山苦瓜萃物暨其區分物及一些苦味分子對腸道內分泌細胞株 STC-1 分泌 GLP-1 之影響

Abstract

第二型糖尿病對健康的影響已成為一個重大的公共健康問題。由腸道 L 細胞分泌的 GLP-1 能刺激胰臟 β 細胞分泌胰島素，並增加 β 細胞增生及減少細胞凋亡，並能藉由延遲胃排空而減緩飯後血糖之上升。因此，GLP-1 類似物或其分解酵素 DPP-4 之抑制劑已成為治療糖尿病之藥物之一。近來研究顯示：苦味受器亦表現於 L 細胞及 STC-1 腸內分泌細胞株。本實驗室先前研究顯示，山苦瓜苦味物質粗萃物及苦味物質 denatonium benzoate 能刺激腸道內分泌細胞株 STC-1 細胞分泌 GLP-1。本研究目的為：以腸道內分泌細胞株 STC-1 模式，篩選偵測不同山苦瓜萃物及其水解物促進 GLP-1 分泌之活性，並測試一些已知之苦味分子是否亦能刺激此細胞分泌 GLP-1。 實驗結果發現，山苦瓜水萃物及富含苦味物質之區分物具有最佳之活性。而除了強酸水解萃物 2Ba15-E 及 3Ba18-E 外，其他萃物水解後之有機溶劑萃物亦具活性。純化合物方面，苦瓜所含之 karavilagenin E 及 oleanolic acid 亦具活性，但人參之 ginsenoside Re 則否。已知 oleanolic acid 為膽酸膜受器 TGR5 之 agonist，推測其可能透過此受器促進 STC-1 分泌 GLP-1。所測試之苦味化合物中，高濃度下具有苦味之人工甜味劑 acesulfame K、saccharin，以及苦味分子 chloroquine、brucine、denatonium benzoate、quinine、caffeine、strychnine、allyl isothiocyanate、limonin 均具有促進腸道內分泌細胞分泌 GLP-1 之活性，salicin 及 phenylthiocarbamide 則否。其中，可專一性活化 TAS2R38 苦味受器之 allyl isothiocyanate 具最佳之活性，然此活性可被 probenecid 所抑制。Probenecid 為 TAS2R16 與 TAS2R38 之 allosteric inhibitor，意味 allyl isothiocyanate 促進 GLP-1 分泌之作用係透過苦味受器 TAS2R38。另一方面，probenecid 亦可部分抑制山苦瓜水萃物及山苦瓜苦味物質粗萃物促進腸道內分泌細胞分泌 GLP-1 之活性，推測山苦瓜中可能含有苦味受器 TAS2R16 或 TAS2R38 之 agonist，並透過苦味受器刺激 GLP-1 分泌。 綜之，本研究結果顯示山苦瓜水萃物、富含苦味物質之區分物及多數之水解萃物以及十種已知苦味物質皆能刺激 GLP-1 分泌。苦味抑制劑添加後對數種樣品之抑制現象，顯示部分苦味物質可能透過腸內分泌細胞之苦味受器之作用，促進 GLP-1 之分泌，對血糖調控有所助益。

The prevalence of type 2 diabetes and impaired glucose homeostasis have become a significant public health concern. Glucagon-like peptide-1 (GLP-1) secreted by intestinal L cells is an enteric hormone that stimulates insulin secretion, increase the β cell proliferation, decreased β cell apoptosis, and slow down the rise in postprandial plasma glucose by delayed gastric emptying. Thus, GLP-1 receptor agonists and DPP-4 inhibitors have been introduced as the newest class of glucose-lowering agents for the treatment of type 2 diabetes mellitus. The intestinal L cells and the enteroendocrine cell line, STC-1, have been shown to express bitter taste receptors (T2Rs). Previous findings in our laboratory revealed that the crude bitter extract of bitter gourd (BGP-bi) and denatonium benzoate could enhance GLP-1 secretion in STC-1 cells. We hypothesized that one of the mechanisms for the hypoglycemic effect of BG might be through enhancing GLP-1 secretion in the enteroendocrine cells via T2Rs. Treatment with Water Extract(WE) and BGP-bi resulted in the highest GLP-1 secretion in the STC-1 cells. Except for the 2 extracts of strong acid hydrolysates, 2Ba15-E and 3Ba18-E, most extracts of hydrolyzed products also had this activity. Karavilagenin E and oleanolic acid, two pure triterpenoid compounds from BG were also active, but not ginsenoside Re. The oleanolic acid is an agonist of TGR5, a bile acid receptor expressed in the STC-1 cells. This receptor might contribute, at least in part, to its activity. Among bitter compounds tested, acesulfame K, saccharin, chloroquine, brucine, denatonium benzoate, quinine, caffeine, strychnine, allyl isothiocyanate, limoin could stimulate GLP-1, but not salicin and phenylthiocarbamide. Among these, allyl isothiocyanate showed highest activity. As this compound is a specific agonist of TAS2R38, ad its activity could be inhibited by probenecid, an allosteric inhibitor of TAS2R16 and TAS2R38, the stimulation of GLP-1 secretion by allyl isothiocyanate might be through TAS2R38. Moreover, probenecid also partially inhibited GLP-1 secretion of BG WE and bitter enriched fraction, suggesting these BG extracts might contain agonists of TAS2R16 or TAS2R38 and stimulate GLP-1 secretion through these receptors. In conclusion, water extracts, bitter rich fractions, most hydrolyzed extracts, Karavilagenin E and oleanolic acid from BG and 10 bitter compounds have been shown to stimulate GLP-1 secretion in STC-1 cells. Based on the fact that the stimulation could be inhibited by a specific inhibitor of taste receptor, it is concluded that bitter taste receptors might be involved in the GLP-1 secretion of STC-1 cells stimulated by bitter compounds, including those in bitter gourd. It would be interested to further study the glucose regulation potential of these bitter compounds as well.