Despite the fact that the ERRs were the first family of orphan nuclear receptors cloned, their biological function has remained uncertain (12
). Recent evidence has implicated ERRα and ERRγ in the transcriptional regulation of cellular energy metabolism. ERRα is enriched in adult mammalian tissues with high oxidative metabolic capacity, such as the heart, slow-twitch skeletal muscle, and brown adipose. ERRα and ERRγ have also recently been shown to serve as functional partners for the PGC-1 family of coactivators (17
), which have emerged as key regulators of mitochondrial metabolism and biogenesis (23
). We hypothesized that ERR isoforms serve as key regulators of heart and skeletal muscle energy metabolism downstream of PGC-1α. To this end, gene expression profiling experiments were conducted in cardiac myocytes. ERRα overexpression was shown to increase the expression of genes involved in multiple pathways involved in cellular fatty acid utilization, including fatty acid uptake and intracellular binding and mitochondrial oxidation. In addition, a subset of genes involved in mitochondrial electron transport and oxidative phosphorylation were upregulated by ERRα.
Heart and slow-twitch skeletal muscle meet high ATP demands predominantly through oxidation of fatty acids in mitochondria. Our results demonstrating ERRα as an activator of oxidative metabolism in myocytes are consistent with the enriched expression of this nuclear receptor in heart and slow-twitch skeletal muscle. Indeed, ERRα null mice exhibit a compensatory increase in PGC-1α and ERRγ expression in heart and a reduction in expression of MCAD, a key fatty acid metabolic target gene, in the soleus, where no change in ERRγ expression was observed. These findings strongly suggest that ERR isoforms contribute to the high-level basal expression of fatty acid utilization genes in oxidative tissues.
Consistent with this conclusion, ERRα was recently shown, in a combined proteomic and gene expression profiling study, to be coregulated with proteins and enzymes physically associated with the mitochondria, further supporting its role as a regulator of energy metabolism (33
). Interestingly, metabolic function in white adipose, a predominantly glycolytic tissue, is impaired in ERRα null animals and is associated with increased MCAD expression (32
). These apparently disparate results support a complex tissue-selective metabolic regulatory function for ERRα, with the activity of ERRα being dependent on the complement of cofactors coexpressed in a given tissue. However, our in vivo gene expression data do not exclude indirect effects, such as metabolic derangements related to the ERRα-deficient state, influencing the expression of some putative gene targets.
We found that the metabolic regulatory effects of ERRα overexpression in cardiac myocytes displayed considerable overlap with those of the nuclear receptor PPARα. The following lines of evidence indicate that this overlap is due, at least in part, to direct activation of PPARα gene expression by ERRα: (i) overexpression of ERRα induced PPARα gene expression in cardiac myocytes, C2C12 skeletal myotubes, and primary mouse fibroblasts; (ii) ERRα directly activated the PPARα gene promoter in transient cotransfection assays through a conserved nuclear receptor response element to which it bound in vitro and in cells; and (iii) ERRα-mediated regulation of a subset of its fatty acid oxidation targets in primary fibroblasts absolutely required the presence of PPARα. Specifically, ERRα overexpression in PPARα null fibroblasts had no effect on the expression of PPARα targets, including M-CPT I, MCAD, or acyl-coenzyme A oxidase, yet ERRα activated these targets in PPARα-expressing cells. These results strongly suggest that in tissues where ERRα and PPARα are coexpressed, like skeletal muscle and heart, activation of PPARα by ERRα is an important mechanism to control the expression of certain genes involved in cellular fatty acid metabolism.
Collectively, our data and the results of recently published studies suggest that ERRα regulates mitochondrial metabolism through the activation of several downstream transcriptional regulatory cascades. While this manuscript was in review, two studies presented additional evidence for ERRα as a key component of the PGC-1α-mediated regulation of mitochondrial metabolism. Schreiber et al. demonstrated that induction of mitochondrial proliferation and respiratory chain enzyme gene expression by PGC-1α is impaired by RNA interference inhibition of ERRα expression (41
), indicating that ERRα is downstream of PGC-1α in regulating certain mitochondrial biogenic programs. Studies by Mootha et al. found a similar role for ERRα downstream of PGC-1α in C2
). The latter study suggested that ERRα activates the NRF cascade through direct activation of the Gabpa
gene promoter, which encodes a component of the NRF-2 complex. These data are consistent with our findings, which demonstrate that ERRα activates the mitochondrial fatty acid oxidation in cardiac myocytes by converging on the PPARα regulatory pathway.
Our data do not exclude the possibility that, in addition to activating PPARα, ERRα plays a direct role in the regulation of certain target genes. The results of our gene expression profiling studies demonstrated that, in addition to the fatty acid oxidation enzyme genes, a number of genes involved in cellular fatty acid uptake and mitochondrial respiration were also activated by ERRα. It is likely that a number of these genes are directly regulated by ERRα. Indeed, our previous work demonstrated that ERRα with PGC-1α directly activates the MCAD
gene promoter in transient transfection assays through NRRE-1, a pleiotropic nuclear receptor response element that has been shown to bind both PPARα and ERRα (14
). We also observed modest activation of the lipoprotein lipase
promoter by ERRα (J. Huss, unpublished observation). Furthermore, recent studies have demonstrated that ERRα with PGC-1α directly activates the ATP synthase
β and cytochrome c
gene promoters through consensus ERRα response elements (41
). It is therefore likely that ERRα regulates cellular metabolism through multiple pathways, including indirect regulation via other transcription factors and direct regulation of metabolic target genes.
Our results and studies by others have shown that ERRα expression is upregulated by PGC-1α in cultured cells (Fig. ) (42
) and in vivo in the hearts of mice overexpressing PGC-1α (39
; L. Russell and J. Huss, unpublished observation). These results place ERRα in a central position within the PGC-1α regulatory network (Fig. ). We propose that ERRα transduces PGC-1α-derived signals to transcription factors such as PPARα and NRFs as well as directly to target genes involved in energy metabolism (34
). The extent of the role of ERR isoforms in mediating the actions of PGC-1α on cellular metabolism and physiology is unknown. However, the inducibility of PGC-1α by fasting, exercise, and cold exposure suggests that this regulatory circuit serves a critical role in the physiologic regulation of cardiac and skeletal muscle energy metabolism. Given that derangements in mitochondrial oxidative metabolism occur in pathophysiologic states such as skeletal muscle insulin resistance and cardiac hypertrophy, ERRs may prove to be an attractive therapeutic target for common human diseases such as diabetes and heart failure.
FIG. 7. Role of ERRα in regulating cellular oxidative capacity. PGC-1α coactivates and regulates the expression of a number of transcription factors, including ERRα, PPARα, and NRFs, involved in mediating PGC-1α effects (more ...)