NBL1 Exacerbates Kidney Injury via TGF-β/Smad Signaling

Background: Elevated circulating levels of NBL1 have been shown to be significantly associated with an increased risk of end-stage kidney disease (ESKD) development in patients with diabetes (Kobayashi H. Sci Transl Med, 2022). The current study aimed to elucidate the biological role of NBL1 in kidney injury using both in vitro and in vivo experimental models. Methods: HK-2 cells were stimulated with NBL1 (500 ng/mL) for 72 hours, and the expression of Epithelial–mesenchymal transition (EMT)-related genes were assessed. For in vivo analysis, NBL1 homozygous knockout (KO) mice on a C57BL/6NCrlCr genetic background and wild-type (WT) littermates were subjected to two models: cisplatin-induced acute kidney injury (AKI; 20 mg/kg intraperitoneally, assessed at day 3) and unilateral ureteral obstruction (UUO)–induced chronic kidney disease (CKD; assessed at day 14). Kidney function, gene and protein expression, PAS staining, TUNEL assays, and immunohistochemistry were performed. Results: In HK-2 cells, NBL1 stimulation significantly increased mRNA levels of TGF-β1, αSMA, fibronectin, and vimentin, and decreased E-cadherin protein expression. Phosphorylation of Smad2/3 was also elevated. In the AKI model, NBL1 expression was upregulated in the kidneys of WT mice after cisplatin treatment, and renal function was more severely impaired in WT than in KO mice. WT mice also exhibited increased expression of EMT-related genes (TGF-β1, αSMA, fibronectin, MCP-1), reduced E-cadherin, elevated αSMA and TGF-β1 proteins, and enhanced p-Smad2/3 levels, all of which were attenuated in KO mice. KO mice showed fewer TUNEL-positive apoptotic cells and reduced Mac-1–positive inflammatory cells. No significant differences in glomerular injury scores (GIS) or tubulointerstitial injury scores (TIS) were observed. In the CKD model, NBL1 mRNA and serum protein levels were significantly increased in UUO-treated WT mice. Expression of TGF-β1, CTGF, αSMA, fibronectin, collagen 4a1, and vimentin was markedly lower in KO mice compared to WT mice, while E-cadherin expression remained higher. Smad2/3 phosphorylation was also suppressed in KO mice. No significant differences in GIS or TIS were observed. Conclusion: NBL1 promotes kidney injury through the TGF-β/Smad signaling pathway. Inhibition of NBL1 may serve as a novel therapeutic target in both acute and chronic kidney injury. Funding: Commercial Support - This work was supported by the Academic Encouragement Award from the Kondo Memorial Medical Foundation; research grants from the Japan Foundation for Applied Enzymology (Vascular Biology Innovation [VBIC] and Cardiovascular Innovative Conference [CVIC]); the 8th Boehringer Ingelheim and Eli Lilly Diabetes Research Grant from the Japan Diabetes Foundation; the Academic Support Program from Bayer Yakuhin, Ltd.; a Grant-in-Aid for Young Scientists from the Japan Society for the Promotion of Science (JSPS); the Young Investigator Research Grant from the Suzuki Manpei Diabetes Foundation; a research grant from the Mochida Memorial Foundation for Medical and Pharmaceutical Research; a research grant from the MSD Life Science Foundation; a clinical research grant from the Clinical Research Promotion Foundation; the Young Researcher Grant from the UBE Scientific Foundation; a research grant from the Life Science Foundation of Japan; a research grant from the Naito Foundation; a Novartis Research Grant; a research grant from Astellas Pharma Inc.; and the Research Award from the Uehara Memorial Foundation., Private Foundation Support