Involvement of JAK-family tyrosine kinases in hematopoietin receptor signal transduction

Three Tyrosine Residues in the Erythropoietin Receptor Are Essential for Janus Kinase 2 V617F Mutant-induced Tumorigenesis

The erythropoietin receptor (EpoR) regulates development of blood cells, and its full activation normally requires the cytokine erythropoietin (Epo). In the case of myeloproliferative neoplasms (MPN), Epo-independent signaling through EpoR can be caused by a point mutation, V617F, in the EpoR-interacting tyrosine kinase Janus kinase 2 (JAK2). In cells expressing the JAK2 V617F mutant, eight tyrosine residues in the intracellular domain of EpoR are phosphorylated, but the functional role of these phosphorylations in oncogenic signaling is incompletely understood. Here, to evaluate the functional consequences of the phosphorylation of these tyrosine residues, we constructed an EpoR-8YF mutant in which we substituted all eight tyrosine residues with phenylalanine. Co-expression of EpoR-8YF with the JAK2 V617F mutant failed to induce cytokine-independent cell proliferation and tumorigenesis, indicating that JAK2-mediated EpoR phosphorylation is the reason for JAK2 V617F mutant-induced oncogenic signaling. An exhaustive mutational analysis of the eight EpoR tyrosine residues indicated that three of these residues, Tyr-343, Tyr-460, and Tyr-464, are required for the JAK2 V617F mutant to exhibit its oncogenic activity. We also showed that phosphorylation at these three residues was necessary for full activation of the transcription factor STAT5, which is a critical downstream factor of JAK2 V617F-induced oncogenic signaling. In contrast, Epo stimulation could moderately stimulate the proliferation of cells expressing wild type JAK2 and EpoR-8YF, suggesting that the requirement of the phosphorylation of these three tyrosine residues seems to be specific for the oncogenic proliferation provoked by V617F mutation. Collectively, these results have revealed that phosphorylation of Tyr-343, Tyr-460, and Tyr-464 in EpoR underlies JAK2 V617F mutant-induced tumorigenesis. We propose that the targeted disruption of this pathway has therapeutic utility for managing MPN.

JAK redux: a second look at the regulation and role of JAKs in the heart

A number of type 1 receptor cytokine family members protect the heart from acute and chronic oxidative stress. This protection involves activation of two intracellular signaling cascades: the reperfusion injury salvage kinase (RISK) pathway, which entails activation of phosphatidylinositol 3-kinase (PI3-kinase) and ERK1/2, and JAK-STAT signaling, which involves activation of transcription factor signal transducer and activator of transcription 3 (STAT3). Obligatory for activation of both RISK and STAT3 by nearly all of these cytokines are the kinases JAK1 and JAK2. Yet surprisingly little is known about how JAK1 and JAK2 are regulated in the heart or how they couple to PI3-kinase activation. Although the JAKs are linked to antioxidative stress programs in the heart, we recently reported that these kinases are inhibited by oxidative stress in cardiac myocytes. In contrast, others have reported that cardiac JAK2 is activated by acute oxidative stress by an undefined process. Here we summarize recent insights into the regulation of JAK1 and JAK2. Besides oxidative stress, inhibitory regulation involves phosphorylation, nitration, and intramolecular restraints. Stimulatory regulation involves phosphorylation and adaptor proteins. The net effect of stress on JAK activity in the heart likely represents the sum of both inhibitory and stimulatory processes, along with their dynamic interaction. Thus the regulation of JAKs in the heart, once touted as the paragon of simplicity, is proving rather complicated indeed, requiring a second look. It is our contention that a better understanding of the regulation of this kinase family that is implicated in cardiac protection could translate into effective therapeutic strategies for preventing myocardial damage or repairing the injured heart.

Prolactin regulation of apoptosis-associated gene expression in T cells

Evidence accumulated over the last two decades indicates important actions for prolactin (PRL) in regulation of several functions of the immune system. That PRL can serve to facilitate immune cell proliferation is well established. In addition, PRL appears to play a salient role in the genesis and/or potentiation of certain autoimmune diseases. Recent evidence from several laboratories has extended the spectrum of PRL actions in immunological systems to include regulation of lymphocyte pool size through the process of apoptosis. Experimental results obtained using lactogen-dependent rat pre-T cell lines, the Nb2 lymphoma, have demonstrated that PRL suppresses cell death mechanisms activated by cytokine/hormone deprivation and cytotoxic drugs such as glucocorticoids. In this paper, we review results from studies conducted to investigate the mechanism(s) underlying PRL-regulated apoptosis suppression. Effects of the hormone on expression of apoptosis-associated genes of the Bcl-2 family as well as the protooncogene pim-1 in proliferating Nb2 sublines and in cells exposed to apoptotic stimuli are presented. It is concluded that PRL-mediated apoptosis suppression in immune cells reflects a complex interaction among several gene products.

The human homologue of the yeast proteins Skb1 and Hsl7p interacts with Jak kinases and contains protein methyltransferase activity

To expand our understanding of the role of Jak2 in cellular signaling, we used the yeast two-hybrid system to identify Jak2-interacting proteins. One of the clones identified represents a human homologue of the Schizosaccaromyces pombe Shk1 kinase-binding protein 1, Skb1, and the protein encoded by the Saccharomyces cerevisiae HSL7 (histone synthetic lethal 7) gene. Since no functional motifs or biochemical activities for this protein or its homologues had been reported, we sought to determine a biochemical function for this human protein. We demonstrate that this protein is a protein methyltransferase. This protein, designated JBP1 (Jak-binding protein 1), and its homologues contain motifs conserved among protein methyltransferases. JBP1 can be cross-linked to radiolabeled S-adenosylmethionine (AdoMet) and methylates histones (H2A and H4) and myelin basic protein. Mutants containing substitutions within a conserved region likely to be involved in AdoMet binding exhibit little or no activity. We mapped the JBP1 gene to chromosome 14q11.2-21. In addition, JBP1 co-immunoprecipitates with several other proteins, which serve as methyl group acceptors and which may represent physiological targets of this methyltransferase. Messenger RNA for JBP1 is widely expressed in human tissues. We have also identified and sequenced a homologue of JBP1 in Drosophila melanogaster. This report provides a clue to the biochemical function for this conserved protein and suggests that protein methyltransferases may have a role in cellular signaling.

Prolactin receptor signaling: shared components with the T-cell antigen receptor in Nb2 lymphoma cells

Previously, we reported that activation of the human prolactin receptor (PRLR) produced a protein phosphorylation pattern strikingly similar to that provoked by Concanavalin A (Con A), an activator of the T-cell antigen receptor (TCR). These results suggested that certain signaling components of the TCR may be shared by the activated PRLR. Additional studies here assessed the levels of TCR expression following PRLR stimulation and the effect of TCR activation on PRL-stimulated proliferation in lactogen-dependent pre-T Nb2-11 lymphoma cells. The results indicated that the TCR was expressed on the surface of approx 4% of exponentially proliferating and prolactin- (PRL) treated cells. In contrast, approx 45% of quiescent cells, cultured in the absence of PRL for 24 h, expressed the TCR at the cell surface, suggesting that lactogen withdrawal may up-regulate TCR cell-surface expression. Moreover, TCR activation with anti-CD3 antibodies attenuated PRL-stimulated Nb2-11 cell proliferation in a concentration-dependent manner. In other experiments, immunoprecipitation and immunoblotting of Nb2-11 lysates revealed that activation of the PRLR resulted in rapid tyrosyl phosphorylation of ZAP-70, a critical TCR-associated tyrosine kinase. In addition, ZAP-70 was found to associate transiently with the putative guanine nucleotide exchange factor and substrate, Vav, in PRL-treated cells. ZAP-70 was also found to associate constitutively with the PRLR; PRL stimulation provoked the transient recruitment of Vav to the complex. These observations suggest that PRL signaling reflects the transient formation of a PRLR-ZAP-70-Vav complex and its immunomodulatory actions involve diverse interactions that affect TCR expression and signaling mechanisms.

Mechanisms of cytokine signal transduction: IL-2, IL-4 and prolactin as hematopoietin receptor models

Cytokines, hormones and hematopoietic growth factors transduce biological signals across the cell membrane via a highly conserved family of single membrane-spanning receptors. The intracellular signal transducing machinery responsible for mediating these responses has remained largely unknown. However, recent identification of a homologous class of tyrosine kinases, Janus Kinases (JAKs), and a related family of transcription factors, signal transducers and activators of transcription (STATs), has shed new light on the molecular mechanisms responsible for mediating hematopoietin signaling and immune response. Current research efforts within the field of cytokine signaling have now shifted to understanding how these molecules are activated by hematopoietic receptors, positively and negatively regulated by kinases and phosphatases, and how they impact on gene transcription to ultimately coordinate cell homeostasis, proliferation and differentiation. This article will review some of our results identifying the involvement of JAKs, STATs, and secondary effector molecules activated following engagement of hematopoietic receptors for IL-2, IL-4, and prolactin. Here, we provide evidence for the ingenious ability of cytokine receptors to selectively recruit and activate these proteins among a repertoire of possible alternative biochemical messengers as a means to affect unique and general cell responses.

Prolactin stimulates serine/tyrosine phosphorylation and formation of heterocomplexes of multiple Stat5 isoforms in Nb2 lymphocytes

Transcription factors of the Stat gene family are selectively activated by many hormones and cytokines. Stat5 originally was cloned as a prolactin-stimulated DNA-binding protein, but is also activated by non-lactogenic cytokines in many cell types. The recent identification of two distinct Stat5 genes, which encode a 94-kDa Stat5a and a 92-kDa Stat5b as well as several lower molecular weight isoforms, suggests additional complexity and combinatorial possibilities for transcriptional regulation. We now report a biochemical analysis of prolactin activation of Stat proteins in Nb2 lymphocytes, which was associated with: 1) rapid tyrosine phosphorylation of Stat5a, Stat5b, a COOH-terminally truncated 80-kDa Stat5 form, Stat1alpha, and Stat3; 2) rapid and selective formation of Stat5a/b heterodimers, without involvement of Stat1alpha or Stat3; 3) marked serine, but not threonine phosphorylation of Stat5a and Stat5b; and 4) the appearance of two qualitatively distinct Stat5 protein complexes, which discriminated between oligonucleotides corresponding to the prolactin response elements of the beta-casein and interferon regulatory factor-1 gene promoters. Collectively, our analyses showed that Stat5a and Stat5b respond similarly to prolactin receptor activation, but also suggested that the two genes have evolved unique properties that may contribute to the specificity of receptors that utilize Stat5 signaling proteins.

Interleukin-13 is a potent activator of JAK3 and STAT6 in cells expressing interleukin-2 receptor-gamma and interleukin-4 receptor-alpha

The lymphocyte growth factors interleukin-2 (IL2), IL4, IL7, IL9 and IL15 use the common IL2 receptor-gamma (IL2R gamma) and activate the IL2R gamma-associated tyrosine kinase JAK3 (Janus kinase 3). IL13 is structurally related to IL4, competes with IL4 for binding to cell surface receptors and exhibits many similar biological effects. The molecular basis for this functional overlap between IL4 and IL13 has been attributed mainly to a shared use of the 140 kDa IL4R alpha, since these cytokines appear to be uniquely different in that, according to several recent reports, IL13 does not recruit the IL2R gamma or JAK3. This notion has been supported by the identification of a novel 70 kDa IL13 receptor in certain IL13-responsive cell lines that lack IL2R gamma. The present study sheds new light on the issue of functional overlap between IL13 and IL4, by demonstrating for the first time that, in cells that express both IL2R gamma and IL4R alpha, IL13 can mimic IL4-induced heterodimerization of IL2R gamma and IL4R alpha, with consequent marked activation of JAK3 and the transcription factor STAT6 (IL4-STAT). Reconstitution experiments in BA/F3 cells showed that both cytokines require the simultaneous presence of IL4R alpha and IL2R gamma to mediate JAK3 and proliferative responses, and analysis of 12 IL4R alpha variants showed that IL4 and IL13 signals were equally affected by mutations of the cytoplasmic domain. We conclude that IL13 activates the IL2R gamma-associated JAK3 tyrosine kinase in appropriate cell types, and propose that IL13 is capable of interacting with multiple receptor subunits in a cell-dependent and combinatorial manner. Consequently, we predict that partial disruption of IL13 signal transduction also contributes to the severe combined immuno-deficiency syndromes associated with inactivation of the IL2R gamma or JAK3 genes.

Prolactin recruits STAT1, STAT3 and STAT5 independent of conserved receptor tyrosines TYR402, TYR479, TYR515 and TYR580

The present study of prolactin (PRL) receptor-mediated recruitment of signal transducers and activators of transcription (STATs) demonstrates that PRL activates STAT3, in addition to STAT1 and STAT5 as previously reported, and that STAT1, STAT3 and STAT5 are mediators of PRL effects in cells whether of lymphoid, myeloid or mammary epithelial origin. Furthermore, receptor mutants M240 and T280 that do not mediate PRL-induced JAK2 activation and cell proliferation, are also unable to mediate STAT activation, supporting the proposed model of JAK2 as the initial effector protein used by PRL receptors. On the other hand, tyrosine phosphorylation analysis and electrophoretic mobility shift assays showed that receptor mutant G328, which lacks four of the five conserved cytoplasmic tyrosine residues of PRL receptors, retained the ability to activate JAK2 and STAT1, STAT3 and STAT5. These results support the notion that phosphotyrosyl residues other than those of the receptor, i.e., JAK2, are involved in recruiting STAT proteins to the activated PRL receptor complex.