Abstract 12533: Mast Cells Induce Vascular Smooth Muscle Cell Apoptosis in Atherosclerotic Plaques via a Toll-Like Receptor 4 Activation Pathway
Rationale Vascular smooth muscle cells (VSMCs) play a crucial role in the stabilization of the fibrous cap of atherosclerotic plaques. Activated mast cells may contribute to vulnerable plaque formation by inducing VSMC apoptosis. Although it has been shown that mast cells can induce VSMC apoptosis by chymase release, the mechanism whereby mast cells release chymase in atherosclerosis is unknown. We hypothesized that mast cell-associated VSMC apoptosis is regulated by Toll-like receptor 4 (TLR4) signaling.
Methods & Results In a vulnerable plaque model using Apolipoprotein E knockout mice, we showed that local recruitment and activation of mast cells with DNP-HSA reduced VSMC content in the cap region by 80% (8.6±2.6% to 1.9± 0.7%; p<0.05, n=8 per group). This was prevented by co-treatment with a TLR4 antagonist (Bartonella LPS; 1.9±0.7% to 7.0±10.3%; p<0.0001). Likewise, co-culture of VSMCs with E. Coli LPS-activated mast cells (MC9 cells), increased VSMC apoptosis by 50% (12.5±0.8% versus 8.2±2.2% with non-activated MC9 cells). Mast cell-associated VSMC apoptosis was inhibited by a TLR4 antagonist and a TLR4 antibody (5.9±1.3%, p<0.002 and 7.9±0.8%, p<0.01 respectively), as well as by Soy Bean Trypsin Inhibitor (SBTI), a specific chymase inhibitor (13.6±0.2% to 10.0±0.3%, p<0.001), indicating that TLR4 activation is involved in chymase release in MC9 cells. This pathway is mediated via IL-6, as TLR4 activation on MC9 cells induced IL-6 production (undetectable in control conditions to 393.5±46.6 pg/ml, p<0.001), which was reduced by pre-incubation with a TLR4 antagonist or antibody (50.2±28.2 pg/ml, p<0.01 and 228.5±29.5 pg/ml, p<0.05 respectively). Furthermore, IL-6 promoted mast cell associated VSMC apoptosis (10.3±0.2% to 16.4±1.2, p<0.01), which was inhibited by pre-incubation with SBTI (16.4±1.2% to 12.0±0.7%, p<0.05). Data are presented as mean ± SEM.
Conclusion To our knowledge, this is the first study to show that mast cells promote VSMC apoptosis in atherosclerotic plaques in vivo. In addition, we conclude that TLR4 signaling may play an important role in chymase release in mast cells and contribute to fibrous cap thinning and plaque destabilization by promoting VSMC apoptosis.
- © 2010 by American Heart Association, Inc.