Our earlier studies found that dietary BPA exposure at TDI level induced hepatic steatosis in CD-1 mice through causing intestinal flora disorder and consequently activating the gut-liver axis. Based on our previous research, we further explored the protective effect of curcumin on BPA-mediated hepatic steatosis. Our current results indicated that curcumin supplementation effectively inhibited BPA-induced hepatic steatosis through improving intestinal flora disorder, and then reducing serum LPS levels and down-regulating the expression of lipogenic genes in the liver. Our study provided a new intervention approach for curcumin as an effective dietary nutrient to prevent BPA-mediated liver steatosis.
As a traditional medicine materials and common food spice, curcumin is often used to prevent and treat various diseases. Many studies have demonstrated that curcumin possesses anti-inflammatory, antioxidant and cholesterol-lowering effects[17–19], and has therapeutic potential for metabolic diseases such as type 2 diabetes, cardiovascular diseases and NAFLD[21, 22]. For example, our previous study showed that curcumin reduced serum and hepatic TG and TC levels, and attenuated liver lipid deposition in mice. We also found that supplementation with curcumin inhibited intestinal cholesterol absorption and lowered serum cholesterol levels in HFD-fed hamsters, and prevented HFD-induced atherosclerosis in apolipoprotein E knockout mice[17, 18]. A double-blind, placebo-controlled clinical trial showed that short-term administration of curcumin improved serum TG and HDL-C levels, liver transaminases and steatosis index in overweight subjects with impaired fasting plasma glucose. In addition, administration of curcumin has been shown to regulate the abundance of gut microbial communities including Bacteroidetes and Rikenellaceae in HFD-fed rats. With the deepening of research, studies found that curcumin can resist the damage caused by BPA. Tandon et al. demonstrated that curcumin protected against BPA-induced neurotoxicity and behavioural deficits through up-regulating the Notch signaling pathway in rats. Curcumin had also been found to alleviate BPA-induced insulin resistance via inhibition of the JNK/p38 signaling pathway in HepG2 cells. Consistent with previous research, our current study revealed that curcumin supplementation substantially attenuated BPA-induced lipid metabolism disorder and hepatic steatosis in CD-1 mice.
As a complex ecosystem, intestinal microbiota is closely associated with energy and lipid metabolism, and play a critical role in the onset and development of NAFLD. In this study, we observed an altered microbial composition induced by BPA. BPA exposure for up to 6 months shifted the structure of gut microbiota in CD-1 mice, including the microbial richness, diversity, and composition, which was markedly reversed by curcumin treatment. Bacteroidetes and Firmicutes are the leading genus of intestinal flora, and are considered to be related to lipid absorption and metabolism. Animal experiment indicated that the gut microbiota in obese male C57 BL/6 mice exhibited a lower Bacteroidetes/Firmicutes ratio compared with normal-weight individuals. Acupuncture protocol was found to improve obesity-related dyslipidemia through increasing the ratio of Bacteroidetes/Firmicutes and promoting the recovery of Akkermansia abundance in the gut microbiome. Likewise, in this study, we observed that the ratio of Bacteroidetes/Firmicutes was remarkably reduced in BPA-exposed mice and was significantly elevated in curcumin-treated mice, indicating curcumin supplementation reversed the decreased Bacteroidetes/Firmicutes ratio induced by BPA.
Moreover, we also found that BPA had a strong inhibitory effect on the flora of Verrucomicrobia and Akkermansia, while curcumin was shown to significantly increase the abundance of these flora. Akkermansia is the only representative of the Verrucomicrobia phylum and is considered as a promising probiotic owing to its health-promoting properties. Animal study demonstrated that Akkermansia administration reduced major adipose tissues weight, adipogenesis and serum TC levels, and improved the liver function, metabolic dysregulation and obesity in HFD-fed mice. Akkermansia was also found to up-regulate the expression of hepatic LDL receptor and alleviate hyperlipidemia in CREHB gene deleted mice . Additionally, Rikenella, a harmful bacteria belonging to Bacteroidetes phylum, was reported to be enriched in diabetic db/db mice, hyperlipidemia mice and diet-induced obese mice . We also found that the relative abundance of Rikenella is much higher in BPA-exposed mice than that in control mice, however, the growth of Rikenella was restrained in curcumin-treated mice. Collectively, curcumin effectively modulated the changes of intestinal flora composition induced by BPA, suppressing the growth of harmful flora and promoting the growth of beneficial flora and then improving dyslipidemia.
The imbalance of intestinal flora can lead to the increase of LPS. It was reported that the relative abundance of Bacteroidetes and the concentration of fecal and plasma LPS in preeclampsia patients were higher than those in healthy control group. Furthermore, oral administration of Akkermansia to C57BL/6 mice reduced blood LPS concentration and prevented obesity and abnormal glucose metabolism . Fecal microbiota transplantation of Akkermansia reduced serum and colon LPS levels in male C57BL/6J mice. Similarly, our current study showed that the relative abundance of Bacteroidetes and Firmicutes was increased and the relative abundance of Akkermansia was decreased in BPA-exposed mice, which was correlated with the increased serum LPS levels. LPS can pass through the intestinal mucosa with the help of chylomicrons and be transported to the liver through lipoproteins. The increase of LPS can stimulate lipogenic gene expression in the liver. Research demonstrated that LPS induced liver LXRα expression in rats, leading to the lipid metabolism disorder. Liver lipogenic gene SREBP-1c was also found to be activated by LPS. Consistent with the activation of hepatic SREBP-1c, liver ACC1 and ACC2 were markedly up-regulated in LPS-treated mice [12, 13].
The liver is the central organ of lipid metabolism and hepatic lipid biosynthesis is transcriptionally regulated by LXRα, SREBP-1c, ACC1 and ACC2. LXRα is a transcription factor that can stimulate SREBP-1c expression by binding to SREBP-1c promoter and lead to liver steatosis. SREBP-1c is a key lipogenic transcription factor that can activate ACC, and is dedicated to fatty acid uptake and triglyceride synthesis. ACC is the first enzyme in liver de novo lipogenesis (DNL) pathway. There are two isozymes of ACC1 and ACC2 in mammals. Combined inhibition of ACC1 and ACC2 results in DNL reduction, leading to decreased TG in liver and substantially improve hepatic steatosis. The activation of hepatic lipogenic pathway is a critical metabolic change required for hepatic steatosis formation. Our present study found that dietary BPA exposure induced obvious liver fat accumulation and hepatic steatosis, which was accompanied by increased serum LPS levels and up-regulation of hepatic LXRα, SREBP-1c, ACC1, ACC2. In accordance with our results, Marmugi et al. also found that low doses of BPA exposure induced gene expression related to lipid synthesis including LXRα, SREBP-1c and ACC1 in adult mice liver. However, curcumin treatment significantly suppressed the expression of hepatic LXRα, SREBP-1c, ACC1 and ACC2 and reduced liver fat accumulation induced by BPA.