Etoposide-induced cancer cell death: roles of mitochondrial VDAC1 and calpain, and resistance mechanisms

Etoposide is a DNA topoisomerase II inhibitor, disrupting an enzyme critical for DNA transcription, replication, and chromosome segregation. While it is well established that etoposide induces cell death through DNA damage, our findings suggest that multiple molecular mechanisms contribute to its apoptotic effects. Specifically, etoposide promotes the overexpression and oligomerization of mitochondrial voltage-dependent anion channel 1 (VDAC1), forming a mega-channel that facilitates the release of pro-apoptotic proteins and initiates apoptosis.
Additionally, etoposide induces the C-terminal truncation of VDAC1 (VDAC1-ΔC) via proteolytic cleavage by calpain-1 and asparagine endopeptidase (AEP). Inhibition of calpain effectively prevents etoposide-induced VDAC1-ΔC formation, apoptosis, and the nuclear translocation of apoptosis-inducing factor (AIF). Etoposide also upregulates key apoptosis regulators (p53, Bax, p21, AIF) alongside calpain and AEP. Notably, VDAC1 truncation and etoposide-induced apoptosis are cell-type dependent, correlating with calpain expression levels and activity.
Moreover, VDAC1-ΔC formation and apoptosis share similar concentration- and time-dependent patterns, are suppressed by calpain and AEP inhibitors, as well as the VDAC1 oligomerization inhibitor VBIT-4, and require intracellular Ca2+. These findings underscore the complexity of etoposide’s mechanisms across different cellular contexts, suggest potential resistance pathways, identify possible biomarkers for guiding etoposide-based therapies, and highlight VDAC1 and calpain as promising therapeutic targets in cancer treatment.