Abstract 468: Monocytes are Specific and Potent Regulators of Endothelial Cell Proliferation through a Contact Dependent Mechanism Independent of VEGF Receptor 2
Accumulating evidence links monocyte recruitment and accumulation within the vascular wall with vascular repair, endothelial regeneration and angiogenesis in ischemia and neoplasia. Although cytokines released by activated monocytes can induce endothelial cell (EC) proliferation the direct effects of monocytes on EC proliferation remain undefined. In the present study we tested the hypothesis that interaction of monocytes with ECs can act as a mechanism for the regulation of EC proliferation.
Methods: We examined the interaction of subconfluent primary human ECs and negatively selected primary human monocytes. All assays were preformed in DMEM 5% inactivated fetal calf serum. Proliferation was determined by cell count, 3H-thymidine incorporation and BRDU incorporation.
Results: Direct monocyte interaction increased 3H-thymidine incorporation by 22.2±2.7 (P<0.05) and 20.2±2 (P<0.05) fold in human umbilical vein and arterial ECs respectively compared with only 1.9±0.23 (P<0.05) and 1.2±.0.14 (P<0.05) fold increase in smooth muscle cells and fibroblasts. Cell counts measured a two fold increase in EC number when cocultured with monocytes for 24 hours compared with untreated group. When monocytes were added in transwell inserts to prevent physical contact the increase in 3H-thymidine incorporation decreased to 2.3±0.28 (P<0.05) and 2.0±0.25 (P<0.05) above controls, but was identical to direct contact on smooth muscle cells and fibroblasts. Monocytes binding induced a single cell cycle in ECs. ECs remain receptive to additional cycles of stimulation but require contact with fresh primary monocytes. Neutralization of VEGF165, FGF2 or PDGF had only minor effect on monocyte induced EC proliferation. Remarkably, neutralization of VEGF receptor 2 by antibodies or the selective receptor inhibitor ZM323881 did not inhibit monocyte induced EC proliferation. Neutralization of β2 integrin and to further extent CD11b inhibited monocyte induced EC proliferation 3 and 6 fold respectively.
Conclusions: Monocyte induction of EC proliferation represents a novel mode of regulation that together with, but distinct from, vascular growth factors create the complex growth kinetics, unique architecture and homeostatic potency of the intact endothelium.