ISSN: 2161-1017
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Short Communication - (2016) Volume 5, Issue 4
Estrogen receptor related receptor alpha (ERRα) was the oldest orphan nuclear receptor with sequence identity to the estrogen receptors, ERα/β. The sequence alignment of the ERRα and the ERs reveals a high similarity (68%) in the DNA-binding domain and a moderate similarity (36%) in other parts of the proteins such as the ligand-binding E domain.
Keywords: ERRa; Cartilage; Osteoclasts; Cholesterol; Aging
Estrogen receptor related receptor alpha (ERRα) was the oldest orphan nuclear receptor with sequence identity to the estrogen receptors, ERα/β [1]. The sequence alignment of the ERRα and the ERs reveals a high similarity (68%) in the DNA-binding domain and a moderate similarity (36%) in other parts of the proteins such as the ligand-binding E domain [1]. If ERRα does not bind estrogen, cholesterol had been recently described as a potential agonist of the receptor [2]. Bone maintenance depends on a balance between bone resorption and bone formation that implicates bone-resorbing cells (osteoclasts), bone-forming cells (osteoblasts) and the osteocytes that modulate response of bone mechanical stress [3]. In skeletal tissues, ERRα plays mainly a functional role in osteoclasts (bone resorbing cells) but also has a role in osteoblasts (bone-forming cells) and chondrocytes [4].
A recent study has reinforced the role for ERRa in osteoclasts differentiation and function [2]. In osteoclastogenesis, ERRa was already known to act as a pro-osteoclastic factor in vivo, the ERRα knockout mice exhibiting osteopetrosis (excess of bone formation) [5]. Concomitantly, osteoclastogenesis was dramatically disturbed in vitro and genes implicated in mitochondrial biogenesis were down regulated (Figure 1A). Moreover, ERRα was also implicated in osteoclasts mobility and actin cytoskeletal organization by regulating the osteopontin (OPN)-integrin b3 chain-activated c-src (phosphorylated at the Tyr416) pathway causing the disruption of the specific actin structure (podosome belt) implicated in osteoclast adhesion, migration and invasion [6] (Figure 1B). Recently, ERRα was shown to mediate the effect of cholesterol on bone resorption and skeletal remodeling [2] (Figure 1C). Many studies have suggested a link between dyslipidemia (such as hypercholesterolemia) and low bone mineral density (a strong predicator of osteoporosis) for postmenopausal women [7]. Interestingly, osteoporosis is mainly due to an excess of osteoclasts since the amount of bone resorbed by the osteoclasts is not restored with the new bone deposited by the osteoblasts, suggesting that cholesterol may directly act through osteoclasts to induce bone loss in postmenopausal women [8]. Moreover, cholesterol had been described as a stimulator of Interleukin 1α (IL1α) secretion by macrophages and of RANKL (receptor activator of the NF-κB ligand) that are both strong pro-osteoclastic factors [9-11]. ERRα has also been link to osteoporosis. Indeed, ERRα expression is stimulated by estrogen in proliferative osteoblasts in vitro and inhibited in bone in vivo in ovariectomized adult rats [12]. Similarly to estrogens/ERs, ERRα may also regulate vascularisation and VEGF expression which is also known to impact osteoblasts and osteoclasts [13-17]. Moreover, the conditional knock-out of ERRα in female mice in pre-osteoblasts and the global ERRα deletion confer resistance to bone loss induced by estrogen-deficiency which suggest that ERRα may contribute to bone loss in osteoporosis [18,19]. It seems that ERRα may also mediate the pharmacological effects of bisphosphonates, the most prescribed antiresorptive drugs for fracture prevention in postmenopausal women [2].
Figure 1: ERRα as a regulator of osteoclastogenesis through its function in both mitochondrial biogenesis (A) and actin organization and resorption capacity (B). Recently, ERRα was shown to mediate the effect of cholesterol on bone resorption (C), and its transcriptional activity was decreased by the reduction of cholesterol synthesis induced by the zoledronate suggesting that ERRα mediates at least in part the anti-resorptive effects of bisphosphonates (D).
Cholesterol also has the ability to recruit coactivators PGC1β to ERRα in osteoclasts [2] (Figure 1C). PGC1β is upregulated during the transition from bone marrow macrophages to pre-osteoclasts, and PGC1β knockout mice exhibited osteopetrosis [5]. It is also downregulated in mice that were deleted in NF-κB proteins in osteoclast precursors [20]. Moreover, similarly to osteoclasts deleted in ERRα, PGC1β-deficient osteoclasts displayed abnormal morphology and their bone resorbing activity was significantly impaired due to a reduction in phosphorylation of c-src at Tyr416 and a decrease in actin ring formation [5,6]. Taken together, these data suggest that targeting ERRα-PGC1β through synthetic molecules like the inverse agonist XCT-790 that was designed to block ERRα activity by preventing its interaction with the PGC1 coactivators, can block the ERRα activation by cholesterol. Consequently, ERRα regulation of the mitochondrial biogenesis and of the actin cytoskeletal organization that are required for osteoclasts formation, migration and resorption capacity could be altered [21].
1. In clinic, treatments that are generally recommended for postmenopausal women are bisphosphonates that bind to bone surfaces, target osteoclasts and decrease bone resorption [22]. Interestingly, the nitrogen-containing-bisphosphonates such as zoledronate inhibits the mevalonate pathway and therefore the production of cholesterol which results in osteoclasts apoptosis [23] (Figure 1D). In mice, Wei, et al. show that the reduction of cholesterol synthesis by the zoledronate decrease ERRα transcriptional activity suggesting that ERRα mediates at least in part the anti-resorptive effects of bisphosphonates [2] (Figure 1D). They also show that the statins that are the most prescribed cholesterol-lowering drugs can also regulate ERRα activity in muscle. In contrast to bisphosphonates, that only target bone matrix surfaces, statins have pleiotropic effects [24]. Indeed beside their cardio-protective properties, statins have also been described to act as pro-osteogenic molecules by increasing the bone mineral density in post-menopausal women [25,26]. Moreover, statins (Simvastatin, atorvastatin) are able to stimulate growth factors secretions such as VEGF in osteoblasts which is also a direct target gene of ERRα [17,27]. Statins (Lovastatin) can also inhibit osteoclasts formation and a defect in ERRα in osteoclasts blocks the effect of Lovastatin [2].
In aging, cholesterol is strongly linked to age-related disorders [28]. ERRα in association with the PGC1 family of coactivators play a main role in the transcriptional control of mitochondrial biogenesis and respiratory function [29]. Deregulation of mitochondrial function is a common feature in multiple aspects of bone loss and cartilage destruction suggesting the involvement of ERRα in skeletal aging [30,31]. The bone phenotype in ERRα knock-out mice is more prevalent in aged mice (10 to 12 month) compared with 4 to 5 monthold mice and mainly due to osteoclasts defects and downregulation of genes implicated in mitochondrial function and biogenesis [5]. Very recently, ERRα-PGC1β had been linked to Sirtuin 3, a major mitochondrial deacetylase (nicotinamide adenine dinucleotide (NAD)-dependent protein deacetylase) that regulates oxidative stress resistance, in bone homeostasis [32,33]. Indeed, mice deficient in SIRT3 exhibit osteopenia due to increased numbers of osteoclasts. Huh et al show that in response to the pro-osteoclastic cytokine RANKL, the osteoclasts progenitors Sirt3-/- undergo increased osteoclastogenesis due to the stimulation of the ERRα-PGC1β at the transcriptional level.
Mitochondria also play a key role in chondrocytes function, survival and oxidative stress [34]. Chondrocytes from osteoarthritis cartilage, the most common chronic joint disease in the elderly population, showed a significant decrease of mitochondrial electron transport chain activity leading to mitochondrial damage of the outer membrane [30,35]. Proteomics study from osteoarthritic (OA) chondrocytes described a decrease in mitochondrial superoxide dismutase (SOD) levels and an increase in intracellular reactive oxygen species (ROS) in OA chondrocytes [36]. Considerable data now support the idea that ERRα, combined with PGC1 family members, regulates ROS production. Indeed, dysregulation of ERRα with the inverse agonist XCT-790 enhanced ROS production in differentiated adipocytes, muscles and breast cancer [37-39] (Figure 2).
Moreover, in DAergic neuronal cells, ERRα was involved in Sirt3 neuroprotective functions by regulating Sirt3 expression via ERRα- PGC1α interaction and binding on Sirt3 promoter. Increase of Sirt3 expression led to interaction with SOD2 that prevented ROS production and DAergic neurons death observed in Parkinson’s disease [40]. Also, the anti-oxidant effect of resveratrol was recently linked to the transcriptional regulation of SOD2 by ERRα in cells deficient in mitochondria Complex I [41] (Figure 2). Currently, no similar data are yet available in osteoarthritic chondrocytes, but these results clearly show a strong link between ERRα-PGCα complex with ROSdetoxifying processes, the mitochondrial SOD2 and the NADdependent deacetylase Sirt3 suggesting that similar transcriptional regulation may occur in cartilage in aging.
In conclusion, we have reviewed the increasing data supporting a role for ERRα in regulation of osteoclasts differentiation and function. Together, the data suggest that ERRα mainly act as a regulator of bone resorption. They also bring new insights into ERRα function and suggest that ERRα may mediate the pharmacological effects of antiresorptive drugs such as bisphosphonates and statins that are both targeting cholesterol metabolism. The fact that cholesterol is also linked to age-related disorders combined with ERRα function in mitochondria and in oxidative stress as a regulator of ROS production, suggest that ERRα may also act as a regulator of the aging process in skeletal tissues.