Autophagy regulates the survival of cells with chromosomal instability

Targeting chromosomal instability in patients with cancer

Chromosomal instability (CIN) is a hallmark of cancer and a driver of metastatic dissemination, therapeutic resistance, and immune evasion. CIN is present in 60-80% of human cancers and poses a formidable therapeutic challenge as evidenced by the lack of clinically approved drugs that directly target CIN. This limitation in part reflects a lack of well-defined druggable targets as well as a dearth of tractable biomarkers enabling direct assessment and quantification of CIN in patients with cancer. Over the past decade, however, our understanding of the cellular mechanisms and consequences of CIN has greatly expanded, revealing novel therapeutic strategies for the treatment of chromosomally unstable tumours as well as new methods of assessing the dynamic nature of chromosome segregation errors that define CIN. In this Review, we describe advances that have shaped our understanding of CIN from a translational perspective, highlighting both challenges and opportunities in the development of therapeutic interventions for patients with chromosomally unstable cancers.

Aneuploidy is Linked to Neurological Phenotypes Through Oxidative Stress

Aneuploidy, having an aberrant genome, is gaining increasing attention in neurodegenerative diseases. It gives rise to proteotoxic stress as well as a stereotypical oxidative shift which makes these cells sensitive to internal and environmental stresses. A growing body of research from numerous laboratories suggests that many neurodegenerative disorders, especially Alzheimer's disease and frontotemporal dementia, are characterised by neuronal aneuploidy and the ensuing apoptosis, which may contribute to neuronal loss. Using Drosophila as a model, we investigated the effect of induced aneuploidy in GABAergic neurons. We found an increased proportion of aneuploidy due to Mad2 depletion in the third-instar larval brain and increased cell death. Depletion of Mad2 in GABAergic neurons also gave a defective climbing and seizure phenotype. Feeding animals an antioxidant rescued the climbing and seizure phenotype. These findings suggest that increased aneuploidy leads to higher oxidative stress in GABAergic neurons which causes cell death, climbing defects, and seizure phenotype. Antioxidant feeding represents a potential therapy to reduce the aneuploidy-driven neurological phenotype.

Chromosomal Instability Causes Sensitivity to Polyamines and One-Carbon Metabolism

Aneuploidy, or having a disrupted genome, is an aberration commonly found in tumours but rare in normal tissues. It gives rise to proteotoxic stress as well as a stereotypical oxidative shift, which makes these cells sensitive to internal and environmental stresses. Using Drosophila as a model, we investigated the changes in transcription in response to ongoing changes to ploidy (chromosomal instability, CIN). We noticed changes in genes affecting one-carbon metabolism, specifically those affecting the production and use of s-adenosyl methionine (SAM). The depletion of several of these genes has led to cell death by apoptosis in CIN cells but not in normal proliferating cells. We found that CIN cells are particularly sensitive to SAM metabolism at least partly because of its role in generating polyamines. Feeding animals spermine was seen to rescue the cell death caused by the loss of SAM synthase in CIN tissues. The loss of polyamines led to decreased rates of autophagy and sensitivity to reactive oxygen species (ROS), which we have shown to contribute significantly to cell death in CIN cells. These findings suggest that a well-tolerated metabolic intervention such as polyamine inhibition has the potential to target CIN tumours via a relatively well-characterised mechanism.

cGAS-STING signalling in cancer: striking a balance with chromosomal instability

Chromosomal instability (CIN) is a hallmark of cancer that drives tumour evolution. It is now recognised that CIN in cancer leads to the constitutive production of misplaced DNA in the form of micronuclei and chromatin bridges. These structures are detected by the nucleic acid sensor cGAS, leading to the production of the second messenger 2'3'-cGAMP and activation of the critical hub of innate immune signalling STING. Activation of this immune pathway should instigate the influx and activation of immune cells, resulting in the eradication of cancer cells. That this does not universally occur in the context of CIN remains an unanswered paradox in cancer. Instead, CIN-high cancers are notably adept at immune evasion and are highly metastatic with typically poor outcomes. In this review, we discuss the diverse facets of the cGAS-STING signalling pathway, including emerging roles in homeostatic processes and their intersection with genome stability regulation, its role as a driver of chronic pro-tumour inflammation, and crosstalk with the tumour microenvironment, which may collectively underlie its apparent maintenance in cancers. A better understanding of the mechanisms whereby this immune surveillance pathway is commandeered by chromosomally unstable cancers is critical to the identification of new vulnerabilities for therapeutic exploitation.

Mitochondrial RNA methyltransferase TRMT61B is a new, potential biomarker and therapeutic target for highly aneuploid cancers

Despite being frequently observed in cancer cells, chromosomal instability (CIN) and its immediate consequence, aneuploidy, trigger adverse effects on cellular homeostasis that need to be overcome by anti-stress mechanisms. As such, these safeguard responses represent a tumor-specific Achilles heel, since CIN and aneuploidy are rarely observed in normal cells. Recent data have revealed that epitranscriptomic marks catalyzed by RNA-modifying enzymes change under various stress insults. However, whether aneuploidy is associated with such RNA modifying pathways remains to be determined. Through an in silico search for aneuploidy biomarkers in cancer cells, we found TRMT61B, a mitochondrial RNA methyltransferase enzyme, to be associated with high levels of aneuploidy. Accordingly, TRMT61B protein levels are increased in tumor cell lines with an imbalanced karyotype as well as in different tumor types when compared to control tissues. Interestingly, while TRMT61B depletion induces senescence in melanoma cell lines with low levels of aneuploidy, it leads to apoptosis in cells with high levels. The therapeutic potential of these results was further validated by targeting TRMT61B in transwell and xenografts assays. We show that TRM61B depletion reduces the expression of several mitochondrial encoded proteins and limits mitochondrial function. Taken together, these results identify a new biomarker of aneuploidy in cancer cells that could potentially be used to selectively target highly aneuploid tumors.

One-Carbon and Polyamine Metabolism as Cancer Therapy Targets

Cancer metabolic reprogramming is essential for maintaining cancer cell survival and rapid replication. A common target of this metabolic reprogramming is one-carbon metabolism which is notable for its function in DNA synthesis, protein and DNA methylation, and antioxidant production. Polyamines are a key output of one-carbon metabolism with widespread effects on gene expression and signaling. As a result of these functions, one-carbon and polyamine metabolism have recently drawn a lot of interest for their part in cancer malignancy. Therapeutic inhibitors that target one-carbon and polyamine metabolism have thus been trialed as anticancer medications. The significance and future possibilities of one-carbon and polyamine metabolism as a target in cancer therapy are discussed in this review.

Emerging roles of mitotic autophagy

Lysosomes exert pleiotropic functions to maintain cellular homeostasis and degrade autophagy cargo. Despite the great advances that have boosted our understanding of autophagy and lysosomes in both physiology and pathology, their function in mitosis is still controversial. During mitosis, most organelles are reshaped or repurposed to allow the correct distribution of chromosomes. Mitotic entry is accompanied by a reduction in sites of autophagy initiation, supporting the idea of an inhibition of autophagy to protect the genetic material against harmful degradation. However, there is accumulating evidence revealing the requirement of selective autophagy and functional lysosomes for a faithful chromosome segregation. Degradation is the most-studied lysosomal activity, but recently described alternative functions that operate in mitosis highlight the lysosomes as guardians of mitotic progression. Because the involvement of autophagy in mitosis remains controversial, it is important to consider the specific contribution of signalling cascades, the functions of autophagic proteins and the multiple roles of lysosomes, as three entangled, but independent, factors controlling genomic stability. In this Review, we discuss the latest advances in this area and highlight the therapeutic potential of targeting autophagy for drug development.

Exploiting aneuploidy-imposed stresses and coping mechanisms to battle cancer

Aneuploidy, an irregular number of chromosomes in cells, is a hallmark feature of cancer. Aneuploidy results from chromosomal instability (CIN) and occurs in almost 90% of all tumours. While many cancers display an ongoing CIN phenotype, cells can also be aneuploid without displaying CIN. CIN drives tumour evolution as ongoing chromosomal missegregation will yield a progeny of cells with variable aneuploid karyotypes. The resulting aneuploidy is initially toxic to cells because it leads to proteotoxic and metabolic stress, cell cycle arrest, cell death, immune cell activation and further genomic instability. In order to overcome these aneuploidy-imposed stresses and adopt a malignant fate, aneuploid cancer cells must develop aneuploidy-tolerating mechanisms to cope with CIN. Aneuploidy-coping mechanisms can thus be considered as promising therapeutic targets. However, before such therapies can make it into the clinic, we first need to better understand the molecular mechanisms that are activated upon aneuploidization and the coping mechanisms that are selected for in aneuploid cancer cells. In this review, we discuss the key biological responses to aneuploidization, some of the recently uncovered aneuploidy-coping mechanisms and some strategies to exploit these in cancer therapy.

Intracellular expression of arginine deiminase activates the mitochondrial apoptosis pathway by inhibiting cytosolic ferritin and inducing chromatin autophagy

BACKGROUND

Based on its low toxicity, arginine starvation therapy has the potential to cure malignant tumors that cannot be treated surgically. The Arginine deiminase (ADI) gene has been identified to be an ideal cancer-suppressor gene. ADI expressed in the cytosol displays higher oncolytic efficiency than ADI-PEG20 (Pegylated Arginine Deiminase by PEG 20,000). However, it is still unknown whether cytosolic ADI has the same mechanism of action as ADI-PEG20 or other underlying cellular mechanisms.

METHODS

The interactions of ADI with other protein factors were screened by yeast hybrids, and verified by co-immunoprecipitation and immunofluorescent staining. The effect of ADI inhibiting the ferritin light-chain domain (FTL) in mitochondrial damage was evaluated by site-directed mutation and flow cytometry. Control of the mitochondrial apoptosis pathway was analyzed by Western Blotting and real-time PCR experiments. The effect of p53 expression on cancer cells death was assessed by siTP53 transfection. Chromatin autophagy was explored by immunofluorescent staining and Western Blotting.

RESULTS

ADI expressed in the cytosol inhibited the activity of cytosolic ferritin by interacting with FTL. The inactive mutant of ADI still induced apoptosis in certain cell lines of ASS- through mitochondrial damage. Arginine starvation also generated an increase in the expression of p53 and p53AIP1, which aggravated the cellular mitochondrial damage. Chromatin autophagy appeared at a later stage of arginine starvation. DNA damage occurred along with the entire arginine starvation process. Histone 3 (H3) was found in autophagosomes, which implies that cancer cells attempted to utilize the arginine present in histones to survive during arginine starvation.

CONCLUSIONS

Mitochondrial damage is the major mechanism of cell death induced by cytosolic ADI. The process of chromatophagy does not only stimulate cancer cells to utilize histone arginine but also speeds up cancer cell death at a later stage of arginine starvation.

Genomic instability and cancer: lessons from Drosophila

Cancer is a genetic disease that involves the gradual accumulation of mutations. Human tumours are genetically unstable. However, the current knowledge about the origins and implications of genomic instability in this disease is limited. Understanding the biology of cancer requires the use of animal models. Here, we review relevant studies addressing the implications of genomic instability in cancer by using the fruit fly, Drosophila melanogaster, as a model system. We discuss how this invertebrate has helped us to expand the current knowledge about the mechanisms involved in genomic instability and how this hallmark of cancer influences disease progression.

Co-Operation between Aneuploidy and Metabolic Changes in Driving Tumorigenesis

Alterations from the normal set of chromosomes are extremely common as cells progress toward tumourigenesis. Similarly, we expect to see disruption of normal cellular metabolism, particularly in the use of glucose. In this review, we discuss the connections between these two processes: how chromosomal aberrations lead to metabolic disruption, and vice versa. Both processes typically result in the production of elevated levels of reactive oxygen species, so we particularly focus on their role in mediating oncogenic changes.

The impact of aneuploidy on cellular homeostasis

Genomic instability is a common feature of tumours that has a wide range of disruptive effects on cellular homeostasis. In this review we briefly discuss how instability comes about, then focus on the impact of gain or loss of DNA (aneuploidy) on oxidative stress. We discuss several mechanisms that lead from aneuploidy to the production of reactive oxygen species, including the effects on protein complex stoichiometry, endoplasmic reticulum stress and metabolic disruption. Each of these are involved in positive feedback loops that amplify relatively minor genetic changes into major cellular disruption or cell death, depending on the capacity of the cell to induce antioxidants or processes such as mitophagy that can moderate the disruption. Finally we examine the direct effects of reactive oxygen species on mitosis and how oxidative stress can compromise centrosome number, cytoskeletal integrity and signalling processes that are vital for mitotic fidelity.

Blockage of autophagic flux is associated with lymphocytosis and higher percentage of tumoral cells in chronic lymphocytic leukemia of B cells

PURPOSE

Autophagy has lately emerged as an important biological process with implications in several hematological pathologies. Recently, a growing body of evidence supports a putative role of autophagy in chronic lymphocytic leukemia; however, no definitive clue has been established so far. To elucidate this issue, we have developed a pilot study to measure autophagic flux in peripheral blood mononuclear cells from chronic lymphocytic leukemia patients, and explored its correlation with classical clinical/analytical parameters.

METHODS/PATIENTS

Thirty-three chronic lymphocytic leukemia patients participated in the study. Autophagic flux in peripheral blood mononuclear cells was determined by western blot measuring the levels of the proteins p62 and lipidated LC3. Moreover, p62 mRNA levels were analyzed by RT-qPCR.

RESULTS

Lymphocytosis and the percentage of tumoral lymphocytes in chronic lymphocytic leukemia patients statistically correlate with a blocked autophagic flux.

CONCLUSION

Alterations in autophagic flux could play an important role in the physiopathology of chronic lymphocytic leukemia.

Chromosomal instability causes sensitivity to protein folding stress and ATP depletion

Aneuploidy _­– having an unbalanced genome – is poorly tolerated at the cellular and organismal level. It gives rise to proteotoxic stress as well as a stereotypical oxidative shift which makes these cells sensitive to internal and environmental stresses. Using Drosophila as a model, we found that protein folding stress is exacerbated by redox stress that occurs in response to ongoing changes to ploidy (chromosomal instability, CIN). We also found that if de novo nucleotide synthesis is blocked, CIN cells are dependent on a high level of lysosome function to survive. Depletion of adenosine monophosphate (AMP) synthesis enzymes led to DNA damage in CIN cells, which showed elevated activity of the DNA repair enzyme activated poly(ADP ribose) polymerase (PARP). PARP activation causes depletion of its substrate, nicotinamide adenine dinucleotide (NAD+) and subsequent loss of Adenosine Tri-Phosphate (ATP), and we found that adding ATP or nicotinamide (a precursor in the synthesis of NAD+) could rescue the observed phenotypes. These findings provide ways to interpret, target and exploit aneuploidy, which has the potential to offer tumour-specific therapies.

Summary: Cells that gain or lose chromosomes during cell division are shown to be sensitive to ATP levels and protein folding stress.

Autophagy, autophagy-associated adaptive immune responses and its role in hematologic malignancies

Autophagy is a tightly regulated catabolic process that leads to the degradation of cytoplasmatic components such as aggregated/misfolded proteins and organelles through the lysosomal machinery. Recent studies suggest that autophagy plays such a role in the context of the anti-tumor immune response, make it an attractive target for cancer immunotherapy. Defective autophagy in hematopoietic stem cells may contribute to the development of hematologic malignancies, including leukemia, myelodysplastic syndrome, and lymphoproliferative disorder. In blood cancer cells, autophagy can either result in chemoresistance or induce autophagic cell death that may act as immunogenic. Based on the successful experimental findings in vitro and in vivo, clinical trials of autophagy inhibitor such as hydroxychloroquine in combination with chemotherapy in patients with blood cancers are currently underway. However, autophagy inactivation might impair autophagy-triggered anticancer immunity, whereas induction of autophagy might become an effective immunotherapy. These aspects are discussed in this review together with a brief introduction to the autophagic molecular machinery and its roles in hematologic malignancies.