Isospongian Diterpenoids from the Leaves of Amomum tsao-ko Promote GLP-1 Secretion via Ca/CaMKII and PKA Pathways and Inhibit DPP-4 Enzyme.

Three uncommon isospongian diterpenoids including a new one, 3-epi-kravanhin A (2), were isolated from the leaves of Amomum tsao-ko. Compounds 2 and 3 dose-dependently promoted GLP-1 secretion on STC-1 cells with promotion ratios of 109.7 % and 186.1 % (60 μM). Mechanism study demonstrated that the GLP-1 stimulative effects of 2 and 3 were closely related with Ca/CaMKII and PKA pathways, but irrelevant to GPBAR1 and GPR119 receptors. Moreover, compound 1 showed DPP-4 inhibitory activity with an IC value of 311.0 μM. Molecular docking verified the binding affinity of 1 with DPP-4 by hydrogen bonds between the γ-lactone carbonyl (C-15) and Arg61 residue. Bioinformatics study indicated that compound 1 exerted antidiabetic effects by improving inflammation, oxidative stress and insulin resistance. This study first disclosed the presence of isospongian diterpenoids in A. tsao-ko, which showed antidiabetic potency by promoting GLP-1 secretion and inhibiting DPP-4 activity.