Role of Chk1 in the differentiation program of hematopoietic stem cells

Checkpoint kinase 1 is essential for fetal and adult hematopoiesis

Checkpoint kinase 1 (CHK1) is critical for S-phase fidelity and preventing premature mitotic entry in the presence of DNA damage. Tumor cells have developed a strong dependence on CHK1 for survival, and hence, this kinase has developed into a promising drug target. Chk1 deficiency in mice results in blastocyst death due to G2/M checkpoint failure showing that it is an essential gene and may be difficult to target therapeutically. Here, we show that chemical inhibition of CHK1 kills murine and human hematopoietic stem and progenitor cells (HSPCs) by the induction of BCL2-regulated apoptosis. Cell death in HSPCs is independent of p53 but requires the BH3-only proteins BIM, PUMA, and NOXA. Moreover, Chk1 is essential for definitive hematopoiesis in the embryo. Noteworthy, cell death inhibition in HSPCs cannot restore blood cell formation as HSPCs lacking CHK1 accumulate DNA damage and stop dividing. Moreover, conditional deletion of Chk1 in hematopoietic cells of adult mice selects for blood cells retaining CHK1, suggesting an essential role in maintaining functional hematopoiesis. Our findings establish a previously unrecognized role for CHK1 in establishing and maintaining hematopoiesis.

Combined inhibition of Chk1 and Wee1 as a new therapeutic strategy for mantle cell lymphoma

Mantle cell lymphoma (MCL) is an aggressive, incurable disease, characterized by a deregulated cell cycle. Chk1 and Wee1 are main regulators of cell cycle progression and recent data on solid tumors suggest that simultaneous inhibition of these proteins has a strong synergistic cytotoxic effect. The effects of a Chk1 inhibitor (PF-00477736) and a Wee1 inhibitor (MK-1775) have been herein investigated in a large panel of mature B-cell lymphoma cell lines. We found that MCL cells were the most sensitive to the Chk1 inhibitor PF-00477736 and Wee1 inhibitor MK-1775 as single agents. Possible involvement of the translocation t(11;14) in Chk1 inhibitor sensitivity was hypothesized. The combined inhibition of Chk1 and Wee1 was strongly synergistic in MCL cells, leading to deregulation of the cell cycle, with increased activity of CDK2 and CDK1, and activation of apoptosis. In vivo treatment with the drug combination of mice bearing JeKo-1 xenografts (MCL) had a marked antitumor effect with tumor regressions observed at non-toxic doses (best T/C%=0.54%). Gene expression profiling suggested effect on genes involved in apoptosis. The strong synergism observed by combining Chk1 and Wee1 inhibitors in preclinical models of MCL provides the rationale for testing this combination in the clinical setting.

Ovarian carcinoma tumor-initiating cells have a mesenchymal phenotype

Solid tumors appear to contain a subpopulation of cells (tumor-initiating cells, TICs) that not only drives and sustains tumor growth, but is possibly responsible for recurrence. We isolated, after enzymatic digestion of primary ovarian carcinoma samples, a subpopulation of cells propagating as non-adherent spheres in medium suitable for tumor stem cells. These cells were able to self-renew in vitro, as suggested by PKH-26 staining studies, were tumorigenic and acquired an epithelial morphology when grown in FBS-supplemented medium, losing their tumorigenic potential. Interestingly, the tumorigenic potential of PKH-26 (high) - and PKH-26 (neg) -sorted cells was similar. These TIC-enriched cultures showed higher levels of genes involved in stemness than differentiated cells derived from them and were more resistant to the cytotoxic effects of some drugs but equally sensitive to others. The higher level of ABCG2 efflux pump could explain increased resistance to taxol and VP16, and higher levels of genes involved in nucleotide excision repair partially explain the resistance to cisplatin. These cells express mesenchymal markers, and epithelial transition could be induced when cultured in differentiating conditions, with a loss of invasive potential. These data suggest that ovarian cancer is a stem cell disease and should help elucidate the role of these cells in the aggressive phenotype of this tumor and find new therapeutic strategies to reduce resistance to current chemotherapeutic drugs.