Curcumin and its cyclohexanone analogue inhibited human Equilibrative nucleoside transporter 1 (ENT1) in pancreatic cancer cells
Our group investigated combining the phytochemical curcumin and gemcitabine in a liposome, to improve gemcitabine's activity against pancreatic tumours. While optimising the curcumin: gemcitabine ratio for co-encapsulation, we found that increasing curcumin concentrations relative to gemcitabine resulted in antagonistic interactions. As curcumin is a promiscuous transporter inhibitor; we suspected that increased resistance occurred via inhibition of Equilibrative nucleoside transporter 1 (ENT1)-mediated gemcitabine uptake. To test our hypothesis, we determined whether curcumin and a related analogue, 2,6-bis((3-methoxy-4-hydroxyphenyl)methylene)-cyclohexanone (or A13), inhibited ENT1-mediated accumulation of [3H]uridine and [3H]gemcitabine into pancreatic cancer cells. We then confirmed the inhibition of gemcitabine accumulation by investigating whether curcumin/A13 could increase gemcitabine resistance in growth inhibition assays. We found that curcumin and A13 concentration-dependently inhibited the ENT1-mediated accumulation of both uridine and gemcitabine in MIA PaCa-2 and PANC-1 cells. We also found that non-toxic concentrations of curcumin and A13 significantly increased the resistance of both cell lines to gemcitabine. Increased resistance only occurred when curcumin/A13 was co-incubated with gemcitabine, and not with sequential exposure (i.e., curcumin first, followed by gemcitabine, or vice versa). We also found that the curcumin analogue (3E,5E)−3,5-bis[(2-fluorophenyl)methylene]−4-piperidinone (or EF24) did not inhibit gemcitabine accumulation, making it more suitable in combinations than curcumin/A13. From these results, we concluded that curcumin and A13 are inhibitors of the ENT1 transporter, but only at high concentrations (2–20 μM). Curcumin is unlikely to inhibit gemcitabine uptake in tumours but may interfere with the oral absorption of ENT1 substrates due to high gut concentrations readily achievable from over-the-counter tablets/capsules.