This study was aimed to determine whether beta-adrenergic receptor ( beta-AR) stimulated by isoproterenol (ISO) activates signal transducers and activators of transcription ( STAT) in mouse heart and, if so, to examine the underlying mechanism. We found that treatment of adult male mice by ISO ( 15 mg/kg body weight, intraperitoneal) caused a delayed STAT3 activation ( at 60 120 min), which was fully abolished by beta-AR antagonist, propranolol. ISO-induced phosphorylation of STAT3 was markedly enhanced by phosphodiesterase inhibitor amrinone, indicating that cAMP is critically involved in beta-AR-mediated STAT3 activation. In addition, beta-AR stimulation significantly increased gene expression of interleukin-6 (IL-6) family of cytokines (IL-6, leukemia inhibitory factor, ciliary neurotrophic factor, and cardiotrophin-1). IL-6 protein levels in serum and mouse myocardium were also significantly increased in response to ISO treatment. In cultured cardiac fibroblasts, IL-6 level was enhanced significantly after ISO (10(-6) mol/liter) stimulation for 2 h and then peaked at 12 h, whereas the response of IL-6 in cultured cardiomyocytes to ISO stimulation was not significant, suggesting that ISO-induced increase in IL-6 is primarily from cardiac fibroblasts rather than cardiomyocytes. Most importantly, IL-6 could activate STAT3 in a time-dependent manner in cultured cardiomyocytes, and inhibition of IL-6 level by anti-IL-6-neutralizing antibody clearly attenuated ISO-induced phosphorylation of STAT3 in myocardium. Taken together, these results indicate that beta-AR stimulation leads to a delayed STAT3 activation via an IL-6 family of cytokine-mediated pathway and that cardiac fibroblasts, but not cardiomyocytes, is probably the predominant source of IL-6 in response to ISO stimulation in mouse myocardium.
