Evidence for Extracellular Secretion of Cyclooxygenase-2

Cyclooxygenase (COX) is the rate limiting enzyme that generates prostanoids from arachidonic acid (AA). This enzyme has two main isoforms: COX-1, which is the constitutive form of the enzyme, and COX-2, whose expression is rapidly and transiently upregulated by a variety of pathological signals. As opposed to COX-1, which resides mostly in the membrane of the endoplasmic reticulum (ER) due to a strong ER retention sequence, COX-2 is also located in the Golgi apparatus. Given this observation we raised the hypothesis that besides its localization inside the cell, certain signals may cause the secretion of COX-2 outside the cell. To test this, we expressed COX-2 in HEK 293, in the absence or presence of AA for 7h, after which the cells were washed, media was replaced with normal media and the cells were allowed to grow overnight. On the next day, we isolated COX-2 from the cells and their overlaying media. Our data show that COX-2 is present outside the cell and that AA treatment increases its presence in the extracellular media. Similar observations were obtained with a cell line that expresses COX-2 endogenously. Furthermore, application of COX-2 conditioned media on to cells that do not express COX-2 endogenously showed its presence 24h post application, suggesting that COX-2 may be transferred through the extracellular media. Together these data propose the existence of a novel COX-2 secretory pathway. We are currently engaged in studying the mechanism that underlies this phenomenon.