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PubMed1999

Role of the Janus kinase (JAK)/signal transducters and activators of transcription (STAT) cascade in advanced glycation end-product-induced cellular mitogenesis in NRK-49F cells.

Advanced glycation end product (AGE) is important in the pathogenesis of diabetic nephropathy, which is characterized by cellular hypertrophy/hyperplasia leading to renal fibrosis. However, the signal transduction pathways of AGE remain poorly understood. The Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway has been associated with cellular proliferation in some extra-renal cells. Because interstitial fibroblast proliferation might be important in renal fibrosis, we studied the role of the JAK/STAT pathway in NRK-49F (normal rat kidney fibroblast) cells cultured in AGE/BSA and non-glycated BSA. We showed that AGE dose-dependently (10-200 microgram/ml) increased cellular mitogenesis in NRK-49F cells at 5 and 7 days. However, cellular mitogenesis was unaffected by the simultaneous presence of BSA. Regarding the JAK/STAT pathway, AGE (100 microgram/ml) induced tyrosine phosphorylation of JAK2 (but not JAK1, JAK3 or TYK2) at 15-60 min; it also induced the tyrosine phosphorylation of STAT1 and STAT3 at 1-2 h and 0.5-4 h respectively. Being a transcription factor, AGE also increased the DNA-binding activities of STAT1 and STAT3 AG-490 (a specific JAK2 inhibitor) (5 microM) inhibited tyrosine phosphorylation of JAK2 and the DNA-binding activities of STAT1 and STAT3. The same results were obtained by using specific 'decoy' oligodeoxynucleotides (ODNs) that prevented STAT1 and STAT3 from binding to DNA. Meanwhile, the STAT1 or STAT3 decoy ODN and AG-490 were effective in reversing AGE-induced cellular mitogenesis. We concluded that the JAK2-STAT1/STAT3 signal transduction pathway is necessary for AGE-induced cellular mitogenesis in NRK-49F cells.

Animals↗
PubMed2003

Prevention of organ allograft rejection by a specific Janus kinase 3 inhibitor.

Because of its requirement for signaling by multiple cytokines, Janus kinase 3 (JAK3) is an excellent target for clinical immunosuppression. We report the development of a specific, orally active inhibitor of JAK3, CP-690,550, that significantly prolonged survival in a murine model of heart transplantation and in cynomolgus monkeys receiving kidney transplants. CP-690,550 treatment was not associated with hypertension, hyperlipidemia, or lymphoproliferative disease. On the basis of these preclinical results, we believe JAK3 blockade by CP-690,550 has potential for therapeutically desirable immunosuppression in human organ transplantation and in other clinical settings.

Animals↗
PubMed1999

Activation of the Janus kinase 3-STAT5a pathway after CD40 triggering of human monocytes but not of resting B cells.

CD40/CD40 ligand interactions play a key role in the immune responses of B lymphocytes, monocytes, and dendritic cells. The signal transduction events triggered by cross-linking of the CD40 receptor have been widely studied in B cell lines, but little is known about signaling following CD40 stimulation of monocytes and resting tonsillar B cells. Therefore, we studied the CD40 pathway in highly purified human monocytes and resting B cells. After CD40 triggering, a similar activation of the NF-kappaB (but not of the AP-1) transcription factor complex occurred in both cell preparations. However, the components of the NF-kappaB complexes were different in monocytes and B cells, because p50 is part of the NF-kappaB complex induced by CD40 triggering in both monocytes and B cells, whereas p65 was only induced in B cells. In contrast, although the Janus kinase 3 tyrosine kinase was associated with CD40 molecules in both monocytes and resting B cells, Janus kinase 3 phosphorylation induction was observed only in CD40-activated monocytes, with subsequent induction of STAT5a DNA binding activity in the nucleus. These results suggest that the activation signals in human B cells and monocytes differ following CD40 stimulation. This observation is consistent with the detection of normal CD40-induced monocyte activation in patients with CD40 ligand+ hyper IgM syndrome in whom a defect in CD40-induced B cell activation has been reported.

B-Lymphocytes↗
PubMed1995

Thrombopoietin induces tyrosine phosphorylation and activation of the Janus kinase, JAK2.

Thrombopoietin (TPO) is a recently characterized growth and differentiation factor for megakaryocytes and platelets that exerts its effects via the receptor, c-MpI. This receptor is a member of the hematopoietin receptor superfamily and is essential for megakaryocyte maturation; however, the molecular mechanisms of TPO and c-MpI action have not been elucidated. Recently, the Janus kinases have emerged as important elements in signaling via this family of receptors. In this report, we show that, in the M07e megakaryocytic cell line, which expresses c-MpI and proliferates in response to TPO, TPO induces phosphorylation of a number of substrates between 80 and 140 kD. Specifically, we show that stimulation with TPO induces the rapid tyrosine phosphorylation of a 130-kD protein that we identify as the Janus kinase, JAK2. However, no detectable tyrosine phosphorylation of JAK1, JAK3, or TYK2 was observed. TPO also induced activation of JAK2 phosphotransferase activity in vitro. Taken together, these data indicate that JAK2 likely plays a key role in TPO-mediated signal transduction.

Cells, Cultured↗
PubMed2006

Janus kinase 2 (V617F) mutation status, signal transducer and activator of transcription-3 phosphorylation and impaired neutrophil apoptosis in myelofibrosis with myeloid metaplasia.

An activating point mutation in Janus kinase 2 (JAK2 V617F) was recently identified in myelofibrosis with myeloid metaplasia (MMM). To further elucidate the pathogenic significance, we examined the JAK2 mutation burden, phosphorylation of JAK2 substrates and neutrophil apoptotic resistance. Immunoblotting revealed phosphorylation of signal transducer and activator of transcription-3 (STAT3) in all four JAK2 with high V617F mutant allele burden and seven of eight with intermediate mutant allele burden, but only one of eight with wild-type JAK2 (P<0.001). In contrast, STAT5 phosphorylation was undetectable in patient MMM neutrophils; and phosphorylation of Akt and extracellular signal-regulated kinases (ERKs) failed to correlate with JAK2 mutation status. Apoptosis was lower in MMM neutrophils (median 41% apoptotic cells, n=50) compared to controls (median 66%, n=9) or other myeloproliferative disorder patients (median 53%, n=11; P=0.002). Apoptotic resistance in MMM correlated with anemia (P=0.01) and the JAK2-V617F (P=0.01). Indeed, apoptotic resistance was greatest in MMM neutrophils with high mutant allele burden (median 22% apoptosis, n=5) than with intermediate burden (median 39%, n=23) or wild-type JAK2 (median 47%, n=22; P=0.008). These results suggest that mutant JAK2 contributes to MMM pathogenesis by constitutively phosphorylating STAT3 and diminishing myeloid cell apoptosis.

Alleles↗
PubMed2006

Inhibition of the janus kinase family increases extracellular signal-regulated kinase 1/2 phosphorylation and causes endoreduplication.

The role of Janus-activated kinase (JAK) signaling in cell cycle transit and maintenance of genomic stability was determined in HL-60 myeloblastic leukemia cells. Inhibition of JAKs, all JAKs (JAK1, JAK2, JAK3, and tyrosine kinase 2), JAK2, or JAK3, caused a significant reduction in cell growth with a major G2-M arrest evident 24 hours after treatment. Targeting all JAKs also caused endoreduplication 48 and 72 hours after treatment. We discovered mitotic cells in both G2 (4N DNA) and G4 (8N DNA) subpopulations of cells treated with an inhibitor of all JAKs as detected by phosphorylated histone H3 expression. Treatment with inhibitors of just JAK2 or JAK3 drastically reduced such mitotic cells. We observed a complete blockage of IFN-gamma and interleukin-6-induced signal transducer and activator of transcription (STAT)-1 and STAT-3 response when all JAKs were inhibited. At the same time, we found baseline phosphorylated extracellular signal-regulated kinase (ERK) 1/2 to be elevated by JAK inhibition, particularly when all JAKs were inhibited. The G2-M arrest and endoreduplication induced by JAK inhibitors were reduced in cells pretreated with PD98059 to inhibit ERK. PD98059 also increased back the expression of the MAD2 cell cycle checkpoint protein that was down-regulated during "all JAKs inhibitor"-mediated endoreduplication. These data suggest that JAK signaling is needed for G2-M transit with inhibition of ERK.

Calcium-Binding Proteins↗
PubMed2004

Modulation of Janus kinase 2 by p53 in ovarian cancer cells.

The constitutive activation of the Janus kinase 2 (JAK2) and mutation of the p53 tumor suppressor are both detected in human cancer. We examined the potential regulation of JAK2 phosphorylation by wild-type (wt) p53 in human ovarian cancer cell lines, Caov-3 and MDAH2774, which harbor mutant form of p53 tumor suppressor gene and high levels of phosphorylated JAK2. The wt p53 gene was re-introduced into the cells using an adenovirus vector. In addition to wt p53, mutant p53 22/23, mutant p53-175, and NCV (negative control virus) were introduced into the cells in the control groups. Expression of wt p53, but not that of p53-175 mutant, diminished JAK2 tyrosine phosphorylation in MDAH2774 and Caov-3 cell lines. Expression of wt p53 or p53 22/23 mutant did not cause a reduction in the phosphorylation of unrelated protein kinases, ERK1 and ERK2 (ERK1/2). The inhibition of JAK2 tyrosine phosphorylation can be reversed by tyrosine phosphatase inhibitor, sodium orthovanadate. Protein tyrosine phosphatase 1-B levels increased with introduction of wt p53 and may be involved in the dephosphorylation of JAK2. These findings present a possible p53-dependent cellular process of modulating JAK2 tyrosine phosphorylation in ovarian cancer cell lines.

Animals↗
PubMed2004

Generation of a conditional knockout allele for the Janus kinase 2 (Jak2) gene in mice.

To study biologically relevant functions of the Janus kinase 2 (Jak2) in multiple cytokine and hormone receptor signal transduction pathways, we generated a conditional knockout (floxed) allele of this gene by placing loxP sites around the first coding exon of Jak2. Homozygous floxed animals developed normally and exhibited no phenotypic abnormalities. The conversion of the floxed allele into a null mutation was achieved by transmitting the targeted allele through the female germline of MMTV-Cre (line A) mice. Embryos that carry two Jak2 null alleles died around midgestation and exhibited impaired definitive erythropoiesis, which is a hallmark of Jak2 deficiency reported previously in conventional knockouts. This observation suggested that the Cre-mediated deletion of the first coding exon results in a true null mutation that is incapable of mediating signals through the erythropoietin receptor. Using mouse embryonic fibroblasts derived from Jak2 null embryos and their wildtype littermate controls, we demonstrated that Jak2-deficiency decouples growth hormone-receptor signaling from its downstream mediators, the signal transducer and activator of transcription (Stat) 5a and 5b.

Animals↗
PubMed1998

A Janus kinase inhibitor, JAB, is an interferon-gamma-inducible gene and confers resistance to interferons.

It has been shown that interferons (IFNs) exert their signals through receptor-associated Janus kinases (JAKs) and signal transducers and activators of transcription (STATs). However, molecular mechanism of regulation of IFN signaling has not been fully understood. We have reported novel cytokine-inducible SH2 protein (CIS) and JAK binding protein (JAB) family genes that can potentially modulate cytokine signaling. Here we report that JAB is strongly induced by IFN-gamma but not by IFN-beta in mouse myeloid leukemia M1 cells and NIH-3T3 fibroblasts. NIH-3T3 cells ectopically expressing JAB but not CIS3 lost responsiveness to the antiviral effect of IFN-beta and IFN-gamma. M1 leukemic cells stably expressing JAB were also resistant to IFN-gamma and IFN-beta-induced growth arrest. In both NIH-3T3 and M1 transformants expressing JAB, IFN-gamma did not induce tyrosine phosphorylation and DNA binding activity of STAT1. Moreover, IFN-gamma-induced activation of JAK1 and JAK2 and IFN-beta-induced JAK1 and Tyk2 activation were inhibited in NIH-3T3 JAB transformants. These results suggest that JAB inhibits IFN signaling by blocking JAK activity. We also found that IFN-resistant clones derived from LoVo cells and Daudi cells expressed high levels of JAB without stimulation. In IFN-resistant Daudi cells, IFN-induced STAT1 and JAK phosphorylation was partially reduced. Therefore, overexpression of JAB could be, at least in part, a mechanism of IFN resistance.

3T3 Cells↗
PubMed1995

Tyrosine phosphorylation and activation of STAT5, STAT3, and Janus kinases by interleukins 2 and 15.

The cytokines interleukin 2 (IL-2) and IL-15 have similar biological effects on T cells and bind common hematopoietin receptor subunits. Pathways that involve Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) have been shown to be important for hematopoietin receptor signaling. In this study we identify the STAT proteins activated by IL-2 and IL-15 in human T cells. IL-2 and IL-15 rapidly induced the tyrosine phosphorylation of STAT3 and STAT5, and DNA-binding complexes containing STAT3 and STAT5 were rapidly activated by these cytokines in T cells. IL-4 induced tyrosine phosphorylation and activation of STAT3 but not STAT5. JAK1 and JAK3 were tyrosine-phosphorylated in response to IL-2 and IL-15. Hence, the JAK and STAT molecules that are activated in response to IL-2 and IL-15 are similar but differ from those induced by IL-4. These observations identify the STAT proteins activated by IL-2 and IL-15 and therefore define signaling pathways by which these T-cell growth factors may regulate gene transcription.

Base Sequence↗
PubMed2000

Janus kinases: components of multiple signaling pathways.

Cytoplasmic Janus protein tyrosine kinases (JAKs) are crucial components of diverse signal transduction pathways that govern cellular survival, proliferation, differentiation and apoptosis. Evidence to date, indicates that JAK kinase function may integrate components of diverse signaling cascades. While it is likely that activation of STAT proteins may be an important function attributed to the JAK kinases, it is certainly not the only function performed by this key family of cytoplasmic tyrosine kinases. Emerging evidence indicates that phosphorylation of cytokine and growth factor receptors may be the primary functional attribute of JAK kinases. The JAK-triggered receptor phosphorylation can potentially be a rate-limiting event for a successful culmination of downstream signaling events. In support of this hypothesis, it has been found that JAK kinase function is required for optimal activation of the Src-kinase cascade, the Ras-MAP kinase pathway, the PI3K-AKT pathway and STAT signaling following the interaction of cytokine/interferon receptors with their ligands. Aberrations in JAK kinase activity, that may lead to derailment of one or more of the above mentioned pathways could disrupt normal cellular responses and result in disease states. Thus, over-activation of JAK kinases has been implicated in tumorigenesis. In contrast, loss of JAK kinase function has been found to result in disease states such as severe-combined immunodeficiency. In summary, optimal JAK kinase activity is a critical determinant of normal transmission of cytokine and growth factor signals.

Animals↗
PubMed1998

Structural and mechanistic aspects of Janus kinases: how the two-faced god wields a double-edged sword.

The Janus family of protein-tyrosine kinases has long been known to function in signal transduction pathways initiated by a host of cytokines. A brief overview of the role of Janus kinases (Jaks) in both cytokine and noncytokine signaling pathways highlights the broad physiologic importance of this kinase family. New insights into the structural and mechanistic regulatory aspects of Janus kinases are rapidly emerging. Recent mutational analyses allow the dissection of Jaks into three distinct structural domains governing receptor affiliation, autoregulation, and catalysis. A fourth domain determining substrate specificity is as yet poorly defined and is, therefore, discussed in the context of known substrates and inhibitors, a collection of molecules that have been expanded recently to include Stam and Jab. The proposed mechanism of the interconversion of Janus kinases from inactive to fully active enzymes involves three states of enzymatic activity. Additional layers of regulation can be independently superimposed on this multistate model, providing a simplified description of the behavior of Janus kinases under normal and pathologic circumstances.

Animals↗
PubMed1997

Janus kinases in interleukin-2-mediated signaling: JAK1 and JAK3 are differentially regulated by tyrosine phosphorylation.

BACKGROUND: Cytokines mediate a variety of effector cell functions, including cellular proliferation, differentiation, and modulation of the immune response. Many cytokines activate receptor-associated Janus kinases (JAKs) that promote tyrosine phosphorylation of signal transducers and activators of transcription (STAT) factors. Although JAK activation has been correlated with phosphorylation, the role of this tyrosine phosphorylation in the regulation of JAK1 and JAK3 remains unclear. Furthermore, the relative roles of JAK1 and JAK3 in the activation of STAT5 by interleukin-2 (IL-2) remain poorly understood. RESULTS: We targeted two conserved tyrosine residues within the activation loop of the JAK1 and JAK3 kinase domains for substitution with phenylalanines. In an overexpression system, the catalytic function of JAK1 strictly required the presence of the first of these tyrosines, Y1033. In contrast, JAK3 retained catalytic activity when either or both of these activation-loop tyrosines were mutated. Analysis of JAK1/3 chimeras demonstrated that JAK activity was also controlled by intramolecular interactions involving the amino-terminal domain of the JAK as well as by the inherent signaling properties of the kinase domain. Finally, we have reconstituted IL-2-dependent STAT5 induction in a cell line that lacks detectable expression of JAK1 and JAK3. Catalytically active versions of both JAK1 and JAK3 must be present for effective induction of STAT5. CONCLUSIONS: JAK1 and JAK3 are differentially regulated by specific tyrosines within their respective activation loops. Additionally, the amino-terminal domain of JAK3 appears to contain regulatory sequences that modify the function of the kinase domain. Finally, both JAK1 and JAK3 must retain catalytic function for IL-2-induced STAT5 activation.

Animals↗
PubMed2002

IL-6 mediated activation of STAT3 bypasses Janus kinases in terminally differentiated B lineage cells.

Cytokine signaling generally occurs through receptors lacking tyrosine kinase activity. Aggregation of receptors leads to activation of receptor associated Janus kinases (Jaks) which in turn phosphorylate members of a family of transcription factors (STATs) that translocate to the nucleus and regulate gene expression. In the case of Interleukin-6 (IL-6), the consensus for signaling in B lineage cells has been that Jak1, Jak2 and Tyk2 are all phosphorylated upon ligand binding and participate in activation of downstream elements, in particular STAT3. In other cell types, Jak1 has been demonstrated to be absolutely required for IL-6 mediated activation of STAT3. In the present studies, we have identified a series of end stage B cell (plasma cell) lines that fail to express Jak1, but phosphorylate STAT3 in response to IL-6. No evidence was found for a requirement of other Jak family members in the activation of STAT3. STAT3 tyrosine phosphorylation was inhibited in a dose dependent manner by the MEK inhibitor U0126, but not by inhibitors of PI-3K or Src kinases. Moreover, STAT3 phosphorylation was similarly inhibited in lines expressing Jak1 wherein Jak1 was phosphorylated upon IL-6 stimulation and Jak1 phosphorylation was not inhibited by U0126. These results indicate that the MAPK pathway plays a critical role in IL-6 mediated tyrosine phosphorylation of STAT3 and suggests that Jak kinases may not be required in this cascade. Thus, it may be important to re-evaluate the role of Jak kinases in other cytokine signaling pathways as well.

Animals↗
PubMed1998

Interaction of Janus kinases JAK-1 and JAK-2 with the insulin receptor and the insulin-like growth factor-1 receptor.

Insulin and insulin-like growth factor-1 (IGF-1) treatment of cells overexpressing the insulin receptor or the IGF-1 receptor promotes phosphorylation and activation of Janus kinases JAK-1 and JAK-2 but not of TYK-2. With insulin, we observed maximal phosphorylation of JAK-1 within 2 min (5.2 +/- 0.6-fold) and maximal phosphorylation of JAK-2 within 10 min (2.4 +/- 0.6-fold). In cells incubated with IGF-1, we found maximal phosphorylation of JAK-2 within 2 min (1.9 +/- 0.2-fold) and of JAK-1 within 5 min (4.5 +/- 0.4-fold). The JAKs from insulin- or IGF-1-stimulated cells were activated, as shown by their autophosphorylation in vitro. Moreover, they were able to phosphorylate in vitro native insulin receptor substrate (IRS)-1 and a fragment of IRS-2 (GST-IRS-2591-786). Comparison of 32P-peptide maps of IRS-1 phosphorylated in vitro by the insulin receptor vs. JAK-1 showed the occurrence of different phosphopeptides, suggesting that different sites are likely to be phosphorylated by the two kinases. Finally, coprecipitation of receptors and JAK-1 was seen, and phosphorylation of both receptors was found to be necessary for receptor binding to JAK-1. Two domains of JAK- 1 are involved in the formation of the complex between receptor and JAK-1, i.e. the N-terminal portion containing JH7 and JH6 domains, and the C-terminal kinase domain (JH1 domain). Taking our data together, we conclude that: 1) insulin and IGF-1 lead to phosphorylation and activation of JAK-1 and JAK-2 in intact cells; 2) phosphorylation of IRS-I by JAK-1 seems to occur on sites different from those phosphorylated by the insulin receptor; 3) JAK-1 interacts directly with phosphorylated insulin and IGF-1 receptors; and 4) the JH7-JH6 and JH1 domains of JAK-1 are responsible for the interaction with insulin and IGF-1 receptors.

3T3 Cells↗
PubMed1999

Tyrosine phosphorylation and activation of Janus kinase 1 and STAT3 by sublytic C5b-9 complement complex in aortic endothelial cells.

The pathway involving Janus kinase (JAK) and signal transducers and activators of transcription (STATs) plays an important role in differentiation and proliferation of cells initiated by receptor activation. In the present study we identified the JAK and STAT proteins activated by C5b-9 in human aortic endothelial cells (AEC). JAK1 but not JAK2 was tyrosine phosphorylated in response to sublytic C5b-9. STAT3 was rapidly tyrosine phosphorylated also by C5b-9. Pertussis toxin inhibited the C5b-9 induced JAK1 activation. However, phosphorylation of STAT3 was not inhibited by Pertussis toxin, although C5b-9 induced a time-dependent nuclear translocation of STAT3. These observations indicated that JAK1 is phosphorylated by C5b-9 through activation of trimeric G proteins of the Gi/Go family. Raf-1 and ERK1 were also activated by C5b-9 in human AEC in a G protein dependent manner. Therefore, JAK1 activity may be involved in activation of Raf-1 and ERK1 via G proteins activated by C5b-9. This study demonstrates the ability of membrane-inserted C5b-9 to activate JAK1 and STAT3 proteins, thus defining new signalling pathway by which C5b-9 may regulate gene activation.

Adult↗
PubMed1994

The Janus kinase family and signaling through members of the cytokine receptor superfamily.

Many cytokines initiate cellular responses through their interaction with members of the cytokine receptor superfamily which contain no catalytic domains in their cytoplasmic domains. Irrespective, ligand binding induces tyrosine phosphorylation, which requires a membrane proximal region of the cytoplasmic domain. Recent studies have shown that members of the Janus kinase (JAK) family of protein tyrosine kinases associate with the membrane proximal region, are rapidly tyrosine phosphorylated following ligand binding and their in vitro kinase activity is activated. The JAKs are 130-kDa proteins which lack SH2/SH3 domains and contain two kinase domains, an active domain and a second kinase-like domain. Individual receptors associate with, or require, one or more of the three known family members including JAK1, JAK2, and tyk2. Substrates of the JAKs include the 91-kDa and 113-kDa proteins of the interferon-stimulated transcription complex ISGF3. These proteins, when tyrosine phosphorylated, migrate to the nucleus and participate in the activation of gene transcription. Recent evidence suggests that the 91- and 113-kDa proteins are members of a large family of genes that are potential substrates of JAK family members and may regulate a variety of genes involved in cell growth, differentiation or function.

Animals↗
PubMed1996

Janus kinases in cytokine signalling.

Hematopoiesis is largely regulated by 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 tyrosine phosphorylation which is critical for all receptor functions. Recent studies have demonstrated that this is mediated through the association and activation of members of the Janus kinase (Jak) family of protein tyrosine kinases. The activated Jaks phosphorylate the receptors, creating docking sites for SH2 containing signalling proteins which are tyrosine phosphorylated following their association with the receptor complex. Among the substrates of tyrosine phosphorylation are members of the signal transducers and activators of transcription family of proteins (Stats). Various cytokines induce the tyrosine phosphorylation and activation of one or more of the six family members. The pattern of Stat activation provides a level of cytokine individuality that is not observed in the activation of other signalling pathways. Although not required for mitogenic responses, it is speculated that the Stats may mediate many of the cytokine specific functional responses of hematopoietic cells.

Cytokines↗