Okanin Suppresses the Growth of Colorectal Cancer Cells by Targeting at Peroxiredoxin 5

Okanin is a natural product with few known biological activities. Its anti-cancer effects and the underlying mechanisms are investigated. It is found that okanin inhibits cancer cell growth (25-50 µm) with minimal effects on non-cancerous colorectal cells except at much higher doses (i.e., > 100 µm). In colorectal HCT116 cancer cells, okanin binds directly to peroxiredoxin 5 (PRDX5) at a site opposite the catalytic domain, which directly inhibits the enzymatic activity and triggers the production of reactive oxygen species, leading to independent apoptosis and ferroptosis. The binding also causes WSB1-mediated ubiquitination degradation of PRDX5, resulting in reduced transcription and SIAH2-mediated ubiquitination degradation of GPX4, which similarly causes apoptosis and ferroptosis. In xenograft mouse models, okanin decreases the PRDX5 level and inhibits the growth of HCT116 cells, both of which are compromised when cells stably overexpressing PRDX5 are used. Okanin does not change the body weight of the animals; in comparison, 5-fluorouracil reduces the body weight, despite being less effective. In conclusion, the results suggest that okanin targets PRDX5, which capacitates it for anti-cancer activity via apoptosis and ferroptosis independently. Okanin is a promising investigational drug. PRDX5 and GPX4 are candidate targets for cancer chemotherapy, at least for colorectal cancer.