Transcriptional regulation of a contractile gene by mechanical forces applied through integrins in osteoblasts

We examined mechanotranscriptional regulation of the contractile gene, alpha-smooth muscle actin (SMA), in osteoblastic cells. Tensile forces were applied through collagen-coated magnetite beads to ROS17/2.8 cells. These cells were desmin-, vimentin+ and expressed low levels of SMA. After force application (480 piconewton/cell), SMA protein and mRNA were increased but beta-actin was unchanged. Beads coated with bovine serum albumin or poly-L-lysine produced no change of SMA. In cells transiently transfected with plasmids containing the SMA promoter fused to beta-galactosidase or green fluorescent protein coding sequences, SMA promoter activity was increased by approximately 60% after 4 h of force, whereas control (Rous sarcoma virus) promoter activity was unaffected. Transfections with beta-galactosidase or green fluorescent protein reporter constructs showed that force-loaded cells exhibited higher beta-galactosidase activity than cells without force. Cytochalasin D and latrunculin B inhibited force-induced increases of SMA promoter activity. Deletion analyses showed that SMA promoter activity was increased approximately 70% after force with a minimal construct containing 155 bp upstream of the translation start site. The force effect on the SMA promoter was abrogated in cells transfected with CArG-B box mutants. Gel mobility shift analyses of nuclear extracts showed strong binding to the CArG-B motif after force. We conclude that the CArG-B box is a force-responsive element in the SMA promoter.