COX-3, a cyclooxygenase-1 variant inhibited by acetaminophen and other analgesic/antipyretic drugs: Cloning, structure, and expression

Abstract

Two cyclooxygenase isozymes, COX-1 and -2, are known to catalyze the rate-limiting step of prostaglandin synthesis and are the targets of nonsteroidal antiinflammatory drugs. Here we describe a third distinct COX isozyme, COX-3, as well as two smaller COX-1-derived proteins (partial COX-1 or PCOX-1 proteins). COX-3 and one of the PCOX-1 proteins (PCOX-1a) are made from the COX-1 gene but retain intron 1 in their mRNAs. PCOX-1 proteins additionally contain an in-frame deletion of exons 5-8 of the COX-1 mRNA. COX-3 and PCOX mRNAs are expressed in canine cerebral cortex and in lesser amounts in other tissues analyzed. In human, COX-3 mRNA is expressed as an Ϸ5.2-kb transcript and is most abundant in cerebral cortex and heart. Intron 1 is conserved in length and in sequence in mammalian COX-1 genes. This intron contains an ORF that introduces an insertion of 30 -34 aa, depending on the mammalian species, into the hydrophobic signal peptide that directs COX-1 into the lumen of the endoplasmic reticulum and nuclear envelope. COX-3 and PCOX-1a are expressed efficiently in insect cells as membrane-bound proteins. The signal peptide is not cleaved from either protein and both proteins are glycosylated. COX-3, but not PCOX-1a, possesses glycosylation-dependent cyclooxygenase activity. Comparison of canine COX-3 activity with murine COX-1 and -2 demonstrates that this enzyme is selectively inhibited by analgesic/antipyretic drugs such as acetaminophen, phenacetin, antipyrine, and dipyrone, and is potently inhibited by some nonsteroidal antiinflammatory drugs. Thus, inhibition of COX-3 could represent a primary central mechanism by which these drugs decrease pain and possibly fever.A cetaminophen is often categorized as a nonsteroidal antiinflammatory drug (NSAID), even though in clinical practice and in animal models it possesses little antiinflammatory activity (1). Like NSAIDs, however, acetaminophen inhibits pain and fever and is one of the world's most popular analgesic/ antipyretic drugs. Despite acetaminophen's long use and popularity it lacks a clear mechanism of action. Flower and Vane showed that acetaminophen inhibited cyclooxygenase (COX) activity in dog brain homogenates more than in homogenates from spleen (2). This gave rise to the concept that variants of COX enzymes exist that are differentially sensitive to this drug and that acetaminophen acts centrally. Yet, even though two isozymes of COX are known, neither isozyme is sensitive to acetaminophen at therapeutic concentrations of the drug in whole cells or homogenates. Instead, COX-1 and -2 in homogenates frequently exhibit the paradoxical property of being stimulated by submillimolar concentrations of acetaminophen and inhibited by supermillimolar levels of the drug (1). This finding suggests that neither isozyme is a good candidate for the site of action of acetaminophen.In analyzing COX-1 and -2 RNA expression in dog tissues, our laboratory observed that the cerebral cortex of dog brain contains two distinct RNAs that hybridized to a canine COX-1 cDNA. ...

A Novel Phenotype of Nonsteroidal Anti-Inflammatory Drug Hypersensitivity

“…Interestingly, among low COX inhibitors, acetaminophen was most commonly incriminated, followed by nimesulide, meloxicam, and less frequently, the coxibs, the more selective drugs sparing COX-1 (Figure 2). This is in agreement with the rates of inhibition of COX-1 in vitro,[11] with the exception of acetaminophen that inhibits another isoenzyme, COX-3, which is closely related to COX-1 [12-14]. …”

15-Hydroxyprostaglandin dehydrogenase (15-PGDH) and lung cancer

“…COX-2 is regarded inducible and is expressed in response to mitogens, tumor promoters and inflammatory cytokines in some inflammatory cells and cancer cells (3). COX-3 is an alternate splicing variant of COX-1 and is expressed in some neural cells (4). It appears that only COX-2 and its ensuing coupling enzyme responsible for the synthesis of PGE 2 , microsomal PGE synthase, were reported to over-express in lung tumors (5,6).…”

Glycosylation regulates turnover of cyclooxygenase‐2

“…COX is bifunctional, converting arachidonic acid (AA), an omega-6 fatty acid, to the precursor prostaglandin G 2 (PGG 2 ) and subsequently converting PGG 2 to the precursor PGH 2 via cyclooxygenase and peroxidase activities, respectively. Three isoforms of COX have been found -the constitutively expressed COX-1 [5][6][7]; COX-2, which can be inducible or constitutive, depending on the tissue [1,[8][9][10]; and the constitutively expressed COX-3, which is believed to be a splice variant of COX-1 [11]. Although COX-1 is considered the housekeeping enzyme expressed in nearly all tissues, COX-2 is generally perceived to be involved in pathological conditions, such as inflammation and cancer [1,12,13].…”