GCN5L1/BLOS1 Links Acetylation, Organelle Remodeling, and Metabolism

In-Cell Testing of Zinc-Dependent Histone Deacetylase Inhibitors in the Presence of Class-Selective Fluorogenic Substrates: Potential and Limitations of the Method

The development of anticancer drugs based on zinc-dependent histone deacetylase inhibitors (HDACi) has acquired great practical significance over the past decade. The most important HDACi characteristics are selectivity and strength of inhibition since they determine the mechanisms of therapeutic action. For in-cell testing of the selectivity of de novo-synthesized HDACi, Western blot analysis of the level of acetylation of bona fide protein substrates of HDACs of each class is usually used. However, the high labor intensity of this method prevents its widespread use in inhibitor screening. We developed an in-cell high-throughput screening method based on the use of three subtype-selective fluorogenic substrates of the general structure Boc-Lys(Acyl)-AMC, which in many cases makes it possible to determine the selectivity of HDACi at the class level. However, we found that the additional inhibitory activity of HDACi against metallo-β-lactamase domain-containing protein 2 (MBLAC2) leads to testing errors.

Mitochondrial CypD Acetylation Promotes Endothelial Dysfunction and Hypertension

BACKGROUND

Nearly half of adults have hypertension, a major risk factor for cardiovascular disease. Mitochondrial hyperacetylation is linked to hypertension, but the role of acetylation of specific proteins is not clear. We hypothesized that acetylation of mitochondrial CypD (cyclophilin D) at K166 contributes to endothelial dysfunction and hypertension.

METHODS

To test this hypothesis, we studied CypD acetylation in patients with essential hypertension, defined a pathogenic role of CypD acetylation in deacetylation mimetic CypD-K166R mutant mice and endothelial-specific GCN5L1 (general control of amino acid synthesis 5 like 1)-deficient mice using an Ang II (angiotensin II) model of hypertension.

RESULTS

Arterioles from hypertensive patients had 280% higher CypD acetylation coupled with reduced Sirt3 (sirtuin 3) and increased GCN5L1 levels. GCN5L1 regulates mitochondrial protein acetylation and promotes CypD acetylation, which is counteracted by mitochondrial deacetylase Sirt3. In human aortic endothelial cells, GCN5L1 depletion prevents superoxide overproduction. Deacetylation mimetic CypD-K166R mice were protected from vascular oxidative stress, endothelial dysfunction, and Ang II-induced hypertension. Ang II-induced hypertension increased mitochondrial GCN5L1 and reduced Sirt3 levels resulting in a 250% increase in GCN5L1/Sirt3 ratio promoting CypD acetylation. Treatment with mitochondria-targeted scavenger of cytotoxic isolevuglandins (mito2HOBA) normalized GCN5L1/Sirt3 ratio, reduced CypD acetylation, and attenuated hypertension. The role of mitochondrial acetyltransferase GCN5L1 in the endothelial function was tested in endothelial-specific GCN5L1 knockout mice. Depletion of endothelial GCN5L1 prevented Ang II-induced mitochondrial oxidative stress, reduced the maladaptive switch of vascular metabolism to glycolysis, prevented inactivation of endothelial nitric oxide, preserved endothelial-dependent relaxation, and attenuated hypertension.

CONCLUSIONS

These data support the pathogenic role of CypD acetylation in endothelial dysfunction and hypertension. We suggest that targeting cytotoxic mitochondrial isolevuglandins and GCN5L1 reduces CypD acetylation, which may be beneficial in cardiovascular disease.

Validation of GCN5L1/BLOC1S1/BLOS1 antibodies using knockout cells and tissue

GCN5L1, also known as BLOC1S1 and BLOS1, is a small intracellular protein involved in many key biological processes. Over the last decade, GCN5L1 has been implicated in the regulation of protein lysine acetylation, energy metabolism, endo-lysosomal function, and cellular immune pathways. An increasing number of published papers have used commercially-available reagents to interrogate GCN5L1 function. However, in many cases these reagents have not been rigorously validated, leading to potentially misleading results. In this report we tested several commercially-available antibodies for GCN5L1, and found that two-thirds of those available did not unambiguously detect the protein by western blot in cultured mouse cells or ex vivo liver tissue. These data suggest that previously published studies which used these unverified antibodies to measure GCN5L1 protein abundance, in the absence of other independent methods of corroboration, should be interpreted with appropriate caution.

GCN5L1-mediated acetylation prevents Rictor degradation in cardiac cells after hypoxic stress

Cardiomyocyte apoptosis and cardiac fibrosis are the leading causes of mortality in patients with ischemic heart disease. As such, these processes represent potential therapeutic targets to treat heart failure resulting from ischemic insult. We previously demonstrated that the mitochondrial acetyltransferase protein GCN5L1 regulates cardiomyocyte cytoprotective signaling in ischemia-reperfusion injury in vivo and hypoxia-reoxygenation injury in vitro. The current study investigated the mechanism underlying GCN5L1-mediated regulation of the Akt/mTORC2 cardioprotective signaling pathway. Rictor protein levels in cardiac tissues from human ischemic heart disease patients were significantly decreased relative to non-ischemic controls. Rictor protein levels were similarly decreased in cardiac AC16 cells following hypoxic stress, while mRNA levels remained unchanged. The reduction in Rictor protein levels after hypoxia was enhanced by the knockdown of GCN5L1, and was blocked by GCN5L1 overexpression. These findings correlated with changes in Rictor lysine acetylation, which were mediated by GCN5L1 acetyltransferase activity. Rictor degradation was regulated by proteasomal activity, which was antagonized by increased Rictor acetylation. Finally, we found that GCN5L1 knockdown restricted cytoprotective Akt signaling, in conjunction with decreased mTOR abundance and activity. In summary, these studies suggest that GCN5L1 promotes cardioprotective Akt/mTORC2 signaling by maintaining Rictor protein levels through enhanced lysine acetylation.

Cardiomyocyte-specific deletion of GCN5L1 reduces lysine acetylation and attenuates diastolic dysfunction in aged mice by improving cardiac fatty acid oxidation

Cardiac mitochondrial dysfunction is a critical contributor to the pathogenesis of aging and many age-related conditions. As such, complete control of mitochondrial function is critical to maintain cardiac efficiency in the aged heart. Lysine acetylation is a reversible post-translational modification shown to regulate several mitochondrial metabolic and biochemical processes. In the present study, we investigated how mitochondrial lysine acetylation regulates fatty acid oxidation (FAO) and cardiac function in the aged heart. We found a significant increase in mitochondrial protein acetylation in the aged heart which correlated with increased level of mitochondrial acetyltransferase-related protein GCN5L1. We showed that acetylation status of several fatty acid and glucose oxidation enzymes (long-chain acyl-coenzyme A dehydrogenase, hydroxyacyl-coA dehydrogenase, and pyruvate dehydrogenase) were significantly up-regulated in aged heart which correlated with decreased enzymatic activities. Using a cardiac-specific GCN5L1 knockout (KO) animal model, we showed that overall acetylation of mitochondrial proteins was decreased in aged KO animals, including FAO proteins which led to improved FAO activity and attenuated cardiac diastolic dysfunction observed in the aged heart. Together, these findings indicate that lysine acetylation regulates FAO in the aged heart which results in improved cardiac diastolic function and this is in part regulated by GCN5L1.

GCN5L1-mediated acetylation prevents Rictor degradation in cardiac cells after hypoxic stress

Cardiomyocyte apoptosis and cardiac fibrosis are the leading causes of mortality in patients with ischemic heart disease. As such, these processes represent potential therapeutic targets to treat heart failure resulting from ischemic insult. We previously demonstrated that the mitochondrial acetyltransferase protein GCN5L1 regulates cardiomyocyte cytoprotective signaling in ischemia-reperfusion injury in vivo and hypoxia-reoxygenation injury in vitro. The current study investigated the mechanism underlying GCN5L1-mediated regulation of the Akt/mTORC2 cardioprotective signaling pathway. Rictor protein levels in cardiac tissues from human ischemic heart disease patients were significantly decreased relative to non-ischemic controls. Rictor protein levels were similarly decreased in cardiac AC16 cells following hypoxic stress, while mRNA levels remained unchanged. The reduction in Rictor protein levels after hypoxia was enhanced by the knockdown of GCN5L1, and was blocked by GCN5L1 overexpression. These findings correlated with changes in Rictor lysine acetylation, which were mediated by GCN5L1 acetyltransferase activity. Rictor degradation was regulated by proteasomal activity, which was antagonized by increased Rictor acetylation. Finally, we found that GCN5L1 knockdown restricted cytoprotective Akt signaling, in conjunction with decreased mTOR abundance and activity. In summary, these studies suggest that GCN5L1 promotes cardioprotective Akt/mTORC2 signaling by maintaining Rictor protein levels through enhanced lysine acetylation.

Fibroblast-to-cardiomyocyte lactate shuttle modulates hypertensive cardiac remodelling

BACKGROUND

Cardiac fibroblasts (CFs) and cardiomyocytes are the major cell populations in the heart. CFs not only support cardiomyocytes by producing extracellular matrix (ECM) but also assimilate myocardial nutrient metabolism. Recent studies suggest that the classical intercellular lactate shuttle may function in the heart, with lactate transported from CFs to cardiomyocytes. However, the underlying mechanisms regarding the generation and delivery of lactate from CFs to cardiomyocytes have yet to be explored.

RESULTS

In this study, we found that angiotensin II (Ang II) induced CFs differentiation into myofibroblasts that, driven by cell metabolism, then underwent a shift from oxidative phosphorylation to aerobic glycolysis. During this metabolic conversion, the expression of amino acid synthesis 5-like 1 (GCN5L1) was upregulated and bound to and acetylated mitochondrial pyruvate carrier 2 (MPC2) at lysine residue 19. Hyperacetylation of MPC2k19 disrupted mitochondrial pyruvate uptake and mitochondrial respiration. GCN5L1 ablation downregulated MPC2K19 acetylation, stimulated mitochondrial pyruvate metabolism, and inhibited glycolysis and lactate accumulation. In addition, myofibroblast-specific GCN5L1-knockout mice (GCN5L1fl/fl: Periostin-Cre) showed reduced myocardial hypertrophy and collagen content in the myocardium. Moreover, cardiomyocyte-specific monocarboxylate transporter 1 (MCT1)-knockout mice (MCT1fl/fl: Myh6-Cre) exhibited blocked shuttling of lactate from CFs to cardiomyocytes and attenuated Ang II-induced cardiac hypertrophy.

CONCLUSIONS

Our findings suggest that GCN5L1-MPC2 signalling pathway alters metabolic patterns, and blocking MCT1 interrupts the fibroblast-to-cardiomyocyte lactate shuttle, which may attenuate cardiac remodelling in hypertension.

Validation of GCN5L1/BLOC1S1/BLOS1 Antibodies Using Knockout Cells and Tissue

GCN5L1, also known as BLOC1S1 and BLOS1, is a small intracellular protein involved in a number of key biological processes. Over the last decade, GCN5L1 has been implicated in the regulation of protein lysine acetylation, energy metabolism, endo-lysosomal function, and cellular immune pathways. An increasing number of published papers have used commercially-available reagents to interrogate GCN5L1 function. However, in many cases these reagents have not been rigorously validated, leading to potentially misleading results. In this report we tested several commercially-available antibodies for GCN5L1, and found that two-thirds of those available did not unambiguously detect the protein by western blot in cultured mouse cells or ex vivo liver tissue. These data suggest that previously published studies which used these unverified antibodies to measure GCN5L1 protein abundance, in the absence of other independent methods of corroboration, should be interpreted with appropriate caution.

Mitochondrial general control of amino acid synthesis 5 like 1 promotes nonalcoholic steatohepatitis development through ferroptosis-induced formation of neutrophil extracellular traps

BACKGROUND

Mitochondria play central roles in metabolic diseases including nonalcoholic steatohepatitis (NASH). However, how mitochondria regulate NASH progression remains largely unknown. Our previous findings demonstrate that mitochondrial general control of amino acid synthesis 5 like 1 (GCN5L1) is associated with mitochondrial metabolism. Nevertheless, the roles of GCN5L1 in NASH are unclear.

AIMS AND METHODS

The GCN5L1 expression was detected in the fatty livers of NASH patients and animals. Hepatocyte-specific GCN5L1 deficiency or overexpression mice were used to induce NASH models by feeding with a high-fat/high-cholesterol or methionine-choline deficient diet. The molecular mechanisms underlying GCN5L1-regulated NASH were further explored and verified in mice.

RESULTS AND CONCLUSIONS

GCN5L1 expression was increased in NASH patients. Upregulated GCN5L1 level was also illustrated in NASH mice. Mice with hepatocyte-specific GCN5L1 conditional knockout improved the inflammatory response compared to GCN5L1flox/flox mice. However, overexpression of mitochondrial GCN5L1 augmented the inflammatory response. Mechanically, GCN5L1 acetylated CypD and enhanced its binding with ATP5B, which induced the opening of mitochondrial permeability transition pores and the release of mitochondrial ROS into the cytoplasm. The increased ROS promoted ferroptosis of hepatocytes and induced accumulation of high mobility group box 1 in the microenvironment, which recruited neutrophils and induced the generation of neutrophil extracellular traps (NETs). NETs block impaired GCN5L1-induced NASH progression. Furthermore, the upregulation of GCN5L1 in NASH was contributed by lipid overload-induced endoplasmic reticulum stress. Together, mitochondrial GCN5L1 has a vital function in promoting NASH progression by regulating oxidative metabolism and the hepatic inflammatory microenvironment. Thus, GCN5L1 might be a potential intervention target in NASH treatment.

Comprehensive analysis of a pyroptosis-related gene signature of clinical and biological values in spinal cord injury

Background

Since some of the clinical examinations are not suitable for patients with severe spinal cord injury (SCI), blood biomarkers have been reported to reflect the severity of SCI. The objective of this study was to screen out the potential biomarkers associated with the diagnosis of SCI by bioinformatics analysis.

Methods

The microarray expression profiles of SCI were obtained from the Gene Expression Omnibus (GEO) database. Core genes correlated to pyroptosis were obtained by crossing the differential genes, and module genes were obtained by WGCNA analysis and lasso regression. The immune infiltration analysis and GSEA analysis revealed the essential effect of immune cells in the progression of SCI. In addition, the accuracy of the biomarkers in diagnosing SCI was subsequently evaluated and verified using the receiver operating characteristic curve (ROC) and qRT-PCR.

Results

A total of 423 DEGs were identified, among which 319 genes were upregulated and 104 genes were downregulated. Based on the WGCNA analysis, six potential biomarkers were screened out, including LIN7A, FCGR1A, FGD4, GPR27, BLOC1S1, and GALNT4. The results of ROC curves demonstrated the accurate value of biomarkers related to SCI. The immune infiltration analysis and GSEA analysis revealed the essential effect of immune cells in the progression of SCI, including macrophages, natural killer cells, and neutrophils. The qRT-PCR results verified that FGD4, FCAR1A, LIN7A, BLOC1S1, and GPR27 were significantly upregulated in SCI patients.

Conclusion

In this study, we identified and verified five immune pyroptosis-related hub genes by WGCNA and biological experiments. It is expected that the five identified potential biomarkers in peripheral white blood cells may provide a novel strategy for early diagnosis.

Retrograde regulation of mitochondrial fission and epithelial to mesenchymal transition in hepatocellular carcinoma by GCN5L1

Metabolic reprogram is crucial to support cancer cell growth and movement as well as determine cell fate. Mitochondrial protein acetylation regulates mitochondrial metabolism, which is relevant to cancer cell migration and invasion. The functional role of mitochondrial protein acetylation on cancer cell migration remains unclear. General control of amino acid synthesis 5 like-1(GCN5L1), as the regulator of mitochondrial protein acetylation, functions on metabolic reprogramming in mouse livers. In this study, we find that GCN5L1 expression is significantly decreased in metastatic HCC tissues. Loss of GCN5L1 promotes reactive oxygen species (ROS) generation through enhanced fatty acid oxidation (FAO), followed by activation of cellular ERK and DRP1 to promote mitochondrial fission and epithelia to mesenchymal transition (EMT) to boost cell migration. Moreover, palmitate and carnitine-stimulated FAO promotes mitochondrial fission and EMT gene expression to activate HCC cell migration. On the other hand, increased cellular acetyl-CoA level, the product of FAO, enhances HCC cell migration. Taken together, our finding uncovers the metastasis suppressor role as well as the underlying mechanism of GCN5L1 in HCC and also provides evidence of FAO retrograde control of HCC metastasis.

The endo-lysosomal regulatory protein BLOC1S1 modulates hepatic lysosomal content and lysosomal lipolysis

BLOC1S1 is a common component of BLOC and BORC multiprotein complexes which play distinct roles in endosome and lysosome biology. Recent human mutations in BLOC1S1 associate with juvenile leukodystrophy. As leukodystrophy is linked to perturbed lysosomal lipid storage we explored whether BLOC1S1 itself modulates this biology. Given the central role of the liver in lipid storage, our investigations were performed in hepatocyte specific liver bloc1s1 knockout (LKO) mice and in human hepatocyte-like lines (HLCs) derived from inducible pluripotential stem cells (iPSCs) from a juvenile leukodystrophy subject's with bloc1s1 mutations and from isogenic corrected iPSCs. Here we show that hepatocyte lipid stores are diminished in parallel with increased lysosomal content, increased lysosomal lipid uptake and lipolysis in LKO mice. The lysosomal lipolysis program was independent of macro- and chaperone-mediated lipophagy but dependent on cellular lysosome content. In parallel, genetic induction of lysosomal biogenesis in a transformed hepatocyte cell line replicated depletion of intracellular lipid stores. Interestingly bloc1s1 mutant and isogenic corrected HLCs both showed normal lysosomal enzyme activity. However, relative to the isogenic corrected HLCs, mutant bloc1s1 HLCs showed reduced lysosomal content and increased lipid storage. Together these data show distinct phenotypes in human mutant HLCs compared to murine knockout cells. At the same time, human blcs1s1 mutation and murine hepatocyte bloc1s1 depletion disrupt lysosome content and the cellular lipid storage. These data support that BLOC1S1 modulates lysosome content and lipid handling independent of autophagy and show that lysosomal lipolysis is dependent on the cellular content of functional lysosomes.

Endosomal trafficking in metabolic homeostasis and diseases

The global prevalences of obesity and type 2 diabetes mellitus have reached epidemic status, presenting a heavy burden on society. It is therefore essential to find novel mechanisms and targets that could be utilized in potential treatment strategies and, as such, intracellular membrane trafficking has re-emerged as a regulatory tool for controlling metabolic homeostasis. Membrane trafficking is an essential physiological process that is responsible for the sorting and distribution of signalling receptors, membrane transporters and hormones or other ligands between different intracellular compartments and the plasma membrane. Dysregulation of intracellular transport is associated with many human diseases, including cancer, neurodegeneration, immune deficiencies and metabolic diseases, such as type 2 diabetes mellitus and its associated complications. This Review focuses on the latest advances on the role of endosomal membrane trafficking in metabolic physiology and pathology in vivo, highlighting the importance of this research field in targeting metabolic diseases.

GCN5L1-mediated TFAM acetylation at K76 participates in mitochondrial biogenesis in acute kidney injury

BACKGROUND

Mitochondrial dysfunction is an important pathogenic event in acute kidney injury (AKI). GCN5L1 is a specific acetyltransferase in mitochondria, which regulates glucose and fatty acid metabolism. However, the role of GCN5L1 in mitochondrial dysfunction and the pathogenesis of ischemic AKI are not fully understood.

METHODS

The protein level of GCN5L1 was detected by western blot assay. Acetylated proteomics was used to explore the level of acetylated TFAM. Duolink proximity ligation assay and co-immunoprecipitation were used to detect the interaction of TFAM and translocase of outer membrane 70 (TOM70). mtDNA copy number, the expression of mitochondrial electron transport chain complexes, the number and morphology of mitochondria were measured. The renal injury of AKI mice was reflected by the levels of creatinine and urea nitrogen and the pathological changes of renal tissue.

RESULTS

We showed that GCN5L1 was highly expressed in vivo and in vitro and renal tubules specific knockdown of GCN5L1 could effectively attenuate AKI-induced mitochondrial impairment. Besides, acetylated proteomics revealed that acetylated TFAM was significantly upregulated in AKI mice kidney, which reminded us that TFAM might be an acetylating substrate of GCN5L1. Mechanistically, we evidenced that GCN5L1 could acetylate TFAM at its K76 site and subsequently inhibited its binding to TOM70, thereby reducing TFAM import into mitochondria and mitochondrial biogenesis. Clinically, GCN5L1 and acetylated TFAM were positively correlated with disease severity (all p < 0.05).

CONCLUSIONS

In sum, these data demonstrated an unrecognized regulating mechanism of GCN5L1 on TFAM acetylation and its intracellular trafficking, and a potential intervening target for AKI associated mitochondrial disorders as well.

Regulation of Blos1 by IRE1 prevents the accumulation of Huntingtin protein aggregates

Huntington's disease is characterized by accumulation of the aggregation-prone mutant Huntingtin (mHTT) protein. Here, we show that expression of exon 1 of mHTT in mouse cultured cells activates IRE1, the transmembrane sensor of stress in the endoplasmic reticulum, leading to degradation of the Blos1 mRNA and repositioning of lysosomes and late endosomes toward the microtubule organizing center. Overriding Blos1 degradation results in excessive accumulation of mHTT aggregates in both cultured cells and primary neurons. Although mHTT is degraded by macroautophagy when highly expressed, we show that before the formation of large aggregates, mHTT is degraded via an ESCRT-dependent, macroautophagy-independent pathway consistent with endosomal microautophagy. This pathway is enhanced by Blos1 degradation and appears to protect cells from a toxic, less aggregated form of mHTT.

Brucella activates the host RIDD pathway to subvert BLOS1-directed immune defense

The phagocytosis and destruction of pathogens in lysosomes constitute central elements of innate immune defense. Here, we show that Brucella, the causative agent of brucellosis, the most prevalent bacterial zoonosis globally, subverts this immune defense pathway by activating regulated IRE1α-dependent decay (RIDD) of Bloc1s1 mRNA encoding BLOS1, a protein that promotes endosome-lysosome fusion. RIDD-deficient cells and mice harboring a RIDD-incompetent variant of IRE1α were resistant to infection. Inactivation of the Bloc1s1 gene impaired the ability to assemble BLOC-1-related complex (BORC), resulting in differential recruitment of BORC-related lysosome trafficking components, perinuclear trafficking of Brucella-containing vacuoles (BCVs), and enhanced susceptibility to infection. The RIDD-resistant Bloc1s1 variant maintains the integrity of BORC and a higher-level association of BORC-related components that promote centrifugal lysosome trafficking, resulting in enhanced BCV peripheral trafficking and lysosomal destruction, and resistance to infection. These findings demonstrate that host RIDD activity on BLOS1 regulates Brucella intracellular parasitism by disrupting BORC-directed lysosomal trafficking. Notably, coronavirus murine hepatitis virus also subverted the RIDD-BLOS1 axis to promote intracellular replication. Our work establishes BLOS1 as a novel immune defense factor whose activity is hijacked by diverse pathogens.

Role of yUbp8 in Mitochondria and Hypoxia Entangles the Finding of Human Ortholog Usp22 in the Glioblastoma Pseudo-Palisade Microlayer

KAT Gcn5 and DUB Ubp8 are required for respiration and mitochondria functions in budding yeast, and in this study we show that loss of respiratory activity is acquired over time. Interestingly, we show that absence of Ubp8 allows cells to grow in hypoxic conditions with altered mitophagy. Comparatively, the aggressive glioblastoma (GBM) multiforme tumor shows survival mechanisms able to overcome hypoxia in the brain. Starting from yeast and our findings on the role of Ubp8 in hypoxia, we extended our analysis to the human ortholog and signature cancer gene Usp22 in glioblastoma tumor specimens. Here we demonstrate that Usp22 is localized and overexpressed in the pseudo-palisade tissue around the necrotic area of the tumor. In addition, Usp22 colocalizes with the mitophagy marker Parkin, indicating a link with mitochondria function in GBM. Collectively, this evidence suggests that altered expression of Usp22 might provide a way for tumor cells to survive in hypoxic conditions, allowing the escape of cells from the necrotic area toward vascularized tissues. Collectively, our experimental data suggest a model for a possible mechanism of uncontrolled proliferation and invasion in glioblastoma.

Disruption of mitochondrial quality control genes promotes caspase-resistant cell survival following apoptotic stimuli

In cells undergoing cell-intrinsic apoptosis, mitochondrial outer membrane permeabilization (MOMP) typically marks an irreversible step in the cell death process. However, in some cases, a subpopulation of treated cells can exhibit a sublethal response, termed "minority MOMP." In this phenomenon, the affected cells survive, despite a low level of caspase activation and subsequent limited activation of the endonuclease caspase-activated DNase (DNA fragmentation factor subunit beta). Consequently, these cells can experience DNA damage, increasing the probability of oncogenesis. However, little is known about the minority MOMP response. To discover genes that affect the MOMP response in individual cells, we conducted an imaging-based phenotypic siRNA screen. We identified multiple candidate genes whose downregulation increased the heterogeneity of MOMP within single cells, among which were genes related to mitochondrial dynamics and mitophagy that participate in the mitochondrial quality control (MQC) system. Furthermore, to test the hypothesis that functional MQC is important for reducing the frequency of minority MOMP, we developed an assay to measure the clonogenic survival of caspase-engaged cells. We found that cells deficient in various MQC genes were indeed prone to aberrant post-MOMP survival. Our data highlight the important role of proteins involved in mitochondrial dynamics and mitophagy in preventing apoptotic dysregulation and oncogenesis.

Post-translational lysine ac(et)ylation in health, ageing and disease

The acetylation/acylation (ac(et)ylation) of lysine side chains is a dynamic post-translational modification (PTM) regulating fundamental cellular processes with implications on the organisms' ageing process: metabolism, transcription, translation, cell proliferation, regulation of the cytoskeleton and DNA damage repair. First identified to occur on histones, later studies revealed the presence of lysine ac(et)ylation in organisms of all kingdoms of life, in proteins covering all essential cellular processes. A remarkable finding showed that the NAD⁺-dependent sirtuin deacetylase Sir2 has an impact on replicative lifespan in Saccharomyces cerevisiae suggesting that lysine acetylation has a direct role in the ageing process. Later studies identified sirtuins as mediators for beneficial effects of caloric/dietary restriction on the organisms' health- or lifespan. However, the molecular mechanisms underlying these effects are only incompletely understood. Progress in mass-spectrometry, structural biology, synthetic and semi-synthetic biology deepened our understanding of this PTM. This review summarizes recent developments in the research field. It shows how lysine ac(et)ylation regulates protein function, how it is regulated enzymatically and non-enzymatically, how a dysfunction in this post-translational machinery contributes to disease development. A focus is set on sirtuins and lysine acyltransferases as these are direct sensors and mediators of the cellular metabolic state. Finally, this review highlights technological advances to study lysine ac(et)ylation.

Loss of the mitochondrial phosphate carrier SLC25A3 induces remodeling of the cardiac mitochondrial protein acylome

Mitochondria are recognized as signaling organelles because, under stress, mitochondria can trigger various signaling pathways to coordinate the cell's response. The specific pathway(s) engaged by mitochondria in response to mitochondrial energy defects in vivo and in high-energy tissues like the heart are not fully understood. Here, we investigated cardiac pathways activated in response to mitochondrial energy dysfunction by studying mice with cardiomyocyte-specific loss of the mitochondrial phosphate carrier (SLC25A3), an established model that develops cardiomyopathy as a result of defective mitochondrial ATP synthesis. Mitochondrial energy dysfunction induced a striking pattern of acylome remodeling, with significantly increased post-translational acetylation and malonylation. Mass spectrometry-based proteomics further revealed that energy dysfunction-induced remodeling of the acetylome and malonylome preferentially impacts mitochondrial proteins. Acetylation and malonylation modified a highly interconnected interactome of mitochondrial proteins, and both modifications were present on the enzyme isocitrate dehydrogenase 2 (IDH2). Intriguingly, IDH2 activity was enhanced in SLC25A3-deleted mitochondria, and further study of IDH2 sites targeted by both acetylation and malonylation revealed that these modifications can have site-specific and distinct functional effects. Finally, we uncovered a novel crosstalk between the two modifications, whereby mitochondrial energy dysfunction-induced acetylation of sirtuin 5 (SIRT5), inhibited its function. Because SIRT5 is a mitochondrial deacylase with demalonylase activity, this finding suggests that acetylation can modulate the malonylome. Together, our results position acylations as an arm of the mitochondrial response to energy dysfunction and suggest a mechanism by which focal disruption to the energy production machinery can have an expanded impact on global mitochondrial function.

Autophagy in metabolism and quality control: opposing, complementary or interlinked functions?

The sensu stricto autophagy, macroautophagy, is considered to be both a metabolic process as well as a bona fide quality control process. The question as to how these two aspects of autophagy are coordinated and whether and why they overlap has implications for fundamental aspects, pathophysiological effects, and pharmacological manipulation of autophagy. At the top of the regulatory cascade controlling autophagy are master regulators of cellular metabolism, such as MTOR and AMPK, which render the system responsive to amino acid and glucose starvation. At the other end exists a variety of specific autophagy receptors, engaged in the selective removal of a diverse array of intracellular targets, from protein aggregates/condensates to whole organelles such as mitochondria, ER, peroxisomes, lysosomes and lipid droplets. Are the roles of autophagy in metabolism and quality control mutually exclusive, independent or interlocked? How are priorities established? What are the molecular links between both phenomena? This article will provide a starting point to formulate these questions, the responses to which should be taken into consideration in future autophagy-based interventions.

Combining exome/genome sequencing with data repository analysis reveals novel gene-disease associations for a wide range of genetic disorders

PURPOSE

Within this study, we aimed to discover novel gene-disease associations in patients with no genetic diagnosis after exome/genome sequencing (ES/GS).

METHODS

We followed two approaches: (1) a patient-centered approach, which after routine diagnostic analysis systematically interrogates variants in genes not yet associated to human diseases; and (2) a gene variant centered approach. For the latter, we focused on de novo variants in patients that presented with neurodevelopmental delay (NDD) and/or intellectual disability (ID), which are the most common reasons for genetic testing referrals. Gene-disease association was assessed using our data repository that combines ES/GS data and Human Phenotype Ontology terms from over 33,000 patients.

RESULTS

We propose six novel gene-disease associations based on 38 patients with variants in the BLOC1S1, IPO8, MMP15, PLK1, RAP1GDS1, and ZNF699 genes. Furthermore, our results support causality of 31 additional candidate genes that had little published evidence and no registered OMIM phenotype (56 patients). The phenotypes included syndromic/nonsyndromic NDD/ID, oral-facial-digital syndrome, cardiomyopathies, malformation syndrome, short stature, skeletal dysplasia, and ciliary dyskinesia.

CONCLUSION

Our results demonstrate the value of data repositories which combine clinical and genetic data for discovering and confirming gene-disease associations. Genetic laboratories should be encouraged to pursue such analyses for the benefit of undiagnosed patients and their families.

General Control of Amino Acid Synthesis 5-Like 1-Mediated Acetylation of Manganese Superoxide Dismutase Regulates Oxidative Stress in Diabetic Kidney Disease

Diabetic kidney disease (DKD) is the major cause of end-stage renal disease (ESRD). In the past few decades, there has been a large amount of evidence to highlight the pivotal role of oxidative stress in the development and progression of DKD. However, the detailed molecular mechanisms are not fully elucidated. A new sight has been established that the mitochondrial acetyltransferase GCN5L1 participates in cellular redox homeostasis maintenance in DKD. Firstly, we found that the expression of GCN5L1 is significantly elevated both in human and mouse kidney tissues with DKD and in hyperglycemic renal tubular epithelial cells (TECs), while deletion of GCN5L1 could effectively ameliorate oxidative stress-induced renal injury in DKD. Furthermore, deletion of GCN5L1 could reduce MnSOD acetylation on lysine 68 and activate its activity, thereby scavenging excessive ROS and relieving oxidative stress-induced renal inflammation and fibrosis. In general, GCN5L1-mediated acetylation of MnSOD exacerbated oxidative stress-induced renal injury, suggesting that GCN5L1 might be a potential intervention target in DKD.

The emerging roles of GCN5L1 in mitochondrial and vacuolar organelle biology

General control of amino acid synthesis 5 like 1 (GCN5L1) was named due to its loose sequence alignment to GCN5, a catalytic subunit of numerous histone N-acetyltransferase complexes. Further studies show that GCN5L1 has mitochondrial and cytosolic isoforms, although functional-domain sequence alignment and experimental studies show that GCN5L1 itself does not possess intrinsic acetyltransferase activity. Nevertheless, GCN5L1 does support protein acetylation in the mitochondria and cytosol and functions as a subunit of numerous intracellular multiprotein complexes that control intracellular vacuolar organelle positioning and function. The majority of GCN5L1 studies have focused on distinct intracellular functions and in this review, we summarize these findings as well as postulate what may be common features of the diverse phenotypes linked to GCN5L1.

BLOS1 mediates kinesin switch during endosomal recycling of LDL receptor

Low-density lipoprotein receptor (LDLR) in hepatocytes plays a key role in plasma clearance of circulating LDL and in whole body cholesterol homeostasis. The trafficking of LDLR is highly regulated in clathrin-dependent endocytosis, endosomal recycling and lysosomal degradation. Current studies focus on its endocytosis and degradation. However, the detailed molecular and cellular mechanisms underlying its endosomal recycling are largely unknown. We found that BLOS1, a shared subunit of BLOC-1 and BORC, is involved in LDLR endosomal recycling. Loss of BLOS1 leads to less membrane LDLR and impairs LDL clearance from plasma in hepatocyte-specific BLOS1 knockout mice. BLOS1 interacts with kinesin-3 motor KIF13A, and BLOS1 acts as a new adaptor for kinesin-2 motor KIF3 to coordinate kinesin-3 and kinesin-2 during the long-range transport of recycling endosomes (REs) to plasma membrane along microtubule tracks to overcome hurdles at microtubule intersections. This provides new insights into RE’s anterograde transport and the pathogenesis of dyslipidemia.

Catalysis by protein acetyltransferase Gcn5

Gcn5 serves as the defining member of the Gcn5-related N-acetyltransferase (GNAT) superfamily of proteins that display a common structural fold and catalytic mechanism involving the transfer of the acyl-group, primarily acetyl-, from CoA to an acceptor nucleophile. In the case of Gcn5, the target is the ε-amino group of lysine primarily on histones. Over the years, studies on Gcn5 structure-function have often formed the basis by which we understand the complex activities and regulation of the entire protein acetyltransferase family. It is now appreciated that protein acetylation occurs on thousands of proteins and can reversibly regulate the function of many cellular processes. In this review, we provide an overview of our fundamental understanding of catalysis, regulation of activity and substrate selection, and inhibitor development for this archetypal acetyltransferase.

Chemical acetylation of mitochondrial transcription factor A occurs on specific lysine residues and affects its ability to change global DNA topology

Chemical acetylation is postulated to occur in mitochondria. Mitochondrial transcription factor A (TFAM or mtTFA), a mitochondrial transcription initiation factor as well as the major mitochondrial nucleoid protein coating the entire mitochondrial genome, is proposed to be acetylated in animals and cultured cells. This study investigated the properties of human TFAM, in conjunction with the mechanism and effects of TFAM acetylation in vitro. Using highly purified recombinant human TFAM and 3 kb circular DNA as a downsized mtDNA model, we studied how the global TFAM-DNA interaction is affected/regulated by the quantitative TFAM-DNA relationship and TFAM acetylation. Results showed that the TFAM-DNA ratio strictly affects the TFAM property to unwind circular DNA in the presence of topoisomerase I. Mass spectrometry analysis showed that in vitro chemical acetylation of TFAM with acetyl-coenzyme A occurs preferentially on specific lysine residues, including those reported to be acetylated in exogenously expressed TFAM in cultured human cells, indicating that chemical acetylation plays a crucial role in TFAM acetylation in mitochondria. Intriguingly, the modification significantly decreased TFAM's DNA-unwinding ability, while its DNA-binding ability was largely unaffected. Altogether, we propose TFAM is chemically acetylated in vivo, which could change mitochondrial DNA topology, leading to copy number and gene expression modulation.

Cardiac metabolism as a driver and therapeutic target of myocardial infarction

Reducing infarct size during a cardiac ischaemic‐reperfusion episode is still of paramount importance, because the extension of myocardial necrosis is an important risk factor for developing heart failure. Cardiac ischaemia‐reperfusion injury (IRI) is in principle a metabolic pathology as it is caused by abruptly halted metabolism during the ischaemic episode and exacerbated by sudden restart of specific metabolic pathways at reperfusion. It should therefore not come as a surprise that therapy directed at metabolic pathways can modulate IRI. Here, we summarize the current knowledge of important metabolic pathways as therapeutic targets to combat cardiac IRI. Activating metabolic pathways such as glycolysis (eg AMPK activators), glucose oxidation (activating pyruvate dehydrogenase complex), ketone oxidation (increasing ketone plasma levels), hexosamine biosynthesis pathway (O‐GlcNAcylation; administration of glucosamine/glutamine) and deacetylation (activating sirtuins 1 or 3; administration of NAD⁺‐boosting compounds) all seem to hold promise to reduce acute IRI. In contrast, some metabolic pathways may offer protection through diminished activity. These pathways comprise the malate‐aspartate shuttle (in need of novel specific reversible inhibitors), mitochondrial oxygen consumption, fatty acid oxidation (CD36 inhibitors, malonyl‐CoA decarboxylase inhibitors) and mitochondrial succinate metabolism (malonate). Additionally, protecting the cristae structure of the mitochondria during IR, by maintaining the association of hexokinase II or creatine kinase with mitochondria, or inhibiting destabilization of F_OF₁‐ATPase dimers, prevents mitochondrial damage and thereby reduces cardiac IRI. Currently, the most promising and druggable metabolic therapy against cardiac IRI seems to be the singular or combined targeting of glycolysis, O‐GlcNAcylation and metabolism of ketones, fatty acids and succinate.

Mitochondrial functions and rare diseases

Mitochondria are dynamic cellular organelles responsible for a large variety of biochemical processes as energy transduction, REDOX signaling, the biosynthesis of hormones and vitamins, inflammation or cell death execution. Cell biology studies established that 1158 human genes encode proteins localized to mitochondria, as registered in MITOCARTA. Clinical studies showed that a large number of these mitochondrial proteins can be altered in expression and function through genetic, epigenetic or biochemical mechanisms including the interaction with environmental toxics or iatrogenic medicine. As a result, pathogenic mitochondrial genetic and functional defects participate to the onset and the progression of a growing number of rare diseases. In this review we provide an exhaustive survey of the biochemical, genetic and clinical studies that demonstrated the implication of mitochondrial dysfunction in human rare diseases. We discuss the striking diversity of the symptoms caused by mitochondrial dysfunction and the strategies proposed for mitochondrial therapy, including a survey of ongoing clinical trials.

Mitochondrial General Control of Amino Acid Synthesis 5 Like 1 Regulates Glutaminolysis, Mammalian Target of Rapamycin Complex 1 Activity, and Murine Liver Regeneration

BACKGROUND AND AIMS

The regenerative capacity of the liver plays a protective role against hepatotoxins and impaired regeneration exacerbates liver dysfunction in nonalcoholic fatty liver disease (NAFLD). Mitochondrial bioenergetic and -synthetic functions are important contributory factors in hepatic regeneration, and the control of mitochondrial protein acetylation is implicated in the mitochondrial susceptibility to liver stressors. Here, we evaluated the role of general control of amino acid synthesis 5 like 1 (GCN5L1), a mediator of mitochondrial metabolism and acetylation, in modulating murine liver regeneration (LR) in response to acute CCl₄ -induced hepatotoxicity.

APPROACH AND RESULTS

Initial metabolomic screening found that liver GCN5L1 knockout (LKO) mice have augmented glutaminolysis. Absence of GCN5L1 modified enzyme activity of liver-enriched glutaminase enzyme (glutaminase 2; GLS2), and GCN5L1 levels modulated GLS2 oligomerization and acetylation. This metabolic remodeling resulted in the elevation of α-ketoglutarate levels, which are known to activate mammalian target of rapamycin complex 1 (mTORC1). This signaling pathway was induced with increased phosphorylation of S6 kinase in LKO hepatocytes, and inhibition of glutaminolysis reversed aberrant mTORC1 signaling. At the same time, glutaminolysis, activity of GLS2, and activation of mTORC1 signaling were reversed by the genetic reintroduction of the mitochondrial isoform of GCN5L1 into LKO primary hepatocytes. Finally, LKO mice had a more robust regenerative capacity in response to CCl₄ hepatoxicity, and this response was blunted by both the mTORC1 inhibitor, rapamycin, and by pharmacological blunting of glutaminolysis.

CONCLUSIONS

These data point to a central role of glutaminolysis in modulating the regenerative capacity in the liver. Furthermore, inhibition of mitochondrial GCN5L1 to augment LR may be a useful strategy in disease states linked to hepatotoxicity.

GCN5L1 controls renal lipotoxicity through regulating acetylation of fatty acid oxidation enzymes

Dyslipidemia is a common risk factor of chronic kidney disease (CKD). Current notion suggests that insufficient intracellular fatty acid oxidation (FAO) and subsequently enhanced fatty acid esterification within renal resident cells, a process termed as renal lipotoxicity, is the key pathogenic event responsible for dyslipidemia-induced kidney injury. However, the detailed mechanism is not fully elucidated. Recently, accumulating data indicated that acetylation modification is an important regulating manner for both mitochondrial function and energy metabolism, while whether acetylation modification is involved in renal lipotoxicity is of little known. In the present study, the expression level of global lysine acetylation was detected by immunohistochemistry in high-fat diet mice and western blot in palmitic acid (PA) stimulated HK-2 cells. The acetylation levels of long-chain acyl-CoA dehydrogenases (LCAD) and β-hydroxyacyl-CoA dehydrogenase (β-HAD) were measured by immunoprecipitation. And a multifunction microplate reader was applied to detect FAO rate, triglyceride and acyl-CoA contents, and the enzyme activities, with cellular lipid accumulation identified by Oil Red O staining. We evidenced the acetylation levels of LCAD and β-HAD that were enhanced, which led to decreased enzymatic activities and impaired FAO rate. Furthermore, renal protein hyperacetylation induced by lipid overload was associated with increased expression of GCN5L1. And the silence of GCN5L1 in tubular epithelial cells resulted in deacetylation and activation of LCAD and β-HAD. Finally, excess lipids induced lipotoxicity and epithelial-mesenchymal transition (EMT) were ameliorated by GCN5L1 suppression, suggesting GCN5L1-mediated mitochondrial LCAD and β-HAD acetylation might be a key pathogenic event underlying excess lipids induced FAO impairment.

Protein Lysine Acetylation: Grease or Sand in the Gears of β-Cell Mitochondria?

Mitochondria carry out many essential functions in metabolism. A central task is the oxidation of nutrients and the generation of ATP by oxidative phosphorylation. Mitochondrial metabolism needs to be tightly regulated for the cell to respond to changes in ATP demand and nutrient supply. Here, we review how protein lysine acetylation contributes to the regulation of mitochondrial metabolism in insulin target tissues and the insulin-secreting pancreatic β-cell. We summarize recent evidence showing that in pancreatic β-cells, lysine acetylation occurs on a large number of proteins involved in metabolism. Furthermore, we give a brief overview of the molecular mechanism that controls lysine acetylation dynamics. We propose that protein lysine acetylation is an important mechanism for the fine-tuning of mitochondrial activity in β-cells during normal physiology. In contrast, nutrient oversupply, oxidative stress, or inhibition of the mitochondrial deacetylase SIRT3 leads to protein lysine hyperacetylation, which impairs mitochondrial function. By perturbing mitochondrial activity in β-cells and insulin target tissues, protein lysine hyperacetylation may contribute to the development of type 2 diabetes.

Crosstalk Between Mitochondrial Hyperacetylation and Oxidative Stress in Vascular Dysfunction and Hypertension

Significance: Vascular dysfunction plays a key role in the development of arteriosclerosis, heart disease, and hypertension, which causes one-third of deaths worldwide. Vascular oxidative stress and metabolic disorders contribute to vascular dysfunction, leading to impaired vasorelaxation, vascular hypertrophy, fibrosis, and aortic stiffening. Mitochondria are critical in the regulation of metabolic and antioxidant functions; therefore, mitochondria-targeted treatments could be beneficial. Recent Advances: Vascular dysfunction is crucial in hypertension pathophysiology and exhibits bidirectional relationship. Metabolic disorders and oxidative stress contribute to the pathogenesis of vascular dysfunction and hypertension, which are associated with mitochondrial impairment and hyperacetylation. Mitochondrial deacetylase Sirtuin 3 (Sirt3) is critical in the regulation of metabolic and antioxidant functions. Clinical studies show that cardiovascular disease risk factors reduce Sirt3 level and Sirt3 declines with age, paralleling the increased incidence of cardiovascular disease and hypertension. An imbalance between mitochondrial acetylation and reduced Sirt3 activity contributes to mitochondrial dysfunction and oxidative stress. We propose that mitochondrial hyperacetylation drives a vicious cycle between metabolic disorders and mitochondrial oxidative stress, promoting vascular dysfunction and hypertension. Critical Issues: The mechanisms of mitochondrial dysfunction are still obscure in human hypertension. Mitochondrial hyperacetylation and oxidative stress contribute to mitochondrial dysfunction; however, regulation of mitochondrial acetylation, the role of GCN5L1 (acetyl-CoA-binding protein promoting acetyltransferase protein acetylation) acetyltransferase, Sirt3 deacetylase, and acetylation of specific proteins require further investigations. Future Directions: There is an urgent need to define molecular mechanisms and the pathophysiological role of mitochondrial hyperacetylation, identify novel pharmacological targets, and develop therapeutic approaches to reduce this phenomenon.

Degradation of Blos1 mRNA by IRE1 repositions lysosomes and protects cells from stress

Bae et al. show that degradation of the mRNA encoding the adaptor Blos1 leads to the repositioning of late endosomes/lysosomes to the microtubule-organizing center in response to ER stress. This repositioning enhances cell survival during stress by promoting the clearance of protein aggregates.

Cells respond to stress in the ER by initiating the widely conserved unfolded protein response. Activation of the ER transmembrane nuclease IRE1 leads to the degradation of specific mRNAs, but how this pathway affects the ability of cells to recover from stress is not known. Here, we show that degradation of the mRNA encoding biogenesis of lysosome-related organelles 1 subunit 1 (Blos1) leads to the repositioning of late endosomes (LEs)/lysosomes to the microtubule-organizing center in response to stress in mouse cells. Overriding Blos1 degradation led to ER stress sensitivity and the accumulation of ubiquitinated protein aggregates, whose efficient degradation required their independent trafficking to the cell center and the LE-associated endosomal sorting complexes required for transport. We propose that Blos1 regulation by IRE1 promotes LE-mediated microautophagy of protein aggregates and protects cells from their cytotoxic effects.

Cardiac-specific deletion of GCN5L1 restricts recovery from ischemia-reperfusion injury

GCN5L1 regulates mitochondrial protein acetylation, cellular bioenergetics, reactive oxygen species (ROS) generation, and organelle positioning in a number of diverse cell types. However, the functional role of GCN5L1 in the heart is currently unknown. As many of the factors regulated by GCN5L1 play a major role in ischemia-reperfusion (I/R) injury, we sought to determine if GCN5L1 is an important nexus in the response to cardiac ischemic stress. Deletion of GCN5L1 in cardiomyocytes resulted in impaired myocardial post-ischemic function and increased infarct development in isolated work-performing hearts. GCN5L1 knockout hearts displayed hallmarks of ROS damage, and scavenging of ROS restored cardiac function and reduced infarct volume in vivo. GCN5L1 knockdown in cardiac-derived AC16 cells was associated with reduced activation of the pro-survival MAP kinase ERK1/2, which was also reversed by ROS scavenging, leading to restored cell viability. We therefore conclude that GCN5L1 activity provides an important protection against I/R induced, ROS-mediated damage in the ischemic heart.

Epigenetic Factors and Mitochondrial Biology in Yeast: A New Paradigm for the Study of Cancer Metabolism?

Bidirectional cross-talk between nuclear and mitochondrial DNA is fundamental for cell homeostasis. Epigenetic mechanisms regulate the inter-organelle communication between nucleus and mitochondria. Recent research highlights not only the retrograde activation of nuclear gene transcription in case of mitochondria dysfunction, but also the role of post-translational modifications of mitochondrial proteins in respiratory metabolism. Here we discuss some aspects and novel findings in Saccharomyces cerevisiae. In yeast, KAT-Gcn5 and DUB-Ubp8 have a role in respiration and are localized, as single proteins, into mitochondria. These findings, beside the canonical and widely known nuclear activity of SAGA complex in chromatin regulation, provide novel clues on promising aspects linking evolutionary conserved epigenetic factors to the re-programmed metabolism of cancer cells.

GCN5L1 interacts with αTAT1 and RanBP2 to regulate hepatic α-tubulin acetylation and lysosome trafficking

Although GCN5L1 (also known as BLOC1S1) facilitates mitochondrial protein acetylation and controls endosomal-lysosomal trafficking, the mechanisms underpinning these disparate effects are unclear. As microtubule acetylation modulates endosome-lysosome trafficking, we reasoned that exploring the role of GCN5L1 in this biology may enhance our understanding of GCN5L1-mediated protein acetylation. We show that α-tubulin acetylation is reduced in GCN5L1-knockout hepatocytes and restored by GCN5L1 reconstitution. Furthermore, GCN5L1 binds to the α-tubulin acetyltransferase αTAT1, and GCN5L1-mediated α-tubulin acetylation is dependent on αTAT1. Given that cytosolic GCN5L1 has been identified as a component of numerous multiprotein complexes, we explored whether novel interacting partners contribute to this regulation. We identify RanBP2 as a novel interacting partner of GCN5L1 and αTAT1. Genetic silencing of RanBP2 phenocopies GCN5L1 depletion by reducing α-tubulin acetylation, and we find that RanBP2 possesses a tubulin-binding domain, which recruits GCN5L1 to α-tubulin. Finally, we find that genetic depletion of GCN5L1 promotes perinuclear lysosome accumulation and histone deacetylase inhibition partially restores lysosomal positioning. We conclude that the interactions of GCN5L1, RanBP2 and αTAT1 function in concert to control α-tubulin acetylation and may contribute towards the regulation of cellular lysosome positioning. This article has an associated First Person interview with the first author of the paper.

The protein acetylase GCN5L1 modulates hepatic fatty acid oxidation activity via acetylation of the mitochondrial β-oxidation enzyme HADHA

Sirtuin 3 (SIRT3) deacetylates and activates several mitochondrial fatty acid oxidation enzymes in the liver. Here, we investigated whether the protein acetylase GCN5 general control of amino acid synthesis 5-like 1 (GCN5L1), previously shown to oppose SIRT3 activity, is involved in the regulation of hepatic fatty acid oxidation. We show that GCN5L1 abundance is significantly up-regulated in response to an acute high-fat diet (HFD). Transgenic GCN5L1 overexpression in the mouse liver increased protein acetylation levels, and proteomic detection of specific lysine residues identified numerous sites that are co-regulated by GCN5L1 and SIRT3. We analyzed several fatty acid oxidation proteins identified by the proteomic screen and found that hyperacetylation of hydroxyacyl-CoA dehydrogenase trifunctional multienzyme complex subunit α (HADHA) correlates with increased GCN5L1 levels. Stable GCN5L1 knockdown in HepG2 cells reduced HADHA acetylation and increased activities of fatty acid oxidation enzymes. Mice with a liver-specific deletion of GCN5L1 were protected from hepatic lipid accumulation following a chronic HFD and did not exhibit hyperacetylation of HADHA compared with WT controls. Finally, we found that GCN5L1-knockout mice lack HADHA that is hyperacetylated at three specific lysine residues (Lys-350, Lys-383, and Lys-406) and that acetylation at these sites is significantly associated with increased HADHA activity. We conclude that GCN5L1-mediated regulation of mitochondrial protein acetylation plays a role in hepatic metabolic homeostasis.

Metabolic Plasticity of Tumor Cell Mitochondria

Mitochondria are dynamic organelles that exchange a multiplicity of signals with other cell compartments, in order to finely adjust key biological routines to the fluctuating metabolic needs of the cell. During neoplastic transformation, cells must provide an adequate supply of the anabolic building blocks required to meet a relentless proliferation pressure. This can occur in conditions of inconstant blood perfusion leading to variations in oxygen and nutrient levels. Mitochondria afford the bioenergetic plasticity that allows tumor cells to adapt and thrive in this ever changing and often unfavorable environment. Here we analyse how mitochondria orchestrate the profound metabolic rewiring required for neoplastic growth.