Diabetic nephropathy (DN) is one of the most dangerous microvascular complications in patients with diabetic metabolic disorders. DN is characterized by persistent proteinuria and progressive decline in renal function, and the main pathological changes are reduction of podocytes, deposition of extracellular matrix, and diffuse thickening of the glomerular basement membrane (Tervaert et al., 2010). Among them, glomerular fibrosis is the most important hallmark of DN progression. The activation of the transforming growth factor-β (TGF-β)/Smad2/3 signaling pathway, which regulates the secretion of collagen IV (COL4) and fibronectin (FN) by mesangial cells, is considered to be the central mechanism of glomerular fibrosis in DN (Sierra-Mondragon et al., 2018; Wada and Makino, 2016). However, the activation mechanism of the signaling has not been fully elucidated in DN. Previous studies have suggested a close association of DN with inflammatory response, oxidative stress, hyperglycemia, and hypertension (Gnudi et al., 2016; Magee et al., 2017; Yaribeygi et al., 2019), implying that DN may be the result of multiple factors acting together, and there may be other factors involved in the progression of DN. Recently years, it has been reported that high blood lipids and blood glucose are the important characteristics of type 2 diabetic mellitus (T2DM). More and more studies have suggested that the DN may be a result of high blood lipids and glucose interaction (Luo et al., 2021; Pan et al., 2018). However, the mechanisms of high blood lipids and glucose in promoting glomerular fibrosis and deteriorating DN are still not completely clear, and there are still no potential targets and effective drugs for improving DN of T2DM.

Transient receptor potential cation channels (TRPCs) are non-selective cation channels expressed in a variety of cell types, which play important physiological roles in regulating smooth muscle contraction, lung endothelial permeability, renal structure, and function (Dietrich and Gudermann, 2014). Its family members include TRPC1-7, of which TRPC6 is predominantly expressed in renal cells and is more sensitive to diacylglycerol (DAG), metabolic and biosynthetic intermediates of some lipid classes (Eder, 2017; Kotapati and Bates, 2021). The nuclear factor of activated T cells (NFATs), important nuclear transcription factors, are downstream of the TRPC6. NFAT family contains five members, among which the transcriptional activity of NFAT1-4 is regulated by calcium-regulated phosphatase of calcineurin (CN). In the resting state, the NFAT is phosphorylated and localized in the cytoplasm with an inactive state. When Ca2+ inward flow increases intracellular calcium levels, which activates CN to dephosphorylate NFAT, leading to the nuclear entry of NFAT to regulate the expression of target genes (Mognol et al., 2016). The NFAT2 is wildly expressed in kidneys and is closely correlated with podocyte injuries in DN. The 11R-VIVIT, an inhibitor of NFAT, is reported to be a therapeutic strategy for the treatment of DN (Zhang et al., 2013). The TRPC6 inhibitor (BI 749327) downregulates the expression of pro-hypertrophic genes in isolated cardiomyocytes by inhibiting NFAT activation (Lin et al., 2019). Unilateral ureteral obstruction significantly increases TRPC6 expression, leading to significant interstitial fibrosis and kidney injury in mice (Wu et al., 2017). Our recent studies also indicated that palmitate (PA) treatment could activate NFAT2 through increasing TRPC6 expression, resulting in human glomerular mesangial cell fibrosis in vitro (Su et al., 2020). However, whether regulation of TRPC6/NFAT2 signaling attenuates renal fibrosis in DN has not been fully elucidated.

Panax ginseng C. A. Meyer (ginseng) is a perennial herb belonging to the family Araliaceae (Liu et al., 2020), and its traditional effects are to enhance the immunity of the body, soothe the nerves, tonify the vital energy and enhance blood circulation, which has excellent modulating effects on the nervous system, cardiovascular system, immune system and many other body systems (Smith et al., 2014). Ginseng has been extensively used worldwide as an adjunctive treatment for a variety of diseases, including diabetes (Chen et al., 2019; Mancuso and Santangelo, 2017) and diabetic nephropathy (Bai et al., 2018; Kang et al., 2006, 2013). Initially, ginseng was found to be applied to the glycemic control of diabetic patients, and the mechanism may be associated with the amelioration of insulin sensitivity in patients (Sotaniemi et al., 1995). With in-depth studies, it was also revealed that it has a promising suppression on the development of diabetic nephropathy. Several types of ginseng have been reported to ameliorate the pathological damage, inflammation, and fibrosis of the kidney in diabetic nephropathy (Karunasagara et al., 2020; Liu et al., 2021; Sen et al., 2012), while its specific medicinal active ingredients and mechanisms are still unclear. Ginsenoside Rg1 (Rg1) is one of the main active components in ginseng. Modern pharmacological research suggested that Rg1 has multiple pharmacological effects such as regulating cell proliferation, differentiation, and regeneration, as well as anti-inflammatory and anti-apoptotic effects (Kim et al., 2017). Increasing studies reported that Rg1 might exert renal protective effects in diabetic nephropathy through the regulation of oxidative stress, autophagy, or inflammation (Du et al., 2018; Shi et al., 2020; Wang et al., 2020a). Our previous study showed that Rg1 could inhibit ROS-mediated dysregulation of calcium homeostasis to attenuate brain ischemia-reperfusion injury in mice (Han et al., 2021). In this study, we hypothesized that Rg1 may protect against glomerular fibrosis in DN by inhibiting the TRPC6/NFAT2 signaling pathway, resulting in the downregulation of TGF-β/Smad2/3 in T2DM mice. In the study, we mainly investigated the effect and mechanism of Rg1 treatment in T2DM-induced renal pathological injury and glomerular fibrosis in mice. Our study may provide new targets and drug options for the treatment of DN.