Different effects of cold exposure and cold acclimation on rat liver mitochondrial fatty acid oxidation and ketone bodies production

Modulation of hepatic transcription factor EB activity during cold exposure uncovers direct regulation of bis(monoacylglycero)phosphate lipids by Pla2g15

Cold exposure is an environmental stress that elicits a rapid metabolic shift in endotherms and is required for survival. The liver provides metabolic flexibility through its ability to rewire lipid metabolism to respond to an increased demand in energy for thermogenesis. We leveraged cold exposure to identify novel lipids contributing to energy homeostasis and found that lysosomal bis(monoacylglycero)phosphate (BMP) lipids were significantly increased in the liver during acute cold exposure. BMP lipid changes occurred independently of lysosomal abundance but were dependent on the lysosomal transcriptional regulator transcription factor EB (TFEB). Knockdown of TFEB in hepatocytes decreased BMP lipid levels. Through molecular biology and biochemical assays, we found that TFEB regulates lipid catabolism during cold exposure and that TFEB knockdown mice were cold intolerant. To identify how TFEB regulates BMP lipid levels, we used a combinatorial approach to identify TFEB target Pla2g15 , a lysosomal phospholipase, as capable of degrading BMP lipids in in vitro liposome assays. Knockdown of Pla2g15 in hepatocytes led to a decrease in BMP lipid species. Together, our studies uncover a required role of TFEB in mediating lipid liver remodeling during cold exposure and identified Pla2g15 as an enzyme that regulates BMP lipid catabolism.

Overheating or overcooling: heat transfer in the spot to fight against the pandemic obesity

The prevalence of obesity has nearly doubled worldwide over the past three and a half decades, reaching pandemic status. Obesity is associated with decreased life expectancy and with an increased risk of metabolic, cardiovascular, nervous system diseases. Hence, understanding the mechanisms involved in the onset and development of obesity is mandatory to promote planned health actions to revert this scenario. In this review, common aspects of cold exposure, a process of heat generation, and exercise, a process of heat dissipation, will be discussed as two opposite mechanisms of obesity, which can be oversimplified as caloric conservation. A common road between heat generation and dissipation is the mobilization of Free Faty Acids (FFA) and Carbohydrates (CHO). An increase in energy expenditure (immediate effect) and molecular/metabolic adaptations (chronic effect) are responses that depend on SNS activity in both conditions of heat transfer. This cycle of using and removing FFA and CHO from blood either for heat or force generation disrupt the key concept of obesity: energy accumulation. Despite efforts in making the anti-obesity pill, maybe it is time to consider that the world's population is living at thermoneutrality since temperature-controlled places and the lack of exercise are favoring caloric accumulation.

NAD+ /sirtuin metabolism is enhanced in response to cold-induced changes in lipid metabolism in mouse liver

The nicotinamide adenine dinucleotide (NAD⁺ )/Sirtuin (SIRT) system is linked to metabolic adaptation. This study aimed to determine the temporal profile of metabolic responses of the liver to cold exposure and changes in the hepatic NAD⁺ /SIRT system. Eight-week-old male C57BL/6 mice were individually housed in conventional cages under cold exposure (4 °C) for up to 5 days. Cold exposure decreased the hepatic triglyceride level and cholesterol level in mice by 1.7- and 1.6-fold, respectively. Lipogenic gene expression was persistently reduced, while gluconeogenic gene expression was transiently increased. Hepatic NAD⁺ /SIRT metabolism was induced during the 'cold remodeling' phase (days 1-3) and correlated with decreasing lipogenic and increasing gluconeogenic gene expression, contributing to the maintenance of whole-body lipid and glucose homeostasis.

Multiple effects of cold exposure on livers of male mice

Cold exposure of mice is a common method to stimulate brown adipose tissue (BAT) activity and induce browning of white adipose tissue (WAT) that has beneficial effects on whole-body lipid metabolism, including reduced plasma triglyceride (TG) concentrations. The liver is a key regulatory organ in lipid metabolism as it can take up as well as oxidize fatty acids. The liver can also synthesize, store and secrete TGs in VLDL particles. The effects of cold exposure on murine hepatic lipid metabolism have not been addressed. Here, we report the effects of 24-h exposure to 4°C on parameters of hepatic lipid metabolism of male C57BL/6J mice. Cold exposure increased hepatic TG concentrations by 2-fold (P < 0.05) but reduced hepatic lipogenic gene expression. Hepatic expression of genes encoding proteins involved in cholesterol synthesis and uptake such as the LDL receptor (LDLR) was significantly increased upon cold exposure. Hepatic expression of Cyp7a1 encoding the rate-limiting enzyme in the classical bile acid (BA) synthesis pathway was increased by 4.3-fold (P < 0.05). Hepatic BA concentrations and fecal BA excretion were increased by 2.8- and 1.3-fold, respectively (P < 0.05 for both). VLDL-TG secretion was reduced by approximately 50% after 24 h of cold exposure (P < 0.05). In conclusion, cold exposure has various, likely intertwined effects on the liver that should be taken into account when studying the effects of cold exposure on whole-body metabolism.

Endocrine and metabolic signaling in retroperitoneal white adipose tissue remodeling during cold acclimation

The expression profiles of adiponectin, resistin, 5'-AMP-activated protein kinase α (AMPK α ), hypoxia-inducible factor-1 α (HIF-1 α ), and key enzymes of glucose and fatty acid metabolism and oxidative phosphorylation in rat retroperitoneal white adipose tissue (RpWAT) during 45-day cold acclimation were examined. After transient suppression on day 1, adiponectin protein level increased following sustained cold exposure. In parallel, on day 1, the protein level of HIF-1 α was strongly induced and AMPK α suppressed, while afterwards the reverse was seen. What is more, after an initial decrease on day 1, a sequential increase in pyruvate dehydrogenase, acyl-CoA dehydrogenase, cytochrome c oxidase, and ATP synthase and a decrease in acetyl-CoA carboxylase (from day 3) were observed. Similar to adiponectin, protein level of resistin showed a biphasic profile: it increased after days 1, 3, and 7 and decreased below the control after 21 days of cold-acclimation. In summary, the data suggest that adiponectin and resistin are important integrators of RpWAT metabolic response and roles it plays during cold acclimation. It seems that AMPK α mediate adiponectin effects on metabolic remodeling RpWAT during cold acclimation.

Oxidative activity in mitochondria isolated from rat liver at different stages of development

The purpose of this study was to evaluate the oxidative capacities in hepatic mitochondria isolated from prepubertal, young adult and adult rats (40, 90 and 180 days of age, respectively). In these rats, mitochondrial respiratory rates using FAD- and NAD-linked substrates as well as mitochondrial protein mass were measured. The results show that only the oxidative capacity of FAD-linked pathways significantly declined in mitochondria from 180-day-old rats compared with those from younger animals. When we consider FAD-linked respiration expressed per g liver, no significant difference was found among rats of different ages because of an increased mitochondrial protein mass found in 180-day-old rats. However, when FAD-linked and lipid-dependent respiratory rates were expressed per 100 g body weight, significant decreases occurred in 180-day-old rats. Therefore, the decrease in liver weight expressed per 100 g body weight rather than an impaired hepatic cellular activity may be the cause of body energy deficit in 180-day-old rats.

Extraperoxisomal targets of peroxisome proliferators: mitochondrial, microsomal, and cytosolic effects. Implications for health and disease

Peroxisome proliferators are a structurally diverse group of compounds that include the fibrate hypolipidemic drugs, the phthalate ester industrial plasticizers, the phenoxy acid herbicides, and the anti-wetting corrosion inhibitors perfluorinated straight-chain monocarboxylic fatty acids. Administration of these chemicals to rodents results in a number of effects, the most prominent being hepatomegaly and induction of peroxisomal enzyme activities. Several of these compounds have also been associated with the production of liver tumors in rodents and are classified as nongenotoxic hepatocarcinogens. Experimental evidence suggests that humans are not susceptible to these effects following exposure to peroxisome-proliferating compounds. This has led to the proposal that an "actual threat to humans" from exposure to one of these compounds seems "rather unlikely". Indeed, recent reports suggest that peroxisome proliferators may prove valuable as antitumor agents in humans. However, this assessment is preliminary given that peroxisome proliferators also produce a myriad of extraperoxisomal effects in livers and other tissues of experimental animals. Such effects include both stimulation and inhibition of mitochondrial and microsomal metabolism and alteration of the activities of various cytosolic enzymes. These responses may be directly or indirectly related to the effects on peroxisomes or may be totally independent of these events. Whether the extraperoxisomal effects of these compounds occur in humans is not known and their potential impact on human health remains to be investigated.