In diabetic nephropathy (DN), high glucose (HG) decreases proximal tubular (PT) Sirt1 prior to the decrease in glomerular Sirt1, leading to a corruption of tubule-podocyte communication. Here, we investigated whether Sglt2 regulates PT Sirt1.Design and Method:
Cultured PTs (HK2) were cultivated on a two-chamber-system consisting of an upper chamber equivalent to the ureteral lumen on the apical side of PTs, a lower chamber corresponding to the vascular lumen on the basolateral side of PTs, and a porous cell culture insert. PTs were cultured with combinations of normal glucose (NG) or HG in each chamber to determine the polarity of HG that affects Sglt2 and Sirt1 expression. We also administered a Sglt2 inhibitor (canagliflozin) to 8-week-old db/db mice and analyzed the resulting changes in Sglt2 and Sirt1 expression.Results:
HG in the lower chamber increased Sglt2 and decreased Sirt1 expression, but HG in the upper chamber had no effect. Thus, high blood sugar (BS), but not glucosuria regulates Sglt2 and Sirt1. Furthermore, in db/db mice, high BS also remarkably elevated Sglt2 and lowered Sirt1 expression in PTs, as confirmed by immunofluorescence and immunoelectron microscopy, and reduced de-acetylation of Foxa2, a Sirt1 target. Decreased Sirt1 expression led to acetylated, activated Foxa2, which supports gluconeogenesis. However, treatment with Sglt2 inhibitors attenuated these changes. Results from DNA microarrays and confirmatory real-time PCR using laser-microdissected (LSM)-PTs from db/db showed an increase in Pck1 and G6Pase in concert with the upregulation of Sglt2 and the downregulation of Sirt1.Conclusions:
Increased Sglt2 expression forms a vicious cycle whereby both exogenous HG mediated by a Sglt2-induced influx of BS and endogenous HG generated by Foxa2-induced renal gluconeogenesis reduce Sirt1 levels. Decreased Sirt1 acetylates and activates Foxa2, resulting in the further promotion of renal gluconeogenesis. However, treatment with Sglt2 inhibitors can block all parts of this process.