Based on the previous report that FXR regulated PEPCK indire

Based on the previous report that FXR regulated PEPCK indirectly [20], we speculated that FXR may regulate gluconeogenesis by regulating some key transcription factors associated with gluconeogenesis. Finally, by using HS218 as a probe, we found that FXR binds to PGC-1α promoter and directly regulates the transcription of PGC-1α, while HS218 inhibits PGC-1α gene expression by inhibiting FXR binding to PGC-1α promoter. Furthermore, we also found HS218 could regulate PGC-1α downstream gene expression, thus inhibiting PPARα/FoxO1 pathway. All modulations of HS218 on FXR and PGC-1α lead to the suppression of gluconeogenesis. At present, liver-specific PGC-1α inhibitor SR-18292 has been developed for the treatment of T2DM, because SR-18292 could increase PGC-1α acetylation and decrease PGC-1α activity, leading to gluconeogenesis inhibition and hyperglycemia improvement in ob/ob and HFD mice [49]. Accordingly, our finding that HS218 can inhibit PGC-1α and improve gluconeogenesis may provide a potent basis for the development of anti-T2DM by targeting PGC-1α inhibitors.
Conclusion In summary, we determined a new FXR small molecule antagonist HS218, which was effective in inhibiting gluconeogenesis and improving blood glucose in db/db and HFD/STZ mice. HS218 regulated gluconeogenesis by inhibiting FXR binding to PGC-1α promoter, leading to the suppression of PGC-1α gene expression and inhibition of its downstream pathway (Fig. 8O). Our results have supplied new evidence for the role of FXR in the regulation of gluconeogenesis and a promising strategy for anti-T2DM drug design. Moreover, HS218 might be used as a potential drug lead compound against diabetes.
Author Contributions
Introduction Cholangiocarcinoma (CCA) is a heterogeneous cancer type affecting bile duct epithelial TCS 5861528 (i.e. cholangiocytes) [1]. Its incidence rate is increasing worldwide and represents the second most common primary liver neoplasia and ~3% of all gastrointestinal tumors [2]. The late diagnosis of these tumors together with their high chemoresistance is limiting the available therapeutic options. To date, surgical tumor resection or liver transplantation are the only potentially curative options; however, most CCAs are diagnosed at advanced stages of the disease, when they are no longer amenable for surgery or transplantation [3], and tumor recurrence is elevated. In this scenario, there is an urgent need for novel effective therapeutic strategies for CCA. The etiology of most CCAs is currently unknown. However, a significant number of CCAs arise more frequently under certain cholestatic liver conditions, such as primary sclerosing cholangitis (PSC) and hepatolithiasis, and CCAs themselves may cause cholestasis by tumor-induced biliary obstruction [1]. This obstruction can result in the accumulation of toxic bile acids (BAs), which in turn may promote the development and progression of different gastrointestinal tumors [4]. Particularly, in cholestatic liver diseases, intrahepatic accumulation of BAs does not induce carcinogenesis directly, but it facilitates a cocarcinogenic effect inducing cholangiocyte inflammation and proliferation [5], [6], [7]. BAs are key molecules regulating proliferation, secretion and survival [8], [9]. To exert some of their functions, BAs specifically bind with different affinities to nuclear and membrane receptors, i.e., the farnesoid X receptor (FXR, gene symbol NR1H4) [10], [11], [12], [13] and the G protein-coupled BA receptor 1 (TGR5, gene symbol GPBAR1) [14], respectively. FXR is the member of the nuclear hormone receptor superfamily that preferentially mediates and regulates BA homeostasis and signaling [10], [15], [16], [17], [18], [19]. FXR expression is elevated in tissues exposed to high BA concentration, including cholangiocytes, and is involved in several processes such as lipid and glucose metabolism, inflammation, immunomodulation, fibrosis, regeneration, cell differentiation and proliferation [20], [21], [22], [23], [24], [25]. TGR5 is ubiquitously expressed and also regulates BA homeostasis [15]. In cholangiocytes, TGR5 modulates the bile composition and stimulates the generation of the so-called “bicarbonate biliary umbrella”, which seems crucial to avoid BA-induced toxicity [26]. Of note, both FXR and TGR5 are relevant in different cancers. FXR functions as a tumor suppressor [25], whereas TGR5 promotes tumorigenesis by stimulating cell proliferation and survival in certain cancers [27], [28]. Therefore, both BA receptors are currently considered potential targets for therapy in cancer. New BA derivatives have been developed to selectively activate FXR or TGR5. Obeticholic acid (OCA) [also known as INT-747 (6α-ethyl-chenodeoxycholic acid)] is a synthetic BA derivative that activates FXR with high affinity and selectivity [29]. OCA is currently under clinical evaluation for the treatment of several diseases such as non-alcoholic steatohepatitis (NASH), PSC and biliary atresia [30], [31], and has recently been approved for the treatment of primary biliary cholangitis (PBC) [32]. On the other hand, INT-777 (6α-ethyl-23(S)-methyl-3α,7α,12α-trihydroxyl-5β-cholan-24-oic acid) [33] is a synthetic BA derivative that, with high affinity and selectivity, activates TGR5. In different animal models, INT-777 has been reported to control glucose homeostasis, hamper atherosclerosis by macrophage regulation, prevent diabetic kidney disease and control weight gain as well as adiposity by modulating cAMP and energy expenditure [34], [35], [36].