Divergence in aerobic capacity influences hepatic and systemic metabolic adaptations to bile acid sequestrant and short-term high-fat/sucrose feeding in rats.

High versus low aerobic capacity significantly impacts the risk for metabolic diseases. Rats selectively bred for high or low intrinsic aerobic capacity differently modify hepatic bile acid metabolism in response to high-fat diets (HFDs). Here we tested if a bile acid sequestrant would alter hepatic and whole body metabolism differently in rats with high and low aerobic capacity fed a 1-wk HFD. Male rats (8 mo of age) that were artificially selected to be high (HCR) and low-capacity runners (LCR) with divergent intrinsic aerobic capacities were transitioned from a low-fat diet (LFD, 10% fat) to an HFD (45% fat) with or without a bile acid sequestrant (BA-Seq, 2% cholestyramine resin) for 7 days while maintained in an indirect calorimetry system. HFD + BA-Seq increased fecal excretion of lipids and bile acids and prevented weight and fat mass gain in both strains. Interestingly, HCR rats had increased adaptability to enhance fecal bile acid and lipid loss, resulting in more significant energy loss than their LCR counterpart. In addition, BA-Seq induced a greater expression of hepatic CYP7A1 gene expression, the rate-limiting enzyme of bile acid synthesis in HCR rats both on HFD and HFD + BA-Seq diets. HCR displayed a more significant reduction of RQ in response to HFD than LCR, but HFD + BA-Seq lowered RQ in both groups compared with HFD alone, demonstrating a pronounced impact on metabolic flexibility. In conclusion, BA-Seq provides uniform metabolic benefits for metabolic flexibility and adiposity, but rats with higher aerobic capacity display adaptability for hepatic bile acid metabolism.NEW & NOTEWORTHY The administration of bile acid sequestrant (BA-Seq) has uniform metabolic benefits in terms of metabolic flexibility and adiposity in rats with high and low aerobic capacity. However, rats with higher aerobic capacity demonstrate greater adaptability in hepatic bile acid metabolism, resulting in increased fecal bile acid and lipid loss, as well as enhanced fecal energy loss.

Oral combined contraceptives induce liver mitochondrial reactive oxygen species and whole-body metabolic adaptations in female mice.

Compared to age-matched men, pre-menopausal women show greater resilience against cardiovascular disease (CVD), hepatic steatosis, diabetes and obesity - findings that are widely attributed to oestrogen. However, meta-analysis data suggest that current use of oral combined contraceptives (OC) is a risk factor for myocardial infarction, and OC use further compounds with metabolic disease risk factors to increase CVD susceptibility. While mitochondrial function in tissues such as the liver and skeletal muscle is an emerging mechanism by which oestrogen may confer its protection, effects of OC use on mitochondria and metabolism in the context of disease risk remain unexplored. To answer this question, female C57Bl/6J mice were fed a high fat diet and treated with vehicle or OCs for 3, 12 or 20 weeks (n = 6 to 12 per group) at a dose and ratio that mimic the human condition of cycle cessation in the low oestrogen, high progesterone stage. Liver and skeletal muscle mitochondrial function (respiratory capacity, H2 O2 , coupling) was measured along with clinical outcomes of cardiometabolic disease such as obesity, glucose tolerance, hepatic steatosis and aortic atherosclerosis. The main findings indicate that regardless of treatment duration, OCs robustly increase hepatic mitochondrial H2 O2 levels, likely due to diminished antioxidant capacity, but have no impact on muscle mitochondrial H2 O2 . Furthermore, OC-treated mice had lower adiposity and hepatic triglyceride content compared to control mice despite reduced wheel running, spontaneous physical activity and total energy expenditure. Together, these studies describe tissue-specific effects of OC use on mitochondria as well as variable impacts on markers of metabolic disease susceptibility. KEY POINTS: Oestrogen loss in women increases risk for cardiometabolic diseases, a link that has been partially attributed to negative impacts on mitochondria and energy metabolism. To study the effect of oral combined contraceptives (OCs) on hepatic and skeletal muscle mitochondria and whole-body energy metabolism, we used an animal model of OCs which mimics the human condition of cessation of hormonal cycling in the low oestrogen, high progesterone state. OC-treated mice have increased hepatic mitochondrial oxidative stress and decreased physical activity and energy expenditure, despite displaying lower adiposity and liver fat at this time point. These pre-clinical data reveal tissue-specific effects of OCs that likely underlie the clinical findings of increased cardiometabolic disease in women who use OCs compared to non-users, when matched for obesity.

Estradiol treatment or modest exercise improves hepatic health and mitochondrial outcomes in female mice following ovariectomy.

We recently reported that compared with males, female mice have increased hepatic mitochondrial respiratory capacity and are protected against high-fat diet-induced steatosis. Here, we sought to determine the role of estrogen in hepatic mitochondrial function, steatosis, and bile acid metabolism in female mice and investigate potential benefits of exercise in the absence or presence of estrogen via ovariectomy (OVX). Female C57BL mice (n = 6 per group) were randomly assigned to sham surgery (sham), ovariectomy (OVX), or OVX plus estradiol replacement therapy (OVX + Est). Half of the mice in each treatment group were sedentary (SED) or had access to voluntary wheel running (VWR). All mice were fed a high-fat diet (HFD) and were housed at thermoneutral temperatures. We assessed isolated hepatic mitochondrial respiratory capacity using the Oroboros O2k with both pyruvate and palmitoylcarnitine as substrates. As expected, OVX mice presented with greater hepatic steatosis, weight gain, and fat mass gain compared with sham and OVX + Est animals. Hepatic mitochondrial coupling (basal/state 3 respiration) with pyruvate was impaired following OVX, but both VWR and estradiol treatment rescued coupling to levels greater than or equal to sham animals. Estradiol and exercise also had different effects on liver electron transport chain protein expression depending on OVX status. Markers of bile acid metabolism and excretion were also impaired by ovariectomy but rescued with estradiol add-back. Together our data suggest that estrogen depletion impairs hepatic mitochondrial function and liver health, and that estradiol replacement and modest exercise can aid in rescuing this phenotype.NEW & NOTEWORTHY OVX induces hepatic steatosis in sedentary mice which can be prevented by modest physical activity (VWR) and/or estradiol treatment. Estrogen impacts hepatic mitochondrial coupling in a substrate-specific manner. OVX mice have impaired fecal bile acid excretion, which was rescued with estradiol treatment.

Sex modulates hepatic mitochondrial adaptations to high-fat diet and physical activity.

The impact of sexual dimorphism and mitophagy on hepatic mitochondrial adaptations during the treatment of steatosis with physical activity are largely unknown. Here, we tested if deficiencies in liver-specific peroxisome proliferative activated-receptor-γ coactivator-1α (PGC-1α), a transcriptional coactivator of biogenesis, and BCL-2/ADENOVIRUS EIB 19-kDa interacting protein (BNIP3), a mitophagy regulator, would impact hepatic mitochondrial adaptations (respiratory capacity, H2O2 production, mitophagy) to a high-fat diet (HFD) and HFD plus physical activity via voluntary wheel running (VWR) in both sexes. Male and female wild-type (WT), liver-specific PGC-1α heterozygote (LPGC-1α), and BNIP3 null mice were thermoneutral housed (29-31°C) and divided into three groups: sedentary-low-fat diet (LFD), 16 wk of (HFD), or 16 wk of HFD with VWR for the final 8 wk (HFD + VWR) (n = 5-7/sex/group). HFD did not impair mitochondrial respiratory capacity or coupling in any group; however, HFD + VWR significantly increased maximal respiratory capacity only in WT and PGC-1α females. Males required VWR to elicit mitochondrial adaptations that were inherently present in sedentary females including greater mitochondrial coupling control and reduced H2O2 production. Females had overall reduced markers of mitophagy, steatosis, and liver damage. Steatosis and markers of liver injury were present in sedentary male mice on the HFD and were effectively reduced with VWR despite no resolution of steatosis. Overall, reductions in PGC-1α and loss of BNIP3 only modestly impacted mitochondrial adaptations to HFD and HFD + VWR with the biggest effect seen in BNIP3 females. In conclusion, hepatic mitochondrial adaptations to HFD and treatment of HFD-induced steatosis with VWR are more dependent on sex than PGC-1α or BNIP3.

Hepatic mitochondrial adaptations to physical activity: impact of sexual dimorphism, PGC1α and BNIP3-mediated mitophagy.

KEY POINTS: Hepatic mitochondrial adaptations to physical activity may be regulated by mitochondrial biogenesis (PGC1α) and mitophagy (BNIP3). Additionally, these adaptations may be sex-dependent. Chronic increase in physical activity lowers basal mitochondrial respiratory capacity in mice. Female mice have higher hepatic electron transport system protein content, elevated respiratory capacity, lowered mitophagic flux, and emit less mitochondrial H2 O2 independent of physical activity. Males require chronic daily physical activity to attain a similar mitochondrial phenotype compared to females. In contrast, females have limited hepatic adaptations to chronic physical activity. Livers deficient in PGC1α and BNIP3 display similar mitochondrial adaptations to physical activity to those found in wild-type mice. ABSTRACT: Hepatic mitochondrial adaptations to physical activity may be regulated by biogenesis- and mitophagy-associated pathways in a sex-dependent manner. Here, we tested if mice with targeted deficiencies in liver-specific peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α; LPGC1α+/- ) and BCL2/adenovirus E1B 19 kDa protein-interacting protein 3 (BNIP3)-mediated mitophagy (BNIP3-/- ) would have reduced physical activity-induced adaptations in respiratory capacity, H2 O2 emission and mitophagy compared to wild-type (WT) controls and if these effects were impacted by sex. Male and female WT, LPGC1α+/- and BNIP3-/- C57BL6/J mice were divided into groups that remained sedentary or had access to daily physical activity via voluntary wheel running (VWR) (n = 6-10/group) for 4 weeks. Mice had ad libitum access to low-fat diet and water. VWR reduced basal mitochondrial respiration, increased mitochondrial coupling and altered ubiquitin-mediated mitophagy in a sex-specific manner in WT mice. Female mice of all genotypes displayed higher electron transport system content, displayed increased ADP-stimulated respiration, produced less mitochondrially derived reactive oxygen species, exhibited reduced mitophagic flux, and were less responsive to VWR compared to males. Males responded more robustly to VWR-induced changes in hepatic mitochondrial function resulting in a match to adaptations found in females. Deficiencies in PGC1α and BNIP3 alone did not largely alter mitochondrial adaptations to VWR. However, VWR restored sex-dependent abnormalities in mitophagic flux in LPGC1α+/- . Finally, BNIP3-/- mice had elevated mitochondrial content and increased mitochondrial respiration putatively through repressed mitophagic flux. In conclusion, hepatic mitochondrial adaptations to physical activity are more dependent on sex than PGC1α and BNIP3.

Heat shock protein 72 regulates hepatic lipid accumulation.

Induction of the chaperone heat shock protein 72 (HSP72) through heat treatment (HT), exercise, or overexpression improves glucose tolerance and mitochondrial function in skeletal muscle. Less is known about HSP72 function in the liver where lipid accumulation can result in insulin resistance and nonalcoholic fatty liver disease (NAFLD). The purpose of this study was 1) to determine whether weekly in vivo HT induces hepatic HSP72 and improves glucose tolerance in rats fed a high-fat diet (HFD) and 2) to determine the ability of HSP72 to protect against lipid accumulation and mitochondrial dysfunction in primary hepatocytes. Male Wistar rats were fed an HFD for 15 wk and were given weekly HT (41°C, 20 min) or sham treatments (37°C, 20 min) for the final 7 wk. Glucose tolerance and insulin sensitivity were assessed, along with HSP72 induction and triglyceride storage, in the skeletal muscle and liver. The effect of an acute loss of HSP72 in primary hepatocytes was examined via siRNA. Weekly in vivo HT improved glucose tolerance, elevated muscle and hepatic HSP72 protein content, and reduced muscle triglyceride storage. In primary hepatocytes, mitochondrial morphology was changed, and fatty acid oxidation was reduced in small interfering HSP72 (siHSP72)-treated hepatocytes. Lipid accumulation following palmitate treatment was increased in siHSP72-treated hepatocytes. These data suggest that HT may improve systemic metabolism via induction of hepatic HSP72. Additionally, acute loss of HSP72 in primary hepatocytes impacts mitochondrial health as well as fat oxidation and storage. These findings suggest therapies targeting HSP72 in the liver may prevent NAFLD.

Aerobic capacity mediates susceptibility for the transition from steatosis to steatohepatitis.

KEY POINTS: Low intrinsic aerobic capacity is associated with increased all-cause and liver-related mortality in humans. Low intrinsic aerobic capacity in the low capacity runner (LCR) rat increases susceptibility to acute and chronic high-fat/high-sucrose diet-induced steatosis, without observed increases in liver inflammation. Addition of excess cholesterol to a high-fat/high-sucrose diet produced greater steatosis in LCR and high capacity runner (HCR) rats. However, the LCR rat demonstrated greater susceptibility to increased liver inflammatory and apoptotic markers compared to the HCR rat. The progressive non-alcoholic fatty liver disease observed in the LCR rats following western diet feeding was associated with further declines in liver fatty acid oxidation and mitochondrial respiratory capacity compared to HCR rats. ABSTRACT: Low aerobic capacity increases risk for non-alcoholic fatty liver disease and liver-related disease mortality, but mechanisms mediating these effects remain unknown. We recently reported that rats bred for low aerobic capacity (low capacity runner; LCR) displayed susceptibility to high fat diet-induced steatosis in association with reduced hepatic mitochondrial fatty acid oxidation (FAO) and respiratory capacity compared to high aerobic capacity (high capacity runner; HCR) rats. Here we tested the impact of aerobic capacity on susceptibility for progressive liver disease following a 16-week 'western diet' (WD) high in fat (45% kcal), cholesterol (1% w/w) and sucrose (15% kcal). Unlike previously with a diet high in fat and sucrose alone, the inclusion of cholesterol in the WD induced hepatomegaly and steatosis in both HCR and LCR rats, while producing greater cholesterol ester accumulation in LCR compared to HCR rats. Importantly, WD-fed low-fitness LCR rats displayed greater inflammatory cell infiltration, serum alanine transaminase, expression of hepatic inflammatory markers (F4/80, MCP-1, TLR4, TLR2 and IL-1ß) and effector caspase (caspase 3 and 7) activation compared to HCR rats. Further, LCR rats had greater WD-induced decreases in complete FAO and mitochondrial respiratory capacity. Intrinsic aerobic capacity had no impact on WD-induced hepatic steatosis; however, rats bred for low aerobic capacity developed greater hepatic inflammation, which was associated with reduced hepatic mitochondrial FAO and respiratory capacity and increased accumulation of cholesterol esters. These results confirm epidemiological reports that aerobic capacity impacts progression of liver disease and suggest that these effects are mediated through alterations in hepatic mitochondrial function.

Acylcarnitines: potential implications for skeletal muscle insulin resistance.

Insulin resistance may be linked to incomplete fatty acid ß-oxidation and the subsequent increase in acylcarnitine species in different tissues including skeletal muscle. It is not known if acylcarnitines participate in muscle insulin resistance or simply reflect dysregulated metabolism. The aims of this study were to determine whether acylcarnitines can elicit muscle insulin resistance and to better understand the link between incomplete muscle fatty acid ß-oxidation, oxidative stress, inflammation, and insulin-resistance development. Differentiated C2C12, primary mouse, and human myotubes were treated with acylcarnitines (C4:0, C14:0, C16:0) or with palmitate with or without carnitine acyltransferase inhibition by mildronate. Treatment with C4:0, C14:0, and C16:0 acylcarnitines resulted in 20-30% decrease in insulin response at the level of Akt phosphorylation and/or glucose uptake. Mildronate reversed palmitate-induced insulin resistance concomitant with an ∼25% decrease in short-chain acylcarnitine and acetylcarnitine secretion. Although proinflammatory cytokines were not affected under these conditions, oxidative stress was increased by 2-3 times by short- or long-chain acylcarnitines. Acylcarnitine-induced oxidative stress and insulin resistance were reversed by treatment with antioxidants. Results are consistent with the conclusion that incomplete muscle fatty acid ß-oxidation causes acylcarnitine accumulation and associated oxidative stress, raising the possibility that these metabolites play a role in muscle insulin resistance.

Unique plasma metabolomic signatures of individuals with inherited disorders of long-chain fatty acid oxidation.

Blood and urine acylcarnitine profiles are commonly used to diagnose long-chain fatty acid oxidation disorders (FAOD: i.e., long-chain hydroxy-acyl-CoA dehydrogenase [LCHAD] and carnitine palmitoyltransferase 2 [CPT2] deficiency), but the global metabolic impact of long-chain FAOD has not been reported. We utilized untargeted metabolomics to characterize plasma metabolites in 12 overnight-fasted individuals with FAOD (10 LCHAD, two CPT2) and 11 healthy age-, sex-, and body mass index (BMI)-matched controls, with the caveat that individuals with FAOD consume a low-fat diet supplemented with medium-chain triglycerides (MCT) while matched controls consume a typical American diet. In plasma 832 metabolites were identified, and partial least squared-discriminant analysis (PLS-DA) identified 114 non-acylcarnitine variables that discriminated FAOD subjects and controls. FAOD individuals had significantly higher triglycerides and lower specific phosphatidylethanolamines, ceramides, and sphingomyelins. Differences in phosphatidylcholines were also found but the directionality differed by metabolite species. Further, there were few differences in non-lipid metabolites, indicating the metabolic impact of FAOD specifically on lipid pathways. This analysis provides evidence that LCHAD/CPT2 deficiency significantly alters complex lipid pathway flux. This metabolic signature may provide new clinical tools capable of confirming or diagnosing FAOD, even in subjects with a mild phenotype, and may provide clues regarding the biochemical and metabolic impact of FAOD that is relevant to the etiology of FAOD symptoms.

Long-chain acylcarnitines activate cell stress and myokine release in C2C12 myotubes: calcium-dependent and -independent effects.

Acylcarnitines, important lipid biomarkers reflective of acyl-CoA status, are metabolites that possess bioactive and inflammatory properties. This study examined the potential for long-chain acylcarnitines to activate cellular inflammatory, stress, and death pathways in a skeletal muscle model. Differentiated C2C12 myotubes treated with l-C14, C16, C18, and C18:1 carnitine displayed dose-dependent increases in IL-6 production with a concomitant rise in markers of cell permeability and death, which was not observed for shorter chain lengths. l-C16 carnitine, used as a representative long-chain acylcarnitine at initial extracellular concentrations ≥25 µM, increased IL-6 production 4.1-, 14.9-, and 31.4-fold over vehicle at 25, 50, and 100 µM. Additionally, l-C16 carnitine activated c-Jun NH2-terminal kinase, extracellular signal-regulated kinase, and p38 mitogen-activated protein kinase between 2.5- and 11-fold and induced cell injury and death within 6 h with modest activation of the apoptotic caspase-3 protein. l-C16 carnitine rapidly increased intracellular calcium, most clearly by 10 µM, implicating calcium as a potential mechanism for some activities of long-chain acylcarnitines. The intracellular calcium chelator BAPTA-AM blunted l-C16 carnitine-mediated IL-6 production by >65%. However, BAPTA-AM did not attenuate cell permeability and death responses, indicating that these outcomes are calcium independent. The 16-carbon zwitterionic compound amidosulfobetaine-16 qualitatively mimicked the l-C16 carnitine-associated cell stress outcomes, suggesting that the effects of high experimental concentrations of long-chain acylcarnitines are through membrane disruption. Herein, a model is proposed in which acylcarnitine cell membrane interactions take place along a spectrum of cellular concentrations encountered in physiological-to-pathophysiological conditions, thus regulating function of membrane-based systems and impacting cell biology.

Acylcarnitines activate proinflammatory signaling pathways.

Incomplete ß-oxidation of fatty acids in mitochondria is a feature of insulin resistance and type 2 diabetes mellitus (T2DM). Previous studies revealed that plasma concentrations of medium- and long-chain acylcarnitines (by-products of incomplete ß-oxidation) are elevated in T2DM and insulin resistance. In a previous study, we reported that mixed D,L isomers of C12- or C14-carnitine induced an NF-κB-luciferase reporter gene in RAW 264.7 cells, suggesting potential activation of proinflammatory pathways. Here, we determined whether the physiologically relevant L-acylcarnitines activate classical proinflammatory signaling pathways and if these outcomes involve pattern recognition receptor (PRR)-associated pathways. Acylcarnitines induced the expression of cyclooxygenase-2 in a chain length-dependent manner in RAW 264.7 cells. L-C14 carnitine (5-25 µM), used as a representative acylcarnitine, stimulated the expression and secretion of proinflammatory cytokines in a dose-dependent manner. Furthermore, L-C14 carnitine induced phosphorylation of JNK and ERK, common downstream components of many proinflammatory signaling pathways including PRRs. Knockdown of MyD88, a key cofactor in PRR signaling and inflammation, blunted the proinflammatory effects of acylcarnitine. While these results point to potential involvement of PRRs, L-C14 carnitine promoted IL-8 secretion from human epithelial cells (HCT-116) lacking Toll-like receptors (TLR)2 and -4, and did not activate reporter constructs in TLR overexpression cell models. Thus, acylcarnitines have the potential to activate inflammation, but the specific molecular and tissue target(s) involved remain to be identified.

Contributions of adipose tissue architectural and tensile properties toward defining healthy and unhealthy obesity.

The extracellular matrix (ECM) plays an important role in the maintenance of white adipose tissue (WAT) architecture and function, and proper ECM remodeling is critical to support WAT malleability to accommodate changes in energy storage needs. Obesity and adipocyte hypertrophy place a strain on the ECM remodeling machinery, which may promote disordered ECM and altered tissue integrity and could promote proinflammatory and cell stress signals. To explore these questions, new methods were developed to quantify omental and subcutaneous WAT tensile strength and WAT collagen content by three-dimensional confocal imaging, using collagen VI knockout mice as a methods validation tool. These methods, combined with comprehensive measurement of WAT ECM proteolytic enzymes, transcript, and blood analyte analyses, were used to identify unique pathophenotypes of metabolic syndrome and type 2 diabetes mellitus in obese women, using multivariate statistical modeling and univariate comparisons with weight-matched healthy obese individuals. In addition to the expected differences in inflammation and glycemic control, approximately 20 ECM-related factors, including omental tensile strength, collagen, and enzyme transcripts, helped discriminate metabolically compromised obesity. This is consistent with the hypothesis that WAT ECM physiology is intimately linked to metabolic health in obese humans, and the studies provide new tools to explore this relationship.

Divergence in aerobic capacity and energy expenditure influence metabolic tissue mitochondrial protein synthesis rates in aged rats.

Age-associated declines in aerobic capacity promote the development of various metabolic diseases. In rats selectively bred for high/low intrinsic aerobic capacity, greater aerobic capacity reduces susceptibility to metabolic disease while increasing longevity. However, little remains known how intrinsic aerobic capacity protects against metabolic disease, particularly with aging. Here, we tested the effects of aging and intrinsic aerobic capacity on systemic energy expenditure, metabolic flexibility and mitochondrial protein synthesis rates using 24-month-old low-capacity (LCR) or high-capacity runner (HCR) rats. Rats were fed low-fat diet (LFD) or high-fat diet (HFD) for eight weeks, with energy expenditure (EE) and metabolic flexibility assessed utilizing indirect calorimetry during a 48 h fast/re-feeding metabolic challenge. Deuterium oxide (D2O) labeling was used to assess mitochondrial protein fraction synthesis rates (FSR) over a 7-day period. HCR rats possessed greater EE during the metabolic challenge. Interestingly, HFD induced changes in respiratory exchange ratio (RER) in male and female rats, while HCR female rat RER was largely unaffected by diet. In addition, analysis of protein FSR in skeletal muscle, brain, and liver mitochondria showed tissue-specific adaptations between HCR and LCR rats. While brain and liver protein FSR were altered by aerobic capacity and diet, these effects were less apparent in skeletal muscle. Overall, we provide evidence that greater aerobic capacity promotes elevated EE in an aged state, while also regulating metabolic flexibility in a sex-dependent manner. Modulation of mitochondrial protein FSR by aerobic capacity is tissue-specific with aging, likely due to differential energetic requirements by each tissue.

Sexually dimorphic hepatic mitochondrial adaptations to exercise: a mini-review.

Exercise is a physiological stress that disrupts tissue and cellular homeostasis while enhancing systemic metabolic energy demand mainly through the increased workload of skeletal muscle. Although the extensive focus has been on skeletal muscle adaptations to exercise, the liver senses these disruptions in metabolic energy homeostasis and responds to provide the required substrates to sustain increased demand. Hepatic metabolic flexibility is an energetically costly process that requires continuous mitochondrial production of the cellular currency ATP. To do so, the liver must maintain a healthy functioning mitochondrial pool, attained through well-regulated and dynamic processes. Intriguingly, some of these responses are sex-dependent. This mini-review examines the hepatic mitochondrial adaptations to exercise with a focus on sexual dimorphism.

Relationship of Muscle Apolipoprotein E Expression with Markers of Cellular Stress, Metabolism, and Blood Biomarkers in Cognitively Healthy and Impaired Older Adults.

BACKGROUND: Individuals with mild cognitive impairment (MCI) have reduced lipid-stimulated mitochondrial respiration in skeletal muscle. A major risk factor for Alzheimer's disease (AD), the apolipoprotein E4 (APOE4) allele, is implicated in lipid metabolism and is associated with metabolic and oxidative stress that can result from dysfunctional mitochondria. Heat shock protein 72 (Hsp72) is protective against these stressors and is elevated in the AD brain. OBJECTIVE: Our goal was to characterize skeletal muscle ApoE and Hsp72 protein expression in APOE4 carriers in relationship to cognitive status, muscle mitochondrial respiration and AD biomarkers. METHODS: We analyzed previously collected skeletal muscle tissue from 24 APOE4 carriers (60y+) who were cognitively healthy (CH, n = 9) or MCI (n = 15). We measured ApoE and Hsp72 protein levels in muscle and phosphorylated tau181 (pTau181) levels in plasma, and leveraged previously collected data on APOE genotype, mitochondrial respiration during lipid oxidation, and VO2 max. RESULTS: Muscle ApoE (p = 0.013) and plasma pTau181 levels (p < 0.001) were higher in MCI APOE4 carriers. Muscle ApoE positively correlated with plasma pTau181 in all APOE4 carriers (R2 = 0.338, p = 0.003). Hsp72 expression negatively correlated with ADP (R2 = 0.775, p = <0.001) and succinate-stimulated respiration (R2 = 0.405, p = 0.003) in skeletal muscle of MCI APOE4 carriers. Plasma pTau181 negatively tracked with VO2 max in all APOE4 carriers (R2 = 0.389, p = 0.003). Analyses were controlled for age. CONCLUSION: This work supports a relationship between cellular stress in skeletal muscle and cognitive status in APOE4 carriers.

Acute exercise rapidly activates hepatic mitophagic flux.

Exercise is critical for improving metabolic health and putatively maintains or enhances mitochondrial quality control in metabolic tissues. Although previous work has shown that exercise elicits hepatic mitochondrial biogenesis, it is unknown if acute exercise activates hepatic mitophagy, the selective degradation of damaged or low-functioning mitochondria. We tested if an acute bout of treadmill running increased hepatic mitophagic flux both right after and 2-h postexercise in 15- to 24-wk-old C57BL/6J female mice. Acute exercise did not significantly increase markers of autophagic flux, however, mitophagic flux was activated 2-h post-treadmill running as measured by accumulation of both LC3-II and p62 in isolated mitochondria in the presence of leupeptin, an inhibitor of autophagosome degradation. Furthermore, mitochondrial-associated ubiquitin, which recruits the autophagy receptor protein p62, was also significantly increased at 2 h. Further examination via Western blot and proteomics analysis revealed that acute exercise elicits a time-dependent, dynamic activation of mitophagy pathways. Moreover, the results suggest that exercise-induced hepatic mitophagy is likely mediated by both polyubiquitination and receptor-mediated signaling pathways. Overall, we provide evidence that acute exercise activates hepatic mitophagic flux while also revealing specific receptor-mediated proteins by which exercise maintains mitochondrial quality control in the liver.NEW & NOTEWORTHY This study provides evidence that acute exercise activates hepatic mitophagic flux and mitochondrial polyubiquitination while additionally revealing specific receptor-mediated proteins by which exercise maintains mitochondrial quality control in the liver.

Acute exercise dynamically modulates the hepatic mitochondrial proteome.

Exercise powerfully increases energy metabolism and substrate flux in tissues, a process reliant on dramatic changes in mitochondrial energetics. Liver mitochondria play a multi-factorial role during exercise to fuel hepatic glucose output. We previously showed acute exercise activates hepatic mitophagy, a pathway to recycle low-functioning/damaged mitochondria, however little is known how individual bouts of exercise alters the hepatic mitochondrial proteome. Here we leveraged proteomics to examine changes in isolated hepatic mitochondria both immediately after and 2 hours post an acute, 1 hour bout of treadmill exercise in female mice. Further, we utilized leupeptin, a lysosomal inhibitor, to capture and measure exercise-induced changes in mitochondrial proteins that would have been unmeasured due to their targeting for lysosomal degradation. Proteomic analysis of enriched hepatic mitochondria identified 3241 total proteins. Functional enrichment analysis revealed robust enrichment for proteins critical to the mitochondria including metabolic pathways, tricarboxylic acid cycle, and electron transport system. Compared to the sedentary condition, exercise elevated processes regulating lipid localization, Il-5 signaling, and protein phosphorylation in isolated mitochondria. t-SNE analysis identified 4 unique expressional clusters driven by time-dependent changes in protein expression. Isolation of proteins significantly altered with exercise from each cluster revealed influences of leupeptin and exercise both independently and cooperatively modulating mitochondrial protein expressional profiles. Overall, we provide evidence that acute exercise rapidly modulates changes in the proteins/pathways of isolated hepatic mitochondria that include fatty acid metabolism/storage, post-translational protein modification, inflammation, and oxidative stress. In conclusion, the hepatic mitochondrial proteome undergoes extensive remodeling with a bout of exercise.

Heat Treatment Improves Hepatic Mitochondrial Respiratory Efficiency via Mitochondrial Remodeling.

Nonacholic fatty liver disease, or hepatic steatosis, is the most common liver disorder affecting the western world and currently has no pharmacologic cure. Thus, many investigations have focused on alternative strategies to treat or prevent hepatic steatosis. Our laboratory has shown that chronic heat treatment (HT) mitigates glucose intolerance, insulin resistance, and hepatic steatosis in rodent models of obesity. Here, we investigate the direct bioenergetic mechanism(s) surrounding the metabolic effects of HT on hepatic mitochondria. Utilizing mitochondrial proteomics and respiratory function assays, we show that one bout of acute HT (42°C for 20 min) in male C57Bl/6J mice (n = 6/group) triggers a hepatic mitochondrial heat shock response resulting in acute reductions in respiratory capacity, degradation of key mitochondrial enzymes, and induction of mitophagy via mitochondrial ubiquitination. We also show that chronic bouts of HT and recurrent activation of the heat shock response enhances mitochondrial quality and respiratory function via compensatory adaptations in mitochondrial organization, gene expression, and transport even during 4 weeks of high-fat feeding (n = 6/group). Finally, utilizing a liver-specific heat shock protein 72 (HSP72) knockout model, we are the first to show that HSP72, a protein putatively driving the HT metabolic response, does not play a significant role in the hepatic mitochondrial adaptation to acute or chronic HT. However, HSP72 is required for the reductions in blood glucose observed with chronic HT. Our data are the first to suggest that chronic HT (1) improves hepatic mitochondrial respiratory efficiency via mitochondrial remodeling and (2) reduces blood glucose in a hepatic HSP72-dependent manner.

Mild Cognitive Impairment and Donepezil Impact Mitochondrial Respiratory Capacity in Skeletal Muscle.

Alzheimer's Disease (ad) associates with insulin resistance and low aerobic capacity, suggestive of impaired skeletal muscle mitochondrial function. However, this has not been directly measured in AD. This study ( n  = 50) compared muscle mitochondrial respiratory function and gene expression profiling in cognitively healthy older adults (CH; n = 24) to 26 individuals in the earliest phase of ad-related cognitive decline, mild cognitive impairment (MCI; n  = 11) or MCI taking the ad medication donepezil (MCI + med; n  = 15). Mitochondrial respiratory kinetics were measured in permeabilized muscle fibers from muscle biopsies of the vastus lateralis. Untreated MCI exhibited lower lipid-stimulated skeletal muscle mitochondrial respiration (State 3, ADP-stimulated) than both CH ( P = .043) and MCI + med (P = .007) groups. MCI also exhibited poorer mitochondrial coupling control compared to CH (P = .014). RNA sequencing of skeletal muscle revealed unique differences in mitochondrial function and metabolism genes based on both MCI status (CH vs MCI) and medication treatment (MCI vs MCI + med). MCI + med modified over 600 skeletal muscle genes compared to MCI suggesting donepezil powerfully impacts the transcriptional profile of muscle. Overall, skeletal muscle mitochondrial respiration is altered in untreated MCI but normalized in donepezil-treated MCI participants while leak control is impaired regardless of medication status. These results provide evidence that mitochondrial changes occur in the early stages of AD, but are influenced by a common ad medicine. Further study of mitochondrial bioenergetics and the influence of transcriptional regulation in early ad is warranted.

Roles for miRNA-378/378* in adipocyte gene expression and lipogenesis.

In this study, we explored the roles of microRNAs in adipocyte differentiation and metabolism. We first knocked down Argonaute2 (Ago2), a key enzyme in the processing of micro-RNAs (miRNAs), to investigate a potential role for miRNAs in adipocyte differentiation and/or metabolism. Although we did not observe dramatic differences in adipogenesis between Ago2 knock-down and control 3T3-L1 cells, incorporation of [(14)C]glucose or acetate into triacylglycerol, and steady-state levels of triacyglycerol were all reduced, suggesting a role for miRNAs in adipocyte metabolism. To study roles of specific miRNAs in adipocyte biology, we screened for miRNAs that are differentially expressed between preadipocytes and adipocytes for the 3T3-L1 and ST2 cell lines. Distinct subsets of miRNAs decline or increase during adipocyte conversion, whereas most miRNAs are not regulated. One locus encoding two miRNAs, 378/378*, contained within the intron of PGC-1beta is highly induced during adipogenesis. When overexpressed in ST2 mesenchymal precursor cells, miRNA378/378* increases the size of lipid droplets and incorporation of [(14)C]acetate into triacylglycerol. Although protein and mRNA expression levels of C/EBPalpha, C/EBPbeta, C/EBPdelta, and PPARgamma1 are unchanged, microarray and quantitative RT-PCR analyses indicate that a set of lipogenic genes are upregulated, perhaps due to increased expression of PPARgamma2. Knock-down of miRNA378 and/or miRNA378* decreases accumulation of triacylglycerol. Interestingly, we made the unexpected finding that miRNA378/378* specifically increases transcriptional activity of C/EBPalpha and C/EBPbeta on adipocyte gene promoters.