Regulator of Calcineurin 1 Aggravates Tubular Epithelial Cell Injury in Diabetic Mice Through Promoting Liver Kinase B1 Ubiquitination

Renal tubular injury is closely related to the occurrence and development of diabetic kidney disease (DKD). Regulator of calcineurin 1 (RCAN1) is an endogenous regulatory factor of the phosphatase calcineurin and plays an important role in cell differentiation and the regulation of mitochondrial function. However, the underlying mechanism of RCAN1 in renal tubular injury in DKD remains unclear. Here, we found that RCAN1 expression is predominantly upregulated in renal tubular epithelial cells (RTECs) of patients with DKD and mice with streptozotocin-induced diabetic kidney injury. RCAN1 overexpression in RTECs significantly exacerbates mitochondrial damage and interstitial fibrosis in diabetic mice. Furthermore, RCAN1 overexpression reduces AMP-activated protein kinase (AMPK) phosphorylation, activates the mTOR/PRAS40/S6K signaling pathway, and promotes lipid deposition. Co-immunoprecipitation and mass spectrometry analysis reveal an interaction between RCAN1 and AMPK upstream kinase liver kinase B1 (LKB1). Mechanistically, RCAN1 directly binds to a deubiquitinase BRCA1-associated protein 1 (BAP1) through the N-terminal residues 1-29. The binding reduces the stability of the LKB1-MO25-STRAD complex in the cytoplasm by inhibiting BAP1-mediated deubiquitination of these proteins. Our findings provide for the first time the role of RCAN1 in regulating protein ubiquitination modification. In conclusion, RCAN1 promotes tubular damage by disrupting the LKB1/AMPK pathway in RTECs under HG conditions. RCAN1 may act as a potential therapeutic target for preventing DKD progression.