Abstract
Secreted frizzled related protein 2 (sFRP2) has been shown to protect against myocardial ischemia primarily mediated by reducing fibrosis. Our hypothesis is that sFRP2 myocardial ischemic protection induced by permanent coronary artery occlusion (CAO) must involve a primary angiogenesis/arteriogenesis mechanism in order to protect the heart in the chronic absence of blood flow. To test this hypothesis, we induced permanent CAO for 1 week in cardiac specific sFRP2 transgenic (TG) mice (n=12), which exhibited reduced infarct size vs. their wild type littermates (WTL; n=13) (16±1 vs. 25±2%) and improved left ventricular ejection fraction (LVEF) (61±6 vs. 45±6%). These salutary effects were accompanied by angiogenesis (170% increase in capillary proliferation and a 54% increase in capillary density) and arteriogenesis (106% increase in arteriole proliferation). sFRP2 increased in vitro tubule formation in HUVEC cells by 46% (p<0.05), compared to vehicle. As expected, sFRP2 TG mice also exhibited decreased remote fibrosis, compared to WTL (1.6±0.1 vs.± 3.7±0.3%; n=4-6/group). ATF6, an ER stress response transcription factor, was upregulated in sFRP2 TG mice. When ATF6 was blocked with AEBSF, sFRP2 TG mice no longer demonstrated cardioprotection with permanent CAO, as reflected by similar infarct size, LV EF, capillary and arteriolar proliferation observed in WTL. cTGF, a pro-angiogenic factor, was also upregulated in sFRP2 TG mice. Blocking cTGF with a cTGF antibody, eliminated the increased tubule formation with sFRP2 treatment. AEBSF also eliminated the increased cTGF in sFRP2 TG, confirming ATF6 as the upstream regulator of cTGF, responsible for the angiogenesis and arteriogenesis observed in the sFRP2 TG mice. To determine if angiogenesis/arteriogenesis was the initiating mechanism inducing cardioprotection, blood flow, measured ultrasonically at 1 day following CAO, was increased significantly more, p<0.05, in the ischemic zone of sFRP2 TG mice (2.1±0.46 A.U.) compared with blood flow in WTL (0.4 ±0.09 A.U.). Thus, angiogenesis/arteriogenesis appears to be the primary cardioprotective mechanism induced by sFRP2, with reduced fibrosis playing a secondary role.
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