Abstract 1223: Mice with cardiomyocyte-restricted Smad4 deletion resist to Angiotensin II induced cardiac hypertrophy
Background: It has been shown that Angiotensin II, a potent hypertrophic stimulus for cardiomyocytes, causes significant increases in gene expression of Transforming Growth Factor β1 (TGFβ1). Smad4 mediates TGF-β signaling via forming a heteromeric complex with other Smads and translocating into the nucleus to regulate expression of TGF-β responsive genes. It has been shown that Smad4 is essential for the heart in embryonic and post-natal development. However, it is unclear if Smad4 is involved in Angiotensin II induced cardiac growth in a post-developmental stage.
Methods and results: To investigate a potential role of Smad4 mediating Angiotensin II signaling in cardiac growth, we generated a cardiomyocyte-restricted Smad4 knockout line (CS4K) using Cre-loxP technology. A previously published mouse line with cardiomyocyte-restricted Cre overexpression (α-MyHC-Cre) was used for the crossing and as control. Up to 90% of Smad4 was eliminated in cardiomyocytes isolated from the CS4K mice. The CS4K mice were overtly normal during the first 5–6 months after birth. No detectable pathophysiological abnormality could be detected during this period of time. Angiotensin II was administered in 2-month-old CS4K and controlled mice with matched age and gender via subcutaneous implantation of mini-osmotic pumps containing a subpressor dose of Angiotensin II in saline (100 ng/kg/min) for 4 weeks. Angiotensin II treatment did not induced blood pressure elevation but induced cardiac hypertrophy in both CS4K and controlled mice with increased heart weight to tibial length (HW/TL) ratio and ANF expression. However, CS4K mice exhibited substantially smaller increased in HW/TL ratios and ANF expression compared with controlled mice (n=9, P<0.05 and P<0.01 respectively). Histological examination also revealed that Angiotensin II-induced fibrosis in CS4K hearts was substantially less than those of controlled hearts.
Conclusion: Together, we conclude that Smad4 is an important signaling mediator in Angiotensin II-induced cardiac growth.