Phosphorylation of Y372 is critical for Jak2 tyrosine kinase activation

The oncogenic mechanisms of the Janus kinase-signal transducer and activator of transcription pathway in digestive tract tumors

Digestive tract tumors are heterogeneous and involve the dysregulation of multiple signaling pathways. The Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway plays a notable role in the oncogenesis of digestive tract tumors. Typically activated by pro-inflammatory cytokines, it regulates important biological processes, such as cell growth, differentiation, apoptosis, immune responses, and inflammation. The aberrant activation of this pathway manifests in different forms, including mutations in JAKs, overexpression of cytokine receptors, and sustained STAT activation, and contributes to promoting the malignant characteristics of cancer cells, including uncontrolled proliferation, resistance to apoptosis, enhanced invasion and metastasis, angiogenesis, acquisition of stem-like properties, and drug resistance. Numerous studies have shown that aberrant activation of the JAK-STAT pathway is closely related to the development and progression of digestive tract tumors, contributing to tumor survival, angiogenesis, changes in the tumor microenvironment, and even immune escape processes. In addition, this signaling pathway also affects the sensitivity of digestive tract tumors to chemotherapy and targeted therapy. Therefore, it is crucial to comprehensively understand the oncogenic mechanisms underlying the JAK-STAT pathway in order to develop effective therapeutic strategies against digestive tract tumors. Currently, several JAK-STAT inhibitors are undergoing clinical and preclinical trials as potential treatments for various human diseases. However, further investigation is required to determine the role of this pathway, as well as the effectiveness and safety of its inhibitors, especially in the context of digestive tract tumors. In this review, we provide an overview of the structure, classic activation, and negative regulation of the JAK-STAT pathway. Furthermore, we discuss the pathogenic mechanisms of JAK-STAT signaling in different digestive tract tumors, with the aim of identifying potential novel therapeutic targets. Video Abstract.

Citrulline depletion by ASS1 is required for proinflammatory macrophage activation and immune responses

Citrulline can be converted into argininosuccinate by argininosuccinate synthetase (ASS1) in the urea cycle and the citrulline-nitric oxide cycle. However, the regulation and biological function of citrulline metabolism remain obscure in the immune system. Unexpectedly, we found that macrophage citrulline declines rapidly after interferon gamma (IFN-γ) and/or lipopolysaccharide (LPS) stimulation, which is required for efficient proinflammatory signaling activation. Mechanistically, IFN-γ and/or LPS stimulation promotes signal transducers and activators of transcription 1 (STAT1)-mediated ASS1 transcription and Janus kinase2 (JAK2)-mediated phosphorylation of ASS1 at tyrosine 87, thereby leading to citrulline depletion. Reciprocally, increased citrulline directly binds to JAK2 and inhibits JAK2-STAT1 signaling. Blockage of ASS1-mediated citrulline depletion suppresses the host defense against bacterial infection in vivo. We therefore define a central role for ASS1 in controlling inflammatory macrophage activation and antibacterial defense through depletion of cellular citrulline and, further, identify citrulline as an innate immune-signaling metabolite that engages a metabolic checkpoint for proinflammatory responses.

Analysis of Janus tyrosine kinase phosphorylation and activation

Activation of Janus kinases (Jaks) occurs through autophosphorylation of key tyrosine residues located primarily within their catalytic domain. Phosphorylation of these tyrosine residues facilitates access of substrates to the active site and serves as an intrinsic indicator of Jak activation. Here, we describe the methods and strategies used for analyzing Jak phosphorylation and activation. Tyrosine-phosphorylated (active) Jaks are primarily detected from cell extracts using anti-phosphotyrosine-directed Western blot analysis of Jak-specific immunoprecipitates. Additionally, receptor pull-down and in vitro kinase assays can also be utilized to measure cellular Jak catalytic activity. In addition to tyrosine phosphorylation, recent evidence indicates Jaks can be serine phosphorylated upon cytokine stimulation, however the lack of commercially available antibodies to detect these sites has hindered their analysis by Western blot. However, phosphoamino acid analysis (PAA) has been employed to monitor Jak serine and threonine phosphorylation. Over the past decade, remarkable advances have been made in our understanding of Jak function and dysfunction, however much remains to be learned about their complex regulatory mechanisms.

Tyrosine 201 is required for constitutive activation of JAK2V617F and efficient induction of myeloproliferative disease in mice

The JAK2V617F mutation has been detected in most cases of Ph-negative myeloproliferative neoplasms (MPNs). The JAK2V617F protein is a constitutively activated tyrosine kinase that leads to transformation of hematopoietic progenitors. Previous studies have shown that several tyrosine residues within JAK2 are phosphorylated on growth factor or cytokine stimulation. However, the role of these tyrosine residues in signaling and transformation mediated by JAK2V617F remains unclear. In this study, we sought to determine the role of tyrosine 201, which is a potential binding site for Src homology 2 domain-containing proteins, in JAK2V617F-induced hematopoietic transformation by introducing a tyrosine-to-phenylalanine point mutation (Y201F) at this site. We observed that the Y201F mutation significantly inhibited cytokine-independent cell growth and induced apoptosis in Ba/F3-EpoR cells expressing JAK2V617F. The Y201F mutation also resulted in significant inhibition of JAK2V617F-mediated transformation of hematopoietic cells. Biochemical analyzes revealed that the Y201F mutation almost completely inhibited constitutive phosphorylation/activation of JAK2V617F. We also show that the Y201 site of JAK2V617F promotes interaction with Stat5 and Shp2, and constitutive activation of downstream signaling pathways. Furthermore, using a BM transduction/transplantation approach, we found that tyrosine 201 plays an important role in the induction of MPNs mediated by JAK2V617F.