Abstract 477: SM22 Deficiency Induces SRF Nuclear Translocation and Promotes Carotid Medial Fibrosis in Response to Injury
Introduction: SM22 (aka transgelin) is known to be an actin-binding protein that is rapidly downregulated in response to vascular injury. Despite prevalent view regarding SM22 as a maker of VSMC dedifferentiation, recently a series of studies including our own demonstrate that SM22 downregulation promotes SMC dedifferentiate into a variety of SMC subtypes involving in proliferation, migration, inflammation, osteochondrogendesis and apoptosis.
Hypothesis: In the present study, we assess the hypothesis that SM22 regulates the expression of col1a2, a fibrotic marker, in SMCs.
Methods: We use molecular and cellular approaches to analyze the regulation of col1a2 in smooth muscle cells upon artery injury using Sm22 knockout mice.
Results: We found that carotid injury induces high expression of Col1a2 in the medial layer of the vessel wall and the nuclear translocation of serum response factor (SRF), a key transcription factor that regulates muscle-specific genes as well as growth factor response genes. To determine whether col1a2, an extracellular matrix protein, is a target of SRF, we performed bioinformatics analysis: we identified an evolutionarily conserved SRF binding sequence, CC(A/T)6GG (known as the CArG box), in the col1a2 promoter. Luciferase transfection assay showed that Vp16-SRF significantly transactivated the col1a2 promoter; mutation of the CArG box abolished most of this stimulating effect. Gel shift and chromatin precipitation assays confirmed that SRF bound to the CArG box of the col1a2 promoter. To investigate whether loss of SM22 in SMCs contributes to increased nuclear SRF expression, we knocked down SM22 in PAC1 cells, a VSMC cell line, using SM22 siRNA. Consistent with the in vivo observation, knockdown SM22 also enhanced nuclear SRF expression. However, cola2 transcription was not upregulated, suggesting that additional signals may be involved in this regulation in vivo.
Conclusions: SM22 deficiency increases the fibrosis in the arterial media, and the induction of nuclear SRF may contribute at least in part to col1a2 transcription in response to artery injury.
- © 2012 by American Heart Association, Inc.