DEPLETION OF LYSOPHOSPHATIDIC ACID TRIGGERS A LOSS OF ORIENTED DETYROSINATED MICROTUBULES IN MOTILE FIBROBLASTS

We reported earlier that isolated plasma membranes trigger a number of responses comprising contact inhibition of motility, including loss o f oriented detyrosinated microtubules (Glu MTs) from the lamella of mo tile fibroblasts, In this study, we show that the membranes trigger th is loss of Glu MTs, not by binding to cells, but by removing an essent ial component from the medium necessary to maintain oriented Glu MTs, Preincubation of membranes with medium containing serum followed by re moval of the membranes by sedimentation rendered the membrane-treated medium capable of triggering the loss of oriented Glu MTs. Membrane ac tivity was inhibited by high concentrations of serum and removal of se rum from medium triggered the loss of oriented Glu MTs similar to that triggered by membranes, These results suggest that the membranes trig ger the loss of Glu MTs by inactivating factors in serum that are requ ired for the maintenance of oriented Glu MTs, By fractionating serum, we have identified lysophosphatidic acid (LPA) as the principal serum factor that is responsible for supporting oriented Glu MTs. The activi ty of LPA to maintain oriented Glu MTs upon serum withdrawal was half maximal at 100 nM and no activity was observed with structurally relat ed phospholipids, Serum LPA levels were sufficient to account for the ability of serum to support oriented Glu MTs, Enzymatic degradation of serum LPA strongly reduced the ability of serum to support oriented G lu MTs, That membranes degrade LPA was shown by the ability of membran es to block L-PA's ability to maintain oriented Glu MTs, and by direct measurement of the loss of radiolabeled LPA after incubation with mem branes in vitro, These results show that isolated plasma membranes tri gger the loss of Glu MTs from the lamella of motile cells by degrading serum EPA, Coupled with earlier results showing that membranes trigge r a number of contact inhibition responses, our data suggest a new mod el for contact inhibition of motility in which local degradation of LP A and/or interference with LPA-stimulated signalling pathways initiate s a contact inhibition response in colliding cells.