Indigodole E from Strobilanthes cusia exhibits anti-IL-17A effect

Abstract One new indazole alkaloid, indigodole E (1), was isolated from a traditional Chinese medicine Qing Dai prepared from the aerial parts of Strobilanthes cusia. The structure of 1 was elucidated by NMR, MS, UV, and IR spectra as well as optical rotation. Additionally, compound 1 could obviously inhibit not only IL-17A protein production at concentrations from 1.25 to 2.5 μg/mL, but also IL-17 gene expression at concentrations from 5.0 to 10.0 μg/mL without cytotoxicity toward Th17 and Jukat cells, respectively. Overall, indazole analogue 1 could be the anti-IL 17 A contributor of Qing Dai in this investigation. Graphical Abstract


Introduction
Indole alkaloids are a kind of natural compounds which were biosynthesized from the precursors of tryptophan or tryptamine and then metabolized to produce a large structural variety of indole analogues (Rosales et al. 2020). Numerous pharmacological properties, including anticancer, anti-inflammatory, antibacterial, antifungal, antiviral, antimalarial, antiparasitic, anti-acetylcholinesteras, and anti-butyrylcolinesterase activities have been reported for indole alkaloids (Rosales et al. 2020).
The aerial part of Strobilanthes cusia (Acanthaceae) owns indole compounds (Sun et al. 2021) and could be processed to manufacture Qing Dai used as a traditional Chinese medicine (TCM) in Asia (Lee et al. 2019). It was found to ameliorate inflammatory bowel disease and psoriasis in clinical studies (Naganuma 2019;Sun et al. 2021). Additionally, Qing Dai was also proved to reverse the IL-17A gene expression in human psoriatic skin lesions (Cheng et al. 2017), and our previous studies indicated that the indole alkaloids, indigodoles A, C, D, cephalandole B, tryptanthrin, and indirubin could contribute to anti-IL-17A properties of Qing Dai (Lee et al. 2019;. Therefore, the active phytochemicals of aforementioned TCM are interesting for us. In this study, continuing on chromatographic fractionation of Qing Dai provided one new alkaloid (1) and its IL-17A inhibitory effect was also discussed within.
In anti-IL-17 bioassay, primary mouse CD4þ T lymphocytes were obtained from BALB/c mice and then polarized into Th17 cells in order to investigate the anti-IL-17 effect of compound 1. Consequently, compound 1 could obviously inhibit IL-17A protein production of Th17 cells at concentrations from 1.25 to 2.5 lg/mL ( Figure 9SB) without cytotoxicity (cell viabilities > approximately 80%; Figure 9SA). In the IL-17 luciferase reporter assay, compound 1 could inhibit the IL-17 gene expression at concentrations from 5.0 to 10.0 lg/mL in Jukat cells (immortalized T lymphocytes) which were transfected with IL-17 luciferase reporters ( Figure 10). Our previous studies indicated that the indole alkaloids, indigodoles A, C, D, cephalandole B, tryptanthrin, and indirubin could contribute to anti-IL-17 properties of Qing Dai (Lee et al. 2019(Lee et al. , 2020. However, compound 1 is an indazole analogue which has a five-membered pyrazole ring instead of the pyrrole ring in indole alkaloids. Indazole alkaloid 1 obtained in this work could be the anti-IL-17 candidate of Qing Dai as well.

Conclusions
In this study, one new indazole alkaloid, indigodole E (1), was obtained from Qing Dai and showed inhibition against IL-17A protein production and gene expression. Consequently, compound 1 could be further researched to apply for IL-17A related diseases, such as psoriasis.

Supplementary materials
The detailed experimental procedures and all spectroscopic data of 1 are available online at https://doi.org/10.1080/14786419.2022.2041633.