The ERR gene lacks the typical TATA and CAAT boxes
The ERRα gene lacks the typical TATA and CAAT boxes, but has multiple consensus Sp1 binding elements in the GC-rich promoter (Shi et al., 1997). Previously, our laboratory has shown that ERRα expression is upregulated in the mouse uterus by estrogen (Shigeta et al., 1997) and recently we identified a multiple hormone response Bromophenol Blue (MHRE), a 57bp region in human and a 34bp region in mouse, that plays a significant role in estrogen-stimulated activity (Liu et al., 2003). The MHRE is a pleiotropic response element for other nuclear receptors and also serves as the binding site for ERRα and ERRγ (Laganiere et al., 2004, Liu et al., 2005, Mootha et al., 2004). It has been proposed that, in response to physiological cues, PGC-1α is induced and partners with ERRα to form an autoregulatory loop in the stimulation of ERRα gene expression. This event stimulates the expression of downstream target genes that are involved in energy production (Laganiere et al., 2004, Mootha et al., 2004). Nonetheless, the mechanism of ERRα gene induction by PGC-1α remains to be elucidated. ERRγ is coexpressed with ERRα in metabolically active tissues such as kidney, skeletal muscle and cardiac muscle, where it binds to the MHRE and is a stronger activator than the ERRα in self-stimulating the ERRα gene promoter (Liu et al., 2005). In particular, PGC-1α interacts with ERRγ and coactivates ERRγ's transactivation function on the βPDGF (SIS) element (Hentschke et al., 2002a) and the ERRα MHRE (Liu et al., 2005). These findings indicate that ERRγ may have a major role in regulation of ERRα gene expression and in energy homeostasis.
Materials and methods
Discussion Expression of ERRα, ERRγ and PGC-1α in metabolically active tissues such as skeletal muscle, heart and kidney has been reported by many laboratories (Bonnelye et al., 1997, Heard et al., 2000, Hong et al., 1999, Huss et al., 2002, Ichida et al., 2002, Knutti et al., 2000, Puigserver et al., 1998, Sanyal et al., 2002, Shi et al., 1997, Shigeta et al., 1997, Sladek et al., 1997), however, they were examined individually. The Northern blot analyses of the present study examined the coexpression of ERRs and PGC-1α in the same poly A RNA preparation of various human tissues. Indeed, ERRα is most abundant in many tissues that require high-energy output in particular the kidney and heart where ERRγ and PGC-1α are also highly expressed. In these tissues, the relative ratio of ERRα, ERRγ and PGC-1α may be important in executing the metabolic program that involves the PGC-1α and ERRα and ERRγ. Recently, mounting evidence demonstrates that PGC-1α functions as a master regulator in executing the energy balance program including the up-regulation of ERRα gene expression (Knutti and Kralli, 2001, Puigserver and Spiegelman, 2003). ERRα cooperates with PGC-1α in stimulation of genes involved in the mitochondrial biogenesis, mitochondria oxidative phosphorylation (Huss et al., 2004, Schreiber et al., 2004), and the mitochondria structure (Cartoni et al., 2005). However, a physiological role for ERRγ in energy metabolism is not established. It is known that ERRγ is a strong activator of selective response elements (Razzaque et al., 2004, Sanyal et al., 2004), interacts with PGC-1α (Hentschke et al., 2002a, Huppunen et al., 2004, Huss et al., 2002) and stimulates the ERRα gene promoter via the MHRE (Liu et al., 2005). Furthermore, treatment of HEC-1B cells with ERRγ siRNA significantly reduced the ERRα mRNA expression (Liu et al., 2005). Our current data showed that ERRγ expression in liver is also under the influence of physiological cue such as fasting. These studies support the hypothesis that ERRγ is one of the major regulators of ERRα gene expression. Since the expression of ERRα in selective tissues is influenced by circadian rhythm (Horard et al., 2004a, Horard et al., 2004b), basal levels of ERRα may fluctuate during the 24h fasting period with maximum in the afternoon. It is not known whether PGC-1α or ERRγ are also under circadian regulation. These factors may influence the level of ERRα, ERRγ and PGC-1α mRNA measured during fasting. Nonetheless, four mice in the control group were killed in the afternoon at the highest level of circadian behavior and significant induction of ERRs and PGC-1α were detected in both 12 and 24h fasting mice. Taken together, ERRγ may involve in regulation of the energy balance program at least by enhancing the ERRα expression. Whether ERRγ regulates the same ERRα downstream target genes in mitochondria oxidative phosphorylation or mitochondria biogenesis requires further investigation.