Veliparib (ABT-888), a PARP inhibitor potentiates the cytotoxic activity of 5-fluorouracil by inhibiting MMR pathway through deregulation of MSH6 in colorectal cancer stem cells
Subarno Paul, Subhajit Chatterjee, Saptarshi Sinha, Somya Ranjan Dash, Rajalaxmi Pradhan, Biswajit Das, Kunal Goutam, Chanakya Nath Kundu
Abstract
OBJECTIVE: Sensitization of mismatch repair (MMR)-deficient colorectal cancer (CRC) cells by 5-Fluorouracil (5-FU) is well-documented. But not much is known about the treatment of MMR-proficient CRC cancer stem cells (CRC-CSCs). Here, we investigated whether a PARP inhibitor (ABT-888) can enhance the 5-FU-mediated apoptosis in CRC-CSCs through MMR pathway inhibition. METHODS: The anti-cancer action of 5-FU+ABT-888 combination in CRC-CSCs has been studied by using in vitro, ex vivo, and in vivo preclinical model systems. RESULTS: 5-FU caused DNA damage in CRC-CSCs, and ABT-888 enhanced the accumulation of DNA mismatches by downregulating the MMR pathway, triggering S-phase arrest, and finally apoptosis and cell death in 5-FU-pre-treated MMR-proficient-CRC-CSCs at much lower concentrations than their individual treatments. After 5-FU treatment, PARylated-PARP1 activated MMR pathway by interacting with MSH6. But, upon ABT-888 treatment in 5-FU-pre-exposed CSCs, PARylation was inhibited, as a result of which PARP1 could not interact with MSH6, and other MMR proteins were downregulated. The role of MSH6 in PARP1-mediated MMR activation, was confirmed by silencing MSH6 gene, which resulted in MMR pathway shutdown. Similar results were obtained in ex vivo and in vivo model systems. CONCLUSIONS: 5-FU+ABT-888 combination enhanced CRC-CSCs death by increasing DNA damage accumulation and simultaneously inhibiting the MMR pathway in MMR-proficient cells. But this study does not discuss whether the combination treatment will increase the sensitivity of MMR-deficient CSCs, for which further research will be performed in the future.