MECHANISMS OF CHONDROCYTE ACTIVATION IN ARTHRITIS

We are working with a synovial factor which induces the synthesis and secretion of prostaglandin E (PGE) and several neutral proteinases, including collagenase, by monolayers of cultured articular chondrocytes. The chondrocyte activating factor (CAF) which does this is closely related to, if not not identical with, catabolin and interleukin-1 (IL-1). The mechanism of chondrocyte activation is completely unknown. From what is known of stimulus-coupled cellular activation in other systems, we may suspect one of several mechanisms. One such mechanism involves binding of the agonist to the plasma membrane with subsequent mediation of a second messenger such as Ca, cyclic AMP or cyclic GMP, producing the activation of one or more protein kinases. Another involves the acquisition of direct protein kinase activity by the receptor in the plasma membrane once it binds the agonist. Certain agents, such as the steroid hormones, penetrate the cell and travel to the nucleus. Along these lines, we will determine the type of mechanism through which chondrocytes are activated by our synovial CAF. Complementary experiments will be conducted with IL-1. Binding studies will be performed to determine whether activation is mediated via a cell surface receptor. To determine whether a "classical" second messenger is involved in signal transduction, we will measure possible fluxes in the cytosolic concentrations of Ca, cGMP and cAMP during activation. Possible mediation of these second messengers will also be examined through the use of specific stimulators and inhibitors of these putative intermediaries. If these experiments fail to identify a classical second messenger, the possibility that the receptor gains protein kinase activity upon binding CAF or IL-1 will be addressed. This information will help to identify ways of therapeutically preventing chondrocyte activation.