Mn porphyrin in combination with ascorbate acts as a pro-oxidant and mediates caspase-independent cancer cell death

Resistance to therapy-mediated apoptosis in inflammatory breast cancer (IBC), an aggressive and distinct subtype of breast cancer, was recently attributed to increased superoxide dismutase (SOD) expression, glutathione (GSH), and decreased accumulation of reactive species. In the present study, we demonstrate the unique ability of two Mn(III) N-substituted pyridylporphyrin (MnP)-based SOD mimics (MnTE-2-PyP5+ and MnTnBuOE-2-PyP5+) to catalyze oxidation of ascorbate, leading to the production of excessive levels of peroxide and in turn cell death. The accumulation of peroxide, as a consequence of MnP + ascorbate treatment, was fully reversed by the administration of exogenous catalase showing that ROS was essential for cell death. Cell death as a consequence of the action of MnP/ascorbate corresponded with decreases in GSH levels, pro-survival signaling (pNF-κB, pERK1/2), and in expression of X-linked inhibitor of apoptosis protein (XIAP), the most potent caspase inhibitor. Although markers of classical apoptosis were observed, including PARP cleavage and Annexin V staining, administration of a pan-caspase inhibitor, QVD-OPh, did not reverse the observed cytotoxicity. MnP + ascorbate treated cells showed nuclear translocation of apoptosis inducing factor (AIF), suggesting the possibility of a mechanism of caspase-independent cell death. Pharmacological ascorbate has already shown promise in recently completed Phase I Clinical Trials, where its oxidation and subsequent peroxide formation was catalyzed by endogenous metalloproteins. The catalysis of ascorbate oxidation by an optimized metal-based catalyst (such as Mn porphyrin) carries a large therapeutic potential as a sole anticancer agent or in combination with other modalities such as radio- and chemo- therapy.