Hematopoietic cytokines stimulate EC angiogenic sprouting in conjunction with VEGF-A or BMP-4, and the EC transcription factor, Runx1, controls hematopoietic cytokine receptor expression that is necessary for EC morphogenic responses in 3D collagen matrices. (A,B) The indicated factors were added to the collagen matrix and ECs were seeded on the collagen gel surface and EC sprouting was quantitated (A) after 24 hours (n ≥ 10; P ≤ .01) and photographed (B) from the side. Arrowheads indicate the monolayer surface. (C) ECs were primed with VEGF-A, FGF-2, VEGF-A/FGF-2, or control and then were seeded on the surface of gels that contained the indicated individual or combined factors (n ≥ 10; P ≤ .01). *Denotes significance over control. (D) SiRNA suppression of the transcription factor Runx1 in ECs with the use of 2 independent siRNAs (Smartpool-SP or Single-SN) directed to Runx1 was tested during EC tube formation (top) or EC sprouting (bottom; n ≥ 10; P ≤ .01). *Denotes significant blockade from Luciferase controls. (E) Reverse transcription polymerase chain reaction analysis was performed on ECs treated with siRNAs to Runx1 versus luciferase controls to determine mRNA expression for each of the hematopoietic cytokine receptors as well as VEGFR2 and controls. (F) Western blot analysis was performed to determine protein expression for each of the hematopoietic cytokine receptors as well as VEGFR2 after siRNA suppression of Runx1 in ECs.