GCN5L1 modulates cross-talk between mitochondria and cell signaling to regulate FoxO1 stability and gluconeogenesis

Drp1 acetylation mediated by CDK5-AMPK-GCN5L1 axis promotes cerebral ischemic injury via facilitating mitochondrial fission

The aberrant acetylation of mitochondrial proteins is involved in the pathogenesis of multiple diseases including neurodegenerative diseases and cerebral ischemic injury. Previous studies have shown that depletion of mitochondrial NAD+, which is necessary for mitochondrial deacetylase activity, leads to decreased activity of mitochondrial deacetylase and thus causes hyperacetylation of mitochondrial proteins in ischemic brain tissues, which results in altered mitochondrial dynamics. However, it remains largely unknown about how mitochondrial dynamics-related protein Drp1 is acetylated in ischemic neuronal cells and brain tissues. Here, we showed that Drp1 and GCN5L1 expression was up-regulated in OGD-treated neuronal cells and ischemic brain tissues induced by dMCAO, accompanied by the increased mitochondrial fission, mtROS accumulation, and cell apoptosis. Further, we confirmed that ischemia/hypoxia promoted Drp1 interaction with GCN5L1 in neuronal cells and brain tissues. GCN5L1 knockdown attenuated, while its overexpression enhanced Drp1 acetylation and mitochondrial fission, indicating that GCN5L1 plays a crucial role in ischemia/hypoxia-induced mitochondrial fission by acetylating Drp1. Mechanistically, ischemia/hypoxia induced Drp1 phosphorylation by CDK5 upregulation-mediated activation of AMPK in neuronal cells, which in turn facilitated the interaction of GCN5L1 with Drp1, thus enhancing Drp1 acetylation and mitochondrial fission. Accordingly, inhibition of AMPK alleviated ischemia/hypoxia- induced Drp1 acetylation and mitochondrial fission and protected brain tissues from ischemic damage. These findings provide a novel insight into the functional roles of GCN5L1 in regulating Drp1 acetylation and identify a previously unrecognized CDK5-AMPK-GCN5L1 pathway that mediates the acetylation of Drp1 in ischemic brain tissues.

Loss of GCN5L1 exacerbates damage in alcoholic liver disease through ferroptosis activation

BACKGROUND AND AIMS

Iron overload, oxidative stress and ferroptosis are associated with liver injury in alcohol-associated liver disease (ALD), however, the crosstalk among these regulatory pathways in ALD development is unclear.

METHODS

ALD mouse model and general control of amino acid synthesis 5 like 1 (GCN5L1) liver knockout mice were generated to investigate the role of GCN5L1 in ALD development. Proteomic screening tests were performed to identify the key factors mediating GCN5L1 loss-induced ALD.

RESULTS

Gene Expression Omnibus data set analysis indicates that GCN5L1 expression is negatively associated with ALD progression. GCN5L1 hepatic knockout mice develop severe liver injury and lipid accumulation when fed an alcohol diet. Screening tests identified that GCN5L1 targeted the mitochondrial iron transporter CISD1 to regulate mitochondrial iron homeostasis in ethanol-induced ferroptosis. GCN5L1-modulated CISD1 acetylation and activity were crucial for iron accumulation and ferroptosis in response to alcohol exposure.

CONCLUSION

Pharmaceutical modulation of CISD1 activity is critical for cellular iron homeostasis and ethanol-induced ferroptosis. The GCN5L1/CISD1 axis is crucial for oxidative stress and ethanol-induced ferroptosis in ALD and is a promising avenue for novel therapeutic strategies.

Mitochondrial GCN5L1 acts as a novel regulator for iron homeostasis to promote sorafenib sensitivity in hepatocellular carcinoma

BACKGROUND

Sorafenib resistance is becoming increasingly common and disadvantageous for hepatocellular carcinoma (HCC) treatment. Ferroptosis is an iron dependent programmed cell death underlying the mechanism of sorafenib. Iron is crucial for synthesis of cofactors essential to mitochondrial enzymes and necessary for HCC proliferation, while mitochondrial iron overload and oxidative stress are associated with sorafenib induced ferroptosis. However, the crosstalk among iron homeostasis and sorafenib resistance is unclear.

METHODS

We conducted bioinformatics analysis of sorafenib treated HCC datasets to analyze GCN5L1 and iron related gene expression with sorafenib resistance. GCN5L1 deleted HCC cell lines were generated by CRISPR technology. Sorafenib resistant HCC cell line was established to validate dataset analysis and evaluate the effect of potential target.

RESULTS

We identified GCN5L1, a regulator of mitochondrial acetylation, as a modulator in sorafenib-induced ferroptosis via affecting mitochondrial iron homeostasis. GCN5L1 deficiency significantly increased sorafenib sensitivity in HCC cells by down-regulating mitochondrial iron transporters CISD1 expression to induce iron accumulation. Mitochondrial iron accumulation leads to an acceleration in cellular and lipid ROS. Sorafenib resistance is related to CISD1 overexpression to release mitochondrial iron and maintaining mitochondrial homeostasis. We combined CISD1 inhibitor NL-1 with sorafenib, which significantly enhanced sorafenib-induced ferroptosis by promoting mitochondrial iron accumulation and lipid peroxidation. The combination of NL-1 with sorafenib enhanced sorafenib efficacy in vitro and in vivo.

CONCLUSIONS

Our findings demonstrate that GCN5L1/CISD1 axis is crucial for sorafenib resistance and would be a potential therapeutic strategy for sorafenib resistant HCC.

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.

Structural Analysis and Novel Mechanism of Enteromorpha prolifera Sulfated Polysaccharide in Preventing Type 2 Diabetes Mellitus

A water-soluble polysaccharide (EP) was purified from edible algae Enteromorpha prolifera. Gel permeation chromatography (GPC), ion chromatography (IC), and fourier transform infrared (FT-IR) were performed to characterize its structure. EP was defined as a low molecular weight (6625 Da) composed of rhamnose, glucose, glucuronic acid, xylose, galactose, arabinose, and mannose. Moreover, it was a sulfated polysaccharide with a degree of substitution (DS) of 1.48. Then, the high-fat diet/streptozotocin (HFD/STZ) induced diabetic mouse model was established to support evidence for a novel hypoglycemic mechanism. Results showed that blood glucose (47.32%), liver index (7.65%), epididymal fat index (16.86%), serum total cholesterol (26.78%) and triglyceride (37.61%) in the high-dose EP (HEP) group were significantly lower than those in the HFD group. Noticeably, the content of liver glycogen in the HEP group was significantly higher (62.62%) than that in the HFD group, indicating the promotion of glycogen synthesis. These beneficial effects were attributed to significantly increased protein kinase B (AKT) phosphorylation and its downstream signaling response. Further studies showed that diabetic mice exhibited excessive O-GlcNAcylation level and high expression of O-linked β-D-N-acetylglucosamine transferase (OGT), which were decreased by 62.21 and 30.43% in the HEP group. This result suggested that EP had a similar effect to OGT inhibitors, which restored AKT phosphorylation and prevented pathoglycemia. This work reveals a novel hypoglycemic mechanism of EP, providing a theoretical basis for further studies on its pharmacological properties in improvement of T2DM.

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.

Hepatocyte TIPE2 is a fasting-induced Raf-1 inactivator that drives hepatic gluconeogenesis to maintain glucose homeostasis

BACKGROUND

The liver regulates metabolic balance during fasting-feeding cycle. Hepatic adaptation to fasting is precisely modulated on multiple levels. Tumor necrosis factor-α-induced protein 8-like 2 (TIPE2) is a negative regulator of immunity that reduces several liver pathologies, but its physiological roles in hepatic metabolism are largely unknown.

METHODS

TIPE2 expression was examined in mouse liver during fasting-feeding cycle. TIPE2-knockout mice, liver-specific TIPE2-knockout mice, liver-specific TIPE2-overexpressed mice were examined for fasting blood glucose and pyruvate tolerance test. Primary hepatocytes or liver tissues from these mice were evaluated for glucose production, lipid accumulation, gene expression and regulatory pathways. TIPE2 interaction with Raf-1 and TIPE2 transcription regulated by PPAR-α were examined using gene overexpression or knockdown, co-immunoprecipitation, western blot, luciferase reporter assay and DNA-protein binding assay.

RESULTS

TIPE2 expression was upregulated in fasted mouse liver and starved hepatocytes, which was positively correlated with gluconeogenic genes. Liver-specific TIPE2 deficiency impaired blood glucose homeostasis and gluconeogenic capacity in mice upon fasting, while liver-specific TIPE2 overexpression elevated fasting blood glucose and hepatic gluconeogenesis in mice. In primary hepatocytes upon starvation, TIPE2 interacted with Raf-1 to accelerate its ubiquitination and degradation, resulting in ERK deactivation and FOXO1 maintenance to sustain gluconeogenesis. During prolonged fasting, hepatic TIPE2 deficiency caused aberrant activation of ERK-mTORC1 axis that increased hepatic lipid accumulation via lipogenesis. In hepatocytes upon starvation, PPAR-α bound with TIPE2 promoter and triggered its transcriptional expression.

CONCLUSIONS

Hepatocyte TIPE2 is a PPAR-α-induced Raf-1 inactivator that sustains hepatic gluconeogenesis and prevents excessive hepatic lipid accumulation, playing beneficial roles in hepatocyte adaptation to fasting.

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.

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.

Periodontitis as a promoting factor of T2D: current evidence and mechanisms

Periodontitis is an infectious disease caused by an imbalance between the local microbiota and host immune response. Epidemiologically, periodontitis is closely related to the occurrence, development, and poor prognosis of T2D and is recognized as a potential risk factor for T2D. In recent years, increasing attention has been given to the role of the virulence factors produced by disorders of the subgingival microbiota in the pathological mechanism of T2D, including islet β-cell dysfunction and insulin resistance (IR). However, the related mechanisms have not been well summarized. This review highlights periodontitis-derived virulence factors, reviews how these stimuli directly or indirectly regulate islet β-cell dysfunction. The mechanisms by which IR is induced in insulin-targeting tissues (the liver, visceral adipose tissue, and skeletal muscle) are explained, clarifying the influence of periodontitis on the occurrence and development of T2D. In addition, the positive effects of periodontal therapy on T2D are overviewed. Finally, the limitations and prospects of the current research are discussed. In summary, periodontitis is worthy of attention as a promoting factor of T2D. Understanding on the effect of disseminated periodontitis-derived virulence factors on the T2D-related tissues and cells may provide new treatment options for reducing the risk of T2D associated with periodontitis.

CerS6 triggered by high glucose activating the TLR4/IKKβ pathway regulates ferroptosis of LO2 cells through mitochondrial oxidative stress

Lipid metabolism disorders and mitochondrial dysfunction contribute to the progression of diabetes and chronic liver disease (CLD). Ferroptosis, as a form of cell death centered on reactive oxygen species (ROS) accumulation and lipid peroxidation, is closely related to mitochondrial dysfunction. However, whether there exists mechanistic links between these processes remains unknown. Here, to explore the molecular mechanism of diabetes complicated with CLD, we showed that high glucose could restrain the activity of antioxidant enzymes, promote mitochondrial ROS (mtROS) production, and induce a state of oxidative stress in the mitochondria of human normal liver (LO2) cells. We demonstrated that high glucose induced ferroptosis and promoted the development of CLD, which was reversed by the ferroptosis inhibitor Ferrostatin-1 (Fer-1). In addition, the mitochondria-targeting antioxidant Mito-TEMPO was used to intervene LO2 cells in high-glucose culture, and ferroptosis was found to be inhibited, whereas markers of liver injury and fibrosis improved. Furthermore, high glucose could promote ceramide synthetase 6 (CerS6) synthesis through the TLR4/IKKβ pathway. The knockout of CerS6 in LO2 cells showed that mitochondrial oxidative stress was attenuated, ferroptosis was inhibited, and markers of liver injury and fibrosis were ameliorated. In contrast, the overexpression of CerS6 in LO2 cells showed the opposite changes and these changes were inhibited by Mito-TEMPO. In short, we positioned the study of lipid metabolism to a specific enzyme CerS6, with a high degree of specificity. Our findings revealed the mechanism by which the mitochondria act as a bridge linking CerS6 and ferroptosis, confirming that under high glucose conditions, CerS6 promotes ferroptosis through mitochondrial oxidative stress, eventually leading to CLD.

Inhibition of 2-arachidonoylglycerol degradation enhances glial immunity by single-cell transcriptomic analysis

BACKGROUND

2-Arachidonoylglycerol (2-AG) is the most abundant endogenous cannabinoid. Inhibition of 2-AG metabolism by inactivation of monoacylglycerol lipase (MAGL), the primary enzyme that degrades 2-AG in the brain, produces anti-inflammatory and neuroprotective effects in neurodegenerative diseases. However, the molecular mechanisms underlying these beneficial effects are largely unclear.

METHODS

Hippocampal and cortical cells were isolated from cell type-specific MAGL knockout (KO) mice. Single-cell RNA sequencing was performed by 10 × Genomics platform. Cell Ranger, Seurat (v3.2) and CellChat (1.1.3) packages were used to carry out data analysis.

RESULTS

Using single-cell RNA sequencing analysis, we show here that cell type-specific MAGL KO mice display distinct gene expression profiles in the brain. Inactivation of MAGL results in robust changes in expression of immune- and inflammation-related genes in microglia and astrocytes. Remarkably, upregulated expression of chemokines in microglia is more pronounced in mice lacking MAGL in astrocytes. In addition, expression of genes that regulate other cellular functions and Wnt signaling in astrocytes is altered in MAGL KO mice.

CONCLUSIONS

Our results provide transcriptomic evidence that cell type-specific inactivation of MAGL induces differential expression of immune-related genes and other fundamental cellular pathways in microglia and astrocytes. Upregulation of the immune/inflammatory genes suggests that tonic levels of immune/inflammatory vigilance are enhanced in microglia and astrocytes, particularly in microglia, by inhibition of 2-AG metabolism, which likely contribute to anti-inflammatory and neuroprotective effects produced by inactivation of MAGL in neurodegenerative diseases.

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.

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.

RING-finger E3 ligases regulatory network in PI3K/AKT-mediated glucose metabolism

The phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway plays an essential role in glucose metabolism, promoting glycolysis and resisting gluconeogenesis. PI3K/AKT signaling can directly alter glucose metabolism by phosphorylating several metabolic enzymes or regulators of nutrient transport. It can indirectly promote sustained aerobic glycolysis by increasing glucose transporters and glycolytic enzymes, which are mediated by downstream transcription factors. E3 ubiquitin ligase RING-finger proteins are mediators of protein post-translational modifications and include the cullin-RING ligase complexes, the tumor necrosis factor receptor-associated family, the tripartite motif family and etc. Some members of the RING family play critical roles in regulating cell signaling and are involved in the development and progression of various metabolic diseases, such as cancer, diabetes, and dyslipidemia. And with the progression of modern research, as a negative or active regulator, the RING-finger adaptor has been found to play an indispensable role in PI3K/AKT signaling. However, no reviews have comprehensively clarified the role of RING-finger E3 ligases in PI3K/AKT-mediated glucose metabolism. Therefore, in this review, we focus on the regulation and function of RING ligases in PI3K/AKT-mediated glucose metabolism to establish new insights into the prevention and treatment of metabolic diseases.

Mitochondrial GCN5L1 regulates glutaminase acetylation and hepatocellular carcinoma

BACKGROUND

Glutaminolysis is a critical metabolic process that promotes cancer cell proliferation, including hepatocellular carcinoma (HCC). Delineating the molecular control of glutaminolysis could identify novel targets to ameliorate this oncogenic metabolic pathway. Here, we evaluated the role of general control of amino acid synthesis 5 like 1 (GCN5L1), a regulator of mitochondrial protein acetylation, in modulating the acetylation and activity of glutaminase to regulate HCC development.

METHODS

Cell proliferation was determined by MTT, 2D and soft agar clone formation assays and orthotopic tumour assays in nude mice. GLS1/2 acetylation and activities were measured in cells and tumours to analyse the correlation with GCN5L1 expression and mTORC1 activation.

RESULTS

Hepatic GCN5L1 ablation in mice markedly increased diethylnitrosamine (DEN)-induced HCC, and conversely, the transduction of mitochondrial-restricted GCN5L1 protected wild-type mice against HCC progression in response to DEN and carbon tetrachloride (CCl₄ ) exposure. GCN5L1-depleted HepG2 hepatocytes enhanced tumour growth in athymic nude mice. Mechanistically, GCN5L1 depletion promoted cell proliferation through mTORC1 activation. Interestingly, liver-enriched glutaminase 2 (GLS2) appears to play a greater role than ubiquitous and canonical tumour-enriched glutaminase 1 (GLS1) in promoting murine HCC. Concurrently, GCN5L1 promotes acetylation and inactivation of both isoforms and increases enzyme oligomerisation. In human HCC tumours compared to adjacent tissue, there were variable levels of mTORC1 activation, GCN5L1 levels and glutaminase activity. Interestingly, the levels of GCN5L1 inversely correlated with mTORC1 activity and glutaminase activity in these tumours.

CONCLUSIONS

Our study identified that glutaminase activity, rather than GLS1 or GLS2 expression, is the key factor in HCC development that activates mTORC1 and promotes HCC. In the Kaplan-Meier analysis of liver cancer, we found that HCC patients with high GCN5L1 expression survived longer than those with low GCN5L1 expression. Collectively, GCN5L1 functions as a tumour regulator by modulating glutaminase acetylation and activity in the development of HCC.

Mitochondrial Dysfunction and Acute Fatty Liver of Pregnancy

The liver is one of the richest organs in mitochondria, serving as a hub for key metabolic pathways such as β-oxidation, the tricarboxylic acid (TCA) cycle, ketogenesis, respiratory activity, and adenosine triphosphate (ATP) synthesis, all of which provide metabolic energy for the entire body. Mitochondrial dysfunction has been linked to subcellular organelle dysfunction in liver diseases, particularly fatty liver disease. Acute fatty liver of pregnancy (AFLP) is a life-threatening liver disorder unique to pregnancy, which can result in serious maternal and fetal complications, including death. Pregnant mothers with this disease require early detection, prompt delivery, and supportive maternal care. AFLP was considered a mysterious illness and though its pathogenesis has not been fully elucidated, molecular research over the past two decades has linked AFLP to mitochondrial dysfunction and defects in fetal fatty-acid oxidation (FAO). Due to deficient placental and fetal FAO, harmful 3-hydroxy fatty acid metabolites accumulate in the maternal circulation, causing oxidative stress and microvesicular fatty infiltration of the liver, resulting in AFLP. In this review, we provide an overview of AFLP and mitochondrial FAO followed by discussion of how altered mitochondrial function plays an important role in the pathogenesis of AFLP.

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.

Feeding-induced resistance to acute lethal sepsis is dependent on hepatic BMAL1 and FXR signalling

In mice, time of day strongly influences lethality in response to LPS, with survival greatest at the beginning compared to the end of the light cycle. Here we show that feeding, rather than light, controls time-of-day dependent LPS sensitivity. Mortality following LPS administration is independent of cytokine production and the clock regulator BMAL1 expressed in myeloid cells. In contrast, deletion of BMAL1 in hepatocytes globally disrupts the transcriptional response to the feeding cycle in the liver and results in constitutively high LPS sensitivity. Using RNAseq and functional validation studies we identify hepatic farnesoid X receptor (FXR) signalling as a BMAL1 and feeding-dependent regulator of LPS susceptibility. These results show that hepatocyte-intrinsic BMAL1 and FXR signalling integrate nutritional cues to regulate survival in response to innate immune stimuli. Understanding hepatic molecular programmes operational in response to these cues could identify novel pathways for targeting to enhance endotoxemia resistance.

Mitochondrial-induced Epigenetic Modifications: From Biology to Clinical Translation

Mitochondria are maternally inherited semi-autonomous organelles that play a central role in redox balance, energy metabolism, control of integrated stress responses, and cellular homeostasis. The molecular communication between mitochondria and the nucleus is intricate and bidirectional in nature. Though mitochondrial genome encodes for several key proteins involved in oxidative phosphorylation, several regulatory factors encoded by nuclear DNA are prominent contributors to mitochondrial biogenesis and function. The loss of synergy between this reciprocal control of anterograde (nuclear to mitochondrial) and retrograde (mitochondrial to nuclear) signaling, triggers epigenomic imbalance and affects mitochondrial function and global gene expressions. Recent expansions of our knowledge on mitochondrial epigenomics have offered novel perspectives for the study of several non-communicable diseases including cancer. As mitochondria are considered beacons for pharmacological interventions, new frontiers in targeted delivery approaches could provide opportunities for effective disease management and cure through reversible epigenetic reprogramming. This review focuses on recent progress in the area of mitochondrial-nuclear cross-talk and epigenetic regulation of mitochondrial DNA methylation, mitochondrial micro RNAs, and post-translational modification of mitochondrial nucleoid-associated proteins that hold major opportunities for targeted drug delivery and clinical translation.

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.

iTRAQ- and LC-MS/MS-based quantitative proteomics reveals Pqlc2 as a potential regulator of hepatic glucose metabolism and insulin signalling pathway during fasting

Glucose homeostasis is tightly controlled by balance between glucose production and uptake in liver tissue upon energy shortage condition. Altered glucose homeostasis contributes to the pathophysiology of metabolic disorders including diabetes and obesity. Here, we aimed to analyse the change of proteomic profile upon prolonged fasting in mice with isobaric tag for relative and absolute quantification (iTRAQ) labelling followed by liquid chromatography-mass spectrometry (LC/MS) technology. Adult male mice were fed or fasted for 16 hours and liver tissues were collected for iTRAQ labelling followed by LC/MS analysis. A total of 322 differentially expressed proteins were identified, including 189 upregulated and 133 downregulated proteins. Bioinformatics analyses, including Gene Ontology analysis (GO), Kyoto encyclopaedia of genes and genomes analysis (KEGG) and protein-protein interaction analysis (PPI) were conducted to understand biological process, cell component, and molecular function of the 322 differentially expressed proteins. Among 322 hepatic proteins differentially expressed between fasting and fed mice, we validated three upregulated proteins (Pqlc2, Ehhadh and Apoa4) and two downregulated proteins (Uba52 and Rpl37) by western-blotting analysis. In cultured HepG2 hepatocellular cells, we found that depletion of Pqlc2 by siRNA-mediated knockdown impaired the insulin-induced glucose uptake, inhibited GLUT2 mRNA level and suppressed the insulin-induced Akt phosphorylation. By contrast, knockdown of Pqlc2 did not affect the cAMP/dexamethasone-induced gluconeogenesis. In conclusion, our study provides important information on protein profile change during prolonged fasting with iTRAQ- and LC-MS/MS-based quantitative proteomics, and identifies Pqlc2 as a potential regulator of hepatic glucose metabolism and insulin signalling pathway in this process.

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.

Murine neonatal ketogenesis preserves mitochondrial energetics by preventing protein hyperacetylation

Ketone bodies are generated in the liver and allow for the maintenance of systemic caloric and energy homeostasis during fasting and caloric restriction. It has previously been demonstrated that neonatal ketogenesis is activated independently of starvation. However, the role of ketogenesis during the perinatal period remains unclear. Here, we show that neonatal ketogenesis plays a protective role in mitochondrial function. We generated a mouse model of insufficient ketogenesis by disrupting the rate-limiting hydroxymethylglutaryl-CoA synthase 2 enzyme gene (Hmgcs2). Hmgcs2 knockout (KO) neonates develop microvesicular steatosis within a few days of birth. Electron microscopic analysis and metabolite profiling indicate a restricted energy production capacity and accumulation of acetyl-CoA in Hmgcs2 KO mice. Furthermore, acetylome analysis of Hmgcs2 KO cells revealed enhanced acetylation of mitochondrial proteins. These findings suggest that neonatal ketogenesis protects the energy-producing capacity of mitochondria by preventing the hyperacetylation of mitochondrial proteins.

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.

SMOC1 is a glucose-responsive hepatokine and therapeutic target for glycemic control

Intertissue communication is a fundamental feature of metabolic regulation, and the liver is central to this process. We have identified sparc-related modular calcium-binding protein 1 (SMOC1) as a glucose-responsive hepatokine and regulator of glucose homeostasis. Acute intraperitoneal administration of SMOC1 improved glycemic control and insulin sensitivity in mice without changes in insulin secretion. SMOC1 exerted its favorable glycemic effects by inhibiting adenosine 3′,5′-cyclic monophosphate (cAMP)–cAMP-dependent protein kinase (PKA)–cAMP response element–binding protein (CREB) signaling in the liver, leading to decreased gluconeogenic gene expression and suppression of hepatic glucose output. Overexpression of SMOC1 in the liver or once-weekly intraperitoneal injections of a stabilized SMOC1-FC fusion protein induced durable improvements in glucose tolerance and insulin sensitivity indb/dbmice, without adverse effects on adiposity, liver histopathology, or inflammation. Furthermore, circulating SMOC1 correlated with hepatic and systemic insulin sensitivity and was decreased in obese, insulin-resistant humans. Together, these findings identify SMOC1 as a potential pharmacological target for the management of glycemic control in type 2 diabetes.

SIRT3 and GCN5L regulation of NADP+- and NADPH-driven reactions of mitochondrial isocitrate dehydrogenase IDH2

Wild type mitochondrial isocitrate dehydrogenase (IDH2) was previously reported to produce oncometabolite 2-hydroxyglutarate (2HG). Besides, mitochondrial deacetylase SIRT3 has been shown to regulate the oxidative function of IDH2. However, regulation of 2HG formation by SIRT3-mediated deacetylation was not investigated yet. We aimed to study mitochondrial IDH2 function in response to acetylation and deacetylation, and focus specifically on 2HG production by IDH2. We used acetylation surrogate mutant of IDH2 K413Q and assayed enzyme kinetics of oxidative decarboxylation of isocitrate, 2HG production by the enzyme, and 2HG production in cells. The purified IDH2 K413Q exhibited lower oxidative reaction rates than IDH2 WT. 2HG production by IDH2 K413Q was largely diminished at the enzymatic and cellular level, and knockdown of SIRT3 also inhibited 2HG production by IDH2. Contrary, the expression of putative mitochondrial acetylase GCN5L likely does not target IDH2. Using mass spectroscopy, we further identified lysine residues within IDH2, which are the substrates of SIRT3. In summary, we demonstrate that 2HG levels arise from non-mutant IDH2 reductive function and decrease with increasing acetylation level. The newly identified lysine residues might apply in regulation of IDH2 function in response to metabolic perturbations occurring in cancer cells, such as glucose-free conditions.

Diurnal regulation of the function of the rat brain glutamate dehydrogenase by acetylation and its dependence on thiamine administration

Glutamate dehydrogenase (GDH) is essential for the brain function and highly regulated, according to its role in metabolism of the major excitatory neurotransmitter glutamate. Here we show a diurnal pattern of the GDH acetylation in rat brain, associated with specific regulation of GDH function. Mornings the acetylation levels of K84 (near the ADP site), K187 (near the active site), and K503 (GTP-binding) are highly correlated. Evenings the acetylation levels of K187 and K503 decrease, and the correlations disappear. These daily variations in the acetylation adjust the GDH responses to the enzyme regulators. The adjustment is changed when the acetylation of K187 and K503 shows no diurnal variations, as in the rats after a high dose of thiamine. The regulation of GDH function by acetylation is confirmed in a model system, where incubation of the rat brain GDH with acetyl-CoA changes the enzyme responses to GTP and ADP, decreasing the activity at subsaturating concentrations of substrates. Thus, the GDH acetylation may support cerebral homeostasis, stabilizing the enzyme function during diurnal oscillations of the brain metabolome. Daytime and thiamine interact upon the (de)acetylation of GDH in vitro. Evenings the acetylation of GDH from control animals increases both IC₅₀ ^GTP and EC₅₀ ^ADP . Mornings the acetylation of GDH from thiamine-treated animals increases the enzyme IC₅₀ ^GTP . Molecular mechanisms of the GDH regulation by acetylation of specific residues are proposed. For the first time, diurnal and thiamine-dependent changes in the allosteric regulation of the brain GDH due to the enzyme acetylation are shown.

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.

Mitochondrial DNA: Epigenetics and environment

Maintenance of the mitochondrial genome is essential for proper cellular function. For this purpose, mitochondrial DNA (mtDNA) needs to be faithfully replicated, transcribed, translated, and repaired in the face of constant onslaught from endogenous and environmental agents. Although only 13 polypeptides are encoded within mtDNA, the mitochondrial proteome comprises over 1500 proteins that are encoded by nuclear genes and translocated to the mitochondria for the purpose of maintaining mitochondrial function. Regulation of mtDNA and mitochondrial proteins by epigenetic changes and post-translational modifications facilitate crosstalk between the nucleus and the mitochondria and ultimately lead to the maintenance of cellular health and homeostasis. DNA methyl transferases have been identified in the mitochondria implicating that methylation occurs within this organelle; however, the extent to which mtDNA is methylated has been debated for many years. Mechanisms of demethylation within this organelle have also been postulated, but the exact mechanisms and their outcomes is still an active area of research. Mitochondrial dysfunction in the form of altered gene expression and ATP production, resulting from epigenetic changes, can lead to various conditions including aging-related neurodegenerative disorders, altered metabolism, changes in circadian rhythm, and cancer. Here, we provide an overview of the epigenetic regulation of mtDNA via methylation, long and short noncoding RNAs, and post-translational modifications of nucleoid proteins (as mitochondria lack histones). We also highlight the influence of xenobiotics such as airborne environmental pollutants, contamination from heavy metals, and therapeutic drugs on mtDNA methylation. Environ. Mol. Mutagen., 60:668-682, 2019. © 2019 Wiley Periodicals, Inc.

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.

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.

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.

The role of YAP/TAZ activity in cancer metabolic reprogramming

In contrast to normal cells, which use the aerobic oxidation of glucose as their main energy production method, cancer cells prefer to use anaerobic glycolysis to maintain their growth and survival, even under normoxic conditions. Such tumor cell metabolic reprogramming is regulated by factors such as hypoxia and the tumor microenvironment. In addition, dysregulation of certain signaling pathways also contributes to cancer metabolic reprogramming. Among them, the Hippo signaling pathway is a highly conserved tumor suppressor pathway. The core oncosuppressive kinase cascade of Hippo pathway inhibits the nuclear transcriptional co-activators YAP and TAZ, which are the downstream effectors of Hippo pathway and oncogenic factors in many solid cancers. YAP/TAZ function as key nodes of multiple signaling pathways and play multiple regulatory roles in cancer cells. However, their roles in cancer metabolic reprograming are less clear. In the present review, we examine progress in research into the regulatory mechanisms of YAP/TAZ on glucose metabolism, fatty acid metabolism, mevalonate metabolism, and glutamine metabolism in cancer cells. Determining the roles of YAP/TAZ in tumor energy metabolism, particularly in relation to the tumor microenvironment, will provide new strategies and targets for the selective therapy of metabolism-related cancers.

Mitochondrial Dysfunction and Signaling in Chronic Liver Diseases

Mitochondria regulate hepatic lipid metabolism and oxidative stress. Ultrastructural mitochondrial lesions, altered mitochondrial dynamics, decreased activity of respiratory chain complexes, and impaired ability to synthesize adenosine triphosphate are observed in liver tissues from patients with alcohol-associated and non-associated liver diseases. Increased lipogenesis with decreased fatty acid β-oxidation leads to the accumulation of triglycerides in hepatocytes, which, combined with increased levels of reactive oxygen species, contributes to insulin resistance in patients with steatohepatitis. Moreover, mitochondrial reactive oxygen species mediate metabolic pathway signaling; alterations in these pathways affect development and progression of chronic liver diseases. Mitochondrial stress and lesions promote cell death, liver fibrogenesis, inflammation, and the innate immune responses to viral infections. We review the involvement of mitochondrial processes in development of chronic liver diseases, such as nonalcoholic fatty, alcohol-associated, and drug-associated liver diseases, as well as hepatitis B and C, and discuss how they might be targeted therapeutically.

GCN5L1/BLOS1 Links Acetylation, Organelle Remodeling, and Metabolism

General control of amino acid synthesis 5 (GCN5) like-1 (GCN5L1) was identified as a novel gene with sequence homology to the histone acetyltransferase Gcn5. Subsequent protein-interaction studies identified GCN5L1 as a subunit of the multiprotein lysosome biogenesis complex, resulting in an alternative designation as biogenesis of lysosome-related organelle complex 1 subunit 1 (BLOS1 or BLOC1S1). Despite the distinct nomenclatures, GCN5L1/BLOS1 has been shown to play crucial roles in mitochondria, endosomes, lysosomes, and synaptic vesicle precursors (SVPs). GCN5L1/BLOS1 controls mitochondrial protein acetylation, modulates metabolic pathways, and orchestrates retrograde mitochondria-to-nucleus signaling. It also contributes to endosome-lysosome and vesicle trafficking and to endolysosomal function. Here we discuss the intracellular roles of GCN5L1/BLOS1 in the hope of linking mitochondria-centric effects to cytosolic vesicle biology.