Copper supplementation amplifies the anti-tumor effect of curcumin in oral cancer cells

BACKGROUND: Oral cancer is the sixth most common cancer worldwide and 90% of oral malignancies are caused by oral squamous cell carcinoma (OSCC). Curcumin, a phytocompound derived from turmeric (Curcuma longa) was observed to have anti-cancer activity which can be developed as an alternative treatment option for OSCC. However, OSCC cells with various clinical-pathological features respond differentially to curcumin treatment.

HYPOTHESIS: Intracellular copper levels have been reported to correlate with tumor pathogenesis and affect the sensitivity of cancer cells to cytotoxic chemotherapy. We hypothesized that intracellular copper levels may affect the sensitivity of oral cancer cells to curcumin.

METHODS: We analysed the correlation between intracellular copper levels and response to curcumin treatment in a panel of OSCC cell lines derived from oral cancer patients. Exogenous copper was supplemented in curcumin insensitive cell lines to observe the effect of copper on curcumin-mediated inhibition of cell viability and migration, as well as induction of oxidative stress and apoptosis. Protein markers of cell migration and oxidative stress were also analysed using Western blotting.

RESULTS: Concentrations of curcumin which inhibited 50% OSCC cell viability (IC50) was reduced up to 5 times in the presence of 250 µM copper. Increased copper level in curcumin-treated OSCC cells was accompanied by the induction of intracellular ROS and increased level of Nrf2 which regulates oxidative stress responses in cells. Supplemental copper also inhibited migration of curcumin-treated cells with enhanced level of E-cadherin and decreased vimentin, indications of suppressed epithelial-mesenchymal transition. Early apoptosis was observed in combined treatment but not in treatment with curcumin or copper alone.

CONCLUSION: Supplement of copper significantly enhanced the inhibitory effect of curcumin treatment on migration and viability of oral cancer cells. Together, these findings provide molecular insight into the role of copper in overcoming insensitivity of oral cancer cells to curcumin treatment, suggesting a new strategy for cancer therapy.