To our knowledge, the present study is the first to demonstrate the involvement of FGF2 in the growth response of cardiac hypertrophy at the in vivo physiological level. Echocardiographic measurements, gravimetry, and cardiomyocyte cross-sectional area reveal that an absence of FGF2 results in a statistically lesser degree of hypertrophy during pressure overload (Figures a, , and ). Owing to their noninvasive technique and the ability to evaluate cardiac disease and function in a serial fashion, M-mode and pulsed-wave Doppler echocardiography are excellent methods for measuring LV mass, LV function, and aortic blood flow velocities (30
). We have used this noninvasive application to assess the weekly progression of cardiac hypertrophy after transverse AC of Fgf2+/+
mice. Our present data support previous findings on the validity of echocardiographic measurements of LV mass and function with other technical applications. This is also the first investigation to show that the echocardiographic measurements of pressure gradient after AC are comparable to the gradients obtained from catheterizations of the LV and femoral artery (Tables and ).
The echocardiographic images from AC Fgf2+/+
mice revealed an eccentric form of cardiac hypertrophy (Figure b). Eccentric LV hypertrophy has been well documented to occur with a volume overload, whereas concentric LV hypertrophy results from a pressure overload situation, such as hypertension (1
). However, recent human and rodent studies have demonstrated that concentric LV hypertrophy and high blood pressure do not necessarily correlate, and, in fact, have indicated that eccentric LV hypertrophy characterizes the early phase of cardiac adaptation to pressure overload (34
). Therefore, the eccentric pattern of LV hypertrophy that we observed may also represent the early phase of adaptation to high blood pressure. If these echocardiographic measurements were continued past 10 weeks after AC, we may have observed the transition to concentric hypertrophy. However, the Fgf2+/+
mice were only analyzed for 10 weeks after AC, because our preliminary echocardiographic data showed that LV function begins to decrease beyond 12 weeks after AC in this mouse model. We did not want to confuse potential roles of FGF2 in the development of hypertrophy with heart failure. The %FS data (Figure c) demonstrate that AC Fgf2+/+
mice are in a compensated stage of cardiac hypertrophy, because cardiac function was not significantly different throughout the time course of hemodynamic load.
FGF2 does not have the signal peptide sequence for its release from cells (37
). Therefore, the mechanism(s) that initiates the release of growth factors remains unclear. The current view is that FGF is synthesized and stored in cardiac myocytes and nonmyocytes, and released in response to a hemodynamic stress (7
). Its release would result in an autocrine or paracrine effect mediated via FGF receptor signaling to regulate gene transcription of the contractile machinery and the hypertrophic growth response (7
). Besides FGF2 (41
), a number of other FGFs, including FGF1 (41
), FGF7 (49
), FGF10 (50
), FGF12 (52
), FGF13 (52
), FGF16 (53
), and FGF18 (54
), along with FGF receptors (55
), have been localized to the heart either during development or in the adult. Despite the many FGFs found in the heart, in the present study, the absence of the Fgf2
gene resulted in a marked reduction of hypertrophy, suggesting that FGF2 has a major role in the growth response to hemodynamic load. It is not surprising that the cardiac hypertrophy was not completely abrogated during hemodynamic stress, as other molecules (e.g., FGF1, TGF-β1
, angiotensin II, catecholamines) have also been implicated in mediating this response (8
). However, Harada et al. (56
) recently demonstrated a lack of involvement of the renin-angiotensin system in a mouse model of pressure overload. This group showed that abdominal AC wild-type and AT1A
receptor–deficient mice achieved the same degree of cardiac hypertrophy, ventricular remodeling, and reexpressed fetal cardiac genes.
A number of reexpressed fetal cardiac genes, including Myhcb
, have been considered as markers for cardiac hypertrophy (3
). Recent work has demonstrated that different hypertrophic stimuli elicit unique cardiac gene profiles (60
). Furthermore, a number of studies have shown that cardiac hypertrophy does not always correlate with the expression pattern of fetal cardiac genes and other hypertrophic indicators (14
). In the present study, the changes in expression and protein levels of the cardiac MHC genes in response to pressure overload differed in an FGF2-independent manner (Figure ). The AC Fgf2+/+
mice demonstrate the typical and extensively documented cardiac gene profile of increased Myhcb
and mildly to greatly decreased Myhca
. In the absence of FGF2, NC mice have a significant level of Myhca
expression, suggesting that this growth factor may have an inhibitory role on cardiac myosin gene expression in the heart under normal conditions. Because the rates at which posttranscriptional regulation converts myosin mRNAs to protein are about the same, regardless of the levels of FGF2 or the degree of pressure overload or hypertrophy, the isoform switch appears to be primarily a function of mRNA expression, not one of an alteration in levels of posttranscriptional regulation. Our data show that FGF2 is a potent inducer of cardiac hypertrophy but that it has no significant effect on isoform switching. Only pressure overload correlates with isoform switching, as it occurs equally as much when there is much less hypertrophy and in the complete absence of FGF2. Also, consistent with the work by Dorn et al. (64
), the degree to which the fetal isoform (β-MHC) increased (15%) in the cardiomyofibril during pressure overload in Fgf2+/+
mice is not enough to decrease cardiac function (%FS), as assessed by echocardiography (Figure c). The depressed rate of contraction (+dP/dt) to β-adrenergic stimulation in AC Fgf2+/+
mice may be an effect of the degree of hypertrophy, as these mice have a 41–52% increase in mass compared with AC Fgf2–/–
mice. However, the rate of relaxation (–dP/dt) was similar between the AC and NC mice.
Over the last 10 years, considerable evidence has accumulated demonstrating a correlative relationship between increased levels of FGF2, cardiac hypertrophy, and reversion of muscle structural mRNA to the fetal isoforms. The results presented here definitively demonstrate that FGF2 is a major stimulatory component of the growth aspects of cardiac hypertrophy, and that hemodynamic stress, rather than FGF2 and hypertrophy, correlates with isoform switching. Finally, although both transcriptional and posttranscriptional controls determine the ratios of myosin isoforms in the cardiomyofibril, the isoform switching that occurs under hemodynamic stress results from alterations in transcriptional, not posttranscriptional, regulation.