Influence of onconase in the therapeutic potential of PARP inhibitors in A375 malignant melanoma cells

Human malignant melanoma is one of the most aggressive cancers, accompanied with poor prognosis, metastatic evolution and high mortality. Many strategies have been developed using BRAF and MEK inhibitors in spite of the classic therapy with alkylating agents, but failure related to the ability of the tumor to activate alternative proliferation pathways occurred after promising initial successes. Poly(ADP-ribose) polymerase (PARP) enzymes are well known to be crucial for DNA damage response, and PARP inhibition results in the accumulation of DNA strand breaks that induce cell injury. For this reason, PARP-inhibitors (PARPi) have become promising tools to counteract many cancer types. One of the most used by clinicians is olaparib, that, however, showed again cancer resistance in patients. Thus, new generation molecules have been designed mainly to counteract this problem. Among them, we chose to test AZD2461 on the particularly aggressive human melanoma A375 cell line. This drug is a PARPi significantly less prone than olaparib to undergo the P-glycoprotein-mediated efflux mechanism, one of those responsible for resistance, that in turn is the main adversity in melanoma therapy. Then, we analysed AZD2461 also together with the enzyme onconase (ONC) on the same A375 cells, to investigate if the combination of drugs could possibly increase the in vitro antitumor activity. ONC is a small amphibian "pancreatic-type" ribonuclease that is able to exert a remarkable antitumor activity against many cancers, either in vitro or in vivo, principally because it can evade the ubiquitous ribonuclease cytosolic inhibitor thanks to its structural determinants. Hence, ONC became relevant in the use of protein-drug strategies against incurable cancers. The studies performed in this work showed that both drugs definitely affect A375 cells viability by inducing cytostatic and pro-apoptotic effects in a time- and dose-dependent manner, either if administered alone or in combination. Although we registered low synergistic effects with the combination of the two drugs, we found that AZD2461 did not induce resistance in A375 after two months treatment with high concentration of this molecule. Moreover, we underline that A375 cells treated for a prolonged time with AZD2461 were definitely more susceptible than parental A375 cells to the pro-apoptotic action of ONC. Considering also the different inhibitory effects of the two drugs on TNF-α gene expression and NF-kB DNA-binding, the tuning of their combined delivery to the A375 tumor cell line might open a promising scenario for future therapeutic applications devoted to defeat human melanoma.