The fibroblast growth factor 23 (FGF23) is a phosphaturic hormone primarily synthesized by osteoblasts that exerts its endocrine function by lowering phosphate reabsorption and active vitamin D synthesis in the kidney. High circulating FGF23 levels are associated with left ventricular hypertrophy (LVH), endothelial dysfunction, atherosclerosis, and impairment of innate immune defense in patients with chronic kidney disease and chronic heart failure. It has been proven in vitro and in vivo, that FGF23 directly induces LVH via FGF receptor 4 binding and subsequent activation of the phospholipase C γ/calcineurin/nuclear factor of activated T cell pathway. Besides the bone, FGF23 is further expressed by cardiac myocytes, whereas the synthesis in other cardiac cell types is not well studied. So far, the role of cardiac FGF23 in cardiac development and function was not investigated and its relevance in high-pressure induced LVH is unknown.
To characterize cardiac FGF23 more precisely, we evaluated the cardiac cell type specific synthesis of FGF23 in C57BL/6 wildtype (WT) mice by immunofluorescence staining and quantified age-dependent Fgf23 mRNA expression by quantitative real-time PCR. To determine the role of cardiac FGF23 on heart function in health, we established a mouse model with a cardiac myocyte specific FGF23 knockout (Fgf23cKO) using Cre-loxP system and investigated heart function by echocardiography. Finally, we performed transverse aortic constriction (TAC) in Fgf23cKO and WT mice to examine whether high-pressure induced cardiac hypertrophy is mediated in part by cardiac FGF23.
In WT mice, FGF23 is co-localized with α-actinin and CD31 positive cells indicating specific synthesis in cardiac myocytes and endothelial cells, respectively. A co-localization with fibroblast specific protein 1 was not observed, thus FGF23 is not expressed in cardiac fibroblasts. Using immunohistochemistry, FGF23 is synthesized in the left and right ventricle, cardiac atria, aorta and truncus pulmonalis. The relative cardiac Fgf23 mRNA expression is 100-fold lower than in bone and steadily increases from neonatal age up to six months.
Examined by echocardiography, Fgf23cKO mice have normal cardiac function and geometry. They do neither show histological features of pathological LVH nor induction of hypertrophic or fibrotic genes. However, the loss of cardiac FGF23 resulted in enhanced expression of FGF2, 10, 12, 16 and 18 in heart tissue of Fgf23cKO mice, indicating a compensatory mechanism.
Two weeks after TAC, Fgf23cKO and WT mice showed a similar increase in cardiac myocyte size, LV fibrosis and hypertrophic gene expression. Importantly, in cardiac magnetic resonance imaging, ejection fraction was more severely reduced in Fgf23cKO mice compared to WT and end-systolic volume and LV systolic diameter were more enhanced. TAC further induced cardiac Fgf23 mRNA expression in WT and Fgf23cKO mice and FGF23/CD31 co-staining revealed increased FGF23 synthesis in endothelial cells.
In health, FGF23 is ubiquitously expressed in cardiac myocytes and endothelial cells over the entire heart at low levels. Pathologically, TAC specifically induces FGF23 in endothelial cells, while FGF23 in cardiac myocytes is not affected. Our data indicate that increased FGF23 synthesis in the endothelium due to high-pressure may stimulate hypertrophic growth of cardiac myocytes in paracrine manner, which has to be confirmed in further studies.