Synthetic Lethality of K‐Ras Mutant Human Colorectal Cancer Cells by Phytochemical Curcumin and FDA‐Approved Targeted Drug Regorafenib

Synthetic lethality refers to the interaction between two co‐essential genes such that inhibiting the function of either gene separately results in cell survival, but inhibiting the function of both genes results in cell death. Targeting synthetic lethal interactions may spare normal cells while selectively killing cancer cells. Curcumin, a major yellow pigment and spice in turmeric and curry, has been demonstrated to have anticarcinogenic effect in vitro , in vivo , and in human clinical trials. Colorectal cancer (CRC) is the third most commonly diagnosed cancer and the forth cause of cancer death worldwide. Mutation of K‐Ras, a Ras superfamily of protein which regulates cell proliferation and survival, has been shown in 35–45% of CRC. Regorafenib is an oral inhibitor that targets multi‐kinase activity (eg. Raf) which involves in tumor angiogenesis, oncogenesis, and tumor microenvironment. Recently, Regorafenib has been approved by the US FDA to treat patients with CRC despite the genotype of K‐Ras after failure of using standard therapies. However, administration of Regorafenib only improved overall survival by nearly 1.4 months in patients with treatment‐refractory metastatic CRC. Here we reveal that curcumin significantly enhanced the cytotoxicity of human CRC HCT 116 cells (K‐Ras mutant) to Regorafenib to a greater extent than that of human CRC HT‐29 cells (K‐Ras wild‐type), producing an additivity or synergy effect on HCT 116 cells and causing an antagonism effect on HT‐29 cells. Flow cytometric analysis displayed that the addition of curcumin elevated the percentages of HCT 116 cells at the sub‐G1 phase, representing induction of apoptosis, as well as greatly increased the cells with acidic vesicular organelles, representing the formation of autolysosomes at the later stage of autophagic flux. Expression of cleaved caspase 3 (at the later stage of apoptosis) and LC3‐II (at the early stage of autophagic process) using Western blot analysis confirmed the induction of apoptosis and autophagy of the HCT 116 cells. No induction of apoptosis or autophagy was observed on HT‐29 cells in response to curcumin, Regorafenib, or combination of these two compounds. It is noteworthy that addition of MEK specific inhibitor (U0126) not only induced apoptosis on Regorafenib‐treated HT‐29 cells, but also behaved like curcumin to enhance Regorafenib‐induced cytotoxicity, apoptosis, and autophagy on HCT 116 cells. Our data suggest that the addition of curcumin may target one more gene other than mutant K‐Ras and enhanced Regorafenib‐induced cytotoxicity (synthetic lethality) on CRC HCT 116 cells, indicating a possible role of curcumin on Regorafenib‐treated K‐Ras mutant CRC. Support or Funding Information (Supported in part by the Ministry of Science and Technology, Taiwan, No. MOST 104‐2320‐B‐003‐007 and National Taiwan Normal University, Taiwan, No. 105T0700).