LPA‐and PDGF‐Stimulated Contraction of Collagen Matrices Involves Neither by Myosin Light Chain Phosphorylation Nor by MLC Kinase

Abstract

Fibroblast‐collagen matrix contraction has been used as a model system to study how cells organize connective tissue. Previous work showed that lysophosphatidic acid (LPA)‐stimulated collagen matrix contraction is independent of Rho kinase whereas platelet‐derived growth factor (PDGF)‐stimulated contraction is Rho kinase dependent. The current studies were carried out to learn more about the molecular motors responsible for LPA‐ and PDGF‐stimulated fibroblast‐collagen matrix contraction. Fibroblasts whose MLC kinase was knocked down using siRNA were used to measure matrix contraction in the presence of LPA or PDGF with or without the Rho kinase inhibitor added. The extent of contraction of MLC kinase‐silenced cells was not detectably different from control cells. Other experiments were carried out to test the effects of LPA and PDGF on MLC phosphorylation with and without Rho kinase inhibitor. After 15 min of growth factor stimulation, levels of diphosphorylated MLC were highest in cells in LPA‐containing medium and lowest in PDGF containing medium. Prior addition of Rho kinase inhibitor markedly reduced phosphorylation in every case. These observations suggested that stimulation of collagen matrix contraction required neither growth factor stimulation of MLC phosphorylation nor MLC kinase.