Chronic kidney disease (CKD) is a health epidemic that increases risk of death due to various comorbidities. Alterations in mineral metabolism are a hallmark of advanced CKD, where hyperphosphatemia and the associated increase in serum concentrations of fibroblast growth factor (FGF) 23 have been suspected to directly contribute to tissue damage and mortality, but the underlying mechanisms are not fully understood. We previously found that by targeting cardiac myocytes via FGF receptor (FGFR) 4, elevated FGF23 activates a specific signaling pathway and induces cardiac hypertrophy in rodent models of CKD. We also reported that by activating this mechanism, FGF23 can directly target hepatocytes. However, the consequences of FGF23?s hepatic action have not been investigated in vivo. Compared to FGF23, much less is known about the direct pathologic actions of elevated serum phosphate. Mechanistic studies have shown that by targeting vascular smooth muscle cells via specific phosphate transporters and signal mediators, elevated phosphate can cause vascular calcification. Direct actions of elevated phosphate concentrations on the heart are not understood, and potential effects of phosphate on the liver have not been analyzed so far. Our preliminary in vitro studies in primary mouse hepatocytes indicate that phosphate and FGF23 can both induce the expression of inflammatory cytokines, such as interleukin 6 (IL6), and of hepcidin.
In Aim 1 we will characterize the mechanism that mediates the phosphate effect on hepatocytes in vitro, and we will study whether phosphate communicates with FGF23/FGFR4 signaling when stimulating IL6 and hepcidin production. Since IL6 is a potent inducer of skeletal muscle atrophy and hepcidin causes functional iron deficiency, we will also determine whether by targeting the liver, elevated serum levels of phosphate and FGF23 contribute to muscle wasting and anemia. To study the contribution of FGF23 in this context, we have generated a novel mouse model with hepatocyte-specific deletion of FGFR4.
In Aim 2, we will determine whether following the induction of kidney injury via administration of an adenine-supplemented diet, these mice are protected from increases in hepatic and systemic levels of IL6 and hepcidin, as well as skeletal muscle injury and anemia. In the Col4a3 knockout model of CKD, we will inject a FGFR4-specific blocking antibody to determine if systemic FGFR4 inhibition has similar protective effects, which would suggest a therapeutic potential for anti-FGFR4 in CKD. In our preliminary studies we also found that increasing phosphate concentrations induce FGF23 production in cultured hepatocytes, and that mouse models of CKD as well as mice on high-phosphate diet express FGF23 in the liver, which is not detected in control mice.
In Aim 3, we will test whether liver-derived FGF23 is required for the in vivo effects of phosphate. We have generated a novel mouse model for the hepatocyte-specific deletion of FGF23, and after administration of an adenine or high-phosphate diet, we will study if IL6 and hepcidin production are reduced as well as potential protective effects on skeletal muscle and iron metabolism.
Patients with advanced chronic kidney disease (CKD) suffer from widespread tissue injuries and develop markedly increased blood levels of phosphate and fibroblast growth factor 23 (FGF23). We have found that phosphate and FGF23 can directly target the liver and cause the release of factors with established pathologic actions, including the induction of anemia, systemic inflammation, and skeletal muscle wasting. In this research proposal, we will determine how exactly phosphate and FGF23 act on liver cells and whether the blockade of these newly identified actions protects mouse models of CKD from anemia and increases muscle strength, which would suggest that CKD patients might benefit from therapies tackling phosphate and FGF23.
