Stat5a and Stat5b proteins have essential and nonessential, or redundant, roles in cytokine responses

Stat5 activation is uniquely associated with cytokine signaling in peripheral T cells

The activation and subsequent proliferation of peripheral T cells requires the engagement of the T cell and a cytokine receptor, typically the IL-2 or IL-4 receptors. Critical to understanding the regulation of peripheral T cells is the knowledge of the unique contributions of each receptor to full T cell activation and cell cycle progression. Mice deficient in Stat5a and Stat5b have demonstrated the essential role that these highly related proteins play in cell cycle progression following peripheral T cell activation. Here we demonstrate that activation of the Stat5 proteins by tyrosine phosphorylation is uniquely contributed by cytokine receptor signaling and specifically does not occur through the T cell receptor complex.

Ineffective erythropoiesis in myelodysplastic syndromes: correlation with Fas expression but not with lack of erythropoietin receptor signal transduction

Ineffective erythropoiesis in myelodysplasia is characterized by a defect in erythroid progenitor growth and by abnormal erythroid differentiation. Increased apoptosis of erythroid, granulocytic and megakaryocytic lineages is thought to account for cytopenias. Erythropoietin (Epo)-induced BFU-E and CFU-E growth was studied in 25 myelodysplastic syndrome (MDS) marrow specimens and found to be drastically diminished. To investigate the functionality of Epo-R in MDS marrow, we focused on Epo-induced STAT5 activation. Epo was able to stimulate STAT5 DNA binding activity in all normal and 12/24 MDS marrows tested, with no correlation between the level of STAT5 activation and the development of erythroid colonies in response to Epo. In contrast, impaired proliferation of erythroid progenitors was related to an increased expression of the transmembrane mediator of apoptotic cell death Fas/CD95 on the glycophorin A+ subpopulation. Therefore we conclude that the stimulation of pro-apoptotic signals rather than the defect of anti-apoptotic pathways resulting from Epo-stimulated Jak2-STAT5 pathway, predominantly accounts for ineffective erythropoiesis in myelodysplasia.

Fetal anemia and apoptosis of red cell progenitors in Stat5a-/-5b-/- mice: a direct role for Stat5 in Bcl-X(L) induction

The erythropoietin receptor (EpoR) is essential for production of red blood cells; a principal function of EpoR is to rescue committed erythroid progenitors from apoptosis. Stat5 is rapidly activated following EpoR stimulation, but its function in erythropoiesis has been unclear since adult Stat5a-/-5b-/- mice have normal steady-state hematocrit. Here we show that Stat5 is essential for the high erythropoietic rate during fetal development. Stat5a-/-5b-/- embryos are severely anemic; erythroid progenitors are present in low numbers, show higher levels of apoptosis, and are less responsive to Epo. These findings are explained by a crucial role for Stat5 in EpoR's antiapoptotic signaling: it mediates the immediate-early induction of Bcl-X(L) in erythroid cells through direct binding to the Bcl-X promoter.

IL-3 dependent regulation of Bcl-xL gene expression by STAT5 in a bone marrow derived cell line

Activation of the Jak/STAT pathway by cytokines has been shown to regulate differentiation, proliferation or apoptosis in hematopoeitic cells. Among the Stat proteins, STAT5 is activated by a broad range of cytokines. In order to study the role of STAT5 in hematopoietic cells, we stably expressed a dominant negative form of STAT5 (STAT5A delta749) in the IL-3 dependent bone marrow derived Ba/F3 cell line. Ba/F3 cells expressing STAT5A delta749 were found to be more sensitive to apoptosis than parental or control Ba/F3 cells after IL-3 withdrawal. Analysis of the expression of the cell death regulators, Bcl-2 and Bcl-x, revealed that the level of Bcl-x was lower in Ba/F3 cells expressing STAT5A delta749 than in control cells. IL-3 regulation of Bcl-x expression at protein and mRNA levels was impaired in these cells while that of Bcl-2 expression was unaffected. We further demonstrated that the Bcl-x gene promoter contained a proximal STAT consensus sequence that bound STAT5. Transactivation of a Bcl-x gene promoter reporter construct by STAT5 was observed in Ba/F3 cells. Introduction of a mutation in the STAT binding site abolished this transactivation. These data indicate that Bcl-x is probably a STAT5 target gene. They also support the involvement of STAT5 in hematopoietic cell survival.

Growth hormone-mediated regulation of insulin-like growth factor I promoter activity in C6 glioma cells

The molecular mechanisms by which GH regulates insulin-like growth factor (IGF-I) gene expression remain obscure. One difficulty has been the lack of established GH-responsive cell lines that express the IGF-I gene. To develop such a cell line, we used rat C6 glioma cells which, as determined by RNase protection assay, express the IGF-I gene but not the GH receptor gene. To confer GH responsiveness, C6 cells were cotransfected with vectors that express the GH receptor (pRc/CMV WTrGHR) and Jak2 (pRc/CMV Jak2). GH responsiveness was demonstrated using luciferase reporter genes containing either the Sis-inducible element from the c-fos gene (pTK81-SIE-Luc) or 6 copies of the GH-responsive GAS-like element (GLE) from the rat spi2.1 gene (pSpi-GLE-Luc). The SIE is activated by binding of STAT1 and 3, whereas the GLE binds STAT5. In cells cotransfected with pRc/CMV WTrGHR, pRc/CMV Jak2, and either pTK81-SIE-Luc or pSpi GLE-Luc, treatment with 500 ng/ml GH for 24 h stimulated a 3.1- and 1.7-fold increase in luciferase activity, respectively. These data suggest that in C6 cells cotransfected with pRc/CMV WTrGHR and pRc/CMV Jak2, GH activates STAT1, 3, and 5. To determine whether GH-responsive IGF-I promoter activity could be demonstrated, C6 cells were cotransfected with pRc/CMV WTrGHR, pRc/ CMV Jak2, and an IGF-I-luciferase fusion gene that contained a fragment of the rat IGF-I gene that extended from -412 in the 5'-flanking region of exon 1 to the Met-22 in exon 3. GH stimulated a modest, but reproducible, 1.7-fold increase in luciferase activity in these cells, suggesting that a GH-responsive element is present in this region of the IGF-I gene. To better localize the GH-responsive element, cells were cotransfected with pRc/CMV WTrGHR, pRc/CMV Jak2 plus one of several IGF-I-luciferase fusion genes containing either fragments of one of the two promoters in the IGF-I gene or a fragment of intron 2 that includes a GH-responsive DNase I hypersensitivity site. For all constructs, treatment with GH for 24 h did not stimulate a significant increase in luciferase activity, suggesting that GH-responsive sequences are not located in these specific regions of the IGF-I gene or that GH-directed transcription of the IGF-I gene is mediated via several different regions of the IGF-I gene and the effect of any one of these regions in isolation was not sufficiently robust to be detected in this model system. In summary, transient expression of the GH receptor and Jak2 in C6 cells creates a GH-responsive system that activates STAT1, 3, and 5. Moreover, a fragment of the IGF-I gene that contains exons 1 and 2, a fragment of exon 3, and introns 1 and 2 is GH responsive using this model system.

Genetic Evidence for an Additional Factor Required for Erythropoietin-Induced Signal Transduction

Erythropoietin (EPO) and its receptor (EPOR) are required for the development of mature erythrocytes. After binding of ligand, the EPOR activates a variety of signaling pathways that ultimately control cellular proliferation, survival, and specific gene expression. Although erythroid progenitors appear to be the principal EPO-responsive cell type in vivo due to the restricted expression of the EPOR, many growth factor–dependent cell lines expressing the EPOR can respond to EPO by activating many or all of these pathways. In the present study, we have identified a cellular context (the interleukin-2 [IL-2]–dependent HT-2 line) in which the EPO stimulation of the EPOR fails to support cellular proliferation, STAT-5 induction, or MAPK activation, despite efficient phosphorylation of the EPOR and JAK2 and inhibition of apoptosis after withdrawal of IL-2. Interestingly, when we fused HT-2 cells expressing the EPOR with Ba/F3 cells in a complementation assay, the resulting hybridomas proliferated and potently activated STAT-5 and MAPK in response to EPO. These data indicate that an unidentified cellular factor is needed to mediate signaling by the EPOR. Moreover, Ba/F3 cells apparently express this factor(s) and somatic fusions can, therefore, confer EPO-responsiveness to HT-2 cells that lack this factor.

Regulation of prolactin receptor (PRLR) gene expression in insulin-producing cells. Prolactin and growth hormone activate one of the rat prlr gene promoters via STAT5a and STAT5b

Expression of the prolactin receptor (PRLR) gene is increased in pancreatic islets during pregnancy and in vitro in insulin-producing cells by growth hormone (GH) and prolactin (PRL). The 5'-region of the rat PRLR gene contains at least three alternative first exons that are expressed tissue-specifically because of differential promoter usage. We show by reverse transcription-polymerase chain reaction analysis that both exon 1A- and exon 1C-containing PRLR transcripts are expressed in rat islets and that human (h)GH, ovine (o)PRL, and bovine (b)GH increase exon 1A expression 6.5 +/- 0. 8-fold, 6.8 +/- 0.7-fold, and 3.9 +/- 0.7-fold and exon 1C expression 4.8 +/- 0.4-fold, 4.4 +/- 0.6-fold, and 2.5 +/- 0.7-fold, respectively. Expression of exon 1B was not detectable. The transcriptional activities of reporter constructs containing the 1A, 1B, or 1C promoter were found to be 22.8-fold, 2.7-fold, and 8. 0-fold, respectively, above that of a promoterless reporter construct when transfected into the insulin-producing INS-1 cells. The transcriptional activity of the 1A promoter construct was increased 8.9 +/- 1.9-fold by 0.5 microgram/ml hGH. Responsiveness to hGH of the 1A promoter was localized to the region from -225 to +81 with respect to the transcription start site. This region contains the sequence TTCTAGGAA that by gel retardation experiments was shown to bind the transcription factors STAT5a and STAT5b in response to stimulation by hGH, oPRL, or bGH. Mutation of this gamma-activated sequence-like element completely abolished transcriptional induction of the 1A promoter by hGH. Our results suggest that GH and PRL increase the levels of exon 1A- and 1C-containing PRLR mRNA species and furthermore that the transcriptional activity of the 1A promoter is increased via activation of STAT5a and STAT5b.

Developmental regulation of insulin-like growth factor-I and growth hormone receptor gene expression

During development, the insulin-like growth factor I (IGF-I) gene is expressed in a tissue specific manner; however, the molecular mechanisms governing its developmental regulation remain poorly defined. To examine the hypothesis that expression of the growth hormone (GH) receptor accounts, in part, for the tissue specific expression of the IGF-I gene during development, the developmental regulation of IGF-I and GH receptor gene expression in rat tissues was examined. The level of IGF-I and GH receptor mRNA was quantified in RNA prepared from rats between day 17 of gestation (E17) and 17 months of age (17M) using an RNase protection assay. Developmental regulation of IGF-I gene expression was tissue specific with four different patterns of expression seen. In liver, IGF-I mRNA levels increased markedly between E17 and postnatal day 45 (P45) and declined thereafter. In contrast, in brain, skeletal muscle and testis, IGF-I mRNA levels decreased between P5 and 4M but were relatively unchanged thereafter. In heart and kidney, a small increase in IGF-I mRNA levels was observed between the early postnatal period and 4 months, whereas in lung, minimal changes were observed during development. The changes in GH receptor mRNA levels were, in general, coordinate with the changes in IGF-I mRNA levels, except in skeletal muscle. Interestingly, quantification of GH receptor levels by Western blot analysis in skeletal muscle demonstrated changes coordinate with IGF-I mRNA levels. The levels of the proteins which mediate GH receptor signaling (STAT1, -3, and -5, and JAK2) were quantified by Western blot analysis. These proteins also are expressed in a tissue specific manner during development. In some cases, the pattern of expression was coordinate with IGF-I gene expression, whereas in others it was discordant. To further define molecular mechanisms for the developmental regulation of IGF-I gene expression, protein binding to IGFI-FP1, a protein binding site that is in the major promoter of the rat IGF-I gene and is important for basal promoter activity in vitro, was examined. Gel shift analyses using a 34-base pair oligonucleotide that contained IGFI-FP1 did not demonstrate changes in protein binding that paralleled those in IGF-I gene expression, suggesting that protein binding to IGFI-FP1 does not contribute to the developmental regulation of IGF-I gene expression, at least in brain and liver. In summary, the present studies demonstrate coordinate expression of the IGF-I gene and GH receptor during development and suggest that GH receptor expression contributes to the tissue specific expression of the IGF-I gene during development.

Identification of SH2-bbeta as a potent cytoplasmic activator of the tyrosine kinase Janus kinase 2

Janus kinases (JAKs) are cytoplasmic tyrosine kinases critical for signaling by growth hormone (GH) and many other ligands that bind to members of the cytokine receptor superfamily. SH2-Bbeta was previously identified as a JAK2-interacting protein that is tyrosyl phosphorylated in response to GH and other cytokines that activate JAK2. In this study, we examined whether SH2-Bbeta alters the activity of JAK2. SH2-Bbeta, when coexpressed with JAK2, significantly increased the tyrosyl phosphorylation of JAK2 and multiple other cellular proteins and stimulated the kinase activity of JAK2 by approximately 20-fold. Coexpression of SH2-Bbeta with JAK2 dramatically increased tyrosyl phosphorylation of signal transducer and activator of transcription (Stat)5B and Stat3, physiological substrates of JAK2. SH2-Bbeta(R555E) with a defective Src homology 2 domain was unable to stimulate JAK2 and JAK2-mediated tyrosyl phosphorylation of Stat5B and Stat3. More importantly, SH2-Bbeta enhanced GH-induced tyrosyl phosphorylation of endogenous JAK2 and ligand-induced tyrosyl phosphorylation of Stat5B by endogenous JAK2. In contrast, SH2-Bbeta did not potentiate the activation of other tyrosine kinases including the receptors for platelet-derived growth factor, epidermal growth factor, or nerve growth factor (TrkA), tyrosine kinases that also bind SH2-Bbeta. These data demonstrate that SH2-Bbeta is a potent cytoplasmic activator of JAK2 and is thereby expected to be an important cellular regulator of signaling by GH and other hormones and cytokines that activate JAK2.

Dominant Negative Mutants Implicate STAT5 in Myeloid Cell Proliferation and Neutrophil Differentiation

STAT5 is a member of the signal transducers and activation of transcription (STAT) family of latent transcription factors activated in a variety of cytokine signaling pathways. We introduced alanine substitution mutations in highly conserved regions of murine STAT5A and studied the mutants for dimerization, DNA binding, transactivation, and dominant negative effects on erythropoietin-induced STAT5-dependent transcriptional activation. The mutations included two near the amino-terminus (W255KR→AAA and R290QQ→AAA), two in the DNA-binding domain (E437E→AA and V466VV→AAA), and a carboxy-terminal truncation of STAT5A (STAT5A/▵53C) analogous to a naturally occurring isoform of rat STAT5B. All of the STAT mutant proteins were tyrosine phosphorylated by JAK2 and heterodimerized with STAT5B except for the WKR mutant, suggesting an important role for this region in STAT5 for stabilizing dimerization. The WKR, EE, and VVV mutants had no detectable DNA-binding activity, and the WKR and VVV mutants, but not EE, were defective in transcriptional induction. The VVV mutant had a moderate dominant negative effect on erythropoietin-induced STAT5 transcriptional activation, which was likely due to the formation of heterodimers that are defective in DNA binding. Interestingly, the WKR mutant had a potent dominant negative effect, comparable to the transactivation domain deletion mutant, ▵53C. Stable expression of either the WKR or ▵53C STAT5 mutants in the murine myeloid cytokine-dependent cell line 32D inhibited both interleukin-3–dependent proliferation and granulocyte colony-stimulating factor (G-CSF)–dependent differentiation, without induction of apoptosis. Expression of these mutants in primary murine bone marrow inhibited G-CSF–dependent granulocyte colony formation in vitro. These results demonstrate that mutations in distinct regions of STAT5 exert dominant negative effects on cytokine signaling, likely through different mechanisms, and suggest a role for STAT5 in proliferation and differentiation of myeloid cells.

Fusion of the ets Transcription Factor TEL to Jak2 Results in Constitutive Jak-Stat Signaling

To study constitutive Janus kinase signaling, chimeric proteins were generated between the pointed domain of the etstranscription factor TEL and the cytosolic tyrosine kinase Jak2. The effects of these proteins on interleukin-3 (IL-3)–dependent proliferation of the hematopoietic cell line, Ba/F3, were studied. Fusion of TEL to the functional kinase (JH1) domain of Jak2 resulted in conversion of Ba/F3 cells to factor-independence. Importantly, fusion of TEL to the Jak2 pseudokinase (JH2) domain or a kinase-inactive Jak2 JH1 domain had no effect on IL-3–dependent proliferation of Ba/F3 cells. Active TEL-Jak2 constructs (consisting of either Jak2 JH1 or Jak2 JH2+JH1 domain fusions) were constitutively tyrosine-phosphorylated but did not affect phosphorylation of endogeneous Jak1, Jak2, or Jak3. TEL-Jak2 activation resulted in the constitutive tyrosine phosphorylation of Stat1, Stat3, and Stat5 as determined by detection of phosphorylation using activation-specific antibodies and by binding of each protein to a preferential GAS sequence in electrophoretic mobility shift assays. Elucidation of signaling events downstream of TEL-Jak2 activation may provide insight into the mechanism of leukemogenesis mediated by this oncogenic fusion protein.

Fusion of the ets Transcription Factor TEL to Jak2 Results in Constitutive Jak-Stat Signaling

To study constitutive Janus kinase signaling, chimeric proteins were generated between the pointed domain of the etstranscription factor TEL and the cytosolic tyrosine kinase Jak2. The effects of these proteins on interleukin-3 (IL-3)–dependent proliferation of the hematopoietic cell line, Ba/F3, were studied. Fusion of TEL to the functional kinase (JH1) domain of Jak2 resulted in conversion of Ba/F3 cells to factor-independence. Importantly, fusion of TEL to the Jak2 pseudokinase (JH2) domain or a kinase-inactive Jak2 JH1 domain had no effect on IL-3–dependent proliferation of Ba/F3 cells. Active TEL-Jak2 constructs (consisting of either Jak2 JH1 or Jak2 JH2+JH1 domain fusions) were constitutively tyrosine-phosphorylated but did not affect phosphorylation of endogeneous Jak1, Jak2, or Jak3. TEL-Jak2 activation resulted in the constitutive tyrosine phosphorylation of Stat1, Stat3, and Stat5 as determined by detection of phosphorylation using activation-specific antibodies and by binding of each protein to a preferential GAS sequence in electrophoretic mobility shift assays. Elucidation of signaling events downstream of TEL-Jak2 activation may provide insight into the mechanism of leukemogenesis mediated by this oncogenic fusion protein.

Dominant Negative Mutants Implicate STAT5 in Myeloid Cell Proliferation and Neutrophil Differentiation

STAT5 is a member of the signal transducers and activation of transcription (STAT) family of latent transcription factors activated in a variety of cytokine signaling pathways. We introduced alanine substitution mutations in highly conserved regions of murine STAT5A and studied the mutants for dimerization, DNA binding, transactivation, and dominant negative effects on erythropoietin-induced STAT5-dependent transcriptional activation. The mutations included two near the amino-terminus (W255KR→AAA and R290QQ→AAA), two in the DNA-binding domain (E437E→AA and V466VV→AAA), and a carboxy-terminal truncation of STAT5A (STAT5A/▵53C) analogous to a naturally occurring isoform of rat STAT5B. All of the STAT mutant proteins were tyrosine phosphorylated by JAK2 and heterodimerized with STAT5B except for the WKR mutant, suggesting an important role for this region in STAT5 for stabilizing dimerization. The WKR, EE, and VVV mutants had no detectable DNA-binding activity, and the WKR and VVV mutants, but not EE, were defective in transcriptional induction. The VVV mutant had a moderate dominant negative effect on erythropoietin-induced STAT5 transcriptional activation, which was likely due to the formation of heterodimers that are defective in DNA binding. Interestingly, the WKR mutant had a potent dominant negative effect, comparable to the transactivation domain deletion mutant, ▵53C. Stable expression of either the WKR or ▵53C STAT5 mutants in the murine myeloid cytokine-dependent cell line 32D inhibited both interleukin-3–dependent proliferation and granulocyte colony-stimulating factor (G-CSF)–dependent differentiation, without induction of apoptosis. Expression of these mutants in primary murine bone marrow inhibited G-CSF–dependent granulocyte colony formation in vitro. These results demonstrate that mutations in distinct regions of STAT5 exert dominant negative effects on cytokine signaling, likely through different mechanisms, and suggest a role for STAT5 in proliferation and differentiation of myeloid cells.

Expression of dominant-negative ErbB2 in the mammary gland of transgenic mice reveals a role in lobuloalveolar development and lactation

Overexpression of the receptor tyrosine kinase ErbB2/HER2/Neu (ErbB2) occurs in 15-40% of human breast cancers. To determine the function of ErbB2 signaling during normal mouse mammary gland development, we expressed a carboxyl-terminal truncated dominant negative allele of ErbB2 (ErbB2deltaIC) in the developing mouse mammary gland. Despite ErbB2deltaIC expression within mammary glands of pubescent virgin and pregnant mice, a phenotype was not observed until late in pregnancy. At 1 day post-partum, lactationally active, distended lobuloalveoli failed to form. This phenotype was exaggerated in multiparous females expressing ErbB2deltaIC. Immunohistochemical staining for ErbB2deltaIC revealed a concordance between high levels of ErbB2deltaIC protein expression and the absence of lactational products within the lumens of ErbB2deltaIC stained lobuloalveoli. These results demonstrate that ErbB2 signaling is required for proper mammary development and lactation at parturition.

The STAT family of proteins in cytokine signaling

It has now been well established that the STAT family of proteins play important roles in cytokine-mediated specific gene activation. Although significant progress has been made toward the understanding of the structure and function of STATs as well as the regulation of STAT signaling pathways, many important questions remain to be answered. STAT PTPase(s) and STAT serine kinase(s) which play important roles in regulating STAT activity have not been identified. In addition, the molecular mechanisms of the negative regulation of STAT signaling by recently discovered protein inhibitors and the crosstalk between STAT and other signal transduction pathways have not been understood. The JAK/STAT field remains to be challenging and exciting.

Protein Kinase B (c-Akt), Phosphatidylinositol 3-Kinase, and STAT5 Are Activated by Erythropoietin (EPO) in HCD57 Erythroid Cells But Are Constitutively Active in an EPO-Independent, Apoptosis-Resistant Subclone (HCD57-SREI Cells)

We found that erythropoietin (EPO) and stem cell factor (SCF) activated protein kinase B (PKB/Akt) in EPO-dependent HCD57 erythroid cells. To better understand signals controlling proliferation and viability, erythroid cells that resist apoptosis in the absence of EPO were subcloned and characterized (HCD57-SREI cells). Constitutive activations of PKB/Akt, STAT5a, and STAT5b were noted in these EPO-independent cells. PI3-kinase activity was an upstream activator of PKB/Akt because the PI3-kinase inhibitor LY294002 blocked both constitutive PKB/Akt and factor-dependent PKB/Akt activity. The LY294002 study showed that proliferation and viability of both HCD57-SREI and HCD57 cells correlated with the activity of PKB/Akt; however, PKB/Akt activity alone did not protect these cells from apoptosis. Treatment of HCD57 cells with SCF also activated PKB/Akt, but did not protect from apoptosis. This result suggested that PKB/PI3-kinase activity is necessary but not sufficient to promote viability and/or proliferation. Constitutive STAT5 activity, activated through an unknown pathway not including JAK2 or EPOR, may act in concert with the constitutive PI3-kinase/PKB/Akt pathway to protect the EPO-independent HCD57-SREI cells from apoptosis and promote limited proliferation.

Stat5a and Stat5b: fraternal twins of signal transduction and transcriptional activation

Stat5a and Stat5b are discretely encoded transcription factors that mediate signals for a broad spectrum of cytokines. Their activation is often an integral component of redundant cytokine signal cascades involving complex cross-talk and pleiotropic gene regulation by Stat5 has been implicated in cellular functions of proliferation, differentiation and apoptosis with relevance to processes of hematopoiesis and immunoregulation, reproduction, and lipid metabolism. Although Stat5a and Stat5b show peptide sequence similarities of > 90%, targeted gene disruptions in mice yield distinctive phenotypes. Prolactin-directed mammary gland maturation fails without functional Stat5a, while disruption of Stat5b in males mitigates growth hormone effects on hepatic function and body mass. The molecular basis for this biologic dichotomy is probably multifaceted. Limited structural dissimilarities between the Stat5a and Stat5b transactivation domains, or subtle differences in the DNA-binding affinities of Stat5 dimer pairs undoubtedly influence gene regulation, but cell-dependent asymmetries in availability of phosphorylated Stat5 can be an underlying factor. Differences in serine phosphorylation(s) of Stat5a and Stat5b, or Stat5 associations with adaptor proteins or co-transcription factors are other potential sources of functional disparity and the signal amplitude, frequency or duration also can be significant. In addition to Stat5 signal attenuation by phosphatase actions or classical feedback inhibition, truncated forms of Stat5 lacking in transactivation capacity may compete upstream for activation and diminish access of full length molecules to DNA binding sites.

Protein Kinase B (c-Akt), Phosphatidylinositol 3-Kinase, and STAT5 Are Activated by Erythropoietin (EPO) in HCD57 Erythroid Cells But Are Constitutively Active in an EPO-Independent, Apoptosis-Resistant Subclone (HCD57-SREI Cells)

We found that erythropoietin (EPO) and stem cell factor (SCF) activated protein kinase B (PKB/Akt) in EPO-dependent HCD57 erythroid cells. To better understand signals controlling proliferation and viability, erythroid cells that resist apoptosis in the absence of EPO were subcloned and characterized (HCD57-SREI cells). Constitutive activations of PKB/Akt, STAT5a, and STAT5b were noted in these EPO-independent cells. PI3-kinase activity was an upstream activator of PKB/Akt because the PI3-kinase inhibitor LY294002 blocked both constitutive PKB/Akt and factor-dependent PKB/Akt activity. The LY294002 study showed that proliferation and viability of both HCD57-SREI and HCD57 cells correlated with the activity of PKB/Akt; however, PKB/Akt activity alone did not protect these cells from apoptosis. Treatment of HCD57 cells with SCF also activated PKB/Akt, but did not protect from apoptosis. This result suggested that PKB/PI3-kinase activity is necessary but not sufficient to promote viability and/or proliferation. Constitutive STAT5 activity, activated through an unknown pathway not including JAK2 or EPOR, may act in concert with the constitutive PI3-kinase/PKB/Akt pathway to protect the EPO-independent HCD57-SREI cells from apoptosis and promote limited proliferation.

Functional roles of STAT family proteins: lessons from knockout mice

STAT (signal transducers and activators of transcription) proteins are activated in response to a large number of cytokines, growth factors, and hormones. Upon activation following the binding of ligands to their receptors, STAT proteins dimerize, translocate to the nucleus, and bind to the promoters of specific target genes. To date, seven mammalian members of the STAT family have been identified. Although some cytokines and growth factors can activate multiple STAT proteins, some STATs are activated with considerable specificity. The physiological role of each individual STAT protein is now being examined through the study of "knockout" mice, harboring a null allele for the particular gene. STAT1-deficient mice exhibit a selective signaling defect in response to interferons. STAT4 and STAT6 are essential for Thl-and Th2-responses, respectively. STAT5a-deficient mice exhibit defective mammary gland development. A study of STAT5b-deficient mice indicates that STAT5b mediates the sexually dimorphic effects of growth hormone in the liver. STAT5a and 5b also play different biological roles in the immune system. STAT3-deficient mice die during early embryogenesis, but the role of STAT3 in adult tissues can be assessed by utilizing the CreloxP recombination system to ablate the gene later in life. Analyses of tissue-specific STAT3-deficient mice indicate that STAT3 plays a crucial role in a variety of biological functions, including cell growth, suppression of apoptosis, and cell motility.

Distinct roles of the phosphatidylinositol 3-kinase and STAT5 pathways in IL-7-mediated development of human thymocyte precursors

Here, we define the IL-7R-activated signal that promotes survival and proliferation of T cell progenitors and demonstrate that it is distinct from the signals that induce differentiation. We show that IL-7 activates PKB and STAT5 in human thymocytes. Into T cell precursors we introduced chimeric receptors with a cytoplasmic domain of the IL-7R that is no longer able to activate PI-3K/PKB and STAT5 and tested the transduced cells in a fetal thymic organ culture. We also examined the T cell precursor activity of progenitors expressing dominant-negative forms of PI-3K or STAT5B. These experiments revealed that PI-3K/PKB activation is essential for the survival and proliferation of T cell precursors and suggest that STAT5 activated by IL-7 mediates T cell differentiation.

Signal transducer and activator of transcription (STAT)5 activation by BCR/ABL is dependent on intact Src homology (SH)3 and SH2 domains of BCR/ABL and is required for leukemogenesis

Signal transducer and activator of transcription (STAT)5 is constitutively activated in BCR/ ABL-expressing cells, but the mechanisms and functional consequences of such activation are unknown. We show here that BCR/ABL induces phosphorylation and activation of STAT5 by a mechanism that requires the BCR/ABL Src homology (SH)2 domain and the proline-rich binding site of the SH3 domain. Upon expression in 32Dcl3 growth factor-dependent myeloid precursor cells, STAT5 activation-deficient BCR/ABL SH3+SH2 domain mutants functioned as tyrosine kinase and activated Ras, but failed to protect from apoptosis induced by withdrawal of interleukin 3 and/or serum and did not induce leukemia in severe combined immunodeficiency mice. In complementation assays, expression of a dominant-active STAT5B mutant (STAT5B-DAM), but not wild-type STAT5B (STAT5B-WT), in 32Dcl3 cells transfected with STAT5 activation-deficient BCR/ABL SH3+SH2 mutants restored protection from apoptosis, stimulated growth factor-independent cell cycle progression, and rescued the leukemogenic potential in mice. Moreover, expression of a dominant-negative STAT5B mutant (STAT5B-DNM) in 32Dcl3 cells transfected with wild-type BCR/ABL inhibited apoptosis resistance, growth factor-independent proliferation, and the leukemogenic potential of these cells. In retrovirally infected mouse bone marrow cells, expression of STAT5B-DNM inhibited BCR/ABL-dependent transformation. Moreover, STAT5B-DAM, but not STAT5B-WT, markedly enhanced the ability of STAT5 activation-defective BCR/ABL SH3+SH2 mutants to induce growth factor-independent colony formation of primary mouse bone marrow progenitor cells. However, STAT5B-DAM did not rescue the growth factor-independent colony formation of kinase-deficient K1172R BCR/ABL or the triple mutant Y177F+R522L+ Y793F BCR/ABL, both of which also fail to activate STAT5. Together, these data demonstrate that STAT5 activation by BCR/ABL is dependent on signaling from more than one domain and document the important role of STAT5-regulated pathways in BCR/ABL leukemogenesis.

Human epidermal keratinocytes upregulate expression of the prolactin receptor after the onset of terminal differentiation, but do not respond to prolactin

Growing and differentiating keratinocytes maintain the epidermal barrier. This is partly controlled by growth factors and hormones. Prolactin (PRL) is named after its hormonal role in mammals during lactation, but is found in all vertebrates where PRL exerts various effects. In serum-free keratinocyte cultures, PRL was thought to be the factor responsible for the proliferative effect of bovine pituitary extract. Here, we evaluated PRL as a clonogenic factor for keratinocytes and found no mitogenic activity. Studying the expression of the PRL receptor by keratinocytes, we found the receptor upregulated only after culture confluence, in differentiating keratinocytes, but we were unable to detect any cellular response to PRL. The hormone does not alter the gene expression of either early (suprabasal keratin) or late (involucrin) differentiation markers by keratinocytes. Accordingly, no activation of the transcription factor Stat5 by PRL can be detected in keratinocytes, Stat5 being nevertheless detected by Western blot.

Differences in phosphorylation of the IL-2R associated JAK/STAT proteins between HTLV-I(+), IL-2-independent and IL-2-dependent cell lines and uncultured leukemic cells from patients with adult T-cell lymphoma/leukemia

To determine activation status of the IL-2R-associated (Jak/STAT) pathway in the HTLV-I infected cells, we examined tyrosine phosphorylation of Jak3, STAT3, and STAT5 in several HTLV-I(+) T-cell lines and in uncultured leukemic T cells isolated from patients with adult T-cell lymphoma/leukemia (ATLL). Constitutive basal phosphorylation of Jak3 and, usually, STAT3 and STAT5 was detected in all four IL-2-independent cell lines tested, but in none of the three IL-2-dependent cell lines. Similarly, there was no detectable basal phosphorylation of Jak3 and STAT5 in the leukemic cells from ATLL patients (0/8 and 0/3, respectively). However, stimulation with IL-2 resulted in Jak3 and STAT5 phosphorylation in both leukemic ATLL cells and IL-2-dependent lines. Furthermore, expression of SHP-1 phosphatase which is a negative regulator of cytokine receptor signaling, was lost in most IL-2 independent cell lines (3/4) but not in the leukemic ATLL cells (0/3). Finally, the HTLV-I(+) T-cell lines (313) but not the control, HTLV-I(-) T-cell lines were resistant to rapamycin and its novel analog RAD. We conclude that (1) HTLV-I infection per se does not result in a constitutive phosphorylation of the Jak3, STAT3, and STAT5 proteins; (2) malignant transformation in at least some cases of ATLL does not require the constitutive, but may require IL-2-induced, activation of the IL-2R Jak/STAT pathway; and (3) there are major differences in T-cell immortalization mechanism(s) which appear to involve SHP-1 and target molecules for rapamycin and RAD.

Design of conditionally active STATs: insights into STAT activation and gene regulatory function

The STAT (signal transducer and activator of transcription) signaling pathway is activated by a large number of cytokines and growth factors. We sought to design a conditionally active STAT that could not only provide insight into basic questions about STAT function but also serve as a powerful tool to determine the precise biological role of STATs. To this end, we have developed a conditionally active STAT by fusing STATs with the ligand-binding domain of the estrogen receptor (ER). We have demonstrated that the resulting STAT-ER chimeras are estrogen-inducible transcription factors that retain the functional and biochemical characteristics of the cognate wild-type STATs. In addition, these tools have allowed us to evaluate separately the contribution of tyrosine phosphorylation and dimerization to STAT function. We have for the first time provided experimental data supporting the model that the only apparent role of STAT tyrosine phosphorylation is to drive dimerization, as dimerization alone is sufficient to unmask a latent STAT nuclear localization sequence and induce nuclear translocation, sequence-specific DNA binding, and transcriptional activity.

Role of interleukin (IL)-2 receptor beta-chain subdomains and Shc in p38 mitogen-activated protein (MAP) kinase and p54 MAP kinase (stress-activated protein Kinase/c-Jun N-terminal kinase) activation. IL-2-driven proliferation is independent of p38 and p54 MAP kinase activation

We have shown recently that interleukin (IL)-2 activates the mitogen-activated protein (MAP) kinase family members p38 (HOG1/stress-activated protein kinase II) and p54 (c-Jun N-terminal kinase/stress-activated protein kinase I). Furthermore, the p38 MAP kinase inhibitor SB203580 inhibited IL-2-driven T cell proliferation, suggesting that p38 MAP kinase might be involved in mediating proliferative signals. In this study, using transfected BA/F3 cell lines, it is shown that both the acidic domain and the membrane-proximal serine-rich region of the IL-2Rbeta chain are required for p38 and p54 MAP kinase activation and that, as for p42/44 MAP kinase, this activation requires the Tyr338 residue of the acidic domain, the binding site for Shc. It is well established that the acidic domain of the IL-2Rbeta chain is dispensable for IL-2-driven proliferation, and thus our observations suggest that neither p38 nor p54 MAP kinase activation is required for IL-2-driven proliferation of BA/F3 cells. In addition, the tetravalent guanylhydrazone inhibitor of proinflammatory cytokine production, CNI-1493, can block the activation of p54 and p38 MAP kinases by IL-2 but has no effect on IL-2-driven proliferation of BA/F3 cells, activated primary T cells, or a cytotoxic T cell line. Furthermore, our observations provide evidence for the existence of an additional, unknown target of the p38 MAP kinase inhibitor SB203580, the activation of which is essential for mitogenic signaling by IL-2.

Distinctive roles of STAT5a and STAT5b in sexual dimorphism of hepatic P450 gene expression. Impact of STAT5a gene disruption

Stat5b gene disruption leads to an apparent growth hormone (GH) pulse insensitivity associated with loss of male-characteristic body growth rates and male-specific liver gene expression (Udy, G. B., Towers, R. P., Snell, R. G., Wilkins, R. J., Park, S. H., Ram, P. A., Waxman, D. J., and Davey, H. W. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 7239-7244). In the present study, disruption of the mouse Stat5a gene, whose coding sequence is approximately 90% identical to the Stat5b gene, resulted in no loss of expression in male mice of several sex-dependent, GH-regulated liver cytochrome P450 (CYP) enzymes. By contrast, the loss of STAT5b feminized the livers of males by decreasing expression of male-specific CYPs (CYP2D9 and testosterone 16alpha-hydroxylase) while increasing to female levels several female-predominant liver CYPs (CYP3A, CYP2B, and testosterone 6beta-hydroxylase). Since STAT5a is thus nonessential for these male GH responses, STAT5b homodimers, but not STAT5a-STAT5b heterodimers, probably mediate the sexually dimorphic effects of male GH pulses on liver CYP expression. In female mice, however, disruption of either Stat5a or Stat5b led to striking decreases in several liver CYP-catalyzed testosterone hydroxylase activities. Stat5a or Stat5b gene disruption also led to the loss of a female-specific, GH-regulated hepatic CYP2B enzyme. STAT5a, which is much less abundant in liver than STAT5b, and STAT5b are therefore both required for constitutive expression in female but not male mouse liver of certain GH-regulated CYP steroid hydroxylases, suggesting that STAT5 protein heterodimerization is an important determinant of the sex-dependent and gene-specific effects that GH has on the liver.

Interleukin-7: physiological roles and mechanisms of action

Interleukin-7 (IL-7), a product of stromal cells, provides critical signals to lymphoid cells at early stages in their development. Two types of cellular responses to IL-7 have been identified in lymphoid progenitors: (1) a trophic effect and (2) an effect supporting V(D)J recombination. The IL-7 receptor is comprised of two chains, IL-7R alpha and gamma(c). Following receptor crosslinking, rapid activation of several classes of kinases occurs, including members of the Janus and Src families and PI3-kinase. A number of transcription factors are subsequently activated including STATs, c-myc, NFAT and AP-1. However, it remains to be determined which, if any, previously identified pathway leads to the trophic or V(D)J endpoints. The trophic response to IL-7 involves protecting lymphoid progenitors from a death process that resembles apoptosis. This protection is partly mediated by IL-7 induction of Bcl-2, however other IL-7-induced events are probably also involved in the trophic response. The V(D)J response to IL-7 is partly mediated through increased production of Rag proteins (which cleave the target locus) and partly by increasing the accessibility of a target locus to cleavage through chromatin remodeling.

Role of Cytokine Signaling Molecules in Erythroid Differentiation of Mouse Fetal Liver Hematopoietic Cells: Functional Analysis of Signaling Molecules by Retrovirus-Mediated Expression

Erythropoietin (EPO) and its cell surface receptor (EPOR) play a central role in proliferation, differentiation, and survival of erythroid progenitors. Signals induced by EPO have been studied extensively by using erythroid as well as nonerythroid cell lines, and various controversial results have been reported as to the role of signaling molecules in erythroid differentiation. Here we describe a novel approach to analyze the EPO signaling by using primary mouse fetal liver hematopoietic cells to avoid possible artifacts due to established cell lines. Our strategy is based on high-titer retrovirus vectors with a bicistronic expression system consisting of an internal ribosome entry site (IRES) and green fluorescent protein (GFP). By placing the cDNA for a signaling molecule in front of IRES-GFP, virus-infected cells can be viably sorted by fluorescence-activated cell sorter, and the effect of expression of the signaling molecule can be assessed. By using this system, expression of cell-survival genes such as Bcl-2 and Bcl-XL was found to enhance erythroid colony formation from colony-forming unit–erythroid (CFU-E) in response to EPO. However, their expression was not sufficient for erythroid colony formation from CFU-E alone, indicating that EPO induces signals for erythroid differentiation. To examine the role of EPOR tyrosine residues in erythroid differentiation, we introduced a chimeric EGFR-EPOR receptor, which has the extracellular domain of the EGF receptor and the intracellular domain of the EPOR, as well as a mutant EGFR-EPOR in which all the cytoplasmic tyrosine residues are replaced with phenylalanine, and found that tyrosine residues of EPOR are essential for erythroid colony formation from CFU-E. We further analyzed the function of the downstream signaling molecules by expressing modified signaling molecules and found that both JAK2/STAT5 and Ras, two major signaling pathways activated by EPOR, are involved in full erythroid differentiation.

Transcriptional regulation of the cyclin D1 promoter by STAT5: its involvement in cytokine-dependent growth of hematopoietic cells

STAT5 is a member of a family of transcription factors that participate in the signal transduction pathways of many hormones and cytokines. Although STAT5 is suggested to play a crucial role in the biological effects of cytokines, its downstream target(s) associated with cell growth control is largely unknown. In a human interleukin-3 (IL-3)-dependent cell line F-36P-mpl, the induced expression of dominant-negative (dn)-STAT5 and of dn-ras led to inhibition of IL-3-dependent cell growth, accompanying the reduced expression of cyclin D1 mRNA. Also, both constitutively active forms of STAT5A (1*6-STAT5A) and ras (H-rasG12V) enabled F-36P-mpl cells to proliferate without added growth factors. In NIH 3T3 cells, 1*6-STAT5A and H-rasG12V individually and cooperatively transactivated the cyclin D1 promoter in luciferase assays. Both dn-STAT5 and dn-ras suppressed IL-3-induced cyclin D1 promoter activities in F-36P-mpl cells. Using a series of mutant cyclin D1 promoters, 1*6-STAT5A was found to transactivate the cyclin D1 promoter through the potential STAT-binding sequence at -481 bp. In electrophoretic mobility shift assays, STAT5 bound to the element in response to IL-3. Furthermore, the inhibitory effect of dn-STAT5 on IL-3-dependent growth was restored by expression of cyclin D1. Thus STAT5, in addition to ras signaling, appears to mediate transcriptional regulation of cyclin D1, thereby contributing to cytokine-dependent growth of hematopoietic cells.

Role of Cytokine Signaling Molecules in Erythroid Differentiation of Mouse Fetal Liver Hematopoietic Cells: Functional Analysis of Signaling Molecules by Retrovirus-Mediated Expression

Erythropoietin (EPO) and its cell surface receptor (EPOR) play a central role in proliferation, differentiation, and survival of erythroid progenitors. Signals induced by EPO have been studied extensively by using erythroid as well as nonerythroid cell lines, and various controversial results have been reported as to the role of signaling molecules in erythroid differentiation. Here we describe a novel approach to analyze the EPO signaling by using primary mouse fetal liver hematopoietic cells to avoid possible artifacts due to established cell lines. Our strategy is based on high-titer retrovirus vectors with a bicistronic expression system consisting of an internal ribosome entry site (IRES) and green fluorescent protein (GFP). By placing the cDNA for a signaling molecule in front of IRES-GFP, virus-infected cells can be viably sorted by fluorescence-activated cell sorter, and the effect of expression of the signaling molecule can be assessed. By using this system, expression of cell-survival genes such as Bcl-2 and Bcl-XL was found to enhance erythroid colony formation from colony-forming unit–erythroid (CFU-E) in response to EPO. However, their expression was not sufficient for erythroid colony formation from CFU-E alone, indicating that EPO induces signals for erythroid differentiation. To examine the role of EPOR tyrosine residues in erythroid differentiation, we introduced a chimeric EGFR-EPOR receptor, which has the extracellular domain of the EGF receptor and the intracellular domain of the EPOR, as well as a mutant EGFR-EPOR in which all the cytoplasmic tyrosine residues are replaced with phenylalanine, and found that tyrosine residues of EPOR are essential for erythroid colony formation from CFU-E. We further analyzed the function of the downstream signaling molecules by expressing modified signaling molecules and found that both JAK2/STAT5 and Ras, two major signaling pathways activated by EPOR, are involved in full erythroid differentiation.

Molecular regulation of cytokine gene expression during the immune response

Cytokine expression by immune system cells plays an important role in the regulation of the immune response. On first encounter with antigen, naive CD4+ T helper (Th) cells differentiate into cytokine-producing effector cells. Two types of effector cells characterized by their distinct expression of cytokine profiles have been described. Th1 cells produce IL-2 and IFN-gamma, whereas Th2 cells produce IL-4, IL-5, IL-6, IL-10, and IL-13. In many pathological situations, the balance between Th1 and Th2 immune responses determines the outcome of diverse immunologically mediated clinical syndromes including infectious, autoimmune, and allergic diseases. However, the molecular basis for the tissue-specific expression of Th1/Th2-like cytokines has remained elusive. In this review we evaluate the possible in vivo role of different transcription factors and transcriptional mechanisms in T cell differentiation and the immune response.

A functional DNA binding domain is required for growth hormone-induced nuclear accumulation of Stat5B

The mechanisms regulating the cellular distribution of STAT family transcription factors remain poorly understood. To identify regions of Stat5B required for ligand-induced nuclear accumulation, we constructed a cDNA encoding green fluorescent protein (GFP) fused to the N terminus of Stat5B and performed site-directed mutagenesis. When co-expressed with growth hormone (GH) receptor in COS-7 cells, GFP-Stat5B is tyrosyl-phosphorylated, forms dimers, and binds DNA in response to GH in a manner indistinguishable from untagged Stat5B. In multiple cell types, laser scanning confocal imaging of GFP-Stat5B co-expressed with GH receptor shows that GFP-Stat5B undergoes a rapid, dramatic accumulation in the nucleus upon GH stimulation. We introduced alanine substitutions in several regions of Stat5B and assayed for GH-dependent nuclear localization. Only the mutation that prevented binding to DNA (466VVVI469) abrogated GH-stimulated nuclear localization. This mutant fusion protein is tyrosyl-phosphorylated and dimerizes in response to GH. These results suggest that either high affinity binding to DNA contributes to nuclear accumulation of Stat5B or that this region is crucial for two functions, namely accumulation of Stat5B in the nucleus and DNA binding. Thus, we have identified a mutant Stat5 defective in nuclear localization despite its ability to be tyrosyl-phosphorylated and to dimerize.

Sustained receptor activation and hyperproliferation in response to granulocyte colony-stimulating factor (G-CSF) in mice with a severe congenital neutropenia/acute myeloid leukemia-derived mutation in the G-CSF receptor gene

In approximately 20% of cases of severe congenital neutropenia (SCN), mutations are found in the gene encoding the granulocyte colony-stimulating factor receptor (G-CSF-R). These mutations introduce premature stop codons, which result in truncation of 82-98 COOH-terminal amino acids of the receptor. SCN patients who develop secondary myelodysplastic syndrome and acute myeloid leukemia almost invariably acquired a GCSFR mutation, suggesting that this genetic alteration represents a key step in leukemogenesis. Here we show that an equivalent mutation targeted in mice (gcsfr-Delta715) results in the selective expansion of the G-CSF- responsive progenitor (G-CFC) compartment in the bone marrow. In addition, in vivo treatment of gcsfr-Delta715 mice with G-CSF results in increased production of neutrophils leading to a sustained neutrophilia. This hyperproliferative response to G-CSF is accompanied by prolonged activation of signal transducer and activator of transcription (STAT) complexes and extended cell surface expression of mutant receptors due to defective internalization. In view of the continuous G-CSF treatment of SCN patients, these data provide insight into why progenitor cells expressing truncated receptors clonally expand in vivo, and why these cells may be targets for additional genetic events leading to leukemia.

Stat5 is required for IL-2-induced cell cycle progression of peripheral T cells

Many cytokines activate two highly homologous Stat proteins, 5a and 5b. Mice deficient in both genes lack all growth hormone and prolactin functions but retain functions associated with cytokines such as erythropoietin. Here, we demonstrate that, while lymphoid development is normal, Stat5a/b mutant peripheral T cells are profoundly deficient in proliferation and fail to undergo cell cycle progression or to express genes controlling cell cycle progression. In addition, the mice lack NK cells, develop splenomegaly, and have T cells with an activated phenotype, phenotypes seen in IL-2 receptor beta chain-deficient mice. These phenotypes are not seen in mice lacking Stat5a or Stat5b alone. The results demonstrate that the Stat5 proteins, redundantly, are essential mediators of IL-2 signaling in T cells.

IL-4 Selectively Inhibits IL-2-Triggered Stat5 Activation, But Not Proliferation, in Human T Cells

IL-2 activates several distinct signaling pathways that are important for T cell activation, proliferation, and differentiation into both Th1 and Th2 phenotypes. IL-4, the major cytokine that promotes differentiation of Th2 cells, has been shown to block signaling of the Th1-promoting cytokine IL-12. As IL-2 synergizes with IL-12 in promoting Th1 differentiation, the effects of IL-4 on IL-2 signal transduction were investigated. IL-4 suppressed activation of DNA binding and tyrosine phosphorylation of the transcription factor Stat5 by IL-2, and suppressed the expression of the IL-2-inducible genes CD25, CIS, the PGE2 receptor, and cytokine responsive (CR) genes CR1 and CR8. Activation of Stat5 by cytokines that share a common γ receptor subunit, IL-2, IL-7, and IL-15, was suppressed by preculture in IL-4. Activation of the Jak1 and Jak3 kinases that are proximal to Stat5 in the IL-2-Jak-STAT signaling pathway was suppressed, and this correlated with inhibition of IL-2Rβ subunit expression. In contrast to suppression of Stat5, proliferative responses to IL-2 were augmented in IL-4-cultured cells, and activation of proliferative pathways leading to activation of mitogen activated protein kinases, induction of expression of Myc, Fos, Pim-1, and cyclin D3, and decreased levels of the cyclin-dependent kinase inhibitor p27 were intact. These results identify molecular mechanisms underlying interactions between IL-4 and IL-2 in T cells and demonstrate that one mechanism of regulation of IL-2 activity is selective and differential modulation of signaling pathways.

Defective Internalization and Sustained Activation of Truncated Granulocyte Colony-Stimulating Factor Receptor Found in Severe Congenital Neutropenia/Acute Myeloid Leukemia

Acquired mutations truncating the C-terminal domain of the granulocyte colony-stimulating factor receptor (G-CSF-R) are found in about 20% of severe congenital neutropenia (SCN) patients, with this cohort of patients predisposed to acute myeloid leukemia (AML). In myeloid cells, such mutations act in a dominant-negative manner leading to hyperproliferation and lack of differentiation in response to G-CSF. However, why these truncated receptors are dominant in function over wild-type receptors has remained unclear. We report that ligand-induced internalization of truncated G-CSF-R is severely impaired compared with the wild-type receptor, which results in sustained activation of STAT proteins. Strikingly, in cells coexpressing both truncated and wild-type forms, the truncated receptors acted dominantly with regard to both internalization and sustained activation. Site-directed mutagenesis of the C-terminus showed that receptor tyrosines in this region were dispensable for internalization, whereas a di-leucine–containing motif in Box B3 played some role. However, loss of the di-leucine motif was not the critical determinant of the sustained activation status of truncated receptors. These data suggest that defective internalization, leading to extended receptor activation, is a major cause of the dominant hyperproliferative effect of truncated G-CSF receptors, which is only partially due to the loss of a di-leucine motif present in the Box B3 region of the full-length receptor.

Defective Internalization and Sustained Activation of Truncated Granulocyte Colony-Stimulating Factor Receptor Found in Severe Congenital Neutropenia/Acute Myeloid Leukemia

Acquired mutations truncating the C-terminal domain of the granulocyte colony-stimulating factor receptor (G-CSF-R) are found in about 20% of severe congenital neutropenia (SCN) patients, with this cohort of patients predisposed to acute myeloid leukemia (AML). In myeloid cells, such mutations act in a dominant-negative manner leading to hyperproliferation and lack of differentiation in response to G-CSF. However, why these truncated receptors are dominant in function over wild-type receptors has remained unclear. We report that ligand-induced internalization of truncated G-CSF-R is severely impaired compared with the wild-type receptor, which results in sustained activation of STAT proteins. Strikingly, in cells coexpressing both truncated and wild-type forms, the truncated receptors acted dominantly with regard to both internalization and sustained activation. Site-directed mutagenesis of the C-terminus showed that receptor tyrosines in this region were dispensable for internalization, whereas a di-leucine–containing motif in Box B3 played some role. However, loss of the di-leucine motif was not the critical determinant of the sustained activation status of truncated receptors. These data suggest that defective internalization, leading to extended receptor activation, is a major cause of the dominant hyperproliferative effect of truncated G-CSF receptors, which is only partially due to the loss of a di-leucine motif present in the Box B3 region of the full-length receptor.

Thymic Stromal Lymphopoietin: A Cytokine That Promotes the Development of IgM+ B Cells In Vitro and Signals Via a Novel Mechanism

A novel cytokine from a thymic stromal cell line (thymic stromal lymphopoietin (TSLP)) promotes the development of B220+/IgM+ immature B cells when added to fetal liver cultures, long term bone marrow cultures, or bone marrow cells plated in semisolid medium. Because the activities of TSLP overlap with those of IL-7 in some in vitro assays, we compared the signaling mechanisms employed by TSLP and IL-7. Proliferation of a factor-dependent pre-B cell line (NAG8/7) in response to either TSLP or IL-7 was inhibited by anti-IL-7Rα mAbs, suggesting that the functional TSLP receptor complex uses IL-7Rα. In contrast, three different Abs to the common cytokine receptor γ-chain had no effect on the response of these cells to TSLP, indicating that the functional TSLP receptor complex does not use the common cytokine receptor γ-chain. Both cytokines induced activation of Stat5, but only IL-7 induced activation of the Janus family kinases Jak1 and Jak3. In fact, TSLP failed to activate any of the four known Janus family kinases, suggesting that Stat5 phosphorylation is mediated by a novel mechanism. Taken together, these data support the idea that TSLP can make unique contributions to B lymphopoiesis and indicate that it does so by mechanisms distinct from IL-7.

Functional association of Nmi with Stat5 and Stat1 in IL-2- and IFNgamma-mediated signaling

Using the coiled-coil region of Stat5b as the bait in a yeast two-hybrid screen, we identified the association of Nmi, a protein of unknown function previously reported as an N-Myc interactor. We further show that Nmi interacts with all STATs except Stat2. We evaluated two cytokine systems, IL-2 and IFNgamma, and demonstrate that Nmi augments STAT-mediated transcription in response to these cytokines. Interestingly, Nmi lacks an intrinsic transcriptional activation domain; instead, Nmi enhances the association of CBP/p300 coactivator proteins with Stat1 and Stat5, and together with CBP/p300 can augment IL-2- and IFNgamma-dependent transcription. Therefore, our data not only reveal that Nmi can potentiate STAT-dependent transcription, but also suggest that it can augment coactivator protein recruitment to at least some members of a group of sequence-specific transcription factors.

STAT5 interaction with the T cell receptor complex and stimulation of T cell proliferation

The role of STAT (signal transducer and activator of transcription) proteins in T cell receptor (TCR) signaling was analyzed. STAT5 became immediately and transiently phosphorylated on tyrosine 694 in response to TCR stimulation. Expression of the protein tyrosine kinase Lck, a key signaling protein in the TCR complex, activated DNA binding of transfected STAT5A and STAT5B to specific STAT inducible elements. The role of Lck in STAT5 activation was confirmed in a Lck-deficient T cell line in which the activation of STAT5 by TCR stimulation was abolished. Expression of Lck induced specific interaction of STAT5 with the subunits of the TCR, indicating that STAT5 may be directly involved in TCR signaling. Stimulation of T cell clones and primary T cell lines also induced the association of STAT5 with the TCR complex. Inhibition of STAT5 function by expression of a dominant negative mutant STAT5 reduced antigen-stimulated proliferation of T cells. Thus, TCR stimulation appears to directly activate STAT5, which may participate in the regulation of gene transcription and T cell proliferation during immunological responses.

Virus-specific immunity after gene therapy in a murine model of severe combined immunodeficiency

Human severe combined immunodeficiency (SCID) can be caused by defects in Janus kinase 3 (JAK3)-dependent cytokine signaling pathways. As a result, patients are at high risk of life-threatening infection. A JAK3 -/- SCID mouse model for the human disease has been used to test whether transplant with retrovirally transduced bone marrow (BM) cells (JAK3 BMT) could restore immunity to an influenza A virus. The immune responses also were compared directly with those for mice transplanted with wild-type BM (+/+ BMT). After infection, approximately 90% of the JAK3 BMT or +/+ BMT mice survived, whereas all of the JAK3 -/- mice died within 29 days. Normal levels of influenza-specific IgG were present in plasma from JAK3 BMT mice at 14 days after respiratory challenge, indicating restoration of B cell function. Influenza-specific CD4(+) and CD8(+) T cells were detected in the spleen and lymph nodes, and virus-specific CD8(+) effectors localized to the lungs of the JAK3 BMT mice. The kinetics of the specific host response correlated with complete clearance of the virus within 2 weeks of the initial exposure. By contrast, the JAK3 -/- mice did not show any evidence of viral immunity and were unable to control this viral pneumonia. Retroviral-mediated JAK3 gene transfer thus restores diverse aspects of cellular and humoral immunity and has obvious potential for human autologous BMT.

Specificity of signaling by hematopoietic cytokine receptors: instructive versus permissive effects

The helical cytokines constitute a family of proteins with a common three-dimensional structure. They exert a wide variety of biological effects with a preference for the hematopoietic system. The effects of helical cytokines are mediated by cell surface receptors, which belong to the cytokine receptor superfamily and signal by activating cytoplasmic tyrosine kinases of the Janus kinase (Jak) family and other downstream signaling pathways. The relevance of each of these pathways for eliciting a specific cellular response remains to be determined. This review will focus on cytokine receptors which play a role in the regulation of hematopoiesis and summarize data the address the question how specificity of signaling is achieved.

Tyrosine-Dependent and -Independent Mechanisms of STAT3 Activation by the Human Granulocyte Colony-Stimulating Factor (G-CSF) Receptor Are Differentially Utilized Depending on G-CSF Concentration

The granulocyte colony-stimulating factor receptor (G-CSF-R) activates multiple STAT proteins. Although the membrane-proximal cytoplasmic region of the G-CSF-R is necessary and sufficient for activation of STAT1 and STAT5, activation of STAT3 requires the membrane distal region that contains four tyrosines. Although one of these (Y704) has previously been shown to be involved in STAT3 activation from a truncated G-CSF-R derived from a patient with severe chronic neutropenia (SCN), this tyrosine is not required for STAT3 activation by the full-length G-CSF-R. To investigate possible alternative mechanisms of STAT3 activation, we generated a series of Ba/F3 cell transfectants expressing the wild-type G-CSF-R or mutant receptors that either completely lack tyrosines or retain just one of the four cytoplasmic tyrosines of the G-CSF-R. We show that, at saturating G-CSF concentrations, STAT3 activation from the full-length G-CSF-R is efficiently mediated by the C-terminal domain in a manner independent of receptor tyrosines. In contrast, at low G-CSF concentrations, Y704 and Y744 of the G-CSF-R play a major role in STAT3 activation. Both tyrosine-dependent and -independent mechanisms of STAT3 activation are sensitive to the Jak2 inhibitor AG-490, follow similar kinetics, and lead to transactivation of a STAT3 reporter construct, indicating functional equivalence. STAT3 activation is also impaired, particularly at nonsaturating G-CSF concentrations, in bone marrow cells from mice expressing a truncated G-CSF-R (gcsfr-▵715). These findings suggest that G-CSF–induced STAT3 activation during basal granulopoiesis (low G-CSF) and “emergency” granulopoiesis (high G-CSF) are differentially controlled. In addition, the data establish the importance of the G-CSF-R C-terminus in STAT3 activation in primary cells, which has implications for understanding why truncated G-CSF-R derived from SCN patients are defective in maturation signaling.

Tyrosine-Dependent and -Independent Mechanisms of STAT3 Activation by the Human Granulocyte Colony-Stimulating Factor (G-CSF) Receptor Are Differentially Utilized Depending on G-CSF Concentration

The granulocyte colony-stimulating factor receptor (G-CSF-R) activates multiple STAT proteins. Although the membrane-proximal cytoplasmic region of the G-CSF-R is necessary and sufficient for activation of STAT1 and STAT5, activation of STAT3 requires the membrane distal region that contains four tyrosines. Although one of these (Y704) has previously been shown to be involved in STAT3 activation from a truncated G-CSF-R derived from a patient with severe chronic neutropenia (SCN), this tyrosine is not required for STAT3 activation by the full-length G-CSF-R. To investigate possible alternative mechanisms of STAT3 activation, we generated a series of Ba/F3 cell transfectants expressing the wild-type G-CSF-R or mutant receptors that either completely lack tyrosines or retain just one of the four cytoplasmic tyrosines of the G-CSF-R. We show that, at saturating G-CSF concentrations, STAT3 activation from the full-length G-CSF-R is efficiently mediated by the C-terminal domain in a manner independent of receptor tyrosines. In contrast, at low G-CSF concentrations, Y704 and Y744 of the G-CSF-R play a major role in STAT3 activation. Both tyrosine-dependent and -independent mechanisms of STAT3 activation are sensitive to the Jak2 inhibitor AG-490, follow similar kinetics, and lead to transactivation of a STAT3 reporter construct, indicating functional equivalence. STAT3 activation is also impaired, particularly at nonsaturating G-CSF concentrations, in bone marrow cells from mice expressing a truncated G-CSF-R (gcsfr-▵715). These findings suggest that G-CSF–induced STAT3 activation during basal granulopoiesis (low G-CSF) and “emergency” granulopoiesis (high G-CSF) are differentially controlled. In addition, the data establish the importance of the G-CSF-R C-terminus in STAT3 activation in primary cells, which has implications for understanding why truncated G-CSF-R derived from SCN patients are defective in maturation signaling.

Prolactin and mammary gland development

Prolactin (PRL) regulates the development of the mammary gland at three stages in the reproductive life history of females. The first stage is mammary gland organogenesis, during which PRL contributes to the maturation of the mammary glands from a primary ductal system, which grows from terminal end buds, to the fully mature nonpregnant gland. The mature mammary gland is characterized by an absence of terminal end buds, and the development of a highly branched architecture, which is decorated by lobular buds. During pregnancy PRL, placental lactogens, and progesterone stimulate the expansion and physiological differentiation of the lobuloalveolar system from the lobular buds. After delivery PRL, in the context of falling progesterone, stimulates the final induction of milk protein gene expression and lactation. PRL acts directly on the mammary epithelium, and indirectly by stimulating luteal progesterone secretion in rodents. Disruption of the genes for PRL and the PRL receptor, as well as those for transcription factors important in mammary gland regulation (Stat proteins), have provided a new set of animal models with which to study normal mammary gland development and the relationships of PRL to breast carcinogenesis. Two major deficiencies in our current knowledge of PRL actions are our understanding of the role of epithelial-stromal interactions in PRL-induced mammary morphogenesis, and the identity of developmentally important genes that are regulated by PRL during normal mammary gland organogenesis.

Interleukin 7 receptor control of T cell receptor gamma gene rearrangement: role of receptor-associated chains and locus accessibility

VDJ recombination of T cell receptor and immunoglobulin loci occurs in immature lymphoid cells. Although the molecular mechanisms of DNA cleavage and ligation have become more clear, it is not understood what controls which target loci undergo rearrangement. In interleukin 7 receptor (IL-7R)alpha-/- murine thymocytes, it has been shown that rearrangement of the T cell receptor (TCR)-gamma locus is virtually abrogated, whereas other rearranging loci are less severely affected. By examining different strains of mice with targeted mutations, we now observe that the signaling pathway leading from IL-7Ralpha to rearrangement of the TCR-gamma locus requires the gammac receptor chain and the gammac-associated Janus kinase Jak3. Production of sterile transcripts from the TCR-gamma locus, a process that generally precedes rearrangement of a locus, was greatly repressed in IL-7Ralpha-/- thymocytes. The repressed transcription was not due to a lack in transcription factors since the three transcription factors known to regulate this locus were readily detected in IL-7Ralpha-/- thymocytes. Instead, the TCR-gamma locus was shown to be methylated in IL-7Ralpha-/- thymocytes. Treatment of IL-7Ralpha-/- precursor T cells with the specific histone deacetylase inhibitor trichostatin A released the block of TCR-gamma gene rearrangement. This data supports the model that IL-7R promotes TCR-gamma gene rearrangement by regulating accessibility of the locus via demethylation and histone acetylation of the locus.