In vitro and in vivo inhibition of Chk1 sensitize lung cancer stem cells to chemotherapy

Abstract

Development of resistance to radiation and chemotherapy turns the treatment of solid cancers into a therapeutic challenge. One of the most exciting breakthroughs being explored in cancer research today is the concept of cancer stem cells (CSCs). CSCs are a minority of cells within a tumor that are the source of tumor cell renewal and thereby determine the behavior of tumors, including proliferation, spreading and response to therapy. CSCs are highly resistant to conventional treatment and are therefore emerging as the preferred target of drug therapies in order to obtain eradication of tumors. In this study, we examined the activation of the DNA damage response pathway in CSCs derived from non-small cell lung cancer (NSCLC-CSCs) and their differentiated counterparts after treatment with chemotherapeutic agents commonly used in clinic for lung cancer treatment. Our data show that NSCLC-CSCs possess a highly active DNA damage pathway compared to differentiated progeny and preferentially activate the checkpoint kinase Chk1 in response to DNA damage caused by chemotherapy. This indicates that Chk1 is most likely the main player of drug resistance in NSCLC-CSCs and its targeting might yield significant therapeutic gains. We demonstrate that chemical Chk1 inhibitors dramatically induce NSCLC-CSC death in vitro in combination with DNA damaging drugs. Cell death is induced through a premature activation of the cell cycle regulatory proteins Cdc2 and Cyclin B1, which in turn forces cells with damaged DNA to enter aberrant mitosis, a mechanism known as mitotic catastrophe. Moreover, our results indicate that final cell death occurs through apoptosis. Combination therapy studies have been successfully carried out also in vivo. Chk1 inhibition enhanced the antitumoral effect of conventional chemotherapy in mice xenograft tumor models by increasing tumor latency, potently abrogating tumor growth and reducing tumor mass. We also found a significant reduction of NSCLC-CSCs in xenograft-derived cells, confirming that combination treatment actually targets and reduces the NSCLC-CSCs compartment in vivo. The importance of DNA repair as a resistance mechanism in cancer is a clinically relevant topic and we believe that the combination of selective Chk1 inhibitors with anti-cancer drugs could represent a new therapeutic approach for targeting NSCLC-CSCs and thereby for effective treatment of lung tumors.