Onconase cytotoxicity in melanoma cancer cells: promising results to be enforced by oligomerizing the protein

Onconase (ONC) is a 104 residues basic protein extracted from the Rana Pipiens frog oocytes which belongs to the “pancreatic-type” ribonuclease super-family. In fact, although RNase A and ONC share less than 30% sequence identity, they display similar folds. Contrarily to most monomeric RNases, ONC evades the cytosolic RNase inhibitor and exerts a remarkable cytotoxic activity against cancer cells1, which display more negatively membranes than normal cells. ONC cytotoxicity has already been tested against some cancer lines and exploited in clinical trials against non-small-lung cancer and unresectable malignant mesothelioma2. Human melanoma, an aggressive malignancy characterized by rapidly growing incidence and high mortality rate, is resistant to radiation therapy and cytotoxic chemotherapy3. Hence, we evaluated here the ONC effects against melanoma registering a remarkable cytotoxic effect in the A375 melanoma cell line, while negligible in NHEM melanocytes. We also investigated with WB some aspects underlying these encouraging results. Then, we recently found that ONC can dimerize through the 3D domain swapping mechanism and that its cytotoxicity increases against pancreatic cancer cells upon dimerization4. Hence, we are evaluating the ONC dimerization effect in A375 cells. Notably, native ONC can dimerize only through the swapping of its N-termini because its C-terminal ends are blocked by the C87-C104 disulfide. Considering that RNase C-swapped oligomers are generally more active than the N-swapped ones5, we are producing some ONC mutants devoted to enhance the oligomerization tendency of the protein. This should increase the ONC cytotoxic activity and contemporarily counteract undesired side-effects.