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Medical Research Council Group in Periodontal Physiology, Faculties of Dentistry and Medicine, University of Toronto, Toronto, Ontario, Canada M5S 3E8
Chronic ventricular pressure overload can regulate expression
of 
-smooth muscle actin (SMA) in cardiac fibroblasts, but it is
unclear if force alone or the concomitant activity of angiotensin II is
the principal regulatory factor. To test if SMA mRNA and protein in rat
cardiac fibroblasts are regulated directly by force, we first induced
SMA expression in cultured cells and then applied magnetically
generated perpendicular forces through focal adhesions using
collagen-coated magnetite beads. Continuous static forces (0.65 pN/µm2) selectively reduced SMA but not 
-actin mRNA
and protein content within 4 h (to 55 ± 9% of controls);
SMA returned to baseline by 8 h. There was no change in SMA
content after force application with either plasma or the cellular
fibronectin IIIA domain, BSA, or poly-L-lysine beads. The
early loss of SMA was apparently due to selective leakage into the cell
culture medium. Treatment with angiotensin II (10 nM) abrogated the
force-induced reduction of SMA and increased the levels of this
protein. The stress kinase p38 was phosphorylated by force, whereas
extracellular signal-regulated kinase 1/2 and c-Jun
NH2-terminal kinase were unaffected. The p38 kinase
inhibitor SB-203580 relieved the force-induced SMA reduction. We
conclude that force-induced inhibition of SMA is mediated through the
p38 kinase pathway, and this pathway antagonizes angiotensin II
regulation of SMA.
mitogen-activated protein kinase; angiotensin II; p38
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