Klotho: a novel phosphaturic substance acting as an autocrine enzyme in the renal proximal tubule

Abstract

Klotho has profound effects on phosphate metabolism, but the mechanisms of how Klotho affects phosphate homeostasis is unknown. We detected Klotho in the proximal tubule cell, brush border, and urinary lumen, where phosphate homeostasis resides. Increasing Klotho in the kidney and urine chronically by transgenic overexpression or acutely by intravenous infusion caused hypophosphatemia, phosphaturia from decreased proximal phosphate reabsorption, and decreased activity and protein of the principal renal phosphate transporter NaPi-2a. The phosphaturic effect was present in FGF23-null mice, indicating a direct action distinct from Klotho's known role as a coreceptor for FGF23. Direct inhibition of NaPi-2a by Klotho was confirmed in cultured cells and in cell-free membrane vesicles characterized by acute inhibition of transport activity followed by decreased cell surface protein. Transport inhibition can be mimicked by recombinant beta-glucuronidase and is associated with proteolytic degradation and reduced surface NaPi-2a. The inhibitory effect of Klotho on NaPi-2a was blocked by beta-glucuronidase inhibitor but not by protease inhibitor. Klotho is a novel phosphaturic substance that acts as an enzyme in the proximal tubule urinary lumen by modifying glycans, which cause decreased transporter activity, followed by proteolytic degradation and possibly internalization of NaPi-2a from the apical membrane.

Xeno-Klotho Inhibits Parathyroid Hormone Signaling

“…(15,16,26) Klotho breaks down sugar side-chains of the type 2 sodium phosphate co-transporter (NaP2), thus decreasing the exposure of NaP2 on apical membranes and reducing phosphate reuptake in proximal tubules. (16) Klotho respectively. An intravenous administration of rh-klotho (100 mg/kg) into rats increased klotho levels in serum (143 AE 10 to 1390 AE 105 pg/mL, p < 0.01) and urine (188 AE 32 to 525 AE 98 pg/mL, p < 0.01) by 10-and threefold, respectively.…”

“…Cells were pretreated with either b-glucuronidase (100 and 300 mg/mL, R&D Systems, Minneapolis, MN, USA) or 1 nM rh-klotho with or without 2,3-didehydro-2-deoxy-N-acetylneuraminic acid (DANA; 10 and 30 pM, Sigma-Aldrich) for 6 hours. (15,16) Then, full-length PTH was added in the media at a concentration of 10 À8 M. Ten minutes later, the medium was taken to evaluate inositol trisphosphate levels and the expression of 1-OH was assessed 12 hours later (n ¼ 5 for each condition).…”

Tumor Suppressor Activity of Klotho in Breast Cancer Is Revealed by Structure–Function Analysis

“…Mutations of klotho putative active sites abolished its effect on TRPV5 and on the renal phosphate transporter NaPi-2a (11,32,33), but did not affect its ability to inhibit Wnt signaling (34). To our knowledge, the role of these residues in mediating klotho tumor suppressor activities has not been studied yet.…”

Serum calcitriol levels in a patient with X-linked hypophosphatemia complicated by autosomal dominant polycystic kidney disease

“…Recently, sKlotho has been shown to exhibit a distinct effect on renal phosphate transport [10] and calcium reabsorption in the renal distal tubules independent of FGF23 actions [13]. In addition, these effects were suggested to be mediated by b-glucuronidase and sialidase associated with sKlotho, respectively [10,13]. Thus, it is plausible that these mechanisms rather than classical action of FGF23 are linked to the peculiar observation of abnormally increased serum 1,25(OH) 2 D levels even in the presence of elevated serum FGF23 levels in this patient.…”

“…The truncated aklotho is the major form of circulating sKlotho [9]. Moreover, it was reported that sKlotho had its own distinct effect on renal phosphate transport independently of transmembrane aklotho [10]. Therefore, the regulatory mechanism of serum 1,25(OH) 2 D levels by FGF23 and sKlotho is complex and intriguing in ADPKD.…”