Downregulation of Akt induces proximal tubule epithelial cell apoptosis via FOXO and BIM pathway in proteinuric States

Proteinuria is a widely utilized surrogate marker in clinical practice for its predictive and prognostic value. The mechanistic link between proteinuria and progression remains elusive. Proximal tubule epithelial cells(PTEC) retrieve albumin in the glomerular filtrate via receptor mediated endocytosis facilitated by megalin-cubilin complex. We reported that cell-survival protein, Akt, phosphorylates cargo binding endocytic adaptor protein to megalin, disabled-2(Dab2). We hypothesize that downregulation of Akt signaling as a result of overwhelmed endocytic machinery in albumin overload is linked to PTEC apoptosis in proteinuric states. We show that cell culture and animal model of albumin overload inhibited phosphorylation of Akt in association with apoptosis in PTEC. Chemical inhibition and overexpression of Akt by constitutively active Akt plasmid exacerbated and alleviated apoptosis respectively in response to albumin overload in PTEC. Mouse with targeted inhibition of Akt1 and Akt2 in PTEC (Akt1/2lox/loxSGLT2cre) displayed perturbed albumin endocytosis at baseline. Albumin overload in Akt1/2 ^lox/lox SGLT2cre mouse led to dephosphorylation and translocation downstream Akt target, Forkhead box O-1 (Foxo1) to nuclei driving transcriptional activation of proapoptotic BIM followed by translocation of proapoptotic Bax and BIM to mitochondria and cytochrome-c to cytosol. In an effort to investigate the role of Akt in progression, we examined kidney biopsy specimens of patients with focal segmental glomerulosclerosis (FSGS) and minimal change disease. Kidney biopsies of patients with FSGS exhibited decreased pSer473-Akt expression in PTEC early in the course of disease, preceding progression to end stage kidney disease. We conclude that downstream dephosphorylation of Foxo and transcriptional activation of BIM and subsequent mitochondrial injury drives apoptosis following Akt downregulation in PTEC albeit inhibition of albumin endocytosis in proteinuric states.