Caspase cascade in the apoptosis induced by camptothein in IOZCAS-Spex-II cells

Camptothecin (CPT) has been found to have insecticidal and fungicidal activities, and gives potential to be developed for use in agrochemicals. Our previous studies demonstrate that caspase-mediated intrinsic apoptotic pathways regulate CPT-induced apoptosis in Spodoptera exigua. However, the underlying mechanisms for caspase cascades remains unclear. In the present study, six caspases (Cas-1 to -6) were identified in IOZCAS-Spex-II cells derived from S. exigua fat bodies. Phylogenetic, biochemical, and immunoblot analyses confirmed a conserved Cas5-Cas1 cascade in apoptosis induced by CPT and its derivative hydroxycamptothecin (HCPT). Cas-5 functioned as the initiator caspase, showing early upregulation and autoactivation. Cas-1 acted as the executioner via proteolytic cleavage. Pre-treatment of cells with Z-VAD-FMK before CPT exposure significantly reduced cytotoxicity by 75.7 % at 48 h and 86.8 % at 72 h. Furthermore, the caspase activities, as well as the cleavage of Cas-1 and Cas-5 triggered by CPT, were effectively suppressed. Additionally, the relative mRNA expression analysis revealed that S. exigua caspases exhibited different responses between CPT and HCPT in IOZCAS-Spex-II cells. Beyond the Cas5-Cas1 cascade activation, HCPT treatment induced stronger upregulation of Cas-2, Cas-3, and Cas-6 compared to CPT treatment. These differential expression profiles suggested that HCPT may engage additional caspase cascade in IOZCAS-Spex-II cells. Given that S. exigua is a major agricultural pest, understanding these apoptotic pathways could reveal potential targets for pest control by disrupting its apoptotic regulation.