Hyperglycemic environment disrupts phosphate transporter function and promotes calcification processes in podocytes and isolated glomeruli

Abstract

Soft tissue calcification is a pathological phenomenon that often occurs in end-stage chronic kidney disease (CKD), which is caused by diabetic nephropathy, among other factors. Hyperphosphatemia present during course of CKD contributes to impairments in kidney function, particularly damages in the glomerular filtration barrier (GFB). Essential elements of the GFB include glomerular epithelial cells, called podocytes. In the present study, we found that human immortalized podocytes express messenger RNA and protein of phosphate transporters, including NaPi 2c (SLC34A3), Pit 1 (SLC20A1), and Pit 2 (SLC20A2), which are sodium-dependent and mediate intracellular phosphate (Pi) transport, and XPR1, which is responsible for extracellular Pi transport. We found that cells that were grown in a medium with a high glucose (HG) concentration (30 mM) expressed less Pit 1 and Pit 2 protein than podocytes that were cultured in a standard glucose medium (11 mM). We found that exposure of the analyzed transporters in the cell membrane of the podocyte is altered by HG conditions. We also found that the activity of tissue nonspecific alkaline phosphatase increased in HG, causing a rise in Pi generation. Additionally, HG led to a reduction of the amount of ectonucleotide pyrophosphatase/phosphodiesterase 1 in the cell membrane of podocytes. The extracellular concentration of pyrophosphate also decreased under HG conditions. These data suggest that a hyperglycemic environment enhances the production of Pi in podocytes and its retention in the extracellular space, which may induce glomerular calcification.

Pit 1 transporter (SLC20A1) as a key factor in the NPP1‐mediated inhibition of insulin signaling in human podocytes

“…We also confirmed our previous observations, in which under HG conditions that mimicked insulin resistance, the amount and membrane exposure of Pit 1 decreased (Kulesza et al, 2022).…”

“…Cell lysates and Western blot analysis were performed as previously described (Kulesza et al, 2022). Primary antibodies that were used in the experiments are listed in Table 1.…”

“…Our recent studies demonstrated that HG conditions alter podocyte function by decreasing the amount of type III sodium-dependent phosphate transporter (Pit 1) and nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) (Kulesza et al, 2022).…”

“…Our recent studies demonstrated that HG conditions alter podocyte function by decreasing the amount of type III sodium‐dependent phosphate transporter (Pit 1) and nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) (Kulesza et al, 2022). Pit 1 belongs to the solute carrier family of proteins, and its crucial function is the transport of phosphate (Pi) into the cell (Kulesza & Piwkowska, 2021).…”

“…Our research established that hyperglycemia causes a decrease in the total amount and plasma membrane exposure of the Pit 1 transporter. NPP1 also abates from the membrane of podocytes, resulting in Pi retention in the extracellular space and a decrease in extracellular PPi concentrations, thereby promoting ectopic calcification in the glomerulus (Kulesza et al, 2022).…”