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Cell Metab. 2015 Dec 1;22(6):1020-32. doi: 10.1016/j.cmet.2015.09.002. Epub 2015 Oct 1.

Activation of Cardiac Fibroblast Growth Factor Receptor 4 Causes Left Ventricular Hypertrophy.

Grabner A, Amaral AP, Schramm K, Singh S, Sloan A, Yanucil C, Li J, Shehadeh LA, Hare JM, David V, Martin A, Fornoni A, Di Marco GS, Kentrup D, Reuter S, Mayer AB, Pavenstädt H, Stypmann J, Kuhn C, Hille S, Frey N, Leifheit-Nestler M, Richter B, Haffner D, Abraham R, Bange J, Sperl B, Ullrich A, Brand M, Wolf M, Faul C.

PMID: 26437603

Summary by Matt Sparks (@Nephro_Sparks)

Background

It is well documented that patients with CKD incur a heavy burden of cardiovascular disease (CVD). However, the underlying pathophysiology driving this is unclear. Several interesting observations linking decreased kidney function to CVD are beginning to emerge. Fibroblast growth factor-23 (FGF-23) is among one of the leading examples. What is notable about patients who go on to eventually require renal replacement therapy is the development of left ventricular hypertrophy (LVH). In this paradigm, it appears that elevated FGF-23 levels is associated with LVH. FGF-23 is a phosphaturic hormone whose main function is to regulate serum phosphate levels. FGF-23 is secreted by osteocytes in response to excess calcitriol, hyperphosphatemia, and PTH (See figure below) and it known to be elevated early in the CKD. Thus, making FGF-23 and it's signaling cascade a potential therapeutic target. In order for FGF-23 to trigger a physiological response by engaging it's receptor (FGFR1-4) and typically the accessory protein Klotho must be present. However, recent literature suggests that in many cell lineages a Klotho independent pathway exists. It is another way in which FGF-23 can result in such pleiotropic effects. In the kidney, FGF-23 stimulates phosphate excretion by the failing kidney in the classic Klotho-dependent manner. However, this may not be the case in cardiomyocytes. This group reported in JCI in 2011 evidence that FGF-23 was capable of inducing cardiac hypertrophy in cultured cardiomyoctes and in mice. They also demonstrated that this effect was Klotho independent. Turns out that global elimination of FGF-23 signaling is not a good thing. This paper, reported in Cell Metabolism aims to identify the specific signaling cascade responsible for the direct cardiac hypertrophy effects of FGF-23.   

What is the signaling cascade utilized by FGF-23 in cardiomyocytes?

Utilizing an in vitro model system (transfected HEK293 cells) that lacks klotho but contains all of the FGF receptors subtypes 1-4 (similar to the cardiomyocyte) the investigators demonstrated that FGF-23 signals by increasing phosphorylated phospholipase C (PLC) gamma. However, when these HEK293 cells are transfected with klotho they signal down the ERK and not the PLC pathway (Figure 1A). Thus, demonstrating that cells which lack klotho (such as the cardiomyocyte) signal down a distinct pathway triggering PLC. They also demonstrate that in the absence of klotho only FGFR4 binds PLC gamma and this binding was prevented by mutating the phosphorylation site (Figure 1B). These results show that FGF-23 is capable of signaling down the PLC gamma pathway via the FGFR4 in cells that lack klotho.

They next moved to cardiomyocytes and show that FGFR4 is expressed in both the mouse and human cardiomyocyte (Figure 2). They next went to the cultured mouse cardiomyocyte and show that similarly to the HEK293 cells in the absence of klotho activate FGFR4 and PLC gamma (Figure 3).

Does FGF receptor 4 signalling mediate cardiac hypertrophy? 

To answer this question the investigators first went to an in vitro model of hypertrophy. Through the use of specific blockers of the different FGF receptors they show that the cardiomyocyte hypertrophy induced by FGF-23 is mediated by FGFR4 in vitro (Figure 4). Luckily a specific FGFR4 blocking antibody already exists! There is also a small molecule inhibitor of FGFR4 being utilized in cancer (apparently FGFR4 is up-regulated in cancer cells that are resistant to chemotherapy). They next moved to a in vivo system. To do this they fed both control and FGFR4 -/- mice a diet high in phosphate (2%) for 12 weeks. This is known to induce cardiac hypertrophy and elevate FGF-23 levels. They found that mice lacking FGFR4 did not undergo cardiac hypertrophy like the control mice did (Figure 5D below). Interestingly, the control mice exhibited increased levels of phosphorylated PLC gamma. They also corroborated these finding by treating neonatal mouse myoctes from control and FGFR4 -/- mice with FGF-23, FGF-2, and Ang II. They show that only FGF-2 and Ang II were able to induce hypertrophy in vitro. 

They next went to a rat model of CKD by utilizing the 5/6 nephrectomy model. This is a commonly used technique in rats. However, because of the renal vasculature branching patterns in mice differ it can be more difficult to pull off in a mouse. They used the FGFR4 blocking antibody via IP injection to disrupt the FGFR4 signaling axis in one group of rats and the others only received vehicle. This was given 1 hour after nephrectomy and then every 3 days through day 12. The rats were analyzed on day 14. Again, they show that even though the rats with CKD who received the FGFR4 antibody has less cardiac hypertrophy and improved diastolic heart function (Figure 6). This was even in the face of similarly impaired kidney function and elevated blood pressure in the group that got FGFR4 blocking antibody. Lastly, they analyzed mice with a constitutively activated FGFR4. This mutation is known to result in more aggressive disease progression in several cancer types. These knockin constitutively activated FGRR4 mice developed spontaneous LVH and cardiac hypertrophy as well. Again, suggesting a direct role of the FGF4R in the development of cardiac hypertrophy.

Conclusion

This is an interesting group of studies with potential therapeutic implications. How much of the cardiac hypertrophy in CKD is a result of this pathway and how much is driven by blood pressure alone. It is interesting to note that cross transplant studies in our lab demonstrated that direct stimulation of the type 1 angiotensin receptor in the heart did not lead to cardiac hypertrophy, but this was driven by the degree of blood pressure elevation. It is also worth mentioning that blood pressures were not measured in both of the mouse studies discussed here so it is not known if the global KO or constituatively activated FGFR4 mice have dysregulated blood pressures. It will also be important to understand what global blockade of this receptor system will do outside of the heart. What are the potential side effects of this therapy? Overall, these are well executed studies and add to our growing knowledge of an important area of investigation.