Increased myosin light chain phosphorylation is not required for growth factor stimulation of collagen matrix contraction.

Previous research suggested the possibility that contraction of floating collagen matrices by human fibroblasts required increased myosin light chain (MLC) phosphorylation. In the current studies, we show that increased MLC phosphorylation was neither necessary for platelet-derived growth factor (PDGF)-dependent matrix contraction nor sufficient for lysophosphatidic acid (LPA)-dependent contraction. In contrast, increased MLC phosphorylation did appear to be coupled to the formation of stress fibers by cells spreading in monolayer culture. Signal transduction pathways required for PDGF- and LPA-dependent matrix contraction involved phosphatidylinositol 3-kinase and the G(i) class of heterotrimeric G proteins, respectively. Our results indicate that PDGF- and LPA-dependent contraction of floating collagen matrices can be uncoupled from an increase in MLC phosphorylation.