Collagen I initiates endothelial cell morphogenesis by inducing actin polymerization through suppression of cyclic AMP and protein kinase A

Collagen I provokes endothelial cells to assume a spindle-shaped morphology and to align into solid cord-like assemblies. These cords closely imitate the solid precapillary cords of embryonic angiogenesis, raising interesting questions about underlying mechanisms. Studies described here identify a critical mechanism beginning with collagen I ligation of integrins alpha(1)beta(1) and alpha(2)beta(1) followed by suppression of cyclic AMP and cyclic AMP (cAMP)-dependent protein kinase X and marked induction of actin polymerization to form prominent stress fibers. In contrast to collagen 1, laminin-1 neither suppressed cAMP nor protein kinase A activity nor induced actin polymerization or changes in cell shape. Moreover, fibroblasts did not respond to collagen I with changes in cAMP, actin polymerization, or cell shape, thus indicating that collagen signaling, as observed in endothelial cells, does not extend to all cell types. Pharmacological elevation of cAMP blocked collagen-induced actin polymerization and formation of cords by endothelial cells; conversely, pharmacological suppression of either cAMP or protein kinase A induced actin polymerization. Collectively, these studies identify a previously unrecognized and critical mechanism, involving suppression of cAMP-dependent protein kinase A and induction of actin polymerization, through which collagen I drives endothelial cell organization into multicellular precapillary cords.