Abstract 3635: Epac1 Protects Heart From Lipopolysaccharide-induced Cardiac Dysfunction by Inhibiting the Toll-like Receptor 4 Signaling Pathway
Prolonged activation of sympathetic nerve activity and thus overproduction of cyclic AMP (cAMP) in cardiac myocytes is known to deteriorate cardiac function in heart failure. Such overproduction of cAMP activates protein kinase A and induces harmful effects on cardiac myocytes. Recent studies have shown that cAMP can activates Epac, exchange protein directly activated by cAMP, as well, while the role of Epac in the pathogenesis of heart failure remains unknown. In order to examine the role of Epac in the heart, we have generated cardiac-specific overexpression of Epac1 (Epac1TG) and examined their response in lipopolysaccharide (LPS)-induced cardiac dysfunction, a well-established model for sepsis-induced cardiac dysfunction. LPS (5mg/kg) elicited a significant and robust reduction of LV ejection fraction in both Epac1TG and non-transgenic mice (NTG). However, the magnitude of this decrease was much less in Epac1TG (19±2.1%) at 6 hrs after injection than that in WT (36±2.4%, P<0.01, n=15–16). Toll-like receptor 4 (TLR4) is activated by LPS and therefore known as the LPS receptor, which is also expressed in the heart and may regulate cardiac function during sepsis. We thus examined the effects of Epac1 on LPS-mediated activation of the TLR4 pathway. The cytosolic localization of TRIF-related adaptor molecule (TRAM), which binds to the TLR4 and translocates to cytosol to propagate signals further downstream, was significantly increased after LPS treatment in WT (40±6.1%) while not in Epac1TG (1.7+6.4%; n=5, P<0.01). The phosphorylation of interferon regulatory factor (IRF)-3 on serine 396, which immunoprecipitates with TRAM, was significantly increased in WT (101±0.4%) after LPS treatment while not in Epac1TG (13±12.2; n=4–7, P<0.01). The phosphorylation of signal transducer and activator of transcription 1 (STAT1) on tyrosine 701, which is a downstream molecule of IRF-3 and harmful to cardiac myocytes, was increased more in WT after LPS treatment than that in Epac1TG (WT vs. Epac1TG; 100±13% vs. 62±9.9%; P<0.05, n=5–8). Taken together, Epac1 protects heart, at least in part, through the inhibition of the TLR4 pathway, suggesting a beneficial role played by sympathetic signal, i.e., antagonizing the TLR4 pathway in heart failure.