A kinase-deficient splice variant of the human JAK3 is expressed in hematopoietic and epithelial cancer cells

JAK-STAT Signaling in Autoimmunity and Cancer

The JAK-STAT pathway is an essential cell survival signaling that regulates gene expressions related to inflammation, immunity and cancer. Cytokine receptors, signal transducer and activator of transcription (STAT) proteins, and Janus kinases (JAKs) are the critical component of this signaling cascade. When JAKs are stimulated by cytokines, STAT phosphorylation, dimerization, and nuclear translocation occur, which eventually impacts gene transcription. Dysregulation of JAK-STAT signaling is linked with various autoimmune diseases, including rheumatoid arthritis, psoriasis, and inflammatory bowel disease. This pathway is constitutively activated in human malignancies and leads to tumor cell survival, proliferation, and immune evasion. Oncogenic mutations in the JAK and STAT genes have been found in solid tumors, leukemia, and lymphoma. Targeting the JAK-STAT pathway is a viable and promising therapeutic strategy for the treatment of autoimmune diseases and cancers.

Brain-Gut and Microbiota-Gut-Brain Communication in Type-2 Diabetes Linked Alzheimer's Disease

The gastrointestinal (GI) tract, home to the largest microbial population in the human body, plays a crucial role in overall health through various mechanisms. Recent advancements in research have revealed the potential implications of gut-brain and vice-versa communication mediated by gut-microbiota and their microbial products in various diseases including type-2 diabetes and Alzheimer's disease (AD). AD is the most common type of dementia where most of cases are sporadic with no clearly identified cause. However, multiple factors are implicated in the progression of sporadic AD which can be classified as non-modifiable (e.g., genetic) and modifiable (e.g. Type-2 diabetes, diet etc.). Present review focusses on key players particularly the modifiable factors such as Type-2 diabetes (T2D) and diet and their implications in microbiota-gut-brain (MGB) and brain-gut (BG) communication and cognitive functions of healthy brain and their dysfunction in Alzheimer's Disease. Special emphasis has been given on elucidation of the mechanistic aspects of the impact of diet on gut-microbiota and the implications of some of the gut-microbial products in T2D and AD pathology. For example, mechanistically, HFD induces gut dysbiosis with driven metabolites that in turn cause loss of integrity of intestinal barrier with concomitant colonic and systemic chronic low-grade inflammation, associated with obesity and T2D. HFD-induced obesity and T2D parallel neuroinflammation, deposition of Amyloid β (Aβ), and ultimately cognitive impairment. The review also provides a new perspective of the impact of diet on brain-gut and microbiota-gut-brain communication in terms of transcription factors as a commonly spoken language that may facilitates the interaction between gut and brain of obese diabetic patients who are at a higher risk of developing cognitive impairment and AD. Other commonality such as tyrosine kinase expression and functions maintaining intestinal integrity on one hand and the phagocytic clarence by migratory microglial functions in brain are also discussed. Lastly, the characterization of the key players future research that might shed lights on novel potential pharmacological target to impede AD progression are also discussed.

STAT5 Is Necessary for the Metabolic Switch Induced by IL-2 in Cervical Cancer Cell Line SiHa

The tumor cells reprogram their metabolism to cover their high bioenergetic demands for maintaining uncontrolled growth. This response can be mediated by cytokines such as IL-2, which binds to its receptor and activates the JAK/STAT pathway. Some reports show a correlation between the JAK/STAT pathway and cellular metabolism, since the constitutive activation of STAT proteins promotes glycolysis through the transcriptional activation of genes related to energetic metabolism. However, the role of STAT proteins in the metabolic switch induced by cytokines in cervical cancer remains poorly understood. In this study, we analyzed the effect of IL-2 on the metabolic switch and the role of STAT5 in this response. Our results show that IL-2 induces cervical cancer cell proliferation and the tyrosine phosphorylation of STAT5. Also, it induces an increase in lactate secretion and the ratio of NAD+/NADH, which suggest a metabolic reprogramming of their metabolism. When STAT5 was silenced, the lactate secretion and the NAD+/NADH ratio decreased. Also, the expression of HIF1α and GLUT1 decreased. These results indicate that STAT5 regulates IL-2-induced cell proliferation and the metabolic shift to aerobic glycolysis by regulating genes related to energy metabolism. Our results suggest that STAT proteins modulate the metabolic switch in cervical cancer cells to attend to their high demand of energy required for cell growth and proliferation.

Circular RNA MAP2K2-modified immunosuppressive dendritic cells for preventing alloimmune rejection in organ transplantation

Long-term patient and graft survival has been achieved in organ transplantation but at the expense of toxic side effects that are associated with long-term use of nonspecific immunosuppressive drugs. Discovering new regulators of dendritic cells is the key for development of an ideal treatment to prevent immune rejection. We hypothesized that knockdown of circMAP2K2 induces immunosuppressive DCs and that treatment with circMAP2K2 silenced-DCs can prevent alloimmune rejection. DCs were cultured and transfected with siRNA for circMAP2K2. circMAP2K2 levels were measured by qRT-PCR. DC's maturation and immune function were assessed by flow cytometry and mixed lymphocyte reactions. The function of circMAP2K2 was illustrated by a series of RIP and IP. The therapeutics of engineered DCs was tested in a mouse heart transplantation model. We found that circMAP2K2 was highly expressed in mature DCs. Knockdown of circMAP2K2 reduced expression of MHCII, CD40 and CD80, attenuated the ability of DCs to activate allogeneic naïve T cells, and enhanced CD4+CD25+FOXP3+ regulatory T cells (Treg). circMAP2K2-induced immunosuppressive DCs by interacting with SENP3. Treatment with circMAP2K2-knockdown DCs attenuated alloimmune rejection and prolonged allograft survival in a murine heart transplantation model. The immune suppression induced in vivo was donor-antigen specific. In conclusion, knockdown of circMAP2K2 can induce immunosuppressive DCs which are able to inhibit overactive immune response, highlighting a new promising therapeutic approach for immune disorder diseases.

New JAK3-INSL3 Fusion Transcript-An Oncogenic Event in Cutaneous T-Cell Lymphoma

Constitutively activated tyrosine kinase JAK3 is implicated in the pathogenesis of cutaneous T-cell lymphomas (CTCL). The mechanisms of constitutive JAK3 activation are unknown although a JAK3 mutation was reported in a small portion of CTCL patients. In this study, we assessed the oncogenic roles of a newly identified JAK3-INSL3 fusion transcript in CTCL. Total RNA from malignant T-cells in 33 patients with Sézary syndrome (SS), a leukemic form of CTCL, was examined for the new JAK3-INSL3 fusion transcript by RT-PCR followed by Sanger sequencing. The expression levels were assessed by qPCR and correlated with patient survivals. Knockdown and/or knockout assays were conducted in two CTCL cell lines (MJ cells and HH cells) by RNA interference and/or CRISPR/Cas9 gene editing. SS patients expressed heterogeneous levels of a new JAK3-INSL3 fusion transcript. Patients with high-level expression of JAK3-INSL3 showed poorer 5-year survival (n = 19, 42.1%) than patients with low-level expression (n = 14, 78.6%). CTCL cells transduced with specific shRNAs or sgRNAs had decreased new JAK3-INSL3 fusion transcript expression, reduced cell proliferation, and decreased colony formation. In NSG xenograft mice, smaller tumor sizes were observed in MJ cells transduced with specific shRNAs than cells transduced with controls. Our results suggest that the newly identified JAK3-INSL3 fusion transcript confers an oncogenic event in CTCL.

Regulation of the JAK/STAT signaling pathway: The promising targets for cardiovascular disease

Individuals have known that Janus kinase (JAK) signal transducer and activator of transcription (STAT) signaling pathway was involved in the growth of the cell, cell differentiation courses advancement, immune cellular survival, as well as hematopoietic system advancement. Researches in the animal models have already uncovered a JAK/STAT regulatory function in myocardial ischemia-reperfusion injury (MIRI), acute myocardial infarction (MI), hypertension, myocarditis, heart failure, angiogenesis and fibrosis. Evidences originating in these studies indicate a therapeutic JAK/STAT function in cardiovascular diseases (CVDs). In this retrospection, various JAK/STAT functions in the normal and ill hearts were described. Moreover, the latest figures about JAK/STAT were summarized under the background of CVDs. Finally, we discussed the clinical transformation prospects and technical limitations of JAK/STAT as the potential therapeutic targets for CVDs. This collection of evidences has essential meanings for the clinical application of JAK/STAT as medicinal agents for CVDs. In this retrospection, various JAK/STAT functions in the normal and ill hearts were described. Moreover, the latest figures about JAK/STAT were summarized under the background of CVDs. Finally, we discussed the clinical transformation prospects and toxicity of JAK/STAT inhibitors as potential therapeutic targets for CVDs. This collection of evidences has essential meanings for the clinical application of JAK/STAT as medicinal agents for CVDs.

The implications of alternative pre-mRNA splicing in cell signal transduction

Cells produce multiple mRNAs through alternative splicing, which ensures proteome diversity. Because most human genes undergo alternative splicing, key components of signal transduction pathways are no exception. Cells regulate various signal transduction pathways, including those associated with cell proliferation, development, differentiation, migration, and apoptosis. Since proteins produced through alternative splicing can exhibit diverse biological functions, splicing regulatory mechanisms affect all signal transduction pathways. Studies have demonstrated that proteins generated by the selective combination of exons encoding important domains can enhance or attenuate signal transduction and can stably and precisely regulate various signal transduction pathways. However, aberrant splicing regulation via genetic mutation or abnormal expression of splicing factors negatively affects signal transduction pathways and is associated with the onset and progression of various diseases, including cancer. In this review, we describe the effects of alternative splicing regulation on major signal transduction pathways and highlight the significance of alternative splicing.

Therapeutic modulation of JAK-STAT, mTOR, and PPAR-γ signaling in neurological dysfunctions

The cytokine-activated Janus kinase (JAK)-signal transducer and activator of transcription (STAT) cascade is a pleiotropic pathway that involves receptor subunit multimerization. The mammalian target of rapamycin (mTOR) is a ubiquitously expressed serine-threonine kinase that perceives and integrates a variety of intracellular and environmental stimuli to regulate essential activities such as cell development and metabolism. Peroxisome proliferator-activated receptor-gamma (PPARγ) is a prototypical metabolic nuclear receptor involved in neural differentiation and axon polarity. The JAK-STAT, mTOR, and PPARγ signaling pathways serve as a highly conserved signaling hub that coordinates neuronal activity and brain development. Additionally, overactivation of JAK/STAT, mTOR, and inhibition of PPARγ signaling have been linked to various neurocomplications, including neuroinflammation, apoptosis, and oxidative stress. Emerging research suggests that even minor disruptions in these cellular and molecular processes can have significant consequences manifested as neurological and neuropsychiatric diseases. Of interest, target modulators have been proven to alleviate neuronal complications associated with acute and chronic neurological deficits. This research-based review explores the therapeutic role of JAK-STAT, mTOR, and PPARγ signaling modulators in preventing neuronal dysfunctions in preclinical and clinical investigations.

The Role of the JAK/STAT Signaling Pathway in the Pathogenesis of Alzheimer's Disease: New Potential Treatment Target

Alzheimer's disease is characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain. However, emerging evidence suggests that neuroinflammation, mediated notably by activated neuroglial cells, neutrophils, and macrophages, also plays an important role in the pathogenesis of Alzheimer's disease. Therefore, understanding the interplay between the nervous and immune systems might be the key to the prevention or delay of Alzheimer's disease progression. One of the most important mechanisms determining gliogenic cell fate is the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway that is influenced by the overactivation of microglia and astrocytes. The JAK/STAT signaling pathway is one of the critical factors that promote neuroinflammation in neurodegenerative diseases such as Alzheimer's disease by initiating innate immunity, orchestrating adaptive immune mechanisms, and finally, constraining neuroinflammatory response. Since a chronic neuroinflammatory environment in the brain is a hallmark of Alzheimer's disease, understanding the process would allow establishing the underlying role of neuroinflammation, then estimating the prognosis of Alzheimer's disease development and finding a new potential treatment target. In this review, we highlight the recent advances in the potential role of JAK/STAT signaling in neurological diseases with a focus on discussing future research directions regarding novel therapeutic approaches and predictive biomarkers for Alzheimer's disease.

Zebrafish Model of Severe Combined Immunodeficiency (SCID) Due to JAK3 Mutation

JAK3 is principally activated by members of the interleukin-2 receptor family and plays an essential role in lymphoid development, with inactivating JAK3 mutations causing autosomal-recessive severe combined immunodeficiency (SCID). This study aimed to generate an equivalent zebrafish model of SCID and to characterize the model across the life-course. Genome editing of zebrafish jak3 created mutants similar to those observed in human SCID. Homozygous jak3 mutants showed reduced embryonic T lymphopoiesis that continued through the larval stage and into adulthood, with B cell maturation and adult NK cells also reduced and neutrophils impacted. Mutant fish were susceptible to lymphoid leukemia. This model has many of the hallmarks of human SCID resulting from inactivating JAK3 mutations and will be useful for a variety of pre-clinical applications.

The JAK/STAT signaling pathway: from bench to clinic

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway was discovered more than a quarter-century ago. As a fulcrum of many vital cellular processes, the JAK/STAT pathway constitutes a rapid membrane-to-nucleus signaling module and induces the expression of various critical mediators of cancer and inflammation. Growing evidence suggests that dysregulation of the JAK/STAT pathway is associated with various cancers and autoimmune diseases. In this review, we discuss the current knowledge about the composition, activation, and regulation of the JAK/STAT pathway. Moreover, we highlight the role of the JAK/STAT pathway and its inhibitors in various diseases.

Role of Janus Kinase 3 in Predisposition to Obesity-associated Metabolic Syndrome

Obesity, a worldwide epidemic, is a major risk factor for the development of metabolic syndrome (MetS) including diabetes and associated health complications. Recent studies indicate that chronic low-grade inflammation (CLGI) plays a key role in metabolic deterioration in the obese population. Previously, we reported that Jak3 was essential for mucosal differentiation and enhanced colonic barrier functions and its loss in mice resulted in basal CLGI and predisposition to DSS induced colitis. Since CLGI is associated with diabetes, obesity, and metabolic syndrome, present studies determined the role of Jak3 in development of such conditions. Our data show that loss of Jak3 resulted in increased body weight, basal systemic CLGI, compromised glycemic homeostasis, hyperinsulinemia, and early symptoms of liver steatosis. Lack of Jak3 also resulted in exaggerated symptoms of metabolic syndrome by western high-fat diet. Mechanistically, Jak3 was essential for reduced expression and activation of Toll-like receptors (TLRs) in murine intestinal mucosa and human intestinal epithelial cells where Jak3 interacted with and activated p85, the regulatory subunit of the PI3K, through tyrosine phosphorylation of adapter protein insulin receptor substrate (IRS1). These interactions resulted in activation of PI3K-Akt axis, which was essential for reduced TLR expression and TLR associated NFκB activation. Collectively, these results demonstrate the essential role of Jak3 in promoting mucosal tolerance through suppressed expression and limiting activation of TLRs thereby preventing intestinal and systemic CLGI and associated obesity and MetS.

STATs: An Old Story, Yet Mesmerizing

Signal transducers and activators of transcription (STATs) are cytoplasmic transcription factors that have a key role in cell fate. STATs, a protein family comprised of seven members, are proteins which are latent cytoplasmic transcription factors that convey signals from the cell surface to the nucleus through activation by cytokines and growth factors. The signaling pathways have diverse biological functions that include roles in cell differentiation, proliferation, development, apoptosis, and inflammation which place them at the center of a very active area of research. In this review we explain Janus kinase (JAK)/STAT signaling and focus on STAT3, which is transient from cytoplasm to nucleus after phosphorylation. This procedure controls fundamental biological processes by regulating nuclear genes controlling cell proliferation, survival, and development. In some hematopoietic disorders and cancers, overexpression and activation of STAT3 result in high proliferation, suppression of cell differentiation and inhibition of cell maturation. This article focuses on STAT3 and its role in malignancy, in addition to the role of microRNAs (miRNAs) on STAT3 activation in certain cancers.

Redox regulation of Janus kinase: The elephant in the room

The redox regulation of Janus kinases (JAKs) is a complex subject. Due to other redox-sensitive kinases in the kinome, redox-sensitive phosphatases, and cellular antioxidant systems and reactive oxygen species (ROS) production systems, the net biological outcomes of oxidative stress on JAK-dependent signal transduction vary according to the specific biological system examined. This review begins with a discussion of the biochemical evidence for a cysteine-based redox switch in the catalytic domain of JAKs, proceeds to consider direct and indirect regulatory mechanisms involved in biological experiments, and ends with a discussion of the role(s) of redox regulation of JAKs in various diseases.

JAK2-STAT5B pathway and osteoblast differentiation

Osteoblast differentiation is a critical step in the maintenance of bone homeostasis. Osteoblast differentiation is generally maintained by growth hormone (GH) and various other endocrine and autocrine/paracrine factors. JAK2-STAT5B pathway is a central axis in the mechanism of GH signaling. Similarly, the autocrine/paracrine signaling factor IGF-1 also mediates its effects through this pathway. Analysis on JAK2-STAT5B pathway showed its importance in the IGF-1/IGF-1R mediated regulation of gene expression and osteoblast differentiation. Persistent activation of STAT5B and inhibition of STAT5B degradation showed increased osteoblastic differentiation and STAT5B/Runx-2 activities. Conditional gene silencing studies showed the importance of the JAK2-STAT5B pathway in stimulation of other transcription factors and expression of various differentiation markers.

Identification of a redox-sensitive switch within the JAK2 catalytic domain

Four cysteine residues (Cys866, Cys917, Cys1094, and Cys1105) have direct roles in cooperatively regulating Janus kinase 2 (JAK2) catalytic activity. Additional site-directed mutagenesis experiments now provide evidence that two of these residues (Cys866 and Cys917) act together as a redox-sensitive switch, allowing JAK2's catalytic activity to be directly regulated by the redox state of the cell. We created several variants of the truncated JAK2 (GST/(NΔ661)rJAK2), which incorporated cysteine-to-serine or cysteine-to-alanine mutations. The catalytic activities of these mutant enzymes were evaluated by in vitro autokinase assays and by in situ autophosphorylation and transphosphorylation assays. Cysteine-to-alanine mutagenesis revealed that the mechanistic role of Cys866 and Cys917 is functionally distinct from that of Cys1094 and Cys1105. Most notable is the observation that the robust activity of the CC866,917AA mutant is unaltered by pretreatment with dithiothreitol or o-iodosobenzoate, unlike all other JAK2 variants previously examined. This work provides the first direct evidence for a cysteine-based redox-sensitive switch that regulates JAK2 catalytic activity. The presence of this redox-sensitive switch predicts that reactive oxygen species can impair the cell's response to JAK-coupled cytokines under conditions of oxidative stress, which we confirm in a murine pancreatic β-islet cell line.

Interleukin-15 plays a central role in human kidney physiology and cancer through the γc signaling pathway

The ability of Interleukin-15 (IL-15) to activate many immune antitumor mechanisms renders the cytokine a good candidate for the therapy of solid tumors, particularly renal cell carcinoma. Although IL-15 is being currently used in clinical trials, the function of the cytokine on kidney's components has not been extensively studied; we thus investigated the role of IL-15 on normal and tumor renal epithelial cells. Herein, we analyzed the expression and the biological functions of IL-15 in normal renal proximal tubuli (RPTEC) and in their neoplastic counterparts, the renal clear cell carcinomas (RCC). This study shows that RPTEC express a functional heterotrimeric IL-15Rαβγc complex whose stimulation with physiologic concentrations of rhIL-15 is sufficient to inhibit epithelial mesenchymal transition (EMT) commitment preserving E-cadherin expression. Indeed, IL-15 is not only a survival factor for epithelial cells, but it can also preserve the renal epithelial phenotype through the γc-signaling pathway, demonstrating that the cytokine possess a wide range of action in epithelial homeostasis. In contrast, in RCC in vitro and in vivo studies reveal a defect in the expression of γc-receptor and JAK3 associated kinase, which strongly impacts IL-15 signaling. Indeed, in the absence of the γc/JAK3 couple we demonstrate the assembly of an unprecedented functional high affinity IL-15Rαβ heterodimer, that in response to physiologic concentrations of IL-15, triggers an unbalanced signal causing the down-regulation of the tumor suppressor gene E-cadherin, favoring RCC EMT process. Remarkably, the rescue of IL-15/γc-dependent signaling (STAT5), by co-transfecting γc and JAK3 in RCC, inhibits EMT reversion. In conclusion, these data highlight the central role of IL-15 and γc-receptor signaling in renal homeostasis through the control of E-cadherin expression and preservation of epithelial phenotype both in RPTEC (up-regulation) and RCC (down-regulation).

Selective functional inhibition of JAK-3 is sufficient for efficacy in collagen-induced arthritis in mice

OBJECTIVE

All gamma-chain cytokines signal through JAK-3 and JAK-1 acting in tandem. We undertook this study to determine whether the JAK-3 selective inhibitor WYE-151650 would be sufficient to disrupt cytokine signaling and to ameliorate autoimmune disease pathology without inhibiting other pathways mediated by JAK-1, JAK-2, and Tyk-2.

METHODS

JAK-3 kinase selective compounds were characterized by kinase assay and JAK-3-dependent (interleukin-2 [IL-2]) and -independent (IL-6, granulocyte-macrophage colony-stimulating factor [GM-CSF]) cell-based assays measuring proliferation or STAT phosphorylation. In vivo, off-target signaling was measured by IL-22- and erythropoietin (EPO)-mediated models, while on-target signaling was measured by IL-2-mediated signaling. Efficacy of JAK-3 inhibitors was determined using delayed-type hypersensitivity (DTH) and collagen-induced arthritis (CIA) models in mice.

RESULTS

In vitro, WYE-151650 potently suppressed IL-2-induced STAT-5 phosphorylation and cell proliferation, while exhibiting 10-29-fold less activity against JAK-3-independent IL-6- or GM-CSF-induced STAT phosphorylation. Ex vivo, WYE-151650 suppressed IL-2-induced STAT phosphorylation, but not IL-6-induced STAT phosphorylation, as measured in whole blood. In vivo, WYE-151650 inhibited JAK-3-mediated IL-2-induced interferon-gamma production and decreased the natural killer cell population in mice, while not affecting IL-22-induced serum amyloid A production or EPO-induced reticulocytosis. WYE-151650 was efficacious in mouse DTH and CIA models.

CONCLUSION

In vitro, ex vivo, and in vivo assays demonstrate that WYE-151650 is efficacious in mouse CIA despite JAK-3 selectivity. These data question the need to broadly inhibit JAK-1-, JAK-2-, or Tyk-2-dependent cytokine pathways for efficacy.

Loss-of-function JAK3 mutations in TMD and AMKL of Down syndrome

Acquired mutations activating Janus kinase 3 (jak3) have been reported in Down syndrome (DS) and non-DS patients with acute megakaryoblastic leukaemia (AMKL). This highlighted jak3-activation as an important event in the pathogenesis of AMKL, and predicted inhibitors of jak3 as conceptual therapeutics for AMKL. Of 16 DS-transient myeloproliferative disorder (TMD)/AMKL patients tested, seven showed JAK3 mutations. Three mutations deleted the kinase (JH1) domain, abolishing the main function of jak3. Another patient displayed a mutation identical to a previously reported inherited loss-of-function causing severe combined immunodeficiency. Our data suggest that both gain-, and loss-of function mutations of jak3 can be acquired in DS-TMD/AMKL.

Activating alleles of JAK3 in acute megakaryoblastic leukemia

Tyrosine kinases are aberrantly activated in numerous malignancies, including acute myeloid leukemia (AML). To identify tyrosine kinases activated in AML, we developed a screening strategy that rapidly identifies tyrosine-phosphorylated proteins using mass spectrometry. This allowed the identification of an activating mutation (A572V) in the JAK3 pseudokinase domain in the acute megakaryoblastic leukemia (AMKL) cell line CMK. Subsequent analysis identified two additional JAK3 alleles, V722I and P132T, in AMKL patients. JAK3(A572V), JAK3(V722I), and JAK3(P132T) each transform Ba/F3 cells to factor-independent growth, and JAK3(A572V) confers features of megakaryoblastic leukemia in a murine model. These findings illustrate the biological importance of gain-of-function JAK3 mutations in leukemogenesis and demonstrate the utility of proteomic approaches to identifying clinically relevant mutations.

Constitutive activation of JAK3/STAT3 in colon carcinoma tumors and cell lines: inhibition of JAK3/STAT3 signaling induces apoptosis and cell cycle arrest of colon carcinoma cells

Signal transducer and activator of transcription 3 (STAT3) has oncogenic potential. The biological effects of STAT3 have not been studied extensively in the pathogenesis of colon cancer, nor has the role of Janus kinase 3 (JAK3), the physiological activator of STAT3, been evaluated. Here, we demonstrate that activated STAT3 (pSTAT3) and activated JAK3 (pJAK3) are expressed constitutively in two colon cancer cell lines, SW480 and HT29. To evaluate the significance of JAK3/STAT3 signaling, we inhibited JAK3 with AG490 and STAT3 with a dominant-negative construct. Inhibition of JAK3 down-regulated pSTAT3. The blockade of JAK3/STAT3 signaling significantly decreased viability of colon cancer cells due to apoptosis and cell-cycle arrest through down-regulation of Bcl-2, Bcl-X(L), Mcl-1, and cyclin D2 and up-regulation of p21(waf1/cip1) and p27(kip1). We also examined histological sections from 22 tumors from patients with stage II or stage IV colon cancer and found STAT3, JAK3, and their activated forms to be frequently expressed. Furthermore, quantitative reverse transcriptase-polymerase chain reaction identified JAK3 mRNA in colon cancer cell lines and primary tumors. Our findings illustrate the biological importance of JAK3/STAT3 activation in the oncogenesis of colon cancer and provide novel evidence that JAK3 is expressed and contributes to STAT3 activation in this malignant neoplasm.

Characterization and analysis of the proximal Janus kinase 3 promoter

Janus kinase 3 (Jak3) is a nonreceptor tyrosine kinase essential for signaling via cytokine receptors that comprise the common gamma-chain (gammac), i.e., the receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Jak3 is preferentially expressed in hemopoietic cells and is up-regulated upon cell differentiation and activation. Despite the importance of Jak3 in lymphoid development and immune function, the mechanisms that govern its expression have not been defined. To gain insight into this issue, we set out to characterize the Jak3 promoter. The 5'-untranslated region of the Jak3 gene is interrupted by a 3515-bp intron. Upstream of this intron and the transcription initiation site, we identified an approximately 1-kb segment that exhibited lymphoid-specific promoter activity and was responsive to TCR signals. Truncation of this fragment revealed that core promoter activity resided in a 267-bp fragment that contains putative Sp-1, AP-1, Ets, Stat, and other binding sites. Mutation of the AP-1 sites significantly diminished, whereas mutation of the Ets sites abolished, the inducibility of the promoter construct. Chromatin immunoprecipitation assays showed that histone acetylation correlates with mRNA expression and that Ets-1/2 binds this region. Thus, transcription factors that bind these sites, especially Ets family members, are likely to be important regulators of Jak3 expression.

Interleukin-7 receptor expression and activation in nonhaematopoietic neoplastic cell lines

The high-affinity human interleukin-7 (IL-7)R is a heterodimeric complex consisting of the IL-7Ralpha and common interleukin-2 receptor gamma (IL-2Rgamma(c)) chains. Activation of the IL-7R complex is associated with tyrosine and serine residue phosphorylation of a number of intracellular substrates leading to proliferation and induction of various cellular differentiation processes. In this study, we demonstrate, by S1 nuclease protection assay, immunoprecipitation and in vitro kinase assay that functional human (h) IL-7R is expressed in haematopoietic and nonhaematopoietic cell lines. The National Cancer Institute (NCI) tumour panel of 60 cell lines (NCI60) was screened for the expression of IL-7R mRNA by S1 nuclease protection assay, and IL-7R mRNA was detected in 9 of 12 leukemia, 3 of 7 lung, 4 of 6 CNS, 2 of 7 melanoma, 2 of 7 renal, 1 of 6 colon and 1 of 6 breast cancer cell lines. Immunoblot analysis of haematopoietic, lung cancer and brain tumour cell lines demonstrated expression of IL-7R, IL-2Rgamma(c) and p59 fyn, suggesting that the components of an IL-7R signalling network are present in nonhaematopoietic neoplastic cells. Immunoprecipitation of IL-7Ralpha followed by an in vitro kinase assay demonstrated functional receptor phosphorylation events in the lung cancer cells but not in the brain tumour cell lines. The expression of functional IL-7R on epithelial tumour cells may represent a potential target for receptor-directed therapy.

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.

Janus kinases and signal transducers and activators of transcription: their roles in cytokine signaling, development and immunoregulation

Cytokines play a critical role in the normal development and function of the immune system. On the other hand, many rheumatologic diseases are characterized by poorly controlled responses to or dysregulated production of these mediators. Over the past decade tremendous strides have been made in clarifying how cytokines transmit signals via pathways using the Janus kinase (Jak) protein tyrosine kinases and the Signal transducer and activator of transcription (Stat) proteins. More recently, research has focused on several distinct proteins responsible for inhibiting these pathways. It is hoped that further elucidation of cytokine signaling through these pathways will not only allow for a better comprehension of the etiopathogenesis of rheumatologic illnesses, but may also direct future treatment options.

Binding of IL-4 to the IL-13Ralpha(1)/IL-4Ralpha receptor complex leads to STAT3 phosphorylation but not to its nuclear translocation

Interleukin-4 (IL-4) is a pleiotropic cytokine, which acts on both hematopoietic and non-hematopoietic cells, through different types of receptor complexes. In this study, we report that in human B cells, IL-4 caused rapid phosphorylation of Janus kinase (JAK) 1 and JAK3 tyrosine kinases. In keratinocytes, the hematopoietic-specific receptor common gamma(c) chain is not expressed and the IL-13 receptor alpha(1) (IL-13Ralpha(1)) participates in IL-4 signal transduction. In keratinocytes, IL-4 induced JAK1 and JAK2 phosphorylation but, unlike in immune cells, IL-4 did not involve JAK3 activation for its signaling. In both cell types, IL-4 induced phosphorylation and DNA binding activation of the signal transducer and activator of transcription (STAT) 6 protein. Furthermore, IL-4 stimulation of keratinocytes also induced tyrosine phosphorylation of STAT3 which was found to bind to the phosphorylated IL-13Ralpha(1). STAT3 however did not significantly translocate to the nucleus, nor did it bind with high affinity to target DNA sequences.

Janus kinases and their role in growth and disease

Janus kinases (JAK) play a crucial role in the initial steps of cytokine signaling. Each of the four members (JAK1, JAK2, JAK3, TYK2) of this non-receptor tyrosine kinase family is indispensable for the effects of distinct cytokines. Moreover, recent reports have added to our knowledge on their highly specific functions: JAK3 knockout mice and JAK3 deficient patients cannot signal through the interleukin-2,4,7,9, or 15 receptors and suffer from severe combined immunodeficiency (SCID). JAK1 and JAK2 knockout mice do not survive, their cells again showing distinct patterns of cytokine signaling deficits. At the other end of the spectrum, JAK fusion proteins have been shown to play a role in leukemias. In addition, a new class of JAK-specific inhibitors was described by several groups, the CIS/SOCS/Jab family. This review on the rapidly growing field focuses on JAK function and regulation, and on their emerging role in development and human disease.

Jaks and STATs: biological implications

Cytokines and interferons are molecules that play central roles in the regulation of a wide array of cellular functions in the lympho-hematopoietic system. These factors stimulate proliferation, differentiation, and survival signals, as well as specialized functions in host resistance to pathogens. Although cytokines are known to activate multiple signaling pathways that together mediate these important functions, one of these pathways, the Jak-STAT pathway, is the focus of this chapter. This pathway is triggered by both cytokines and interferons, and it very rapidly allows the transduction of an extracellular signal into the nucleus. The pathway uses a novel mechanism in which cytosolic latent transcription factors, known as signal transducers and activators of transcription (STATs), are tyrosine phosphorylated by Janus family tyrosine kinases (Jaks), allowing STAT protein dimerization and nuclear translocation. STATs then can modulate the expression of target genes. The basic biology of this system, including the range of known Jaks and STATs, is discussed, as are the defects in animals and humans lacking some of these signaling molecules.

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.

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.

Advances in the understanding of cytokine signal transduction: the role of Jaks and STATs in immunoregulation and the pathogenesis of immunodeficiency

Cytokines are of great importance in the growth and differentiation of hematopoietic and other cells. Moreover, they are also crucial in immunoregulation and in host defense. Although our understanding of the molecular basis of cytokine action is far from complete, recent advances have substantially improved our knowledge of cytokine-dependent signal transduction. The delineation of the structure of cytokine receptors and the signaling pathways they utilize has provided clues as to how the strikingly specific effects of cytokines are achieved. Additionally, the basis of some of the pleiotropic and redundant effects of cytokines has also become clear. The discovery of the Janus family of protein tyrosine kinases (Jaks) and the STATs (signal transducers and activators of transcription) has also provided key insights into the mechanism by which intracellular signals are transduced. The following paradigm has emerged: cytokines induce dimerization of receptor subunits that are constitutively associated with Jaks. This activates the Jaks, which then phosphorylate the receptors. The phosphorylated receptors are bound by SH2-containing proteins, one class of which is the STATs. Activated STATs, then, translocate to the nucleus to effect gene transcription. Though the Jaks do not explain much in terms of specificity in signaling, the function of the STATs does. The discovery of patients with autosomal recessive severe combined immunodeficiency due to mutations of a particular Jak, Jak3, and the phenotype of knockout mice lacking Jak3 and various STATs demonstrate the specific and critical roles of these molecules in the development and function of the immune system.

The role of Jak3 in lymphoid development, activation, and signaling

Mutations in a number of lymphoid signaling molecules lead to immunodeficiencies in mice and humans. Among these, one very pleiotropic syndrome results from deficiencies in an array of cytokine signaling pathways utilizing a cytokine receptor common gamma chain, gammac, and the tyrosine kinase Jak3. Recent advances in our understanding of the role of gammac and Jak3 in lymphocyte development and function highlight the importance of cytokine receptor signaling pathways in regulating lymphoid homeostasis and responsiveness.

Genomic structure and promoter region of the murine Janus-family tyrosine kinase, Jak3

Genomic DNA sequences encoding the murine Janus family tyrosine kinase Jak3 were isolated to determine the intron-exon structure of the gene and to investigate the phylogeny of Jak-family kinases. The murine Jak3 gene comprises approximately 15 kbp of genomic DNA and consists of 23 exons. The organization of sequences encoding the pseudo-kinase domain of Jak3 is similar to the intron-exon structure encoding catalytic domains of Src-family tyrosine kinases, whereas the pattern of introns-exons encoding the Jak3 kinase domain shows no structural similarity to that of other tyrosine kinase genes. Genomic analysis further indicates that alternative splicing gives rise to different forms of the murine Jak3 mRNA encoding different isoforms of the Jak3 protein. Analysis of Jak3 intron-exon structure also suggests that a mutation in the human JAK3 gene responsible for a severe combined immune deficiency (SCID) phenotype results from aberrant splicing of the JAK3 transcript. Finally, potential regulatory sequences in the upstream region of the murine Jak3 gene were analyzed and are discussed in relation to the known expression pattern of Jak3.

Leukocyte protein tyrosine kinases: potential targets for drug discovery

Intracellular signal transduction following the extracellular ligation of a wide variety of different types of surface molecules on leukocytes involves the activation of protein tyrosine kinases. The dependence of successful intracellular signaling on the functions of the nontransmembrane class of protein tyrosine kinases coupled with the cell type-specific expression patterns for several of these enzymes makes them appealing targets for therapeutic intervention. Development of drugs that can interfere with the catalytic functions of the nontransmembrane protein tyrosine kinases or that can disrupt critical interactions with regulatory molecules and/or substrates should find clinical applications in the treatment of allergic diseases, autoimmunity, transplantation rejection, and cancer.

Interleukin-4-induced STAT6 recognizes and activates a target site in the promoter of the interleukin-4 receptor gene

This study addresses the function of STAT6 in interleukin-4-stimulated gene expression. A specific STAT6 DNA-binding target site has been identified in the promoter of the interleukin-4 receptor gene, and STAT6 is shown to be involved in mediating activation of gene expression via this site. STAT6 can mediate transcription of a heterologous reporter gene construct containing the interleukin-4 receptor STAT6 binding site. In addition, evidence is provided that demonstrates a distinct effect of STAT6 DNA binding specificity on transcriptional regulation since transcription was not stimulated from a competent but different DNA binding site. To confirm the role of STAT6 in gene activation, STAT6 mutant proteins were generated and analyzed for their ability to function in interleukin-4-induced transcription. Although the interleukin-2 gamma chain receptor subunit has been demonstrated to be a component of the interleukin-4 receptor, it is not required for specific gene activation.

IL4 and IL13 receptors share the gamma c chain and activate STAT6, STAT3 and STAT5 proteins in normal human B cells

IL13 induces the same biological effects as IL4 in normal human B cells. We show that as in the IL4R complex, both IL4R alpha and IL2R gamma c are components of the IL13R and that both cytokines induced STAT6, STAT3 and STAT5 activation in B cells. In spite of this similar downstream signalling, IL4 and IL13 used a different set of Janus kinases: IL13 is unable to activate JAK1 and JAK3.

Immunodeficiencies caused by genetic defects in protein kinases

The recognition that defects of ZAP-70 and, more recently, of JAK3 kinase in humans result in severe combined immunodeficiency, and the demonstration that targeting of these and other protein-kinase genes in mice also leads to immunodeficiency, have highlighted the crucial role that these proteins play in T-cell differentiation and activation.

Expression of Janus kinase 3 in human endothelial and other non-lymphoid and non-myeloid cells

Members of the Janus kinase (Jak) family of protein tyrosine kinases have recently been implicated in the proximal signal transduction events of cytokine receptors. Jak3, a newly discovered member of this family, is believed to be normally limited in its expression to cells of the lymphoid and myeloid lineages. Herein we show that Jak3 is expressed in primary human vascular cells, as well as other non-lymphoid and non-myeloid cell types. Reverse transcriptase-polymerase chain reaction and Northern blot analysis revealed that Jak3 mRNA was expressed at low levels in human umbilical vein endothelial cells (HUVEC), human aortic smooth muscle cells (HASMC), A549 (human lung carcinoma), and DLD-1 (human colon adenocarcinoma) cells. Higher basal levels of Jak3 mRNA were detected in HMEC-1 (human microvascular cell line) and HepG2 (human hepatocellular carcinoma) cells. Jak3 mRNA expression was induced in HUVEC, HMEC-1, and HASMC by treatment with interleukin-1beta, tumor necrosis factor-alpha, interferon-gamma, and lipopolysaccharide. Jak3 protein was detectable at low levels in untreated HMEC-1, and these levels increased significantly with cytokine treatment. Furthermore, Jak3 protein was phosphorylated upon treatment of these cells with interleukin-4. This work shows that Jak3 is expressed or inducible in human vascular endothelial, vascular smooth muscle, and other non-lymphoid and non-myeloid cells, suggesting a broader role for Jak3 in the cytokine signal transduction of these cells.