NCF2/p67phox

p53 is best known for its role as a potent tumor suppressor and transcription factor that induces processes such as apoptosis or cell cycle arrest in response to a variety of stresses. The decision to survive or to die is most likely dependent on the stimulus and the duration or severity of that stimulus (reviewed in ref. 1). Low amounts of stresses are thought to favor the induction of p53-dependent cell cycle arrest via p21 or 14-3-3 proteins to allow for DNA repair. Interestingly, such proteins have also been shown to actively inhibit apoptosis and favor survival. In fact, in recent years it has become apparent that p53 can employ multiple strategies to promote survival, including inducing a multitude of anti-apoptotic genes.² A small number of these genes can function by decreasing ROS levels, including MnSOD (Manganese superoxide dismutase), ALDH4 (aldehyde dehydrogenase 4), GPX (glutathione peroxidase), sestrins, which have antioxidant activities themselves, or TIGAR (p53-induced glycolysis and apoptosis inhibitor), which inhibits glycolysis and, thus, lowers ROS (Fig. 1).

Figure 1. p53 can both induce and inhibit apoptosis by regulating ROS levels. Some of the pro- (red box) and anti- (green box) apoptotic genes regulated by p53 are depicted in gray.

In the December 15, 2012 issue of Cell Cycle, Italiano et al. explored the possibility that NCF2/p67phox is a novel p53 target gene that could potentially be added to the list of anti-apoptotic p53 target genes (Fig. 1).³ Overexpression of p53 or doxorubicin-mediated stabilization of p53 induced the expression of NCF2/ p67 phox. Furthermore, using luciferase assays and chromatin IPs, they demonstrate the presence of a p53 response element in the promotor region of NCF2/p67 phox. Although p53 shares a number of target genes with its family members p63 and p73, and all family members have a similar consensus response element, the authors show that neither p63 or p73 could induce NCF2/p67 phox expression. NCF2/p67 phox is the activating unit of the NAD(P)H oxidase enzyme complex 2 (Nox2) localized at the plasma membrane and endosomes, involved in generating NADP+ or NAD+ and, thus, increasing ROS. Not surprisingly, siRNA-mediated loss of NCF2/p67 phox expression resulted in a decrease in ROS levels. However, and contrary to ROS generated by the p53 target genes Puma or Bax via the mitochondrial-dependent apoptotic pathway, a decrease in ROS by loss of NCF2/p67 phox expression coincided with increased apoptosis. These results suggest that ROS generated from different sources or possibly at different cellular localizations have differential effects on apoptosis. Although Nox2 has been shown to drive ROS-dependent cell death, others have demonstrated pro-survival and anti-apoptotic effects of ROS production by Nox2⁴ or the Nox family member Nox4⁵ that inhibited apoptosis by enhancing the AKT-mediated phosphorylation of apoptosis signal kinase 1 (ASK1). The data by Italiano et al. add to the complexity of p53 signaling toward ROS, and it will be interesting to further characterize the orchestration of all the p53 signaling routes leading to ROS and apoptosis during various stresses.

As NOXes, including Nox2, have also been shown to induce signaling cascades that enhance cell migration and proliferation,⁶ and Nox activity seems required for H-Ras transformation,⁷ the work of Italiano et al. gives room for some further speculations. Frequently, mutations in p53 give rise to a mutant p53 protein that drives cell migration and proliferation. Although these mutant p53 proteins have often lost the ability to regulate the pro-apoptotic genes, some mutants still retain the ability to regulate some target genes.^8,9 It will be interesting to further explore whether some mutant p53 proteins can still upregulate NCF2/p67 phox, and whether the transforming abilities of mutant p53, like H-Ras, require Nox activity.

Italiano D, Lena AM, Melino G, Candi E. Identification of NCF2/p67phox as a novel p53 target gene. Cell Cycle. 2012;11:4589–96. doi: 10.4161/cc.22853.