Protein kinase Cδ-dependent phosphorylation of syndecan-4 regulates cell migration

Endothelial cell (EC) migration is a complex process requiring exquisitely coordinated focal adhesion assembly and disassembly. Protein kinase C (PKC) is known to regulate focal adhesion formation. Because lysophosphatidylcholine (lysoPC), a major lipid constituent of oxidized low-density lipoprotein, can activate PKC and inhibit EC migration, we explored the signaling cascade responsible for this inhibition. LysoPC increased PKC delta activity, measured by in vitro kinase activity assay, and increased PKC delta phosphorylation. Decreasing PKC delta activation, using pharmacological inhibitors or antisense oligonucleotides, diminished the antimigratory effect of lysoPC. LysoPC-induced PKC delta activation was followed by increased phosphorylation of the transmembrane proteoglycan, syndecan-4, and decreased binding of PKC alpha to syndecan-4, with a concomitant decrease in PKC alpha activity. A reciprocal relationship was noted between the interaction of PKC alpha and alpha-actinin with syndecan-4. These changes were temporally related to the observed changes in cell morphology and the inhibition of migration of ECs incubated with lysoPC. The data suggested that generalized activation of PKC delta by lysoPC initiated a cascade of events, including phosphorylation of syndecan-4, displacement and decreased activity of PKC alpha, binding of alpha-actinin to syndecan-4, and disruption of the time- and site-specific regulation of focal adhesion complex assembly and disassembly required for normal cell migration.