Illuminating the Link between Bone and Metabolism

Bone Tree
Image: Flickr – Livin-Lively
Two back-to-back studies published in the July 23rd issue of Cell, one from Columbia University Medical Center and the other from Johns Hopkins researchers, further the hypothesis that metabolic control and bone remodelling are inextricably linked. Both studies point to osteocalcin, a hormone released by bone, as a key mediator of this link.

The Johns Hopkins study used a conditional knock-out in mice to specifically suppress the expression of the insulin receptor in osteoblasts, the bone-forming cells of the skeletal system. As the mutant mice aged they became fat, had elevated blood sugar, and were glucose intolerant and resistant to insulin, mirroring the picture of diabetes in humans. The researchers found that the mutant mice had fewer osteoblasts, reduced bone formation and lower levels of circulating undercarboxylated osteocalcin (the active form of the hormone). The study showed that signalling via the insulin receptor in osteoblasts suppressed Twist2, an inhibitor of osteoblast development, and enhanced expression of osteocalcin, a mediator of insulin sensitivity and secretion.

The Columbia study links the complete bone remodelling process to energy regulation. Osteocalcin is released from osteoblasts predominantly in an inactive, carboxylated form. The researchers demonstrated that insulin signalling in osteoblasts stimulates release of inactive osteocalcin and activates osteoclasts, which activate the osteocalcin via decarboxylation in a bone-resorption-dependent manner.

The studies clearly have potential impact on human therapy, although significant questions remain. As yet the receptor for undercarboxylated osteocalcin is unknown, so the mechanism by with the hormone stimulates insulin release is unclear. Further work will be necessary to understand the interplay between skeletal- and metabolic-homeostasis in humans.

The two papers are previewed in Cell.

PPARγ– A New Twist in the Tale

Image: Flickr - alexdecarvalho
Obesity and related disorders such as diabetes have reached epidemic proportions. Although the anti-diabetic thiazolidinediones (glitazones) are effective insulin sensitizers, some members of the class have been withdrawn or had their use restricted because of safety concerns. Increased responsiveness to insulin is believed to be mediated by activation of the nuclear receptor, PPARγ but differences in clinically important side effects suggest subtle differences in pharmacology, even amongst full agonists.

Researchers at the Scripps Research Institute and the Dana-Farber Cancer Institute at Harvard University have now shown that cyclin-dependent kinase 5 (Cdk5) in adipose tissue is activated in obese mice fed a high-fat diet, resulting in phosphorylation of PPARγ. This has no effect on the adipogenic capacity of PPARγ but does alter the expression of a large number of obesity-related genes, including a reduction in expression of the insulin-sensitizing adipokine, adiponectin. Phosphorylation of PPARγ by Cdk5 was blocked both in vitro and in vivo by the full agonist, rosiglitazone, and by the partial agonist, MRL-24, leading to increased adiponectin production. The anti-diabetic effect of rosiglitazone in obese patients was also found to be closely associated with inhibition of PPARγ phosphorylation, suggesting that this may be a mechanism of insulin resistance. The authors of the study, which is published in the journal Nature, suggest that drugs that inhibit PPARγ phosphorylation by Cdk5, without necessarily activating the receptor, may provide an improved generation of anti-diabetic drugs.

High Altitude Metabolism

High Altitude
Image: Flickr – Blue Turban Photography
The cellular responses to low oxygen levels (hypoxia), as occurs at high altitude, are critical for survival. The transcription factor, hypoxia inducible factor 1 (HIF-1), is a key player in this setting, upregulating genes that preserve function. Included in these are glycolysis enzymes, which allow ATP synthesis in an O2-independent manner, and vascular endothelial growth factor (VEGF), which promotes angiogenesis. HIFs are also important in development and deletion of HIF-1 in mammals is perinatally lethal.

HIF-1 occurs as a heterodimer of HIF-1α and the constitutively expressed HIF-1β. Under normal oxygen conditions, HIF-1α is a substrate for HIF-1 prolyl hydroxylases and the asparagine hydroxylase, factor inhibiting HIF-1α (FIH). The action of the prolyl hydroxylases results in the targeting of HIF-1α by an E3 ubiquitin ligase and subsequent degradation by the proteasome, whilst hydroxylation by FIH represses activity of its carboxy terminal transactivation domain (CAD). Both hydroxylation processes therefore serve to down-regulate the activity of HIF-1. When oxygen levels are low, however, the prolyl hydroxylases and FIH become inactive since they are dependent on O2.

A team led by researchers at University of California at San Diego have now reported on a FIH-knockout mouse. Despite the importance of HIF-1 in development, the FIH-deleted mice were healthy, although smaller than wild-type littermates. Where they differed significantly was in their metabolic profile. The FIH-null mice exhibited elevated metabolic rate, enhanced insulin sensitivity, hyperventilation and improved lipid and glucose homeostasis. On a high-fat diet, the animals were resistant to weight gain and had reduced central adiposity.

The team went on to explore the effects of tissue-specific FIH deletion, demonstrating that most of the features of the metabolic phenotype of the FIH-null mice could be replicated when only neuronal FIH was deleted.

The study, published in Cell Metabolism, identifies FIH as an essential regulator of metabolism and opens up the possibility of FIH inhibitors for the treatment of metabolic disorders.

Lipid Genes Linked to Cancer

Image: Flickr - Lida Rose
Although clinical and epidemiological studies have linked cancer with other chronic conditions such as inflammatory and metabolic diseases, the pathways linking different diseases are poorly understood. Inflammation is commonly associated with the development and progression of cancer and increased cancer risk is also linked to metabolic syndrome which encompasses obesity, type II diabetes, high cholesterol, and atherosclerosis. To overcome limitations in previous approaches to identifying common genes and signalling pathways, researchers at Harvard Medical School have carried out transcriptional profiling in two experimental isogenic models of cellular transformation. Using two isogenic models in which the transformed and non-transformed tissues are genetically identical makes it is easier to identify genes involved in transformation.

The team used a computational approach to organise the genes identified by transcriptional profiling into networks with central nodes. Comparison with a gene set describing metabolic syndrome revealed a high overlap between the central nodes of cancer and metabolic syndrome. Inflammatory factors such as IFN-γ, IL-1β, IL-6, and NF-κB as well as insulin and low-density lipoprotein (LDL) appeared as central nodes in cancer gene networks, suggesting the importance of inflammatory processes in both cancer and metabolic diseases and also a link between protein and lipid metabolism and cellular transformation. Lipid-related genes that had not previously been linked to cancer included OLR1, SNAP23, VAMP4, SCD1, SREBP1 and GALNT2.

The similarities between the pathways in cancer and metabolic diseases led the team to test whether drugs used to treat inflammation or aspects of metabolic disease might also affect cellular transformation and tumorigenicity. Four of the compounds that performed best in the cell experiments, metformin, sulindac, simvastatin, and cerulenin were tested for their ability to suppress tumour growth in nude mice: tumour growth was completely suppressed by metformin and sulindac and significantly delayed by cerulenin and simvastatin, suggesting that drugs designed to combat metabolic diseases may also be useful in treating some types of cancer.

The study is published in Cancer Cell.

Controlling Fat Distribution

Image: Patrick Hoesly
Extreme accumulation of fat in muscle tissue is associated with cardiovascular disease and is a contributory factor in insulin resistance and type II diabetes. It is therefore important to understand the mechanisms by which fat is taken up from the bloodstream and metabolised by tissues. Surprisingly, the role of blood vessels themselves in the transport of lipids has not been clearly established. Researchers at the Karolinska Institutet have now identified a role for vascular endothelial growth factor-B (VEGF-B) in endothelial targeting of lipids to peripheral tissues.

The VEGFs and their receptors are major regulators of angiogenesis and pharmacological intervention, for example with bevacizumab (a monoclonal antibody specific for VEGF-A), has been successfully exploited in oncology. This latest study has shown that VEGF-B, in mice, controls endothelial uptake of fatty acids via transcriptional regulation of vascular fatty acid transport proteins. Mice that were deficient in VEGF-B (Vegfb-/-) showed reduced uptake and accumulation of lipids in muscle, heart and brown adipose tissue. Instead, the Vegfb-/- mice preferentially transported lipids to white adipose tissue, resulting in a small weight increase. This regulation was mediated by VEGF receptor 1 and neuropilin 1 expressed by the endothelium.

The authors of the study, published in Nature, propose that this new role for VEGF-B could potentially lead to novel strategies to modulate pathological lipid accumulation in diabetes, obesity and cardiovascular diseases.

Eating for How Many?

Image: Flickr - Beneneuman
Soon after birth, the human body is colonised by bacteria. Trillions of bacteria take up residence in the gut and perform a range of useful functions such as helping with digestion and absorption of nutrients, producing vitamins, preventing growth of pathogenic bacteria, and developing the immune system. In 2006, it was shown that the proportion of Bacteroidetes relative to Firmicutes was reduced in the guts of obese people compared with lean individuals and also in the guts of genetically obese mice compared with lean littermates. Researchers at Emory University have now shown that mice engineered to lack toll-like receptor 5 (TLR5) – a component of the innate immune system that is expressed in the gut mucosa and that helps defend against infection – are 20% heavier than normal mice and have elevated triglycerides, cholesterol and blood pressure as well as slightly elevated blood sugar and a decreased response to insulin. TLR5-deficient mice consume about 10% more food than wild type mice and, although they lose weight when food is restricted, they still show insulin resistance. On a high fat diet, TLR5-deficient mice gain more weight than normal mice and develop full-blown diabetes and fatty liver disease, mimicking “metabolic syndrome” which increases the risk of developing heart disease and diabetes in humans.

Treating TLR5-deficient mice with antibiotics to kill most of the bacteria in the intestine reduced their metabolic abnormalities and, conversely, transfer of intestinal bacteria from TLR5-deficient mice to germ-free wild type mice transferred many of the characteristics of metabolic syndrome, including increased appetite, obesity, elevated blood sugar, and insulin resistance. Although earlier studies had shown that greater numbers of Firmicutes bacteria lead to more calories being extracted from the diet, the TLR5-deficient mice had normal proportions of Firmicutes and Bacteroidetes but differed in the composition of bacterial species in the two families. The new study shows that, as well as influencing how well energy is absorbed from food, gut flora can also influence appetite and may contribute to human obesity and metabolic disease.

The study is published in Science Express.

Blocking Lipid Droplet Hydrolysis May Be Beneficial

oil drops
Image: Flickr - Fox Kiyo
Fatty acids can be stored as triacylglycerol in lipid droplets, typically within adipose tissue, and then later released by the action of triacylglycerol hydrolase (TGH, also known as carboxylesterase-3, Ces3). Under normal circumstances, the released fatty acids provide an energy source, but excessive accumulation of triacylglycerol in peripheral tissues is associated with obesity and is a risk factor for type II diabetes and cardiovascular disease.

Researchers at the University of Alberta, Canada, reasoned that blocking the action of TGH would lead to better blood lipid profiles, but might also result in accumulation of triacylglycerol in the liver. However, they have found that mice lacking TGH (tgh-/-) display global metabolic benefits with no obvious down-side. In both fasted- and fed-states, the animals had reduced plasma triacylglycerol, apolipoprotein B, and fatty acid levels. Despite the attenuation of very low-density lipoprotein (VLDL) secretion, TGH deficiency did not increase hepatic triacylglycerol levels. The tgh-/- mice exhibited increased food intake and energy expenditure without change in body weight, and these metabolic changes are accompanied by improved insulin sensitivity and glucose tolerance.

The authors of the study, published in Cell Metabolism, suggest that pharmacological inhibition of TGH could be a useful therapeutic target, although cautioning that further work is required. It may be desirable to target TGH in specific tissues (e.g. hepatic versus adipose) but those subtleties have yet to be established.

Capacity for Physical Endurance comes at a Price

Image: Wikimedia - Wikimol
Image: Wikimedia - Wikimol
Energy-conserving mechanisms that evolved as protective measures in an environment of restricted food supply and high demand for physical activity promote obesity in times of abundant food and low physical activity. ATP-sensitive potassium (KATP) channels in heart and skeletal muscle act as safety valves that limit action potentials to prevent energy depletion and are essential for survival and stress adaptation, but researchers at the Mayo Clinic, the University of Iowa, New York University School of Medicine and the University of Connecticut have now found that the channels also regulate cellular energy use under non-stressed physiological conditions and contribute to fat deposition and obesity.

Both when the animals were at rest or normally active, heart and skeletal muscles of mice lacking the KATP channel dissipated more energy as heat than those of wild type mice and the animals were resistant to increases in body weight caused by a Western-style high fat diet. However, since the animals’ muscles are also less efficient when exercising, they show lower endurance and are less capable of maintaining physical performance than wild type animals.

The authors hope that therapies that reduce the activity of KATP channels in a tissue-specific manner may have the potential to reduce obesity by making muscles more thermogenic at rest and less fuel efficient during exercise.

The study is published in the journal Cell Metabolism.

Inhibition of Sirt1 May be the Way to Control Obesity

Image: Flickr – Bogenfreund
Image: Flickr – Bogenfreund
Activation of the NAD+-dependent deacetylase, sirtuin-1 (Sirt1), has been linked to increased longevity in various species although further studies are needed to establish its role in human ageing. The beneficial effects of calorie restriction on lifespan and the proposed anti-ageing properties of resveratrol have both been linked to activation of sirtuins, although not without controversy. In a significant departure from previous studies which have focussed on activating Sirt1, research carried out at Brown University and Rhode Island Hospital has now suggested that inhibiting Sirt1 may be a way to control obesity.

EX527, a selective inhibitor of Sirt1 that does not inhibit histone deacetylase (HDAC) or other sirtuin deacetylase family members
EX527, a selective inhibitor of Sirt1 that does not inhibit histone deacetylase (HDAC) or other sirtuin deacetylase family members
In the first in-depth study of the metabolic role of Sirt1 in the brain, the researchers found that inhibiting Sirt1 appears to help control food intake. Calorie restriction increases expression of Sirt1 specifically in the hypothalamus, the primary brain centre that regulates food intake and body weight so the team hypothesised that hypothalamic Sirt1 is a metabolic factor controlling food intake. ICV administration of the selective Sirt1 inhibitor, EX527, in fasted rats resulted in decreased food intake and body weight gain. The weight gain was less than that of pair-fed counterparts suggesting that the decrease in weight gain was not exclusively due to the reduced food intake. The effects were shown to be Sirt1-specific since both were reversed by co-administration of a Sirt1 activator at a dose which alone did not change either food intake or weight gain. Knock-down of Sirt1 expression by infusion of Sirt1 specific siRNAs directly into the arcuate nucleus of the hypothalamus also led to lower food consumption and smaller weight gain. Co-administration of the melanocortin antagonist, SHU9119, with EX527 completely attenuated the lower food intake and reduced weight gain caused by EX527, indicating a role for melanocortin signalling in mediating the effects of Sirt1 on energy balance. Inhibition of hypothalamic Sirt1 activity was also shown to reverse fasting-induced decreases in S6 kinase signalling and to increase levels of serum thyroid hormones, which are strong stimulators of basic metabolic rate and thermogenesis.

The authors propose that central Sirt1 senses the nutritional status of the body and regulates hypothalamic melanocortin signalling together with the S6K pathway to govern food intake and body weight, and suggest that agents targeting this pathway may show promise for the treatment of obesity and associated metabolic disorders.

The study is published in PLoSone.

Snacking Leads to Inactivity, Obesity

couch potato cartoon
Image: todaywelive
‘Breakfast like a king, lunch like a prince, sup like a pauper’ is an old and well known proverb but a recently published study gives new insights into why following this advice might help to fight obesity and diabetes.

Researchers at ETH have suggested that eating snacks – even healthy ones – between meals leads to a vicious circle of physical inactivity and overeating, and could ultimately lead to diabetes. The team have identified a novel mechanism by which insulin regulates both metabolic and behavioural responses to food intake. Insulin produced by the pancreas as a result of feeding inhibits the forkhead box transcription factor, Foxa2. Foxa2 regulates fat metabolism in the liver but also influences neurons in the lateral hypothalamic area of the brain which is considered to be the classic ‘feeding centre’, controlling feeding, diurnal rhythm, sleep and sexual behaviour. In the fasted state, Foxa2 is active and promotes synthesis of melanin-concentrating hormone (MCH) and orexin, proteins with roles in controlling food intake and motivated behaviour. In obese mice, Foxa2 was found to be non-functional, regardless of whether the animals were fasted or fed. Genetically modified mice with permanently active Fox2a in their brains have more MCH and orexin, eat more and have increased insulin sensitivity. The levels of physical activity after feeding are also significantly higher, and more closely resemble those of fasted animals. Conditional activation of Foxa2 in the brains of obese mice also resulted in improved glucose homeostasis, decreased fat and increased lean body mass.

The authors suggest that periods of fasting are important to ensure correct body weight since each time food is consumed, Fox2a is suppressed which reduces the motivation for physical activity and, consequently, energy expenditure. Prevention of Foxa2 phosphorylation may lead to increased levels of physical activity and could be a potential pharmacological target for the treatment of obesity and diabetes.

The study is published in the journal Nature.