Background-Preconditioning the heart before an ischemic insult has been shown to protect against contractile dysfunction, arrhythmias, and infarction. Pharmacological studies have suggested that fibroblast growth factor-2 (FGF2) is involved in cardioprotection. However, because of the number of FGFs expressed in the heart and the promiscuity of FGF ligand-receptor interactions, the specific role of FGF2 during ischemia-reperfusion injury remains unclear. Methods and Results-FGF2-deficient (Fgf2 knockout) mice and mice with a cardiac-specific overexpression of all 4 isoforms of human FGF2 (FGF2 transgenic [Tg]) were compared with wild-type mice to test whether endogenous FGF2 elicits cardioprotection. An ex vivo work-performing heart model of ischemia was developed in which murine hearts were subjected to 60 minutes of low-flow ischemia and 120 minutes of reperfusion.

BACKGROUND: Our laboratory showed that overexpression of fibroblast growth factor-2 (FGF2) protected the heart against ischemia-reperfusion injury. FGF2 has different protein isoforms (low [LMW] and high [HMW] molecular weight isoforms) produced from alternative translation start sites. However, which FGF2 isoform(s) mediates this cardioprotection, and which signaling pathway (i.e., mitogen-activated protein kinase (MAPK)) elicits FGF2 isoform-induced cardioprotection remains to be elucidated.

Fibroblast growth factor 2 (FGF2) consists of multiple protein isoforms (low [LMW] and high molecular weight [HMW]), which are localized to different cellular compartments, indicating unique biological activity. We previously showed that the LMW isoform is important in protecting the heart from myocardial dysfunction associated with ischemia-reperfusion (I/R) injury, but the roles of the HMW isoforms remain unknown. To elucidate the role of HMW isoforms in I/R and cardioprotection, hearts from novel mouse models,in which the murine FGF2 HMWs are knocked out (HMWKO) or the human FGF2 24 kDa HMW isoform is overexpressed (HMW Tg) and their wildtype (Wt) or non-transgenic (NTg) cohorts were subjected to an ex vivo work-performing heart model of I/R. There was a significant improvement in post-ischemic recovery of cardiac function in HMWKO hearts (76±5%, p<0.05) compared to Wt hearts (55±5%), with a corresponding decrease in HMW Tg function (line 20: 38±6% and line 28: 33±4%, p<0.05) compared to non-transgenic hearts (68±9%). FGF2 LMW isoform was secreted from Wt and HMWKO hearts during I/R, and a FGF receptor (FGFR) inhibitor, PD173074 caused a decrease in cardiac function when administered in I/R in Wt and FGF2 HMWKO hearts (p<0.05), indicating that FGFR is involved in FGF2 LMW isoform's biological effect in ischemia-reperfusion injury. Moreover, overexpression of HMW isoform reduced FGFR1 phosphorylation/activation with no further decrease in the phosphorylation state in the presence of the FGFR inhibitor. Overall, our data indicate that HMW isoforms have a detrimental role in the development of post-ischemic myocardial dysfunction.