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

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.

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 regulates pulmonary surfactant production by modulating lamellar body biogenesis and trafficking in mouse alveolar epithelial cells

BACKGROUND

The pulmonary surfactant that lines the air-liquid surface within alveoli is a protein-lipid mixture essential for gas exchange. Surfactant lipids and proteins are synthesized and stored in the lamellar body (LB) before being secreted from alveolar type II (AT2) cells. The molecular and cellular mechanisms that regulate these processes are incompletely understood. We previously identified an essential role of general control of amino acid synthesis 5 like 1 (GCN5L1) and the biogenesis of lysosome-related organelle complex 1 subunit 1 (BLOS1) in surfactant system development in zebrafish. Here, we explored the role of GCN5L1 in pulmonary surfactant regulation.

METHOD

GCN5L1 knockout cell lines were generated with the CRISPR/Cas9 system. Cell viability was analyzed by MTT assay. Released surfactant proteins were measured by ELISA. Released surfactant lipids were measured based on coupled enzymatic reactions. Gene overexpression was mediated through lentivirus. The RNA levels were detected through RNA-sequencing (RNA-seq) and quantitative reverse transcription (qRT)- polymerase chain reaction (PCR). The protein levels were detected through western blotting. The cellular localization was analyzed by immunofluorescence. Morphology of the lamellar body was analyzed through transmission electron microscopy (TEM), Lysotracker staining, and BODIPY phosphatidylcholine labeling.

RESULTS

Knocking out GCN5L1 in MLE-12 significantly decreased the release of surfactant proteins and lipids. We detected the downregulation of some surfactant-related genes and misregulation of the ROS-Erk-Foxo1-Cebpα axis in mutant cells. Modulating the activity of the axis or reconstructing the mitochondrial expression of GCN5L1 could partially restore the expression of these surfactant-related genes. We further showed that MLE-12 cells contained many LB-like organelles that were lipid enriched and positive for multiple LB markers. These organelles were smaller in size and accumulated in the absence of GCN5L1, indicating both biogenesis and trafficking defects. Accumulated endogenous surfactant protein (SP)-B or exogenously expressed SP-B/SP-C in adenosine triphosphate-binding cassette transporterA3 (ABCA3)-positive organelles was detected in mutant cells. GCN5L1 localized to the mitochondria and LBs. Reconstruction of mitochondrial GCN5L1 expression rescued the organelle morphology but failed to restore the trafficking defect and surfactant release, indicating specific roles associated with different subcellular localizations.

CONCLUSIONS

In summary, our study identified GCN5L1 as a new regulator of pulmonary surfactant that plays a role in the biogenesis and positioning/trafficking of surfactant-containing LBs.

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.

Cytochrome c lysine acetylation regulates cellular respiration and cell death in ischemic skeletal muscle

Skeletal muscle is more resilient to ischemia-reperfusion injury than other organs. Tissue specific post-translational modifications of cytochrome c (Cytc) are involved in ischemia-reperfusion injury by regulating mitochondrial respiration and apoptosis. Here, we describe an acetylation site of Cytc, lysine 39 (K39), which was mapped in ischemic porcine skeletal muscle and removed by sirtuin5 in vitro. Using purified protein and cellular double knockout models, we show that K39 acetylation and acetylmimetic K39Q replacement increases cytochrome c oxidase (COX) activity and ROS scavenging while inhibiting apoptosis via decreased binding to Apaf-1, caspase cleavage and activity, and cardiolipin peroxidase activity. These results are discussed with X-ray crystallography structures of K39 acetylated (1.50 Å) and acetylmimetic K39Q Cytc (1.36 Å) and NMR dynamics. We propose that K39 acetylation is an adaptive response that controls electron transport chain flux, allowing skeletal muscle to meet heightened energy demand while simultaneously providing the tissue with robust resilience to ischemia-reperfusion injury.

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.

Reduced acetylation of TFAM promotes bioenergetic dysfunction in the failing heart

General control of amino acid synthesis 5-like 1 (GCN5L1) was previously identified as a key regulator of protein lysine acetylation in mitochondria. Subsequent studies demonstrated that GCN5L1 regulates the acetylation status and activity of mitochondrial fuel substrate metabolism enzymes. However, the role of GCN5L1 in response to chronic hemodynamic stress is largely unknown. Here, we show that cardiomyocyte-specific GCN5L1 knockout mice (cGCN5L1 KO) display exacerbated heart failure progression following transaortic constriction (TAC). Mitochondrial DNA and protein levels were decreased in cGCN5L1 KO hearts after TAC, and isolated neonatal cardiomyocytes with reduced GCN5L1 expression had lower bioenergetic output in response to hypertrophic stress. Loss of GCN5L1 expression led to a decrease in the acetylation status of mitochondrial transcription factor A (TFAM) after TAC in vivo, which was linked to a reduction in mtDNA levels in vitro. Together, these data suggest that GCN5L1 may protect from hemodynamic stress by maintaining mitochondrial bioenergetic output.

GPER-dependent estrogen signaling increases cardiac GCN5L1 expression

Reversible lysine acetylation regulates the activity of cardiac metabolic enzymes, including those controlling fuel substrate metabolism. Mitochondrial-targeted GCN5L1 and SIRT3 have been shown to regulate the acetylation status of mitochondrial enzymes, but the role that lysine acetylation plays in driving metabolic differences between male and female hearts is not currently known. In this study, we describe a significant difference in GCN5L1 levels between male and female mouse hearts, and in the hearts of women between post- and premenopausal age. We further find that estrogen drives GCN5L1 expression in a cardiac cell line and uses pharmacological approaches to determine the mechanism to be G protein-coupled estrogen receptor (GPER) activation, via translational regulation.NEW & NOTEWORTHY We demonstrate here for the first time that mitochondrial protein acetylation is increased in female hearts, associated with an increase in GCN5L1 levels through a GPER-dependent mechanism. These findings reveal a new potential mediator of divergent cardiac mitochondrial function between men and women.

Perfluorooctanoic acidinduced lipid metabolism disorder in SD rat liver and its effect on the expression of fatty acid metabolismrelated proteins

OBJECTIVES

Perfluorooctanoic acid (PFOA) can cause lipid metabolism disorders in animal body and affect the lipolysis and synthesis of fatty acids. Peroxisome proliferators-activated receptor (PPAR) plays an extremely important role in this process. This study aims to explore the effects of PFOA on liver lipid metabolism disorders in Sprague Dewley (SD) rats and the expression of PPAR.

METHODS

A total of 40 male SD rats were randomly divided into 4 groups (n=10 in each group): a control group (ddH2O), a low-dose PFOA group [PFOA 1.25 mg/(kg·d)], a middle-dose PFOA group [PFOA 5.00 mg/(kg·d)], and a high-dose PFOA group [PFOA 20.00 mg/(kg·d)]. The rats were fed with normal diet, and PFOA exposure were performed by oral gavage for 14 days, and the rats were observed, weighted and recorded every day during the exposure. After the exposure, the blood was collected, and the livers were quickly stripped after the rats were killed. Part of the liver tissues were fixed in 4% paraformaldehyde for periodic acid-schiff (PAS) staining; the contents of HDLC, LDLC, TG, TC in serum and liver tissues, as well as the activities of their related enzymes were assayed; The expression levels of cyclic adenosine monophosphate-response element binding protein (Cbp), general control of amino acid synthesis 5-like 2 (Gcn5L2), peroxidation peroxisome proliferation factor activated receptor γ (PPAR), silent information regulator 1 (Sirt1) and human retinoid X receptor alpha 2 (Rxrα2) ) were detected by Western blotting.

RESULTS

After 14 days of PFOA exposure, the PAS staining positive particles in the cytoplasm and nucleus of SD rats in the medium and high dose groups were significantly reduced compared with the control group. The serum levels of LDLC and TC in the low-dose and middle-dose groups were significantly reduced compared with the control group (all P<0.05), while the high-dose group showed an increasing tendency, without siginificant difference (P>0.05), there was no significant difference in HDLC and TG (both P>0.05). The activities of alkaline phosphatase (AKP) and alanine aminotransferase (ALT) were increased significantly (both P<0.05) compared with control group; the ratio of ALT/aspartate aminotransferase (AST) in the high-dose group was increased significantly (P<0.05), there was no significant difference in LDH and TG (both P>0.05); the HDLC content in the liver tissues in the high-dose group was significantly reduced, compared with the control group (P<0.05); the TC contents in the liver tissues in the low, medium and high-dose groups were significantly increased (all P<0.05), there was no significant difference in LDLC and TG (both P>0.05); the AKP activity in the livers in the medium and high-dose groups was significantly increased (both P<0.05), there was no siginificant difference in LDH, ALT, and the ratio of ALT/AST (all P>0.05); the protein expression levels of Ppar γ, Cbp and Rxrα2 in the liver in the high dose groups were significantly down-regulated compared with the control group (all P<0.05), while the protein expression levels of Sirt1 were significantly up-regulated (all P<0.05).

CONCLUSIONS

PFOA exposure can cause lipid metabolism disorder and glycogen reduction in SD rat livers, which may be related to the activation of Sirt1 and inhibition of Ppar γ expression, leading to affecting the normal metabolism of fatty acids and promoting glycolysis.

Diet-induced obese mice are resistant to improvements in cardiac function resulting from short-term adropin treatment

Previous studies have shown that treatment with recombinant adropin, a circulating peptide secreted by the liver and brain, restores glucose utilization in the hearts of diet-induced obese mice. This restoration of fuel substrate flexibility, which is lost in obese and diabetic animals, has the potential to improve contractile function in the diabetic heart. Using an ex vivo approach, we examined whether short-term adropin treatment could enhance cardiac function in a mouse model of diet-induced obesity. Our study showed that acute adropin treatment reduces inhibitory phosphorylation of pyruvate dehydrogenase in primary neonatal cardiomyocytes, and leads to moderate improvements in ex vivo cardiac function in mice fed a low fat diet. Conversely, short-term exposure to adropin led to a small decrease in cardiac function in mice fed a long-term high fat diet. Insulin treatment did not significantly alter cardiac function in adropin treated hearts from either low or high fat diet mice, however acute adropin treatment did moderately restore some aspects of downstream insulin signaling in high fat diet fed mice. Overall, these data suggest that in an ex vivo setting, acute adropin treatment alone is not sufficient to promote improved cardiac function in obese animals.

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Adropin promotes cardiac glucose utilization in hearts from diet-induced obese mice.

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In this study, we examined whether adropin treatment improves ex vivo cardiac function in obesity.

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While adropin improves contractile function in lean mice, its impact is lost after exposure to a long-term high fat diet.

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Adropin fails to fully restore cardiac insulin signaling in obese mouse hearts.

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.

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.

Increased fatty acid oxidation enzyme activity in the hearts of mice fed a high fat diet does not correlate with improved cardiac contractile function

Changes in the acetylation status of mitochondrial proteins have been linked to the development of metabolic dysfunction in a number of tissues. Increased lysine acetylation has been reported in the hearts of obese mice, and is associated with changes in fuel metabolism, redox status, and mitochondrial oxidative phosphorylation. In this study, we examined whether diet-induced changes in the acetylation of mitochondrial acyl-CoA dehydrogenases affected fatty acid oxidation enzyme activity and contractile function in the obese mouse heart. Exposure to a long-term high fat diet in wildtype mice led to the hyperacetylation of short- and long-chain acyl-CoA dehydrogenases SCAD and LCAD, which correlated with their increased enzymatic activity in vitro. Cardiomyocyte-specific deletion of the mitochondrial acetyltransferase-related protein GCN5L1 prevented both the hyperacetylation and increased activity of these enzymes under the same conditions of dietary excess. Despite the potential for increased cardiac fatty acid oxidation activity, wildtype mice did not display any increase in cardiac contractility following exposure to a high fat diet. We conclude that the potential energetic benefits of elevated fatty acid oxidation activity are not sufficient to counter the various deleterious effects of a high fat diet on cardiac function.

Cardiomyocyte-specific deletion of GCN5L1 in mice restricts mitochondrial protein hyperacetylation in response to a high fat diet

Mitochondrial lysine acetylation regulates several metabolic pathways in cardiac cells. The current study investigated whether GCN5L1-mediated lysine acetylation regulates cardiac mitochondrial metabolic proteins in response to a high fat diet (HFD). GCN5L1 cardiac-specific knockout (cKO) mice showed significantly reduced mitochondrial protein acetylation following a HFD relative to wildtype (WT) mice. GCN5L1 cKO mice did not display any decrease in ex vivo cardiac workload in response to a HFD. In contrast, ex vivo cardiac function in HFD-fed WT mice dropped ~ 50% relative to low fat diet (LFD) fed controls. The acetylation status of electron transport chain Complex I protein NDUFB8 was significantly increased in WT mice fed a HFD, but remained unchanged in GCN5L1 cKO mice relative to LFD controls. Finally, we observed that inhibitory acetylation of superoxide dismutase 2 (SOD2) at K122 was increased in WT (but not cKO mice) on a HFD. This correlated with significantly increased cardiac lipid peroxidation in HFD-fed WT mice relative to GCN5L1 cKO animals under the same conditions. We conclude that increased GCN5L1 expression in response to a HFD promotes increased lysine acetylation, and that this may play a role in the development of reactive oxygen species (ROS) damage caused by nutrient excess.

New insights into the role of mitochondria in cardiac microvascular ischemia/reperfusion injury

As reperfusion therapies have become more widely used in acute myocardial infarction patients, ischemia-induced myocardial damage has been markedly reduced, but reperfusion-induced cardiac injury has become increasingly evident. The features of cardiac ischemia-reperfusion (I/R) injury include microvascular perfusion defects, platelet activation and sequential cardiomyocyte death due to additional ischemic events at the reperfusion stage. Microvascular obstruction, defined as a no-reflow phenomenon, determines the infarct zone, myocardial function and peri-operative mortality. Cardiac microvascular endothelial cell injury may occur much earlier and with much greater severity than cardiomyocyte injury. Endothelial cells contain fewer mitochondria than other cardiac cells, and several of the pathological alterations during cardiac microvascular I/R injury involve mitochondria, such as increased mitochondrial reactive oxygen species (mROS) levels and disturbed mitochondrial dynamics. Although mROS are necessary physiological second messengers, high mROS levels induce oxidative stress, endothelial senescence and apoptosis. Mitochondrial dynamics, including fission, fusion and mitophagy, determine the shape, distribution, size and function of mitochondria. These adaptive responses modify extracellular signals and orchestrate intracellular processes such as cell proliferation, migration, metabolism, angiogenesis, permeability transition, adhesive molecule expression, endothelial barrier function and anticoagulation. In this review, we discuss the involvement of mROS and mitochondrial morphofunction in cardiac microvascular I/R injury.

Loss of GCN5L1 in cardiac cells disrupts glucose metabolism and promotes cell death via reduced Akt/mTORC2 signaling

GCN5L1 regulates protein acetylation and mitochondrial energy metabolism in diverse cell types. In the heart, loss of GCN5L1 sensitizes the myocardium to injury from exposure to nutritional excess and ischemia/reperfusion injury. This phenotype is associated with the reduced acetylation of metabolic enzymes and elevated mitochondrial reactive oxygen species (ROS) generation, although the direct molecular targets of GCN5L1 remain largely unknown. In this study, we sought to determine the mechanism by which GCN5L1 impacts energy substrate utilization and mitochondrial health. We find that hypoxia and reoxygenation (H/R) leads to a reduction in cell viability and Akt phosphorylation in GCN5L1 knockdown AC16 cardiomyocytes, in parallel with elevated glucose utilization and impaired fatty acid use. We demonstrate that glycolysis is uncoupled from glucose oxidation under normoxic conditions in GCN5L1-depleted cells. We show that GCN5L1 directly binds to the Akt-activating mTORC2 component Rictor, and that loss of Rictor acetylation is evident in GCN5L1 knockdown cells. Finally, we show that restoring Rictor acetylation in GCN5L1-depleted cells reduces mitochondrial ROS generation and increases cell survival in response to H/R. These studies suggest that GCN5L1 may play a central role in energy substrate metabolism and cell survival via the regulation of Akt/mTORC2 signaling.