CHK2 Promotes Anoikis and is Associated with the Progression of Papillary Thyroid Cancer

Cell cycle checkpoint kinase 2 (CHK2) performs essential cellular functions and might be associated with tumorigenesis and tumor progression. Here, we explored the function and molecular mechanisms of CHK2 in the progression of papillary thyroid cancer (PTC).

The expression levels of both total CHK2 and activated CHK2 (p-CHK2) in tissues from 100 PTC patients were detected and evaluated using immunohistochemistry. The roles of CHK2 on cell proliferation, invasion, migration, apoptosis and cancer stem cell (CSC) markers were investigated by CCK-8, Transwell, flow cytometry, western blot and ALDEFLOUR assay. PTC cells cultured in suspension conditions were assayed for anoikis. The anchorage-independent condition was further detected by soft agar colony formation assay. Furthermore, anoikis associated regulatory proteins were explored by western blot and verified by forced downregulation experiment, respectively.

We found that the levels of both CHK2 and p-CHK2 were significantly upregulated in PTC cancer tissues compared with those in tumor-adjacent tissues. The overexpression of p-CHK2 in primary tumor tissues was associated with tumor aggressiveness and metastatic potential. However, the levels of both CHK2 and p-CHK2 were decreased in metastatic lymph nodes. Our results showed that CHK2 upregulated the levels of CSC markers with no effect on cell proliferation, invasion and migration. Interestingly, we revealed a previously undescribed anoikis-promoting role for CHK2 in PTC. Specifically, the detachment of PTC cells from the extracellular matrix (ECM) triggers CHK2 degradation. Then, the forced downregulation of CHK2 rescued PTC cells from anoikis, but no effect was observed on the apoptosis of adherent PTC cells. Additionally, as a novel regulator of anoikis, CHK2 can induce cell death in a p53-independent manner via the regulation of PRAS40 activation.

High expression levels of CHK2 and p-CHK2 were associated with the progression of PTC. Our results defined an unexpected role for CHK2 as a mediator of anoikis that functions through the regulation of PRAS40 activation, which may be associated with the survival of circulating tumor cells and metastatic behavior.