高糖化對 TRPC 通道之影響及甘草次酸經由抑制 TRPC 調控 sRAGE 釋放之機轉

Impairment of blood glucose is a major cause of receptor for advanced glycation endproducts (RAGE) activation, increasing the probability of diabetic complications. Recent studies have suggested that soluble RAGE (sRAGE) improved diabetes mellitus by down-regulation of RAGE signaling cascades. For this reason, sRAGE is considered as a potential protein for diabetes therapy. sRAGE is an isoform of RAGE, which is classified into esRAGE (endogenous secretory RAGE) and cRAGE (cleaved RAGE). The mechanisms of sRAGE secretions are regulated by calcium. Calcium channels, canonical transient receptor potential channels (TRPC), were associated with oxidative stress-activated diabetic complications. Nevertheless, the role of TRPC channels is unkown in diabetes mellitus. 18β-glycyrrhetinic acid (18βGA), which is poduced form glycyrrhizin in intestines, has been shown to possess several beneficial pharmacological activities, such as anti-ulcerative, anti-inflammatory and anti-diabetic properties. Thus, the aim of this study was to investigate the regulatory effect of 18βGA on esRAGE and cRAGE secretions via TRPC channels in high glucose (HG)-induced THP-1 cells. The results showed that high glucose enhanced the mRNA and protein expressions of TRPC3 and TRPC6, resulting in elevation of the intracellular [Ca2+]. Moreover, high glucose increased the protein expressions of NADPH oxidase 2 (Nox2) and inducible nitric synthase (iNOS), while decreased the protein expression of uncoupling protein 2 (UCP2). The excessive intracellular reactive oxygen species (ROS) led to attenuation of esRAGE secretion in HG-induced THP-1 cells. On the other hand, elevating intracellular [Ca2+] induced the innitial protein expression of ADAM10 (≦ 6 h), while decreased the final protein expression of ADAM10 (≧ 12 h). Meanwhile, the productions of sRAGE and cRAGE were diminished. In addition, the inhibititions of TRPC3 and TRPC6 retarded the intracellular [Ca2+] to raise the levels of sRAGE. The results implied that TRPC played an important role in the production of sRAGE. 18βGA suppressed the protein expressions of TRPC3 and TRPC6 to retarded the intracellular [Ca2+]. Lower intracellular [Ca2+] decreased the protein expressions of Nox2 and iNOS, but increased the protein expression of UCP2. The intracellular ROS was also decreased, resulting in esRAGE enhancement. Moreover, 18βGA increased the level of cRAGE through inhibition of HG-induced inflammation. Taken together, our research demonstrates that 18βGA enhances the secretion of sRAGE via suppressing TRPC channels in HG-induced THP-1 cells. Therefore, 18βGA provides a potential natural complement to diabetes mellitus and its complications. Keywords: sRAGE, TRPC, UCP2, ADAM10, 18β-glycyrrhetinic acid