Exploring the Biochemical Mechanism Beyond the Cytotoxic Activity of Sesquiterpene Lactones from Sicilian Accession of Laserpitium siler Subsp. siculum (Spreng.) Thell

<i>Laserpitium siler</i> subsp. <i>siculum</i> (Apiaceae) is a Mediterranean plant with a long history of traditional medicinal use. In this study, the chemical composition and anticancer potential of three novel (and one new to the genus) sesquiterpene lactones (SLs) isolated from its roots were investigated. The structural characterization, carried out through NMR and HPLC-MS analyses, identified unique guaianolide-type lactones. The biological activity of these compounds was evaluated in vitro using MDA-MB-231 cells, a triple-negative breast cancer (TNBC) cell line. Cell viability assays demonstrated that all SLs tested reduced TNBC cell viability in a dose- and time-dependent manner, with SL-1 exhibiting the highest cytotoxicity. Light microscopy analyses and acridine orange/ethidium bromide staining confirmed the induction of apoptotic cell death, further supported by Western blot analyses showing caspase-3 activation and PARP-1 cleavage. Additional experiments indicated that SL-1 induced oxidative stress, as evidenced by increased ROS production and upregulation of the levels of the antioxidant enzymes MnSOD and HO-1. Moreover, JC-1 staining and Western blot analyses revealed mitochondrial membrane depolarization as well as a significant reduction in VDAC-1 expression, suggesting mitochondrial dysfunction as a key event in the cytotoxic mechanism. These findings highlight <i>L</i>. <i>siler</i> subsp. <i>siculum</i> as a promising source of bioactive compounds with anticancer potential. The ability of its sesquiterpene lactones to induce oxidative stress and mitochondrial impairment provides new insights into their mode of action, supporting further research into their therapeutic applications for TNBC treatment.