Inflammation is the initiator of RF, and persistent chronic inflammation is considered a hallmark feature of CKD (Meng, 2019). In response to pathogenic factors, damaged renal TECs recruit inflammatory cells to the renal interstitial region, and these inflammatory cells then produce large amounts of proinflammatory and profibrotic cytokines (Gewin et al., 2017). Various active ingredients in Cordyceps have anti-inflammatory effects (Phull et al., 2022). They have been shown to protect against liver fibrosis through anti-inflammatory effects (Xu et al., 2021) (Ying-Mei et al., 2020). Cordyceps extract attenuates renal histological changes, reduces Scr and blood urea nitrogen (BUN) levels, and decreases inflammatory factors in acute kidney injury (AKI), including renal ischemia/reperfusion injury (Han et al., 2020) and cisplatin-induced AKI (Deng et al., 2020). In RF related to streptozotocin (STZ), or alloxan monohydrate–induced DN, the active components of Cordyceps, Cordyceps cicadae polysaccharides (CCP) or N⁶-(2-hydroxyethyl) adenosine (HEA), are effective in reducing the expression of proinflammatory factors TNF-α, IL-1β, and IL-6 in serum and kidney and in reducing the expression of interstitial fibrosis-associated proteins α-SMA and collagen I (Wang et al., 2019; Yang et al., 2020). In vitro, CCP also reduces the release of lipopolysaccharide-induced proinflammatory factors and TGF-β-induced activation of fibroblasts (Yang et al., 2020). Unilateral ureteral obstruction (UUO) is a classical RF model, and Cordyceps extract HEA reduces fibrosis-associated proteins TGF-β1, α-SMA, collagen I, and fibronectin, and inflammatory factors TNF-α, IL-6, and IL-1β in renal tissue 14 days after UUO (Zheng et al., 2018). Zheng et al.’s (2018) study initially explored the effect of HEA on inflammatory cells and found that HEA blocked the accumulation of M1 macrophages and induced the accumulation of M2 macrophages in the kidney as reflected by the positive detection of the F4/80 antigen. The role of M2 macrophages in fibrosis is controversial (Ricardo et al., 2008) (Lin et al., 2010), but the M1/M2 macrophage ratio is an important stage in the development of RIF. However, this study did not further distinguish between the subtypes of M2.

The abovementioned studies have explored the regulatory mechanism of Cordyceps extract against inflammation in RF by inhibiting the TGF-β1/Smad and TLR4/NF-κB signaling pathways (Zheng et al., 2018; Yang et al., 2020). Sun et al. found that p-TLR4, TLR4, p-NF-κB, NF-κB, IL-1 β, and TNF-α levels were effectively reduced by Cordycepin treatment in human embryonic kidney 293T cells. However, Cordycepin did not reduce the levels of these molecules when TLR4 was silenced. These results suggest that Cordycepin may affect the NF-κB signaling pathway through TLR4 (Sun et al., 2019). TLRs/NF-κB activate the downstream inflammasome NOD-like receptor family pyrin domain containing 3 (NLRP3) (Xue et al., 2019), which promotes the maturation of proinflammatory factors IL-1β and IL-18 by activating Caspase-1 (Liston and Masters, 2017). Multiple studies have shown that the NLRP3 inflammasome and its downstream pyroptosis and inflammation play an important role in the development of RF (Ma et al., 2022) (Ram et al., 2022; Wang et al., 2022). It has been shown that Ophiocordyceps sinensis causes inhibition of mRNA and protein expression of NLRP3 inflammasome and downstream effectors IL-1β and IL-18 in a rat model of DN (Wang et al., 2018). Thus, Cordyceps and related products are effective in relieving inflammation in RF through multiple mechanisms.

The kidney is a highly metabolically active organ with mitochondria rich in oxidative reactions and susceptible to oxidative stress damage. Oxidative stress and inflammation interact to play a key role in renal tissue destruction, irreversible loss of renal function, and progression of RF (Xu et al., 2015; Darby and Hewitson, 2016; Richter and Kietzmann, 2016). Normal cells produce small amounts of reactive oxygen species (ROS), which play an important physiological role. Free radical–scavenging enzymes and antioxidants maintain oxygen metabolism homeostasis by activating transcription factors, regulating physiologically active substances and inflammatory immunity, and promoting cell proliferation and differentiation. Oxidative stress occurs when the balance between ROS and reactive nitrogen species and the antioxidant defense system is disrupted, that is, when the production of pro-oxidants or ROS exceeds the endogenous antioxidant capacity (Sies et al., 2017). It has been shown that oxidative stress promotes the progression of RF (Aranda-Rivera et al., 2021); if oxidative stress is suppressed, RF can be attenuated (Liao et al., 2022; Lo et al., 2022). Antioxidant enzymes, such as superoxide dismutase (SOD), catalase, and glutathione peroxidase (GSH-Px), protect cells from damage caused by oxygen free radicals. Malondialdehyde (MDA) is one of the products of the reaction between lipids and oxygen free radicals, and it accumulates during oxidative stress. Cordyceps improves the redox properties of CKD by affecting the levels of NO, SOD, and MDA in serum. In rat models of DN (Liu et al., 2016; Wang et al., 2019) and membranous nephropathy (Song et al., 2016), both C. militaris and HEA showed excellent ability to attenuate oxidative stress, elevated SOD and GSH, and decreased MDA levels. Using diclofenac or meloxicam to induce oxidative stress, HEA intervention significantly reduced the level of ROS in human proximal tubular cells (HK-2) cells (Chyau et al., 2021). In rats with adenine-induced chronic renal failure, Cordyceps cicadae and Paecilomyces cicadae effectively reduced serum urea and creatinine levels, improved renal histopathology, inhibited oxidative stress, and enhanced antioxidant capacity (Li et al., 2019). In a CKD mouse model established by adenine gavage, Ophiocordyceps lanpingensis polysaccharides elevated SOD and GSH-PX, decreased ROS and MDA, improved histopathological staining, and decreased fibrosis-related proteins TGF-β1, α-SMA, and collagen I (Zhou et al., 2021). Cordyceps and related products have a favorable ability to attenuate oxidative stress, but the mechanisms involved are not well understood.

A variety of kidney injury factors may trigger apoptosis, including ischemia/reperfusion injury (Xu et al., 2019), cisplatin-induced kidney injury (Yang et al., 2018), and DN (Peng et al., 2015). Apoptosis (a programmed cell death) of glomerular ECs, podocytes, and TECs is closely associated with RF (Thomas et al., 1998; Docherty et al., 2006). Inhibition of apoptosis as a therapeutic target can alleviate fibrosis (Chen et al., 2021; Xia et al., 2021; Park et al., 2022). Cordyceps downregulates apoptosis in ischemia/reperfusion injury and Cyclosporine A–induced renal tubular dysfunction (Shahed et al., 2001; Chyau et al., 2014). Apoptosis is also considered an important mechanism in contrast-induced nephropathy (CIN). C. sinensis prevents CIN in diabetic rats by decreasing the expression of apoptosis-related proteins Caspase-3 and Bax and increasing the expression of antiapoptotic protein Bcl-2. Mechanistically, C. sinensis decreases the expression of JNK protein and increases the expression of ERK protein (Zhao et al., 2018). Renal tubular interstitial fibrosis is a typical pathological feature of hypertensive kidney injury. In a spontaneous hypertension rat model, C. cicadae reduces interstitial fiber deposition, α-SMA expression, apoptosis, and Caspase-3 activity by regulating the SIRT1/p53 signaling pathway (Huang et al., 2020). In vitro, the major renal damage caused by hepatitis B virus infection is hepatitis B virus X (HBx)–induced apoptosis of renal TECs, which is related to the increased Caspase-3 and Caspase-9 activity and increased PI3K/Akt pathway activity. C. sinensis attenuates all of these HBx-induced responses, at least in part by inhibiting the PI3K/Akt signaling pathway (He et al., 2020). Cordyceps and related products have been shown to inhibit apoptosis in vivo and in vitro, but the mechanisms involved have not been unified.

Autophagy is a “self-consuming” cell death pathway that degrades most cytoplasmic components by forming autophagosomes and autolysosomes (Yang and Klionsky, 2010). Basal autophagy in the kidney is critical for maintaining renal homeostasis, structure, and function. Namely, basal autophagy removes potentially dysfunctional organelles and long-lived proteins to maintain cellular homeostasis. In response to environmental and intracellular stress, autophagy may serve as an adaptive response to ensure cell survival. However, autophagy may also play a role in cellular dysfunction and organ lesions (Tang et al., 2020), such as AKI (Kaushal and Shah, 2016), AKI-CKD (Baisantry et al., 2016), and DN (Lenoir et al., 2015). The role of autophagy in RF is controversial. It has been reported that sustained activation of autophagy in renal tubular cells promotes interstitial fibrosis, and inhibition of autophagy with chloroquine or selective deletion of Atg7 in proximal tubules alleviates RF in UUO mice (Livingston et al., 2016). Several studies have also reported antifibrotic effects of autophagy, whereas inhibition of autophagy by 3-methyladenine exacerbated RF in a UUO rat model, suggesting that autophagy may exert antifibrotic effects by attenuating renal tubular cell injury (Kim et al., 2012). In STZ-induced DN mice, C. militaris polysaccharides (CMP) decrease collagen deposition in the kidney and increase the rate of autophagy; promote the expression of autophagy-specific protein Atg5, Beclin1, and LC3; and decrease the expression of p62 protein in the kidney. This suggests that CMP administration significantly enhances autophagy (Chen et al., 2019a). In a hypertensive nephropathy model, Cordyceps cicadae may attenuate the expression of fibrosis-associated proteins. It ameliorates RF induced by hypertension by downregulating autophagy-related proteins LC3II and Beclin-1, thereby significantly inhibiting autophagic vesicles and attenuating autophagic stress. This effect may be attributed to the regulation of SIRT1 pathway-mediated autophagic stress and is achieved by modulating the autophagy regulator forkhead box class O3a (FOXO3a) and oxidative stress (Cai et al., 2021). The regulatory effect of Cordyceps on autophagy is indisputable, but autophagy itself is diverse and can be divided into microautophagy, chaperone-mediated autophagy, and macroautophagy, and macroautophagy can be divided into nonselective autophagy and selective autophagy depending on whether the substrate is selective or not. Because autophagy is environment dependent and may play different roles at different stages of the disease, different cell types do not respond consistently to the autophagy activation, resulting in a bidirectional effect of cellular autophagy on the process of RF. The regulatory role of autophagy in fibrosis remains to be further investigated.

Aberrant activation and proliferation of fibroblasts are believed to be key causes of the progression of RF (Zhou et al., 2017). Activated fibroblasts, which promote the production and release of ECM collagens I, III, and IV and fibronectin (Sun et al., 2016), are the main source of ECM during scar tissue formation. Myofibroblasts in RF can be derived from mesenchymal fibroblasts, bone marrow–derived fibroblasts, renal TECs, ECs, pericytes, and macrophages. TECs can produce stroma through the EMT to fibroblasts and myofibroblasts (Yan et al., 2021). In this process, TGF-β is the main driving factor. TGF-β induces fibroblast activation and proliferation as well as excessive synthesis and accumulation of ECM through downstream signaling pathways such as SMAD, BMMP-7, and CTGF, thereby promoting fibrosis. Targeted inhibition of TGF-β and its downstream signaling pathways delay fibrosis (Meng et al., 2016; Walton et al., 2017; Ma and Meng, 2019). In STZ-induced diabetic mice and high glucose–exposed HK-2 cells, the nucleoside/nucleobase–rich extract from Cordyceps increased E-cadherin expression; decreased α-SMA, fibronectin, and collagen I expression; inhibited EMT and ECM deposition; and improved the fibrotic morphology of tissues and cells. Cordyceps extract effectively inhibited the phosphorylation of p38 and ERK, did not affect the phosphorylation level of JNK, and had a synergistic effect on fibrosis with p38 or ERK inhibitors (Dong et al., 2019). Ergosterol peroxide from Cordyceps cicadae improves TGF-β1-induced renal fibroblast activation via the mitogen-activated protein kinases signaling pathway (Zhu et al., 2014). In DN C57BL/6J mice and 5/6 nephrectomy rats, C. militaris (Yu et al., 2016) and C. sinensis (Pan et al., 2013) counteracted RF and reduced the expression of fibrosis-associated proteins by inhibiting the TGF-β1 pathway. Further exploring downstream mechanisms, 3′-deoxyadenosine interfered with TGF-β and bone morphogenetic protein signaling by downregulating Smads at the transcriptional level. In that way, it induced decreases in collagen I and α-SMA, mesenchymal myofibroblast number, and fibrotic area (Gu et al., 2013). Cordyceps and its extracts are potential therapeutic strategies against fibrosis via the TGF-β/Smad and other pathways.

In addition to the above mechanisms, C. sinensis attenuates RF in the UUO model by inhibiting Bcl-2-associated athanogene 3 (BAG3), which has been reported to be involved in cell proliferation, apoptosis, adhesion and migration, and EMT processes (Du et al., 2015).

Recent studies have reported that gut microbiota mediates RF (Liu et al., 2021). CMP reduce renal injury by regulating intestinal flora imbalance (Song and Zhu, 2020). In a study of CCP to attenuate fibrosis in DN, high-throughput pyrophosphate sequencing of 16S rRNA indicated that CCP regulated dysbiosis of the intestinal flora by increasing the relative abundance and proliferation of probiotic bacteria, thereby exerting a beneficial effect on tubulointerstitial fibrosis in DN rats (Yang et al., 2020).

Normal energy metabolism is particularly important for maintaining the structure and function of the kidney, and metabolic reprogramming is generally considered a key process in activating fibrosis in different organs, as it is in the kidney (Barcena-Varela et al., 2021). The balance of fatty acid oxidation and glycolysis directly affects ECM production and degradation or indirectly affects RF through inflammation and hypoxia (Zhu et al., 2021). Metabolomic analysis has been used to explore the metabolic regulatory effects of C. sinensis in a db/db diabetic mice model. Hirsutella sinensis—the anamorph of the traditional Chinese medicine C. sinensis—attenuates RF, ameliorates system-wide disorders of glucolipid metabolism and amino acid deficiency, and regulates excessive activation energy and nucleotide metabolism (Lu et al., 2019). This suggests that Cordyceps may have a potential role in the regulation of renal metabolism and, consequently, may ameliorate fibrosis.