Abstract 3469: Unphosphorylated STAT3 Mediates Pathological Gene Expression Associated With Angiotensin II Type 1 Receptor-induced Cardiac Hypertrophy
Angiotensin II (Ang II) modulates cardiovascular effects primarily through type 1 receptor (AT1R). Cardiomyocyte overexpression of AT1R in mouse (TG) leads to development of cardiac hypertrophy and heart failure (HF) but the mechanism is unclear. To gain insights into the mechanisms, we used a broad spectrum of approaches including echocardiography, immunohistostaining, immunoblotting, real-time RT-PCR, and microarray analysis. A significant increase of STAT3 (signal transducer and activator of transcription 3) mRNA associated with higher expression of total but not phosphorylated STAT3 protein was found in TG, indicating a net increase of unphosphorylated STAT3 (U-STAT3). The nuclear accumulation of U-STAT3 is 4 times higher in TG than in non-transgenic mice (NTG), increased with the development of cardiac hypertrophy and age of the TG mice and negatively correlated with cardiac function (p<0.0001, r=−0.985). In cultured cardiac fibroblasts, neonatal cardiomyocytes, and AT1R overexpressing HEK293 cells (AT1R-HEK), Ang II treatment caused persistent accumulation of total STAT3 in the nucleus, but phosphorylated STAT3 (P-STAT3) peaked in 1 hour then returned to basal level, indicating a differential between P-STAT3 and U-STAT3 nuclear accumulation. By comparing several sets of microarray gene expression data, regulator of G-protein signaling 2, osteopontin, connective tissue growth factor, and tissue inhibitors of metalloproteinases 1 were identified as potential downstream targets of U-STAT3. Expression of these genes was significantly altered in TG compared to NTG. siRNA based knock-down of STAT3 in AT1R-HEK caused the down-regulation of these genes. Ang II treatment in AT1R-HEK expressing phosphorylation deficient mutant STAT3, Y705F, caused nuclear accumulation of the mutant U-STAT3 and physically associated with p300 and p65. Thus, Ang II/AT1R induced expression of STAT3 promotes nuclear accumulation of U-STAT3 leading to pathogenic gene expression and subsequent development of cardiac hypertrophy through interaction with p300 and p65. We will present a model that explains the dual function of STAT3 in regulating physiological and pathological gene expression programs with both protective and deleterious cardiac effects.