KAP Degradation by Calpain Is Associated with CK2 Phosphorylation and Provides a Novel Mechanism for Cyclosporine A-Induced Proximal Tubule Injury

Olga Tornavaca,Gloria Pascual,Maria Plana,M Teresa Salcedo,M Angeles Montero,Anna Meseguer,Eduard Sarró,Joan López-Hellin,Emilio Itarte,Beatriz Bardaji,Jean-Claude Dussaule

doi:10.1371/journal.pone.0025746

Olga Tornavaca, Gloria Pascual + Show 9 more

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https://doi.org/10.1371/journal.pone.0025746

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Abstract

The use of cyclosporine A (CsA) is limited by its severe nephrotoxicity that includes reversible vasoconstrictor effects and proximal tubule cell injury, the latter associated whith chronic kidney disease progression. The mechanisms of CsA-induced tubular injury, mainly on the S3 segment, have not been completely elucidated. Kidney androgen-regulated protein (KAP) is exclusively expressed in kidney proximal tubule cells, interacts with the CsA-binding protein cyclophilin B and its expression diminishes in kidneys of CsA-treated mice. Since we reported that KAP protects against CsA toxicity in cultured proximal tubule cells, we hypothesized that low KAP levels found in kidneys of CsA-treated mice might correlate with proximal tubule cell injury. To test this hypothesis, we used KAP Tg mice developed in our laboratory and showed that these mice are more resistant to CsA-induced tubular injury than control littermates. Furthermore, we found that calpain, which was activated by CsA in cell cultures and kidney, is involved in KAP degradation and observed that phosphorylation of serine and threonine residues found in KAP PEST sequences by protein kinase CK2 enhances KAP degradation by calpain. Moreover, we also observed that CK2 inhibition protected against CsA-induced cytotoxicity. These findings point to a novel mechanism for CsA-induced kidney toxicity that might be useful in developing therapeutic strategies aimed at preventing tubular cell damage while maintaining the immunosuppressive effects of CsA.

Highlights

Kidney androgen-regulated protein (KAP) is a highly specific, tightly regulated protein of kidney proximal tubule cells [1]
KAP Tg mice exposed to the same treatments that caused tubular injury in littermates did not express KIM-1 (Fig. 1 right panels, and Fig. 1C) and no changes were observed in PCNA levels (Fig. 1B left panels, and Fig. 1D), thereby indicating that KAP protects proximal tubule cells from cyclosporine A (CsA)-induced toxicity in vivo
Since KIM-1 and PCNA levels were not increased in CsA-treated Tg mice (See Figure 1), we postulate that maintenance of KAP levels in proximal tubule cells prevents Tg mice from CsA-induced tubular damage

Summary

Introduction

Kidney androgen-regulated protein (KAP) is a highly specific, tightly regulated protein of kidney proximal tubule cells [1]. We studied KAP transcriptional regulation in mouse kidney and reported a fine-tuned regulation of its mRNA by thyroid and sexual steroid hormones, growth hormone (GH) and insulin-like growth factor 1 (IGF-1) in proximal tubule segments [1,2,3,4,5,6,7,8]. Using specific antibodies raised against KAP-derived synthetic peptides, we identified an apparent 20kDa molecular-weight protein that paralleled KAP mRNA in terms of cell distribution and androgen regulation [10]. We found that KAP interacts with the cyclosporine A (CsA) binding protein cyclophilin B (CypB) [10], and observed that KAP protein levels are lowered in kidneys of CsA-treated mice [10]. KAP protected from CsAinduced toxicity when transfected to the proximal tubule-derived PCT3 cell line [10]

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