Adaptive thermogenesis and uncoupling proteins: a reappraisal of their roles in fat metabolism and energy balance

Abstract

After decades of controversies about the quantitative importance of autoregulatory adjustments in energy expenditure in weight regulation, there is now increasing recognition that even subtle variations in thermogenesis could, in dynamic systems and over the long term, be important in determining weight maintenance in some and obesity in others. The main challenge nowadays is to provide a mechanistic explanation for the role of adaptive thermogenesis in attenuating and correcting deviations of body weight and body composition, and in the identification of molecular mechanisms that constitute its effector systems. This workshop paper reconsiders what constitutes adaptive changes in thermogenesis and reassesses the role of the sympathetic nervous system (SNS) and uncoupling proteins (UCP1, UCP2, UCP3, UCP5/BMCP1) as the efferent and effector components of the classical one-control system for adaptive thermogenesis and fat oxidation. It then reviews the evidence suggesting that there are in fact two distinct control systems for adaptive thermogenesis, the biological significance of which is to satisfy-in a lifestyle of famine-and-feast-the needs to suppress thermogenesis for energy conservation during weight loss and weight recovery even under environmental stresses (e.g., cold, infection, nutrient imbalance) when sympathetic activation of thermogenesis has equally important survival value.Keywords: Obesity; Diabetes; Cachexia; Catecholamines; UCP Resistance to obesity in an obesigenic environmentOne of the greatest challenges towards understanding the aetiology of human obesity is to explain how in environments that promote overeating of high-fat foods and discourage physical activity, there is always a section of the population who, apparently without conscious effort, do not become obese. How do they resist obesity? How is constancy of body weight achieved over decades in these individuals? In addressing the issue of human susceptibility to leanness and fatness, there are three cardinal features of body weight regulation that need to be underlined: (i) Humans cannot escape the laws of thermodynamics.Whatever theory is put forward to explain body weight and body composition regulation, it is undeniable that changes in body energy stores (and ultimately body weight) cannot occur unless there is a difference between energy intake and energy expenditure. (ii) Subtle perturbations in energy balance can lead to obesity. To put it another way, long-term constancy of body weight can only be achieved if the matching between energy intake and energy expenditure is extremely precise, since an error of only 1% between input and output of energy, if persistent, will lead to a gain or loss of 1 kg per year or some 40 kg between the age of 20 to 60 years. Yet, a difference of 5% between energy intake and energy expenditure is hardly measurable with available techniques. (iii) Body weight is a fluctuating and oscillatory phenomenon. Even in individuals that maintain a relatively stable lean body weight over decades, the...

Altered Skeletal Muscle Subsarcolemmal Mitochondrial Compartment During Catch-Up Fat After Caloric Restriction

“…These findings are clearly not in accord with a role for altered skeletal muscle proton leak in the mechanisms by which thermogenesis is suppressed. In addition, no change in UCP3 in SS or IMF muscle mitochondria was found during semistarvation or refeeding, confirming the dissociation between UCP3 and the regulation of mitochondrial proton leak (25,26). The present data on proton leak are much more consistent with an increasingly advocated role for proton leak in the control of mitochondrial ROS.…”

Leptin intake during the suckling period improves the metabolic response of adipose tissue to a high-fat diet

“…It is noteworthy the significant increase (285%) in UCP3 mRNA levels occurring in this depot in leptin-treated rats under HF-diet feeding. UCP3 has been proposed to have a primary function in the regulation of the use of lipids as fuel substrate, 36 then the increased transcription of this gene may be indicative of increased fatty acid oxidation.…”

Substrate cycling between de novo lipogenesis and lipid oxidation: a thermogenic mechanism against skeletal muscle lipotoxicity and glucolipotoxicity

“…To date, however, the molecular mechanisms underlying thermogenesis in tissues other than the brown adipose tissue remains elusive, amid continuing controversies concerning the physiological roles of UCP2 and UCP3 (homologs of UCP1) as effectors of skeletal muscle thermogenesis. 76,77 The energy-dissipating substrate cycle that links glucose and lipid metabolism to thermogenesis in skeletal muscle (depicted in Figure 3) provides a novel molecular mechanism of thermogenesis through which this abovementioned neuroendocrine networkFoperating through insulin, leptin and catecholaminesFoverlaps in the regulation of body weight, blood glucose and intramyocellular lipids, and hence in the protection against obesity, hyperglycemia and lipotoxicity. Perturbations in this neuroendocrine network (and intracellular signaling system) that exerts control over this substrate cycling may thus be early events that lead to intramyocellular lipid accumulation, and hence to the consequential effects of this state of lipotoxicity on the onset and exacerbation of insulin resistance.…”