Limited proteolysis of cyclooxygenase-2 enhances cell proliferation

The cyclooxygenase-2 (COX-2) enzyme catalyzes the rate-limiting step in the conversion of arachidonic acid to prostanoids that play central roles in cardiovascular, immunological and neurological function. However, recent evidence suggest that COX-2 has additional cellular functions that are independent of its classical enzymatic activity. Here we show that cancer-derived cell lines and colorectal tumors present with at least three COX-2 immunoreactive bands that are of lower molecular weight than that of the full-length protein. We further find that a point mutation at the C-terminus of COX-2 (K598R) elicits a similar fragmented COX-2 profile, suggesting that under certain conditions the protein undergoes a process of limited proteolysis. One of the main lower MW weight bands corresponds in size to the catalytic domain of COX-2, without the N-terminal domains that anchor it to the ER and nuclear membranes. Introduction of this mutant into cells causes increased presence of COX-2 fragments in the nucleus and induces a significant increase in proliferation, in an activity independent manner. Transcriptome and proteome analyses of cells expressing the K598R COX-2 mutant reveal significant changes in the expression of 462 genes, most of which are related to regulation of DNA transcription. Lastly, the genes that are specific to K598R-expressing samples are significantly over-represented in several types of cancer including colon, lung, breast and liver. Together these data suggest that limited proteolysis of COX-2 can generate protein fragments that affect proliferation. The identity of COX-2 proteases and the mechanism by which COX-2 fragments enhance cell division are under investigation.