In the largest human study of FGF23 and cardiac structure to date, we confirm previous reports of an independent association between elevated FGF23 levels and greater risk of LVH, which was present in 52% of patients compared with 15%–21% of the general population (4
). In a prospective analysis of those patients with CKD who had normal left ventricular geometry at baseline, elevated FGF23 levels were also associated with increased future risk of new-onset LVH. Using in vitro studies and complementary animal models, we extended these results by demonstrating that FGF23 causes pathological cardiac hypertrophy directly. We showed that the cardiac hypertrophic effects of FGF23 are mediated by FGFR-dependent activation of the calcineurin-NFAT signaling cascade but do not require klotho as coreceptor. We further demonstrated that blocking FGFR signaling can prevent LVH independently of blood pressure in an established animal model of CKD that is characterized by elevated FGF23 levels, severe hypertension, and LVH. By demonstrating direct pathological effects of FGF23 on the heart, we believe that these data uncover a novel mechanism of LVH and novel aspects of the biology of FGF23 that reposition it from biomarker of cardiovascular risk to mechanism of disease and, thus, potential target for clinical intervention.
Klotho is a transmembrane protein expressed primarily in the kidney and parathyroid glands, in which it acts as a coreceptor to increase the binding affinity of FGF23 for FGFR (25
). In addition, a circulating soluble form of klotho that is derived from cleavage of the extracellular domain or alternative splicing acts as an endocrine hormone involved in calcium and phosphate homeostasis (67
). Although it has been assumed that FGF23 cannot exert biologically relevant effects in the absence of klotho, we demonstrate that FGF23 induces hypertrophy of cardiomyocytes that do not express klotho and is associated with LVH in mice that lack klotho. The observation of LVH in klotho-deficient mice is particularly important, because it minimizes the likelihood that soluble klotho acted as a coreceptor for FGFR signaling in the mice that developed LVH after intramyocardial and intravenous injection of FGF23. Another finding which be believe to be novel is that klotho heterozygotes manifest FGF23 levels and a cardiac phenotype that are intermediate between those of klotho-deficient mice and wild-type mice. Unlike klotho-deficient mice, LVH develops in the heterozygotes in the absence of a significant increase in serum phosphate. The identification of dose-response effects for both FGF23 levels and endowment of normal klotho gene copies that are independent of serum phosphate strengthens our results.
In its classic target organs, FGF23 binds FGFR-klotho complexes and activates the MAPK cascade, leading to increased ERK activation and Egr-1 expression (25
). The widely held view that other organs are not capable of responding to FGF23 emerged from studies that defined FGF23 responsiveness based on presence or absence of organ-specific activation of ERK and Egr-1 after intravenous administration of FGF23 (25
). We confirm that FGF23 does not increase cardiac expression of Egr-1 (25
); however, it exerts biological effects nonetheless. We conclude that analyses of klotho expression, ERK activity, or Egr-1 expression alone may be inadequate reporter assays to assess FGF23 responsiveness. Furthermore, since the presence of klotho is not an absolute prerequisite for biological activity of FGF23, we believe our results open new avenues for investigating FGF23 “toxicity” in other nonclassic target organs.
We propose 2 mechanisms to explain klotho-independent, FGF23-induced LVH. First, low-affinity binding of FGF23 to FGFR (50
) may be adequate to induce LVH when there is a protracted period of cardiac exposure to high concentrations of FGF23, such as in CKD. FGF23 toxicity may be accentuated further in patients with CKD in whom klotho expression is downregulated in the kidney and parathyroid glands (67
), which could enhance promiscuous binding of FGF23 to FGFR in other tissues. Thus, high circulating FGF23 concentrations combined with decreased klotho expression could represent an especially cardiotoxic blend in patients with CKD. Alternatively, FGF23-induced LVH could be mediated by higher-affinity binding to specific cardiac FGFRs, such as FGFR4, which we and others detected in isolated cardiomyocytes and in total heart tissue (71
). Unlike FGFR1–FGFR3, FGFR4 is capable of binding FGF23 with high affinity even in the absence of klotho, similar to the other endocrine FGFs in its subfamily, FGF19 and FGF21 (49
). Additional studies are needed to determine which specific FGFR mediates the cardiac effects of FGF23.
Although FGF23 and FGF2 induce a similar hypertrophic phenotype in isolated NRVMs, they use different downstream signaling pathways. FGF2-induced hypertrophy depends primarily on activation of ERK, whereas FGF23-induced hypertrophy is partially ERK dependent but primarily requires PLCγ-calcineurin-NFAT activation. The PI3K-Akt pathway appears to contribute only modestly to FGF2- and not to FGF23-induced hypertrophy. Based on these findings, we conclude that FGF23 and FGF2 cause pathological hypertrophy by activating different branches of canonical FGFR signaling in the heart (Figure ). To our knowledge, this is the first study to identify an FGF that induces cardiac PLCγ-calcineurin-NFAT signaling, which is a classic signaling cascade in pathological LVH (58
). A recent report, in which cyclosporine attenuated LVH in uremic rats (65
) that are known to have elevated FGF23 levels (66
), lends further indirect support to the central role of this pathway.
Schematic representation of FGF23 signaling in classic target cells and cardiomyocytes.
In contrast to CKD, in which FGF23 levels rise as a secondary response to impaired phosphate excretion, X-linked hypophosphatemia (XLH) is a prototypical disease of primary FGF23 excess (73
). Data on cardiac structure in XLH are sparse and inconsistent, with one but not all studies reporting a high rate of LVH (74
). We can speculate several possibilities to reconcile these findings with ours. Patients with CKD are often older and harbor several cardiovascular risk factors. This could accentuate the prohypertrophic effects of FGF23 relative to XLH, which primarily affects children with few cardiovascular risk factors. Second, the magnitude of the increase in FGF23 levels is typically less dramatic in XLH than in CKD (30
). Perhaps variably increased FGF23 levels in XLH might explain their variable manifestation of LVH. Finally, hypophosphatemia and severe phosphate depletion are associated with impaired cardiac function (75
). Therefore, phosphate depletion in XLH could modulate the cardiac response to FGF23 relative to that in CKD, in which phosphate levels are normal to high. Additional studies are needed to explore these possibilities.
While many factors are involved in the complex pathogenesis of LVH, the results of this study indicate that FGF23 is one contributing molecular mediator. Given the strong association between LVH and future mortality (77
) and the growing number of patients with CKD with elevated FGF23 levels and high rates of LVH, these data suggest that a component of cardiovascular risk in patients with CKD could be directly attributable to FGF23. However, quantifying the magnitude of effect of elevated FGF23 levels on LVH relative to that of other risk factors, such as hypertension, will require placebo-controlled randomized trials of FGF23 reduction. Our finding that blocking FGFR attenuates LVH in a classic animal model of CKD without altering the animals’ severe hypertension suggests an important blood pressure-independent, cardiac effect of FGF23. These mechanistic data lend support for a randomized trial to test whether dietary phosphorus restriction or administration of phosphate binders that reduce FGF23 (78
), or monoclonal antibodies or small molecules that block its actions (79
), can slow the progression of LVH, reduce cardiovascular events, and improve survival in patients with CKD.