MYC Can Induce DNA Breaks In vivo and In vitro Independent of Reactive Oxygen Species

Abstract

MYC overexpression is thought to initiate tumorigenesis by inducing cellular proliferation and growth and to be restrained from causing tumorigenesis by inducing cell cycle arrest, cellular senescence, and/or apoptosis. Here we show that MYC can induce DNA breaks both in vitro and in vivo independent of increased production of reactive oxygen species (ROS). We provide an insight into the specific circumstances under which MYC generates ROS in vitro and propose a possible mechanism. We found that MYC induces DNA double-strand breaks (DSBs) independent of ROS production in murine lymphocytes in vivo as well as in normal human foreskin fibroblasts (NHFs) in vitro in normal (10%) serum, as measured by ;H2AX staining. However, NHFs cultured in vitro in low serum (0.05%) and/or ambient oxygen saturation resulted in ROS-associated oxidative damage and DNA single-strand breaks (SSBs), as measured by Ape-1 staining. In NHFs cultured in low versus normal serum, MYC induced increased expression of CYP2C9, a gene product well known to be associated with ROS production. Specific inhibition of CYP2C9 by small interfering RNA was shown to partially inhibit MYC-induced ROS production. Hence, MYC overexpression can induce ROS and SSBs under some conditions, but generally induces widespread DSBs in vivo and in vitro independent of ROS production. (Cancer Res 2006; 66(13): 6598-605)

Many roads lead to oncogene-induced senescence

“…While ROS can induce DNA damage, Sun et al (2007) have also shown that ROS may trigger senescence via a kinase cascade involving p38 and one of its effector kinases known as PRAK, which can phosphorylate and activate p53. Despite these findings, not all oncogenes increase ROS (for example, Raf, myc; Ray et al, 2006;Dolado et al, 2007), and it is still unknown whether a single activated Ras allele is capable of inducing sufficiently high levels of ROS. Therefore, the relative involvement of ROS in triggering the senescence response might be dictated by the specific genetic event and the resulting signal intensity.…”

The Ever Expanding Role for c-Myc in Promoting Genomic Instability

“…Recently, Ray et al 30 have studied in more detail the molecular basis of ROS production by c-Myc in normal human fibroblasts (NHFs) maintained in normal or low serum (10% vs. 0.05%). Notably, they found that activation of MycER induced ROS only when cells were maintained in the lower serum concentration and at ambient oxygen concentrations (20% vs. 2%).…”

“…In fact, breakage was observed in c-Myc over-expressing cells maintained in higher serum and/or lower O2 concentrations, where ROS were not increased over baseline. 30 DNA breaks mediated by c-Myc thus can occur independently of ROS generation as well as through ROS-dependent pathways.…”

“…Using a candidate gene approach, Ray et al 30 focused on CYP2C9, a member of the cytochrome P450 family and a significant source of ROS in many cell types. 31 Specific genetic or pharmacologic inhibition of CYP2C9 expression ablated the production of ROS by activated MycER.…”

Models of reactive oxygen species in cancer

“…Upon treatment of c-Myc activated cells with NAC, a significant decrease in ROS and reduction of DNA damage were observed [15]. A major source of ROS generation in cells over-expressing c-Myc is from the mitochondria, but the role of ROS in causing DNA breaks remains to be defined [16].…”