obesity – Drug Discovery Opinion

Low-Fat or Low-Carb?

There has long been debate about the relative merits of a low-carbohydrate diet, as popularised by Atkins, compared to the more traditional low-fat approach to weight loss. A low-carbohydrate diet has also been anecdotally associated with adverse effects on health.

A newly published clinical study, led by researchers at the Center for Obesity Research and Education at Temple University, Philadelphia, has now shown remarkably little difference between the two regimes. The study followed over 300 subjects randomly assigned to either diet over a two year period and, importantly, combined the diets with comprehensive behavioural treatment.

In the low-carb group, carbohydrate intake was limited to 20 g/d for 3 months in the form of low–glycemic index vegetables with unrestricted consumption of fat and protein. After 3 months, participants were allowed to increase their carbohydrate intake (5 g/d per wk) until a stable and desired weight was achieved. The low-fat diet consisted of limited energy intake (1200 to 1800 kcal/d) with less than 30% of the calories derived from fat. For the behavioural treatment, each participant attended group sessions weekly for the first 20 weeks of the study, every other week for the next 20 weeks, and once every other month for the remainder of the study. In each session, participants discussed topics such as goal setting, self-monitoring, and limiting triggers to overeating.

Although attrition was high at 2 years, there were no differences in weight, body composition, or bone mineral density between the groups at any time point. Weight loss was approximately 11 kg (11%) at 1 year and 7 kg (7%) at 2 years. The low-carbohydrate diet group had greater increases in high-density lipoprotein cholesterol (“good” cholesterol) levels at all time points, increasing by approximately 23% at 2 years, suggesting that a low-carb diet may have some cardiovascular benefit.

Gary Foster, Director of Temple’s Center for Obesity Research and Education and lead author of the study said:

When comparing these two popular weight loss plans, none of the existing research had included a comprehensive, long-term, behavioural support component. This research tells us that people wanting to manage their weight need to be less concerned with which diet they choose, and more concerned with incorporating behavioural changes into their plan.

The study is published in Annals of Internal Medicine.

New Obesity Target

The presence of multiple redundant and compensatory pathways controlling energy homeostasis has, so far, limited the effectiveness of anti-obesity treatments and suggests that combination therapy may be the best approach for treating the worldwide obesity epidemic. Writing in the journal Cell Metabolism, researchers at Merck have now demonstrated a role for the orphan bombesin receptor subtype 3 (BRS-3) in controlling energy balance.

Using a selective BRS-3 agonist (Bag-1) and antagonist (Bantag-1), the team have established a role for BRS-3 in the regulation of food intake, metabolic rate, and body weight. Intracerebroventricular infusion of the peptide Bantag-1 led to higher food intake and a progressive increase in adipose mass and body weight whereas oral administration of Bag-1 increased metabolic rate and reduced food intake, adipose weight, and body weight. Prolonged high levels of brain receptor occupancy by agonist increased weight loss, suggesting a lack of tachyphylaxis.

As well as suggesting a potential new target for the treatment of obesity, the discovery of selective BRS-3 agonists and antagonists will allow investigation of the mechanisms by which BRS-3 regulates energy metabolism as well as exploration of other aspects of BRS-3 biology.

Capacity for Physical Endurance comes at a Price

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 (K_ATP) 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 K_ATP 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 K_ATP 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

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

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.

MKP-1, Muscle Fibres and Obesity

The fibres that make up skeletal muscle broadly fall into two groups: Type I, ‘slow-twitch’ myofibres, and Type II, ‘fast-twitch’ myofibres. Slow-twitch fibres utilise oxidative metabolism for energy generation and are associated with endurance, whilst fast-twitch fibres use a mixture of oxidative and anaerobic metabolism but are quicker to fatigue. Sprinters have up to 80 per cent type II fibres while marathon runners have up to 90 per cent type I fibres. Those that have a more sedentary lifestyle have about the same percentage of both.

Obese individuals have relatively fewer Type I fibres compared to average-weight counterparts and Type I fibres are believed to confer resistance to obesity. Consistent with this, the numbers of Type I fibres decrease in response to inactivity and or a high-fat diet (HFD). Conversely, numbers of Type I fibres increase in response to exercise.

Researchers at Yale University School of Medicine have now identified MAPK phosphatase-1 (MKP-1) as a key player in the Type I/Type II shift in response to HFD. The group had previously shown that mice deficient in MKP-1 displayed increased energy expenditure and were resistant to diet-induced obesity. This new study, published in the Journal of Clinical Investigation, found that MKP-1 was overexpressed in skeletal muscle of mice in response to excess dietary fat.

MKP-1 dephosphorylates, and consequently deactivates, MAP kinases in the nucleus. In the study, MKP-1 overexpression reduced p38 MAPK-mediated phosphorylation of PPARγ coactivator 1α (PGC-1α), which plays a central role in maintaining levels of Type I myofibres. The phosphorylation of PGC-1α is believed to stabilise the protein and, consistent with this, MKP-1 deficient mice had higher levels of PGC-1α in skeletal muscle than did wild-type mice and were refractory to the loss of Type I myofibres when fed a high-fat diet.

New Insights into Adipogenesis

The nuclear receptor, PPARγ, has hitherto been regarded as the master regulator of adipogenesis, without which new adipose tissue cannot be formed. Adipogenesis plays a key role in obesity and associated metabolic diseases such as type II diabetes and the thiazolidines, which target PPARγ are widely used to treat type II diabetes. Researchers at the University of Central Florida have now discovered, however, that monocyte chemotactic protein-1 (MCP-1)-induced protein (MCPIP), can trigger adipogenesis without involvement of PPARγ. The authors had previously shown that binding of MCP-1 to its receptor, CCR2, leads to induction of the Zn-finger protein, MCPIP. In the present study, MCP-1 was found to be produced, and MCPIP to be induced, before induction of PPARγ or other transcription factors in fibroblasts undergoing differentiation into adipocytes. Knockdown of MCPIP using siRNA was found to inhibit both gene induction and adipogenesis whereas treatment with MCP-1 or forced expression of MCPIP induced adipogenesis. Forced expression of MCPIP was also shown to induce adipogenesis in PPARγ-/- mouse embryonic fibroblasts, further demonstrating that MCPIP can act independently of PPARγ.

Obesity is well known to increase MCP-1 levels and the finding that MCPIP is able to induce adipogenesis without involvement of PPARγ provides new mechanistic evidence for the role of MCP-1/CCR2 in obesity and type II diabetes. Since there is experimental evidence that MCPIP promotes angiogenesis, MCP-1/CCR2 interaction could promote the formation of new blood vessels to supply blood to growing adipose tissue as well as promoting development of the adipocytes. A drug that could block the function of MCPIP may thus have the potential to treat obesity and type II diabetes.

The study is published in the online edition of Journal of Biological Chemistry.

Allergy Drugs May Treat Obesity and Type 2 Diabetes

Mast cells are best known for their role in allergic responses but a new study by researchers at Brigham and Women’s Hospital and colleagues has now shown a link with diet-induced obesity and type 2 diabetes. Writing in the July edition of Nature Medicine, they show that mast cells are far more abundant in white adipose tissue from obese humans and mice than in tissue from normal weight individuals. ketotifen and cromolyn structures

In mice on a high calorie diet, treatment for two months with either of the allergy treatments, ketotifen fumarate or cromolyn, led to significant weight loss and improvement in diabetic markers compared with control animals. More dramatic improvements were seen if the animals were also switched to a reduced fat diet.

In further studies, the team showed that mice which lack mature mast cells neither became obese nor developed diabetes over a three month period, despite being fed a Western diet rich in sugars and fats. As a next step towards possible testing in humans, the researchers plan to study the effect of the compounds on obese and diabetic non-human primates.

Ketotifen fumarate and cromolyn are both used in anti-allergy eye drops, and to prevent asthma attacks. Although both stabilise mast cells, the exact mechanisms by which they achieve this are somewhat different.

Brain Peptidase Controls Appetite

The polypeptide hormone precursor, pro-opiomelanocortin (POMC) undergoes extensive, tissue-specific, post-translational modification to yield a number of peptides with diverse biological activities. POMC neurons in the hypothalamus convert POMC via a multi-enzyme process into the 13 residue α-melanocyte-stimulating hormone (α-MSH_1-13). As well as stimulating production and release of melanin in skin and hair, α-MSH_1-13 acts on postsynaptic melanocortin receptors in the brain to suppress appetite. In the late 1990s, mutations in MCR4 that prevent α-MSH_1-13 signalling were linked to inherited human obesity but, although the pathways leading to activation of MCR4 are relatively well understood, little was known about how signalling is terminated.

Writing in the Journal of Clinical Investigation, researchers at Yale University School of Medicine and the University of California Davis now show that, in mice, α-MSH_1-13 is inactivated by removal of the amidated C-terminal valine residue by the enzyme prolylcarboxypeptidase (PRCP). PRCP is a serine protease that cleaves the C-terminal residue from peptides with a penultimate proline residue that also converts the vasopressor peptide, angiotensin II, into the inactive angiotensin 1-7. Mice lacking PRCP had increased levels of α-MSH_1-13 in the hypothalamus, were leaner and shorter than normal mice, and did not become obese when fed a high-fat diet. Additionally, icv administration of a PRCP inhibitor, Boc-prolylprolinal, reduced food intake in rats, as did systemic administration to obese, leptin-deficient mice.

On the basis of their studies, the authors propose that regulation of PRCP activity in the hypothalamus may be a target for the treatment of obesity and related disorders but an accompanying commentary cautions that brain-penetrating drugs would need to be developed to be useful for the treatment of obesity. Skin and hair colour could be affected by inhibiting PRCP, as well as levels of the vasoactive peptides, angiotensin II and bradykinin. Since PRCP is widely expressed, there may be other – not yet unidentified – substrates, and inhibitors of PRCP could have other, unforeseen, effects.

Put Vinegar on those Chips!

fish and chips Vinegar can be made from almost any carbohydrate source – the action of yeast first ferments the natural sugars to alcohol which is then converted into acetic acid by acetic acid bacteria. Vinegars typically contain around 5% acetic acid together with a variety of other components including polyphenols and organic acids.

Vinegars have been used for thousands of years as food preservatives and flavourings and, over the years, have also acquired a reputation for many health benefits. The human evidence for many of these claims remains equivocal although several studies have confirmed the anti-glycaemic properties of vinegar.

Largely anecdotal evidence has pointed to positive effects on weight loss and at a recent meeting of the Japan Society of Nutrition and Food Science, scientists at the Mizkan Central Research Institute have now described reductions in waist size, abdominal fat and blood neutral fat in overweight (BMI 25-30) men and women who drank 30mL of an apple vinegar-based drink (containing 1.5g acetic acid) each day for 12 weeks. Those who drank 15mL of the vinegar-based drink also saw reductions, whilst control subjects who did not take the drink saw no changes.

Writing in the Journal of Agricultural and Food Chemistry, researchers at Mizkan have also described studies in mice that highlight the mechanisms responsible for the changes in abdominal fat. Mice on a high-fat diet were divided into three groups and treated with 0.3% or 1.5% acetic acid solutions or water for 6 weeks. The groups treated with the acetic acid solutions consumed the same amount of food as the control group but showed reduced accumulation of body fat (about 10%) and hepatic lipids, with no changes in skeletal muscle weight. Significant increases in the expression of genes for peroxisome-proliferator-activated receptor-α (PPARα) and for fatty acid oxidation- and thermogenesis-related proteins were seen in the livers of the treatment groups. Similar up-regulation of gene expression was observed in vitro on addition of acetate to HepG2 cells. The effects were not observed in cells depleted of α2 5′-AMP-activated protein kinase (AMPK) by siRNA. The authors conclude that acetic acid suppresses accumulation of body fat and liver lipids by up-regulating genes for PPARα and fatty-acid-oxidation-related proteins in the liver via an α2 AMPK mediated process.

If larger clinical studies confirm fat reduction and positive effects on metabolism in overweight humans, vinegar could be set to take a share of a very large weight-loss market.