Co-ordinate decrease in the expression of the mitochondrial genome and nuclear genes for mitochondrial proteins in the lactation-induced mitochondrial hypotrophy of rat brown fat

The adaptation of maternal energy metabolism to lactation and its underlying mechanisms

Maternal energy metabolism undergoes a singular adaptation during lactation that allows for the caloric enrichment of milk. Changes in the mammary gland, changes in the white adipose tissue, brown adipose tissue, liver, skeletal muscles and endocrine pancreas are pivotal for this adaptation. The present review details the landmark studies describing the enzymatic modulation and the endocrine signals behind these metabolic changes. We will also update this perspective with data from recent studies showing transcriptional and post-transcriptional mechanisms that mediate the adaptation of the maternal metabolism to lactation. The present text will also bring experimental and observational data that describe the long-term consequences that short periods of lactation impose to maternal metabolism.

Role of sex hormones in modulation of brown adipose tissue activity

The recent demonstration that metabolically active brown adipose tissue (BAT) is present with a high prevalence in humans undoubtedly represents one of the major advancements in the field of metabolic research in the last few years. The increasing interest in BAT is justified by preclinical observations highlighting an important role of this tissue in energy dissipation and metabolic clearance of substrates from the blood. These findings imply that stimulation of BAT activity may represent a new therapeutic approach for obesity and associated comorbidities. However, before proposing BAT as a target organ for therapeutics in a clinical setting, many further notions about BAT function and modulation need to be explored. Keeping in mind the importance of sex dimorphism in energy metabolism control under physiological and pathological conditions, sex hormones may play a relevant role in the regulation of BAT activity in both males and females. Much of the evidence acquired in the past supports the concept of an important role for different sex hormones in BAT thermogenesis and indicates that this tissue mediates the ability of sex hormones to modulate energy balance. These findings make it plausible that a modified interaction between BAT and sex hormones may contribute to the development and the maintenance of obesity and associated metabolic complications.

Negative correlation between milk production and brown adipose tissue gene expression in lactating mice

It has been proposed that the performance of lactating animals is limited by the capacity of the female to dissipate body heat – the heat dissipation limit (HDL) theory. This theory predicts that milk production might be constrained not by intrinsic properties of the mammary glands but rather by competitive heat production such as thermogenesis in brown adipose tissue (BAT). To test this prediction, we measured the expression of genes linked to thermogenesis in BAT of lactating laboratory mice. The applicability of BAT gene expression to reflect thermogenic activity of BAT was confirmed by a positive relationship between expression levels of several BAT genes (summarised by the first principal component following principal component analysis) and daily energy expenditure in virgin mice. Milk production at peak lactation was strongly and negatively associated with the expression of thermogenic genes in BAT. Downregulation of these genes during lactation was correlated with low levels of circulating leptin and high levels of circulating prolactin. Our results are consistent with the HDL theory. However, we cannot discount the converse interpretation that milk production may reduce BAT activity. If the reduction in BAT activity does facilitate increased milk production, then reducing the heat generated by competitive processes may be a more productive route to increase lactational performance than attempts to improve mammary gland performance in isolation from the other body systems.

Sex steroid receptor expression profile in brown adipose tissue. Effects of hormonal status

Recent investigations suggest that sex hormones play an important role in the brown adipose tissue (BAT) thermogenic program by acting on several steps of the lipolytic signal cascade and on the UCP1 transcription control. However, the number of studies focusing on steroid receptor status in brown adipose tissue is negligible. In the present study, we analyze steroid receptor mRNA levels in brown adipose tissue in male and female rats and in pregnant and lactating females, all of them models with a different hormonal background. The direct effect of sex hormones on the expression of their receptors was studied in vitro in primary culture of brown adipocytes. Oestrogen receptor (ERalpha) and androgen receptor (AR) densities were higher in male than in female BAT. PR A+B mRNA expression was downregulated in lactation, suggesting a role of progesterone signalling in thermogenesis impairment at this stage. In vitro studies showed that progesterone decreased PR A+B mRNA and that testosterone downregulated ERalpha mRNA. The results highlighted in this study demonstrate the presence of steroid receptor mRNA in BAT and in brown cell cultured adipocytes, supporting the idea that changes in steroid receptor expression would be important for the understanding of sex hormone effects on BAT physiology.

Dietary n-3 fatty acids affect mRNA level of brown adipose tissue uncoupling protein 1, and white adipose tissue leptin and glucose transporter 4 in the rat

We examined the effect of dietary fats rich in n-3 polyunsaturated fatty acids (PUFA) on mRNA levels in white and brown adipose tissues in rats. Four groups of rats were fed on a low-fat diet (20 g safflower oil/kg) or a high-fat diet (200 g/kg) containing safflower oil, which is rich in n-6 PUFA (linoleic acid), or perilla (α-linolenic acid) or fish oil (eicosapentaenoic and docosahexaenoic acids), both of which are rich in n-3 PUFA, for 21 d. Energy intake was higher in rats fed on a high-safflower-oil diet than in those fed on low-fat or high-fish-oil diet, but no other significant differences were detected among the groups. Perirenal white adipose tissue weight was higher and epididymal white adipose tissue weight tended to be higher in rats fed on a high-safflower-oil diet than in those fed on a low-fat diet. However, high-fat diets rich in n-3 PUFA, compared to a low-fat diet, did not increase the white adipose tissue mass. High-fat diets relative to a low-fat diet increased brown adipose tissue uncoupling protein 1 mRNA level. The increases were greater with fats rich in n-3 PUFA than with n-6 PUFA. A high-safflower-oil diet, compared to a low-fat diet, doubled the leptin mRNA level in white adipose tissue. However, high-fat diets rich in n-3 PUFA failed to increase it. Compared to a low-fat diet, high-fat diets down-regulated the glucose transporter 4 mRNA level in white adipose tissue. However, the decreases were attenuated with high-fat diets rich in n-3 PUFA. It is suggested that the alterations in gene expression in adipose tissue contribute to the physiological activities of n-3 PUFA in preventing body fat accumulation and in regulating glucose metabolism in rats.

Carboxyatractyloside effects on brown-fat mitochondria imply that the adenine nucleotide translocator isoforms ANT1 and ANT2 may be responsible for basal and fatty-acid-induced uncoupling respectively

In brown-fat mitochondria, fatty acids induce thermogenic uncoupling through activation of UCP1 (uncoupling protein 1). However, even in brown-fat mitochondria from UCP1-/- mice, fatty-acid-induced uncoupling exists. In the present investigation, we used the inhibitor CAtr (carboxyatractyloside) to examine the involvement of the ANT (adenine nucleotide translocator) in the mediation of this UCP1-independent fatty-acid-induced uncoupling in brown-fat mitochondria. We found that the contribution of ANT to fatty-acid-induced uncoupling in UCP1-/- brown-fat mitochondria was minimal (whereas it was responsible for nearly half the fatty-acid-induced uncoupling in liver mitochondria). As compared with liver mitochondria, brown-fat mitochondria exhibit a relatively high (UCP1-independent) basal respiration ('proton leak'). Unexpectedly, a large fraction of this high basal respiration was sensitive to CAtr, whereas in liver mitochondria, basal respiration was CAtr-insensitive. Total ANT protein levels were similar in brown-fat mitochondria from wild-type mice and in liver mitochondria, but the level was increased in brown-fat mitochondria from UCP1-/- mice. However, in liver, only Ant2 mRNA was found, whereas in brown adipose tissue, Ant1 and Ant2 mRNA levels were equal. The data are therefore compatible with a tentative model in which the ANT2 isoform mediates fatty-acid-induced uncoupling, whereas the ANT1 isoform may mediate a significant part of the high basal proton leak in brown-fat mitochondria.

Mitochondrial biogenesis in mtDNA-depleted cells involves a Ca2+-dependent pathway and a reduced mitochondrial protein import

Alterations in mitochondrial activity resulting from defects in mitochondrial DNA (mtDNA) can modulate the biogenesis of mitochondria by mechanisms that are still poorly understood. In order to study mitochondrial biogenesis in cells with impaired mitochondrial activity, we used rho-L929 and rho(0)143 B cells (partially and totally depleted of mtDNA, respectively), that maintain and even up-regulate mitochondrial population, to characterize the activity of major transcriptional regulators (Sp1, YY1, MEF2, PPARgamma, NRF-1, NRF-2, CREB and PGC-1alpha) known to control the expression of numerous nuclear genes encoding mitochondrial proteins. Among these regulators, cyclic AMP-responsive element binding protein (CREB) activity was the only one to be increased in mtDNA-depleted cells. CREB activation mediated by a calcium-dependent pathway in these cells also regulates the expression of cytochrome c and the abundance of mitochondrial population as both are decreased in mtDNA-depleted cells that over-express CREB dominant negative mutants. Mitochondrial biogenesis in mtDNA-depleted cells is also dependent on intracellular calcium as its chelation reduces mitochondrial mass. Despite a slight increase in mitochondrial mass in mtDNA-depleted cells, the mitochondrial protein import activity was reduced as shown by a decrease in the import of radiolabeled matrix-targeted recombinant proteins into isolated mitochondria and by the reduced mitochondrial localization of ectopically expressed HA-apoaequorin targeted to the mitochondria. Decrease in ATP content, in mitochondrial membrane potential as well as reduction in mitochondrial Tim44 abundance could explain the lower mitochondrial protein import in mtDNA-depleted cells. Taken together, these results suggest that mitochondrial biogenesis is stimulated in mtDNA-depleted cells and involves a calcium-CREB signalling pathway but is associated with a reduced mitochondrial import for matrix proteins.

Reverse Transcriptase Inhibitors Alter Uncoupling Protein-1 and Mitochondrial Biogenesis in Brown Adipocytes

Objective

Human adipose depots contain remnant brown adipocytes interspersed among white adipocytes, and disturbances of brown with respect to white adipocyte biology have been implicated in highly active antiretroviral therapy (HAART)-induced lipo matosis. Brown adipocytes express the uncoupling protein-1 (UCP1) and contain a large number of mitochondria, potential targets of HAART toxicity. The aim of this study was to evaluate the effects of reverse transcriptase inhibitors (RTIs) on primary brown adipocytes differentiated in culture.

Design and methods

We analysed the effects of RTIs, nucleoside analogues (NRTIs: stavudine, zidovudine, didanosine and lamivudine) and non-nucleoside analogues (NNRTIs: nevirapine and efavirenz), on differentiation, mitochondrial biogenesis and gene expression in brown adipocytes.

Results

None of the NRTIs altered brown adipocyte differentiation whereas NNTRIs had differing effects. Efavirenz blocked lipid deposition and expression of adipose marker genes but nevirapine induced lipid accumulation and adipose gene expression, promoted mitochondrial biogenesis and increased UCP1. Stavudine, zidovudine and didanosine reduced mitochondrial DNA (mtDNA) content. However, mitochondrial genome expression was only impaired in didanosine-treated adipocytes. Stavudine, but not zidovudine, induced expression of the mitochondrial transcription factors and this may explain compensatory mechanisms for the depletion of mtDNA by up-regulating mtDNA transcription. Stavudine caused a specific induction of UCP1 gene expression through direct interaction with a retinoic acid-dependent pathway.

Conclusions

Specific disturbances in brown adipocytes in adipose depots may contribute to HAART-induced lipomatosis. Mitochondrial depletion does not appear to be the only mechanism explaining adverse effects in brown adipocytes because there is evidence of compensatory mechanisms that maintain mtDNA expression, and the expression of the UCP1 gene is specifically altered.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Mitochondrial biogenesis and thyroid status maturation in brown fat require CCAAT/enhancer-binding protein alpha

Brown fat differentiation in mice is fully achieved in fetuses at term and entails the acquisition of not only adipogenic but also thermogenic and oxidative mitochondrial capacities. The present study of the mice homozygous for a deletion in the gene for CCAAT/enhancer-binding protein alpha (C/EBPalpha-null mice) demonstrates that C/EBPalpha is essential for all of these processes. Developing brown fat from C/EBPalpha-null mice showed a lack of uncoupling protein-1 expression, impaired adipogenesis, and reduced size and number of mitochondria per cell when compared with wild-type mice. Furthermore, immature mitochondrial morphology was found in brown fat, but not in liver or heart, from C/EBPalpha-null mice. Concordantly, expression of both nuclear and mitochondrial genome-encoded genes for mitochondrial proteins was reduced in C/EBPalpha-null brown fat, although expression of mitochondrial rRNA and mitochondrial DNA content were unaltered. Expression of nuclear respiratory factor-2, thyroid hormone nuclear receptors, and peroxisome proliferator-activated receptor gamma coactivator-1, was delayed in C/EBPalpha-null brown fat. Iodothyronine 5'-deiodinase activity and thyroid hormone content were also reduced in brown fat from C/EBPalpha-null mice, indicating for the first time a crucial role for C/EBPalpha in controlling thyroid status in developing brown fat, which may contribute to impaired mitochondrial biogenesis and cell differentiation. When survival of C/EBPalpha-null mice was achieved by transgenically expressing C/EBPalpha only in the liver, a substantial recovery in brown fat differentiation was found by day 7 of postnatal age, which is associated with a compensatory overexpression of C/EBPdelta and C/EBPbeta.

Differential regulation of expression of genes encoding uncoupling proteins 2 and 3 in brown adipose tissue during lactation in mice

Thermogenic activity in brown adipose tissue (BAT) decreases during lactation; the down-regulation of the gene encoding uncoupling protein 1 (UCP1) is involved in this process. Our studies show that UCP2 mRNA expression does not change during the breeding cycle in mice. In contrast, UCP3 mRNA is down-regulated in lactation but it recovers after weaning, in parallel with UCP1 mRNA. This leads to a decrease in the content of UCP3 in BAT mitochondria during lactation. Lowering the energy-sparing necessities of lactating dams by decreasing litter size or feeding with a high-fat diet prevented the down-regulation of UCP1 mRNA and UCP3 mRNA. In most cases this resulted in a less marked decrease in UCP1 and UCP3 protein in BAT mitochondria owing to lactation. Fasting for 24 h caused a different response in UCP1 and UCP3 mRNA expression: it decreased UCP1 mRNA levels but had no effect on UCP3 mRNA abundance in virgin mice; it even increased UCP3 mRNA expression in lactating dams. These changes did not lead to modifications in UCP1 or UCP3 protein abundance. Whereas acute treatment with peroxisome-proliferator-activated receptor (PPAR)alpha and PPARgamma agonists increased UCP1 mRNA levels only in lactating dams, UCP3 mRNA expression was induced by both kinds of PPAR activator in lactating dams and by PPARalpha agonists in virgin mice. It is concluded that modifications of UCP2 mRNA levels are not part of the physiological adaptations taking place in BAT during lactation. In contrast, the down-regulation of UCP3 mRNA expression and mitochondrial UCP3 content is consistent with a role for the gene encoding UCP3 in the decrease in metabolic fuel oxidation and thermogenesis in BAT during lactation.

Temperature-dependent expression of cytochrome-c oxidase in Antarctic and temperate fish

Seasonal acclimation versus permanent adaptation to low temperatures leads to a differential response in the expression of cytochrome-c oxidase (CCO) in temperate and Antarctic eelpouts. Although eurythermal eelpout from the North Sea (Zoarces viviparus) displayed a cold-induced rise of CCO activity in white muscle, enzyme activity in the cold stenothermal Antarctic eelpout Pachycara brachycephalum failed to reflect such a compensatory increase. In Antarctic eelpout, CCO activity correlates with transcript levels of mitochondrial encoded subunits of CCO (CCO I and CCO II), whereas cold-acclimated eelpout from the North Sea show lower enzyme activities than expected on the basis of mitochondrial mRNA levels. In these animals, CCO expression at low temperatures may be limited either by nuclear CCO transcripts or by posttranscriptional processes. These may comprise translation of the subunits or assembly of the CCO holoenzyme. mRNA levels of CCO IV, one of the nuclear encoded subunits, increased strongly during cold acclimation, indicating that the expression of CCO is likely not message limited in cold-acclimated Z. viviparus. Our data suggest that seasonal cold acclimation of Z. viviparus results in a modification of the relationship between transcription and translation or posttranslational processes. In permanently cold-adapted P. brachycephalum, on the other hand, CCO expression shows similar characteristics as in the warm-acclimated confamilial species, e.g., low levels of enzyme activity correlated with low levels of mitochondrial message.

Regulation of mitochondrial biogenesis in brown adipose tissue: nuclear respiratory factor-2/GA-binding protein is responsible for the transcriptional regulation of the gene for the mitochondrial ATP synthase beta subunit

The regulation of transcription of the gene for the beta subunit of the FoF1 ATP synthase (ATPsynbeta) in brown adipose tissue has been studied as a model to determine the molecular mechanisms for mitochondrial biogenesis associated with brown adipocyte differentiation. The expression of the ATPsynbeta mRNA is induced during the brown adipocyte differentiation that occurs during murine prenatal development or when brown adipocytes differentiate in culture. This induction occurs in parallel with enhanced gene expression for other nuclear and mitochondrially-encoded components of the respiratory chain/oxidative phosphorylation system (OXPHOS). Transient transfection assays indicated that the expression of the ATPsynbeta gene promoter is higher in differentiated HIB-1B brown adipocytes than in non-differentiated HIB-1B cells. A major transcriptional regulatory site was identified between nt -306 and -266 in the ATPsynbeta promoter. This element has a higher enhancer capacity in differentiated brown adipocyte HIB-1B cells than in non-differentiated cells. Electrophoretic shift analysis indicated that Sp1and nuclear respiratory factor-2/GA-binding protein (NRF2/GABP) were the main nuclear proteins present in brown adipose tissue that bind this site. Double-point mutant analysis indicated a major role for the NRF2/GABP site in the enhancer capacity of this element in brown fat cells. It is proposed that NRF2/GABP plays a pivotal role in the co-ordinated enhancement of OXPHOS gene expression associated with mitochondrial biogenesis in brown adipocyte differentiation.

A cold-inducible coactivator of nuclear receptors linked to adaptive thermogenesis

Adaptive thermogenesis is an important component of energy homeostasis and a metabolic defense against obesity. We have cloned a novel transcriptional coactivator of nuclear receptors, termed PGC-1, from a brown fat cDNA library. PGC-1 mRNA expression is dramatically elevated upon cold exposure of mice in both brown fat and skeletal muscle, key thermogenic tissues. PGC-1 greatly increases the transcriptional activity of PPARgamma and the thyroid hormone receptor on the uncoupling protein (UCP-1) promoter. Ectopic expression of PGC-1 in white adipose cells activates expression of UCP-1 and key mitochondrial enzymes of the respiratory chain, and increases the cellular content of mitochondrial DNA. These results indicate that PGC-1 plays a key role in linking nuclear receptors to the transcriptional program of adaptive thermogenesis.

Hypothermia preserves function and signaling for mitochondrial biogenesis during subsequent ischemia

Hypothermia is known to protect myocardium during ischemia, but its role in induction of a protective stress response before ischemia has not been evaluated. As cold incites stress responses in other tissues, including heat shock protein induction and signaling mitochondrial biogenesis, we postulated that hypothermia in perfused hearts would produce similar phenomena while reducing injury during subsequent ischemia. Studies were performed in isolated perfused rabbit hearts (n = 77): a control group (C) and a hypothermic group (H) subjected to decreasing infusate temperature from 37 to 31 degrees C over 20 min. Subsequent ischemia during cardioplegic arrest at 34 degrees C for 120 min was followed by reperfusion. At 15 min of reperfusion, recovery of left ventricular developed pressure (LVDP), maximum first derivative of left ventricular pressure (LV dP/dtmax), LV -dP/dtmax, and the product of heart rate and LVDP was significantly increased in H (P < 0.01) compared with C hearts. Ischemic contracture started later in H (97.5 +/- 3.6 min) than in C (67.3 +/- 3.3 min) hearts. Myocardial ATP preservation and repletion during ischemia and reperfusion were higher in H than in C hearts. mRNA levels of the nuclear-encoded mitochondrial proteins adenine nucleotide translocase isoform 1 (ANT1) and beta-F1-adenosine-triphosphatase (beta-F1-ATPase) normalized to 28S RNA decreased in C hearts but were preserved in H hearts after reperfusion. Inducible heat shock protein (HSP70-1) mRNA was elevated nearly 4-fold after ischemia in C hearts and 12-fold in H hearts. These data indicate that hypothermia preserves myocardial function and ATP stores during subsequent ischemia and reperfusion. Signaling for mitochondrial biogenesis indexed by ANT1 and beta-F1-ATPase mRNA levels is also preserved during a marked increase in HSP70-1 mRNA.

Muscle/heart isoform of mitochondrial adenine nucleotide translocase (ANT1) is transiently expressed during perinatal development in rat liver

A postnatal increase in the content of mitochondrial ANT in rat liver which is related to the maturation of mitochondrial function has previously been reported [Schönfeld et al., Biochim. Biophys Acta 1144 (1993) 353-358]. In order to define the contribution of the ANT isoforms to this postnatal increase we have studied the expression of ANT1 and ANT2 isoforms in the liver during this period. The results show that in contrast to adult liver, perinatal liver expressed the ANT1 isoform at the mRNA and protein level, and that during this period the expression of ANT1 increased to a similar extent as total ANT content. It is concluded that the postnatal increase in ANT is mainly due to the ANT1 isoform and therefore, a role for the ANT1 isoform in the postnatal maturation of mitochondrial respiration in rat liver is suggested.

ATP synthase subunit c expression: physiological regulation of the P1 and P2 genes

Pre-translational regulation of subunit c has been suggested to control the biosynthesis of mitochondrial ATP synthase (ATPase) in brown adipose tissue (BAT). Subunit c is encoded by the genes P1 and P2, which encode identical mature proteins. We have determined here the levels of P1 and P2 mRNAs in different tissues, in response to cold acclimation in rats, during ontogenic development of BAT in hamsters, and following thyroid hormone treatment in rat BAT and liver. Quantitative ribonuclease protection analysis showed that both the P1 and P2 mRNAs were present in all rat tissues measured. Their total amount in each tissue corresponded well with the ATPase content of that tissue. While the P1/P2 mRNA ratio is high in ATPase-rich tissues, the P2 mRNA dominates in tissues with less ATPase. Cold acclimation affects P1 but not P2 gene expression in rat BAT. A rapid and transient increase in P1 mRNA is followed by sustained depression, which is accompanied by a decrease in ATPase content. Similarly, ontogenic suppression of ATPase content in hamster BAT was accompanied by suppression of the P1 mRNA levels, while P2 expression was virtually unchanged. Furthermore, when hypothyroid rats were treated with thyroid hormone, the steady-state level of P1 but not of P2 mRNA was significantly increased in liver. BAT was unaffected. We conclude that the P1 and P2 genes for subunit c are differentially regulated in vivo. While the P2 gene is expressed constitutively, the P1 gene responds to different physiological stimuli as a means of modulating the relative content of ATP synthase.