DNA damage and autophagy

Abstract

Both exogenous and endogenous agents are a threat to DNA integrity. Exogenous environmental agents such as ultraviolet (UV) and ionizing radiation, genotoxic chemicals and endogenous byproducts of metabolism including reactive oxygen species can cause alterations in DNA structure (DNA damage). Unrepaired DNA damage has been linked to a variety of human disorders including cancer and neurodegenerative disease. Thus, efficient mechanisms to detect DNA lesions, signal their presence and promote their repair have been evolved in cells. If DNA is effectively repaired, DNA damage response is inactivated and normal cell functioning resumes. In contrast, when DNA lesions cannot be removed, chronic DNA damage triggers specific cell responses such as cell death and senescence. Recently, DNA damage has been shown to induce autophagy, a cellular catabolic process that maintains a balance between synthesis, degradation, and recycling of cellular components. But the exact mechanisms by which DNA damage triggers autophagy are unclear. More importantly, the role of autophagy in the DNA damage response and cellular fate is unknown. In this review we analyze evidence that supports a role for autophagy as an integral part of the DNA damage response.

Figure 1. DNA damage response

A. Formation of a double-stranded DNA break end (DSB) leads to the recruitment of the MRN (meiotic recombination protein-11 [MRE11]–RAD50–Nijmegen breakage syndrome protein-1 [NBS1]) complex activating ataxia-telangiectasia mutated (ATM) by phosphorylation. The activated ATM phosphorylates downstream targets, including checkpoint kinase-1 (CHK2) and p53. B. Formation of single-stranded DNA breaks (SSB) activates two complexes: RAD9–RAD1–HUS1 (also known as 9-1-1) and one comprising the single-stranded DNA Replication Factor C (RFC), which binds to RAD17. RAD17–replication factor C (RFC) loads the 9-1-1 complex. Loading of the 9-1-1 complex brings the ATR to the damaged site leading to the phosphorylation of the downstream checkpoint kinase-1 (CHK1) and other ATR effectors that include breast cancer-1 (BRCA1). ATM- or ATR-induced phosphorylation of downstream targets leads to cell-cycle arrest, cell death pathways activation and DNA repair.

Figure 2. Autophagy (macroautophagy) pathway

Autophagy is induced by a variety of different stimuli including DNA damage. Autophagy involves the sequestration of cytosolic proteins and organelles within double-membrane structures termed autophagosomes and further degradation via lysosomal hydrolases. Atg12 ubiquitin-like system activity takes place during elongation of the phagophore mediating LC3 lipidation (LC3-II) and its localization to the autophagosome membrane. Fusion of the autophagosome and the lysosome is driven by intraorganelle acidification. Ultimately, autophagosome and lysosome fusion leads to a) the degradation of cytosolic components with ulterior generation of amino acids and fatty acids to be recycled for cellular survival; or b) the removal of potential damaging proteins and organelles. Under certain conditions autophagy might induce cell death to destroy impaired/damaged cells.

Figure 3. Autophagy as part of the DNA damage response

p53 is involved in both apoptosis and autophagy pathways in response to DNA damage. DNA damage-induced apoptosis is regulated by alterations in pro-apoptotic and anti-apoptotic Bcl-2 protein levels suggesting a crosstalk between autophagy and apoptosis signaling by regulation of Bcl-2 family members. For example, nuclear p53 induces Bax and Bak expression leading to apoptosis, and in the mitochondria, p53 can also induce apoptosis by oligomerization with Bax and Bak. In adition, p53 can bind directly to the anti-apoptotic Bcl-2 proteins, Bcl-2 and Bcl-xl. Because autophagy is inhibited by the interaction of the anti-apoptotic proteins Bcl-2/Bcl-xl with Beclin 1 p53 might promote autophagy by prevention of Bcl-2/Bcl-xl-induced Beclin 1 inhibition. Interestingly, p53 plays a dual role in autophagy regulation. Under DNA-damaging conditions, nuclear p53 activates AMPK, PTEN and/or DRAM to inhibit mTOR and induce cell survival via autophagy. Contrary, cytoplasmic p53 functions as a repressor of autophagy as well. ATM regulates autophagy in response to genotoxic and oxidative stress through signaling of AMPK and TSC2. FOXO3a transactivates ATM and mediates transcriptional regulation of Atg genes (not depicted in the figure). Mitochondrial induced reactive oxygen species (ROS) are a source for oxidative DNA damage. Mitophagy is the major degradative pathway for mitochondrial turnover which contributes to the regulation of ROS production. Thus, autophagy might protect against oxidative DNA damage by ROS. Inhibition of autophagy potentiates apoptosis induced by DNA damage agents suggesting that autophagy acts as a protective mechanism against DNA damage-induced cell death. Failure in the DNA damage response might increase toxicity and mutations with ulterior development of cancer and/or neurodegenerative diseases.