High- but not low-molecular weight FGF-2 causes cardiac hypertrophy in vivo; possible involvement of cardiotrophin-1

The heart expresses high and low molecular weight (hmw, lmw) fibroblast growth factor 2 (FGF-2) isoforms. While the injury-repair-related activities of lmw-FGF-2 have been studied extensively, those of hmw-FGF-2 have not. Thus, we investigated the effects of hmw-FGF-2 on acute as well as chronic responses to myocardial infarction (MI) induced by irreversible coronary occlusion in the rat. Hmw- or lmw-FGF-2 was injected into the ischemic zone during acute evolving MI. Both isoforms were equally effective in reducing infarct size (at 24 h post-MI) and improving heart function up to 6 weeks post-MI, compared to a vehicle-treated infarcted group. Lmw-FGF-2 alone upregulated vascularization in the infarct. Hmw-FGF-2 elicited significant hypertrophy, compared to the vehicle-treated group, at 4-8 weeks post-MI, assessed by ultrasound, heart morphometry and cardiomyocyte cross-sectional area. In addition, hmw- (but not lmw-) FGF-2-treated hearts displayed increased accumulation of the cytokine cardiotrophin-1 and its signal transducer gp130. In culture, hmw- (but not lmw-) FGF-2 increased cardiomyocyte protein synthesis and cell size as well as upregulated cardiotrophin-1 released by cardiac fibroblasts, pointing to similar activities in vivo. Thus, hmw- and lmw-FGF-2 exert isoform-specific effects in the heart and only hmw-FGF-2 triggers cardiomyocyte hypertrophic growth. Direct effects of hmw-FGF-2 on cardiomyocytes, becoming reinforced and sustained by upregulation of cardiotrophin-1 and acting in concert with other factors, are likely to contribute to post-MI hypertrophy.