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MitoNews: Volume 9, Issue 6

Mitochondria and Immunometabolism, September 2013

Edited by James Murray, PhD and Adam Campbell PhD

Thousands of researchers around the world are studying the connections between mitochondria, metabolism and disease. MitoNews summarizes a selection of the latest published findings and highlights how Abcam's Mitosciences range of research tools has contributed to this effort. Read the full list of 40 original research papers published in the last two months.

Immunometabolism integrates the fields of immunology, obesity, metabolism, nutrition, and exercise science.

Once thought of as a problem of only high-income countries, obesity has reached global epidemic proportions. To draw attention to this problem the American medical association declared obesity a disease It is now widely accepted by the healthcare and scientific community that obesity affects the immune system and the associated systemic, low-grade inflammation that results contributes to metabolic diseases such as Type 2 diabetes, cancer and cardiovascular disease.

In classic inflammation a pathogen is recognized by the immune system. In response immune cells activate pro-inflammatory pathways and downstream kinases JNK, IKK-epsilon, and PKR. These pathways result in expression of cytokines, recruitment of immune cells as well as a metabolic shift to a "hot" or high energy expending state rendering the immune cells resistant to insulin, this resolves itself once the pathogen is cleared.

In obese individuals a chronic low-grade, or "cold", inflammation is observed in the absence of a pathogen. Excess nutrients in the blood trigger the same pro-inflammatory pathways leading to expression of cytokines, recruitment of immune cells, resistance to insulin but without the corresponding increase in energy expenditure observed in “hot” inflammation. This “cold” inflammation fails to resolve and persists long term, leads to tissue remodeling, systemic metabolic deterioration.

The IKK-nuclear factor-kappaB (NFkappaB) pathway links obesity and inflammation. This pathway includes two non-canonical inhibitors of kappaB kinase family members, IKK-epsilon and TANK-binding kinase 1 (TBK1), which have the ability to activate NFkappaB signaling in immune cells.

The Saltiel laboratory reported that mice fed a high fat diet (HFD) showed induction of IKK-epsilon in stromal and immune cells in adipose and liver tissues. The recent work of Reilly et al. builds upon the importance of TBK1 and IKK-epsilon through the use of the selective inhibitor amlexanox, an anti-inflammatory anti-allergic immune-modulator. They were able to show that mice on HFD treated with amlexanox, to selectively inhibit TBK1 and IKK-epsilon function, had reduced weight gain compared to vehicle/HFD and had equivalent weight to vehicle/control diet mice. When obese mice on HFD were treated they observed a significant loss in body weight after 4 wks, without a reduction in food intake with a return to obese weight once amlexanox was withdrawn.

Amlexanox treatment significantly improved insulin sensitivity in mice with established dietary induced obesity (DIO). This improved sensitivity led to improved phosphorylation of Akt and enhanced suppression of free fatty acids released by white adipose tissue (WAT) in response to insulin. As a result of HFD the mice develop hepatomegaly, postmortem investigations on treated DIO mice revealed that this effect was reversed.

When they investigated the WAT and brown adipose tissue (BAT) they observed a decreased infiltration of inflammatory macrophages in treated DIO mice. This was accompanied by a decrease in pro-inflammatory cytokines in serum. They observed a decrease in phosphorylation of proteins known to become hyper-phosphorylated in states of obesity. As well as an increase in lipid oxidation and oxygen consumption which the authors attribute as a sign that amlexanox leads to increased energy expenditure in those tissues.

The effects of increased energy expenditure and oxygen consumption by amlexanox were lost in mouse embryonic fibroblast (MEFs) isolated from TBK1 and IKK-epsilon double knockout mice. These findings support the notion that TBK1 and IKK-epsilon act as important mediators for inflammatory signaling and energy expenditure.

An inhibitor of the protein kinases TBK1 and IKK-ɛ improves obesity-related metabolic dysfunctions in mice Nat Med. 2013. Reilly SM, Chiang SH, Decker SJ, Chang L, Uhm M, Larsen MJ, Rubin JR, Mowers J, White NM, Hochberg I, Downes M, Yu RT, Liddle C, Evans RM, Oh D, Li P, Olefsky JM, Saltiel AR.
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Obesity and Diabetes Linked To Defects in Oxidative Metabolism

Histone deacetylases (HDACs) are a family of proteins that regulate transcription of genes by removing acetyl groups from histone proteins in the nucleus. This leads to a shift to a more positive charge and causes tighter binding to the negatively charged DNA molecule. This renders the DNA strand less accessible to the transcriptional machinery thus potentially reducing transcription.

In a recent paper published by Galmozzi et al., they investigated the effects of sodium butyrate, a known inhibitor of HDACs, as potential therapy to treat metabolic conditions such as obesity and type 2 diabetes that are associated with suppressed oxidative metabolism.

To evaluate if targeting class I and/or class II HDACs resulted in enhanced mitochondrial function. The authors treated C2C12 differentiated to myotubes with compounds MS275 (class I HDAC inhibitor), MC1568 (class II HDAC inhibitor) or SAHA (pan HDAC inhibitor) in tissue culture experiments. They found that inhibition of class I HDACs led to an increase in mitochondrial density, increase in mitochondrial-related transcription factors, increase in mitochondrial biogenesis, increased mitochondrial respiratory complex chain proteins (Complex I and V), and increased oxygen consumption rates compared to myotubes treated with class II inhibitor.

Next they tested if these observations in tissue culture translated to in vivo mouse experiments. They treated leptin receptor defective mice, db/db mice, with the same HDAC inhibitors used in the myotubes experiments. In the db/db mice treated with pan-or class I HDAC inhibitor they observed the mice had a significant reduction in body weight without a reduction in food intake. They also observed a reduction in circulating insulin, blood triglycerides, and fasting blood glucose levels. Mice treated with the class II inhibitor showed no improvement in any measurements made, suggesting that all beneficial effects were due to the class I HDAC inhibition.

Furthermore inhibition of class I HDACs led to an increase in gene expression of transcription factors and cofactors that regulate mitochondrial function, as well as genes involved in glucose metabolism, lipid metabolism, TCA cycle and oxidative phosphorylation. A corresponding increase in mitochondrial complex I and II proteins and increased staining for succinate dehydrogenase indicating greater oxidative capacity in the muscle fibers was observed. They observed that mice treated with class I HDAC inhibitor had an increased rate of oxygen consumption and heat production without a change to locomotive activity.

The researchers performed ChIP assays on C2C12 myotubes, db/db mouse biopsies of skeletal muscle, BAT and WAT. They found that treatment with the class I and pan-HDAC inhibitors had reduced the recruitment of HDAC3 to the Pgc-1alpha promoter in all three tissue types. They hypothesize that this reduction of HDAC3 recruitment may drive increased expression of genes that encode proteins responsible for greater oxidative activity in muscle and BAT while driving the WAT to take on a more BAT phenotype.

Inhibition of class I histone deacetylases unveils a mitochondrial signature and enhances oxidative metabolism in skeletal muscle and adipose tissue. Diabetes 2013. Galmozzi A, Mitro N, Ferrari A, Gers E, Gilardi F, Godio C, Cermenati G, Gualerzi A, Donetti E, Rotili D, Valente S, Guerrini U, Caruso D, Mai A, Saez E, De Fabiani E, Crestani M.

Mitochondrial Assembly and Activity Associated With Insulin Sensitivity

Recent mouse studies have indicated that reduction in mitochondrial electron transport chain (ETC) function leads to reduced ageing and therefore increased lifespan. Mouse knockout studies of the complex IV assembly protein surfeit locus protein 1 (Surf1) have shown an increase in lifespan. Surf1 is a nuclear encoded inner mitochondrial membrane protein involved in the assembly of complex IV, and it remains unclear how Surf1 knockout leads to extended lifespan. Recent investigations using C. elegans mitochondrial mutants suggest that hypoxia inducible factor-1 alpha and mitochondrial unfolded protein response play a role in lifespan extension.

Previous studies using mouse knockout models with known mild mitochondrial dysfunction revealed that the mice exhibit extended lifespan and increased insulin sensitivity. Deepa et al. recently published a paper investigating if extended lifespan of Surf1 knockout mice was due to increased insulin sensitivity via reduced complex IV activity.

The authors first investigated the body composition of Surf1-/- mice compared to normal mice. They found that Surf1-/- had similar amounts of food intake, but had a 15% reduction in body weight compared to wild type mice. Using quantitative magnetic resonance (QMR) they observed that the mice had nearly 20% reduction in fat mass and 7% increase in muscle mass compared to wild type. Using H&E staining the authors were able to determine that overall fat cell size was reduced and the number of fat cells was not changed compared to control group. They observed that during dark cycle the respiratory quotient (RQ), the ratio of oxygen consumption to carbon dioxide production, suggested a shift in metabolism toward greater fatty acid oxidation and increased energy expenditure compared to controls.

The researchers then investigated if lower body weight and increased energy expenditure could be due to impaired adipogenesis or dysregulated expression of key genetic markers. They observed that in Surf1-/- mice there was no difference in the levels of several mature adipocyte differentiation markers suggesting that adipogenesis was unaffected. However they did observe that proteins involved in fatty acid oxidation, specifically phosphorylated ACC, phosphorylated AMPK, PPAR alpha, and CPT1 were elevated in Surf1-/- mice. These finding suggest that in Surf1-/- mice decreased fat accumulation may be due to increased fatty acid oxidation.

When the mice underwent glucose and insulin tolerance tests they found that Surf1-/- mice had similar levels of circulating blood glucose. The binding of insulin to its receptor activates a series of downstream signaling events in target cells and that phosphorylated Akt levels were higher in WAT, liver, and skeletal muscle indicating insulin sensitivity. In addition they found elevated levels of phosphorylated insulin receptor substrate 1 and 2 (IRS1, IRS2) and glucose transporter type 4 (GLUT4) in WAT exclusively; indicating that WAT had increased insulin sensitivity in Surf1-/- mice.

The authors found in Surf1-/- mice the anticipated decrease in complex IV activity, but also an elevation in the levels of PGC-1 alpha protein and its subsequent target genes. Target genes which regulates a number of mitochondrial electron transport chain, mitochondrial genome copy number and mitochondrial structural proteins. These data indicates that in Surf1-/- mice there is an increase in mitochondrial number as a compensatory mechanism for reduced complex IV activity leading to increased insulin sensitivity and fatty acid oxidation.

Improved insulin sensitivity associated with reduced mitochondrial complex IV assembly and activity. FASEB J 2013. Deepa SS, Pulliam D, Hill S, Shi Y, Walsh ME, Salmon A, Sloane L, Zhang N, Zeviani M, Viscomi C, Musi N, Van Remmen H.

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