Chronic beta2-adrenergic receptor stimulation increases proliferation of human cardiac fibroblasts via an autocrine mechanism.

OBJECTIVE The aim of this study was to determine whether chronic beta-adrenergic receptor (beta-AR) stimulation induces proliferation of human cardiac fibroblasts and to investigate the mechanism(s) involved. METHODS AND RESULTS In vitro cultures of human cardiac fibroblasts were established from biopsies of right atrial appendage. RT-PCR analysis and pharmacological studies demonstrated that these cells express predominantly the beta(2)-AR subtype coupled to activation of adenylyl cyclase and p44/42 mitogen-activated protein kinase (MAPK). Proliferation was determined by cell counting over a 6-day period in medium containing 2.5% fetal calf serum (control) or supplemented with the non-selective beta-AR agonist isoproterenol (ISO). ISO induced a concentration-dependent increase in cardiac fibroblast proliferation, which was maximal at 1 micromol/l. This increased proliferation was inhibited by the beta(2)-AR-selective antagonist ICI-118,551, but not the beta(1)-AR-selective antagonist atenolol. Direct activation of adenylyl cyclase alone (0.1-10 micromol/l forskolin) stimulated cyclic AMP production and MAPK activation, but did not induce cell proliferation. Since catecholamines are not considered to be 'classical' growth factors, we subsequently investigated whether beta(2)-AR stimulation results in secretion of growth factors that are able to stimulate proliferation in an autocrine manner. Conditioned medium obtained from cardiac fibroblasts treated with ISO for 48 h increased proliferation of parallel cultures of fibroblasts in the presence of the beta-AR antagonist alprenolol. Heat-treatment of this conditioned medium fully prevented the increase in cell proliferation, indicating that the autocrine factor(s) are heat-sensitive proteins. CONCLUSIONS Chronic beta(2)-AR stimulation increases proliferation of human cardiac fibroblasts via a mechanism involving increased secretion of heat-sensitive growth factors.