REN and renal fibrosis: Numerous studies have demonstrated that renal fibrosis was associated with the dysbiosis or dysregulation of gut microbiota, non-coding RNAs, renin–angiotensin system, aryl hydrocarbon receptor, IƙB/NF-ƙB, Keap1/Nrf2, TGF-β/Smad, and Wnt/β-catenin signaling pathways (Ma et al., 2018a; Chen et al., 2019b; Garg and Maurya, 2019; Zhao et al., 2019; Hu et al., 2020a; Miao et al., 2021b; Wang et al., 2021d; Wu et al., 2021; Zhou et al., 2021) as well as metabolite disorders including tryptophan metabolism and lipid metabolism (Zhao, 2013a; Zhao et al., 2015; Wang et al., 2019; Liu et al., 2021a).