SGK1 induces vascular smooth muscle cell calcification through NF-κB signaling

Medial vascular calcification, associated with enhanced mortality in chronic kidney disease (CKD), is fostered by osteo-/chondrogenic transdifferentiation of vascular smooth muscle cells (VSMCs). Here, we describe that serum- and glucocorticoid-inducible kinase 1 (SGK1) was upregulated in VSMCs under calcifying conditions. In primary human aortic VSMCs, overexpression of constitutively active SGK1S422D, but not inactive SGK1K127N, upregulated osteo-/chondrogenic marker expression and activity, effects pointing to increased osteo-/chondrogenic transdifferentiation. SGK1S422D induced nuclear translocation and increased transcriptional activity of NF-κB. Silencing or pharmacological inhibition of IKK abrogated the osteoinductive effects of SGK1S422D. Genetic deficiency, silencing, and pharmacological inhibition of SGK1 dissipated phosphate-induced calcification and osteo-/chondrogenic transdifferentiation of VSMCs. Aortic calcification, stiffness, and osteo-/chondrogenic transdifferentiation in mice following cholecalciferol overload were strongly reduced by genetic knockout or pharmacological inhibition of Sgk1 by EMD638683. Similarly, Sgk1 deficiency blunted vascular calcification in apolipoprotein E-deficient mice after subtotal nephrectomy. Treatment of human aortic smooth muscle cells with serum from uremic patients induced osteo-/chondrogenic transdifferentiation, effects ameliorated by EMD638683. These observations identified SGK1 as a key regulator of vascular calcification. SGK1 promoted vascular calcification, at least partly, via NF-κB activation. Inhibition of SGK1 may, thus, reduce the burden of vascular calcification in CKD.

SGK1-Sensitive Regulation of Cyclin-Dependent Kinase Inhibitor 1B (p27) in Cardiomyocyte Hypertrophy

BACKGROUND/AIMS

The serum- and glucocorticoid-inducible kinase SGK1 participates in the orchestration of cardiac hypertrophy and remodeling. Signaling linking SGK1 activity to cardiac remodeling is, however, incompletely understood. SGK1 phosphorylation targets include cyclin-dependent kinase inhibitor 1B (p27), a protein which suppresses cardiac hypertrophy. The present study explored how effects of SGK1 on nuclear p27 localization might modulate the hypertrophic response in cardiomyocytes.

METHODS

Experiments were performed in HL-1 cardiomyocytes and in SGK1-deficient (sgk1-/-) and corresponding wild-type (sgk1+/+) mice following pressure overload by transverse aortic constriction (TAC). Transcript levels were quantified by RT-PCR, protein abundance by Western blotting and protein localization by confocal microscopy.

RESULTS

In HL-1 cardiomyocytes, overexpression of constitutively active SGK1 (SGK1S422D) but not of inactive SGK1 (SGK1K127N) increased significantly the cell size and transcript levels encoding Acta1, a molecular marker of hypertrophy. Those effects were paralleled by almost complete relocation of p27 in the cytoplasm. Treatment of HL-1 cardiomyocytes with isoproterenol was followed by up-regulation of SGK1 expression. Moreover, isoproterenol treatment stimulated the hypertrophic response and was followed by disappearance of p27 from the nuclei, effects prevented by the SGK1 inhibitor EMD638683. The effect of SGK1S422D overexpression on Acta1 mRNA levels was disrupted by overexpression of p27 and of the p27T197A mutant lacking the SGK1 phosphorylation site, but not of the phosphomimetic p27T197D mutant. In sgk1+/+ mice, TAC increased significantly SGK1 and Acta1 mRNA levels and decreased the nuclear to cytoplasmic protein ratio of p27 in cardiac tissue, effects blunted in the sgk1-/- mice.

CONCLUSION

SGK1-induced hypertrophy of cardiomyocytes involves p27 phosphorylation at T197, which fosters cytoplasmic p27 localization.

Inhibition of Phosphate-Induced Vascular Smooth Muscle Cell Osteo-/Chondrogenic Signaling and Calcification by Bafilomycin A1 and Methylamine

BACKGROUND/AIMS

Excessive phosphate concentrations trigger vascular calcification, an active process promoted by osteoinduction of vascular smooth muscle cells (VSMCs) with increased expression and activity of transcription factor RUNX2 (Core-binding factor α1, CBFA1), alkaline phosphatase (ALPL), TGFß1, transcription factor NFAT5, and NFAT5-sensitive transcription factor SOX9. The osteoinductive signaling and vascular calcification of hyperphosphatemic klotho-hypomorphic mice could be reversed by treatment with NH4Cl, effects involving decrease of TGFß1 and inhibition of NFAT5-dependent osteoinductive signaling. Known effects of NH4Cl include alkalinization of acidic cellular compartments. The present study explored whether osteo-/chondrogenic signaling could be influenced by alkalinization of acidic cellular compartments following inhibition of the vacuolar H+ ATPase with bafilomycin A1 or following dissipation of the pH gradient across the membranes of acidic cellular compartments with methylamine.

METHODS

Primary human aortic smooth muscle cells (HAoSMCs) were treated with high phosphate to trigger osteo-/chondrogenic signaling and calcification in the absence or presence of bafilomycin A1 or methylamine. Calcium content was determined using a QuantiChrom Calcium assay, ALP activity by a colorimetric assay and transcript levels by quantitative RT-PCR.

RESULTS

High phosphate increased significantly the calcium deposition, CBFA1 and ALPL mRNA expression as well as alkaline phosphatase activity in HAoSMCs, all effects ameliorated by both, bafilomycin A1 and methylamine. High phosphate further significantly up-regulated the mRNA levels of TGFB1, NFAT5 and SOX9, effects significantly blunted by additional treatment with bafilomycin A1 or methylamine. Treatment of HAoSMCs with human TGFß1 protein or high phosphate up-regulated NFAT5, SOX9, CBFA1 and ALPL mRNA expression to similarly high levels which could not be further increased by combined treatment with high phosphate and TGFß1. Bafilomycin A1 failed to reverse the osteo-/chondrogenic signaling triggered by high phosphate together with TGFß1.

CONCLUSIONS

Inhibition of the vacuolar H+ ATPase or dissipation of the pH gradient across the membranes of acidic cellular compartments both disrupt osteo-/chondrogenic signaling and calcium deposition in VSMCs, observations supporting the hypothesis that vascular calcification requires acidic cellular compartments.

Inhibitory effect of NH4Cl treatment on renal Tgfß1 signaling following unilateral ureteral obstruction

BACKGROUND/AIMS

Consequences of obstructive nephropathy include tissue fibrosis, a major pathophysiological mechanism contributing to development of end-stage renal disease. Transforming growth factor β 1 (Tgfβ1) is involved in the progression of renal fibrosis. According to recent observations, ammonium chloride (NH4Cl) prevented phosphate-induced vascular remodeling, effects involving decrease of Tgfβ1 expression and inhibition of Tgfβ1-dependent signaling. The present study, thus, explored whether NH4Cl influences renal Tgfβ1-induced pro-fibrotic signaling in obstructive nephropathy induced by unilateral ureteral obstruction (UUO).

METHODS

UUO was induced for seven days in C57Bl6 mice with or without additional treatment with NH4Cl (0.28 M in drinking water). Transcript levels were determined by RT-PCR as well as protein abundance by Western blotting, blood pH was determined utilizing a blood gas and chemistry analyser.

RESULTS

UUO increased renal mRNA expression of Tgfb1, Tgfβ-activated kinase 1 (Tak1) protein abundance and Smad2 phosphorylation in the nuclear fraction of the obstructed kidney tissues, effects blunted in NH4Cl treated mice as compared to control treated mice. The mRNA levels of the transcription factors nuclear factor of activated T cells 5 (Nfat5) and SRY (sex determining region Y)-box 9 (Sox9) as well as of tumor necrosis factor α (Tnfα), interleukin 6 (Il6), plasminogen activator inhibitor 1 (Pai1) and Snai1 were up-regulated in the obstructed kidney tissues following UUO, effects again significantly ameliorated following NH4Cl treatment. Furthermore, the increased protein and mRNA expression of α-smooth muscle actin (α-Sma), fibronectin and collagen type I in the obstructed kidney tissues following UUO were significantly attenuated following NH4Cl treatment.

CONCLUSION

NH4Cl treatment ameliorates Tgfβ1-dependent pro-fibrotic signaling and renal tissue fibrosis markers following obstructive nephropathy.