ISG20L1 is a p53 family target gene that modulates genotoxic stress-induced autophagy

Abstract

BackgroundAutophagy is characterized by the sequestration of cytoplasm and organelles into multimembrane vesicles and subsequent degradation by the cell's lysosomal system. It is linked to many physiological functions in human cells including stress response, protein degradation, organelle turnover, caspase-independent cell death and tumor suppression. Malignant transformation is frequently associated with deregulation of autophagy and several tumor suppressors can modulate autophagic processes. The tumor suppressor p53 can induce autophagy after metabolic or genotoxic stress through transcriptionally-dependent and -independent mechanisms. In this study we expand on the former mechanism by functionally characterizing a p53 family target gene, ISG20L1 under conditions of genotoxic stress.ResultsWe identified a p53 target gene, ISG20L1, and show that transcription of the gene can be regulated by all three p53 family members (p53, p63, and p73). We generated an antibody to ISG20L1 and found that it localizes to the nucleolar and perinucleolar regions of the nucleus and its protein levels increase in a p53- and p73-dependent manner after various forms of genotoxic stress. When ectopically expressed in epithelial cancer-derived cell lines, ISG20L1 expression decreased clonogenic survival without a concomitant elevation in apoptosis and this effect was partially rescued in cells that were ATG5 deficient. Knockdown of ISG20L1 did not alter 5-FU induced apoptosis as assessed by PARP and caspase-3 cleavage, sub-G1 content, and DNA laddering. Thus, we investigated the role of ISG20L1 in autophagy, a process commonly associated with type II cell death, and found that ISG20L1 knockdown decreased levels of autophagic vacuoles and LC3-II after genotoxic stress as assessed by electron microscopy, biochemical, and immunohistochemical measurements of LC3-II.ConclusionsOur identification of ISG20L1 as a p53 family target and discovery that modulation of this target can regulate autophagic processes further strengthens the connection between p53 signaling and autophagy. Given the keen interest in targeting autophagy as an anticancer therapeutic approach in tumor cells that are defective in apoptosis, investigation of genes and signaling pathways involved in cell death associated with autophagy is critical.

Autophagy and cancer – issues we need to digest

“…This indicates that p73 is able to modulate autophagy through a different target gene or genes. More recent studies have shown that apoptosis enhancing nuclease (AEN, also known as ISG20L1) is a target gene of the three p53 family members, p53, p63 and p73 and is a modulator of stress-induced autophagy (Eby et al, 2010). How AEN-induced autophagy is affected by DRAM1 and vice versa in the context of p53-driven autophagy is yet to be determined and is an area worthy of further investigation.…”

The autophagic paradox in cancer therapy

“…Accumulating evidence suggests that p53 can regulate autophagy in a dual fashion, the outcome of which depends on its subcellular localization . Nuclear p53 as a transcription factor stimulates autophagy through transcriptional upregulation of its pro-autophagic target genes, including damage-regulated autophagy modulator (DRAM; a lysosomal protein), Sestrin2, Skp2, PUMA, Bax, ULK1/2 and ISG20L1 (Crighton et al, 2006;Maiuri et al, 2009;Yee et al, 2009;Barre and Perkins, 2010;Eby et al, 2010;Gao et al, 2011). In addition, p53 promotes the dissociation of the Beclin 1/Bcl-2 complex and inhibits mTOR signaling (Feng and Levine, 2010;Lorin et al, 2010).…”

Autophagy and genomic integrity

“…88,89 Several chemo therapeutic agents induce TA-p73a and TA-p73b expression, 90 which are sufficient to activate autophagy through direct transcriptional regulation of the above-mentioned genes. Moreover, p73 induces the expression of several DNA repair genes, 91 thus positioning p73 in the interface between DNA repair and autophagy.…”

“…74,75,77 Induces autophagy in response to DNA damage through transcription of ULK1, ULK2, DRAM, Sestrins 1/2 and ISG20L1. 13,14,78,88 In the cytoplasm, inhibits autophagy through AMPK inhibition. 81,127 p73 Promotes apoptosis in response to chemotherapeutic-induced DNA damage.…”