Thiol antioxidants sensitize malabaricone C induced cancer cell death via reprogramming redox sensitive p53 and NF-κB proteins in vitro and in vivo

Specific focus on redox cancer therapy by targeting drugs to redox homeostasis of the cancer cells is growing rapidly. Recent clinical studies showed that N-acetyl cysteine (NAC) treatment significantly decreased the metabolic heterogeneity and reduced Ki67 (a proliferation marker) with simultaneous enhancement in apoptosis of tumor cells in patients. However, it is not yet precisely known how thiol antioxidants enhance killing of cancer cells in a context dependent manner. To this end, we showed that a dietary compound, malabaricone C (mal C) generated copious amounts of reactive oxygen species (ROS) and also reduced GSH level in lung cancer cells. Paradoxically, although antioxidants supplementation reduced mal C-induced ROS, thiol-antioxidants (NAC/GSH) restored intracellular GSH level but enhanced DNA DSBs and apoptotic cell death induced by mal C. Our results unraveled two tightly coupled biochemical mechanisms attributing this sensitization process by thiol antioxidants. Firstly, thiol antioxidants enable the catechol-quinone redox cycle of mal C and ameliorate ROS generation and bio-molecular damage (DNA and protein). Secondly, thiol antioxidants cause rapid glutathionylation of transcription factors [p53, p65 (NF-kappa B) etc.], oxidized by mal C, and abrogates their nuclear sequestration and transcription of the anti-apoptotic genes. Furthermore, analyses of the mitochondrial fractions of p53 expressing and silenced cells revealed that cytoplasmic accumulation of glutathionylated p53 (p53-SSG) triggers a robust mitochondrial death process. Interestingly, mutation of redox sensitive cysteine residues at 124, 141 and 182 position in p53 significantly reduces mal C plus NAC mediated sensitization of cancer cells. The preclinical results, in two different tumor models in mice, provides further support our conclusion that NAC is able to sensitize mal C induced suppression of tumor growth in vivo.