SGK3 promotes vascular calcification via Pit-1 in chronic kidney disease.

Rationale: Vascular calcification (VC) is a life-threatening complication in patients with chronic kidney disease (CKD) caused mainly by hyperphosphatemia. However, the regulation of VC remains unclear despite extensive research. Although serum- and glucocorticoid-induced kinase 3 (SGK3) regulate the sodium-dependent phosphate cotransporters in the intestine and kidney, its effect on VC in CKD remains unknown. Additionally, type III sodium-dependent phosphate cotransporter-1 (Pit-1) plays a significant role in VC development induced by high phosphate in vascular smooth muscle cells (VSMCs). However, it remains unclear whether SGK3 regulates Pit-1 and how exactly SGK3 promotes VC in CKD via Pit-1 at the molecular level. Thus, we investigated the role of SGK3 in the certified outflow vein of arteriovenous fistulas (AVF) and aortas of uremic mice. Methods and Results: In our study, using uremic mice, we observed a significant upregulation of SGK3 and calcium deposition in certified outflow veins of the AVF and aortas, and the increase expression of SGK3 was positively correlated with calcium deposition in uremic aortas. In vitro, the downregulation of SGK3 reversed VSMCs calcification and phenotype switching induced by high phosphate. Mechanistically, SGK3 activation enhanced the mRNA transcription of Pit-1 through NF-κB, downregulated the ubiquitin-proteasome mediated degradation of Pit-1 via inhibiting the activity of neural precursor cells expressing developmentally downregulated protein 4 subtype 2 (Nedd4-2), an E3 ubiquitin ligase. Moreover, under high phosphate stimulation, the enhanced phosphate uptake induced by SGK3 activation was independent of the increased protein expression of Pit-1. Our co-immunoprecipitation and in vitro kinase assays confirmed that SGK3 interacts with Pit-1 through Thr468 in loop7, leading to enhanced phosphate uptake. Conclusion: Thus, it is justifiable to conclude that SGK3 promotes VC in CKD by enhancing the expression and activities of Pit-1, which indicate that SGK3 could be a therapeutic target for VC in CKD.

The Physiopathologic Roles of Calcium Signaling in Podocytes.

Calcium (Ca2+) plays a critical role in podocyte function. The Ca2+-sensitive receptors on the cell surface can sense changes in Ca2+ concentration, and Ca2+ flow into podocytes, after activation of Ca2+ channels (such as transient receptor potential canonical (TRPC) channels and N-type calcium channels) by different stimuli. In addition, the type 2 ryanodine receptor (RyR2) and the voltage-dependent anion channel 1 (VDAC1) on mitochondrial store-operated calcium channels (SOCs) on the endoplasmic reticulum maintain the Ca2+ homeostasis of the organelle. Ca2+ signaling is transmitted through multiple downstream signaling pathways and participates in the morphogenesis, structural maintenance, and survival of podocytes. When Ca2+ is dysregulated, it leads to the occurrence and progression of various diseases, such as focal segmental glomerulosclerosis, diabetic kidney disease, lupus nephritis, transplant glomerulopathy, and hypertensive renal injury. Ca2+ signaling is a promising therapeutic target for podocyte-related diseases. This review first summarizes the role of Ca2+ sensing, Ca2+ channels, and different Ca2+-signaling pathways in the biological functions of podocytes, then, explores the status of Ca2+ signaling in different podocyte-related diseases and its advances as a therapeutic target.

Curcumin ameliorates focal segmental glomerulosclerosis by inhibiting apoptosis and oxidative stress in podocytes.

Focal segmental glomerulosclerosis (FSGS), a podocyte disease, is the leading cause of end-stage renal disease (ESRD). Nevertheless, the current effective treatment for FSGS is deficient. Curcumin (CUR) is a principal curcuminoid of turmeric, which is a member of the ginger family. Previous studies have shown that CUR has renoprotective effects. However, the mechanism of CUR in anti-FSGS is not clear. This study aimed to explore the mechanism of CUR against FSGS through a combination of network pharmacological methods and verification of experiments. The analysis identified 98 shared targets of CUR against FSGS, and these 98 targets formed a network of protein-protein interactions (PPI). Of these 98 targets, AKT1, TNF, IL-6, VEGFA, STAT3, MAPK3, HIF1A, CASP3, IL1B, and JUN were identified as the hub targets. Molecular docking suggested that the best binding to CUR is MAPK3 and AKT1. Apoptotic process and cell proliferation were identified as the main biological processes of CUR against FSGS by gene ontology (GO) analysis. The most enriched signaling pathway in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis was the PI3K-AKT signaling pathway. Western blots and flow cytometry showed that CUR could inhibit adriamycin (ADR) induced apoptosis, oxidative stress damage, and attenuate podocyte epithelial-mesenchymal transition (EMT) by repressing the AKT signaling pathway. Collectively, our study demonstrates that CUR can attenuate apoptosis, oxidative stress damage, and EMT in FSGS in vitro. These results supply a compelling basis for future studies of CUR for the clinical treatment of FSGS.

Role of the SLIT-ROBO signaling pathway in renal pathophysiology and various renal diseases.

SLIT ligand and its receptor ROBO were initially recognized for their role in axon guidance in central nervous system development. In recent years, as research has advanced, the role of the SLIT-ROBO signaling pathway has gradually expanded from axonal repulsion to cell migration, tumor development, angiogenesis, and bone metabolism. As a secreted protein, SLIT regulates various pathophysiological processes in the kidney, such as proinflammatory responses and fibrosis progression. Many studies have shown that SLIT-ROBO is extensively involved in various aspects of kidney development and maintenance of structure and function. The SLIT-ROBO signaling pathway also plays an important role in different types of kidney disease. This article reviews the advances in the study of the SLIT-ROBO pathway in various renal pathophysiological and kidney disorders and proposes new directions for further research in this field.

Contributions of SGK3 to transporter-related diseases.

Serum- and glucocorticoid-induced kinase 3 (SGK3), which is ubiquitously expressed in mammals, is regulated by estrogens and androgens. SGK3 is activated by insulin and growth factors through signaling pathways involving phosphatidylinositol-3-kinase (PI3K), 3-phosphoinositide-dependent kinase-1 (PDK-1), and mammalian target of rapamycin complex 2 (mTORC2). Activated SGK3 can activate ion channels (TRPV5/6, SOC, Kv1.3, Kv1.5, Kv7.1, BKCa, Kir2.1, Kir2.2, ENaC, Nav1.5, ClC-2, and ClC Ka), carriers and receptors (Npt2a, Npt2b, NHE3, GluR1, GluR6, SN1, EAAT1, EAAT2, EAAT4, EAAT5, SGLT1, SLC1A5, SLC6A19, SLC6A8, and NaDC1), and Na+/K+-ATPase, promoting the transportation of calcium, phosphorus, sodium, glucose, and neutral amino acids in the kidney and intestine, the absorption of potassium and neutral amino acids in the renal tubules, the transportation of glutamate and glutamine in the nervous system, and the transportation of creatine. SGK3-sensitive transporters contribute to a variety of physiological and pathophysiological processes, such as maintaining calcium and phosphorus homeostasis, hydro-salinity balance and acid-base balance, cell proliferation, muscle action potential, cardiac and neural electrophysiological disturbances, bone density, intestinal nutrition absorption, immune function, and multiple substance metabolism. These processes are related to kidney stones, hypophosphorous rickets, multiple syndromes, arrhythmia, hypertension, heart failure, epilepsy, Alzheimer's disease, amyotrophic lateral sclerosis, glaucoma, ataxia idiopathic deafness, and other diseases.

TOPK Activation Exerts Protective Effects on Cisplatin-induced Acute Kidney Injury.

OBJECTIVE: T-LAK-cell-originated protein kinase (TOPK), a PSD95-Disc large-ZO1 (PDZ) binding kinase (PBK), is a novel member of the mitogen-activated protein kinase (MAPK) family. Studies have shown that TOPK plays a critical role in the function of tumor cells, including apoptosis and mitosis. However, little is known on the effect of TOPK in cisplatin-induced acute kidney injury (CP-AKI). This study aimed to investigate the role and mechanism of TOPK in CP-AKI. METHODS: Cisplatin was administered to C57BL/6 mice and cultured kidney tubular epithelial cells (TECs) to establish the CP-AKI murine or cellular models. TECs were then stimulated with the specific inhibitor of TOPK OTS514 or transfected with the recombinant-activated plasmid TOPK-T9E to inhibit or activate TOPK. The TECs were treated with AKT inhibitor VIII following stimulation with OTS514 or cisplatin. Western blotting and flow cytometry were used to evaluate the cell cycle and apoptosis of TECs. RESULTS: The analysis revealed that the TOPK activity was significantly suppressed by cisplatin, both in vivo and in vitro. Furthermore, the pharmacological inhibition of TOPK by OTS514, a specific inhibitor of TOPK, exacerbated the cisplatin-induced cell cycle arrest in the G2/M phase and apoptosis of cultured TECs. Moreover, the TOPK activation via the TOPK-T9E plasmid transfection could partially reverse the cell cycle arrest at the G2/M phase and apoptosis of cisplatin-treated TECs. In addition, AKT/protein kinase B (PKB), as a TOPK target protein, was inhibited by cisplatin in cultured TECs. The pharmaceutical inhibition of AKT further aggravated the apoptosis of TECs induced by cisplatin or TOPK inhibition. TOPK systematically mediated the apoptosis via the AKT pathway in the CP-AKI cell model. CONCLUSION: These results indicate that TOPK activation protects against CP-AKI by ameliorating the G2/M cell cycle arrest and cell apoptosis.

Mechanistic insights into the renoprotective role of curcumin in cisplatin-induced acute kidney injury: network pharmacology analysis and experimental validation.

Cisplatin-induced acute kidney injury (CP-AKI) is a severe complication in patients receiving CP chemotherapy. However, effective therapies for CP-AKI are currently lacking. Curcumin (CUR), a natural polyphenol, is extracted from the rhizome of turmeric and has been reported to have nephroprotective activity. However, the role of CUR in CP-AKI remains unclear. This study aimed to explore the mechanism of CUR in CP-AKI by combining a network pharmacology approach with experimental validations. The analysis revealed 176 potential targets of CUR based on the HERB database and 1,286 related targets of CP-AKI from the GeneCards, DrugBank, and OMIM databases. Further, 106 common targets of CUR against CP-AKI were obtained, and these common targets constructed a protein-protein interaction (PPI) network. In addition, the core targets were screened from the PPI network using Cytoscape. Molecular docking revealed that CUR displayed the best binding to AKT1. Gene Ontology (GO) analysis indicated that the primary biological processes of CUR against CP-AKI included cellular response to chemical stress and apoptotic regulation. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis suggested that the PI3K-Akt signaling pathway was most significantly enriched in CUR against CP-AKI. Western blotting and flow cytometry showed that CUR inhibited apoptosis induced by CP by activating the Akt signaling pathway in human kidney tubular epithelial cells (HK-2). Altogether, our findings demonstrated that CUR alleviated apoptosis by activating the Akt signaling pathway in CP-AKI in vitro. These data provide a scientific basis for future investigations into the clinical application of CUR against CP-AKI.

Serum and Glucocorticoid-Inducible Kinase 3/Nedd4-2 Signaling Pathway Participates in Podocyte Injury by Regulating the Stability of Nephrin.

Serum and glucocorticoid-inducible kinase 3 (SGK3) is involved in maintaining podocyte function by regulating the protein levels of podocin and CD2-associated protein. Nephrin is also one of the slit diaphragm proteins of podocytes, but whether SGK3 participates in podocyte injury by regulating the levels of nephrin remains unclear. In this study, we focused on whether SGK3 affects nephrin levels and the mechanisms involved in the same. In the kidneys of adriamycin (ADR)-induced podocyte injury mouse model, the protein levels of SGK3 and nephrin were significantly decreased. Furthermore, the expression of SGK3 was negatively correlated with the output of proteinuria, and positively correlated with the levels of nephrin. In ADR-treated conditionally immortalized mouse podocyte cells (MPCs), the protein levels of nephrin and SGK3 were inhibited, while the constitutive expression of SGK3 reversed the ADR-induced decline in nephrin protein levels. Furthermore, ADR treatment or SGK3 inactivation enhanced the ubiquitin-proteasome degradation of nephrin in MPCs, and dramatically activated downstream effector proteins of SGK3, neural precursor cells expressing developmentally downregulated protein 4 subtype 2 (Nedd4-2) and glycogen synthase kinase-3 ß (GSK3ß). Similarly, Nedd4-2 or GSK3ß overexpression resulted in increased activity of Nedd4-2 or GSK3ß, and significantly downregulated nephrin levels. Interestingly, ubiquitin-mediated protein degradation of nephrin was regulated by Nedd4-2, rather than by GSK3ß. In summary, SGK3 inactivation downregulated the levels of nephrin by increasing Nedd4-2 and GSK3ß activity in ADR-induced podocyte injury model; in particular, the SGK3/Nedd4-2 signaling pathway was found to be involved in ubiquitin-mediated proteasome degradation of nephrin.