Overexpression of long or short FGFR-1 results in FGF-2-mediated proliferation in neonatal cardiac myocyte cultures

Objective: The type 1 fibroblast growth factor receptor (FGFR-1) is the onl y high affinity receptor for fibroblast growth factor-2 (FGF-2) in the rat myocardium, and is essential for normal growth and development of the heart . Levels of FGFR-1 are developmentally regulated, being high in embryonic c ardiac myocytes. Also, FGFR-1 exists as both 'long' and 'short' isoforms, a nd there is a switch from predominant expression of the 'long' isoform in t he embryo to the 'short' isoform in the adult heart. Both the decrease in r eceptor levels and the isoform switch in postnatal cardiac myocytes correla te with a loss of proliferative potential. We investigated whether an incre ase in either 'long' or 'short' FGFR-1 isoforms could stimulate proliferati on in postnatal rat cardiac myocyte cultures. Methods and Results: Previous ly we cloned cDNAs corresponding to 'long' (L) and 'short' (S) FGFR-1 isofo rms from embryonic mouse hearts. Hybrid FGFR-1(L) and (S) genes, directed b y a myosin light chain-2 promoter and SV40 enhancer sequences, were generat ed and used to transiently transfect neonatal rat cardiac myocytes. Overexp ression of FGFR-1 mRNA and protein was detected by RNA blotting and immunoc ytochemistry. Ligand-crosslinking confirmed the presence of specific recept ors capable of binding FGF-2 on the cell membrane. Overexpression of either FGFR-1(L) or (S) was associated with stimulation of proliferation as asses sed by significant increases in bromodeoxyuridine uptake (DNA synthesis) an d cell number. To determine whether this response was FGF-2 specific, the l evel of FGF-2 was assessed in the culture medium of cardiac myocytes overex pressing FGFR-1 isoforms. A three-fold increase was detected in the media o f cardiac myocytes overexpressing either FGFR-1(L) or (S) compared to contr ol levels. Neutralization of this FGF-2 with antibodies inhibited the proli ferative response. Conclusion: Overexpression of either FGFR-1(L) or (S) re sulted in an increase in FGF-2-mediated proliferation of postnatal rat card iac myocytes. (C) 1999 Elsevier Science B.V. All rights reserved.