Four unprecedented macrocyclic nonapeptides, orberryamides A-D (1-4), were isolated from Glycosmis pentaphylla (orangeberry) and structurally characterized by obtaining solid data from numerous analytical measurements. Compound 1 incorporated a new amino acid residue, named orangeberrine (Orgb), compounds 2 and 3 integrated the rare, nonproteinogenic amino acid residues (Kyn and Dioia, respectively), and compound 4 existed as two major conformational isomers in solution at ambient temperature. The biosynthetic pathways proposed for compounds 1-4 are of considerable biological significance for the modification and metabolism of tryptophan (Trp) and/or Trp containing proteins in nature. Besides, compounds 1-4 suppressed Th17 differentiation significantly, and the effects of 1-3 was achieved through targeting the ligand-binding domain (LBD) of the retinoic acid-related orphan receptor gamma t (RORγt).

RORγt is the master transcription factor of IL-17 cytokine expression and Th17 lymphocyte differentiation, which are responsible for the induction of many autoimmune diseases. Recently, RORγt has become an attractive target for drug development to treat these types of diseases, and the field of RORγt antagonist research is now extremely competitive. In our current study, molecular docking was applied to demonstrate that cardenolides, including uscharin, calactin, and calotropin derived from Calotropis gigantea, probably directly bind to RORγt. Therefore, the inhibitory effect was further validated using a luciferase reporter assay. Because RORγt is the key transcriptional factor for Th17 differentiation, the effects of these compounds on Th17 differentiation were studied by flow cytometry. The results showed that uscharin, calactin, and calotropin inhibited Th17 differentiation from 100 to 500 nM. Furthermore, uscharin had a better effect than digoxin, a well-known inverse agonist of RORγt, in reducing Th17 polarization. Additionally, the effects of the cardenolides on the differentiation of other Th lineages, including Th1, Th2, and Treg, were investigated. Uscharin suppressed Th1, Th2, and Treg cell differentiation, while calactin suppressed the differentiation of Th1 cells, and calotropin did not influence the other T cell subsets, indicating that calactin suppressed Th1 and Th17 differentiation, and calotropin selectively quenched Th17 polarization. Structural analysis of the three compounds showed that the selectivity of uscharin, calactin, and calotropin on the suppression of the different subsets of T cells is correlated to the minor differences in their chemical structures. Collectively, calactin and calotropin have greater potential to be developed as lead compounds than uscharin to treat autoimmune diseases mediated by Th17 and/or Th1 cells.

Primary bile acids (BAs), products of cholesterol metabolism and clearance, are synthesized in the liver and released into the intestine to facilitate the digestion and absorption of lipids. BAs are further converted by gut commensal bacteria into secondary colonic BAs and the metabolism disorder is closely linked to cholestatic liver diseases via regulating immune response. However, the effect and underlying mechanism of these host‐microorganism biliary metabolites on T lymphocyte remain unclear. In the current study, we synthesized a sulfated product of lithocholic acid (LCA), lithocholic acid 3‐sulfate (LCA‐3‐S), and investigated the binding affinity of the BAs metabolites on RORγt, the transcription factor of IL‐17A. Our results demonstrated that the sulfate of LCA, LCA‐3‐S, exhibited better effect than its oxidated metabolite, 3‐oxo‐LCA, binding to RORγt. The results further demonstrated that LCA‐3‐S selectively suppressed Th17 cell differentiation without influence on Th1, Th2, and Treg cells. Collectively, we synthesized the sulfated biliary metabolite LCA‐3‐S and demonstrated that LCA‐3‐S selectively inhibited Th17 cell differentiation by targeting RORγt, indicating that metabolite disorder of BAs resulting in the decrease of LCA‐3‐S probably contributes to the pathogenesis of cholestatic liver diseases.