A long-term HFD induces obesity [24, 25], and causes lipid hypermetabolism in the liver [26]. In the present study, weight regain after weight loss in mice was induced by switching HFD to ND and then re-switching to HFD. We mainly analyzed the physiological and metabolic changes in mice, and the underlying mechanism. Transcriptomic analysis showed that lipid metabolism-related genes were differentially expressed in the liver during weight fluctuation of obese mice. We later found that the E4bp4-Cyp3a11 axis maybe responsible for weight regain after weight loss in obese mice.
Our data revealed that after switching HFD to ND, obese mice presented weight loss, and significant reduction in FBG, glucose area under the curve and blood lipids, and lipid droplet accumulation in the liver. After HFD re-feeding, obese mice experienced weight regain, although their body weight, blood lipids and lipid droplet accumulation in the liver were significantly lower than those in the mice continuously fed with HFD. A relevant study has consistently illustrated that the body weight of mice fed with low-fat diet for 7 weeks and then HFD for 4 weeks does not rebound to the level before weight loss, but remains lower than that of HFD-fed mice; moreover, their glucose and lipid metabolism are also more healthier than those in HFD-fed mice [15]. However, Kyung et al. [27] have reported that body weight, FBG and cholesterol re-elevate in mice fed with HFD for 8 weeks, followed by the feeding of ND for 4 weeks and HFD for another 4 weeks, suggesting the potential harm of weight rebound after weight loss. The controversial findings may be attributed to differences in dietary intervention and weight loss duration that influence the glucose and lipid metabolism and the degree of weight rebound.
The emergence of transcriptomics makes it easier to explore phenotypes of diseases. Through comparing transcriptomic differences between normal sates and pathological states, transcriptomics has been used to identify disease-related diagnostic or therapeutic genes or targets, to predict the function of the target genes, and infer the pathogenic mechanism and drug targets. Since liver is the core organ responsible for metabolism, the present study investigated the impacts of transcriptomic changes during weight fluctuations on glycolipid metabolism and hepatic lipid deposition, with the aim of providing new targets for the treatment of obesity, NAFLD, and other metabolic diseases. Here, the transcriptomic profile in the liver was analyzed during the process of weight rebound after weight loss. It is found that signaling pathways associated with lipid metabolism were enriched in a large number of DEGs between WG and WR group, including the steroid hormone biosynthesis pathway. We speculated that the liver transcriptome may be responsible for weight regain after weight loss via mediating lipid metabolism in mouse liver. RNA-seq further identified that Cyp3a11 was differentially expressed during weight fluctuation. Cyp3a11 (human homolog Cyp3a4) is a member of the cytochrome P450 (Cyp450) superfamily and widely present in mouse liver. Functionally, Cyp3a11 is involved in drug detoxification and lipid metabolism in the liver [28, 29].
Zeng et al. [30] have reported that the mRNA and protein levels of Cyp3a11 are downregulated in the liver of HFD-fed mice, palmitic acid-induced primary mice hepatocytes, palmitic acid-induced HepG2 cells, as well as the liver of NAFLD patients [31]. Consistently, our data also showed that the mRNA and protein levels of Cyp3a11 were significantly downregulated in the liver of HFD-fed mice compared with ND-fed mice. Compared with that of obese mice, the expression level of Cyp3a11 was significantly higher in the liver of obese mice experiencing weight regain after weight loss. Hence, we believed that Cyp3a11 is involved in liver transcriptomic change during weight fluctuation induced by dietary interventions.
The correlation of Cyp3a11 with weight fluctuation in obese mice may be mediated by its upstream transcription factors. In the present study, E4bp4 was identified as a DEG among the four groups. E4bp4, also known as nuclear factor interleukin 3-regulated (NFIL3), is a member of the basic leucine zipper (bZIP) family [32]. It is widely distributed in the liver, serving as a vital regulator for lipid metabolism [33]. Inhibiting the activation of E4bp4 in the liver has been demonstrated as a potential therapeutic target for hepatic steatosis [33, 34]. In high-fat, low-methionine, and choline-deficient (HFLMCD) diet-induced nonalcoholic steatohepatitis (NASH) mouse model, liver-specific E4bp4 knockout (E4bp4-LKO) mice present a lower degree of lipid droplet accumulation in the liver and a better liver function compared with those of E4bp4flox/flox mice [34].
A latest study has elucidated the key role of E4bp4 in regulating the expression and activity of Cyp3a11 in mice [32]. Cyp3a11 is significantly upregulated in E4bp4−/− mice than that of wild-type mice. Moreover, E4bp4 overexpression significantly downregulates, whereas its knockdown upregulates the mRNA and protein levels of Cyp3a11 in Hepa-1c1c7 cells in vitro [32]. As an upstream transcription factor, E4bp4 negatively regulates the expression level of Cyp3a11 both in vivo and in vitro. Their clinical significances in lipid metabolism in the liver, however, require a thorough exploration in the future.
During the process of weight fluctuation in obese mice, E4bp4 was significantly upregulated in WG group compared to NC group, but significantly downregulated in WL and WR groups compared to WG group. We also validated the opposite change in the expression level of E4bp4 to that of Cyp3a11 in the mouse liver. Therefore, we speculate that the E4bp4-Cyp3a11 axis is closely linked with weight fluctuation in obese mice.
Taken together, HFD induced weight gain, glucose and lipid metabolism disorders in mice, all of which were alleviated after feeding ND. Obese mice experienced weight regain after weight loss by re-feeding HFD, but their glucose and lipid metabolism disorders were milder than those induced by persistent obesity. The liver transcriptomic profile varied between mice with weight regain after weight loss and those with persistent obesity. During the process of weight fluctuation, E4bp4 and Cyp3a11 undergone simultaneous changes in the mouse liver. The E4bp4-Cyp3a11 axis may responsible for alleviating lipid droplet accumulation, weight loss and preventing weight rebound via mediating lipid metabolism in the liver. In the future, our findings should be validated by counterfactual experiments and a long-term observation. The underlying mechanism of the E4bp4-Cyp3a11 axis in regulating lipid metabolism and weight management requires a thorough analysis as well.
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