Josiane Seydoux

Peroxisome proliferator–activated receptor α mediates the adaptive response to fasting

Prolonged deprivation of food induces dramatic changes in mammalian metabolism, including the release of large amounts of fatty acids from the adipose tissue, followed by their oxidation in the liver. The nuclear receptor known as peroxisome proliferator‐activated receptor α (PPARα) was found to play a role in regulating mitochondrial and peroxisomal fatty acid oxidation, suggesting that PPARα may be involved in the transcriptional response to fasting. To investigate this possibility, PPARα-null mice were subjected to a high fat diet or to fasting, and their responses were compared with those of wildtype mice. PPARα-null mice chronically fed a high fat diet showed a massive accumulation of lipid in their livers. A similar phenotype was noted in PPARα-null mice fasted for 24 hours, who also displayed severe hypoglycemia, hypoketonemia, hypothermia, and elevated plasma free fatty acid levels, indicating a dramatic inhibition of fatty acid uptake and oxidation. It is shown that to accommodate the increased requirement for hepatic fatty acid oxidation, PPARα mRNA is induced during fasting in wildtype mice. The data indicate that PPARα plays a pivotal role in the management of energy stores during fasting. By modulating gene expression, PPARα stimulates hepatic fatty acid oxidation to supply substrates that can be metabolized by other tissues.

Uncoupling protein-3: a new member of the mitochondrial carrier family with tissue-specific expression.

Brown adipose tissue (BAT) and skeletal muscle are important sites of nonshivering thermogenesis. The uncoupling protein‐1 (UCP1) is the main effector of nonshivering thermogenesis in BAT and the recently described ubiquitous UCP2 [1] has been implicated in energy balance. In an attempt to better understand the biochemical events underlying nonshivering thermogenesis in muscle, we screened a human skeletal muscle cDNA library and isolated three clones: UCP2, UCP3L and UCP3S. The novel UCP3 was 57% and 73% identical to human UCP1 and UCP2, respectively, highly skeletal muscle‐specific and its expression was unaffected by cold acclimation. This new member of the UCP family is a candidate protein for the modulation of the respiratory control in skeletal muscle.

Adipose angiotensinogen is involved in adipose tissue growth and blood pressure regulation

White adipose tissue and liver are important angiotensinogen (AGT) production sites. Until now, plasma AGT was considered to be a reflection of hepatic production. Because plasma AGT concentration has been reported to correlate with blood pressure, and to be associated with body mass index, we investigated whether adipose AGT is released locally and into the blood stream. For this purpose, we have generated transgenic mice either in which adipose AGT is overexpressed or in which AGT expression is restricted to adipose tissue. This was achieved by the use of the aP2 adipocyte‐specific promoter driving the expression of rat agt cDNA in both wild‐type and hypotensive AGT‐deficient mice. Our results show that in both genotypes, targeted expression of AGT in adipose tissue increases fat mass. Mice whose AGT expression is restricted to adipose tissue have AGT circulating in the blood stream, are normotensive, and exhibit restored renal function compared with AGT‐deficient mice. Moreover, mice that overexpress adipose AGT have increased levels of circulating AGT, compared with wild‐type mice, and are hypertensive. These animal models demonstrate that AGT produced by adipose tissue plays a role in both local adipose tissue development and in the endocrine system, which supports a role of adipose AGT in hypertensive obese patients.

Green tea and thermogenesis: interactions between catechin-polyphenols, caffeine and sympathetic activity

The thermogenic effect of tea is generally attributed to its caffeine content. We report here that a green tea extract stimulates brown adipose tissue thermogenesis to an extent which is much greater than can be attributed to its caffeine content per se, and that its thermogenic properties could reside primarily in an interaction between its high content in catechin-polyphenols and caffeine with sympathetically released noradrenaline (NA). Since catechin-polyphenols are known to be capable of inhibiting catechol-O-methyl-transferase (the enzyme that degrades NA), and caffeine to inhibit trancellular phosphodiesterases (enzymes that break down NA-induced cAMP), it is proposed that the green tea extract, via its catechin-polyphenols and caffeine, is effective in stimulating thermogenesis by relieving inhibition at different control points along the NA–cAMP axis. Such a synergistic interaction between catechin-polyphenols and caffeine to augment and prolong sympathetic stimulation of thermogenesis could be of value in assisting the management of obesity.

Role of UCP homologues in skeletal muscles and brown adipose tissue: mediators of thermogenesis or regulators of lipids as fuel substrate?

The mRNA expressions of UCP2 and UCP3, two newly described genes with high sequence homology to the uncoupling protein UCP1 in brown adipose tissue (BAT), were examined in two skeletal muscles (gastrocnemius and soleus) as well as in interscapular BAT (IBAT) of the rat in response to food deprivation and controlled refeeding. In IBAT (a tissue highly dependent on lipids for thermogenesis), the pattern of mRNA expression of UCP2 and UCP3 closely follows that of UCP1: it was markedly down‐regulated during food deprivation (when this tissues thermogenesis and lipid fuel requirements are decreased) and restored to control levels by day 5 of refeeding. By contrast, in the gastrocnemius muscle (a mixed fiber type muscle with a high capacity to shift between glucose and lipids as fuel substrate), mRNA expression of both UCP2 and UCP3 mRNA was found to be markedly up‐regulated during food deprivation (when this tissues thermogenesis is also decreased but its lipid fuel utilization is increased). The expressions were subsequently found to be markedly down‐regulated upon transition to refeeding, with mRNA levels remaining below control levels on days 3, 5, and 10 of refeeding (period of enhanced efficiency of body fat deposition). In the soleus muscle (an oxidative type muscle with higher dependency on lipids than the gastrocnemius, and hence with a lower capacity to shift between lipids and glucose as fuel substrate), UCP homologues were also found to be up‐regulated during food deprivation, but changes in their mRNA expression contrast with those in the gastrocnemius muscle both in their much lower magnitude of response to food deprivation and in their more rapid restoration to control levels during refeeding. Up‐regulation of UCP2 and UCP3 gene expressions in skeletal muscle during food deprivation was found to persist at thermoneutrality (i.e., under conditions of reduced thermoregulatory thermogenesis). Together, these tissue‐dependent differential mRNA expressions of the UCP homologues in IBAT, gastrocnemius, and soleus muscles during food deprivation and refeeding are much more consistent with a role for UCP2 and UCP3 in the regulation of lipids as fuel substrate rather than as mediators of regulatory thermogenesis.—Samec, S., Seydoux, J., Dulloo, A. G. Role of UCP homologues in skeletal muscles and brown adipose tissue: mediators of thermogenesis or regulators of lipids as fuel substrate? FASEB J. 12, 715–724 (1998)

Uncoupling Protein-3 Expression in Rodent Skeletal Muscle Is Modulated by Food Intake but Not by Changes in Environmental Temperature

A new member of the uncoupling protein (UCP) family called UCP3 has recently been cloned and shown to be highly expressed in skeletal muscle of rodents and humans. In the present study, UCP3 was overexpressed in C2C12myoblasts where it acts as an uncoupling protein. Changes in UCP3 mRNA expression were examined in rodent muscles under conditions known to modulate thermogenesis in brown adipose tissue. In skeletal muscle, UCP3 expression did not change in response to 48 h of cold exposure (6 °C), whereas it was decreased by 81% or increased 5.6-fold by 1 week of 50% food restriction or fasting, respectively. It was also decreased by 36% in soleus muscle of obese (fa/fa) as compared with lean Zucker rats. The unexpected rise of UCP3 mRNA level induced by fasting did not change in vitro muscle basal heat production rate but decreased by 31% the capacity to produce heat in response to the uncoupler carbonylcyanidep-trifluoromethoxyphenylhydrazone. This decrease may reflect underlying uncoupling by UCP3. Up-regulation of UCP3 mRNA after a 24-h fast was still observed in mice exposed at thermoneutrality. These results show that the increase in UCP3 expression induced by fasting is associated with the maintenance of thermogenesis measured in muscle in vitro and is not modulated by environmental temperature. The notion that UCP3 expression is modulated by food intake is of importance to better understand the pathophysiology of obesity in humans.

Arachidonic acid and prostacyclin signaling promote adipose tissue development a human health concern

High fat intake is associated with fat mass gain through fatty acid activation of peroxisome proliferator-activated receptors δ and γ, which promote adipogenesis. We show herein that, compared to a combination of specific agonists to both receptors or to saturated, monounsaturated, and ω-3 polyunsaturated fatty acids, arachidonic acid (C20:4, ω-6) promoted substantially the differentiation of clonal preadipocytes. This effect was blocked by cyclooxygenase inhibitors and mimicked by carbacyclin, suggesting a role for the prostacyclin receptor and activation of the cyclic AMP-dependent pathways that regulate the expression of the CCAAT enhancer binding proteins β and δ implicated in adipogenesis. During the pregnancy-lactation period, mother mice were fed either a high-fat diet rich in linoleic acid, a precursor of arachidonic acid (LO diet), or the same isocaloric diet enriched in linoleic acid and α-linolenic acid (LO/LL diet). Body weight from weaning onwards, fat mass, epididymal fat pad weight, and adipocyte size at 8 weeks of age were higher with LO diet than with LO/LL diet. In contrast, prostacyclin receptor-deficient mice fed either diet were similar in this respect, indicating that the prostacyclin signaling contributes to adipose tissue development. These results raise the issue of the high content of linoleic acid of i) ingested lipids during pregnancy and lactation, and ii) formula milk and infant foods in relation to the epidemic of childhood obesity.

UCP2 and UCP3 rise in starved rat skeletal muscle but mitochondrial proton conductance is unchanged.

The relationship between UCP2 and UCP3 expression and mitochondrial proton conductance of rat skeletal muscle was examined. Rats were starved for 24 h and the levels of UCP2 and UCP3 mRNA and UCP3 protein were determined by Northern and Western blots. Proton conductance was measured by titrating mitochondrial respiration rate and membrane potential with malonate. Starvation increased UCP2 and UCP3 mRNA levels more than 5‐fold and 4‐fold, respectively, and UCP3 protein levels by 2‐fold. However, proton conductance remained unchanged. These results suggest either that Northern and Western blots do not reflect the levels of active protein or that these UCPs do not catalyse the basal proton conductance in skeletal muscle mitochondria.

Tissue-dependent upregulation of rat uncoupling protein-2 expression in response to fasting or cold.

The control of uncoupling protein‐2 (UCP2) mRNA expression in rat brown adipose tissue (BAT), heart and skeletal muscles was examined. Cold exposure (48 h) increased UCP2 mRNA in BAT, heart and soleus muscle by 2.4‐, 4.3‐ and 2.6‐fold, respectively. Fasting (48 h) had no effect on UCP2 mRNA expression neither in BAT nor in heart, but markedly increased it in skeletal muscles. While the upregulation of UCP2 mRNA in response to cold exposure is in line with a putative uncoupling role for this protein in thermoregulatory thermogenesis, the unexpected upregulation of UCP2 in skeletal muscles in response to fasting seems inconsistent with its role as an uncoupling protein involved in dietary regulation of thermogenesis.

Angiotensinogen-Deficient Mice Exhibit Impairment of Diet-Induced Weight Gain with Alteration in Adipose Tissue Development and Increased Locomotor Activity

White adipose tissue is known to contain the components of the renin-angiotensin system, which gives rise to angiotensin II from angiotensinogen (AGT). Recent evidence obtained in vitro and ex vivo is in favor of angiotensin II acting as a trophic factor of adipose tissue development. To determine whether AGT plays a role in vivo in this process, comparative studies were performed in AGT-deficient (agt−/−) mice and control wild-type mice. The results showed that agt−/− mice gain less weight than wild-type mice in response to a chow or high fat diet. Adipose tissue mass from weaning to adulthood appeared altered rather specifically, as both the size and the weight of other organs were almost unchanged. Food intake was similar for both genotypes, suggesting a decreased metabolic efficiency in agt−/− mice. Consistent with this hypothesis, cellularity measurement indicated hypotrophy of adipocytes in agt−/− mice with a parallel decrease in the fatty acid synthase activity. Moreover, AGT-deficient mice exhibit...