JNK–NQO1 axis drives TAp73-mediated tumor suppression upon oxidative and proteasomal stress

Abstract

Hyperproliferating cancer cells produce energy mainly from aerobic glycolysis, which results in elevated ROS levels. Thus aggressive tumors often possess enhanced anti-oxidant capacity that impedes many current anti-cancer therapies. Additionally, in ROS-compromised cancer cells ubiquitin proteasome system (UPS) is often deregulated for timely removal of oxidized proteins, thus enabling cell survival. Taken that UPS maintains the turnover of factors controlling cell cycle and apoptosis – such as p53 or p73, it represents a promising target for pharmaceutical intervention. Enhancing oxidative insult in already ROS-compromised cancer cells appears as an attractive anti-tumor scenario. TAp73 is a bona fide tumor suppressor that drives the chemosensitivity of some cancers to cisplatin or γ-radiation. It is an important drug target in tumors where p53 is lost or mutated. Here we discovered a novel synergistic mechanism leading to potent p73 activation and cancer cell death by oxidative stress and inhibition of 20S proteasomes. Using a small-molecule inhibitor of 20S proteasome and ROS-inducer – withaferin A (WA), we found that WA-induced ROS activates JNK kinase and stabilizes phase II anti-oxidant response effector NF-E2-related transcription factor (NRF2). This results in activation of Nrf2 target – NQO1 (NADPH quinone oxidoreductase), and TAp73 protein stabilization. The observed effect was ablated by the ROS scavenger – NAC. Concurrently, stress-activated JNK phosphorylates TAp73 at multiple serine and threonine residues, which is crucial to ablate TAp73/MDM2 complex and to promote TAp73 transcriptional function and induction of robust apoptosis. Taken together our data demonstrate that ROS insult in combination with the inhibition of 20S proteasome and TAp73 activation endows synthetic lethality in cancer cells. Thus, our results may enable the establishment of a novel pharmacological strategy to exploit the enhanced sensitivity of tumors to elevated ROS and proteasomal stress to kill advanced tumors by pharmacological activation of TAp73 using molecules like WA.

Treatment of adult and pediatric high-grade gliomas with Withaferin A: antitumor mechanisms and future perspectives

“…In addition to targeting the UPS, investigators also studied the activation of TAp73, a tumor suppressor that causes chemosensitivity to Cisplatin or CT when p53 is lost or mutated [49]. TAp73 is also modulated through MDM2 concentrations in a relationship similar to the p53/MDM2 complex.…”

“…Additionally, cancer cells with mutated p53 proteins are often ROS-compromised; the ubiquitin proteasome system (UPS) is deregulated in the removal of oxidized proteins, allowing pro cell survival [49,50]. In addition to targeting the UPS, investigators also studied the activation of TAp73, a tumor suppressor that causes chemosensitivity to Cisplatin or CT when p53 is lost or mutated [49].…”

“…WA-induced ROS activates JNK kinases to phosphorylate TAp73, thereby inhibiting the TAp73/MDM2 complex. The synergistic effect of WA-induced ROS production and the inhibition of the TAp73/MDM2 complex allowed TAp73 to induce apoptosis in cancer cells [49].…”

“…Some therapies, such as Cisplatin administration, are dependent on regular p53 function [48]. WA is able to stabilize p53 through activating ARF, which functions to prevent MDM2 binding to p53 [49].…”

TAp73 transcriptionally represses BNIP3 expression

“…Therefore our current data, although still preliminary, might indicate TAp73/BNIP3 negative axis as a novel pathway for TAp73 tumor suppressor 100,101 BNIP3 upregulation as a consequence of TAp73 loss might therefore contribute to TAp73-dependent mitochondrial phenotype and be associated to the complex involvement of p73 [102][103][104] and the other family members in regulation of mitochondrial activity, [105][106][107][108] cell metabolism [109][110][111][112][113][114] and redox homeostasis. [115][116][117][118] However, currently it is still unclear whether the complex integration of all the p53 family members, in particular the truncated isoform of p73, DNp73, and the cancer-associated mutants of p53, impacts and affects the TAp73-dependent antagonism of BNIP3 expression and more generally of hypoxia response. Future studies are demanded to address these aspects.…”