factor in type I IFN mediated anti-viral and anti-proliferative signaling. STAT2 function is regulated by tyrosine phosphorylation, which is the trigger for STAT-dimerization, subsequent nuclear translocation, and transcriptional activation of IFN stimulated genes. Evidence of additional STAT2 phosphorylation sites has emerged as well as novel roles for STAT2 separate from the classical ISGF3-signaling. This review aims to summarize knowledge of phosphorylation-mediated STAT2-regulation and future avenues of related STAT2 research.

Hepatic growth hormone resistance after acute injury

Severe injury and infection are often followed by accelerated protein catabolism and acute insulin resistance. This results in several effects that complicate and prolong recovery, including weakness, immobility, impaired wound healing, and organ dysfunction. Recent studies have demonstrated the development of GH resistance during severe inflammation, providing a potential mechanism for the protein loss that follows injury and infection. To understand this GH resistance, we recently developed a murine model of acute injury. Mice were subjected to soft-tissue injury, alone or combined with hemorrhage, and injected iv with GH 30, 60, or 90 minutes later. Hepatic GH signaling was measured via Western analysis. GH-induced signal transducer and activator of transcription 5 phosphorylation was decreased immediately after completion of the trauma procedure, and at 30 and 60 minutes, but further decreased by 90 minutes after trauma. Combined trauma and hemorrhage resulted in severely decreased GH-induced signal transducer and activator of transcription 5 phosphorylation compared with trauma alone, and this was true at all time points studied. Western analysis revealed an apparent decrease in the molecular weight of the hepatic GH receptor (GHR) after trauma and hemorrhage, but not trauma alone. Additional studies determined that the hemorrhage-induced decrease in receptor size was not due to changes in GHR N-linked glycosylation. These results suggest that GH sensitivity is rapidly impaired after acute injury and that trauma combined with hemorrhage results in a more severe form of GH resistance resulting from alteration or inactivation of hepatic GHR.

Forskolin-inducible cAMP pathway negatively regulates T-cell proliferation by uncoupling the interleukin-2 receptor complex

Cytokine-mediated regulation of T-cell activity involves a complex interplay between key signal transduction pathways. Determining how these signaling pathways cross-talk is essential to understanding T-cell function and dysfunction. In this work, we provide evidence that cross-talk exists between at least two signaling pathways: the Jak3/Stat5 and cAMP-mediated cascades. The adenylate cyclase activator forskolin (Fsk) significantly increased intracellular cAMP levels and reduced proliferation of the human T-cells via inhibition of cell cycle regulatory genes but did not induce apoptosis. To determine this inhibitory mechanism, effects of Fsk on IL-2 signaling was investigated. Fsk treatment of MT-2 and Kit 225 T-cells inhibited IL-2-induced Stat5a/b tyrosine and serine phosphorylation, nuclear translocation, and DNA binding activity. Fsk treatment also uncoupled IL-2 induced association of the IL-2Rβ and γc chain, consequently blocking Jak3 activation. Interestingly, phosphoamino acid analysis revealed that Fsk-treated cells resulted in elevated serine phosphorylation of Jak3 but not Stat5, suggesting that Fsk can negatively regulate Jak3 activity possibly mediated through PKA. Indeed, in vitro kinase assays and small molecule inhibition studies indicated that PKA can directly serine phosphorylate and functionally inactivate Jak3. Taken together, these findings suggest that Fsk activation of adenylate cyclase and PKA can negatively regulate IL-2 signaling at multiple levels that include IL-2R complex formation and Jak3/Stat5 activation.

Methylsulfonylmethane suppresses breast cancer growth by down-regulating STAT3 and STAT5b pathways

Breast cancer is the most aggressive form of all cancers, with high incidence and mortality rates. The purpose of the present study was to investigate the molecular mechanism by which methylsulfonylmethane (MSM) inhibits breast cancer growth in mice xenografts. MSM is an organic sulfur-containing natural compound without any toxicity. In this study, we demonstrated that MSM substantially decreased the viability of human breast cancer cells in a dose-dependent manner. MSM also suppressed the phosphorylation of STAT3, STAT5b, expression of IGF-1R, HIF-1α, VEGF, BrK, and p-IGF-1R and inhibited triple-negative receptor expression in receptor-positive cell lines. Moreover, MSM decreased the DNA-binding activities of STAT5b and STAT3, to the target gene promoters in MDA-MB 231 or co-transfected COS-7 cells. We confirmed that MSM significantly decreased the relative luciferase activities indicating crosstalk between STAT5b/IGF-1R, STAT5b/HSP90α, and STAT3/VEGF. To confirm these findings in vivo, xenografts were established in Balb/c athymic nude mice with MDA-MB 231 cells and MSM was administered for 30 days. Concurring to our in vitro analysis, these xenografts showed decreased expression of STAT3, STAT5b, IGF-1R and VEGF. Through in vitro and in vivo analysis, we confirmed that MSM can effectively regulate multiple targets including STAT3/VEGF and STAT5b/IGF-1R. These are the major molecules involved in tumor development, progression, and metastasis. Thus, we strongly recommend the use of MSM as a trial drug for treating all types of breast cancers including triple-negative cancers.

HIV infection perturbs interleukin-7 signaling at the step of STAT5 nuclear relocalization

OBJECTIVE

Interleukin-7 (IL-7) responses are impaired in CD4(+) T cells from HIV-infected patients, which may play a significant role in the loss of CD4(+) T-cell homeostasis. We set to investigate the nature of IL-7-dependent signaling defects in patients with progressive HIV-1 infection.

DESIGN AND METHODS

IL-7 signaling was compared in CD4(+) T cells from viremic patients with a viral load more than 10,000 copies of HIV RNA/ml (n = 23) and from healthy blood donors (n = 23). Phosphorylation of the transcription factor STAT5 on the regulatory serine S726 and the key tyrosine Y694 was monitored by intracellular flow cytometry. Phospho-STAT5 relocalization to the nucleus was analyzed by quantitative immunofluorescence imaging.

RESULTS

In control CD4(+) T cells, S726 phosphorylation was mostly constitutive and inducible by IL-7 to a limited extent (1.3x, P < 0.05). In contrast, phosphorylation at Y694 was highly inducible by IL-7 (12.6x, P < 0.0001). Progressive HIV infection led to hyperphosphorylation of both S726 and Y694 in naive CD4(+) T cells, with these changes correlating together (R = 0.66, P = 0.01). Quantitative image analysis revealed an impairment in the nuclear relocalization of both forms of phospho-STAT5 in patient cells (P < 0.005 for S726; P < 0.05 for Y694). The nuclear relocalization defect correlated with increased HLA-DR expression (R = 0.75, P < 0.01), suggesting a role for chronic immune activation in perturbed IL-7 signal transduction.

CONCLUSION

HIV infection perturbs IL-7 signaling by impairing the access of STAT5 to the nuclear compartment. This defect may contribute to the loss of CD4(+) T-cell populations in patients with chronically high immune activation.

Stat5a serine 725 and 779 phosphorylation is a prerequisite for hematopoietic transformation

Stat5 transcription factors are essential gene regulators promoting proliferation, survival, and differentiation of all hematopoietic cell types. Mutations or fusions of oncogenic tyrosine kinases often result in constitutive Stat5 activation. We have modeled persistent Stat5 activity by using an oncogenic Stat5a variant (cS5). To analyze the hitherto unrecognized role of Stat5 serine phosphorylation in this context, we have generated cS5 constructs with mutated C-terminal serines 725 and 779, either alone or in combination. Genetic complementation assays in primary Stat5(null/null) mast cells and Stat5(DeltaN) T cells demonstrated reconstitution of proliferation with these mutants. Similarly, an in vivo reconstitution experiment of transduced Stat5(null/null) fetal liver cells transplanted into irradiated wild-type recipients revealed that these mutants exhibit biologic activity in lineage differentiation. By contrast, the leukemogenic potential of cS5 in bone marrow transplants decreased dramatically in cS5 single-serine mutants or was completely absent upon loss of both serine phosphorylation sites. Our data suggest that Stat5a serine phosphorylation is a prerequisite for cS5-mediated leukemogenesis. Hence, interference with Stat5a serine phosphorylation might provide a new therapeutic option for leukemia and myeloid dysplasias without affecting major functions of Stat5 in normal hematopoiesis.

Protein phosphatase 2A regulates interleukin-2 receptor complex formation and JAK3/STAT5 activation

Reversible protein phosphorylation plays a key role in interleukin-2 (IL-2) receptor-mediated activation of Janus tyrosine kinase 3 (JAK3) and signal transducer and activator of transcription 5 (STAT5) in lymphocytes. Although the mechanisms governing IL-2-induced tyrosine phosphorylation and activation of JAK3/STAT5 have been extensively studied, the role of serine/threonine phosphorylation in controlling these effectors remains to be elucidated. Using phosphoamino acid analysis, JAK3 and STAT5 were determined to be serine and tyrosine-phosphorylated in response to IL-2 stimulation of the human natural killer-like cell line, YT. IL-2 stimulation also induced serine/threonine phosphorylation of IL-2Rbeta, but not IL-2Rgamma. To investigate the regulation of serine/threonine phosphorylation in IL-2 signaling, the roles of protein phosphatase 1 (PP1) and 2A (PP2A) were examined. Inhibition of phosphatase activity by calyculin A treatment of YT cells resulted in a significant induction of serine phosphorylation of JAK3 and STAT5, and serine/threonine phosphorylation of IL-2Rbeta. Moreover, inhibition of PP2A, but not PP1, diminished IL-2-induced tyrosine phosphorylation of IL-2Rbeta, JAK3, and STAT5, and abolished STAT5 DNA binding activity. Serine/threonine phosphorylation of IL-2Rbeta by a staurosporine-sensitive kinase also blocked its association with JAK3 and IL-2Rgamma in YT cells. Taken together, these data indicate that serine/threonine phosphorylation negatively regulates IL-2 signaling at multiple levels, including receptor complex formation and JAK3/STAT5 activation, and that this regulation is counteracted by PP2A. These findings also suggest that PP2A may serve as a therapeutic target for modulating JAK3/STAT5 activation in human disease.

Regulation of rapid signal transducer and activator of transcription-5 phosphorylation in the resting cells of the growth plate and in the liver by growth hormone and feeding

GH has physiological functions in many tissues, but the cellular targets for direct effects of GH remain ill defined in complex tissues such as the growth plate in which the contribution of direct vs. indirect actions of GH remains controversial. The Janus kinase (Jak)-signal transducer and activator of transcription (STAT)-5 pathway is activated by GH, so we developed a method to visualize nuclear Stat5b and phosphorylated Stat5 in single cells in response to a pulse of GH. Hep2 cells did not show a Stat5 phosphorylation (pY-Stat5) response to GH except in cells transfected to express GH receptors. ATDC5 cells express GH receptors and showed GH-induced pY-Stat5 responses, which varied with their state of chondrocyte differentiation. In vivo, Stat5b(+ve) nuclei were seen in the resting and prehypertrophic chondrocytes of the growth plate. After a single ip pulse of human GH or mouse GH, but not prolactin, pY-Stat5 responses were visible in cells in the resting zone and groove of Ranvier, 10-45 min later. Prehypertrophic chondrocytes showed no pY-Stat5 response to GH. GH target cells were also identified in other tissues, and a marked variability in spatiotemporal pY-Stat5 responses was evident. Endogenous hepatic pY-Stat5 was detected in mice with intact GH secretion but only during a GH pulse. Fasting and chronic exposure to GH attenuated the pY-Stat5 response to an acute GH injection. In conclusion, pY-Stat5 responses to GH vary in time and space, are sensitive to nutritional status, and may be inhibited by prior GH exposure. In the growth plate, our data provide direct in vivo support for an early role of GH to regulate the fate of immature chondrocytes.

Nuclear factor-kappaB mediates the inhibitory effects of tumor necrosis factor-alpha on growth hormone-inducible gene expression in liver

TNF inhibits serine protease inhibitor 2.1 (Spi 2.1) and IGF-I gene expression by GH in CWSV-1 hepatocytes. The current study describes construction of a GH-inducible IGF-I promoter construct and investigates mechanisms by which TNF and nuclear factor-kappaB (NFkappaB) inhibit GH-inducible gene expression. CWSV-1 cells were transfected with GH-inducible Spi 2.1 or IGF-I promoter luciferase constructs, incubated with TNF signaling inhibitors (fumonisin B1 for sphingomyelinase and SP600125 for c-Jun N-terminal kinase), treated with or without TNF, and then stimulated with recombinant human GH. The 5- to 6-fold induction of Spi 2.1 and IGF-I promoter activity by GH was inhibited by TNF. Neither fumonisin B1 nor SP600125 prevented the inhibitory effects of TNF on GH-inducible promoter activity. Dominant-negative inhibitor-kappaBalpha (IkappaBalpha) expression vectors (IkappaBalphaS/A or IkappaBalphaTrunc), p65 and p50 expression vectors, and p65 deletion constructs were used to investigate the NFkappaB pathway. IkappaBalphaS/A and IkappaBalphaTrunc ameliorated the inhibitory effects of TNF on GH-inducible Spi 2.1 and IGF-I promoter activity. Cotransfection of CWSV-1 cells with expression vectors for p65 alone or p50 and p65 together inhibited GH-inducible Spi 2.1 and IGF-I promoter activity. Cotransfection with a C-terminal p65 deletion (1-450) enhanced GH-inducible promoter activity, whereas the N-terminal deletion (31-551) was inhibitory for IGF-I but not Spi 2.1. Cycloheximide did not antagonize the inhibitory effects of TNF on GH-inducible IGF-I expression. We conclude the inhibitory effects of TNF on GH-inducible promoter activity are mediated by NFkappaB, especially p65, by a mechanism that does not require protein synthesis.

S731 in the transactivation domain modulates STAT5b activity

As mediators of cytokine and growth factor signaling, signal transducers and activators of transcription (STATs) transmit signals from the membrane and cytoplasm to the nucleus. While Y699 phosphorylation is required for STAT5b transcriptional activity, our previous studies show that mutation of two tyrosines in the transactivation domain of STAT5b (Y740/743F) increases Y699 phosphorylation leading to increased transcriptional activity and DNA synthesis in breast cancer cells [A.M. Weaver, C.M. Silva, Modulation of signal transducer and activator of transcription 5b activity in breast cancer cells by mutation of tyrosines within the transactivation domain, Molecular Endocrinology 20 (2006) 2392-2405]. In many instances, phosphorylation of serines in the transactivation domain also modulates STAT5b activity. Here, we demonstrate for the first time that EGF stimulation enhances S731 phosphorylation. Furthermore, we show that the increased activity of the Y740/743F STAT5b mutant requires S731. As STAT5b is implicated in several cancers, understanding how its activity is regulated through tyrosine and serine phosphorylation is vital for the development of potential novel cancer therapeutics.

Endotoxin attenuates growth hormone-induced hepatic insulin-like growth factor I expression by inhibiting JAK2/STAT5 signal transduction and STAT5b DNA binding

Gram-negative sepsis with release of endotoxin is a frequent cause of cachexia that develops partly because of resistance to growth hormone (GH) with reduced insulin-like growth factor-I (IGF-I) expression. We set out to more fully characterize the mechanisms for the resistance and to determine whether in addition to a defect in the janus kinase 2 (JAK2)-signal transducer and activator of transcription (STAT) 5b pathway, required for GH-induced IGF-I expression, there might also be a more distal defect. Conscious rats were given endotoxin and studied 4 h later. In liver of these animals, GH-induced JAK2 and STAT5 phosphorylation was impaired and appeared to be caused, at least in part, by a marked increase in hepatic tumor necrosis factor-alpha and interleukin-6 mRNA expression accompanied by elevated levels of inhibitors of GH signaling, namely cytokine-inducible suppressors of cytokine signaling-1 and -3 and cytokine-inducible SH2 protein (CIS). Nuclear phosphorylated STAT5b levels were significantly depressed to 61% of the control values and represent a potential cause of the reduced GH-induced IGF-I expression. In addition, binding of phosphorylated STAT5b to DNA was reduced to an even greater extent and averaged 17% of the normal control value. This provides a further explanation for the impaired IGF-I gene transcription. Interestingly, when endotoxin-treated rats were treated with GH, there was a marked increase in proinflammatory cytokine gene expression in the liver. If such a response were to occur in humans, this might provide a partial explanation for the adverse effect of GH treatment reported in critically ill patients.

Stat5 in the mammary gland: controlling normal development and cancer

The signal transducer and activator of transcription (Stat5) funnels extracellular signals of cytokines, hormones, and growth factors into transcriptional activity in the mammary gland. Postnatal development and functionality of this tissue is synchronized with the reproductive cycle. Consequently, Stat5 involvement in lobuloalveolar development, milk-protein synthesis, or tissue remodeling is dictated by the particular reproductive stage. Latent deregulation of Stat5 activity during the reproductive cycle predisposes the tissue to tumorigenesis at a later stage, when the female is no longer fertile. Accumulating data from studies with mouse models and breast-cancer specimens demonstrate a dual role for Stat5 in this context. It causes tumorigenesis, but delays metastasis progression. Consequently, Stat5 activity in breast-cancer specimens marks a better prognosis for survival.

Signalling cross-talk between hepatocyte nuclear factor 4alpha and growth-hormone-activated STAT5b

In the present study, we have characterized signalling cross-talk between STAT5b (signal transducer and activator of transcription 5b) and HNF4alpha (hepatocyte nuclear factor 4alpha), two major regulators of sex-dependent gene expression in the liver. In a HepG2 liver cell model, HNF4alpha strongly inhibited beta-casein and ntcp (Na+/taurocholate cotransporting polypeptide) promoter activity stimulated by GH (growth hormone)-activated STAT5b, but had no effect on interferon-gamma-stimulated STAT1 transcriptional activity. By contrast, STAT5b synergistically enhanced the transcriptional activity of HNF4alpha towards the ApoCIII (apolipoprotein CIII) promoter. The inhibitory effect of HNF4alpha on STAT5b transcription was associated with the inhibition of GH-stimulated STAT5b tyrosine phosphorylation and nuclear translocation. The short-chain fatty acid, butyrate, reversed STAT5b transcriptional inhibition by HNF4alpha, but did not reverse the inhibition of STAT5b tyrosine phosphorylation. HNF4alpha inhibition of STAT5b tyrosine phosphorylation was not reversed by pervanadate or by dominant-negative phosphotyrosine phosphatase 1B, suggesting that it does not result from an increase in STAT5b dephosphorylation. Rather, HNF4alpha blocked GH-stimulated tyrosine phosphorylation of JAK2 (Janus kinase 2), a STAT5b tyrosine kinase. Thus STAT5b and HNF4alpha exhibit bi-directional cross-talk that may augment HNF4alpha-dependent gene transcription while inhibiting STAT5b transcriptional activity via the inhibitory effects of HNF4alpha on JAK2 phosphorylation, which leads to inhibition of STAT5b signalling initiated by the GH receptor at the cell surface.

Tumor necrosis factor inhibits growth hormone-mediated gene expression in hepatocytes

Growth hormone (GH) stimulates STAT5 phosphorylation by JAK2, which activates IGF-I and serine protease inhibitor 2.1 (Spi 2.1) transcription, whereas STAT5 dephosphorylation by protein tyrosine phosphatases (PTPs) terminates this signal. We hypothesized that the inhibitory effects of TNF on GH signaling and gene transcription were responsible for hepatic GH resistance. CWSV-1 hepatocytes were treated with TNF, pervanadate (a PTP inhibitor), or both, before GH stimulation. Total and tyrosine-phosphorylated JAK2, STAT5, ERK1/2, SHP-1 and SHP-2, IGF-I, and Spi 2.1 mRNA levels were measured. GH stimulated STAT5 and ERK1/2 phosphorylation, IGF-I, and Spi 2.1 mRNA expression. TNF attenuated JAK2/STAT5 and ERK1/2 phosphorylation and IGF-I and Spi 2.1 mRNA expression following GH stimulation. SHP-1 and SHP-2 protein levels were unaltered by TNF or GH, and the GH-induced increase in SHP-1 PTP activity was not further increased by TNF. In TNF-treated cells, pervanadate restored STAT5 and ERK1/2 phosphorylation to control levels following GH stimulation but did not restore IGF-I or Spi 2.1 mRNA induction. Cells transfected with a Spi 2.1 promoter-luciferase vector demonstrate a 50-fold induction in luciferase activity following GH stimulation or cotransfection with a constitutively active STAT5 vector. TNF prevented the induction of Spi 2.1 promoter activity by GH and the STAT5 construct. We conclude that TNF does not inhibit GH activity by inducing SHP-1 or -2 expression and that correction of GH signaling defects in TNF-treated cells by pervanadate does not restore GH-induced gene expression. The inhibitory effects of TNF on GH-mediated gene transcription appear independent of STAT5 activity and previously identified abnormalities in JAK2/STAT5 signaling.

Growth hormone regulation of sex-dependent liver gene expression

The liver is a primary target for the action of GH, a pituitary protein hormone that regulates a broad range of physiological processes, including long bone growth, fatty acid oxidation, glucose uptake, and hepatic steroid and foreign compound metabolism. GH exerts sex-dependent effects on the liver in many species, with many hepatic genes, most notably genes coding for cytochrome P450 (CYP) enzymes, being transcribed in a sex-dependent manner. Sex differences in CYP expression are most striking in rats and mice (up to 500-fold male-female differences), but are also seen, albeit to a much smaller degree, in humans, where they are an important determinant of the sex dependence of hepatic drug and steroid metabolism. This article examines the mechanisms whereby GH, via its sex-dependent temporal patterns of pituitary release, activates intracellular signaling leading to the sexually dimorphic transcription of CYPs and other liver-expressed genes. Recent findings implicating the GH-regulated transcription factor STAT5b (signal transducer and activator of transcription 5b), hepatocyte nuclear factors 3beta, 4alpha and 6, and sex differences in DNA methylation and chromatin structure in the sex-dependent actions of GH are reviewed, and current mechanistic models are evaluated.

Role of the cytokine-induced SH2 domain-containing protein CIS in growth hormone receptor internalization

The cytokine-inducible SH2 domain-containing protein CIS inhibits signaling from the growth hormone (GH) receptor (GHR) to STAT5b by a proteasome-dependent mechanism. Here, we used the GH-responsive rat liver cell line CWSV-1 to investigate the role of CIS and the proteasome in GH-induced GHR internalization. Cell-surface GHR localization and internalization were monitored in GH-stimulated cells by confocal immunofluorescence microscopy using an antibody directed against the GHR extracellular domain. In GH naïve cells, GHR was detected in small, randomly distributed granules on the cell surface and in the cytoplasm, with accumulation in the perinuclear area. GH treatment induced a rapid (within 5 min) internalization of GH.GHR complexes, which coincided with the onset of GHR tyrosine phosphorylation and the appearance in the cytosol of distinct granular structures containing internalized GH. GHR signaling to STAT5b continued for approximately 30-40 min, however, indicating that GHR signaling and deactivation of the GH.GHR complex both proceed from an intracellular compartment. The internalization of GH and GHR was inhibited by CIS-R107K, a dominant-negative SH2 domain mutant of CIS, and by the proteasome inhibitors MG132 and epoxomicin, which prolong GHR signaling to STAT5b. GH pulse-chase studies established that the internalized GH.GHR complexes did not recycle back to the cell surface in significant amounts under these conditions. Given the established specificity of CIS-R107K for blocking the GHR signaling inhibitory actions of CIS, but not those of other SOCS/CIS family members, these findings implicate CIS and the proteasome in the control of GHR internalization following receptor activation and suggest that CIS-dependent receptor internalization is a prerequisite for efficient termination of GHR signaling.

Phospho-STAT5 accumulation in nuclear fractions from vitamin A-deficient rat liver

The growth hormone (GH)-responsive cytochrome P450 (CYP) 2C11 is down-regulated in vitamin A-deficient (VAD) rat liver. This study assessed the impact of a VAD diet on the hepatic Janus kinase-Signal Transducers and Activators of Transcription (JAK-STAT) system that mediates GH signalling. Nuclear tyrosine- and serine-phosphorylated STAT5 accumulated in VAD liver, whereas nuclear JAK2 tyrosine kinase and SHP-1 phosphatase were decreased. Tyrosine-phosphorylated SHP-1 was decreased to 36+/-14% of control (P<0.01), indicating its impaired activation in VAD liver. Episodic GH pulses increased nuclear phospho-STAT5, especially in control liver, but nuclear phospho-JAK2 and phospho-SHP-1 were not restored. CYP2C11 protein and testosterone 16alpha-hydroxylation were decreased in VAD liver to 67+/-16% and 76+/-19% of control, and were further decreased by GH to 32+/-8% and 30+/-14% of control. Thus, hypo-responsiveness of JAK-STAT in VAD liver is associated with impaired nuclear phospho-STAT dephosphorylation.

ERBB4/HER4 potentiates STAT5A transcriptional activity by regulating novel STAT5A serine phosphorylation events

The epidermal growth factor receptor family member ERBB4 is required for mammary gland development and lactation. ERBB4 activities in the breast are mediated through the signal transducer and activator of transcription (STAT) family member STAT5A, and ERBB4 directly activates STAT5A, in part, through phosphorylation of STAT5A at the regulatory Tyr-694. Here we show that STAT5A regulation by ERBB4 is also mediated through STAT5A serine phosphorylation. Using a reverse-phase high performance liquid chromatography tandem mass spectrometry analysis of proteolytically digested STAT5A coexpressed with ERBB4, we identified STAT5A serine phosphorylations at the previously described Ser-779 and at the novel Ser-127/Ser-128. Immunohistochemistry of wild-type and ERBB4-null mammary glands at late pregnancy showed that ERBB4 expression was required for STAT5A phosphorylation at Ser-779. Independent serine-to-alanine residue substitutions in full-length STAT5A revealed that although STAT5A Ser-779 phosphorylation was dispensable for phosphorylation of STAT5A at Tyr-694 and subsequent DNA binding, Ser-779 was required to stabilize an interaction with ERBB4 and mediate ERBB4-induced STAT5A stimulation of gene expression. STAT5A Ser-127/Ser-128, on the other hand, was required for ERBB4-induced phosphorylation of Tyr-694, whereas Ser-779 and as yet unidentified tyrosine residues were phosphorylated in the absence of Ser-127/Ser-128. In addition, STAT5A S127A/S128A remained associated with ERBB4 but failed to bind DNA or activate transcription in response to ERBB4 coexpression. Our studies demonstrate that phosphorylation of STAT5A at Ser-127/Ser-128 and Ser-779 are obligatory events regulating ERBB4-mediated activation of STAT5A.

The Cytoplasmic Tyrosine Kinase Pyk2 as a Novel Effector of Fibroblast Growth Factor Receptor 3 Activation

Activating mutations within fibroblast growth factor receptor 3 (FGFR3), a receptor tyrosine kinase, are responsible for human skeletal dysplasias including achondroplasia and the neonatal lethal syndromes thanatophoric dysplasia types I and II. Several of these same FGFR3 mutations have also been identified somatically in human cancers, including multiple myeloma, bladder carcinoma, and cervical cancer. The molecular pathways exploited by FGFR3 to stimulate abnormal proliferation during neoplasia are unclear. The nonreceptor protein-tyrosine kinase Pyk2 (proline-rich tyrosine kinase 2) has been shown previously to regulate apoptosis in multiple myeloma cells. Here we describe a novel interaction between FGFR3 and Pyk2, mediated by the juxtamembrane domain of FGFR3 and the kinase domain of Pyk2. Within the FGFR family, Pyk2 also interacted significantly with FGFR2. Overexpression of Pyk2 alone led to its spontaneous activation and tyrosine phosphorylation, resulting in activation of Stat5B, indicated by the reporter GFP-Stat5B. These effects were completely dependent upon Tyr(402), the autophosphorylation site of Pyk2, which allows recruitment of Src family members for further activating phosphorylations at other sites on Pyk2. In the presence of activated FGFR3, the activation of Pyk2 itself became independent of Tyr(402), indicating that FGFR3 activation circumvents the requirement for c-Src recruitment at Tyr(402) of Pyk2. We also examined the role of the tyrosine phosphatase Shp2 in antagonizing Pyk2 activation. Taken together, these results suggest that signaling pathways regulated by FGFR3 may converge with Pyk2-dependent pathways to provide maximal activation of Stat5B.

Interleukin 4 Regulates Phosphorylation of Serine 756 in the Transactivation Domain of Stat6

Lymphokines interleukin-4 (IL4) and IL13 exert overlapping biological activities via the shared use of the IL4 receptor alpha-chain and signal transducer and activator of transcription 6 (Stat6). Stat6 is critical for T-helper 2 cell differentiation, B-cell Ig class switch, and allergic diseases; thus, understanding its regulation is of central importance. Phosphorylation is crucial for Stat activity. Whereas Stat6 is phosphorylated on Tyr(641), less is known about serine or threonine. We demonstrate in primary human T-cells (>95% CD3+) that IL4 and for the first time IL13 induce Stat6 serine but not threonine phosphorylation that closely paralleled early IL4 receptor alpha-chain activation (10 min). Stat6 uniquely fails to share a positionally conserved Stat serine phosphorylation sequence; however, known phosphoacceptor sites are proline-flanked. Alanine substitutions of these conserved residues revealed that the transactivation domain, which localized Ser(756) but not Ser(827) or Ser(176), is the IL4-regulated site based on phosphoamino acid analysis. Tyr(641) was dispensable for IL4-mediated serine phosphorylation, suggesting that dimerization is not preconditional. Only Stat6 Y641F variant showed a significant effect on IL4-inducible Cepsilon DNA-binding and reporter gene expression. Lastly, recent work has shown that protein phosphatase 2A negatively regulates Stat6. We propose this target residue(s) is distinct from Ser(756) and may be proximal to Tyr(641) at Thr(645), a residue conserved only among Stat6 members. The phosphomimic variants T645E or T645D ablated Stat6 activation, whereas polar uncharged substitutions (Gln or Asn) and additional mutants (Ala, Val, or Phe) showed no effect. These findings suggest that Stat6 has mechanisms of regulation distinct from other Stats.

RhoA/ROCK activation by growth hormone abrogates p300/histone deacetylase 6 repression of Stat5-mediated transcription

We demonstrate here that growth hormone (GH) stimulates the activation of RhoA and its substrate Rho kinase (ROCK) in NIH-3T3 cells. GH-stimulated formation of GTP-bound RhoA requires JAK2-dependent dissociation of RhoA from its negative regulator p190 RhoGAP. Inactivation of RhoA does not affect GH-stimulated JAK2 tyrosine phosphorylation nor p44/42 MAPK activity. However, RhoA and ROCK activities are required for GH-stimulated, Stat5-mediated transcription. RhoA-dependent enhancement of GH-stimulated, Stat5-mediated transcription is due to repression of histone deacetylase 6 activity recruited by transcription cofactor p300 that negatively regulates GH-stimulated, Stat5-mediated transcription. We also demonstrate that RhoA is the pivot for cAMP-dependent protein kinase inhibition of GH-stimulated, Stat5-mediated transcription as a consequence of cAMP-dependent protein kinase inactivation of RhoA through serine residue 188 of RhoA. We have therefore provided a novel mechanism by which a Ras-like small GTPase, RhoA, can regulate Stat5-mediated transcription.

Genomic structure and promoter characterization of the human Sprouty4 gene, a novel regulator of lung morphogenesis

The expression of Sprouty4 (Spry4), an intracellular FGF receptor antagonist, shows a temporally and spatially restricted pattern in embryonic lung and is induced by ERK signaling. To clarify the molecular mechanisms regulating Spry4 transcription, the genomic structure of the human Sprouty4 (hSpry4) gene was first determined by using the GenomeWalker kit. The hSpry4 gene spans > 14 kb and is organized in three exons and two introns. Multiple transcription start sites were subsequently mapped by 5'-rapid amplification of cDNA ends. Analysis of up to 4 kb of sequence in the 5'-flanking region of the gene showed the presence of multiple potential transcription factor binding sites but no TATA or CAAT boxes. Transient transfection using luciferase reporter gene constructs with progressive deletions of the hSpry4 5'-flanking region revealed that the core promoter activity is located within the proximal 0.4-kb region, whereas the minimal ERK-inducible promoter activity is between -69 and -31. Homology analysis further showed that the core promoter region of the hSpry4 gene exhibits significant similarity to the 5'-flanking region of the mouse gene.

Role of Stat5 in type I interferon-signaling and transcriptional regulation

Type I interferons are pleiotropic cytokines that transduce signals via activation of multiple downstream signaling cascades, including the Jak-Stat pathway. Although the roles of Stat1 and Stat2 in Type I interferon signaling are well established, the roles that other Stat-family members play in the induction of IFN-responses remain to be defined. In previous studies, we have shown that Stat5 associates with the CrkL adapter and forms a signaling complex that binds DNA. In the present study, we provide evidence that Stat5 is phosphorylated on serines 725/730 in an IFNalpha- and IFNbeta-dependent manner, providing direct evidence that serine phosphorylation of the protein is a component of an interferon signaling cascade. Such serine phosphorylation of Stat5 is Map kinase- and PI 3(')-kinase independent, while the activation of the serine kinase that phosphorylates Stat5 is regulated by upstream tyrosine kinase activity. Using mouse embryonic fibroblasts with targeted disruption of the Stat5a and Stat5b genes, we demonstrate that full activation of Stat5 is required for Type I interferon-dependent gene transcription via GAS elements. Altogether, our data provide evidence that Stat5 plays an important role in IFN-signaling and participates in the induction of Type I IFN-dependent responses. Furthermore, our results strongly suggest that, in addition to phosphorylation on tyrosine residues, phosphorylation on serine residues exhibits regulatory effects on the transcriptional capacity of Stat5.

EGF receptor-mediated, c-Src-dependent, activation of Stat5b is downregulated in mitogenically responsive hepatocytes

Signal transducer and activator of transcription (STAT) proteins may be activated by epidermal growth factor (EGF), but their role in EGF receptor-mediated mitogenic signaling is not clear. We previously showed that Stat5b was activated by EGF in rat hepatocytes in primary monolayer culture. In the present study, we found that EGF induced tyrosine phosphorylation of Stat5b both on Tyr-699, which correlated with specific DNA binding activity, and also on other tyrosine residues. The Src tyrosine kinase inhibitor CGP77675 blocked the EGF-induced activation of Stat5b, but did not affect the Stat5b activation by growth hormone (GH) or prolactin (PRL). The Stat5b response to EGF was most pronounced soon (3 h) after plating (early G(1)) and at high cell density (50,000 hepatocytes per cm(2)). However, at this cell density EGF did not stimulate DNA synthesis. In hepatocytes at 24 h of culturing (mid/late G(1)) with 20,000 cells per cm(2), EGF induced strong phosphorylation of the EGF receptor, as well as Shc and ERK, and stimulated DNA synthesis, but did not activate Stat5b, although the Stat5b response to GH or PRL was retained. A strong GH-induced Stat5b activation neither influenced the DNA synthesis alone nor enhanced the mitogenic effect of EGF. The results show that EGF induces tyrosine phosphorylation and DNA-binding activity of Stat5b in a manner different from GH and PRL, apparently by a Src-dependent mechanism. The data also provide further evidence that Stat5b is not required for mitogenic signaling from the EGF receptor.

The mechanisms by which nitric oxide affects mammary epithelial growth and differentiation

Nitric oxide (NO) enhances prolactin-stimulated DNA synthesis and inhibits prolactin-induced differentiation in mouse mammary epithelium. The molecular pathways used by NO were determined by employing specific inhibitors of the transducers utilized by NO. Inhibitors of the Jun N-terminal kinase (JNK) blocked the effect of NO on DNA synthesis, although this appeared to involve a protein kinase G (PKG)-independent pathway. In contrast, inhibitors of the extracellular signal-regulated kinase (ERK) prevented NO from suppressing alpha-lactalbumin accumulation and this effect was PKG-dependent. NO can also elevate cAMP through the inhibition of phosphodiesterase 3 and cAMP mimicks the actions of NO on both DNA synthesis and differentiation. However, suppression of cAMP levels did not prevent the effects of NO. Therefore, NO uses two separate pathways to affect mammary epithelium: it stimulates growth via JNK and inhibits differentiation through ERK.

Naturally occurring dominant-negative Stat5 suppresses transcriptional activity of estrogen receptors and induces apoptosis in T47D breast cancer cells

Signal transducer and activator of transcription (Stat) 5 regulates growth, differentiation, and survival of mammary and hematopoietic cells. The role of Stat5 in breast cancer has not been established, although Stat5 is critical for some hematopoietic malignancies. We detected for the first time that Stat5b is constitutively activated in human breast cancer cell lines, and analysed the role of Stat5 in estrogen receptor(ER)-positive breast cancer cell lines using dominant-negative variants of Stat5. Two distinct carboxyl-truncated Stat5a derivatives were generated. Stat5aDelta740 corresponded to a naturally occurring alternative splice variant, and Stat5aDelta713 was analogous to an 80 kDa Stat5a product of a nuclear protease. Stat5aDelta740 and Stat5aDelta713 displayed comparable dominant-negative properties and suppressed transcriptional activity of wild-type Stat5a and Stat5b equally well. Cotransfection experiments revealed that Stat5aDelta740 completely blocked transcriptional activity of endogenous estrogen receptor in T47D and MCF7 cells, and of both ER alpha and ER beta in COS-7 cells. Stat5aDelta740 was selected for adenoviral delivery, and high-efficiency expression of tyrosine phosphorylated Stat5aDelta740 was achieved in infected cells. Adenoviral-mediated Stat5aDelta740 induced apoptosis in T47D cells but not in caspase-3-negative MCF7 cells. The present study indicates that overexpression of a dominant-negative variant of Stat5 suppresses ER transcriptional activity and induces apoptosis in estrogen-responsive breast cancer tissue culture cells.

Signal transducer and activator of transcription 5a (STAT5a) is required for eosinophil differentiation of human cord blood-derived CD34+ cells

Signal transducers and activators of transcription (STATs) have been reported to play a critical role in the differentiation of several myeloid cell lines, although the importance of STATs in the differentiation of primary human hematopoietic cells remains to be established. Terminal eosinophil differentiation is induced by interleukin-5 (IL-5), which has also been demonstrated to activate STAT5. We have investigated whether STAT5 plays a critical role during eosinophil differentiation using umbilical cord blood-derived CD34(+) cells. In this ex vivo system, STAT5 expression and activation are high early during differentiation, and STAT5 protein expression is down-regulated during the final stages of eosinophil differentiation. Retroviral transductions were performed to ectopically express wild-type and dominant-negative STAT5a (STAT5aDelta750) in CD34(+) cells. Transduction of cells with STAT5a resulted in enhanced proliferation compared with cells transduced with empty vector alone. Interestingly, ectopic expression of STAT5a also resulted in accelerated differentiation. In contrast, ectopic expression of STAT5aDelta750 resulted in a block in differentiation, whereas proliferation was also severely inhibited. Similar results were obtained with dominant-negative STAT5b. Forced expression of STAT5a enhanced expression of the STAT5 target genes Bcl-2 and p21(WAF/Cip1), suggesting they may be important in STAT5a-mediated eosinophil differentiation. These results demonstrate that STAT5 plays a critical role in eosinophil differentiation of primary human hematopoietic cells.

STAT5a activation mediates the epithelial to mesenchymal transition induced by oncogenic RhoA

The involvement of Rho GTPases in signal transduction pathways leading to transcription activation is one of the major roles of this family of GTPases. Thus, the identification of transcription factors regulated by Rho GTPases and the understanding of the mechanisms of their activation and its biological outcome are of great interest. Here, we provide evidence that Rho GTPases modulate Stat5a, a transcription factor of the family of signal transducers and activators of transcription. RhoA triggers tyrosine phosphorylation (Y696) of Stat5a via a JAK2-dependent mechanism and promotes DNA-binding activity of Stat5a. Tyrosine phosphorylation of Stat5a is also stimulated physiologically by lysophosphatidic acid (LPA) in a Rho-dependent manner. Simultaneously, RhoA reduces serine phosphorylation of Stat5a at both serine residues S726 and S780, resulting in a further increase of activity as defined by mutagenesis experiments. Furthermore, serine dephosphorylation of Stat5a by RhoA does not take place by down-modulation of either JNK1, MEK1, or p38 MAP kinases, as determined by transfection experiments or chemical inhibition of both MEK1, p38, and JNK serine kinases. Thus, RhoA regulates Stat5a via tyrosine phosphorylation and via a yet to be determined novel down-modulating pathway that involves serine dephosphorylation. Finally, we provide evidence for a role of Stat5a in RhoA-induced epithelial-to-mesenchymal transition with concomitant increase in vimentin expression, E-cadherin down-regulation, and cell motility.