The role of apurinic/apyrimidinic endonuclease on the progression of streptozotocin-induced diabetic nephropathy in rats

Streptozotocin induces renal proximal tubular injury through p53 signaling activation

Streptozotocin (STZ), an anti-cancer drug that is primarily used to treat neuroendocrine tumors (NETs) in clinical settings, is incorporated into pancreatic β-cells or proximal tubular epithelial cells through the glucose transporter, GLUT2. However, its cytotoxic effects on kidney cells have been underestimated and the underlying mechanisms remain unclear. We herein demonstrated that DNA damage and subsequent p53 signaling were responsible for the development of STZ-induced tubular epithelial injury. We detected tubular epithelial DNA damage in NET patients treated with STZ. Unbiased transcriptomics of STZ-treated tubular epithelial cells in vitro showed the activation of the p53 signaling pathway. STZ induced DNA damage and activated p53 signaling in vivo in a dose-dependent manner, resulting in reduced membrane transporters. The pharmacological inhibition of p53 and sodium-glucose transporter 2 (SGLT2) mitigated STZ-induced epithelial injury. However, the cytotoxic effects of STZ on pancreatic β-cells were preserved in SGLT2 inhibitor-treated mice. The present results demonstrate the proximal tubular-specific cytotoxicity of STZ and the underlying mechanisms in vivo. Since the cytotoxic effects of STZ against β-cells were not impaired by dapagliflozin, pretreatment with an SGLT2 inhibitor has potential as a preventative remedy for kidney injury in NET patients treated with STZ.

The antioxidant and DNA-repair enzyme apurinic/apyrimidinic endonuclease 1 limits the development of tubulointerstitial fibrosis partly by modulating the immune system

Apurinic/apyrimidinic endonuclease 1 (APE1) is a multifunctional protein that controls the cellular response to oxidative stress and possesses DNA-repair functions. It has important roles in the progression and outcomes of various diseases; however, its function and therapeutic prospects with respect to kidney injury are unknown. To study this, we activated APE1 during kidney injury by constructing an expression vector (pCAG-APE1), using an EGFP expression plasmid (pCAG-EGFP) as a control. We performed unilateral ureteral obstruction (UUO) as a model of tubulointerstitial fibrosis on ICR mice before each vector was administrated via retrograde renal vein injection. In this model, pCAG-APE1 injection did not produce any adverse effects and significantly reduced histological end points including fibrosis, inflammation, tubular injury, and oxidative stress, as compared to those parameters after pCAG-EGFP injection. qPCR analysis showed significantly lower expression of Casp3 and inflammation-related genes in pCAG-APE1-injected animals compared to those in pCAG-EGFP-injected UUO kidneys. RNA-Seq analyses showed that the major transcriptional changes in pCAG-APE1-injected UUO kidneys were related to immune system processes, metabolic processes, catalytic activity, and apoptosis, leading to normal kidney repair. Therefore, APE1 suppressed renal fibrosis, not only via antioxidant and DNA-repair functions, but also partly by modulating the immune system through multiple pathways including Il6, Tnf, and chemokine families. Thus, therapeutic APE1 modulation might be beneficial for the treatment of renal diseases.

MicroRNA-23b Targets Ras GTPase-Activating Protein SH3 Domain-Binding Protein 2 to Alleviate Fibrosis and Albuminuria in Diabetic Nephropathy

Diabetic nephropathy (DN) is a frequent and severe complication of diabetes that is structurally characterized by glomerular basement membrane thickening, extracellular matrix accumulation, and destabilization of podocyte foot processes. MicroRNAs (miRNAs) are dysregulated in DN, but identification of the specific miRs involved remains incomplete. Here, we confirm that the peripheral blood from patients with diabetes and the kidneys of animals with type 1 or 2 diabetes have low levels of miR-23b compared with those of their nondiabetic counterparts. Furthermore, exposure to high glucose downregulated miR-23b in cultured kidney cells. In contrast, renal expression of Ras GTPase-activating protein SH3 domain-binding protein 2 (G3BP2), a putative miR-23b target, increased in DN. In vitro, overexpression of miR-23b decreased, and inhibition of miR-23b increased, G3BP2 expression levels. Bioinformatics analysis also revealed p53 binding sites in the miR-23b promoter; in vitro inhibition of p53 or the upstream p38 mitogen-activated protein kinase (p38MAPK) upregulated miR-23b expression in high-glucose conditions. In turn, inhibition of G3BP2 or overexpression of miR-23b downregulated p53 and p38MAPK expression in high-glucose conditions. In vivo, overexpression of miR-23b or inhibition of p53 in db/db mice reversed hyperalbuminuria and kidney fibrosis, whereas miR-23b antagomir treatment promoted renal fibrosis and increased albuminuria in wild-type mice. These data suggest that hyperglycemia regulates pathogenic processes in DN through an miR-23b/G3BP2 feedback circuit involving p38MAPK and p53. In conclusion, these results reveal a role for miR-23b in DN and indicate a novel potential therapeutic target.

Apurinic/apyrimidinic endonuclease immunoreactivity in germ cells of experimental varicocele-induced rat testes

Increased germ cell apoptosis is related to oxidative DNA damage; therefore, we investigated whether there was a significant change in apurinic/apyrimidinic endonuclease (APE) in varicoceles. Experimental varicoceles were created by partial ligation of the left renal vein of adult male Sprague-Dawley rats, which were sacrificed at 1, 3 and 6 weeks after varicocele creation. Testicular tissues were sampled for TUNEL, Western blotting and immunohistochemistry. There was a significant increase in apoptotic germ cells in the ipsilateral testes 6 weeks after varicocele creation. Increased activation of p53, Bax and cleaved caspase-3 in the left testes was also noted. APE increased activation until 3 weeks after varicocele creation, and then decreased at 6 weeks after varicocele surgery. The spermatocytes were immunostained for both 8-hydroxy-2'-deoxyguanosine and APE, but the spermatogonia revealed only APE immunopositivity in the defective tubules. These results suggest that repression of APE is an underlying mechanism of augmented p53-dependent apoptosis in varicocele-induced rat testes and that remaining APE in the spermatogonia plays a decisive role in regaining testicular spermatogenic function after varicocelectomy.

Notch pathway is involved in high glucose-induced apoptosis in podocytes via Bcl-2 and p53 pathways

Recent studies have shown that Notch pathway plays a key role in the pathogenesis of diabetic nephropathy (DN), however, the exact mechanisms remain elusive. Here we demonstrated that high glucose (HG) upregulated Notch pathway in podocytes accompanied with the alteration of Bcl-2 and p53 pathways, subsequently leading to podocytes apoptosis. Inhibition of Notch pathway by chemical inhibitor or specific short hairpin RNA (shRNA) vector in podocytes prevented Bcl-2- and p53-dependent cell apoptosis. These findings suggest that Notch pathway mediates HG-induced podocytes apoptosis via Bcl-2 and p53 pathways.