The nuclear factor κB (NF-κB) family of transcription factors comprises five members forming hetero- and homo-dimers that play essential roles in physiological and pathological processes (Oeckinghaus and Ghosh, 2009; Zhang et al., 2017). Proinflammatory cytokines, viruses, bacterial components, and DNA damage activate the classical NF-κB signaling pathway (Hoesel and Schmid, 2013). These stimulations lead to the activation of the inhibitor of NF-κB (IκB) kinase complex, which phosphorylates IκBα (Chen and Greene, 2004). Phosphorylated IκBα is immediately ubiquitinated and degraded by the 26S proteasome, liberating NF-κB dimers to translocate to the nucleus (Bhoj and Chen, 2009). NF-κB dimers promote the expression of diverse genes involved in inflammation, immune responses, proliferation, differentiation, and survival (Karin and Greten, 2005; Hoesel and Schmid, 2013). Therefore, the investigation and development of new small-molecule compounds targeting the NF-κB signaling pathway have attracted extensive research attention.

Alantolactone (Fig. 1A) is a eudesmane-type sesquiterpene lactone isolated from herbal plants belonging to the Asteraceae family (Babaei et al., 2021; Cai et al., 2021; Liu et al., 2021). To date, alantolactone has been shown to exert a number of biological effects, including antitumor and anti-inflammatory activities, in various models (Babaei et al., 2021; Cai et al., 2021; Liu et al., 2021). Regarding its anti-inflammatory activity, previous studies reported that alantolactone inhibited the NF-κB-dependent pathway (Chun et al., 2012; Khan et al., 2012; Lei et al., 2012; Wei et al., 2013; Wang et al., 2017; Cui et al., 2018; Liu et al., 2018; He et al., 2019; Ren et al., 2019; Tan et al., 2019; Dang et al., 2020; Zhu et al., 2020; Chuo et al., 2021). Alantolactone possesses an α-methylene-γ-lactone moiety in its chemical structure. The α-methylene-γ-lactone moiety undergoes a Michael-type addition with biological nucleophiles and forms covalent bonds with cysteine residues via sulfhydryl groups in many functional proteins and enzymes (Kupchan et al., 1970). In the NF-κB signaling pathway, natural and synthetic compounds possessing the α-methylene-γ-lactone moiety have been reported to target IκB kinase β (Cys-179) and the NF-κB family member RelA (Cys-38) (Rossi et al., 2000; García-Piñeres et al., 2001; Kwok et al., 2001). A molecular docking study revealed that alantolactone targeted the ATP-binding site of IκB kinase β (Wang et al., 2017).

The RIKEN Natural Products Depository (NPDepo) has constructed chemical libraries with a collection of natural products and their derivatives (Kato et al., 2012). To search for novel anti-inflammatory compounds, the NPDepo chemical library was used for screening based on cell-ELISA assays to quantitate the expression of intercellular adhesion molecule-1 (ICAM-1) on human umbilical vein endothelial cells (HUVEC) stimulated with proinflammatory cytokines and ligands of Toll-like receptors. In the course of this screening, alantolactone derivatives (Fig. 1B) were identified as inhibitors of ICAM-1 expression induced by inflammatory cytokines and ligands of Toll-like receptors. In the present study, the biological activities of alantolactone derivatives were further investigated in human lung adenocarcinoma A549 cells stimulated with tumor necrosis factor-α (TNF-α). The results obtained showed that alantolactone derivatives prevented the TNF-α-induced NF-κB pathway by a different mechanism from alantolactone.