Cellular confluence determines injury-induced prostaglandin E 2 synthesis by human keratinocyte cultures

Abstract

When keratinocyte cultures become confluent, their prostaglandin E 2 synthesis is suppressed. To determine whether the injury response is characterized by increased prostaglandin E 2 synthesis, an in vitro injury model was developed. When confluent keratinocyte cultures were focally lethally irradiated using ultraviolet light B, a dose-dependent increase in prostaglandin E 2 synthesis was induced by the injury. After irradiation, confluent cultures' prostaglandin E 2 synthesis increased for 2 days to 8-fold more than controls, then decreased to control values by day 6. Increased prostaglandin E 2 synthesis was first detected 8 h after injury. Focal irradiation of non-confluent cultures (killing isolated colonies) caused no change in prostaglandin E 2 synthesis, indicating that culture continuity must be disrupted before synthesis increases. In addition, partial irradiations of petri dishes demonstrated that enhanced metabolism was confined to cells adjacent to the injury site and was not mediated by a soluble factor. When confluent and injured cultures were incubated with [ 14C]arachidonic acid, and the products formed analyzed by thin layer chromatography, 10-fold more prostaglandin E 2 Jug protein was seen in irradiated cultures relative to confluent controls. The products formed by each group were the same, and no consistent increases in metabolites other than prostaglandin E 2 were observed. The increased synthesis of prostaglandin E 2 by injured cultures was apparently due to an increase in cyclooxygenase activity as determined by kinetic experiments. These data indicate that the pattern of metabolism of arachidonic acid seen in non-confluent cultures is similar to that seen in injury, and that cell-cell contact modulates enhanced prostaglandin E 2 synthesis.