Villars, F., B. Guillotin, T. Amé dé e, S. Dutoya, L. Bordenave, R. Bareille, and J. Amé dé e. Effect of HUVEC on human osteoprogenitor cell differentiation needs heterotypic gap junction communication. Am J Physiol Cell Physiol 282: C775-C785, 2002; 10.1152/ajpcell.00310.2001.-Bone development and remodeling depend on complex interactions between bone-forming osteoblasts and other cells present within the bone microenvironment, particularly vascular endothelial cells that may be pivotal members of a complex interactive communication network in bone. Our aim was to investigate the interaction between human umbilical vein endothelial cells (HUVEC) and human bone marrow stromal cells (HBMSC). Cell differentiation analysis performed with different cell culture models revealed that alkaline phosphatase activity and type I collagen synthesis were increased only by the direct contact of HUVEC with HBMSC. This "juxtacrine signaling" could involve a number of different heterotypic connexions that require adhesion molecules or gap junctions. A dye coupling assay with Lucifer yellow demonstrated a functional coupling between HUVEC and HBMSC. Immunocytochemistry revealed that connexin43 (Cx43), a specific gap junction protein, is expressed not only in HBMSC but also in the endothelial cell network and that these two cell types can communicate via a gap junctional channel constituted at least by Cx43. Moreover, functional inhibition of the gap junction by 18␣-glycyrrhetinic acid treatment or inhibition of Cx43 synthesis with oligodeoxyribonucleotide antisense decreased the effect of HUVEC cocultures on HBMSC differentiation. This stimulation could be mediated by the intercellular diffusion of signaling molecules that permeate the junctional channel.coupling; intercellular messenger; connexin43; osteoblast; endothelial cells ANGIOGENESIS is a tightly regulated process involved in growth, repair, and bone remodeling (9, 11-13, 23, 28, 50, 54, 56). Several studies have shown that there is a reciprocal regulation and functional relationship between endothelial cells and osteoblast-like cells during osteogenesis, in which systemic hormones and paracrine growth factors or cytokines play an active role (4,24,53,55). In a previous study (51), we showed that osteoblast progenitor cells [human bone marrow stromal cells (HBMSC)] behave differently in terms of proliferation and differentiation when cultured in association with endothelial cells [human umbilical vein endothelial cells (HUVEC)] in various coculture models (coculture with or without direct contact, conditioned medium) than when they are cultured alone. An increase in alkaline phosphatase activity (Al-P) was observed only when HBMSC were cocultured in direct contact with HUVEC. The enhancement of this early osteoblastic marker is not provided when HBMSC and HUVEC are cocultured separately with the use of a semipermeable membrane (51) or when HBMSC are seeded onto a matrix obtained from HUVEC cultures (51).Therefore, the intercommunication between endothelial cells and os...
