GOAT belongs to the super family of

GOAT belongs to the super family of membrane-bound O-acyltransferases (MBOATs). It is encoded by the gene MBOAT4 in human. The protein GOAT is highly conserved from zebrafish to human (Yanagi et al., 2018). GOAT is ubiquitously expressed with abundant expression in the stomach and intestine (Sakata et al., 2009). Like ghrelin, its expression fluctuates in response to the feeding status and diet, indicating the ghrelin-GOAT system is responding to the energy status (Xu et al., 2009; Gahete et al., 2010). The molecular mechanisms by which ghrelin-producing Lomustine sense nutrient intake and regulate ghrelin expression and secretion are less clear. Previously, we demonstrated that mTORC1 signaling pathway coordinates the extracellular cues and ghrelin production/secretion (Xu et al., 2009). We later identified histone deacetylase 5 (HDAC5) as a critical regulator of ghrelin production. HDAC5 functions as an upstream regulator of mTORC1 signaling activity through interaction and deacetylation of Raptor, the essential component of mTORC1 complex (Ma et al., 2015). The downstream effector of mTORC1 signaling in the regulation of ghrelin is still unknown. ETV5 is an E-twenty-six (ETS) transcription factor belonging to the polyoma enhancer activator 3 (PEA3)-subfamily, consisting of ETV1, ETV4, and ETV5 (also named ER81, PEA3, and ERM). It is ubiquitously expressed and its biological functions have been implicated in tumorigenesis, fertility, organ development and immune function (Chen et al., 2005; Lu et al., 2009; Mao et al., 2009; Kuure et al., 2010; Oh et al., 2012). Recently several genome-wide association studies (GWAS) have revealed that a single-nucleotide polymorphism (SNP) within ETV5 is associated with human obesity (Thorleifsson et al., 2009). Consistent with human findings, ETV5 deficient mice exhibit reduced body weight and resistance to diet induced obesity. Interestingly, ETV5 knockout mice also have less food intake and reduced adiposity (Gutierrez-Aguilar et al., 2014), indicating that ETV5 is involved in the control of energy homeostasis which involves the coordinated modulation of energy intake and expenditure. Furthermore, the expression of ETV5 in the brain, especially in the hypothalamic region, is altered in response to different nutritional status (Boender et al., 2012; Mechaly et al., 2017). These results suggest an important role of ETV5 in regulating food intake and energy balance. In the present study, we examined the role of ETV5 in GOAT/ghrelin system using the CLU122 cells, a murine hypothalamic cell line previously used for ghrelin secretion study (Ma et al., 2015), as well as in mice. We found that ETV5 significantly increased acyl ghrelin but not total ghrelin production. This occurred through the enhanced expression of GOAT enzyme. ETV5 transactivated the GOAT promoter, leading to the subsequent increase of its expression. mTORC1 signaling inhibited the expression of ETV5. Deficiency of ETV5 significantly attenuated the effect of mTORC1 signaling on GOAT/ghrelin production. Therefore, ETV5 is a new transcription factor regulating GOAT expression and GOAT/ghrelin system. ETV5 could be a new pharmacological target for the appetite control and obesity treatment.
Material and methods
Results
Discussion In this study, we identified the transcription factor ETV5 as a nuclear effector in response to the external nutrient states in ghrelin secreting cells (Fig. 6). ETV5 increased GOAT transcriptional activity and protein expression, thus enhancing the production of acyl ghrelin which is critical for the biological functions of ghrelin system. ETV5's expression was negatively regulated by mTORC1 signaling activity via post-transcriptional regulation mechanism. The effect of mTORC1 on GOAT/ghrelin axis was, at least partially, mediated by ETV5. Therefore, we have demonstrated a novel mTORC1-ETV5-GOAT/ghrelin axis in the ghrelin production. Ghrelin is mainly produced in the gastric X/A-like cells. It has pleiotropic functions, including stimulating appetite, growth control, regulating gastric motility and glucose homeostasis (Yanagi et al., 2018). Although the biological functions of ghrelin have been extensively studied, the mechanisms of ghrelin biosynthesis, post-translational modification, and secretion are less clear. Mature ghrelin needs to be post-translationally modified by O-acylation with octanoate, which is required for its binding and activation of the ghrelin receptors to exert its biological functions. GOAT belongs to MBOAT family of membrane-bound O-acyltransferases. It is co-expressed in the ghrelin-producing cells and catalyzes the acylation process once ghrelin is produced (Gutierrez et al., 2008; Yang et al., 2008). Since ghrelin acylation depends on GOAT, therefore the abundance and activity of GOAT are important in the regulation of ghrelin's biofunction, such as energy balance and glucose homeostasis (Li et al., 2016). Modulating the activity or abundance of GOAT may provide a promising approach for the metabolic control.