The Renoprotective Effect, Nephrotoxicity, and Molecular Mechanisms of Emodin

INTRODUCTION Renal diseases, including acute kidney injury (AKI) and chronic kidney disease (CKD), with high morbidity and mortality rates affect 15%-20% of the global population.[1] Emodin is a bioactive anthraquinone found in diverse medicinal plants, such as Rheum officinale, Rheum palmatum, Polygonum cuspidatum, Polygonum multiflorum, Aloe vera, and Cassia obtusifolia.[1-4] It exhibits a renoprotective effect, mediated by anti-inflammatory, antioxidant, and anti-fibrotic mechanisms,[2,3] and has been shown to improve renal function and protect against renal fibrosis.[1,5] However, emodin is also nephrotoxic.[2,4] Here, we highlight the renoprotective effect, nephrotoxicity, and molecular mechanisms of emodin and provide insight into a promising therapeutic strategy and new perspective for targeting renal fibrosis. THE RENOPROTECTIVE EFFECT AND NEPHROTOXICITY OF EMODIN Numerous studies have shown that emodin improves renal function and reduces tubulointerstitial inflammation and renal fibrosis.[1,5-8] Emodin’s renoprotective effect There is accumulating evidence that emodin reduces serum creatinine and urea levels and inhibits renal fibrosis.[1,5-8] Pathologically, emodin ameliorates inflammatory cell infiltration, glomerulosclerosis, and tubulointerstitial fibrosis by alleviating mesangial expansion, collagen deposition, and glomerular basement membrane thickening in CKD.[5,7,8] Furthermore, it has been shown to reduce macrophage infiltration, decrease the proportion of M1-polarized macrophages, and downregulate α-smooth muscle actin (α-SMA), fibronectin, and collagen I in mice with unilateral ureteral obstruction (UUO).[8] Although studies have shown that emodin has a renoprotective effect, clinical studies and applications are lacking. Therefore, clinical research studies should be designed and performed in the future. Emodin’s nephrotoxicity Studies have indicated that emodin is nephrotoxic.[2,3] Specifically, emodin was found to reduce the viability of human kidney 2 cell line (HK-2 cells) and increase the incidence of renal tubular pigmentation and nephropathy in emodin-exposed mice, with female animals demonstrating greater susceptibility.[2] Notably, while emodin can induce apoptosis through caspase-3-dependent and mitochondrial pathways within the concentration range of 40-80 µM, and significantly inhibit cell proliferation at 130.65 µM, thereby causing nephrotoxicity.[2,3] A structure-function relationship analysis attributed the nephrotoxicity of emodin to its hydroxyl groups, with dihydroxylation reducing the nephrotoxic effect.[4] Structural modifications combined with advanced delivery systems, such as colon-targeted emodin in situ gels that reduce systemic absorption, demonstrate promise in lowering toxicity.[9] Therefore, optimizing the structure of emodin-based candidates and developing targeted delivery platforms should be prioritized to achieve an optimal therapeutic risk-benefit profile. MOLECULAR MECHANISMS UNDERLYING EMODIN’S RENOPROTECTIVE AND NEPHROTOXIC EFFECTS Molecular mechanisms of emodin’s renoprotective effect In both AKI and CKD, emodin’s renoprotective effect is mediated by multi-target and multi-pathway mechanisms [Table 1], and these can be used to develop four core intervention strategies.[1,8,10] First, emodin has shown anti-fibrotic activity in mice with UUO and rats with DKD that was associated with the inhibition of the transforming growth factor-beta (TGF-β)/suppressor of Mothers against Decapentaplegic (Smad) signaling pathway and the induction of estrogen receptor alpha (ERα) ubiquitination and degradation.[1,7,8] It was found to suppress the nuclear translocation of phosphorylated Smad3 and thereby block its pro-fibrotic effect.[1] Moreover, emodin downregulates α-SMA and fibronectin expression by inhibiting the TGF-β/Smad pathway [Figure 1]. Simultaneously, it activates the anti-fibrotic hepatocyte growth factor (HGF)/mesenchymal-epithelial transition factor pathway and promotes collagen II expression, demonstrating a complementary effect in maintaining fibrotic balance.[8] Furthermore, emodin functions as a “molecular adhesive” that facilitates the interaction between ERα and synoviolin 1 (SYVN1), thereby promoting ERα degradation. This leads to sirtuin 6 (SIRT6) expression, enhanced hypoxia-inducible factor-1α (HIF-1α) degradation, and suppression of osteogenic factor expression.[7] Finally, emodin upregulates Smad7 expression while inhibiting the pro-fibrotic TGF-β1/Smad ubiquitination regulatory factor 2 pathway and Nod-like receptor (NLR) family pyrin domain-containing protein 3 (NLRP3) inflammasome activation, which further contributes to its anti-fibrotic effect.[6] Table 1 - Molecular mechanisms underlying the renoprotective effect of emodin in AKI, CKD, and DKD Disease Experimental model Underlying molecular mechanisms and key targets Ref AKI Renal IRI in mice and hypoxia/reoxygenation-induced HK-2 cells Reduced mitochondria-mediated apoptosis, mitochondrial reactive oxygen species, and phosphorylation of CaMKII and Drp1 at Ser616 [1] AKI Lipopolysaccharide-induced NRK-52E cells Downregulation of TLR2, NF-κB, TNF-α, and IL-1β mRNA and protein [1] AKI, CKD RAW 264.7 macrophages Reduced TNF-α, IL-6, and IL-1βInhibition of M1 macrophage polarization and EGFR, Ras, and phosphorylated ERK1/2 expression [10] CKD UUO in mice and bone marrow-derived macrophages Inhibition of collagen, α-SMA, fibronectin, and AIM2 inflammasome expressionSuppression of K27-linked polyubiquitination of AIM2 by acting on K64 [6] CKD CKD in mice and sodium phosphate-induced A7R5 and MOVAS cells Enhanced ERα-SYVN1 interaction, accelerated ERα ubiquitination and degradation, and inhibition of HIF-1α deacetylation by SIRT6 [7] CKD UUO in mice and TGFβ-induced HK-2 cells Increased HGF-induced collagen II expression and inhibition of fibronectin, α-SMA, and Smad2 expression [8] DKD HG-induced rat mesangial cells Downregulation of fibronectin and phosphorylation of p38MAPK and phosphorylated cyclic adenosine monophosphate response element-binding protein, with upregulation of PPARγ [1] DKD HG-induced rat mesangial cells Suppression of TGF-β1 and fibronectin expression by inhibiting NF-κB activation [1] DKD Unilateral nephrectomy combined with STZ in rats Inhibition of apoptosis and enhancement of podocyte autophagy by regulating AMPK/mTOR pathway [1] DKD STZ-induced diabetic rats Inhibition of PI3K signaling and apoptosis pathways [1] DKD STZ-induced diabetic rats and GH-induced HK-2 cells Inhibition of ferroptosis by upregulating GPX4 and Nrf2 [5] AIM2, absent in melanoma 2; AKI, acute kidney injury; AMPK, adenosine monophosphate-activated protein kinase; CaMKII, calmodulin-dependent protein kinase II; CKD, chronic kidney disease; DKD, diabetic kidney disease; Drp1, dynamin-related protein 1; EGFR, epidermal growth factor receptor; ERK1/2, extracellular signal-regulated kinase; ERα, estrogen receptor alpha; GPX4, glutathione peroxidase 4; HG, high glucose; HGF, hepatocyte growth factor; HIF-1α, hypoxia-inducible factor-1α; HK-2, human kidney 2; IL-1β, interleukin-1; IRI, ischemia-reperfusion injury; MAPK, mitogen-activated protein kinase; mRNA, messenger RNA; mTOR, mammalian target of rapamycin; NF-κB, nuclear factor kappa B; NRK-52E, normal rat Kidney-52E; PI3K, phosphoinositide 3-kinase; PPARγ, peroxisome proliferator-activated receptor gamma; Ras, rat sarcoma virus; Ser616, Serine at position 616; SIRT6, sirtuin 6; STZ, streptozotocin; SYVN1, synoviolin 1; TGF-β1, transforming growth factor β1; TLR2, Toll-like receptor 2; TNF-α, tumor necrosis factor alpha; UUO, unilateral ureteral obstruction; α-SMA, α-smooth muscle actin. Figure 1.: Molecular mechanisms underlying the renoprotective effect and nephrotoxicity of emodin. AKI and CKD involve the activation of the pro-inflammatory/NF-κB and impaired Keap1/Nrf2 pathways, which are regulated by emodin. Emodin also inhibits the TGF-β1/Smad pathway activated in renal disease. Additionally, emodin mitigates its own nephrotoxicity, primarily by inhibiting the mitochondrial apoptotic pathway. 12-LOX, 12-lipoxygenase; ARE, antioxidant response element; Bax, Bcl-2-associated X protein; Bcl-2, B-cell lymphoma 2; COX-2, cyclooxygenase-2; CTCAGC, cytosine-thymine-cytosine-adenine-guanine-cytosine; CUL3, cullin-3; Cyt C, cytochrome C; DLG, D-Leu-Gly; ECM, extracellular matrix; EpRE, electrophile response element; ETGE, Glu-Thr-Gly-Glu; HO-1, heme oxygenase-1; IκB, Inhibitor of kappa B; Keap1, Kelch-like ECH-associated protein 1; MCP-1, monocyte chemoattractant protein-1; MnSOD, manganese superoxide dismutase; NF-κB, nuclear factor kappa B; NQO1, nicotinamide adenine dinucleotide phosphate quinone dehydrogenase 1; Nrf2, nuclear factor erythroid 2-related factor 2; Rbx1, ring-box 1; sMaf, small Musculoaponeurotic Fibrosarcoma protein; TGAG, thymine-guanine-adenine-guanine; TGF-β1, transforming growth factor β1; Ub, ubiquitin.Second, emodin exerts anti-inflammatory and immunomodulatory effects. It suppresses inflammation and regulates immune-cell functions by inhibiting the nuclear factor κB (NF-κB) pathway, suppressing absent in melanoma 2 (AIM2) inflammasome activation, and regulating macrophage polarization [Figure 1].[1,6,10] Emodin suppresses the Toll-like receptor 2 (TLR2)/NF-κB signaling cascade, which leads to reduced levels of tumor necrosis factor alpha (TNF-α) and interleukin-1β (IL-1β).[1] In addition, it interacts with K64 of AIM2, inhibiting K27 ubiquitination.[6] This prevents oligomerization and interaction between AIM2 and apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), which inhibits caspase-1 activation and the subsequent release of IL-1β and IL-18.[6] Emodin suppresses M1 macrophage polarization by suppressing the epidermal growth factor receptor (EGFR)/mitogen-activated protein kinase (MAPK) pathway.[10] Third, emodin displays a cytoprotective effect and inhibits cell death by helping maintain the structural integrity and functional stability of podocytes, renal tubular epithelial cells, and vascular smooth muscle cells, which are associated with inhibiting ferroptosis, protecting mitochondrial function, reducing ischemia-reperfusion injury (IRI), and suppressing vascular calcification.[1,5,7] For instance, emodin inhibited ferroptosis by activating the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway and increasing glutathione peroxidase 4 (GPX4) and solute carrier family 7 member 11 (SLC7A11) expression in streptozotocin (STZ)-induced diabetic rats and high glucose (HG)-induced HK-2 cells [Figure 1].[5] In another study, emodin reduced phosphorylated Ca2+/calmodulin-dependent protein kinase II (CaMKII) expression by affecting dynamin-related protein 1 (Drp1) phosphorylation at serine 616 (S616), indicating that emodin plays a protective role in IRI by inhibiting CaMKII expression.[1] Moreover, emodin improves the osteogenic transdifferentiation of vascular smooth muscle cells by inhibiting vascular calcification.[7] The molecular mechanism study reveals that emodin promotes interaction between ERα and SYVN1, facilitating ERα ubiquitination and degradation, which results in increasing SIRT6 expression, suppression of HIF-1α signaling, and the subsequent downregulation of RUNX family transcription factor 2 and bone morphogenetic protein 2, thereby preventing osteogenic-like transdifferentiation.[7] Finally, emodin improves aberrant glucose and lipid metabolism while activating autophagy. Emodin enhances insulin signaling and glucose metabolism by upregulating phosphorylated glycogen synthase kinase-3β (GSK-3β), activating the adenosine monophosphate-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR)-mediated autophagy pathway.[1] Additionally, these effects are inhibited by emodin-induced autophagy via the modulation of the phosphoinositide 3-kinase (PI3K)/AMPK/mTOR pathway.[1] Collectively, these findings demonstrate that emodin’s renoprotective effect is the result of synergistic mechanisms involving multiple targets and pathways. They also indicate that using emodin to regulate specific immune factors and metabolic pathways is a promising therapeutic strategy for treating renal fibrosis. Molecular mechanism of emodin’s nephrotoxicity Critically, emodin exhibits dose- and time-dependent nephrotoxicity, as evidenced by its capacity to induce apoptosis in renal cells.[4] Mechanistically, this apoptosis is primarily caused by the coordinated activation of mitochondria-mediated pathways and caspase-3-dependent execution, which specifically induces programmed cell death in renal tubular epithelial cells [Figure 1].[2,3] CONCLUSION Due to its capacity to regulate various inflammatory, apoptotic, and autophagic pathways, emodin has promise as a therapeutic agent for the treatment of renal diseases. There is considerable pharmacological evidence that emodin could be an effective agent in the management of renal diseases, and concept-driven therapeutic strategies can be derived from existing findings. However, research in this area has predominantly been limited to animal models and cell-based studies, with a lack of clinical trials and practical applications. This may be attributed to the significant challenges associated with the clinical translation of emodin—it exhibits poor solubility, low bioavailability, suboptimal intestinal absorption, long-term dose dependence on specific targets, and potential hepatotoxicity or nephrotoxicity. Therefore, Structure- and function-based design, high-throughput screening, computer-aided design, and rigorous safety and efficacy studies should be utilized to develop emodin-based candidates that effectively and safely treat renal diseases. Financial support and sponsorship This study was supported by the National Natural Science Foundation of China (No. 82274192). Author contributions Liu J: Methodology, Investigation, Resources, Writing—Original Draft. Vaziri ND: Data curation, Methodology, Supervision, Investigation, Writing—Review and Editing. Miao H: Conceptualization, Methodology, Investigation, Resources, Funding Acquisition, Formal Analysis, Writing—Original Draft, Writing—Review and Editing. Zhao YY: Conceptualization, Methodology, Investigation, Resources, Formal Analysis, Writing—Original Draft, Writing—Review and Editing. All authors have read and approved the final manuscript. Ethics approval and consent to participate Not applicable. Conflict of interest Yingyong Zhao is an editorial board member of the journal. The article was subject to the journal’s standard procedures, with peer review handled independently of the editor and the affiliated research groups. Data availability statement No additional data.