Mitochondrial respiratory chain (RC) disorders are the most prevalent inborn metabolic diseases and remain without effective treatment to date. Up-regulation of residual enzyme activity has been proposed as a possible therapeutic approach in this group of disorders. As resveratrol (RSV), a natural compound, was proposed to stimulate mitochondrial metabolism in rodents, we tested the effect of this compound on mitochondrial functions in control or in Complex I (CI)- or Complex IV (CIV)-deficient patients' fibroblasts. We show that RSV stimulates the expression of a panel of proteins representing structural subunits or assembly factors of the five RC complexes, in control fibroblasts. In moderate RC-deficient patients' cells, RSV treatment increases the amount of mutated proteins and stimulates residual enzyme activities. In these patients' cells, we establish that up-regulation of RC enzyme activities induced by RSV translates into increased cellular O2 consumption rates and results in the correction of RC deficiencies. Importantly, RSV also prevents the accumulation of lactate that occurred in RC-deficient fibroblasts. Different complementary approaches demonstrate that RSV induces a mitochondrial biogenesis that might underlie the increase in mitochondrial capacities. Finally, we showed that, in human fibroblasts, RSV stimulated mitochondrial functions mainly in a SIRT1- and AMPK-independent manner and that its effects rather involved the estrogen receptor (ER) and estrogen-related receptor alpha (ERRα) signaling pathways. These results represent the first demonstration that RSV could have a beneficial effect on inborn CI and CIV deficiencies from nuclear origin, in human fibroblasts and might be clinically relevant for the treatment of some RC deficiencies.
Inborn respiratory chain (RC) deficiencies, first identified in the 80's, are now considered as the most common inborn metabolic disorder in humans, with an estimated incidence of 1:5000 to 1:8000 individuals (1). These diseases are characterized by an extraordinary diversity of phenotypes, possibly affecting almost any tissue, with any age of onset. Molecular analysis of these disorders initially identified disease-causing mutations in the mitochondrial DNA (mtDNA), and, more recently, in a growing number of nuclear genes. Despite advances in our understanding of the molecular and biochemical basis of these disorders, the development of effective therapies has been so far extremely limited (2). Accordingly, the vast majority of mitochondrial disorders still remain without treatment, to date. Until now, current pharmacological therapies essentially focused on vitamin or co-factors supplementation, administration of electron acceptors or use of free-radical scavengers. Agents susceptible to improve lactic acidosis, in particular dichloroacetate, which acts by inhibiting pyruvate dehydrogenase kinase (3), were also tested. These treatments are largely supportive and aim at relieving the symptoms of the disease, rather than the cause, i.e. the RC deficiency. Overall, recent reviews of clinical trials concluded that none of these compounds were deemed beneficial for the therapy of mitochondrial disorders (2).
In recent years, new strategies started to emerge, based on a fully different rationale. Indeed, progress in the characterization of master regulators of mitochondrial oxidative metabolism stimulated the research of compounds capable to target these regulatory factors, in order to ‘boost’ mitochondrial energy production. In line with this, pharmacological enhancement of mitochondrial function is now admitted to carry significant implications for the treatment of common metabolic or neurodegenerative diseases associated with mitochondrial dysfunction. This might also represent a promising approach for the therapy of inborn mitochondrial disorders. Indeed, in 2008, our group reported the first proof-of-concept that, in patient cells harboring CI or CIV deficiency owing to nuclear gene mutations, treatment by bezafibrate was able to stimulate residual enzyme activity of the deficient RC complex and led to restore normal oxygen consumption rates in some treated cells (4). Similar conclusions on the potential of bezafibrate were drawn from the study of transmitochondrial cybrids carrying common pathogenic mtDNA mutations (5). Interpretation of these data converges on the pivotal role of the nuclear co-activator PGC-1α, whose expression is strongly induced by bezafibrate, in mediating OXPHOS up-regulation. The importance of PGC-1α as a putative therapeutic target in mitochondrial disorders has also been shown in various models of mice with mitochondrial chain defects (6,7).
Induction of PGC-1α promotes the recruitment of the main transcription factors governing the expression of nuclear RC genes, such as Nuclear Respiratory Factors 1 and 2 (NRF1 and NRF2), which also control the expression of the mitochondrial transcription factor Tfam (8). Over the last decade, numerous studies demonstrated that the PGC-1α signaling cascade could mediate increases in respiratory rates and mitochondrial biogenesis and serves critical regulatory functions to activate energy metabolism in tissues with high oxidative capacity, such as heart, slow-twitch skeletal muscle or brown adipose tissue (8,9).
Accordingly, delineating the signaling pathways involved in the activation of PGC-1α has focused much attention in recent years (10). It was thus shown that posttranslational modification played an essential role in up-regulating PGC-1α activity, in particular through deacetylation and phosphorylation. While the deacetylation is catalyzed by SIRT1, a member of the NAD-dependent histone deacetylases sirtuin family, the phosphorylation can be catalyzed by different protein kinases including AMP-activated protein kinase (AMPK) (10). A number of small molecules, including resveratrol (RSV), were proposed to act as allosteric activators of SIRT1 (11), opening new prospects for a possible use of these compounds in the treatment of metabolic disorders. However, during the past years, the assertion that SIRT1 was a direct target of RSV has been challenged; it was recently proposed that RSV rather indirectly activated SIRT1. Indeed, Park et al. reported that RSV might directly inhibit the cAMP-specific phosphodiesterase, resulting in the activation of AMPK and in increased levels of NAD+, which, subsequently would activate SIRT1 (12). Therefore, the precise sequence of events mediating PGC-1α activation in response to RSV is still debated (13).
However, long before attracting the attention as a putative SIRT1 activator, RSV was classified as a phytoestrogen capable to bind and activate estrogen receptors (ERs) (14). Interestingly, in recent years, there has been increasing evidence that estrogens and ERs might be important regulator of mitochondrial RC (15). ERs belong to the superfamily of nuclear receptors, which also include the estrogen-related receptors (ERRs). As their name implies ERRs share close structural homology with ERs but do not bind natural estrogens and are still considered as orphan receptors (16). Importantly, among the three existing ERR (α, β and γ) isoforms, ERRα has been identified as a master regulator of mitochondrial energy metabolism and biogenesis. Thus, in mouse liver, ERRα occupies the extended promoter region of many genes encoding enzymes and proteins of the RC (16).
Accordingly, our study aimed at testing whether RSV could stimulate mitochondrial functions and biogenesis in a panel of patient cells harboring Complex I (CI) deficiency, one of the commonest inborn mitochondrial RC disorder, or Complex IV (CIV) deficiency. Furthermore, we analyzed the involvement of SIRT1, AMPK, ER and ERRα in mediating the effects of RSV in primary human fibroblasts. There are yet limited data, especially in primary human cells, connecting these signaling pathways to the RC. This study establishes the proof of concept that RSV can correct moderate CI and CIV deficiencies and provides evidence that the effects of RSV in this cell type are mainly SIRT1- and AMPK-independent and rather involve the ER and ERRα signaling pathways.
Resveratrol stimulates the expression of RC proteins in control and in moderate RC-deficient fibroblasts
The optimal conditions for cell treatment by RSV (75 µm, 48 or 72 h) were established from dose-response experiments (Fig. 1), performed in control and in patient cells; 75 µm was chosen as the most effective dose, without cell toxicity. Using these conditions, we then assessed the effects of RSV on the expression of a panel of proteins representing structural subunits or assembly factors of the five RC complexes, in control fibroblasts. As shown in Figure 2, western blot analysis revealed that, in control fibroblasts, the levels of the thirteen RC proteins considered were all significantly increased in response to RSV. Protein induction by RSV ranged from +22% (NDUFS3) to +141% (core 2) and was observed for nuclear and mitochondrial-encoded (ND1, COX2) subunits.
We then evaluated the effects of RSV on the expression of mutated RC proteins, in CI- and in CIV-deficient fibroblasts. Six CI-deficient cell lines harboring distinct mutations of NDUFV1 gene were first compared (Fig. 3A). In the absence of treatment, five of them (Patients 2–6) exhibited a marked NDUFV1 protein deficiency, whereas Patient 1 was in the normal range. Cell treatment with RSV resulted in a significant increase in NDUFV1 protein levels in three out of six patients' cell lines, with inductions varying from +68% (Patient 1; P < 0.001) to +117% (Patient 3; P < 0.001).
The other CI-deficient cells came from patients harboring NDUFV2, NDUFS1 or NDUFS3 gene mutations (Fig. 3B–D). In these fibroblasts, the levels of the various mutant proteins were generally much lower than normal in the absence of treatment. Treatment with RSV stimulated the expression of mutant NDUFV2 protein in Patient 7 fibroblasts (+43%, P < 0.001; Fig. 3B). In contrast, RSV had no effect on NDUFS1 protein levels in Patients 8 and 9 (Fig. 3C) or on NDUFS3 protein level in Patient 10 cells (Fig. 3D).
Three CIV-deficient cells were considered in this study, corresponding to two patients with distinct SURF1 gene mutant genotypes (Patients 15 and 16), and one patient with COX10 gene mutations (Patient 14). Patients 15 and 16 presented a profound SURF1 protein deficiency, which was found unchanged after treatment by RSV (Fig. 3E). For the COX10-deficient patient, in the absence of commercially available COX10 antibody, we analyzed the expression of two CIV subunits, the mitochondrial-encoded COX2 and the nuclear-encoded COX4 subunits. As shown in Figure 3F, the protein levels of both COX subunits were strongly reduced in untreated cells, and these patient fibroblasts exhibited a marked response to RSV, which induced a +121 and +84% increase in COX2 and COX4 protein levels, respectively.
Resveratrol increases CI and CIV enzyme activity and can correct RC deficiencies
Under baseline conditions, variable levels of CI deficiency were found in our panel of patient fibroblasts (Fig. 4A). In control cells, treatment with RSV resulted in a marked increase in CI enzyme activity (+41%; P < 0.001). In CI-deficient cells, treatment with RSV resulted in a significant (P < 0.001) increase in CI enzyme activity in five patient cell lines, including three NDUFV1-deficient (Patients 1–3), one NDUFV2-deficient (Patient 7) and one cell line with unknown mutations (Patient 13). No significant changes in CI enzyme activity occurred in the other cell lines in response to RSV. In the RSV-responsive cells, the inductions of CI enzyme activity were at least +25% (Patient 2) and reached +90% in Patient 13, compared with untreated cells. Notably, cells from Patients 2 and 13 exhibited normal CI enzyme activity values after treatment with RSV. As observed for CI, CIV enzyme activity value was found significantly augmented (+60%; P < 0.001) in control cells following treatment with RSV (Fig. 4B). Among the CIV-deficient patient fibroblasts, the two SURF1-deficient cells (Patients 15 and 16) did not respond to RSV. In contrast, treatment with RSV induced a significant increase (+33%, P < 0.001) in CIV enzyme activity in COX10-deficient fibroblasts.
Further experiments were then performed to determine whether up-regulation of RC enzyme activities induced by RSV translated into changes in the cellular O2 consumption rates. Indeed, measurements of oxygen consumption in patient cells provide a reliable index to evaluate the functional consequences of RC complex deficiencies on the RC activity, i.e. to reveal oxidative phosphorylation deficiencies in living cells. We therefore used a new method based on oxygen-sensitive fluorescent probes, which allowed measuring maximal respiration rates in fibroblasts. This method (17), which has already been used to measure cell respiration in suspensions of CHO cell line (18), in adherent HepG2 (19) and Caco2 cells (18) and in isolated mitochondria (20), was adapted to suspensions of primary human fibroblasts. Preliminary experiments were performed (Fig. 5) to check that the oxygen uptake rates (O.U.R.) were linear to the amount of cells (Fig. 5A), were measured at optimal concentrations of uncoupling agent [carbonyl cyanide 3-chlorophenyl hydrazone (CCCP)] (Fig. 5B) and were fully (>95%) rotenone-, and KCN-, sensitive (Fig. 5C).
We then analyzed the O.U.R. in control, CI or CIV-deficient fibroblasts, and their variations in response to RSV (Fig. 6A and B). All the CI-deficient cells previously shown to respond to RSV were considered in these experiments (Group 1; Patients 1, 2, 3, 7 and 13), along with some non-responsive patient CI-deficient fibroblasts, taken as negative controls (Group 2, Patients 5, 6, 8 and 12). Under baseline conditions, O.U.R. values were ∼70% of normal value in Group 1 cells, versus 10–30% in Group 2 cells (Fig. 6A). In this latter group, treatment with RSV induced no changes in oxygen consumption. In contrast, exposure to RSV had a marked stimulatory effect on O.U.R. in control (+37%, P < 0.001), and in Group 1 fibroblasts. In these patients, O.U.R. values were restored to the control range following treatment with RSV (Fig. 6A).
Finally, similar experiments were performed in the COX10-deficient cell line in which RSV was previously shown to increase COX enzyme activity. As seen in Figure 6B, these patient cells exhibited basal O2 consumption representing 75% of normal values and responded to RSV by an increase in their O2 consumption rate. Interestingly, and as observed in some CI-deficient cells, treatment by RSV was able to restore O2 consumption value to the normal range in CIV-deficient Patient 14 fibroblasts.
Resveratrol improves lactate/pyruvate ratio
Another informative index of whole cell energy metabolism in living cells is the lactate-to-pyruvate (L/P) ratio, reflecting the cellular NADH/NAD ratio, which we compared in CI-deficient or control cells (Fig. 6C). In OXPHOS-deficient patients, a severe lactic acidosis, which could be fatal, is commonly encountered (21). In control cells under baseline conditions, the mean L/P ratio value was 33.1 with little inter-individual variation (±10%). As expected, CI deficiency was associated with high L/P ratio, varying from ∼43 up to 78 in the patient cells studied. Treatment with RSV resulted in a decrease in the L/P value in control fibroblasts and remarkably in these five CI-deficient cells in which RSV restored normal oxygen consumption rates, treatment by RSV resulted in the restoration of normal L/P values.
Resveratrol induces mitochondrial biogenesis
By use of different complementary approaches, we then addressed the question whether treatment by RSV could stimulate the mitochondrial biogenesis in human fibroblasts. Experiments using the Mitotracker green probe clearly revealed increases in staining intensity in both control and patients' fibroblasts upon exposure to RSV (Fig. 7A). Quantification of the data confirmed this, showing increased fluorescence in response to RSV in patients (+30–50%; P < 0.05) and in control (+80%; P < 0.05) fibroblasts.
Measurements of citrate synthase enzyme activity, commonly considered as a marker of mitochondrial density, were also performed in control cells, and in a panel of ‘responsive’ and ‘non-responsive’ fibroblasts. Interestingly, treatment with RSV was found to increase citrate synthase activity (+30 to +90%) both in control cells and in all the RC-deficient patient cells tested (Fig. 7B).
Last, we analyzed in the same panel of cells the protein expression of Tfam, the mitochondrial transcription factor A, which co-localizes with mtDNA in the mitochondrial nucleoids (22). Western blot analysis revealed a general stimulation of Tfam protein expression in all the cell lines, in response to RSV (Fig. 7C).
Resveratrol effects on RC capacity in human fibroblasts are mainly SIRT1- and AMPK-independent
In order to analyze the molecular mechanisms underlying the effects of RSV on RC in our cell system, we first evaluated SIRT1 involvement. Knockdown of SIRT1 (siRNA from Dharmacon) was performed in control cells and in Patient 3 fibroblasts, and western blots confirmed the total suppression of SIRT1 expression (Fig. 8A). Two readouts were chosen to study the effects of siSIRT1: the NDUFV1 protein levels and the oxygen consumption rates. In control fibroblasts, NDUFV1 protein levels were significantly increased in vehicle-treated cells transfected with siSIRT1, compared with non-target siRNA (Fig. 8A), suggesting that SIRT1 might have an inhibitory effect on basal NDUFV1 expression. However, in vehicle-treated patient cells, this difference in NDUFV1 levels between siSIRT1 and non-target siRNA was not found significant. Overall, in the absence of treatment by RSV, there was a trend toward higher oxygen consumption rates in fibroblasts transfected with siSIRT1, compared with the corresponding non-target siRNA groups (Fig. 8B). Our results clearly show that SIRT1 knockdown only partially abolished the response to RSV. Indeed, in control and Patient 3 fibroblasts lacking SIRT1, there were still significant increases in NDUFV1 proteins after treatment with RSV (Fig. 8A) (+88 and +82%, respectively, versus +146 and 104% in the corresponding non-target siRNA fibroblasts). From a functional point of view, the changes in cellular respiration paralleled those observed in protein levels (Fig. 8B). Accordingly, silencing of SIRT1 led to reduce, but did not prevent, the increases in O.U.R. in response to RSV. It should be mentioned that the same results were obtained after transfecting control and Patient 3 cells with another siSIRT1 (from Sigma), ruling out possible off-target effects.
A recent study proposed that SIRT1 would be required to mediate the effects of low doses of RSV whereas the cellular response to high doses of RSV could be SIRT1 independent (13). However, in experiments using a lower RSV dose (20 µm), no significant increases in O.U.R. were obtained in control or in Patient 3 fibroblasts (data not shown).
As some authors proposed that RSV effects could primarily be mediated through activation of AMPK (12), we tested this hypothesis and performed siAMPK experiments in control and in patient cells. Western blot analysis and measurements of respiratory rates showed that robust silencing of AMPK did not prevent the increase in NDUFV1 protein levels (Fig. 8C) and oxygen consumption (Fig. 8D) in response to RSV. Finally, as there is an obvious crosstalk between SIRT1 and AMPK, and to rule out possible compensatory mechanisms between the two pathways, we also performed a double knockdown of SIRT1 and AMPK. Western blot analysis revealed that the RSV-induced increases in NDUFV1 were still significant, and these data therefore confirmed that the RSV stimulatory effects on RC did not mainly require SIRT1 and AMPK (Fig. 8E).
ER and ERRα are likely involved in the effects of resveratrol on RC capacity in human fibroblasts
Given the fact that RSV was identified as a phytoestrogen and binds ERs, and considering the proposed role of ERRα in regulating the expression of RC genes, we explored a possible role of ER and of ERRα in the cell response to RSV. We used the specific ER antagonist ICI182780, or the specific ERRα inverse agonist XCT790, in conjunction with RSV, and measured O.U.R. to determine whether or not the increases triggered by RSV persisted in the presence of these inhibitors. As can be seen in Figure 9A, in control and patients' fibroblasts incubated with ICI182780 or with XCT790, the inductions of O.U.R. by RSV treatment were fully abolished, and the values of oxygen consumption were not statistically different from those measured without RSV treatment. These data clearly indicate the involvement of both receptors in mediating the stimulatory effects of RSV on oxygen consumption in human fibroblasts.
We speculated from these results that RSV might regulate ERRα expression. Western blot analysis showed detectable levels of ERRα protein in vehicle-treated cells that did not differ between control and patient cells. Interestingly, it appeared clearly that RSV treatment led to up-regulate, by ∼50%, ERRα receptor expression in all treated fibroblasts (Fig. 9B). Accordingly, our results indicate that RSV can positively modulate the levels of mitochondrial RC complexes and oxygen consumption, but also the expression of a key regulatory factor involved in its own signaling pathway, namely ERRα, which is known to represent a master regulator of mitochondrial RC and biogenesis.
The present study shows that, in control human fibroblasts, treatment by RSV up-regulated the overall expression of the RC, as indicated by the coordinate increases in the levels of proteins representative of complexes I to V, together with the increase in oxygen consumption.
In patients' cells with various nuclear gene mutations affecting CI or CIV, studies of oxygen consumption revealed RSV-induced increases in cell respiratory rates, with restoration to the control range in 6 out of 16 cell lines. In these fibroblasts, it can therefore be concluded that enzyme activities were restored to a level allowing normal OXPHOS activity. Regarding CI defect, this was corroborated by analysis of lactate-to-pyruvate ratio (L/P), reflecting the cellular NADH/NAD ratio.
Improvement or correction of RC defect was only observed in CI-deficient cell lines in which significant up-regulation of the mutant protein was achieved in response to RSV. This clearly suggests that mutations harbored in these cell lines did not induce marked protein instability and did not greatly hamper its ability to functionally interact with the other CI subunits. Accordingly, these cells can be considered to exhibit moderate CI deficiencies. On the other hand, severe CI deficiencies exhibited very low mutant protein levels unchanged after treatment with RSV, which emphasizes a strong destabilizing effect of some of the mutations considered. Accordingly, the diversity of responses to RSV among the patient cell lines might, for a large part, be ascribed to the variable effects of the different mutations on protein stability, with highly unstable mutant proteins being no longer inducible by RSV. Altogether, these data show that RSV could correct moderate but not severe CI or CIV deficiency in human fibroblasts.
Several studies suggest that RSV stimulates energy metabolism via a stimulation of mitochondrial biogenesis (23,24); however, this has hardly been documented in human cells (25). In the present study, analysis of several markers of mitochondrial density consistently indicated that RSV stimulated the mitochondrial biogenesis in control, and in all the RC-deficient fibroblasts. Thus, in patients' cells in which a correction of RC defect was observed after treatment with RSV, mitochondrial biogenesis might underlie the increase in RC capacity. However, taking citrate synthase activity and Tfam abundance as indices, it is noteworthy that a mitochondrial biogenesis was uniformly found in all the patients' cells, including those in which RSV did not improve the RC complex deficiency. This points out that the stimulation of mitochondrial biogenesis triggered by RSV is not, by itself, sufficient to induce improvement of CI or CIV deficiency in deficient cell models and that up-regulation of residual enzyme activity is a pre-requisite.
The exact cascade of activation leading to the stimulation of mitochondrial energy metabolism is presently a matter of controversy (26). In particular, several authors attempted to identify the primary target of RSV and proposed either that RSV directly activates SIRT1 that subsequently activates AMPK (13) or, alternatively, that RSV works primarily by activating AMPK (27) through indirect mechanisms, like inhibition of PDEs (12). In this latter hypothesis, activation of SIRT1 is proposed to be indirect via elevation of NAD+ level (28). However, whatever the molecular mechanism is in reality, there seems to be a consensus on the involvement of SIRT1 and AMPK to explain the beneficial effects of RSV on mitochondrial functions. We therefore used siRNA approaches to investigate the requirement of SIRT1 and AMPK in the observed effects. Concerning SIRT1, we repeatedly observed increased NDUFV1 expression and oxygen consumption values in untreated control fibroblasts after transfection with siSIRT1, indicating a potential repressor role of SIRT1 under basal conditions. Though surprising, these results are in agreement with a recent study, showing that SIRT1 KO MEF cells exhibited higher mitochondrial cellular respiration than wild-type MEFs (29). Concerning the involvement of SIRT1 in the response to RSV, silencing SIRT1 abolished only a part of RSV effects on protein levels and respiration. Accordingly, in human fibroblasts, RSV acts mainly via a SIRT1-independent signaling pathway to stimulate mitochondrial energy metabolism. Recent data suggest that the implication of SIRT1 in mediating the response to RSV might vary depending on the dose of RSV used for cell treatment. Indeed, in the rodent C2C12 cell line, a moderate dose of RSV (25 µm) elicited mitochondrial effects that were found entirely SIRT1 dependent, whereas at higher RSV doses (50 µm), the effects appeared SIRT1 independent (13). This is in agreement with our observations and with the results of Csiszar et al. who showed that in human endothelial cells, a mitochondrial biogenesis could be triggered by 10 µm of RSV and that this effect was SIRT1 dependent (25). However, at this point, it should be recalled that the dose of 75 µm RSV was determined by dose-response experiments and was chosen as the most effective to stimulate CI enzyme activity in control and patient fibroblasts, without toxic effects. In our hands, fibroblasts usually require higher concentrations of active molecule than human myoblasts (30,31). Nevertheless, we also performed experiments at 20 µm RSV and showed that this concentration is not an efficient dose to stimulate mitochondrial respiration. Therefore, in our cell culture conditions, a stimulation of O.U.R. in primary human fibroblasts is only achieved at high RSV concentrations. We next tested a potential involvement of AMPK in mediating the effects of RSV. Our data clearly indicate that in our cell models, AMPK is not required. Finally, and given the literature data clearly showing an interaction between SIRT1 and AMPK pathways, not only in response to RSV but also in situations like exercise and fasting, we silenced both proteins and demonstrated that up-regulation of mitochondrial expression by RSV was still present. There are abundant data claiming the involvement of SIRT1 in the signaling cascade triggered by RSV that leads to stimulate energy metabolism, both in vivo and in vitro. However, it is worth mentioning that many of these studies support their conclusions by showing a concomitant increase in SIRT1 mRNA, SIRT1 protein or SIRT1 activity in response to RSV, which does not prove the actual implication of SIRT1 in the signaling cascade. Some authors used SIRT1 KO mice or SIRT1 silencing approaches to sustain their conclusions (13,24,32). In these cases, the apparent discrepancy with our own conclusions might be due to species differences between humans and rodents, to cell-specific action of RSV, because many of these results were obtained in C2C12 cells, or as already discussed, to a RSV-induced SIRT1 activation that could be dose dependent.
These unexpected results led us to consider other targets in different signaling pathways, which might explain the beneficial effects of RSV on mitochondrial RC in our cell system. Our search revealed that RSV was known to bind and activate ERα and ERβ (33), which, interestingly, are present in the nucleus but also in the mitochondria in number of cell types and tissues (15). Moreover, in recent years, there has been increasing evidence showing that ERs regulate the mitochondrial RC expression and biogenesis (15). The regulation of mitochondrial RC genes by ERs probably involves direct and indirect mechanisms (Fig. 9C)—a direct regulation of mitochondrial genes by ERs that are localized in mitochondria in many cells and tissues and—an indirect regulation through induction of NRFs, TFAM and PGC-1α. We therefore sought to identify the role of ERs in RSV-induced up-regulation of RC functions using the specific ERα and ERβ inhibitor ICI182780. The results obtained support the hypothesis that RSV effects on RC are mediated through its interaction with ER (Fig. 9D). ERβ very likely mediates these effects because its presence has been demonstrated in human skin fibroblasts whereas ERα is not expressed (34,35). Interestingly, it has been established that RSV regulates the mitochondrial superoxide dismutase via its interaction with ERβ (36). Finally, estrogens have recently been shown to ameliorate mitochondrial functions in Friedreich ataxia (35) and in Leber's hereditary optic neuropathy (34).
The search of RSV's targets naturally led us to consider the possible involvement of ERRα because of its kinship with ERs and its recognized role in the regulation of a wide range of mitochondrial pathways, including RC (16). We investigated the effects of XCT790 and showed that the effects of RSV on oxygen consumption were abolished in the presence of this specific ERRα inverse agonist (Fig. 9E). XCT790 has been shown to block the expression of PGC-1α target genes (37) and is often used to demonstrate a role of ERRα cellular respiration in various cells (16,38). Therefore, both ER and ERRα appear to mediate the effects of RSV. We cannot completely rule out the possibility that RSV might act as an ERRα agonist. Indeed, ERRα is still considered to be an orphan receptor but Suetsugi et al. have shown that some phytoestrogens were agonists of ERRα (39), even though RSV was not considered in their study. Nevertheless, we rather favor an indirect effect of RSV on ERRα mediated by ER. This hypothesis is supported by our data showing an up-regulation of ERRα expression after RSV treatment in human fibroblasts and is strengthened by literature data showing that ERRα gene is a downstream target of ERs in breast cancer cells (40).
In conclusion, we provide, in this study, the first evidence that RSV could have a beneficial effect on inborn CI and CIV deficiencies from nuclear origin, in human fibroblasts. This natural product, with no known adverse effects, is presently tested in several clinical trials in various diseases. Importantly, in a recent paper, RSV (resVida) given to obese individuals for 30 days was shown to improve mitochondrial function in skeletal muscle (41). This supports the hypothesis that the stimulatory effects of RSV observed in the present study might be clinically relevant for the treatment of some RC deficiencies in humans. From a fundamental point of view, our study and many data of the literature indicate that RSV has wide-ranging effects on cell biology. Therefore, not surprisingly, numerous signaling pathways have been reported to underlie the effects of RSV. In our primary human fibroblasts, up-regulation of RC observed after treatment by RSV appears mainly SIRT1- and AMPK-independent but rather requires the ER and ERRα signaling pathways. Altogether, this study unravels new therapeutic targets for the correction of mitochondrial disorders.
MATERIALS AND METHODS
Patients and control fibroblasts
The CI- and CIV-deficient human fibroblasts used in our study originate from skin biopsies performed from 15 patients that have been previously described and 1 patient for which disease-causing mutations are still unknown. Mutations data are summarized in Table 1.
|Patients||Mutated gene||Nucleotide change||Amino acid change||Nucleotide change||Amino acid change||References|
|2||NDUFV1||c.640G>A||E214K||c.1162+4A>C(IVS8)||ex 8 del||(45)|
|7||NDUFV2||IVS2+5_+8delGTTA||ex 2 del||IVS2+5_+8delGTTA||ex 2 del||(48)|
|8||NDUFS1||Del entire gene||c.2119A>G||M707V||(45)|
|11||NDUFS6||Unknown||ex 3 and 4 del||Unknown||ex 3 and 4 del||(50)|
|Patients||Mutated gene||Nucleotide change||Amino acid change||Nucleotide change||Amino acid change||References|
|2||NDUFV1||c.640G>A||E214K||c.1162+4A>C(IVS8)||ex 8 del||(45)|
|7||NDUFV2||IVS2+5_+8delGTTA||ex 2 del||IVS2+5_+8delGTTA||ex 2 del||(48)|
|8||NDUFS1||Del entire gene||c.2119A>G||M707V||(45)|
|11||NDUFS6||Unknown||ex 3 and 4 del||Unknown||ex 3 and 4 del||(50)|
Patients 1–13: CI-deficient patients and Patients 14–16: CIV-deficient patients.
Cell culture and treatments
Human skin fibroblasts from control and patients were cultured at 37°C, 5% CO2 in RPMI with Glutamax™ (Gibco) supplemented with 10% (V/V) fetal bovine serum, 100 U/ml penicillin, 0.1 mg/ml streptomycin and 5 µg/ml plasmocin (Invivogen). For treatment, the media were removed and cells were incubated with fresh media containing 75 µm of trans-RSV (Cayman chemical) or RSV together with ICI182780 (Fulvestrant, Sigma) or with XCT790 (Sigma) or equivalent amounts of dimethylsulfoxide (0.04% DMSO, vehicle).
Small interfering RNAs (siRNAs) targeting SIRT1 sequence were obtained either from Dharmacon (ON-TARGET plus SMARTpool) or from Sigma, as well as control non-target siRNAs. Fibroblasts were transfected with 30 nm siRNAs 48 h prior to RSV treatment, using Lipofectamine® RNAiMAX Reagent according to the manufacturer's instructions.
Western blot analysis
Western blots were performed as previously described (42). Proteins were detected with the following antibodies: NDUFV1 (Proteintech); NDUFS3, Ip, Core1, Core2, COX2, COX4, SURF1, mtTFA, ERRα (Abcam); NDUFV2 (Sigma); NDUFS1, SIRT1 (Santa Cruz); Fp (Molecular Probe); β-actin (Millipore); ND1 (Kindly provided by A. Lombes, France).
Oxygen consumption assay
Fibroblasts oxygen consumption was measured using Oxoplates®OP96U, 96-well microplates with integrated optical oxygen sensors (PreSens, Germany). Fibroblasts were harvested, and Oxoplates® wells were filled with 180 000 to 200 000 cells (60 to 70 µg) suspended in RPMI 1640 without glucose (Gibco) supplemented with 1.25 µm of CCCP. Cell suspensions were then covered with 200 µl pre-warmed (37°C) heavy mineral oil (Sigma) to seal them from ambient oxygen. Finally, Oxoplates® were read out from the bottom side every 30 s for 1 h by a fluorescence intensity microplate reader (infinite®M200, Tecan). The kinetics of fluorescence intensities was analyzed with Microsoft Excel according to the manufacturer's instruction manual. In each well, O.U.R. were assessed by determining the maximal slope of oxygen partial pressure decrease.
Determination of lactate and pyruvate
Fibroblasts were seeded in 24-well plates at 70 000 cells/well. Seventy-two hours after treatment with RSV, the media was removed, wells were rinsed twice with PBS and cells were incubated for 4 h with 200 µl of Krebs Henseleit bicarbonate buffer pH = 7.4 (20 mm Hepes buffer pH = 7.4, 26 mm NaHCO3, 124 mm NaCl, 5 mm KCl, 1.2 mm KH2PO4, 1.2 mm MgSO4 and 2.5 mm CaCl2) supplemented with 11 mm glucose. At the end of the incubation, supernatants were collected. Lactate and pyruvate contents were measured by fluorimetric assays according to Passoneau and Lowry (43). Results were normalized to the amount of proteins in each well.
Enzyme activity measurements
CI enzyme activity was measured using the CI Enzyme Activity Microplate Assay Kit (MitoSciences) from Abcam, following the manufacturer's recommendations. CIV and citrate synthase enzyme activities were measured according to the methods described in (44) and (43), respectively.
Fibroblasts were stained with 100 nm of MitoTracker Green FM, and the fluorescence intensity of the mitochondria relative to the cell volume was calculated using the ImageJ software.
Differences between groups were analyzed by one-way ANOVA and the Fisher test or by the paired t-test.
This work was supported by grants of the Association Française contre les Myopathies (AFM) and the Agence Nationale de la Recherche (ANR, grant ANR-09-GENO-024-01). Alexandra Lopes Costa is a recipient of a Cifre Fellowship from GlaxoSmithKline.
We thank Dr Anne Lombes for providing the ND1 antibody.
Conflict of Interest statement. None declared.