ATG12 deficiency results in intracellular glutamine depletion, abrogation of tumor hypoxia and a favorable prognosis in cancer

Hypoxia is a common feature of solid tumors and is associated with increased tumor progression, resistance to therapy and increased metastasis. Hence, tumor hypoxia is a prognostic factor independent of treatment modality. To survive hypoxia, cells activate macroautophagy/autophagy. Paradoxically, in several cancer types, mutations or loss of essential autophagy genes have been reported that are associated with earlier onset of tumor growth. However, to our knowledge, the phenotypic and therapeutic consequences of autophagy deficiency have remained unexplored. In this study, we determined autophagy-defects in head and neck squamous cell carcinoma (HNSCC) and observed that expression of ATG12 (autophagy related 12) was lost in 25%-40% of HNSCC. In line, ATG12 loss is associated with absence of hypoxia, as determined by pimonidazole immunohistochemistry. Hence, ATG12 loss is associated with improved prognosis after therapy in two independent HNSCC cohorts and 7 additional cancer types. In vivo, ATG12 targeting resulted in decreased hypoxia tolerance, increased necrosis and sensitivity of the tumor to therapy, but in vitro ATG12-deficient cells displayed enhanced survival in nutrient-rich culture medium. Besides oxygen, delivery of glucose was hampered in hypoxic regions in vivo, which increases the reliance of cells on other carbon sources (e.g., L-glutamine). We observed decreased intracellular L-glutamine levels in ATG12-deficient cells during hypoxia and increased cell killing after L-glutamine depletion, indicating a central role for ATG12 in maintaining L-glutamine homeostasis. Our results demonstrate that ATG12^low tumors represent a phenotypically different subtype that, due to the lowered hypoxia tolerance, display a favorable outcome after therapy.

Abbreviations: ARCON:accelerated radiotherapy with carbogen and nicotinamide; ATG: autophagy related; BrdUrd: bromodeoxyuridine; CA9/CAIX: carbonic anhydrase 9; HIF1A/HIF1α: hypoxia inducible factor 1 subunit alpha; HNSCC: head and neck squamous cell carcinoma; HPV: human papilloma virus; HR: hazard ratio; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MEF: mouse embryonic fibroblast; mRNA: messenger ribonucleic acid; PCR: polymerase chain reaction; SLC2A1/GLUT1: solute carrier family 2 member 1; TCGA: the Cancer Genome Atlas; TME: tumor microenvironment; UTR: untranslated region; VEGF: vascular endothelial growth factor

EGFRvIII expression triggers a metabolic dependency and therapeutic vulnerability sensitive to autophagy inhibition

Expression of EGFRvIII is frequently observed in glioblastoma and is associated with increased cellular proliferation, enhanced tolerance to metabolic stresses, accelerated tumor growth, therapy resistance and poor prognosis. We observed that expression of EGFRvIII elevates the activation of macroautophagy/autophagy during starvation and hypoxia and explored the underlying mechanism and consequence. Autophagy was inhibited (genetically or pharmacologically) and its consequence for tolerance to metabolic stress and its therapeutic potential in (EGFRvIII+) glioblastoma was assessed in cellular systems, (patient derived) tumor xenopgrafts and glioblastoma patients. Autophagy inhibition abrogated the enhanced proliferation and survival advantage of EGFRvIII+ cells during stress conditions, decreased tumor hypoxia and delayed tumor growth in EGFRvIII+ tumors. These effects can be attributed to the supporting role of autophagy in meeting the high metabolic demand of EGFRvIII+ cells. As hypoxic tumor cells greatly contribute to therapy resistance, autophagy inhibition revokes the radioresistant phenotype of EGFRvIII+ tumors in (patient derived) xenograft tumors. In line with these findings, retrospective analysis of glioblastoma patients indicated that chloroquine treatment improves survival of all glioblastoma patients, but patients with EGFRvIII+ glioblastoma benefited most. Our findings disclose the unique autophagy dependency of EGFRvIII+ glioblastoma as a therapeutic opportunity. Chloroquine treatment may therefore be considered as an additional treatment strategy for glioblastoma patients and can reverse the worse prognosis of patients with EGFRvIII+ glioblastoma.

Autophagy is required during cycling hypoxia to lower production of reactive oxygen species

BACKGROUND AND PURPOSE

Human tumors are characterized by the presence of cells that experience periodic episodes of hypoxia followed by reoxygenation. These cells are exposed to reactive oxygen species (ROS) upon reoxygenation and require adaptation to this stress by lowering ROS production or enhancing ROS-clearance for their survival. We hypothesized that autophagy, a lysosomal degradation pathway, may be involved in reducing ROS during periodic hypoxia through removal of ROS producing species.

MATERIALS AND METHODS

Human tumor cells (MCF-7, HT29, U373) were exposed to cycles of hypoxia (O(2)<0.02%) and reoxygenation in the absence or presence of the autophagy inhibitor chloroquine (CQ). Clonogenic survival, ROS production and mitochondrial-DNA content were assessed. In addition, A549 cells overexpressing wild-type or K63-mutated ubiquitin (K63R) were analyzed for ROS production.

RESULTS

Our data indicate that CQ treatment sensitizes cells to cycling hypoxia, due to increased production of ROS, associated with an incapacity to reduce mitochondrial content. Addition of the ROS-scavenger N-acetyl-cysteine increased cell viability and neutralized CQ-effects. Additionally, genetic prevention of K63-linked ubiquitin chains that are required for the removal of toxic protein aggregates by autophagy, resulted in increased ROS production.

CONCLUSIONS

Inhibition of autophagy substantially increases cell death induced by cycling hypoxia through increased ROS production, providing an opportunity to decrease the hypoxic fraction within tumors and enhance tumor therapy.

Identification and characterization of two antisense transcripts from the major immediate early region of rat cytomegalovirus

We have identified and characterized two antisense transcripts from the rat cytomegalovirus (RCMV) major immediate early (MIE) region. These transcripts, designated IE-AS1 and IE-AS2, are complementary to part of the sense IE1 transcript. The IE-AS transcripts were first detected in peripheral blood leukocytes (PBL) of RCMV-infected rats at 7 days post-infection (pi) in the absence of IE1 transcription. Nevertheless, both the IE1 and IE-AS transcripts were found at the same time in the salivary glands of RCMV-infected rats at 7 and 120 days pi as well as in RCMV-infected rat embryo fibroblasts (REFs) at 48 h pi.

The rat cytomegalovirus R78 G protein-coupled receptor gene is required for production of infectious virus in the spleen

The rat cytomegalovirus (RCMV) R33 and R78 genes are conserved within members of the subfamily Betaherpesvirinae and encode proteins (pR33 and pR78, respectively) that show sequence similarity with G protein-coupled receptors. Previously, the biological relevance of these genes was demonstrated by the finding that R33- and R78-deleted RCMV strains are severely attenuated in vivo. In addition, R78-deleted strains were found to replicate less efficiently in cell culture. To monitor of the expression of R33- and R78-encoded proteins, recombinant RCMV strains, designated RCMV33G and RCMV78G, were generated. These recombinants expressed enhanced green fluorescent protein (EGFP)-tagged versions of pR33 and pR78 instead of native pR33 and pR78, respectively. Here it is reported that, although RCMV33G replicates as efficiently as wt virus in rat embryo fibroblast cultures, strain RCMV78G produces virus titres that are 3- to 4-fold lower than wt RCMV in the culture medium. This result indicates that the pR78-EGFP protein, as expressed by RCMV78G, does not completely functionally replace its native counterpart (pR78) in vitro. Interestingly, in infected rats, infectious RCMV33G was produced in significantly lower amounts than infectious wt RCMV, as well as RCMV78G, in the salivary glands. Conversely, although RCMV33G replicated to similar levels as wt virus in the spleen, both RCMV78G and an R78 knock-out strain (RCMV Delta R78a) replicated poorly in this organ. Together, these data indicate that R78 is crucial for the production of infectious RCMV in the spleen of infected rats.