Guaianolide Sesquiterpene Lactones from Globe Artichoke (Cynara scolymus L.) Induce Nrf2-Associated Antioxidant Signaling

Background/Objectives: Oxidative and electrophilic stress-response pathways contribute to cellular resilience across various chronic diseases, including neurodegenerative, hepatic, and metabolic disorders. Pharmacological activation of the nuclear factor erythroid 2-related factor 2 (Nrf2)/antioxidant response element (ARE) pathway is a validated strategy to counteract oxidative damage, as evidenced by the clinical approval of dimethyl fumarate (DMF) and monomethyl fumarate (MMF) for relapsing forms of multiple sclerosis; both electrophilic compounds activate Nrf2 via covalent Keap1 (Kelch-like ECH-associated protein 1) modification. Cynara scolymus L. contains structurally related electrophilic metabolites; however, their contribution to Nrf2-associated signaling remains undefined. This study aimed to identify and characterize the constituents responsible for Nrf2-inducing activity and benchmark their potency against DMF and MMF. Methods: Bioactivity-guided fractionation combining Soxhlet extraction, Nrf2/ARE luciferase reporter screening, semi-preparative HPLC, and spectroscopic identification was employed. Functional validation included extracellular thiol quantification, H2O2 cytoprotection assays, and Western blot analysis of heme oxygenase-1 (HO-1). Results: The dichloromethane extract exhibited the highest Nrf2-inducing activity (54.4-fold). Fractionation yielded five guaianolide sesquiterpene lactones (1–5), four of which were active. The α-methylene-γ-lactone moiety was essential for activity. Aguerin B (3) exhibited the highest activity (39.14 ± 11.13-fold), while TBA analysis identified cynaropicrin (2) as the dominant extract-level contributor (62.9% of total activity). Notably, aguerin B (3) and cynaropicrin (2) induced greater reporter activity than DMF and MMF. Downstream pathway induction was confirmed by concentration- and time-dependent HO-1 upregulation, elevated extracellular glutathione and cysteinylglycine levels, and significant protection against H2O2-induced cytotoxicity without intrinsic toxicity. Conclusions: Guaianolide sesquiterpene lactones are the primary mediators of Nrf2-associated antioxidant signaling in C. scolymus. Cynaropicrin (2) exhibited stronger in vitro ARE-reporter induction than fumarates, supporting its relevance for further pharmacological investigation.