Abstract P1162: Nedd4-2 Inactivation Underlies The Increase In Cardiac Sodium-dependent Glucose Cotransporter-1 In Diabetic Hearts

Abstract

Rationale: Type-2 diabetes (T2D) is a major risk factor for developing heart failure. While the underlying mechanisms are poorly understood, recent evidence suggests that upregulation of the Na + /glucose cotransporter 1 (SGLT1) plays an important role, as it causes myocyte Na + overload. Objective: To identify the mechanisms involved in promoting an increase in SGLT1 protein level in diabetic hearts. Methods/Results: We compared hearts from patients with and without T2D and hearts from rats with late-onset T2D caused by overexpression of human amylin in pancreatic β-cells versus hearts from wild-type littermates. SGLT1 protein level was significantly higher in hearts from both humans and rats with T2D. In contrast, the SGLT1 mRNA levels were comparable in hearts from T2D and non-diabetic patients and rats. This result suggests that theT2D-induced increase in cardiac SGLT1 protein occurs at a post-transcriptional level. Co-immunoprecipitation of SGLT1 with ubiquitin was reduced in diabetic human and rat hearts, which indicates that SGLT1 degradation is impaired. The E3 ubiquitin ligase Nedd4-2 is responsible for ubiquitination of several cardiac membrane transporters. Pharmacological inhibition of Nedd4-2 with Heclin in HL-1 cells resulted in significantly higher SGLT1 protein levels, suggesting that Nedd4-2 also mediates SGLT1 degradation. Nedd4-2 is inactivated upon phosphorylation by several kinases, including the serum and glucocorticoid-regulated kinase-1 (SGK1). Using western blotting, we found increased Nedd4-2 phosphorylation in hearts from humans and rats with T2D compared to their non-diabetic counterparts. Moreover, hearts from T2D patients exhibited higher levels of SGK1 and its activated phosphorylated form compared to their non-diabetic counterparts. Conclusion/New Hypothesis: The increase in SGLT1 protein in the diabetic heart is regulated post-transcriptionally by an impairment in Nedd4-2 dependent ubiquitination and degradation of SGLT1.