Induction of GLP-1 secretion from STC-1 cells. (A) Cells were treated with different concentrations of curcumin, unstable analogue 1, stable analogue 6, and KCl as positive control, and release of GLP-1 to the supernatant was quantified by ELISA. (B) Cells were treated with unstable analogues (1–5) of curcumin (C) and stable analogues (BCP, 6, 7) at 20 μM concentration, and the release of GLP-1 was quantified by ELISA. (C) An MTT assay was performed to test viability of STC-1 cells treated with curcumin and analogs for 2 h. Data represent the mean ± S.D. of 3 independent experiments. *p < 0.05 or **p < 0.005 versus control (Dunnett test).