Application of Herbal Traditional Chinese Medicine in the Treatment of Acute Kidney Injury

Acute kidney injury (AKI) is a clinical syndrome characterized by a rapid loss of renal function, which may further develop into chronic kidney damage (CKD) or even end-stage renal disease (ESRD). AKI is a global health problem associated with high morbidity and costly treatments, and there is no specific or effective strategy to treat AKI. In recent years, Traditional Chinese Medicine (TCM) has attracted more attention, with lines of evidence showing that application of TCM improved AKI, and the mechanisms of action for some TCMs have been well illustrated. However, reviews summarizing the progress in this field are still lacking. In this paper, we reviewed TCM preparations and TCM monomers in the treatment of AKI over the last 10 years, describing their renal protective effects and mechanisms of action, including alleviating inflammation, programmed cell death, necrosis, and reactive oxygen species. By focusing on the mechanisms of TCMs to improve renal function, we provide effective complementary evidence to promote the development of TCMs to treat AKI. Moreover, we also summarized TCMs with nephrotoxicity, which provides a more comprehensive understanding of TCMs in the treatment of AKI. This review may provide a theoretical basis for the clinical application of TCMs in the future.


INTRODUCTION
Acute kidney injury (AKI), characterized by an abrupt decline of renal function, can be induced by numerous causes including renal ischemia reperfusion injury (IRI), nephrotoxic insults and infection of sepsis (Waikar et al., 2008;Linkermann, 2016). Accumulating evidence shows that AKI is a global public health concern and a pivotal threat to human health, especially in hospitalized patients, as it impacts more than 13 million patients per year (Chertow et al., 2005;Lameire et al., 2013;Thomas et al., 2015;Yang et al., 2015;Allison, 2016). Excessive inflammatory responses, oxidative stress, and the imbalance of the damage and repair of renal tubules, are all highly involved in the pathological process of AKI, however, specific targets and effective therapies are still lacking (Sancho-Martinez et al., 2015;Yang et al., 2016;Zuk and Bonventre, 2016).
Traditional Chinese medicine (TCM) has been widely used for the treatment of AKI and its complications in China and neighboring countries, including Japan and Korea, for a long time. Some TCM-based therapies show good results and high efficacy in inhibiting inflammatory responses, programmed cell death and oxidative stress. In this regard, the therapeutic effects of TCMs have widely been tested in animal models of AKI and even in patients. For instance, the Xuebijing Injection is effective in improving clinical symptoms of sepsis-induced AKI patients after the Wenchuan Earthquake (Yuxi et al., 2017). Our recent study showed that wogonin not only protects cisplatin-induced AKI, but also preserves and even promotes the anti-tumor effect of cisplatin (Meng et al., 2018). However, it is noteworthy that some TCMs, such as aristolochic acids and other plant alkaloids, are nephrotoxic, (Yang et al., 2018). So, the application of TCM should be carefully evaluated.
In this paper, we reviewed the therapeutic effects of TCMs on AKI and the mechanism of action based on the assessment of evidence that supports hypotheses, additionally, TCMs with nephrotoxicity have also been discussed.

APPLICATIONS OF TCM IN AKI TCM Preparations in AKI
Until now, several TCM preparations have been tested in the treatment of AKI. There are shown in Table 1.

Astragalus membranaceus var. mongholicus and Angelica sinensis (A&A)
Decoctions of roots from A&A can improve renal blood flow in a murine model of acute ischemic renal injury, possibly by increasing NO production by activating eNOS and scavenging ROS, therefore accelerating renal repair after ischemic injury (Meng et al., 2007). Moreover, the therapeutic effect of A&A may be JNK-dependent (Cai et al., 2001).

Xuebijing Injection
It consists of chuan dome, radix paeoniae rubra, safflower, Salvia miltiorrhiza, and Chinese angelica. Administration of a Xuebijing injection can suppress the production and release of high mobility group box-1 protein (HMGB1) in the kidney, thereby alleviating serious scald injury-induced AKI . In addition, an intravenous injection of Xuebijing attenuates the inflammatory response in AKI rats with paraquat poisoning . Importantly, Xuebijing improved the clinical symptoms of patients with sepsis-induced AKI after the Wenchuan Earthquake (Yuxi et al., 2017).

TCM Monomers in AKI
Compared with TCM preparations, TCM Monomers have recently attracted more attention in the treatment of diseases because they have certain molecular structures, clear mechanisms of action, predicted pharmacological effects and less drugdrug interactions. In the kidney, numerous TCM monomers have been applied in treating renal diseases including AKI caused by different stimuli. Therefore, we list TCMs that have comprehensively been studied to protect against AKI in recent years.

Astaxanthin (ATX)
Astaxanthin (ATX) is a natural carotenoid extracted from marine organisms which are widely applied because of their strong antioxidant effect. Current studies demonstrate the renoprotective effects of ATX in many AKI models. ATX (5 mg/kg for 14 days via oral gavage) can improve I/Rinduced AKI by exerting antioxidant activity and inhibiting tubular apoptosis/necrosis via scavenging free radicals . Moreover, ATX (40 mg/kg for 5 days by intraperitoneal injection) attenuated arsenic-induced AKI by fulfilling antioxidant functions and reducing As accumulation , and ATX (50 mg/kg for 12 h by gavage) ameliorated HgCl 2 -induecd AKI by exerting anti-oxidant activity and preventing lipid and protein oxidation (Augusti et al., 2008). ATX (20 mg/kg 12 h via tail intravenous injection) consistently improved early AKI, following a severe burn, by modulating antioxidant activity and Akt/Bad/Caspases-mediated mitochondrial-apoptotic pathway (Guo et al., 2015).

Baicalin
Baicalin is a Scutellaria baicalensis-derived flavonoid which has been tested in multiple types of AKI models. In clinical Gentamicin-induced renal injury Attenuating apoptosis By limiting caspase-3 activation (Hsu et al., 2014) trials, Baicalin protects against AKI in pediatric sepsis by inhibiting renal cell apoptosis . In ischemiareperfusion injured kidney, Baicalin (10 µmol/L for 24 h) exerts protective effects by inhibiting TLR2/4-mediated inflammation and mitochondrial stress-induced apoptosis of tubular epithelial cells (Ji et al., 2014). Administration of Baicalin (100 µmol/L) in HK-2 cells consistently reduced H 2 O 2 -induced cytotoxicity by activating downstream Nrf2 signaling and attenuating ER stress . These findings are supported by recent findings that Baicalin (50 mg/kg, i.p. for 2 weeks) prevents Lead (Pb)-induced renal injury and pediatric sepsis-induced AKI by blocking oxidative stress and apoptosis Zhu et al., 2016). Moreover, it is of note that baicalin is a novel PPARγ activator which may suppress NF-κB-mediated inflammation effectively (Lim et al., 2012).

Cordyceps sinensis (CS)
Cordyceps sinensis (CS) is used as a tonic food which is derived from an entomogenous fungus (Zhu et al., 1998). CS (5 g/kg via intragastric for 2 days) improves the outcome of I/Rinduced AKI via different mechanisms including modulating SDF-1/CXCR4-signaling axis , up-regulating expression of HIF-1α, down-regulating the expression of NGAL (Yu et al., 2012) and reducing the expression level of TLR-4 (Zhou and Hu, 2010). Moreover, treatment of CS (1.5 mg/200 µl) significantly alleviates stress responses and tissue damage by reducing autophagy and apoptosis in LPS-induced AKI (Wu et al., 2011). Additionally, CSP, as the mycelia glycoproteins of Cordyceps sobolifera, significantly suppresses cyclosporine A (CsA)-induced apoptosis and protects against nephron loss via increasing magnesium reabsorption (Chyau et al., 2014).

Epigallocatechin Gallate (EGCG)
As a major component of green tea, EGCG is famous for its anti-inflammatory and anti-apoptotic properties. EGCG, as a potent inducer of HO-1, can suppress renal injury by reducing oxidative stress and inflammation in several AKI models induced by contrast (EGCG 20 mg/kg intravenously) , I/R (EGCG 50 mg/kg i.p. for 24 h) (Lv et al., 2015) and cisplatin (EGCG 100 mg/kg i.p. for 12 days) (Sahin et al., 2010), respectively. Furthermore, underlying mechanisms have extensively been explored in cisplatin nephropathy, EGCG (100 mg/kg i.p. for 2 days) prevented activation of ERK, the NF-κB pathway and caspase-12 while down-regulating the Fas-conducted extrinsic pathway and Bcl-2/Bax ratio, thereby reducing the apoptosis of tubular epithelial cells Chen B. et al., 2015;Pan et al., 2015).

Ginsenoside Rd (GSRd)
Ginsenoside Rd (GSRd) is isolated from the root of Panax ginseng and applied to protect cells in various types of diseases especially ischemia diseases . It is noteworthy that GSRd has an impact on different cell types which are involved in AKI. For instance, previous studies identified that GSRd (50 mg/kg i.p. for 2 days) prevented M1 macrophage polarization in I/R-injured kidney . Additionally, GSRd (5 mg/kg i.p. for 30 days) protected proximal tubule cells against I/R model-induced hypoxia-reoxygenation by inhibiting oxygen free radicals from attacking cell membranes (Yokozawa et al., 1998). The renoprotective effect of GSRd (5 mg/kg i.p. for 30 days) was further determined in cisplatin and glycerolinduced AKI models, treatment of GSRd decreased apoptosistriggered DNA fragmentation and oxidative stress (Yokozawa and Liu, 2000;Yokozawa and Dong, 2001;Zhou et al., 2014).

Resveratrol (RSV)
Resveratrol (RSV), a popular natural phenolic compound which is abundant in wines and grape skins, protects against multiple types of AKI due to its low toxicity, powerful antioxidants, and anti-inflammatory properties. Resveratrol (100 mg/kg for 20 h by oral gavage) can attenuate LPS-induced AKI by suppressing inflammation and apoptosis driven by macrophages (Chen L. et al., 2015). Resveratrol (10 mg/kg i.p. for 12 h) consistently protects against sepsis-induced tubular epithelium injury by restoring the renal microcirculation and scavenging reactive nitrogen species (Holthoff et al., 2012). In addition, resveratrol (3 mg/kg for 6 days via the forearm vein) ameliorates arsenic trioxide (As 2 O 3 )-induced nephrotoxicity by antagonizing oxidative stress and facilitating arsenic metabolism (Yu et al., 2013). Moreover, resveratrol is proven to be an anti-inflammatory agent in glycerol (RSV 25 mg/kg/day for 4 days via gastric intubation)-and cisplatin (RSV 25 mg/kg/day i.p. for 2/5 days)induced AKI (de Jesus Soares et al., 2007;Do Amaral et al., 2008). Furthermore, previous studies demonstrated that resveratrolmediated activation of SIRT1 improved cisplatin (RSV 10 mg/kg orally once a day for 7 days)-induced AKI by deacetylating p53 and reducing apoptosis , and RSV also inhibited sepsis (RSV 10 mg/kg i.p. for 3 days)-induced AKI and renal inflammation through NF-κB de-acetylation (Gan et al., 2017) or SIRT3-mediated deacetylation of SOD2 (Xu et al., 2016). Resveratrol (30 mg/kg i.p. for 12 h) protected against early sepsisinduced AKI by inhibiting the endoplasmic reticulum stress (IRE1)-activated NF-κB pathway . A previous study showed that RSVA405 (3 mg/kg i.p. for 24 h) and RSVA314 (3 mg/kg i.p. for 24 h), two biologically active resveratrol analogs (RSVAs), attenuated I/R-induced AKI by exerting antioxidative and anti-inflammatory effects, indicating that RSV and its derivatives may be promising agents to prevent and/or treat AKI with high efficiency (Khader et al., 2015).

Tetramethylpyrazine (TMP)
Tetramethylpyrazine (TMP) is a natural product isolated from the Chinese herb Ligusticum wallichii Franch., which is famous for its antioxidative and anti-inflammatory effects. Previous studies showed that treatment with TMP protects against arsenic (TMP 100 µM for 6 h)-induced nephrotoxicity by targeting HO-1 and ARS2, which was further evidenced by the findings that TMP (20 mg/kg/day i.p. for 7 days) relieves gentamicin-induced AKI by enhancing Hax-1 mitochondrial localization in HO-1-dependent mechanisms (Sue et al., 2009;Gong et al., 2016). Additionally, by suppressing ROS production and the consequential inflammatory response, TMP protected against cisplatin (80 mg/kg/day orally for 7 days) or arsenic (100 µM for 24 h)-induced AKI (Ali et al., 2008;Gong et al., 2015). Moreover, a recent study showed that TMP (80 mg/kg/day i.p. for 4 days) suppressed the apoptosis of renal cells by targeting FoxO1, a pro-apoptotic transcription factor, to prevent contrast-induced AKI (Gong et al., 2013). Interestingly, TMP exerted a renoprotective role by downregulating P-selectin, which has been accepted as a key modulator of neutrophil infiltration in I/R kidney injury (Chen et al., 2003). Our group also tested the therapeutic potential of traditional Chinese medicine in the treatment of AKI. We screened 10 kinds of Chinese herbal medicine with anti-inflammatory effects and found that wogonin and protocatechuic aldehyde had significant therapeutic effects. Wogonin inhibits cisplatininduced renal damage by inhibiting RIPK1-mediated necroptosis and attenuates inflammation (Meng et al., 2018), whereas protocatechuic aldehyde (PA) not only inhibits necroptosis, but also effectively reduces cisplatin-induced over-production of ROS (Gao L. et al., 2016). Interestingly, we all know that cisplatin is commonly used as an anti-cancer drugs in clinic, and these two TCMs could even promote anti-tumor effects of cisplatin, so wogonin and protocatechuic aldehyde may be renoprotective adjuvants for cisplatin-based anticancer therapy.
There are many other TCMs to treat AKI, and these are listed in Table 2.
Mechanisms Involved in the Therapeutic Effect of TCMs in AKI As shown in Figure 1, the TGF-β receptor, Toll-like receptors (TLRs), TNF receptor, and FASL/Death receptors are stimulated by LPS, cisplatin and I/R, etc., then these receptors activate downstream pathways, further triggering ROS production and inflammatory responses, eventually leading to kidney damage. TCMs suppress cisplatin/LPS/I/R-stimulated TLRs including TLR2/4, or by activating PPAR, further inhibiting the NF-κB pathway and reducing inflammation. Moreover, TCMs reduce apoptosis by inhibiting TGF-β, PI3K/AKT and ERK/JNK/P38MARK pathways. In addition, TCMs inhibit autophagy by targeting AMPK/mTOR. In recent years, new cell death mechanisms like programmed necrosis and ferroptosis have also attracted attention. Wogonin and protocatechuic aldehyde can effectively inhibit RIPK1/RIPK3-mediated necroptosis. Breviscapine can reduce ferroptosis by increasing glutathione peroxidase levels. Additionally, TCMs can inhibit H 2 O 2 -induced endoplasmic reticulum (ER) stress and further reduce ROS production. In the AKI model, ROS, HMGB1, P53, Nrf2, HO-1, and SIRT1/3 are regarded as potential therapeutic targets of TCMs.

Nephrotoxicity of TCMs
It has been recorded that up to 25% of all cases of AKI may be correlated to nephrotoxic medications (Bentley et al., 2010). As shown in Table 3, previous studies indicate that some TCMs, including aristolochic acid, anthraquinones, flavonoids, and glycosides from herbs, non-steroidal anti-inflammatory drugs, aminoglycosides, cytostatic drugs, osmotic agents, radiocontrast, and phosphate salts, may lead to kidney damage and induce AKI (Liangos, 2012;Yang et al., 2018). Several reasons contributing to nephrotoxicity of TCMs include the intrinsic toxicity of herb medicines, incorrect dosing, interactions between herbs and medications, adulteration, incorrect processing and storage, and contamination by heavy metals (Yang et al., 2018).
It was reported that two patients took sciadopitysin, a kind of flavonoid extracted from Taxus celebica to treat diabetes mellitus, and suffered acute tubular necrosis and acute interstitial nephritis (Lin and Ho, 1994). Andrographolide (Chuan Xin Lian) is widely used in China for the treatment of dysentery and respiratory tract infection. It is noteworthy that a systemic analysis, based on clinical cases reported in Chinese literature from January 1978 to August 2013, revealed 26 patients with AKI induced by andrographolide. The major pathologic features in these patients were acute tubular necrosis. The mechanism is still obscure . Aristolochia acids (AA) is   or TLR4 (Zhu et al., 2012) Cisplatin-induced AKI Inhibiting apoptosis and activating autophagy.

Green tea
Contrast-induced AKI Alleviating apoptosis, oxidative stress and inflammation.

Propolis and Alpinia officinarum
Cisplatin-induced AKI Attenuating oxidative stress, inflammation, and cell death.

Ingredient in licorice
LPS-induced renal injury Inhibiting cell apoptosis, oxidative stress.

Ginseng stem and leaves
LPS-induced AKI Inhibiting inflammatory and oxidative stress.
By suppressing programmed cell death (Gong et al., 2015) Contrast-induced AKI Suppressing autophagy and apoptosis.

Scutellaria baicalensis Georgi
Cisplatin-induced AKI Attenuating inflammatory response. By targeting RIPK1-mediated necroptosis (Meng et al., 2018) extracted from Aristolochia fangchi, within which aristolochic acid I (AAI) and aristolochic acid II (AAII) are well known. AA has widely been used as an anti-inflammatory agent. However, a series of studies demonstrated that AA could induce early and transient acute tubular necrosis and progressive tubulointerstitial injury, which finally lead to renal fibrosis (Sato et al., 2004;Debelle et al., 2008;DeBroe, 2012). AA also caused nephropathy, by inducing DNA adduct formation (Allard et al., 2013). Some drugs related with AA are nephrotoxic due to the intrinsic toxicity of herbs and the misidentification of potentially toxic compounds. The root of asarum (also known as Xi Xin) contains low levels of AA and has widely been used as an analgesic for headache, toothache, and inflammatory diseases. But the whole asarum plant contains high levels of AA (Drew et al., 2002). Triptolide, isolated from Tripterygium wilfordii Hook.f (TWHf)-derived diterpenoid, has been commonly used for its immunosuppressive and anti-cancer properties (Carter et al., 2006). However, administration of triptolide may result in severe kidney injury by impairing the antioxidant system, promoting production of reactive oxygen species and inducing apoptosis of tubular epithelial cells, which may limit the application of triptolide in the clinic .

Prevention and Treatment of TCMs-Induced AKI
The first principle of effective therapy is to acknowledge and prevent or minimize nephrotoxicity of TCMs. In this regard, several strategies should be applied: (1) In view of FIGURE 1 | The molecular pathways targeted by the TCMs covered in this review are summarized. TGF-β receptor, Toll-like receptors (TLRs), TNF receptor, and FASL/Death receptors are stimulated by LPS, cisplatin, I/R, etc. These receptors are then activated by the downstream pathway, further triggering ROS production and an inflammatory response, eventually leading to kidney damage. TCMs suppress cisplatin/LPS/I/R-stimulated Toll-like receptors (TLR2/4), or by activating PPAR-γ, further inhibiting the NF-κB pathway and reducing inflammation. Additionally, apart from targeting caspase3/9, TCMs reduce apoptosis by inhibiting the TGF-β receptor, the ERK/JNK/P38MARK pathway and by promoting PI3K/AKT. In addition, TCMs inhibit autophagy by targeting inhibition of AMPK/mTOR. In addition to the traditional apoptosis, autophagy, programmed necrosis and ferroptosis are also caused during AKI. Wogonin and protocatechuic aldehyde can effectively inhibit RIPK1 in the RIPK1/RIPK3/MLKL of necroptosis. TLR2/4-mediated TRAF2 has a stimulant effect on RIPK3. Induction of HMGB2 by necroptosis and TLR2/4-regulated MyD88 aggravates the inflammatory response of acute kidney injury, while TCMs significantly improve this phenomenon via direct or indirect effects. Breviscapine can reduce ferroptosis by increasing glutathione peroxidase levels. In acute kidney injury, the production of ROS, the multiple roles of P53, the protective effects of eNOS, Nrf2, HO-1, and SIRT1/3 all become therapeutic targets of TCMs.
the intrinsic toxicity of some herbs, researchers could modify molecular structure of TCMs to lower the toxic effects without affecting their therapeutic effects. It is essential to reveal the

Names Origins Functions Mechanisms
Aristolochia acids (AA)

Aristolochia species
Increasing oxidative stress and inflammatory Increasing Nox2 and reducing NO bioavailability (Sato et al., 2004;Debelle et al., 2008) Andrographide A herbaceous plant in the family Acanthaceae Promoting cell necrosis Unclear  Sciadopitysin

Accelerating oxidative stress and inducing apoptosis
Inducing production of ROS  compound/phytochemicals present in the formulations which are correlated with the toxicity in AKI.
(2) As for the incorrect identification, processing and storage, standardization of herbal products need to be emphasized. We should also ensure safe manufacturing processes to avoid contamination from heavy metals and other ingredients.
(3) We should determine and limit the dosing and duration of drugs usage through adequate preclinical trials and dose conversions between animals and humans. Safe and effective dose ranges for humans as well as appropriate monitoring for adverse effects are also needed. (4) It is worth mentioning that pharmacists and doctors should clearly know the interactions between TCMs and other medications before prescribing these drugs to patients. (5) For patients with special conditions, like chronic kidney disease and liver disease, their medication needs to be carefully considered.

CONCLUSION AND PERSPECTIVES
Collectively, previous studies showed that numerous types of TCMs protect against AKI via different mechanisms of action, including inhibiting inflammation, cell apoptosis, necroptosis, ferroptosis, and restraining oxidative stress etc. These data support the potential application of these TCMs as novel therapeutic agents in treating patients with AKI. Although some TCMs have entered preclinical trials, it is essential to initiate preclinical pharmacologic and toxicologic trials and clinical trials to evaluate the efficacy and safety of TCMs usage. Moreover, considering that some TCMs are deleterious to the kidney, they should be attracted more attention when utilized. In addition, it is believed that western medicines always relieve symptoms quickly while TCMs exert therapeutic effects gently and fundamentally. In this regard, the combination of TCMs and western medicines may become a promising treatment strategy for AKI by taking advantages of both and by limiting side effects. The interaction between medicines should also be considered. In conclusion, from a holistic point of view, TCM-based anti-AKI therapy should be emphasized and extensively explored, as this may help to minimize the morbidity and mortality of AKI and prolong the survival of patients.

AUTHOR CONTRIBUTIONS
JL, XM, and CH designed the theme and direction of the manuscript. LZ and XL critically revised the manuscript. HL drafted the manuscript.