Heterotypic contact reveals a COX-2-mediated suppression of osteoblast differentiation by endothelial cells: A negative modulatory role for prostanoids in VEGF-mediated cell: cell communication?

Clarkin, C E and Garonna, E and Pitsillides, A A and Wheeler-Jones, C P D (2008) Heterotypic contact reveals a COX-2-mediated suppression of osteoblast differentiation by endothelial cells: A negative modulatory role for prostanoids in VEGF-mediated cell: cell communication? Experimental Cell Research, 314 (17). pp. 3152-3161.

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Abstract

in bone, angiogenesis most be initiated appropriately, but limited once remodelling or repair is complete. Our recent findings have Supported a role for prostaglandins (PG), known modulators of osteoblast (OB) and endothelial cell (EC) behaviour, in facilitating VEGF-mediated paracrine communication from OBs to 'remotely located' ECs, but the mechanism(s) regulating OB:EC crosstalk when these cells are closely opposed are undefined. In this study we have examined: (i) the effects of exogenous PGE(2) on VEGF-driven events in ECs, and (it) the role of endogenous COX-2-derived prostanoids in mediating communication between intimately opposed OBs and ECs in direct contact. Exposure of ECs to PGE(2) increased ERK1/2 phosphorylation, COX-2 induction, 6-keto-PGF(1 alpha),, release and EC proliferation. In contrast, PGE2 attenuated VEGF(165)- induced VEGFR2/Flk1 phosphorylation, ERK1/2 activation and proliferation of ECs, Suggesting that exogenous PGE2 restricts the actions of VEGF. However, the COX-2-selective inhibitor, NS398, also attenuated VEGF-induced proliferation, implying a distinct role for endogenous COX-2 activity in regulating EC behaviour. To examine the effect of OB:EC proximity and the role of COX-2 products further, we Used a confrontational co-culture model. These studies showed that COX-2 blockade with NS398 enhanced EC-dependent increases in OB differentiation, that this effect was reversed by exogenous PGH(2) (immediate COX-2 product), and that exogenous VEGF did not influence EC-dependent OB differentiation under these conditions. Our findings indicate that locally produced prostanoids may serve distinct roles depending,g on OB:EC proximity and negatively modulate VEGF-mediated changes in EC behaviour when these cells are closely opposed to control angiogenesis during bone (re)modelling. (C) 2008 Elsevier Inc. All rights reserved.