Abstract 33: β-Catenin Is Essential for Vascular Smooth Muscle Cell Survival and Artery Formation

Abstract

Background: The molecular mechanisms required for formation of a competent vessel wall are not fully understood. Although β-catenin (β-ctn) has an essential developmental function in various cell types, its specific contributions to vascular smooth muscle cell (SMC) biology have not been fully determined. In preliminary studies, we found that SMC-selective β-ctn inactivation (KO) in mice was incompatible with postnatal survival. Hypothesis: β-ctn is a key modulator of vascular SMC function during development. Methods: We analyzed mouse embryos from stages E9.5-19.5. β-ctn expression and endothelial and SMC markers were tested by immunohistochemistry. Proliferation and apoptosis were assessed by phospho-histone 3 and TUNEL assays, respectively. In studies of cultured SMCs lacking β-ctn, we followed cell population growth and assessed proliferation and cell cycle by BrdU uptake and FACS analysis. Cell death was induced by serum starvation, hydrogen peroxide, or agonistic anti-Fas antibody, and assessed by LIVE/DEAD assay or FACS. Results: β-ctn inactivation in SMCs caused embryonic lethality by E12.5. Paired and fused dorsal aortae in KO embryos had thinner walls (wall/lumen area ratio 0.38 vs 0.18 in control vs. KO at E11.5, p<0.001, n=4). KO vessels showed intact endothelial layers, negligible β-ctn expression, few SMCs, slightly reduced cell proliferation, and more apoptosis (0.30% vs 1.24% in control vs. KO, p<0.05, n=11). By E12.5, KO vessels were severely disrupted, allowing extravasation. Cultured SMCs lacking β-ctn grew more slowly, plateaued at lower cell density than control (p<0.0001, n=3), and showed decreased cell cycle progression and more death upon serum starvation (12.0% vs. 21.8% in control vs. KO, p=0.052, n=3). Interestingly, cell death induction by other stimuli (oxidative stress or Fas ligand activation) had the opposite effect, with loss of β-ctn conferring protection (p<0.01, n=3). Conclusion: β-ctn is essential for SMC survival and formation of a competent arterial vessel wall during development, but its survival function in SMCs appears to be stimulus-specific. β-ctn may oppose or promote SMC death depending on specific cellular and environmental conditions and stimuli.