Isothiocyanates in Brassica Vegetables Induce Glucagon Like Peptide-1 Secretion in Enteroendocrine Cells Through Intestinal Bitter Taste Receptors

Bitter taste receptors (T2Rs) have been found in the gastrointestinal tract and their activation was linked with improving glucose homeostasis, mainly through inducing the secretion of gut hormones. Brassicaceae vegetables are rich in glucosinolates (GSLs), which upon hydrolysis, produce bioactive isothiocyanates (ITCs) with a bitter and pungent flavor. We hypothesized that GSLs and/or ITCs may interact with the intestinal T2Rs and induce the secretion of glucagon-like peptide-1 (GLP-1). The murine enteroendocrine cell line STC-1 cells were treated with allyl isothiocyanate (AITC), phenethyl isothiocyanate (PEITC), benzyl isothiocyanate (BITC), sulforaphane (SF), or sinigrin (SN) at 6.25, 12.5, and 25.0 μM, or vehicle (0.1% DMSO) for 1 hour for active GLP-1 measurement. To test the mechanism of ITCs interacting with T2Rs to induce GLP-1 secretion, 2-APB (IP3 receptor antagonist, 50 μM), A967079 (TRPA1 channel inhibitor, 30 μM), TPPO (TRPM5 channel inhibitor, 10 μM) and EGTA (extracellular Ca2+ chelator,1 mM) were added with AITC (12.5 μM) or vehicle (0.1% DMSO) to cells for 1 hour and active GLP-1 levels were measured. Student's t-test was used to compare two treatments. A P-value < 0.05 was considered statistically significant. AITC (25.0 μM), BITC (12.5, 25.0 μM), and SF (6.25, 12.5, 25.0 μM) increased GLP-1 secretion compared to the vehicle (P < 0.05), whereas PEITC and SN showed no impact (P > 0.05). When comparing different ITCs, SF (6.25 μM) tended to induce higher GLP-1 secretion than AITC (6.25 μM) (P = 0.05). 2-APB, A967079 and EGTA significantly reduced GLP-1 secretion in AITC treated cells compared to AITC only (P < 0.05), whereas no changes were observed in GLP-1 levels in the vehicle group with/without the addition of the inhibitors 2-APB and EGTA. AITC at 25.0 μM is more potent at inducing GLP-1 secretion compared to its parent GSL, SN. Other ITCs, including BITC and SF, also increased GLP-1 secretion. AITC induces GLP-1 secretion, potentially through T2R pathways mediated by IP3, TRPA1, and the extracellular Ca2+. This study may provide evidence for a novel therapeutic mechanism explaining brassica vegetables improving glucose homeostasis. This study was supported in part by the MOE program, DNS, UIUC. Anqi Zhao was supported by the USDA NIFA Grant (2019–38,420-28,973) to the DNS, UIUC.