Nuclear interactor of ARF and Mdm2 regulates multiple pathways to activate p53

The p53 tumor suppressor is controlled by an interactive network of factors that stimulate or inhibit its transcriptional activity. Within that network, Mdm2 functions as the major antagonist of p53 by promoting its ubiquitylation and degradation. Conversely, Tip60 activates p53 through direct association on target promoters as well as acetylation of p53 at lysine 120 (K120). This study examines the functional relationship between Mdm2 and Tip60 with a novel p53 regulator, NIAM (nuclear interactor of ARF and Mdm2). Previous work showed NIAM can suppress proliferation and activate p53 independently of ARF, indicating that other factors mediate those activities. Here, we demonstrate that NIAM is a chromatin-associated protein that binds Tip60. NIAM can promote p53 K120 acetylation, although that modification is not required for NIAM to inhibit proliferation or induce p53 transactivation of the p21 promoter. Notably, Tip60 silencing showed it contributes to but is not sufficient for NIAM-mediated p53 activation, suggesting other mechanisms are involved. Indeed, growth-inhibitory forms of NIAM also bind to Mdm2, and increased NIAM expression levels disrupt p53-Mdm2 association, inhibit p53 polyubiquitylation, and prevent Mdm2-mediated inhibition of p53 transcriptional activity. Importantly, loss of NIAM significantly impairs p53 activation. Together, these results show that NIAM activates p53 through multiple mechanisms involving Tip60 association and Mdm2 inhibition. Thus, NIAM regulates 2 critical pathways that control p53 function and are altered in human cancers, implying an important role for NIAM in tumorigenesis.

FOXO transcription factors enforce cell cycle checkpoints and promote survival of hematopoietic cells after DNA damage

The PI3K/AKT signaling pathway contributes to cell cycle progression of cytokine-dependent hematopoietic cells under normal conditions, and it is absolutely required to override DNA damage-induced cell cycle arrest checkpoints in these cells. Phosphatidylinositol-3-kinase (PI3K)/AKT activity also correlates with Cdk2 activity in hematopoietic cells, suggesting that Cdk2 activation may be a relevant end point for this signaling pathway. However, mediators downstream of AKT in this pathway have not been defined. The forkhead transcription factor O (FOXO) family are negatively regulated by AKT-dependent phosphorylation and are known regulators of genes affecting cell cycle progression. We show that enhanced FOXO activity replicates the effect of PI3K inhibitors in enforcing G(1) and G(2) phase arrest after DNA damage. Conversely, knockdown of endogenous FOXO proteins increased Cdk2 activity and overrode DNA damage checkpoints in cells lacking PI3K activity. Moreover, loss of FOXO activity caused an increase in sensitivity to cisplatin-induced cell death, which was associated with failure to arrest cell cycle progression in the face of DNA damage caused by this chemotherapeutic agent. These cell cycle arrests were dependent on p27 expression when mediated by FOXO3a alone, but also involve p27-independent mechanisms when promoted by endogenous FOXO proteins. Together, these observations show that FOXO proteins enforce DNA damage-induced cell cycle arrest in hematopoietic cells. Inhibition of FOXO activity by cytokine-induced PI3K/AKT signaling is sufficient to override these DNA damage-induced cell cycle checkpoints, but may negatively impact hematopoietic cell viability.

Phosphoinositide 3-kinase signaling overrides a G2 phase arrest checkpoint and promotes aberrant cell cycling and death of hematopoietic cells after DNA damage

DNA damage activates arrest checkpoints to halt cell cycle progression in G(1) and G(2) phases. These checkpoints can be overridden in hematopoietic cells by cytokines, such as erythropoietin, through the activation of a phosphoinositide 3-kinase (PI3K) signaling pathway. Here, we show that PI3K activity specifically overrides delayed mechanisms effecting permanent G(1) and G(2) phase arrests, but does not affect transient checkpoints arresting cells up to 10 hours after gamma-irradiation. Assessing the status of cell cycle regulators in hematopoietic cells arrested after gamma-irradiation, we show that Cdk2 activity is completely inhibited in both G(1) and G(2) arrested cells. Despite the absence of Cdk2 activity, cells arrested in G(2) phase did retain detectable levels of Cdk1 activity in the absence of PI3K signaling. However, reactivation of PI3K promoted robust increases in both Cdk1 and Cdk2 activity in G(2)-arrested cells. Reactivation of Cdks was accompanied by a resumption of cell cycling, but with strikingly different effectiveness in G(1) and G(2) phase arrested cells. Specifically, G(1)-arrested cells resumed normal cell cycle progression with little loss in viability when PI3K was activated after gamma-irradiation. Conversely, PI3K activation in G(2)-arrested cells promoted endoreduplication and death of the entire population. These observations show that cytokine-induced PI3K signaling pathways promote Cdk activation and override permanent cell cycle arrest checkpoints in hematopoietic cells. While this activity can rescue irradiated cells from permanent G(1) phase arrest, it results in aberrant cell cycling and death when activated in hematopoietic cells arrested at the G(2) phase DNA damage checkpoint.

Prolactin and heregulin override DNA damage-induced growth arrest and promote phosphatidylinositol-3 kinase-dependent proliferation in breast cancer cells

Heregulin (HRG), a ligand of ErbB receptor tyrosine kinases, is a potent mitogenic factor for breast cancer cells. Prolactin (PRL) has also been reported to regulate proliferation in breast cancer cells through its receptor, a member of the type I cytokine receptor family. Cytokine receptors are potent mitogens in hematopoietic cells, where they also override DNA damage-induced growth arrest checkpoints through activation of a phosphatidylinositol-3 kinase (PI3K) signaling pathway. In this study, we assessed the effect of gamma-irradiation on the mitogenic activity of HRG and PRL in breast cancer cells. HRG and PRL enhanced the proliferation of non-irradiated breast cancer cell lines in association with their ability to activate PI3K signaling pathways. Both growth factors also overrode irradiation-induced growth arrest in T47D cells, which resulted in decreased viability after irradiation. An inhibitor of PI3K, LY294002, abrogated growth factor-induced proliferation and the activity of cell cycle-dependent kinases in non-irradiated and irradiated cells. Thus, growth factors acting through distinct receptor families share a similar PI3K-dependent ability to promote proliferation and override DNA damage-induced growth arrest in breast cancer cells. These observations also suggest that selective activation of PI3K-dependent signaling can enhance radiosensitivity in breast cancer cells.

Nucleophosmin (B23) targets ARF to nucleoli and inhibits its function

The ARF tumor suppressor is a nucleolar protein that activates p53-dependent checkpoints by binding Mdm2, a p53 antagonist. Despite persuasive evidence that ARF can bind and inactivate Mdm2 in the nucleoplasm, the prevailing view is that ARF exerts its growth-inhibitory activities from within the nucleolus. We suggest ARF primarily functions outside the nucleolus and provide evidence that it is sequestered and held inactive in that compartment by a nucleolar phosphoprotein, nucleophosmin (NPM). Most cellular ARF is bound to NPM regardless of whether cells are proliferating or growth arrested, indicating that ARF-NPM association does not correlate with growth suppression. Notably, ARF binds NPM through the same domains that mediate nucleolar localization and Mdm2 binding, suggesting that NPM could control ARF localization and compete with Mdm2 for ARF association. Indeed, NPM knockdown markedly enhanced ARF-Mdm2 association and diminished ARF nucleolar localization. Those events correlated with greater ARF-mediated growth suppression and p53 activation. Conversely, NPM overexpression antagonized ARF function while increasing its nucleolar localization. These data suggest that NPM inhibits ARF's p53-dependent activity by targeting it to nucleoli and impairing ARF-Mdm2 association.

Cytokine-induced phosphoinositide 3-kinase activity promotes Cdk2 activation in factor-dependent hematopoietic cells

Cytokine growth factors regulate the proliferation of hematopoietic cells through activation of several distinct signaling pathways. We have assessed the contribution of phosphoinositide 3-kinase (PI3K) pathways to erythropoietin (Epo) and interleukin (IL)-3-induced proliferation of factor-dependent hematopoietic cells. Lack of cytokine-induced PI3K activation caused by receptor mutation or treatment with a specific inhibitor (LY294002) did not prevent proliferation but resulted in an increase in the G1 phase content and doubling time of cell cultures. The reduced proliferation of cells lacking cytokine-induced PI3K activity could be partially restored by overexpressing constitutively active Akt. Inhibition of PI3K activity decreased the proportion of cytokine-treated cells entering S phase and was associated with a significant reduction in cytokine-induced phosphorylation and activation of Cdk2. By contrast, Cdk4 activity and p27(Kip1) expression were not significantly altered by inhibition of PI3K. Together, these observations identify a mechanism through which cytokine-activated PI3K contributes to G1 to S phase progression in factor-dependent hematopoietic cells by enhancing the phosphorylation and activation of Cdk2.

Erythropoietin receptors associate with a ubiquitin ligase, p33RUL, and require its activity for erythropoietin-induced proliferation

The proliferation and survival of hematopoietic cells is strictly regulated by cytokine growth factors that act through receptors of the Type I cytokine receptor family, including erythropoietin (Epo) and its receptor, EpoR. Mitogenic signaling by these receptors depends on activation of Jak tyrosine kinases. However, other required components of this pathway have not been fully identified. In a screen for proteins that interact with EpoR and Jak2, we identified a novel member of the U-box family of ubiquitin ligases. This receptor-associated ubiquitin ligase, RUL, co-precipitated with EpoR from mammalian cells and mediated ubiquitination of EpoR. Also, endogenously expressed RUL was rapidly and transiently phosphorylated on serine after cytokine treatment of factor-dependent hematopoietic cells. Expression of ubiquitin ligase-deficient mutants of RUL inhibited Epo-induced expression of c-myc and bcl-2, two immediate-early genes normally associated with Epo-induced cell growth. Consistent with that finding, expression of mutant RUL also inhibited Epo-dependent proliferation and survival of factor-dependent cells. Together, these observations suggest that RUL is a required component of mitogenic signaling by EpoR. We also show that RUL is phosphorylated in response to growth factors that act through non-cytokine receptors, suggesting that RUL may function as a common regulator of mitogenesis.

Cytokine activation of phosphoinositide 3-kinase sensitizes hematopoietic cells to cisplatin-induced death

Cytokine growth factors regulate the normal proliferation of hematopoietic cells but can also override irradiation-induced growth arrest checkpoints through activation of a phosphoinositide 3-kinase (PI3K) signaling pathway. In the present study, we assessed the effect that erythropoietin and interleukin-3 have on cisplatin-treated hematopoietic cells. When cultured in the presence of cytokine, cisplatin-treated 32D cells transiently accumulated in a G(2)-M phase arrest and ultimately died by a nonapoptotic mechanism. By comparison, reduction of cytokine-induced PI3K activity, either through cytokine receptor mutation or direct inhibition with LY294002, caused cisplatin-treated cells to enter a biphasic G(1) and G(2)-M arrest. The arrest of these cells coincided with an absence of cyclin-dependent kinase (Cdk)1 and Cdk2 activity and significantly reduced cell death during cisplatin treatment. Indeed, LY294002 treatment during cisplatin exposure allowed the recovery of a viable, proliferating cell population after removal of cisplatin. In contrast, Cdks remained active in the G(2)-M-arrested population of cisplatin-treated cells with continuous cytokine activation of PI3K, and even transient exposure to cisplatin resulted in death of the entire population. These data suggest that cytokine activation of PI3K signaling pathways overrides cisplatin-induced growth arrest checkpoints, thereby sensitizing hematopoietic cells to DNA damage-induced death.

Dna damage-induced G(1) arrest in hematopoietic cells is overridden following phosphatidylinositol 3-kinase-dependent activation of cyclin-dependent kinase 2

Exposure of hematopoietic cells to DNA-damaging agents induces p53-independent cell cycle arrest at a G(1) checkpoint. Previously, we have shown that this growth arrest can be overridden by cytokine growth factors, such as erythropoietin or interleukin-3, through activation of a phosphatidylinositol 3-kinase (PI 3-kinase)/Akt-dependent signaling pathway. Here, we show that gamma-irradiated murine myeloid 32D cells arrest in G(1) with active cyclin D-cyclin-dependent kinase 4 (Cdk4) but with inactive cyclin E-Cdk2 kinases. The arrest was associated with elevated levels of the Cdk inhibitors p21(Cip1) and p27(Kip1), yet neither was associated with Cdk2. Instead, irradiation-induced inhibition of cyclin E-Cdk2 correlated with absence of the activating threonine-160 phosphorylation on Cdk2. Cytokine treatment of irradiated cells induced Cdk2 phosphorylation and activation, and cells entered into S phase despite sustained high-level expression of p21 and p27. Notably, the PI 3-kinase inhibitor, LY294002, completely blocked cytokine-induced Cdk2 activation and cell growth in irradiated 32D cells but not in nonirradiated cells. Together, these findings demonstrate a novel mechanism underlying the DNA damage-induced G(1) arrest of hematopoietic cells, that is, inhibition of Cdk2 phosphorylation and activation. These observations link PI 3-kinase signaling pathways with the regulation of Cdk2 activity.

DNA damage-induced cell-cycle arrest of hematopoietic cells is overridden by activation of the PI-3 kinase/Akt signaling pathway

Exposure of hematopoietic cells to DNA-damaging agents induces cell-cycle arrest at G1 and G2/M checkpoints. Previously, it was shown that DNA damage-induced growth arrest of hematopoietic cells can be overridden by treatment with cytokine growth factors, such as erythropoietin (EPO) or interleukin-3 (IL-3). Here, the cytokine-activated signaling pathways required to override G1 and G2/M checkpoints induced by gamma-irradiation (gamma-IR) are characterized. Using factor-dependent myeloid cells stably expressing EPO receptor (EPO-R) mutants, it is shown that removal of a minimal domain required for PI-3K signaling abrogated the ability of EPO to override gamma-IR-induced cell-cycle arrest. Similarly, the ability of cytokines to override gamma-IR-induced arrest was abolished by an inhibitor of PI-3K (LY294002) or by overexpression of dominant-negative Akt. Moreover, the ability of EPO to override these checkpoints in cells expressing defective EPO-R mutants could be restored by overexpression of a constitutively active Akt. Thus, activation of a PI-3K/Akt signaling pathway is required for cytokine-dependent suppression of DNA-damage induced checkpoints. Together, these findings suggest a novel role for PI-3K/Akt pathways in the modulation of growth arrest responses to DNA damage in hematopoietic cells. (Blood. 2001;98:834-841)

Cytokine rescue of p53-dependent apoptosis and cell cycle arrest is mediated by distinct Jak kinase signaling pathways

Exposure of hematopoietic progenitors to gamma-irradiation (IR) induces p53-dependent apoptosis and a p53-independent G2/M cell cycle arrest. These responses to DNA-damage can be inhibited by treatment with cytokine growth factors. Here we report that gamma-IR-induced apoptosis and cell cycle arrest are suppressed by specific cytokines (e.g., erythropoietin and interleukin-3) and that activation of the Jak kinase is necessary and sufficient for these effects. Using myleoid cells expressing a series of erythropoietin receptor (EpoR) mutants, we have demonstrated that Jak kinase-dependent signals initiated from the membrane proximal domain of EpoR were sufficient to prevent IR-induced apoptotic cell death, but failed to prevent cell cycle arrest. Cell survival by Epo did not require activation of other known signaling pathways including PI-3 kinase, PLC-gamma, Ras or Stats. Signaling targets of Jak kinase pathways included members of the Bcl-2 family of anti-apoptotic proteins, and enforced expression of Bcl-2 or Bcl-xL was as effective as cytokine treatment in blocking IR-induced apoptosis but did not prevent growth arrest. A distinct signal derived from a membrane distal domain of EpoR is required to overcome growth arrest associated with DNA damage. These findings functionally link the Jak signaling pathway to suppression of p53-mediated cell death by cytokines and demonstrate that the apoptotic and growth arrest responses to DNA damage in hematopoietic cells are modulated by distinct, cytokine specific signal transduction pathways.

Jaks and Stats in cytokine signaling

Hematopoiesis is regulated through the binding of cytokines to receptors of the cytokine receptor superfamily. Although lacking catalytic domains, members of the cytokine receptor superfamily mediate ligand-dependent activation of protein tyrosine phosphorylation through their association and activation of members of the Janus kinase (Jak) family of protein tyrosine kinases. The activated Jaks phosphorylate the receptors which creates docking sites for SH2-containing signaling proteins which are tyrosine phosphorylated following their association with the complex. Among the substrates of tyrosine phosphorylation are members of the signal transducers and activators of the transcription family of proteins (Stats). Various cytokines induce the tyrosine phosphorylation and activation of one or more of the seven family members. The pattern of Stat activation provides a level of cytokine individuality that is not observed in the activation of other signaling pathways. The role of various Stats in the biological responses to cytokines has been assessed through the analysis of receptor mutations which disrupt Stat activation and more recently by disruption of the genes in mice. Our results have demonstrated that the activation of Stat5a and Stat5b by erythropoietin is critical for the activation of a number of immediate early genes but is not required for a mitogenic response. Mice in which the genes for Stat4 and Stat6 are disrupted are viable but lack functions that are mediated by interleukin 12 (IL-12) or IL-4, respectively, suggesting that these Stats perform very specific functions in immune responses.

Lack of IL-4-induced Th2 response and IgE class switching in mice with disrupted Stat6 gene

Signal transducers and activators of transcription (Stats) are activated by tyrosine phosphorylation in response to cytokines, and are thought to mediate many of their functional responses. Stat6 is activated in response to interleukin (IL)-4 and may contribute to various functions including mitogenesis, T-helper cell differentiation and immunoglobulin isotype switching. To evaluate the role of Stat6, we generated Stat6-null mice (Stat6 -/-) by gene disruption in embryonic stem cells. The mice were viable, indicating the lack of a non-redundant function in normal development. Although naive lymphoid cell development was normal, Stat6 -/- mice were deficient in IL-4-mediated functions including Th2 helper T-cell differentiation, expression of cell surface markers, and immunoglobulin class switching to IgE. In contrast, IL-4-mediated proliferation was only partly affected.

Erythropoietin induces activation of Stat5 through association with specific tyrosines on the receptor that are not required for a mitogenic response

The cytoplasmic domain of the erythropoietin receptor (EpoR) contains a membrane-distal region that is dispensable for mitogenesis but is required for the recruitment and tyrosine phosphorylation of a variety of signaling proteins. The membrane-proximal region of 96 amino acids is necessary and sufficient for mitogenesis as well as Jak2 activation, induction of c-fos, c-myc, cis, the T-cell receptor gamma locus (TCR-gamma), and c-pim-1. The studies presented here demonstrate that this region is also necessary and sufficient for the activation of Stat5A and Stat5B. The membrane-proximal domain contains a single tyrosine, Y-343, which when mutated eliminates the ability of the receptor to couple Epo binding to the activation of Stat5. Furthermore, peptide competitions demonstrate that this site, when phosphorylated, can disrupt Stat5 DNA binding activity, consistent with a role of Y-343 as a site of recruitment to the receptor. Cells expressing the truncated, Y343F mutant (a mutant with a Y-to-F alteration at position 343) proliferate in response to Epo in a manner comparable to that of the controls. However, in these cells, Epo stimulation does not induce the appearance of transcripts for cis, TCR-gamma, or c-fos, suggesting a role for Stat5 in their regulation.

Interleukin-9 induces tyrosine phosphorylation of insulin receptor substrate-1 via JAK tyrosine kinases

Interleukin (IL)-9 stimulates the proliferation of a variety of hematopoietic lineages through its interaction with a receptor of the cytokine receptor superfamily. In the studies presented here, we have begun to characterize the downstream signaling pathways activated by IL-9. In addition to the activation of JAK1 and JAK3 tyrosine kinases, IL-9, unlike most hematopoietic cytokines but similar to IL-4, induces the tyrosine phosphorylation of a 170-kDa protein that is related to the insulin receptor substrate-1 (IRS-1). We further demonstrate for the first time that IRS-1 is not only associated with JAK1 but also tyrosine phosphorylated and functionally involved in IL-9 signaling in TS1 lymphocytes transfected with the murine IRS-1 cDNA. Cotransfection studies and in vitro experiments directly demonstrate that JAK1, JAK2, or JAK3 is capable of tyrosine phosphorylating IRS-1, suggesting a functional role for these kinases in vivo. Lastly, we demonstrate that IL-9 induces the tyrosine phosphorylation of Stat3 and in this regard differs from IL-4, which triggers tyrosine phosphorylation of Stat6. Taken together, these results strongly suggest that IL-9 and IL-4 utilize common and unique signaling pathways via inducing the similar and distinct tyrosine-phosphorylated proteins.

Distribution of the mammalian Stat gene family in mouse chromosomes

Studies of transcriptional activation by interferons and a variety of cytokines have led to the identification of a family of proteins that serve as signal transducers and activators of transcription, Stats. Here, we report that the seven mouse Stat loci map in three clusters, with each cluster located on a different mouse autosome. The data suggest that the family has arisen via a tandem duplication of the ancestral locus, followed by dispersion of the linked loci to different mouse chromosomes.

Phosphorylation and activation of the DNA binding activity of purified Stat1 by the Janus protein-tyrosine kinases and the epidermal growth factor receptor

The activation of Janus protein-tyrosine kinases (Jaks) and the subsequent phosphorylation and activation of latent signal transducers and activators of transcription (Stats) are common elements in signal transduction through the cytokine receptor superfamily. To assess the role and specificity of Jaks in Stat activation, we have utilized baculovirus expression systems to produce Stat1 and the Jaks. Co-expression of Stat1 with Tyk2, Jak1, or Jak2 resulted in the specific tyrosine phosphorylation of Stat1 at Tyr701, the residue phosphorylated in mammalian cells stimulated with interferon gamma. Alternatively, Stat1, purified to apparent homogeneity from insect cell extracts, was phosphorylated at Tyr701 in Jak immune complex kinase reactions. Phosphorylation of purified Stat1 was necessary and sufficient for the acquisition of DNA binding activity. The specificity in both systems was indicated by the inability of a Jak2 catalytically inactive mutant (Jak2-Glu882) or the Tec protein-tyrosine kinase to phosphorylate Stat1. However, immune complex-purified epidermal growth factor receptor was capable of phosphorylating purified Stat1 at Tyr701 and activating its DNA binding activity in in vitro reactions.

Cloning of murine Stat6 and human Stat6, Stat proteins that are tyrosine phosphorylated in responses to IL-4 and IL-3 but are not required for mitogenesis

By searching a database of expressed sequences, we identified a member of the signal transducers and activators of transcription (Stat) family of proteins. Human and murine full-length cDNA clones were obtained and sequenced. The sequence of the human cDNA was identical to the recently published sequence for interleukin-4 (IL-4)-Stat (J. Hou, U. Schindler, W.J. Henzel, T.C. Ho, M. Brasseur, and S. L. McKnight, Science 265:1701-1706, 1994), while the murine Stat6 amino acid and nucleotide sequences were 83 and 84% identical to the human sequences, respectively. Using Stat6-specific antiserum, we demonstrated that Stat6 is rapidly tyrosine phosphorylated following stimulation of appropriate cell lines with IL-4 or IL-3 but is not detectably phosphorylated following stimulation with IL-2, IL-12, or erythropoietin. In contrast, IL-2, IL-3, and erythropoietin induce the tyrosine phosphorylation of Stat5 while IL-12 uniquely induces the tyrosine phosphorylation of Stat4. Inducible tyrosine phosphorylation of Stat6 requires the membrane-distal region of the IL-4 receptor alpha chain. This region of the receptor is not required for cell growth, demonstrating that Stat6 tyrosine phosphorylation does not contribute to mitogenesis.

Signaling through the hematopoietic cytokine receptors

Hematopoiesis is regulated through the interaction of a variety of growth factors with specific receptors of the cytokine receptor superfamily. Although lacking catalytic domains, all the receptors couple ligand binding to the rapid induction of protein tyrosine phosphorylation. This is mediated through a novel family of protein tyrosine kinases termed the Janus kinases (Jaks) which associate with the receptors and are activated following ligand binding. Depending upon the cytokine/receptor system, one or more of the four known Jaks (Jak1, Jak2, Jak3, Tyk2) is/are involved. The activated Jaks phosphorylate both themselves and the receptor subunits, creating docking sites for SH2-containing proteins including SHC, which couples receptor engagement to activation of the ras pathway, and HCP, a protein tyrosine phosphatase which negatively affects the response. In addition, the Jaks phosphorylate one or more of a family of signal transducers and activators of transcription (Stats). Phosphorylation of Stats induces their nuclear translocation and DNA-binding activity. Activation of Stats is independent of activation of the ras pathway and represents a novel signaling pathway correlated with mitogenesis.

Induction of tyrosine phosphorylation of Vav and expression of Pim-1 correlates with Jak2-mediated growth signaling from the erythropoietin receptor

The receptor for erythropoietin (Epo) belongs to the cytokine receptor family and lacks a tyrosine kinase domain. However, it has been hypothesized that a tyrosine kinase, Jak2, associates with the membrane proximal cytoplasmic region of Epo receptor (EpoR) and mediates the growth signaling from the receptor through tyrosine phosphorylation of cellular substrates. To explore the growth signaling pathways from the EpoR, we analyzed substrates of tyrosine phosphorylation induced by Epo stimulation in cells expressing various mutant EpoRs. The vav proto-oncogene product was found to be tyrosine phosphorylated after Epo stimulation in cells expressing the wild-type EpoR or a truncated receptor, H mutant, that retains the growth signaling function. In these cells, Epo also induced the expression of a serine/threonine kinase, Pim-1. However, Epo stimulation did not have any effect on Vav or Pim-1 in cells expressing a mutant EpoR, PM4 mutant, inactivated by a point mutation, Trp282 to Arg, in the membrane proximal region, which abrogates the interaction with Jak2. On the other hand, both tyrosine phosphorylation of Vav and expression of Pim-1 were observed constitutively in cells expressing a mutant EpoR that is constitutively activated by a point mutation, Arg 129 to Cys, in the extracellular domain. Jak2 was also constitutively tyrosine phosphorylated and activated in cells expressing this mutant, which confirms the crucial role of Jak2 in growth signaling from the EpoR. Taken together, these observations suggest that the tyrosine phosphorylation of Vav and the expression of Pim-1 may play important roles in growth signaling from the EpoR.

Erythropoietin induces association of the JAK2 protein tyrosine kinase with the erythropoietin receptor in vivo

Protein tyrosine phosphorylation has been hypothesized to play a key role in the growth signaling induced by erythropoietin (Epo), although the Epo receptor (EpoR), a member of the cytokine receptor superfamily, lacks a tyrosine kinase domain. Recently, the JAK2 tyrosine kinase was shown to be activated on Epo stimulation and to bind to the cytoplasmic domain of EpoR in vitro. To further explore the mechanisms of activation of JAK2 in EpoR-mediated signal transduction, we assessed the conditions for association of JAK2 with EpoR in vivo. Epo stimulation rapidly induced association of JAK2 with the EpoR in an interleukin 3 (IL-3)-dependent cell line transfected with the wild-type EpoR. On Epo stimulation JAK2 also associated with a truncated mutant EpoR (H-mutant), which is mitogenetically active but not tyrosine phosphorylated, indicating that association does not require receptor phosphorylation and occurs in the membrane proximal region. However, association was not detected with mutant receptors inactivated by an internal deletion or a point mutation, Trp282 to Arg, in a membrane-proximal cytoplasmic region (PB or PM4 mutant, respectively). Immune complex kinase assays of anti-EpoR immunoprecipitates also revealed that activated JAK2 associates with the EpoR in Epo-stimulated cells. By this approach, association also occurred with the mitogenically active H mutant but not with the mitogenically inactive PB or PM4 mutants. In the immune complex kinases assays, EpoR, JAK2, and a 150-kD protein were phosphorylated on tyrosine. Taken together, the results further support the hypothesis that, on Epo stimulation, JAK2 associates with the membrane-proximal cytoplasmic region of the EpoR to be activated and induces tyrosine phosphorylation of cellular substrates, including the EpoR, to transduce a growth signal.

Stat4, a novel gamma interferon activation site-binding protein expressed in early myeloid differentiation

Interferon regulation of gene expression is dependent on the tyrosine phosphorylation and activation of the DNA-binding activity of two related proteins of 91 kDa (STAT1) and/or 113 kDa (STAT2). Recent studies have suggested that these proteins are substrates of Janus kinases and that proteins related in STAT1 are involved in a number of signalling pathways, including those activated in myeloid cells by erythropoietin and interleukin-3 (IL-3). To clone STAT-related proteins from myeloid cells, degenerate oligonucleotides were used in PCRs to identify novel family members expressed in myeloid cells. This approach allowed the identification and cloning of the Stat4 gene, which is 52% identical to STAT1. Unlike STAT1, Stat4 expression is restricted but includes myeloid cells and spermatogonia. In the erythroid lineage, Stat4 expression is differentially regulated during differentiation. Functionally, Stat4 has the properties of other STAT family genes. In particular, cotransfection of expression constructs for Stat4 and Jak1 and Jak2 results in the tyrosine phosphorylation of Stat4 and the acquisition of the ability to bind to the gamma interferon (IFN-gamma)-activated sequence of the interferon regulatory factor 1 (IRF-1) gene. Stat4 is located on mouse chromosome 1 and is tightly linked to the Stat1 gene, suggesting that the genes arose by gene duplication. Unlike Stat1, neither IFN-alpha nor IFN-gamma activates Stat4. Nor is Stat4 activated in myeloid cells by a number of cytokines, including erythropoietin, IL-3, granulocyte colony-stimulating factor, stem cell factor, colon-stimulating factor 1, hepatocyte growth factor, IL-2, IL-4, and IL-6.