**5. The role of ERR in lipid metabolism**

The functions of ERRs have been defined by their interactions with PGC-1 s as coactivators. Beside inducing OXPHOS and supply the cellular energy demand, ERRs also activate transcription of numerous genes involved in oxidative metabolism that depends on mitochondrial respiration. ERRE is located in the promoter region of the gene that encodes carnitine-acylcarnitine carrier (SLC25A29), which is involved in the net transport of fatty acyl units to the mitochondrial matrix, where they are oxidized through β-oxidation. In addition, ERRE is located in the 5′-flanking region of the gene encoding for MCAD, a key enzyme involved in the initial step of mitochondrial β-oxidation [10]. AMPK-mediated expression of ERRα and PGC-1β and subsequent expression of MCAD/CPT1 is found to play a role in fatty acid β-oxidation in tamoxifen-resistant MCF-7 cells.

Genetic deletion of ERRα or ERRγ in mice confirmed the role of these ERRs in mitochondrial biogenesis and oxidative capacity, particularly in tissues with high energy demand [26, 76]. In cardiac muscles and brown adipose tissue where mitochondrial biogenesis and bioenergetic is needed for the function of the tissue, impaired adaptation to hemodynamic stressors and thermogenesis is observed respectively when ERRα is lost [77, 78]. Loss of ERRγ also resulted in the failure to switch to a oxidative transcriptome [76].

Given these positive regulatory roles of ERRs in mitochondrial respiration and fatty acid oxidation, loss of ERRs is expected to inhibit catabolic metabolism. However, deletion of ERRα, while viable and fertile, exhibited reduced fat mass and resistant to high-fat diet induced obesity [26]. Although mice lacking ERRβ or ERRγ are not viable to adulthood due to placenta and cardiac failures respectively [50, 58, 61, 76], pharmacological inhibition of either ERRγ (the dominant form of ERR in cardiac and skeletal muscles) or ERRα have led to improved insulin response and better tolerance to diet induced metabolic changes [79, 80]. Consistent with the reduced fat mass phenotype, lipogenic genes such as fatty acid synthase (Fasn) and elongase (Elov3) in the adipose tissue are all inhibited when ERRα is absent [26]. In agreement with this putative function of ERRs in lipogenesis, ERRα and PGC1-α expression are concurrently upregulated in response to adipogenic inductions [81]. ERR and PGC1-α together are found to be required for adipogenic differentiation induced by glucocorticoid, cAMP and insulin [81, 82]. Consistent with these observations, ChIP-on-Chip and ChIP-seq analysis indeed show that ERRα can occupy the promoter regions of Fasn and acetyl-CoA carboxylase (ACC), the two rate-limiting enzymes in the lipogenic pathways [83]. Inhibition of ERRα led to reduced triglyceride (TG) content in the liver accompanied by attenuated expression of Fasn and ACC [84].

The storage of lipid into TG starts with esterification of long-chain fatty acids to glycerol 3-phosphate [85] (**Figure 4**). This committed step is catalyzed by GPATs, the rate-limiting enzymes for the process. Acylation at carbon 1 leads to formation of lysophosphatidic acid (LPA) which is converted to phosphatidic acid (PA) via the action of AGPAT. During biosynthesis of triglycerides, PA is converted to

#### **Figure 4.**

*Glycerolipid biosynthesis pathway. Arrows labeled 1 describe the steps leading to the formation of triacylglycerol. Hepatic de novo lipogenesis results in the synthesis of fatty acids through acylation, which is catalysed by GPAT. G3P and acyl-CoA is converted to lysophosphatidic acid with the help of GPAT. Following this, LPA is converted to phosphatidic acid, which is catalysed by AGPAT. Dephosphorylation of PA by PAP generates diacylglycerol, which further serves as the substrate for DGATs in the synthesis of triglycerides. Arrows labeled 2 indicate a catalysis reaction. Arrows labeled 3 indicate the transcriptional regulation of GPAT enzymes by PGC-1α, and arrow 4 indicates how GPAT4 is transcriptionally regulated by ERRα and co-activator PGC-1α.*

diacylglyceride (DAG) via the actions of LIPIN, a group of enzymes recently gained significant attention relating to their functions in lipid particle formation and autophagy. The final step of TG biosynthesis is catalyzed by the actions of DGATs. The preferentially ERRE binding motif, which has a high frequency of 5′GGTCA-3′ was screened for and found in the promoters of *Esrra*, *Dgat1*, *Gpat4*, *Agpat1* and *Agpat3*. Histone marks were also recognized for *Esrra*, *Dgat1*, and *Gpat4*, suggesting that ERRα is being recruited near the transcription initiation sites [84].

Together, these studies thus suggest that ERRα is a broad spectrum of regulator for lipid metabolism including fatty acid b-oxidation, *de novo* lipogenesis as well as glycerolipid biosynthesis. However, how ERRα may play roles in lipid metabolism may be dependent on the metabolic state and physiological stimuli. For example, in liver steatosis induced by rapamycin treatment, lack of ERRα was shown to impair fatty acid oxidation while buildup of citrate due to downregulation of the TCA cycle is redirected towards lipid biosynthesis [83]. Thus, unexplored transcriptional roles of ERRs, at least ERRα in lipid biosynthesis vs. β-oxidation likely play a role in the *in vivo* phenotype observed with ERR function under different metabolic conditions.
