Fibroblast growth factor-2 mediates pressure-induced hypertrophic response

Abstract

In vitro, fibroblast growth factor-2 (FGF2) has been implicated in cardiomyocyte growth and reexpression of fetal contractile genes, both markers of hypertrophy. However, its in vivo role in cardiac hypertrophy during pressure overload is not well characterized. Mice with or without FGF2 (Fgf2(+/+) and Fgf2(-/-), respectively) were subjected to transverse aortic coarctation (AC). Left ventricular (LV) mass and wall thickness were assessed by echocardiography preoperatively and once a week postoperatively for 10 weeks. In vivo LV function during dobutamine stimulation, cardiomyocyte cross-sectional area, and recapitulation of fetal cardiac genes were also measured. AC Fgf2(-/-) mice develop significantly less hypertrophy (4-24% increase) compared with AC Fgf2(+/+) mice (41-52% increase). Cardiomyocyte cross-sectional area is significantly reduced in AC Fgf2(-/-) mice. Noncoarcted (NC) and AC Fgf2(-/-) mice have similar beta-adrenergic responses, but those of AC Fgf2(+/+) mice are blunted. A lack of mitotic growth in both AC Fgf2(+/+) and Fgf2(-/-) hearts indicates a hypertrophic response of cardiomyocytes. Consequently, FGF2 plays a major role in cardiac hypertrophy. Comparison of alpha- and beta-cardiac myosin heavy chain mRNA and protein levels in NC and AC Fgf2(+/+) and Fgf2(-/-) mice indicates that myosin heavy chain composition depends on hemodynamic stress rather than on FGF2 or hypertrophy, and that isoform switching is transcriptionally, not posttranscriptionally, regulated.

Figure 1

Echocardiogram of a Fgf2^+/+ mouse 10 weeks after AC. Two-dimensional image depicting the ascending aorta (ASC), the coarctation site (arrow), and the descending aorta (DESC). Image was captured with an 8.5-MHz transducer. Banding of the aorta was produced by tying a 7-0 silk suture around a 27-gauge needle. The AC occurred at the aortic arch between the left common carotid artery and brachiocephalic trunk.

Figure 2

Serial echocardiographic results for Fgf2^+/+ (open bars) and Fgf2^–/– (filled bars) mice before and after AC. All values are expressed as mean ± SEM. *P < 0.05 vs. prebanded. ^#P < 0.05 vs. Fgf2^+/+ coarct. n = 6 for Fgf2^+/+ and Fgf2^–/–. (a) Calculated LV mass for prebanded and 1- to 10-week AC Fgf2^+/+ and Fgf2^–/– mice showed similar increases at 8 and 9 weeks, but by 10 weeks, that increase in LV mass was significantly less for Fgf2^–/– compared with AC Fgf2^+/+. LV mass was calculated from the echocardiographic measurements of SWT, PWT, and LVED. (b) Relative wall thickness (h/r) for prebanded and 1- to 10-week AC Fgf2^+/+ and Fgf2^–/– mice remained constant, indicating an eccentric form of cardiac hypertrophy during pressure overload. Relative wall thickness was extrapolated from the echocardiographic measurements of PWT and LVED. (c) Fractional shortening (FS) for prebanded and 1- to 10-week AC Fgf2^+/+ and Fgf2^–/– mice showed a preservation of function, suggesting a compensated stage of hypertrophy. FS was determined from the echocardiographic measurements of LVED and LVES.

Figure 3

Tissue weights (a) and tissue/body weight ratios (b) for NC and 10-week AC Fgf2^+/+ and Fgf2^–/– mice showed increases in heart weight for AC mice compared with NC mice; however, AC Fgf2^–/– mice had a significantly lesser increase in heart weight compared with AC Fgf2^+/+ mice. All values are expressed as mean ± SEM. *P < 0.01 vs. NC. ^#P < 0.01 vs. AC Fgf2^+/+. n = 6 for NC and AC Fgf2^+/+ and Fgf2^–/–.

Figure 4

(a) Representative images of hearts from NC and 10-week AC Fgf2^+/+ and Fgf2^–/– mice. Ten-week AC Fgf2^+/+ heart depicting fibrosis (arrow) and an increase in cardiomyocyte size. Picture taken at ×500. Scale bar: 20 μm. (b) Cardiomyocyte cross-sectional area of 10-week AC Fgf2^+/+ mice was significantly increased compared with NC Fgf2^+/+ and 10-week AC Fgf2^–/– mice. Each column represents approximately 100 myocytes from each of 3 hearts per group. *P < 0.05 vs. NC. ^#P < 0.05 vs. AC Fgf2^+/+.

Figure 5

Hemodynamic parameters during dobutamine dose response for NC and 10-week AC Fgf2^+/+ and Fgf2^–/– mice. *P < 0.05 vs. baseline. n = 5 for NC and AC Fgf2^+/+ and Fgf2^–/–. (a) LVP for NC and 10-week AC Fgf2^+/+ and Fgf2^–/– mice during dobutamine stimulation. Ten-week AC Fgf2^+/+ and Fgf2^–/– mice had similar increases in LVP. (b) SBP for NC and 10-week AC Fgf2^+/+ and Fgf2^–/– mice during dobutamine stimulation. Systemic SBP was decreased in 10-week AC mice. (c) +dP/dt for NC and 10-week AC Fgf2^+/+ and Fgf2^–/– mice during dobutamine stimulation. AC Fgf2^–/– mice had a similar increase in the rate of contractility as NC mice during β-adrenergic stimulation; however, AC Fgf2^+/+ had a blunted response to increasing dobutamine levels.

Figure 6

Expression pattern of MHC mRNA and protein levels in NC and 10-week AC Fgf2^+/+ and Fgf2^–/– mice. *P < 0.05 vs. NC. ^§P < 0.05 vs. NC Fgf2^+/+. n = 4 for NC; n = 3 for AC Fgf2^+/+ and Fgf2^–/–. (a) Compared with NC Fgf2^+/+, NC Fgf2^–/– mice had a significantly higher degree of Myhca (α-MHC; open bars) and Myhcb (β-MHC; filled bars) message. Ten-week AC caused a significant increase in Myhcb mRNA expression and a significant decrease in Myhca in Fgf2^+/+ and Fgf2^–/–, respectively. (b) NC Fgf2^+/+ and Fgf2^–/– mice have similar levels of α-MHC (open bars) and β-MHC (filled bars) protein. Ten-week AC caused a significant decrease in α-MHC and a significant increase in β-MHC protein levels in Fgf2^+/+ and Fgf2^–/– mice.