Heteronemin induces ROS-dependent apoptosis and mitophagy-associated responses through SDHB-associated mitochondrial Complex II dysfunction in oral squamous cell carcinoma.

Oral squamous cell carcinoma (OSCC) is difficult to treat because of its aggressive growth and limited therapeutic response. Heteronemin (HET), a marine sesterterpenoid, has anticancer activity, but its mitochondrial effects in OSCC remain incompletely defined. In this study, we investigated the relationship between HET-induced cytotoxicity, succinate dehydrogenase complex iron-sulfur subunit B (SDHB)-related Complex II dysfunction, and reactive oxygen species (ROS)-dependent mitochondrial injury using OSCC cells (HSC-3 and Ca9-22), normal oral cells (HGF-1), and zebrafish xenografts. HET selectively suppressed OSCC cell viability and colony formation while inhibiting cell-cycle progression. HET increased intracellular ROS and mitochondrial superoxide, impaired mitochondrial respiration, reduced SDHB abundance, and decreased succinate dehydrogenase activity. SDHB knockdown altered HET-induced ROS accumulation and limited the additional cytotoxic effect of HET, supporting the functional involvement of SDHB-associated mitochondrial dysfunction. N-acetyl-L-cysteine and MitoTEMPO attenuated oxidative stress, mitochondrial depolarization, apoptosis, and cell death. HET also increased mitophagy-associated proteins, including LC3B, PINK1, Parkin, BNIP3, and Nix, and promoted autophagosome/autolysosome formation. Transmission electron microscopy showed damaged mitochondria enclosed within autophagic vacuoles. Furthermore, NAC reduced these mitophagy-related changes, In zebrafish xenografts, HET reduced OSCC tumor without overt toxicity and increased mitophagy-associated markers. Together, these findings indicate that HET-induced OSCC cytotoxicity is associated with SDHB-related Complex II dysfunction, ROS accumulation, mitochondrial injury, intrinsic apoptosis, and mitophagy-associated responses.