Curcumin-Induced G2/M Cell Cycle Arrest in Colorectal Cancer Cells

Curcumin, a diet-derived chemopreventive and chemotherapeutic agent has been shown to induce G2/M cell cycle arrest, and previous studies suggested that microtubule depolymerisation may be linked to M-phase arrest. However, mechanisms involved have not been elucidated and often non-physiological concentrations of curcumin were used. The goal of this study was to characterise in more detail curcumin-induced cell cycle arrest using a panel of human colorectal cancer cell (CRC) lines, HT-29, SW480, HCT116 p53+/+, HCT116 p53-/- and HCT116 p21-/-. Using physiologically relevant concentrations of curcumin (5-10μM), achievable in the gut tissue following oral ingestion, cell cycle analysis showed that treatment for 12 hours results in significant G2/M arrest in all five cell lines. Curcumin treatment significantly increased the number of cells in M phase in 4 out of the 5 lines tested for this duration, and those with microsatellite instability (HCT116) were found to have a higher mitotic index than those with chromosomal instability. Pre-treatment with caffeine abrogated mitotic arrest in these cell lines, indicating the involvement of the ATM/ATR kinases. Activating phosphorylation of the Chk1 kinase was increased and total protein levels of CDC25C reduced, further implicating the DNA damage pathway in the induction of arrest. Higher levels of HSP70 were also found, indicating proteotoxic stress such as proteasomal inhibition. Image analysis revealed impaired mitotic progression, and significantly higher levels of mitotic spindle abnormalities following curcumin treatment. Aurora B mislocalisation and significantly lower levels of centrosomal separation were found in the HCT116 p53+/+ line. Furthermore, the high levels of pH2A.X staining seen in curcumin-treated mitotic but not interphase cells suggest that aberrant mitosis may result in DNA damage. This proteotoxic and genotoxic stress incurred following curcumin treatment may contribute to the upregulation of NKG2D ligands on the cell surface, leading to CRC lysis and enhancement of the anti-cancer immune response.