Curcumin-induced heme oxygenase-1 expression plays a negative role for its anti-cancer effect in bladder cancers

► Curcumin inhibits cell growth and metastasis of human bladder cancer cells 5637. ► Curcumin induces HO-1 mRNA and protein expression in bladder cancer cells. ► HO-1 knockdown or inhibition enhances curcumin-inhibited cancer cell invasion. ► Curcumin induces HO-1 expression in the lung and bladder tumor tissue in vivo. ► HO-1 inhibitor should be suggested to enhance the anti-cancer effect of curcumin.