Collagen I Initiates Endothelial Cell Morphogenesis by Inducing Actin Polymerization through Suppression of Cyclic AMP and Protein Kinase A

Abstract

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 ␣ 1 ␤ 1 and ␣ 2 ␤ 1 , followed by suppression of cyclic AMP and cyclic AMP (cAMP)-dependent protein kinase A, and marked induction of actin polymerization to form prominent stress fibers. In contrast to collagen I, 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.During angiogenesis, proliferating endothelial cells (ECs) 1 organize to form new three-dimensional capillary networks. This process has been studied extensively in the embryo, establishing that an early stage of angiogenesis involves transition of endothelial precursor cells to a spindle-shape morphology (1) in combination with alignment into solid, multicellular, pre-capillary, cord-like structures (2, 3). Moreover, these cordlike structures are interconnected to form a polygonal network (1, 4). Solid pre-capillary cords have also been observed during angiogenesis in the adult (5). These solid cords subsequently mature into tubes with hollow lumens for the transport of blood (1, 5).Three-dimensional type I collagen provokes ECs in culture to undergo marked shape changes that closely imitate pre-capillary formation during embryonic angiogenesis. Within hours after addition of collagen I to confluent cultures, ECs partially retract and exhibit a spindle-shaped morphology, together with re-alignment to form solid cords organized in a polygonal pattern (6 -10). Subsequently, over the course of several days, these structures mature to form tubes with hollow lumens through a process involving development and coalescence of intracellular vacuoles (11).Consistent with the importance of collagens in regulating EC shape and multicellular organization into pre-capillary cords, there exists considerable evidence that interactions between collagens and ECs are highly relevant in vivo. For example, in the developing embryo, blood vessels arise from the organization of EC precursors within an extracellular matrix r...

DNAzymes to mouse β1 integrin mRNA in vivo: targeting the tumor vasculature and retarding cancer growth

“…Moreover, collagen I ligation of a1b1 and a2b1 is a decisive step in initiating cord formation of endothelial cells. 37 Similarly, avb3, avb5 and a2b1 are scarcely present in quiescent vessels but are common in sprouts 38 and only avb3 expression is enhanced upon bFGF treatment. 39 Integrins also play a dominant role in metastasis, a process characterized by altered cancer cell motility and adhesion to surrounding cells and tissues.…”

Protein Kinase A-regulated Nucleocytoplasmic Shuttling of Id1 during Angiogenesis

“…Stimulation and Inhibition of Protein Kinase A-To stimulate the PKA pathway, HUVECs were treated with an analogue of cAMP N6, 2Ј-O-dibutyryladenosine-3Ј, 5Ј-cyclic monophosphate sodium salt (Bt 2 cAMP, 500 M) (Nacalai Tesque) or forskolin, a stimulator of adenylyl cyclase, (100 M) (Sigma) (26,27) for 2 h, 20 -24 h after cultivation on Matrigel. To inhibit the PKA pathway, HUVECs cultured on culture slides were treated with H-89, a selective inhibitor of PKA (1 M) (Sigma), or MDL 12,330A, an inhibitor of adenylyl cyclase (30 M) (Calbiochem), (26) for 4 h. In a subset of experiments, HUVECs were pretreated with an inhibitor of protein synthesis, cycloheximide (CHX, 100 ng/ml) (Wako), and a proteasome inhibitor, MG132 (20 M) (Biomol) (23), for 30 min prior to PKA stimulation or inhibition.…”

“…Suppression of PKA via certain integrins negatively regulates actin polymerization and EC assembly into capillary-like structures in vitro (26,36,37) and also inhibits EC migration and survival (27,38). In addition, PKA stimulation inhibits cytokine-induced proliferation (37), thereby suppressing angiogenesis in vitro and in vivo (27,39).…”

An in vitro model of hemogenic endothelium commitment and hematopoietic production

“…The pieces of PSM exhibited a poor capacity to spread in the absence of extracellular matrix (not shown). Since collagen was reported to positively regulate EC commitment and migration (Whelan and Senger, 2003;Lamalice et al, 2007), we compared collagen I and IV in their efficacy to support endothelial differentiation. Based on cell morphology (Hirashima et al, 2003;Guo et al, 2007;Eilken et al, 2009), type I collagen-coated dishes offered a better differentiation of PSM cells than type IV collagen-coated dishes (not shown).…”