Downregulation of Akt induces proximal tubule epithelial cell apoptosis via Foxo-1-BIM pathway in proteinuric states

TL;DR

This study shows that reduced Akt activity in kidney cells leads to cell death in proteinuric conditions, offering a potential target for treatment.

Contribution

The study identifies the Akt-Foxo1-BIM pathway as a novel mechanism linking proteinuria to proximal tubule cell apoptosis.

Findings

Akt downregulation in albumin overload leads to PTEC apoptosis via the Foxo1-BIM pathway.

Inhibition of Akt in mice models caused impaired albumin endocytosis and mitochondrial damage.

Human kidney biopsies showed reduced Akt activity in early stages of FSGS.

Abstract

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.

There is a well-recognized relationship between the degree of proteinuria and progression in glomerular diseases, but the mechanism is poorly understood. In this study, we show that expression of survival protein Akt is downregulated in PTEC in response to albumin overload and in human glomerular disease. We demonstrate that inhibition of Akt promotes dephosphorylation of its downstream target Foxo1 and nuclear translocation of propaptotic BIM, leading to intrinsic apoptosis. We propose that interventions to restore Akt signaling will impede tubular injury by inhibiting PTEC apoptosis in proteinuric states.

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