Effects of soluble Klotho and active vitamin D on FGF23-induced cardiac hypertrophy
Final Report Abstract
The risk for cardiovascular diseases is extremely high in patients suffering from chronic kidney disease (CKD). Over 90% of patients with end-stage renal disease develop pathologic changes of the heart, including cardiac hypertrophy. To date the precise molecular mechanisms underlying this interrelationship remain not fully understood. Fibroblast growth factor (FGF)23 is a bone-derived hormone that exerts its biological functions via binding to cell surface FGF receptors (FGFR). In mammals, four different FGFR isoforms exist, FGFR1-4. FGF23`s main function is to maintain serum levels of phosphate. To do so, FGF23 increases renal phosphate excretion by targeting the kidney via FGFR1 and a co-receptor named Klotho. The transmembrane protein Klotho is predominantly expressed in the kidney. In addition to the membrane-bound form, a soluble Klotho form (sKL) exists in blood, urine, and cerebrospinal fluid. In the progression of CKD, serum levels of FGF23 increase to compensate for the rise in serum phosphate concentrations. In parallel, CKD is a state of Klotho and active vitamin D (1,25D) deficiency. High FGF23 levels are associated with the development of cardiovascular disease and mortality rate in CKD patients. It has been shown that FGF23 directly induces cardiac hypertrophy via binding to FGFR4 and activation of a pro-hypertrophic signaling cascade in cardiac myocytes. Since the heart does not express Klotho, the FGF23-induced pathological effects are independent of Klotho. On the other hand, studies have shown that sKL and 1,25D exert beneficial effects on the heart by preventing the induction of cardiac hypertrophy. In this study we show that 1,25D improves the development of cardiac hypertrophy and reduces hypertrophic gene expression in vivo. Blockade of FGF23-mediated signaling via administration of FGFR inhibitors further increased the beneficial cardiac effect of 1,25D. This experimental finding combined with our human data suggests that the benefits of 1,25D therapy depend on the 1,25D dose relative to the preexisting serum FGF23 levels. In contrast, the benefit of 1,25D is independent of hypertension. We also show that sKL can independently bind both, FGF23 and specific FGFR isoforms. We found that FGFR1c, FGFR3c and FGFR4 could co-precipitate with sKL and FGF23, but we assume that FGF23 and sKL must have pre-formed. In the absence of endogenous Klotho, the complex of FGF23 with sKL induces FGFR/Ras/mitogen-activated protein kinase (MAPK) signaling. Furthermore, our data indicate that sKL can inhibit FGF23`s pathologic actions on cardiac myocytes. We demonstrate that we are able to produce stable, bioactive recombinant sKL protein for further use in pre-clinical studies in animal models. Additionally, we provide a novel assay to detect Klotho in its bioactive form in serum. Overall, we conclude that our findings indicate a potential protective function of 1,25D and sKL in scenarios with high endogenous FGF23 levels, such as CKD. For 1,25D, the beneficial effects are counterbalanced by the serum FGF23 levels. The therapeutic potential and dose response of both 1,25D and sKL as antihypertrophic drugs need be further investigated. Since the effect of FGF23 for the induction of fibrosis is not fully known yet, 1,25D and/or sKL could be beneficial as anti-fibrotic reagents as well.
Publications
- “FGF23 actions on target tissues – with and without klotho.” Frontiers in Endocrinology 2018, 9:189
Richter B & Faul C
(See online at https://doi.org/10.3389/fendo.2018.00189)
