Macro‐autophagy is an ancient and highly conserved self‐degradative process that plays a homeostatic role in normal cells by eliminating organelles, pathogens, and protein aggregates. Autophagy, as ...it is routinely referred to, also allows cells to maintain metabolic sufficiency and survive under conditions of nutrient stress by recycling the by‐products of autophagic degradation, such as fatty acids, amino acids, and nucleotides. Tumor cells are more reliant than normal cells on autophagy for survival in part due to their rapid growth rate, altered metabolism, and nutrient‐deprived growth environment. How this dependence of tumor cells on autophagy affects their progression to malignancy and metastatic disease is an area of increasing research focus. Here, we review recent work identifying critical functions for autophagy in tumor cell migration and invasion, tumor stem cell maintenance and therapy resistance, and cross‐talk between tumor cells and their microenvironment.
Autophagy is upregulated as tumors progress to become malignant and this review addresses how autophagy modulates cancer metastasis. Specifically, we examine the critical role played by autophagy in tumor cell migration and invasion, in maintaining the cancer stem cell phenotype and tumor cell dormancy as well as how autophagy affects interactions of tumor cells with components of the tumor microenvironment. Finally, we discuss how these functions make autophagy inhibition an attractive therapeutic option for metastatic cancers.
Autophagy is a conserved catabolic process that plays a housekeeping role in eliminating protein aggregates and organelles and is activated during nutrient deprivation to generate metabolites and ...energy. Autophagy plays a significant role in tumorigenesis, although opposing context-dependent functions of autophagy in cancer have complicated efforts to target autophagy for therapeutic purposes. We demonstrate that autophagy inhibition reduces tumor cell migration and invasion in vitro and attenuates metastasis in vivo. Numerous abnormally large focal adhesions (FAs) accumulate in autophagy-deficient tumor cells, reflecting a role for autophagy in FA disassembly through targeted degradation of paxillin. We demonstrate that paxillin interacts with processed LC3 through a conserved LIR motif in the amino-terminal end of paxillin and that this interaction is regulated by oncogenic SRC activity. Together, these data establish a function for autophagy in FA turnover, tumor cell motility, and metastasis.
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•Autophagy is required for the migration and invasion of metastatic tumor cells•Autophagy promotes the degradation of paxillin and focal adhesion turnover•Paxillin interacts with LC3B through a conserved LIR in a Src-regulated manner•Autophagy is required for Src-regulated tumor cell motility
Sharifi et al. describe a role for autophagy in focal adhesion turnover and cell motility mediated by the interaction of processed LC3 with paxillin in a manner regulated by oncogenic Src. These studies suggest potential therapeutic value in targeting autophagy to inhibit metastatic dissemination.
BNip3 is a hypoxia‐inducible protein that targets mitochondria for autophagosomal degradation. We report a novel tumor suppressor role for BNip3 in a clinically relevant mouse model of mammary ...tumorigenesis. BNip3 delays primary mammary tumor growth and progression by preventing the accumulation of dysfunctional mitochondria and resultant excess ROS production. In the absence of BNip3, mammary tumor cells are unable to reduce mitochondrial mass effectively and elevated mitochondrial ROS increases the expression of Hif‐1α and Hif target genes, including those involved in glycolysis and angiogenesis—two processes that are also markedly increased in BNip3‐null tumors. Glycolysis inhibition attenuates the growth of BNip3‐null tumor cells, revealing an increased dependence on autophagy for survival. We also demonstrate that BNIP3 deletion can be used as a prognostic marker of tumor progression to metastasis in human triple‐negative breast cancer (TNBC). These studies show that mitochondrial dysfunction—caused by defects in mitophagy—can promote the Warburg effect and tumor progression, and suggest better approaches to stratifying TNBC for treatment.
Synopsis
This study shows that BNip3 loss and the ensuing defects in mitophagy lead to ROS production, Hif transcriptional responses and mammary tumor progression. BNIP3 deletion is a prognostic marker of metastatic potential in triple negative breast cancer.
Elevated ROS production by dysfunctional mitochondria in BNip3 null tumors results in increased Hif‐1α levels and increased tumor progression to invasiveness.
This novel negative feedback loop between BNip3 and Hif‐1α limits the oncogenic activity of Hif‐1 in glycolysis and angiogenesis.
Defective mitochondria and aerobic glycolysis arising from loss of BNip3 is associated with increased dependence on autophagy for survival.
BNIP3 is focally deleted in triple negative breast cancer and, together with high HIF‐1α levels, strongly predicts progression to metastasis in TNBC patients.
This study shows that BNip3 loss and the ensuing defects in mitophagy lead to ROS production, Hif transcriptional responses and mammary tumor progression. BNIP3 deletion is a prognostic marker of metastatic potential in triple negative breast cancer.
Endocrine therapy has been the standard of care for patients with metastatic hormone receptor (HR)‐positive, HER2‐negative breast cancer since the 1970s, improving survival while avoiding the ...toxicities associated with cytotoxic chemotherapy. However, all HR‐positive tumors ultimately develop resistance to endocrine therapy. Cyclin‐dependent kinase 4 and 6 (CDK4/6) inhibitors have more recently become an important component of the management of this breast cancer subtype, significantly delaying time to the disease progression and improving survival when combined with endocrine therapy. However, as with endocrine therapy alone, treatment resistance remains a universal phenomenon. As more women receive CDK4/6 inhibitors as part of their treatment, the management of de novo and acquired resistance to combined CDK4/CDK6 inhibitor plus endocrine therapy regimens has emerged as an important clinical challenge. Several resistance mechanisms have been described, including alterations in the CDK4/6/cyclin D complex or its major effector retinoblastoma protein (pRb), bypass signaling through other cyclin/CDK complexes and activation of upstream signaling pathways, in particular the PI3K/mTOR pathway, but robust biomarkers to predict resistance remain elusive, and the role for continuing CDK4/6 inhibitors after progression remains under investigation. Novel strategies being evaluated in clinical trials include the continuation of CDK4/6 inhibitors through progression, as well as triplet therapy combinations with PI3K inhibitors or immune checkpoint inhibitors.
As more women receive CDK4/6 inhibitors as part of their treatment for metastatic hormone receptor‐positive breast cancer, the management of de novo and acquired resistance to combined CDK4/6 inhibitor plus endocrine therapy regimens has emerged as an important clinical challenge. Novel strategies being evaluated in clinical trials include the continuation of CDK4/CDK6 inhibitors through progression as well as triplet therapy combinations with PI3K inhibitors or immune checkpoint inhibitors.
Chlamydomonas reinhardtii is a unicellular green alga that is a key model organism in the study of photosynthesis and oxidative stress. Here we describe the large‐scale generation of a population of ...insertional mutants that have been screened for phenotypes related to photosynthesis and the isolation of 459 flanking sequence tags from 439 mutants. Recent phylogenomic analysis has identified a core set of genes, named GreenCut2, that are conserved in green algae and plants. Many of these genes are likely to be central to the process of photosynthesis, and they are over‐represented by sixfold among the screened insertional mutants, with insertion events isolated in or adjacent to 68 of 597 GreenCut2 genes. This enrichment thus provides experimental support for functional assignments based on previous bioinformatic analysis. To illustrate one of the uses of the population, a candidate gene approach based on genome position of the flanking sequence of the insertional mutant CAL027_01_20 was used to identify the molecular basis of the classical C. reinhardtii mutation ac17. These mutations were shown to affect the gene PDH2, which encodes a subunit of the plastid pyruvate dehydrogenase complex. The mutants and associated flanking sequence data described here are publicly available to the research community, and they represent one of the largest phenotyped collections of algal insertional mutants to date.
Triple negative breast cancer (TNBC) is characterized by high rates of disease recurrence after definitive therapy, and median survival of less than 18 months in the metastatic setting. Systemic ...therapy options for TNBC consist primarily of cytotoxic chemotherapy-containing regimens, and while recently FDA-approved chemo-immunotherapy combinations and antibody-drug conjugates such as Sacituzumab govitecan have improved clinical outcomes, there remains an unmet need for more effective and less toxic therapies. A subset of TNBC expresses the androgen receptor (AR), a nuclear hormone steroid receptor that activates an androgen-responsive transcriptional program, and gene expression profiling has revealed a TNBC molecular subtype with AR expression and luminal and androgen responsive features. Both preclinical and clinical data suggest biologic similarities between luminal AR (LAR) TNBC and ER+ luminal breast cancer, including lower proliferative activity, relative chemoresistance, and high rates of oncogenic activating mutations in the phosphatidylinositol-3-kinase (PI3K) pathway. Preclinical LAR-TNBC models are sensitive to androgen signaling inhibitors (ASIs), and particularly given the availability of FDA-approved ASIs with robust efficacy in prostate cancer, there has been great interest in targeting this pathway in AR+ TNBC. Here, we review the underlying biology and completed and ongoing androgen-targeted therapy studies in early stage and metastatic AR+ TNBC.
The phosphoinositol-3 kinase (PI3K) pathway is highly dysregulated in squamous cell carcinoma of the head and neck (SCCHN). While inhibitors of the PI3K/AKT pathway are being developed in cancer, ...their efficacy does not appear to be related to the presence of mutations or amplification in pathway genes. The PI3K pathway is a major regulator of macro-autophagy, an evolutionarily conserved catabolic process that degrades cellular materials to promote cellular homeostasis and survival under stress. Employing a panel of SCCHN cell lines, we observed a significant correlation between the activity of PI3K/AKT inhibitors and their ability to induce autophagy. More specifically, resistance to these inhibitors was associated with accumulation of p62/SQSTM1, a pleotropic protein that is consumed during autophagy, while loss of autophagy was, for the first time, found to be due to silencing of an essential autophagy gene, ATG7. Moreover, modulating ATG7 and p62/SQSTM1 could regulate sensitivity to PI3K/AKT inhibitors, underscoring a mechanistic link between autophagy and drug sensitivity. Analysis of human tissues revealed progressive accumulation of p62/SQSTM1 in a significant proportion of cancer samples compared to normal tissue, suggesting that defective autophagy has relevance to SCCHN. These findings are further validated by analysis of TCGA data confirming homozygous deletion and mRNA down-regulation of ATG7 in 10.0% of SCCHN samples. Taken together, these data indicate that p62/SQSTM1 levels modulate sensitivity to PI3K/AKT inhibitors; cancers vary in their capacity to undergo autophagy through epigenetic modification and, when deficient, accumulate p62/SQSTM1; and expression of autophagy-related proteins may serve as markers for resistance to PI3K/AKT inhibitors in SCCHN.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Photosynthetic organisms synthesize carotenoids for harvesting light energy, photoprotection, and maintaining the structure and function of photosynthetic membranes. A light-sensitive, ...phytoene-accumulating mutant, pds1-1, was isolated in Chlamydomonas reinhardtii and found to be genetically linked to the phytoene desaturase (PDS) gene. PDS catalyzes the second step in carotenoid biosynthesis--the conversion of phytoene to ζ-carotene. Decreased accumulation of downstream colored carotenoids suggested that the pds1-1 mutant is leaky for PDS activity. A screen for enhancers of the pds1-1 mutation yielded the pds1-2 allele, which completely lacks PDS activity. A second independent null mutant (pds1-3) was identified using DNA insertional mutagenesis. Both null mutants accumulate only phytoene and no other carotenoids. All three phytoene-accumulating mutants exhibited slower growth rates and reduced plating efficiency compared to wild-type cells and white phytoene synthase mutants. Insight into amino acid residues important for PDS activity was obtained through the characterization of intragenic suppressors of pds1-2. The suppressor mutants fell into three classes: revertants of the pds1-1 point mutation, mutations that changed PDS amino acid residue Pro64 to Phe, and mutations that converted PDS residue Lys90 to Met. Characterization of pds1-2 intragenic suppressors coupled with computational structure prediction of PDS suggest that amino acids at positions 90 and 143 are in close contact in the active PDS enzyme and have important roles in its structural stability and/or activity.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Metastasis requires tumor cells to overcome a series of challenges to successfully travel to and colonize new microenvironments. As an adaptive (or maladaptive) response to stress, ...macroautophagy/autophagy has garnered increasing interest with respect to cancer metastasis, supported by clinical observations of increased autophagic flux in distant metastases relative to primary tumors. Recently, we identified a new role for autophagy in tumor cell motility through the turnover of focal adhesions, large multi-protein structures that link extracellular matrix-bound integrins to the cytoskeleton. The disassembly of focal adhesions at the cell rear is critical to forward movement and successful migration/invasion. We demonstrated that the focal adhesion protein PXN (paxillin), which serves as a crucial scaffolding and signal integrator, binds directly to LC3B through a conserved LC3-interacting region (LIR) motif to stimulate focal adhesion disassembly and metastasis and that this interaction is further promoted by oncogenic SRC.
The oligomeric Mg chelatase (MgCh), consisting of the subunits CHLH, CHLI, and CHLD, is located at the central site of chlorophyll synthesis, but is also thought to have an additional function in ...regulatory feedback control of the tetrapyrrole biosynthesis pathway and in chloroplast retrograde signaling. In Arabidopsis thaliana and Chlamydomonas reinhardtii, two genes have been proposed to encode the CHLI subunit of MgCh. While the role of CHLI1 in A. thaliana MgCh has been substantially elucidated, different reports provide inconsistent results with regard to the function of CHLI2 in Mg chelation and retrograde signaling. In the present report, the possible functions of both isoforms were analyzed in C. reinhardtii. Knockout of the CHLI1 gene resulted in complete loss of MgCh activity, absence of chlorophyll, acute light sensitivity, and, as a consequence, down-regulation of tetrapyrrole biosynthesis and photosynthesis-associated nuclear genes. These observations indicate a phenotypical resemblance of chli1 to the chlh and chld C. reinhardtii mutants previously reported. The key role of CHLI1 for MgCh reaction in comparison with the second isoform was confirmed by the rescue of chli1 with genomic CHLI1. Because CHLI2 in C. reinhardtii shows lower expression than CHLI1, strains overexpressing CHLI2 were produced in the chli1 background. However, no complementation of the chli1 phenotype was observed. Silencing of CHLI2 in the wild-type background did not result in any changes in the accumulation of tetrapyrrole intermediates or of chlorophyll. The results suggest that, unlike in A. thaliana, changes in CHLI2 content observed in the present studies do not affect formation and activity of MgCh in C. reinhardtii.
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