SM22α Loss Contributes to Apoptosis of Vascular Smooth Muscle Cells via Macrophage-Derived circRasGEF1B.

Vascular smooth muscle cell (VSMC) apoptosis is a major defining feature of abdominal aortic aneurysm (AAA) and mainly caused by inflammatory cell infiltration. Smooth muscle (SM) 22α prevents AAA formation through suppressing NF-κB activation. However, the role of SM22α in VSMC apoptosis is controversial. Here, we identified that SM22α loss contributed to apoptosis of VSMCs via activation of macrophages. Firstly, deficiency of SM22α enhanced the interaction of VSMCs with macrophages. Macrophages were retained and activated by Sm22α -/- VSMCs via upregulating VCAM-1 expression. The ratio of apoptosis was increased by 1.62-fold in VSMCs treated with the conditional media (CM) from activated RAW264.7 cells, compared to that of the control CM (P < 0.01), and apoptosis of Sm22α -/- VSMCs was higher than that of WT VSMCs (P < 0.001). Next, circRasGEF1B from activated macrophages was delivered into VSMCs promoting ZFP36 expression via stabilization of ZFP36 mRNA. Importantly, circRasGEF1B, as a scaffold, guided ZFP36 to preferentially bind to and decay Bcl-2 mRNA in a sequence-specific manner and triggered apoptosis of VSMCs, especially in Sm22α -/- VSMCs. These findings reveal a novel mechanism by which the circRasGEF1B-ZFP36 axis mediates macrophage-induced VSMC apoptosis via decay of Bcl-2 mRNA, whereas Sm22α -/- VSMCs have a higher sensitivity to apoptosis.