The Antimicrobial Peptide gamma-Thionin from Habanero Chile (Capsicum chinense) Induces Caspase-Independent Apoptosis on Human K562 Chronic Myeloid Leukemia Cells and Regulates Epigenetic Marks

Cancer is a relevant health problem worldwide. In 2020, leukemias represented the 13th most commonly reported cancer cases worldwide but the 10th most likely to cause deaths. There has been a progressive increase in the efficacy of treatments for leukemias; however, these still generate important side effects, so it is imperative to search for new alternatives. Defensins are a group of antimicrobial peptides with activity against cancer cells. However, the cytotoxic mechanism of these peptides has been described mainly for animal defensins. This study shows that defensin gamma-thionin (Capsicum chinense) is cytotoxic to the K562 leukemia cells with an IC50 = 290 mu g/mL (50.26 mu M) but not for human peripheral blood mononuclear cells. Results showed that gamma-thionin did not affect the membrane potential; however, the peptide modified the mitochondrial membrane potential (Delta Psi m) and the intracellular calcium release. In addition, gamma-thionin induced apoptosis in K562 cells, but the activation of caspases 8 and 9 was not detected. Moreover, the activation of calpains was detected at one hour of treatment, suggesting that gamma-thionin activates the caspase-independent apoptosis. Furthermore, the gamma-thionin induced epigenetic modifications on histone 3 in K562 cells, increased global acetylation (similar to 2-fold), and specific acetylation marks at lysine 9 (H3K9Ac) (similar to 1.5-fold). In addition, gamma-thionin increased the lysine 9 methylation (H3K9me) and dimethylation marks (H3K9me2) (similar to 2-fold), as well as the trimethylation mark (H3K9me3) (similar to 2-fold). To our knowledge, this is the first report of a defensin that triggers caspase-independent apoptosis in cancer cells via calpains and regulating chromatin remodelation, a novel property for a plant defensin.