| Connective tissue cells survive mechanical forces by inducing the expression of filamin A, an actin cross-linking protein that inhibits force-induced cell death. Filamin A expression is dependent on p38 activation and Sp-1 binding to the filamin promoter. Force application through integrins increased filamin promoter activity by >5-fold but was blocked by mutations to Sp1 binding sites in the filamin promoter. In Sp deficient SL2 cells, transfection with wild-type but not mutant Sp1 caused phosphorylation of Sp1 at both Thr453 and Thr739 and enhanced binding of nuclear extracts to a filamin promoter. Sp1 phosphorylated at Thr453 and Thr739 by p38 bound to filamin promoter more than unphosphorylated Sp1. Recombinant active p38 phosphorylated wild-type Sp1 but not the Sp1T453A-T739A double mutant protein in vitro. We conclude that filamin A expression is increased by p38-induced phosphorylation of Sp1 at specific threonine residues that in turn enhances binding to the filamin promoter. |
