Bile acid metabolism has been shown to play an important role in the progression of obesity, which is associated with the energy metabolism, glucose and lipid metabolism, gut microflora interactions, and etc. (Donkers 2020, McGlone 2019, Wei 2020) Therefore, we further explored the regulation of bile acid metabolism at the gene level. In this study, we downloaded the mRNA transcriptomic data from obese patients and normal subjects in public databases, and obtained 59 DEGs and 843 obesity related module genes by bioinformatics analyses. Then, based on 13 overlapping genes, we obtained three bile acid metabolism related biomarkers of obesity, namely PEMT, CP and SLC27A2, and further constructed a diagnostic model of obesity.
PEMT (Phosphatidylethanolamine N-Methyltransferase) is related to glycerophospholipid biosynthesis and one-carbon metabolism and related pathways (Li 2023). In mice, the reduced expression of PEMT could catalyze the production of phosphatidylcholine, causing steatosis, inflammation, and etc. In humans, common PEMT variants impair phosphatidylcholine synthesis (Piras 2022). As we all know, choline is one of the components of the bile acid, which deficiency causes hepatic fat accumulation (Zeisel 1991). Therefore, we could expect that the reduction in PEMT is able to cause fat accumulation leading to obesity by impairing the synthesis of choline and bile acids. Notably, PEMT is overexpressed in obese patients, perhaps because of the existence of feedback regulatory mechanisms in this gene, but so far, no study has demonstrated this.
CP (Ceruloplasmin) is a protein coding gene that has been implicated in the transport of inorganic cations/anions and amino acids/oligopeptides and regulation of insulin-like growth factor (IGF) transport and uptake by insulin-like growth factor binding proteins (IGFBPs) (Liu 2022, Safavi 2012), which was consistent with our GSEA results. Erik Arner et al. have pointed that CP is a novel adipokine which is overexpressed in adipose tissue of obesity subjects and in obesity-associated cancer cells (Arner 2014). Oh Yoen Kim et al. have also showed that elevated plasma CP levels were significantly associated with obesity, which could be suggested to be a marker of obesity (Kim 2011). In addition, Zhidong Liu et al. have pointed out that CP was associated with the formation of free radicals in tissues during inflammation and infection, and has the diagnostic effects on metabolic diseases (including obesity)(Liu 2022). In our results, the gene expression level of CP was significantly higher in the obesity group than that in normal group, which is consistent with previous studies. Encourspiringly, the most recent study by Quanxin Jiang et al. have showed that the deletion of CP effectively remodeled bile acid metabolism by upregulating Cyp7a1 and Cyp8b1, which subsequently leads to enhanced bile acid, further decreased lipid accumulation and curbed inflammation (Jiang 2024). In other words, perhaps we could think that the significantly increasing of CP in serum and tissues of obesity patients might further affect the accumulation of bile acids by hindering or disrupting the bile acid metabolism, and finally leading to lipid accumulation.
SLC27A2 (Solute Carrier Family 27 Member 2) was associated with fatty acid metabolism and the synthesis of bile acids and bile salts, and many studies have showed that SLC27A2 could promote fat differentiation and development (Wu 2018, Yang 2004). Undoubtedly, the expression of SLC27A2 in peripheral blood mononuclear cells also could be a molecular marker for overweight development (Caimari 2010). Interestingly, Wenjing Wu et al. have suggested that LGALS12 knockdown could inhibit adipogenesis by downregulating lipogenic genes and activating the PKA-Erk1/2 signaling pathway (Wu 2018). Romina Mancinelli et al. have suggestted that PKA-Erk1/2TGF-β1/VEGF axis played an important role in the accumulation of bile acids (Mancinelli 2022). Therefore, we speculate that SLC27A2, as well as CP, is associated with the accumulation of bile acids. Interestingly, all of these three genes were associated with insulin sensitivity (Liu 2022, Wan 2019, Yang 2011). We have mentioned earlier that bile acids could exert the regulation of glucose metabolism and the influence on insulin sensitivity through TGR5 (Chavez-Talavera 2017), which further implies the correctness of our speculation about the mechanism of biomarkers affecting obesity.
Next, we predicted that there were 33 miRNAs, 4 miRNAs and 11 miRNAs of CP, PEMT, and SLC27A2, respectively, and totals of 238 TFs were screened out, specially, HNF4A, MLXIPL, and TCF2 could regulate CP, PEMT, and SLC27A2 simultaneously. Lei Chen et al. have suggested that HNF4 could regulate SLC27A2 and impact on fatty acid oxidation (Chen 2020), which was in agreement with our findings. Besides, Qi Yin et al. have pointed that HNF4A was associated with lipid metabolic homeostasis (Yin 2022). A Louise Hunter et al. have pointed out that HNF4A could modulate glucocorticoid action and further impact the regulation of energy metabolism and inflammation (Hunter 2022). Yuhui Wang et al. have showed that MLXIPL was involved in the transcriptional regulation of liver fat, and was significantly associated with energy metabolism and insulin resistance of obesity (Wang 2015). These results all imply the possibility that these TFs regulate biomarkers to influence the progression of obesity through fatty acid metabolism. Furthermore, we also predicted 16 targeted drugs of biomarkers, among them, Isoflavone (Nakai 2020, Perez-Torres 2021), Penicillamine (Stickford 2020), Anakinra (Maculewicz 2022), Ampicillin (Awujoola 2023), Protoporphyrin (Galbraith 1990), Isoproterenol (Nureki 2022) and etc. have been reported that either for clinical remission or treatment of obesity. Our study provides a further rationale for these drugs in clinically treating obesity, namely that they act by targeting biomarkers.
In conclusion, this study obtained three biomarkers (CP, PEMT and SLC27A2) related to bile acid metabolism in obesity, which were associated with the synthesis or accumulation of bile acids, and further affected the progression of obesity by affecting energy metabolism, glucose and lipid metabolism, insulin resistance and etc. Moreover, we constructed a new diagnostic model of obesity based on these three biomarkers, and provided further theoretical support for clinical treatment of obesity with soflavone, Penicillamine, and so on. Nevertheless, our hypothesis on the regulatory mechanism of biomarkers has not been confirmed, we will further verify the regulatory role of TFs (HNF4A, MLXIPL, and TCF2) on CP, PEMT, SLC27A2 through cell experiments, and finally construct a complete regulatory axis of bile acid metabolism affecting obesity.