Cyclooxygenases and prostaglandin E ₂ receptors in growth plate chondrocytes in vitro and in situ – prostaglandin E ₂ dependent proliferation of growth plate chondrocytes

Abstract

Prostaglandin E ₂ (PGE ₂) plays an important role in bone development and metabolism. To interfere therapeutically in the PGE ₂ pathway, however, knowledge about the involved enzymes (cyclooxygenases) and receptors (PGE ₂ receptors) is essential. We therefore examined the production of PGE ₂ in cultured growth plate chondrocytes in vitro and the effects of exogenously added PGE ₂ on cell proliferation. Furthermore, we analysed the expression and spatial distribution of cyclooxygenase (COX)-1 and COX-2 and PGE ₂ receptor types EP1, EP2, EP3 and EP4 in the growth plate in situ and in vitro. PGE ₂ synthesis was determined by mass spectrometry, cell proliferation by DNA [ ³H]-thymidine incorporation, mRNA expression of cyclooxygenases and EP receptors by RT-PCR on cultured cells and in homogenized growth plates. To determine cellular expression, frozen sections of rat tibial growth plate and primary chondrocyte cultures were stained using immunohistochemistry with polyclonal antibodies directed towards COX-1, COX-2, EP1, EP2, EP3, and EP4. Cultured growth plate chondrocytes transiently secreted PGE ₂ into the culture medium. Although both enzymes were expressed in chondrocytes in vitro and in vivo, it appears that mainly COX-2 contributed to PGE ₂-dependent proliferation. Exogenously added PGE ₂ stimulated DNA synthesis in a dose-dependent fashion and gave a bell-shaped curve with a maximum at 10 ^-8 M. The EP1/EP3 specific agonist sulprostone and the EP1-selective agonist ONO-D1-004 increased DNA synthesis. The effect of PGE ₂ was suppressed by ONO-8711. The expression of EP1, EP2, EP3, and EP4 receptors in situ and in vitro was observed; EP2 was homogenously expressed in all zones of the growth plate in situ, whereas EP1 expression was inhomogenous, with spared cells in the reserve zone. In cultured cells these four receptors were expressed in a subset of cells only. The most intense staining for the EP1 receptor was found in polygonal cells surrounded by matrix. Expression of receptor protein for EP3 and EP4 was observed also in rat growth plates. In cultured chrondrocytes, however, only weak expression of EP3 and EP4 receptor was detected. We suggest that in growth plate chondrocytes, COX-2 is responsible for PGE ₂ release, which stimulates cell proliferation via the EP1 receptor.