Melanoma Xenografts Growth Inhibition by Solanum Tuberosum Aspartic Proteases 3 (StAP3) Treatment

Abstract

The swaposin domain is present in plant aspartic proteases (APs) inserted into the C-terminal domain as an extra region known as plant-specific insert (PSI). The PSI domain interacts with the plasma membrane, causing cell permeabilization, thus killing plant and human pathogens. A typical Solanum tuberosum AP (StAP) named as StAP3 was demonstrated to have cytotoxic activity against cancer cells and no effect in normal cells in vitro. The toxicity of StAP3 was assessed in a mice model, showing no signs of systemic toxicity. Malignant melanoma (MM) is one of the most aggressive cancers, with high metastatic ability and resistant to therapies. Thus, the pursuing of novel agents against MM is still challenging. Herein, we aimed to evaluate the in vivo antitumor effect of StAP3 in MM. Subcutaneous A375 human melanoma xenografts in athymic nude mice were induced. Once tumors developed (mean larger dimension = 3.8 ± 0.09 mm), mice were treated with StAP3 (6 µg/mg body weight, subcutaneously under the tumor at a single dose) or physiologic solution (controls). Animal experiments complied with the ARRIVE guidelines and were performed with protocols approved by the Argentine National Atomic Energy Commission Animal Care in accordance with the EU Directive 2010/63/EU and NIH Publications No. 8023, revised 1978. A significant inhibition of MM tumors growth was observed in StAP3-treated mice (p < 0.05) vs. controls. This was detected immediately after treatment and was sustained until 15 days post-treatment, with a maximum inhibition of 76%, when control tumors reached 200 mm3 and animals were sacrificed. As far as we know, this is the first report showing the in vivo effect of a plant AP whose action mechanism would be mediated by membrane destabilization, which may be explained by the plasma membrane composition with high levels of phosphatidylserine in the outer leaflet of cancer cells. These results suggest the potential of these plant proteases as anticancer agents.