Novel insights into the mechanism of action of grayanotoxin III

Grayanotoxins are natural products that occur in species belonging to the Ericaceae family and contribute to plant toxicity [1]. They bind to ion channels and cause membrane depolarization [2]. Ion channels are implicated in the progression of cancer [3,4,5]. Therefore, we investigated the effect of grayanotoxin III (GTX III) on cell viability and induction of apoptosis, as well as the underlying mechanisms of GTX III triggered cell death in the HL-60 leukemia cell line. Cell viability decreased as evaluated by WST-1 assay and Western blot analyses indicated caspase cleavage after GTX III exposure. In addition, p38 MAP kinase was phosphorylated pointing to a p38 regulated apoptosis pathway. Preincubation with BAPT AM (cell permeable calcium chelator), Ruthenium Red (blocker of Ca²⁺ uptake and release from mitochondria), MDL 28170 (calpain inhibitor) and dibucaine (voltage-gated sodium channel blocker) before GTX III treatment decreased the cleavage of caspase-9 to its active form. These data suggest an increase in intracellular Ca²⁺ following GTX III exposure, as well as the implication of ion channels and Ca²⁺ in GTX III-induced cell death signaling. Thus, our findings show that GTX III decreased cell viability and induced p38 MAP kinase, Ca²⁺ and voltage-gated sodium channel dependent apoptosis in the HL-60 cell line.

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