The Role of JAK2-CHK2 Signaling in the Mitotic Spindle Assembly
It’s acknowledged that enzymatic regulation of kinases and substrates plays an important role in different cellular signaling pathways of tumorigenesis and cancer. And biologically, cells usually follow a well-regulated cell cycle and cell scanning process to maintain genomic stability, in which the occurrence of genetic mutation proves to be the main underlying cause of human cancers. So a closer look at these checkpoints could provide insights into disease management and the development of treatment strategies.
Recent Research on Checkpoint Kinase
The spindle assembly checkpoint is key to preventing uneven chromosome segregation and aneuploidy, which often leads to neoplastic transformation. Studies have found that checkpoint kinase 2 (CHK2) has a protective effect on mitotic progression. Other than the nucleus, it’s reported that Drosophila CHK2/Mnk localizes to centrosomes, interkinetochore/centromere region, midbody, and pseudocleavage furrows. However, little is understood about the regulation mechanism.
A recent investigation identified a new mitotic phosphorylation site on CHK2 Tyr156 as well as a kinase responsible for it — JAK2, alteration of which proves to be associated with the preferential deletion or amplification of cancer-related genes. Meanwhile, a new JAK2-CHK2 signaling axis is also discovered to be able to maintain genome integrity and may exert influence on genomic stability in the mitotic spindle assembly.
Researchers immunoprecipitated ectopically expressed CHK2 from control or nocodazole (Noc)-treated HEK293T cells and then performed mass spectrometry analysis to determine M phase-specific CHK2 phosphorylation. It turned out that there were numerous phosphorylated residues, in which two phosphorylated tyrosine residues — Tyr156 (Y156) and Tyr159 (Y159) were noticed. Indeed, tyrosine phosphorylation is a relatively rare event. Therefore, researchers further conducted immunoblot analysis with phospho-Tyr (pY)-specific antibodies. Results indicated that CHK2 Y156 phosphorylation might be a mitosis-specific event and importantly, pY156 was excluded from cancer cell-specific events.
Moreover, as previous studies had shown that CHK2 depletion can cause mitotic progression delay, abnormal chromosome alignment, and spindle assembly checkpoint (SAC) deficiency in HCT116 cells, researchers designed experiments and found that CHK2 Y156 phosphorylation is necessary for proper mitotic progression, particularly in protecting the assembly of mitotic spindles.
Then, investigators took advantage of bioinformatics approaches to identify candidate tyrosine kinases that may phosphorylate CHK2 Y156. Among these candidates, JAK2 caught the eye of researchers because losing JAK2 may result in mitotic errors in past reports, and active JAK2, just like active CHK2, localizes to the centrosome. Data drawn from in vitro kinase assays demonstrated that JAK2 can directly phosphorylate CHK2 at Y156. Meanwhile, JAK2 and CHK2 prove to act in the same pathway through the phosphorylation of CHK2 Y156 to ensure proper mitotic progression.
Clinical Implications
It’s confirmed that this study is of clinical significance after analyzing public cancer patient datasets. In accordance with the observations of HeLa cells, loss of JAK2 in human cancers is linked to increased genome alterations, and vice versa, and JAK2 changes, including deep deletion, amplification, and mutations, were identified in a wide range of human metastatic cancers. Higher expression of CHEK2 and JAK2 proves to have a close connection with better survival for patients with lung squamous cell carcinoma, breast cancer, rectal adenocarcinoma, ovarian cancer, and several other cancer types.
In summary, genome alterations caused by lower or lost CHEK2 and JAK2 expression may accelerate cancer progression and predict poor patient survival. JAK2-mediated CHK2 Y156 phosphorylation participates in the regulation of mitotic spindle assembly and protection of the genomic stability, leading to suppression of tumor progression.