Hypoxia-Inducible Factor-1α Promotes Epithelial-to-Mesenchymal Transition Through Upregulation of Cathepsin S Expression in Diabetic Kidney Disease

Background: Tubulointerstitial fibrosis (TIF) plays an important role in the pathogenesis of diabetic kidney disease (DKD). Epithelial-to-mesenchymal transition (EMT) in tubular epithelial cells (TECs) leads to TIF in the progression of DKD. Hypoxia inducible factor-1α (HIF-1α) has been elucidated to promote EMT and TIF through inducing TGF-β1 pathway. However, it still remains unknown by which mediation process the HIF-1α could lead to EMT. In this study, we aimed to identify key genes mostly correlated to HIF-1α through bioinformatics analysis and explored the mediation effect of the key gene in HIF-1α induced EMT in DKD. Methods: Tubulointerstitial gene expression profiling data from DKD patients and healthy controls were acquired from the GEO database. R packages were used for data processing, in which hub differentially expressed genes mostly correlated to DKD would be defined as key genes if they are strongly associated with the expression of HIF-1α. The proximal TEC line (HK-2) was used to validate the bioinformatic findings. Results: We identified CTSS as the key gene. Firstly, upregulation of HIF-1α and CTSS were confirmed in high glucose induced-HK-2 cells. Further gene set enrichment analysis demonstrated that the function of both HIF-1α and CTSS were positively enriched in T cell activity and extracellular matrix receptor interaction, suggesting that HIF-1α might induce TIF and T cell activation to deteriorate DKD through upregulating CTSS expression. Secondly, HIF-1α inhibitor (YC-1) could inhibit the expression of both HIF-1α and CTSS while CTSS inhibitor (Z-FL-COCHO) could only lower the expression of CTSS in high glucose-induced HK-2 cells, suggesting that CTSS plays a role in downstream of HIF-1α. Inhibition of HIF-1α or CTSS could both significantly lower the expression level of α-SMA, type IV collagen and fibronectin which were upregulated in high glucose condition, while inhibition of CTSS had a better effect in restoring E-cadherin compared to inhibition of HIF-1α. These results validated that HIF-1α promotes EMT and TIF through upregulating CTSS in high glucose-induced HK-2 cells. Conclusion: We identified CTSS as the key gene strongly associated with the expression of HIF-1α involved in the pathogenic process of DKD, and HIF-1α might induce EMT and TIF to deteriorate DKD through upregulating CTSS.