Nuclear receptor ERRα and coactivator PGC-1β are effectors of IFN-γ-induced host defense

Macrophages are the principal phagocytic cells in the immune system, and thus play a key defense role during bacterial infection. Upon infection, macrophages ingest bacterial pathogens through phagocytosis. Phagosomes capturing pathogens eventually mature into phago-lysosomes within which pathogens are destroyed. The antibacterial activities of macrophages are triggered by interferon-γ (IFN-γ) (Nathan et al. 1983), a proinflammatory cytokine that exerts its effects through activation of the JAK/STAT-1 pathway (Darnell et al. 1994) and transcriptional induction of antibacterial genes including nitric oxide synthase (iNOS) (MacMicking et al. 1995) and GTP-binding protein LRG-47 (MacMicking et al. 2003; Gutierrez et al. 2004). In addition, IFN-γ alters the expression of up to 10% of the genome in macrophages (Ehrt et al. 2001), although the contribution of most of the IFN-γ inducible genes to IFN-γ-induced host defense is largely unknown. One of the genes whose expression in the macrophage was found to be inducible by IFN-γ was nuclear receptor ERRα (estrogen-related receptor α, NR3B1) (Barish et al. 2005), which belongs to a superfamily of ligand-activated transcription factors (Chawla et al. 2001). Historically, ERRα was the first orphan nuclear receptor to be identified and it shares a high degree of sequence homology with two estrogen receptors (ERα and ERβ), as well as two other ERR subfamily members (ERRβ and ERRγ) (Giguere et al. 1988; Giguere 2002). It can bind to consensus ER-binding sites in vitro, an inverted-repeat of the core element AGGTCA separated by three nucleotides, as a homodimer and is believed to have the potential to influence estrogen signaling pathways such as bone formation and breast cancer (Giguere 2002). It also binds to an extended consensus half site TCAAGGTCA or its variants found in the promoter of many genes involved in mitochondrial function (Sladek et al. 1997; Mootha et al. 2004; Schreiber et al. 2004; Dufour et al. 2007). Indeed, ERRα has been identified as a transcriptional regulator of MCAD, a key enzyme in mitochondrial fatty acid β-oxidation (FAO) (Sladek et al. 1997; Vega and Kelly 1997), and more recently has been shown to be a target for ligand-independent coactivators peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and PGC-1β and to control broad aspects of mitochondria biology including mitochondria biogenesis, FAO, and oxidative respiration (Huss et al. 2002; Kamei et al. 2003; Schreiber et al. 2003, 2004; Mootha et al. 2004; Wende et al. 2005, Dufour et al. 2007). PGC-1α and a related protein, PGC-1β, have been identified as master regulators of mitochondrial oxidative metabolism (Lin et al. 2005; Finck and Kelly 2006). They both activate all of the ERR subtypes as well as other transcription factors implicated in mitochondrial gene regulation such as PPARα, PPARδ, nuclear respiratory factor-1, and GABPa/b. In addition, a variety of other transcriptional factors have been proposed to mediate biological activity elicited by PGC-1 proteins. Consistent with their role as a regulator of oxidative metabolism, they are expressed most highly in oxidative tissues, such as brown adipose tissue, cardiac and skeletal muscles, kidney, and brain. Thus, knockout (KO) mice for PGC-1α or PGC-1β exhibit a variety of metabolic defects (Lin et al. 2004; Leone et al. 2005; Lelliott et al. 2006; Vianna et al. 2006; Sonoda et al. 2007). In addition, expression of PGC-1β in macrophages was recently reported, although its physiological importance in this cell type is not well understood (Vats et al. 2006). A few studies have linked mitochondrial reactive oxygen species (ROS) production in modulation of macrophage-mediated innate immune function. UCP-2 is a ROS-activated mitochondrial uncoupling protein believed to function to reduce mitochondrial ROS production by releasing membrane potential (Echtay et al. 2002). UCP-2 overexpression in macrophages suppresses lipo-polysaccharide-induced intracellular ROS accumulation (Kizaki et al. 2002). UCP-2-deficient mice produce an increased level of ROS in macrophages and are resistant to infection of intracellular pathogens Toxoplasma gondii (Arsenijevic et al. 2000) and Listeria monocytogenes (LM) (Rousset et al. 2006). However, whether mitochondrial ROS production is a regulated process during infection is not clear. In this report, we show that ERRα and PGC-1β act as downstream effectors for IFN-γ, regulating mitochondrial output in macrophages. The loss of ERRα or PGC-1β results in decreased mitochondrial gene expression, intracellular ROS level, and bacterial clearance in IFN-γ-activated macrophages. These results not only uncover a previously unappreciated downstream effect elicited by IFN-γ, but also suggest a link between mitochondrial oxidative metabolism and macrophage-driven antibacterial immunity.