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Most chemotherapeutic drugs used clinically evoke cell apopt
Most chemotherapeutic drugs used clinically evoke cell apoptosis by inducing DNA damage. However, the high efficiency of DNA repair due to the overexpression of DNA repair proteins in cancer thymidylate synthase inhibitors reduces the drug efficacy significantly (Fink et al., 1996, 1998). For example, the expression level of DNA polymerase beta (Pol β) has been correlated with resistance of cancer cells to chemotherapeutic drugs (Lawson et al., 2011). Cells with higher levels of DNA ligase IV exhibit reduced levels of γ-H2AX foci (an early marker of DNA damage in cells) upon treatment with DNA damage agents (Srivastava et al., 2012). Moreover, patients with DNA repair efficiency defects are more sensitive to chemotherapy (Riballo et al., 1999). Based on the roles of FEN1 in DNA repair, we speculate that inhibition of FEN1 could lead to the generation of DNA lesions, thus sensitize cancer cells to chemotherapy.
Breast cancer remains the most common cancer in females, and its incidence continues to rise (Hutchinson, 2010). There is an urgent demand for novel drugs effective in treating breast cancer. In this study, we showed that FEN1 is overexpressed in breast cancer. Using the MCF7 breast cancer cell line as a research model, we demonstrated that FEN1 is essential for proliferation and drug resistance in breast cancer cells. Furthermore, we identified a FEN1 inhibitor, SC13. SC13 blocks FEN1 activity specifically and impairs DNA replication and repair in vitro and in cells. SC13 suppresses cell growth, resulting in the accumulation of DNA double strand breaks (DSBs) in cells, thereby culminating into cytotoxicity. Finally, using mouse cancer models, we showed that SC13 impedes progression of cancer growth, causing a significant increase in the sensitivity of cancers toward chemotherapy.
Materials and Methods
Results
Discussion
Chemotherapy remains the most powerful tool for cancer treatment (Nguyen et al., 2014). Cancer cells are characterized by their ability to divide rapidly. As a critical enzyme for DNA replication, overexpression of FEN1 is believed to be a biomarker of cancer cells (Wang et al., 2014; Abdel-Fatah et al., 2014). Indeed, we and other groups demonstrated that FEN1 is overexpressed in various types of cancers (Fig. 1). We hypothesized that inhibition of FEN1 could suppress cancer growth by blocking DNA synthesis. Supporting our hypothesis, siRNA-mediated downregulation of FEN1 in MCF7 cells retarded cell proliferation significantly (Fig. 2c). The FEN1 inhibitor SC13 also impaired DNA replication and cell growth, leading to the accumulation of cells in G1 phase (Fig. 5a–c).
Besides blocking cell division, most anticancer agents used clinically cause DNA lesions to induce cell apoptosis. However, in many cases, an elevated DNA repair capacity in cancer cells leads to drug resistance and severely limits the efficacy of chemotherapeutic drugs (Kaina and Christmann, 2002). Therapeutic efficacy is determined by the balance between DNA damage and repair. Given the roles of FEN1 in DNA repair, we speculated that manipulation of FEN1 in cancer cells could alter the response of cancer cells to DNA damage agents. Indeed, we demonstrated that cancer cells with high FEN1 levels are more resistant to DNA damage agents such as TMZ, cisplatin, and 5FU (Fig. 2i–j). At the same time, FEN1knockdown sensitizes cancer cells to DNA damage. Furthermore, applying the FEN1 inhibitor SC13 augmented the cytotoxicity of anticancer drugs, both in cells and in a xenograft cancer mouse model (Figs. 5-7).
The dual function of FEN1 makes it an ideal target for cancer therapy. The FEN1 inhibitor can be used independently to suppress DNA replication, inducing DNA damage and apoptosis of cancer cells. Moreover, FEN1 inhibitor can also be combined with other DNA damage-inducing agents. In addition to suppressing cell growth by inhibiting DNA replication, FEN1 inhibitor can also lead to the accumulation of DNA DSBs and genome instability (Fig. 5h, j). Thus, the cytotoxicity induced by FEN1 inhibitor could be due to the combination of defects in DNA replication and repair.