Unligated α5β1-mediated death is PKA dependent. (a) PKA activity was measured in HUVECs attached to poly-L-lysine, fibronectin, or vitronectin in the presence or absence of integrin antagonists. (b) HUVECs were plated on vitronectin-coated or poly-L-lysine–coated culture plates in the presence or absence of anti-α5β1 or anti-αvβ3 Ab’s, a selective PKA inhibitor (1 μM HA1004), or anti-integrin Ab’s in combination with 1 μM HA1004. After 24 hours, the percentage of FITC-annexin–positive cells was determined. (c) Cell lysates from b were immunoblotted with anti–caspase-3 and anti–cleaved caspase-3 Ab’s. Relative caspase-3 cleavage was determined by densitometry.