So close and yet so far: mitochondria and peroxisomes are one but with specific talents

Segregation of pathways leading to pexophagy

Peroxisomes are organelles with key roles in metabolism including long-chain fatty acid production. Their metabolic functions overlap and interconnect with those of mitochondria, with which they share an overlapping but distinct proteome. Both organelles are degraded by selective autophagy processes termed pexophagy and mitophagy. Although mitophagy has received intense attention, the pathways linked to pexophagy and associated tools are less well developed. We have identified the neddylation inhibitor MLN4924 as a potent activator of pexophagy and show that this is mediated by the HIF1α-dependent up-regulation of BNIP3L/NIX, a known adaptor for mitophagy. We show that this pathway is distinct from pexophagy induced by the USP30 deubiquitylase inhibitor CMPD-39, for which we identify the adaptor NBR1 as a central player. Our work suggests a level of complexity to the regulation of peroxisome turnover that includes the capacity to coordinate with mitophagy, via NIX, which acts as a rheostat for both processes.

Chronic stress targets mitochondrial respiratory efficiency in the skeletal muscle of C57BL/6 mice

Episodes of chronic stress can result in psychic disorders like post-traumatic stress disorder, but also promote the development of metabolic syndrome and type 2 diabetes. We hypothesize that muscle, as main regulator of whole-body energy expenditure, is a central target of acute and adaptive molecular effects of stress in this context. Here, we investigate the immediate effect of a stress period on energy metabolism in Musculus gastrocnemius in our established C57BL/6 chronic variable stress (Cvs) mouse model. Cvs decreased lean body mass despite increased energy intake, reduced circadian energy expenditure (EE), and substrate utilization. Cvs altered the proteome of metabolic components but not of the oxidative phosphorylation system (OXPHOS), or other mitochondrial structural components. Functionally, Cvs impaired the electron transport chain (ETC) capacity of complex I and complex II, and reduces respiratory capacity of the ETC from complex I to ATP synthase. Complex I-OXPHOS correlated to diurnal EE and complex II-maximal uncoupled respiration correlated to diurnal and reduced nocturnal EE. Bioenergetics assessment revealed higher optimal thermodynamic efficiencies (ƞ-opt) of mitochondria via complex II after Cvs. Interestingly, transcriptome and methylome were unaffected by Cvs, thus excluding major contributions to supposed metabolic adaptation processes. In summary, the preclinical Cvs model shows that metabolic pressure by Cvs is initially compensated by adaptation of mitochondria function associated with high thermodynamic efficiency and decreased EE to manage the energy balance. This counter-regulation of mitochondrial complex II may be the driving force to longitudinal metabolic changes of muscle physiological adaptation as the basis of stress memory.

Fatty Liver Due to Increased de novo Lipogenesis: Alterations in the Hepatic Peroxisomal Proteome

In non-alcoholic fatty liver disease (NAFLD) caused by ectopic lipid accumulation, lipotoxicity is a crucial molecular risk factor. Mechanisms to eliminate lipid overflow can prevent the liver from functional complications. This may involve increased secretion of lipids or metabolic adaptation to ß-oxidation in lipid-degrading organelles such as mitochondria and peroxisomes. In addition to dietary factors, increased plasma fatty acid levels may be due to increased triglyceride synthesis, lipolysis, as well as de novo lipid synthesis (DNL) in the liver. In the present study, we investigated the impact of fatty liver caused by elevated DNL, in a transgenic mouse model with liver-specific overexpression of human sterol regulatory element-binding protein-1c (alb-SREBP-1c), on hepatic gene expression, on plasma lipids and especially on the proteome of peroxisomes by omics analyses, and we interpreted the results with knowledge-based analyses. In summary, the increased hepatic DNL is accompanied by marginal gene expression changes but massive changes in peroxisomal proteome. Furthermore, plasma phosphatidylcholine (PC) as well as lysoPC species were altered. Based on these observations, it can be speculated that the plasticity of organelles and their functionality may be directly affected by lipid overflow.

Inactivation of SREBP-1a Phosphorylation Prevents Fatty Liver Disease in Mice: Identification of Related Signaling Pathways by Gene Expression Profiles in Liver and Proteomes of Peroxisomes

The key lipid metabolism transcription factor sterol regulatory element-binding protein (SREBP)-1a integrates gene regulatory effects of hormones, cytokines, nutrition and metabolites as lipids, glucose, or cholesterol via phosphorylation by different mitogen activated protein kinase (MAPK) cascades. We have previously reported the impact of SREBP-1a phosphorylation on the phenotype in transgenic mouse models with liver-specific overexpression of the N-terminal transcriptional active domain of SREBP-1a (alb-SREBP-1a) or a MAPK phosphorylation site-deficient variant (alb-SREBP-1a∆P; (S63A, S117A, T426V)), respectively. In this report, we investigated the molecular basis of the systemic observations by holistic analyses of gene expression in liver and of proteome patterns in lipid-degrading organelles involved in the pathogenesis of metabolic syndrome, i.e., peroxisomes, using 2D-DIGE and mass spectrometry. The differences in hepatic gene expression and peroxisomal protein patterns were surprisingly small between the control and alb-SREBP-1a mice, although the latter develop a severe phenotype with visceral obesity and fatty liver. In contrast, phosphorylation site-deficient alb-SREBP-1a∆P mice, which are protected from fatty liver disease, showed marked differences in hepatic gene expression and peroxisomal proteome patterns. Further knowledge-based analyses revealed that disruption of SREBP-1a phosphorylation resulted in massive alteration of cellular processes, including signs for loss of targeting lipid pathways.

Proteomics for studying the effects of L. rhamnosus LV108 against non-alcoholic fatty liver disease in rats

Probiotics show protective effects against non-alcoholic fatty liver disease (NAFLD). However, their efficacy against NAFLD and the mechanisms are still unknown. In this study, Tandem Mass Tag (TMT) relative quantitative proteomics was utilized to track the changes in liver protein expression in rats fed with Lactobacillus rhamnosus LV108. A total of 4155 corresponding proteins were identified by MS. A total of 26 differentially expressed proteins were found between the L. rhamnosus LV108 treatment group and mode group, and there are 16 proteins up-regulated and 10 proteins down-regulated. Most of the differentially expressed proteins were involved in apoptosis and lipid metabolism. The key differentially expressed proteins (BFAR and Cyt-C) were verified by parallel reaction monitoring to be reliable. Our study is the first attempt to analyze the protein profile of probiotic-treated NAFLD model rats by quantitative proteomics. The identified proteins in this study will likely contribute to a better understanding of the molecular mechanisms of the effect of probiotics on NAFLD.

Probiotics show protective effects against non-alcoholic fatty liver disease (NAFLD).

Alteration of Liver Peroxisomal and Mitochondrial Functionality in the NZO Mouse Model of Metabolic Syndrome

PURPOSE

Metabolic syndrome (MetS) consists of five risk factors: elevated blood pressure and fasting glucose, visceral obesity, dyslipidemia, and hypercholesterinemia. The physiological impact of lipid metabolism indicated as visceral obesity and hepatic lipid accumulation on MetS is still under debate. One major cause of disturbed lipid metabolism might be dysfunction of cellular organelles controlling energy homeostasis, i.e., mitochondria and peroxisomes.

EXPERIMENTAL DESIGN

The New Zealand Obese (NZO) mouse model exhibits a polygenic syndrome of obesity, insulin resistance, triglyceridemia, and hypercholesterolemia that resembles human metabolic syndrome. We applied a multi-omics approach combining lipidomics with liver transcriptomics and top-down MS based organelle proteomics (2D-DIGE) of highly enriched mitochondria and peroxisomes in male mice, to investigate molecular mechanisms related to the impact of lipid metabolism in the pathophysiology of the metabolic syndrome.

CONCLUSIONS AND CLINICAL RELEVANCE

Proteome analyses of liver organelles indicate differences in fatty acid and cholesterol metabolism, mainly influenced by PG-C1α/PPARα and other nuclear receptor mediated pathways. These results are in accordance with altered serum lipid profiles and elevated organelle functionality. These data emphasize that metabolic syndrome is accompanied with increased mitochondria and peroxisomal activity to cope with dyslipidemia and hypercholesterinemia driven hepatic lipid overflow in developing a fatty liver.

Dysregulation of redox pathways in liver fibrosis

Reactive oxygen species are implicated in physiological signaling and cell fate decisions. In chronic liver diseases persistent and increased production of oxidative radicals drives a fibrogenic response that is a common feature of disease progression. Despite our understanding the biology of the main prooxidant enzymes, their targets, and antioxidant mechanisms in the liver, there is still lack of knowledge concerning their precise role in the pathogenesis of fibrosis. This review will examine the role of physiological redox signaling in the liver, provide an overview on recent advances in prooxidant and antioxidant pathways that are dysregulated during fibrosis, and highlight possible novel treatment targets.

Peroxisomes compensate hepatic lipid overflow in mice with fatty liver

Major causes of lipid accumulation in liver are increased import or synthesis or decreased catabolism of fatty acids. The latter is caused by dysfunction of cellular organelles controlling energy homeostasis, i.e., mitochondria. Peroxisomes also appear to be an important organelle in lipid metabolism of hepatocytes, but little is known about their role in the development of non-alcoholic fatty liver disease (NAFLD). To investigate the role of peroxisomes alongside mitochondria in excessive hepatic lipid accumulation, we used leptin-resistant db/db mice on C57BLKS background, a mouse model that develops hyperphagia-induced diabetes with obesity and NAFLD. Proteome and gene expression analyses along with lipid analyses in the liver revealed differential expression of genes related to lipid metabolism and β-oxidation, whereas genes for peroxisomal proteins were predominantly regulated.

CONCLUSION

Our investigations show that in fatty liver disease in combination with obesity and diabetes, the hepatocyte-protecting organelle peroxisome is altered. Hence, peroxisomes might indicate a stage of pre-NAFLD, play a role in the early development of NAFLD and appear to be a potential target for treatment and prevention of NAFLD.

Phosphorylation of sterol regulatory element-binding protein (SREBP)-1c by p38 kinases, ERK and JNK influences lipid metabolism and the secretome of human liver cell line HepG2

The transcription factor sterol regulatory element binding protein (SREBP)-1c plays a pivotal role in lipid metabolism. In this report we identified the main phosphorylation sites of MAPK-families, i.e. p38 stress-activated MAPK (p38), ERK-MAPK (ERK) or c-JUN N-terminal protein kinases (JNK) in SREBP-1c. The major phosphorylation sites of p38, i.e. serine 39 and threonine 402, are identical to those we recently identified in the splice-variant SREBP-1a. In contrast, ERK and JNK phosphorylate SREBP-1c at two major sites, i.e. threonine 81 and serine 93, instead of one site in SREBP-1a. Functional analyses of the biological outcome in the human liver cell line HepG2 reveals SREBP-1c phosphorylation dependent alteration in lipid metabolism and secretion pattern of lipid transporting proteins, e.g. ApoE or ApoA1. These results suggest that phosphorylation of SREBP-1c by different MAPKs interferes with lipid metabolism and the secretory activity of liver cells.