Transcription factor specificity protein (SP) family in renal physiology and diseases

The role of SP1 in renal ischemia/reperfusion injury

Renal ischemia/reperfusion (I/R) injury often occurs on patients suffering from the perioperative period, resulting in rapid kidney dysfunction and a high rate of mortality (Barrera-Chimal et al., 2015; Granata et al., 2022; Wu, Siow & Karmin, 2010). It was reported that significant reductions appeared in both SP1 and its downstream regulatory Klotho levels in renal tubular epithelial cells (RTECs) subjected to I/R injury (Hu et al., 2010; Huang et al., 2021; Li et al., 2020). Furthermore, exogenously administered SP1 or Klotho demonstrated individual protective roles in the context of H/R injury (Qian et al., 2018; Yuan et al., 2017). Although the total protein of SP1 in I/R kidney is similar to that in the sham-control kidney, serine-phosphorylation of SP1 was remarkably elevated in the kidney subjected to I/R, which is activated via the phosphorylated ERK and microRNA-204 (Chen et al., 2017; Wu, Siow & Karmin, 2010; Yang et al., 2021a). SP1 also binds to and activates the promotor region of sphingosine kinase (SK) 1 directly in HK-2 cells, subsequently increasing sphinganine-1-phosphate (S1P) formation, which alleviates renal I/R injury (Yuan et al., 2017). Besides, the mRNA and protein expression of folate transporters including folate receptor 1 (FOLR1) and folate carrier (RFC) are decreased under I/R stimulation, which can be downregulated by SP1 in tubular cells (Yang et al., 2021a). Notably, folate is an essential micronutrient for human body. Altogether, the above evidence mentioned suggests that SP1 is tightly associated with I/R injury, indicating that SP1 activation may be beneficial for I/R-associated acute kidney injury.

The role of SP1 in diabetic nephropathy

Diabetic nephropathy (DN) is a prominent cause of end-stage renal disease (ESRD), characterized by interstitial fibrosis and glomerular sclerosis (Flyvbjerg, 2017; Webster et al., 2017). Recent research has extensively examined the role of SP1 in the pathogenesis and progression of DN. It has been discerned that its primary influence on DN development predominantly stems from its capacity to modulate inflammation, oxidative stress, and pro-fibrogenesis. SP1 has also been implicated in regulating high glucose (HG)-induced gene expression in the aortic endothelial cells (Du et al., 2000). Besides, bioinformatics analysis reveals that SP1 may be related to diabetic nephropathy and WNT/β-catenin pathway (Hu et al., 2021; McKay et al., 2016). The protein expression of SP1 is significantly up-regulated in the renal cortical tissue of diabetic db/db mice, and further elevated with the aging of db/db mice (Chen et al., 2014; Li et al., 2023b). In both in vivo and in vitro settings, Zhang et al. (2017) have found that SP1 expression significantly increases in podocytes under diabetic conditions. However, another study shows that mRNA and protein expression of SP1 are both decreased in MPC5 cells under the HG treatment (Zhang et al., 2021). Additionally, advanced glycation end products-modified bovine serum albumin (AGE-BSA) have been found to inhibit Neuropilin-1 (NRP1) promoter transcriptional activity in podocytes by reducing the binding capacity of the SP1, affecting the development of DN (Bondeva & Wolf, 2009; Bondeva & Wolf, 2015). Furthermore, Jeong has introduced a novel molecular strategy using ring SP1 decoy ODNs delivered via HVJ-liposome gene-delivery technique, showing potential in preventing and treating diabetic nephropathy (Kang et al., 2008). Consequently, targeting SP1 may emerge as a viable therapeutic strategy for managing diabetic nephropathy. In diabetic nephropathy, persistent hyperglycemia activates NF-κB, which assumes a central role in inflammation. Chen et al. (2014) observed an interaction between elevated SP1 and NFκB-p65 within the podocyte nucleus upon exposure to high glucose. Furthermore, they identify the direct binding between SP1 and the DNMT1 promoter region, indicating the SP1/NFκB p65-Dnmt1 pathway may be a therapeutic target for protecting against podocyte injury in DN (Zhang et al., 2017).

Abundant evidence suggests that oxidative stress is heightened and mitochondrial dysfunction is exacerbated in diabetic nephropathy, ultimately resulting in renal injury (Forbes & Thorburn, 2018; Hallan & Sharma, 2016). Zhang et al. (2021) have proposed that SP1 can bind to the three SP1 response elements within the peroxiredoxin 6 (Prdx6) promoter, thereby directly influencing the transcriptional activation of Prdx6 in podocytes. The SP1-mediated upregulation of Prdx6 expression serves as a protective mechanism against podocyte injury in diabetic nephropathy by mitigating oxidative stress and inhibiting ferroptosis.

SP1 plays a crucial role in orchestrating key downstream processes associated with extracellular matrix (ECM) accumulation in high-glucose conditions or hyperglycemia. In such environments, there is an upregulation of the tubular-specific enzyme myo-inositol oxygenase (MIOX) (Sharma et al., 2017). Electrophoretic mobility shift assays have demonstrated a robust binding of the SP1 to the MIOX promoter, particularly with the unmethylated SP1 oligo, under high-glucose conditions. Treatment with SP1 siRNA leads to a reduction in mRNA and protein levels of both SP1 and MIOX, as well as the generation of reactive oxygen species originating from NADPH oxidase (NOX)-4 and mitochondrial sources. This intervention also leads to a decrease in hypoxia-inducible factor-1α (HIF-1α) and fibronectin expression, an extracellular matrix protein known to be elevated in diabetic nephropathy and tubulopathy. Additionally, Gao et al. (2016) have proposed that c-Jun and SP1 are pivotal upstream regulators of TGFβ1 activation during the fibrotic progression of DN. It has been reported that the renoprotective factor nuclear factor erythroid 2-related factor 2 (NRF2) can reverse TGFβ1-mediated glomerular fibrosis, but only in the early stages of DN (Jiang et al., 2010). Nrf2 delays the progression of DN by repressing TGFβ1 in a c-Jun and SP1-dependent way (Gao et al., 2014; Gao et al., 2016).

SP1 is also implicated in the dysregulation of multiple RNAs in diabetic nephropathy. For example, Chen et al. (2018b) noted increased expression of long non-coding RNA ZFAS1, which was activated by SP1. Yang et al. (2021b) showed that SP1 overexpression in diabetic nephropathy increases mitochondrial RNAase P (Rmrp) levels, which sponges miR-1a-3p to promote JunD expression. Furthermore, bioinformatics assays, luciferase reporter assays, and chromatin immunoprecipitation (ChIP) assays confirmed that SP1 could bind to the promoter region of Rmrp (Yang et al., 2021b). Piyush also found a significant reduction in the expression of miR29b in a model of diabetic nephropathy in renal proximal tubular epithelial cells (RTECs) exposed to high glucose. At the same time, there was an observed upregulation in the expression of DNA methyltransferases (DNMTs), specifically DNMT1, DNMT3A, DNMT3B and SP1. Further investigation revealed that miR29b targets DNMT1 by regulating its transcription factor, SP1 (Gondaliya et al., 2018).

Thus, modulating expression of SP1 may serve as a therapeutic option to treat diabetic nephropathy.

The role of SP1 in renal interstitial fibrosis

SP1, a widely distributed transcription factor, plays a pivotal role in various pathophysiological contexts, encompassing cell cycle regulation, apoptosis, angiogenesis and embryogenesis (Chae et al., 2006; Zhang et al., 2003). The protein level of SP1 is markedly increased in the unilateral ureteral obstruction (UUO) kidney (Kim et al., 2013). The occurrence and progress of renal fibrosis usually attribute to three or more cell types, including the injury of renal tubular epithelial cells, activation of myofibroblasts, proliferation of mesangial cells, and infiltration of immune cells (Humphreys, 2018; Tang, Nikolic-Paterson & Lan, 2019; Zhao, 2019). Anomalies in the function of RTECs constitute a central factor contributing to aberrant tubular reabsorption and renal fibrosis (Liu et al., 2019; Yao et al., 2017). Notably, SP1 has been previously implicated in regulating CD2AP promoter activity and expression in RTECs, underscoring its functional role in this cell type (Lu et al., 2008). Electrophoretic mobility shift assays (EMSA) conducted with nuclear extracts from HK-2 cells have identified SP1 and multiple SP3 isoforms, whose co-mutation suppresses the gene transcription of macrophage, which can mediate the progression of renal fibrosis (Hughes et al., 2002; Li, Fu & Liu, 2022). Furthermore, the transition of quiescent glomerular mesangial cells (MCs) into a highly proliferative phenotype with myofibroblast-like characteristics is a common process in inflammatory renal glomerular diseases, ultimately leading to glomerulosclerosis (Liu et al., 2023). This transition is mediated by the membrane-associated enzyme membrane type 1 (MT1)-MMP (Turck et al., 1996). Alfonso-Jaume, Mahimkar & Lovett (2004) reported that MT1-MMP transcription in glomerular mesangial cells is cooperatively regulated by nuclear factor of activated T cells (NFAT) c1, along with the zinc finger transcription factors SP1, SP3, and Egr-1.

As is known to all, TGFβ1/Smad signaling is a classic pathway closely associated with renal fibrosis. It is noteworthy that specific binding to TGFβ1 promoter region is identified by SP1 (Geiser et al., 1993). Synthetic double-stranded oligodeoxynucleotides containing consensus sequences that bind to specific transcription factors can block mRNA transcription at the DNA level. Sung et al. (2013) found that SP1 decoy ODNs reduced SP1-DNA and Smad-DNA complexes in a murine model of UUO-induced renal fibrosis, decreasing TGF-β1, IL-1β, p-Smad, and type I collagen expression. Consistent with this, a new chimeric (Chi) ODN method also support that simultaneous inhibition of the transcription factors NF-κB and SP1 attenuates tubulointerstitial fibrosis in a UUO model (Kim et al., 2013). Caveolin-1 (cav-1), an integral membrane protein, plays a crucial role in the synthesis of matrix proteins by glomerular mesangial cells (MC) (Fujita et al., 2004). Follistatin, a widely expressed and secreted glycoprotein, counteracts the profibrotic and proinflammatory actions of several TGFβ-1 isoforms, particularly activins (de Kretser et al., 2012; Hedger & de Kretser, 2013). The absence of cav-1 results in increased nuclear levels of SP1, which regulates the activation of the follistatin promoter in cav-1 knockout MCs. This occurs through enhanced phosphoinositide 3-kinase (PI3K) activity and downstream protein kinase C (PKC) zeta-mediated phosphorylation (Mehta et al., 2019). In addition, many biological processes mediated by TGFβ-1 rely on SP1 in cooperation with Smad proteins, such as Smad3 and Smad4 (Datta, Blake & Moses, 2000; Wu et al., 2007). For instance, SP1 collaborates with Smad proteins to mediate TGFβ-1-induced fibrogenesis and is essential for the production of type I collagen induced by miR-29c downregulation in NRK-52E cells (Jiang et al., 2013). Recent research also has shown that p-Smad2/3 interacts with SP1 and p300 to transmit TGFβ-1 signals in human glomerulonephritis (Kassimatis et al., 2010). Interestingly, the deletion of Smad7 promotes Ang II-induced upregulation of SP1 in hypertensive kidneys. The enhanced SP1-TGFβ1/Smad3-NF-κB signaling pathway may be the primary mechanistic route by which the deletion of Smad7 exacerbates Ang II-mediated renal fibrosis and inflammation (Liu et al., 2013). Furthermore, Li et al. (2020) reported that SP1 overexpression mitigated TGFβ1-induced fibrosis in HK-2 cells by inducing Klotho expression. SP1 was found to directly regulate Klotho expression in kidney cells by binding to a specific CG-rich site within the Klotho promoter region. This finding provides valuable insights into the transcriptional regulation of Klotho in renal disease models.

Overall, SP1 is an important pro-fibrotic target for renal fibrosis, and many treating strategies based on SP1 has been developed until now.

The role of SP1 in lupus nephritis

Lupus nephritis (LN) is a prominent manifestation of systemic lupus erythematosus (SLE), which has an impact on morbidity and mortality (Mackay et al., 2006; Shen et al., 2024). Despite advancements in management, as many as 20% of patients ultimately progress to end-stage renal disease (ESRD) (Almaani, Meara & Rovin, 2017). In the early stages of renal fibrogenesis, evidence from exosomal urinary miRNA and cellular pellets derived from patients with biopsy-proven SLE indicates that miR-29c interacts with SP1 to regulate collagen production in tubular epithelial cells (Solé et al., 2019). Conversely, miR-150 and miR-21 expression contribute to the perpetuation and amplification of the fibrotic process leading to ESRD. This occurs through a Smad3/TGFβ pathway, independent of SP1, during later stages of LN progression. Axl, a member of the TAM (Tyro-3, Axl, and Mer) family of receptor tyrosine kinases (RTKs), exerts influence over various disease states, including lupus-like autoimmune disease (Cohen et al., 2002; Lu & Lemke, 2001), which are strongly upregulated in both mouse models of nephritis and streptozotocin-induced diabetic nephropathy (Yanagita et al., 2002; Zhen, Priest & Shao, 2016). In the context of mesangial Axl expression regulation, Zhen et al. (2018) identified the transcription factor SP1 as a crucial regulator.

The role of SP1 in renal cell carcinoma

Growing evidence indicates that specificity protein transcription factors, including SP1, SP3, and SP4, are overexpressed in tumors and play a crucial role in regulating genes essential for cancer cell death and survival (Chadalapaka et al., 2010, 2008). SP1 is highly expressed in most tumors and has a malignant phenotype, whose abnormal activation can be involved in the pathogenesis of many tumors (Black, Black & Azizkhan-Clifford, 2001; Chang & Hung, 2012). In renal cell carcinoma (RCC), Zhu et al. (2020) found that the knockout of the SP1 gene significantly inhibits cell proliferation and induces cell cycle arrest at the G1 phase. Liu et al. (2020a) identified that the VHL/HIF-2α/SMYD3 signaling cascade mediates EGFR hyperactivity in RCC, and SP1 works with SMYD3 to promote EGFR expression and amplify its downstream signaling activity. Disrupting SP1 accumulation at the EGFR locus can downregulate SMYD3, thereby inhibiting tumor growth. Clear cell renal cell carcinoma (ccRCC) is the most common subtype of renal cell carcinoma (RCC), accounting for 70–80% of all RCC cases (Zhang et al., 2024). Wang et al. (2024) indicated that ubiquitin B regulated the expression of VEGFA in a SP1-dependent manner, thereby modulating the angiogenic capability of RCC cells. ChIP-qPCR assay confirmed that SP1 binds to the promoter region of FBXL6, and FBXL6 depletion restrains ccRCC progression. So, targeting the regulation of SP may provide new insights for exploring targeted therapies for ccRCC based on F-box proteins and substrates (Yu et al., 2022). In another report, it was also demonstrated that SP1 induces upregulation of lncRNA SNHG14 as a competitive RNA, thereby promoting the migration and invasion of ccRCC by modulating N-WASP (Liu et al., 2017).