Checkpoint kinase 1 (Chk1) is a cell cycle regulator and a heat shock protein 90 (Hsp90) client. It is essential for cell proliferation and survival.
Accumulating evidence indicates that Checkpoint kinase 1 (CHEK1) plays an essential role in tumor cells and that it could induce cell proliferation and could be related to prognosis in multiple types of cancer. However, the biological role and molecular mechanism of CHEK1 in GBM still remain unclear.
In a study, Bai et al. identified that CHEK1 expression was enriched in glioblastoma (GBM) tumors and was functionally required for tumor proliferation and that its expression was associated to poor prognosis in GBM patients. Mechanically, CHEK1 induced radio resistance in GBM cells, and CHEK1 knockdown increased cell apoptosis when combined with radiotherapy via regulation of the DNA repair/recombination protein 54L (RAD54L) expression. Therapeutically, we found that CHEK1 inhibitor attenuated tumor growth both in vitro and in vivo. Collectively, CHEK1 promotes proliferation, induces radio resistance in GBM, and could become a potential therapeutic target for GBM 1).
CHK1, both at the mRNA and protein level, is highly expressed in medulloblastoma and elevated CHK1 expression in Group3 medulloblastoma is an adverse prognostic marker. CHK1 inhibition with the small-molecule drug AZD7762, results in decreased cell growth, increased DNA damage and cell apoptosis. Furthermore, AZD7762 acts in synergy with cisplatin in reducing cell proliferation in medulloblastoma. Similar phenotypic changes were observed with another CHK1 inhibitor, PF477736, as well as genetic knockdown using siRNA against CHK1. Treatments with small-molecule inhibitors of CHK1 profoundly modulated the expression of both upstream and downstream target proteins within the CHK1 signaling pathways. This suggests the presence of a feedback loop in activating CHK1. Overall, our results demonstrate that small-molecule inhibition of CHK1 in combination with, cisplatin, is more advantageous than either treatment alone, especially for Group 3 medulloblastoma, and therefore this combined therapeutic approach serves as an avenue for further investigation 2).
Cellular sensing of DNA damage, along with concomitant cell cycle arrest, is mediated by a great many proteins and enzymes. One focus of pharmaceutical development has been the inhibition of DNA damage signaling, and checkpoint kinases (Chks) in particular, as a means to sensitize proliferating tumor cells to chemotherapies that damage DNA. 7-Hydroxystaurosporine, or UCN-01, is a clinically relevant and well-studied kinase activity inhibitor that exerts chemosensitizing effects by inhibition of Chk1, and a multitude of Chk1 inhibitors have entered development. Clinical development of UCN-01 has overcome many initial obstacles, but the drug has nevertheless failed to show a high level of clinical activity when combined with chemotherapeutic agents. One very likely reason for the lack of clinical efficacy of Chk1 inhibitors may be that the inhibition of Chk1 causes the compensatory activation of ATM and ERK1/2 pathways. Indeed, inhibition of many enzyme activities, not necessarily components of cell cycle regulation, may block Chk1 inhibitor-induced ERK1/2 activation and enhance the toxicity of Chk1 inhibitors. This review examines the rationally hypothesized actions of Chk1 inhibitors as cell cycle modulatory drugs as well as the impact of Chk1 inhibition upon other cell survival signaling pathways. An understanding of Chk1 inhibition in multiple signaling contexts will be essential to the therapeutic development of Chk1 inhibitors 3).
Yang et al. investigated the radiosensitivity can be enhanced with treatment of Chk1 inhibitor, AZD7762 in lung cancer cell lines and xenograft models of lung cancer brain metastasis. Clonogenic survival assays showed enhancement of radiosensitivity with AZD7762 after irradiation of various doses. AZD7762 increased ATR/ATM-mediated Chk1 phosphorylation and stabilized Cdc25A, suppressed cyclin A expression in lung cancer cell lines. In xenograft models of lung cancer (PC14PE6) brain metastasis, AZD7762 significantly prolonged the median survival time in response to radiation. Depletion of Chk1 using shRNA also showed an enhancement of sensitivity to radiation in PC14PE6 cells. The results of this study support that Chk1 can be a good target for enhancement of radiosensitivity 4).
Transient expression of dominant-negative CHK1 raised basal ERK1/2 activity and prevented CHK1 inhibitors from activating ERK1/2. CHK1 inhibitors modestly increased the levels of PARP1 ADP ribosylation and molecular or small-molecule inhibition of PARP1 blocked CHK1 inhibitor-stimulated histone H2AX phosphorylation and activation of ERK1/2. Stimulated histone H2AX phosphorylation was ataxia telangiectasia-mutated protein-dependent. Multiple CHK1 inhibitors interacted in a greater than additive fashion with multiple PARP1 inhibitors to cause transformed cell-killing in short-term viability assays and synergistically killed tumor cells in colony-formation assays. Overexpression of BCL-xL or loss of BAX/BAK function, but not the function of BID, suppressed CHK1 inhibitor + PARP1 inhibitor lethality. Inhibition of BCL-2 family protein function enhanced CHK1 inhibitor + PARP1 inhibitor lethality and restored drug-induced cell-killing in cells overexpressing BCL-xL. Thus, PARP1 plays an important role in regulating the ability of CHK1 inhibitors to activate ERK1/2 and the DNA damage response. An inability of PARP1 to modulate this response results in transformed cell death mediated through the intrinsic apoptosis pathway 5).
Nomura et al. analyzed the mechanisms of Chk1 regulation in U87MG glioblastoma cells using Geldanamycin (GA), which interferes with the function of Hsp90. GA reduced Chk1 protein level but not its mRNA level in glioblastoma cells. Co-treatment with GA and cycloheximide (CHX), a protein synthesis inhibitor, induced a decrease of half-life of the Chk1 protein to 3h and resulted in Chk1 down-regulation. CHX alone induced only 32% reduction of Chk1 protein even after 24h. These findings indicated that reduction of Chk1 by GA was due to destabilization and degradation of the protein. In addition, GA-induced down-regulation of Chk1 was reversed by MG132, a specific proteasome inhibitor. And it was revealed that Chk1 was ubiquitinated by GA. These results have indicated that degradation of Chk1 by GA was mediated by the ubiquitin-proteasome pathway in U87MG glioblastoma cells 6).