Downregulation of smooth muscle alpha-actin expression by bacterial lipopolysaccharide.
OBJECTIVE
Smooth muscle alpha-actin (SMA) is a cytoskeletal protein characteristic to vascular smooth muscle cells (VSMC), and it serves to facilitate cell contraction and migration. Bacterial lipopolysaccharide (LPS), a major mediator of septic shock secondary to infection, is known to directly affect VSMC. The objective of this study was to investigate the effect of LPS on the expression levels of SMA in VSMC.


METHODS
This study was performed on cultured VSMC derived from human aorta, human coronary artery, or rat aorta.


RESULTS
We show that SMA expression in VSMC, induced by endothelin-1 (ET1) or transforming growth factor-beta (TGF-beta), is potently inhibited by a LPS. This parallels a decreased migration of VSMC after LPS treatment. Downregulation of SMA by LPS is not a result of altered signaling of ET1 or TGF-beta receptors, and it is not mediated by canonical (for LPS) mechanisms, such as production of prostaglandins or nitric oxide, or secretion of other endocrine factors. On a molecular level, downregulation of SMA expression by LPS occurs at the level of transcription, as both SMA mRNA levels and SMA promoter activity are inhibited by LPS. The SMA promoter is controlled largely by two major regulatory elements-CArG boxes activated by serum response factor (SRF), and TGF-beta control elements (TCE). LPS does not affect the activity of SRF, but it potently inhibits both basal and inducible TCE activation.


CONCLUSIONS
We show for the first time that LPS attenuates SMA transcription and protein expression in VSMC likely through inhibition of a TCE element on the SMA promoter.
Sandbo
Nathan
N
Department of Medicine, The University of Chicago, Chicago, IL 60637, United States.
Taurin
Sebastien
S
Yau
Douglas M
DM
Kregel
Steven
S
Mitchell
Richard
R
Dulin
Nickolai O
NO
D000199:Actins / Q000378:metabolism*; D000818:Animals; D001011:Aorta; D002465:Cell Movement / Q000187:drug effects; D002478:Cells, Cultured; D003331:Coronary Vessels; D015536:Down-Regulation; D015870:Gene Expression / Q000187:drug effects; D006801:Humans; D008070:Lipopolysaccharides / Q000494:pharmacology*; D032389:Myocytes, Smooth Muscle / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D051381:Rats; D011921:Rats, Inbred WKY; D012772:Shock, Septic / Q000378:metabolism* / Q000473:pathology; D014158:Transcription, Genetic / Q000187:drug effects; D016212:Transforming Growth Factor beta / Q000378:metabolism
D000199:Actins; D008070:Lipopolysaccharides; D016212:Transforming Growth Factor beta; C498862:Acta2 protein, rat