ABSTRACT
OBJECTIVE: The intracellular bacterium Chlamydia pneumoniae is present in many atherosclerotic lesions, where it could promote inflammation. This study determined whether monocyte chemoattractant protein 1 (MCP-1) release is stimulated in vascular smooth muscle cells (VSMCs) that are exposed to or infected by C pneumoniae and whether toll-like receptor 2 (TLR2) or TLR4 mediate these effects. METHODS AND RESULTS: TLR2 mRNA was expressed constitutively and was upregulated by C pneumoniae exposure in mouse aortic SMC and was inducible by C pneumoniae and TLR3 and TLR4 agonists in human coronary artery SMCs. Exposure to inactivated or viable extracellular C pneumoniae evoked a robust increase in MCP-1 release and activated nuclear factor-kappaB and extracellular signal-regulated kinase 1/2 in wild-type and TLR4 signaling-deficient mouse aortic SMCs but not in TLR2-deficient SMCs, probably because of TLR2-mediated recognition of a chlamydial antigen. Brief exposure to viable C pneumoniae led to active infection of VSMCs, shown by chlamydial protein synthesis, and caused a persistent (>48-hour) MCP-1 release that was also TLR2 dependent. CONCLUSIONS: The results show that VSMCs express functional TLR2 and that TLR2 mediates both a persistent activation of chemokine release in C pneumoniae-infected VSMCs and its acute stimulation by extracellular C pneumoniae. Therefore, TLR2 expressed in VSMCs may promote inflammation within the arterial wall.
Subject(s)
Chemokines/metabolism , Chlamydia Infections/physiopathology , Chlamydophila pneumoniae , Muscle, Smooth, Vascular/microbiology , Toll-Like Receptor 2/metabolism , Vasculitis/physiopathology , Animals , Aorta/cytology , Cells, Cultured , Chlamydia Infections/immunology , Chlamydia Infections/metabolism , Coronary Vessels/cytology , Humans , Mice , Mice, Inbred C57BL , Muscle, Smooth, Vascular/cytology , Muscle, Smooth, Vascular/physiology , RNA, Messenger/analysis , Toll-Like Receptor 2/genetics , Toll-Like Receptor 4/genetics , Toll-Like Receptor 4/metabolism , Vasculitis/immunology , Vasculitis/microbiologyABSTRACT
Recent evidence supports a role of Toll-like receptor (TLR) signaling in the development of atherosclerotic lesions. In this study, we tested whether TLR4 signaling promotes a proinflammatory phenotype in human and mouse arterial smooth muscle cells (SMC), characterized by increased cytokine and chemokine synthesis and increased TLR expression. Human arterial SMC were found to express mRNA encoding TLR4 and the TLR4-associated molecules MD-2 and CD14 but not TLR2 mRNA. Mouse aortic SMC, on the other hand, expressed both TLR2 and TLR4 mRNA constitutively. Human SMC derived from the coronary artery, but not those from the pulmonary artery, were found to express cell surface-associated CD14. Low concentrations (ng/ml) of Escherichia coli LPS, the prototypical TLR4 agonist, markedly stimulated extracellular regulated kinase 1/2 (ERK1/2) activity, induced release of monocyte-chemoattractant protein-1 (MCP-1) and interleukin (IL)-6, and stimulated IL-1alpha expression in human aortic SMC, and exogenous CD14 enhanced these effects. Expression of a dominant negative form of TLR4 in human SMC attenuated LPS-induced ERK1/2 and MCP-1 release. LPS was a potent inducer of NF-kappaB activity, ERK1/2 phosphorylation, MCP-1 release, and TLR2 mRNA expression in wild-type mice but not in TLR4-signaling deficient mouse aortic SMC. These studies show that TLR4 signaling promotes a proinflammatory phenotype in vascular smooth muscle cells (VSMC) and suggest that VSMC may potentially play an active role in vascular inflammation via the release of chemokines, proinflammatory cytokines, and increased expression of TLR2.
