OAT beta-glucan and Lactobacillus rhamnosus GG regulated fatty acid metabolism in mice fed a high-fat diet and demonstrated anti-inflammatory and antioxidant potentials

Abstract

Oat beta-glucan (OAT) and Lactobacillus rhamnosus GG (LGG) are well-known pre- and probiotic supplements that regulate host metabolic health against high-fat diet (HFD) by maintaining the bacteria that could produce health-benefiting metabolites such as short-chain fatty acids (SCFA). Current studies analyze primarily the fecal SCFA, but it is their content in the tissues that could help to elucidate their metabolism and regulatory roles in various tissues. In addition, the effect of OAT and LGG on the composition and peroxidation of polyunsaturated fatty acid (PUFA), which are associated with inflammation and oxidative stress, has not been well-studied. It is thus worthwhile to investigate the regulatory effects of LGG and OAT on the profiles of SCFA, PUFA and their oxidized products. Therefore, this study highlights a new approach to investigate the regulatory roles of LGG and OAT on inflammation and oxidative status by examining the fatty acid profiles. First, this study modified and validated the pre-existing SCFA extraction method for the extraction from tissue samples, namely the caecum, liver, and brain tissues. This newly adapted method was used to investigate the SCFA-regulating effects of OAT and LGG in HFD-fed mice. The results showed that HFD significantly reduced SCFA levels along the gut-liver-brain axis. However, acetate production in the caecum was regulated when LGG was supplemented. In addition, both LGG and OAT upregulated the levels of acetate and propionate in the liver, suggesting the attenuation of de novo lipogenesis and gluconeogenesis, and hence their potentials to manage metabolic disorders and non-alcoholic fatty liver disease (NAFLD). Secondly, this study proceeded to investigate the regulatory roles of OAT and LGG on the metabolism of PUFA and lipid mediators. HFD significantly suppressed PUFA content in the caecum and the liver while elevating the n-6/n-3 PUFA ratios. In addition, HFD increased pro-inflammatory lipid mediators in the caecum and the brain by activating the cytochrome P450 (CYP) and lipoxygenase (LOX), respectively. Furthermore, HFD increased non-enzymatic peroxidation of docosahexaenoic acid (DHA), which indicates the induction of oxidative stress in the brain. LGG and OAT supplementations reinstated PUFA metabolism in the caecum and displayed homeostatic control on the n-6/n-3 ratios along the gut-liver-brain axis. They also selectively upregulated 4-F4t-neuroprostane and phytoprostanes, which are bioactive lipid mediators that show anti-inflammatory and antioxidant properties. LGG alone activated LOX in the liver as a way of compensating HFD-induced loss of LOX-dependent lipid mediators. Finally, LGG and SCFA (the fermentative end products of oat beta-glucan in the gut) also synergistically regulated the mRNA expressions in the Nrf2-KEAP1-ARE pathway in the lipopolysaccharide (LPS)-challenged Caco-2 cells, which demonstrates antioxidant capacity. The normalized mRNA expression of Nrf2 did not quench the overexpression of the RelA, indicating no crosstalk between Nrf2-NFκB pathways. The LPS-induced suppression of NFKB1 was counteracted by SCFA, which led to the normalization of the downstream markers. The combined result of this study suggests that LGG and OAT to some extent, are capable of regulating SCFA and PUFA metabolism, thereby demonstrating their anti-inflammatory and antioxidant potentials against prolonged intake of HFD.