A major development of carcinogenesis research in the past 20 years has been the discovery of significant levels of DNA damage arising from endogenous cellular sources. Dramatic improvements in analytical chemistry have provided sensitive and specific methodology for identification and quantitation of DNA adducts. Application of these techniques to the analysis of nuclear DNA from human tissues has debunked the notion that the human genome is pristine in the absence of exposure to environmental carcinogens. Much endogenous DNA damage arises from intermediates of oxygen reduction that either attack the bases or the deoxyribosyl backbone of DNA. Alternatively, oxygen radicals can attack other cellular components such as lipids to generate reactive intermediates that couple to DNA bases. Endogenous DNA lesions are genotoxic and induce mutations that are commonly observed in mutated oncogenes and tumor suppressor genes. Their mutagenicity is mitigated by repair via base excision and nucleotide excision pathways. The levels of oxidative DNA damage reported in many human tissues or in animal models of carcinogenesis exceed the levels of lesions induced by exposure to exogenous carcinogenic compounds. Thus, it seems likely that oxidative DNA damage is important in the etiology of many human cancers. This review highlights some of the major accomplishments in the study of oxidative DNA damage and its role in carcinogenesis. It also identifies controversies that need to be resolved. Unraveling the contributions to tumorigenesis of DNA damage from endogenous and exogenous sources represents a major challenge for the future.

The presence and function of CB2 receptors in central nervous system (CNS) neurons are controversial. We report the expression of CB2 receptor messenger RNA and protein localization on brainstem neurons. These functional CB2 receptors in the brainstem were activated by a CB2 receptor agonist, 2-arachidonoylglycerol, and by elevated endogenous levels of endocannabinoids, which also act at CB1 receptors. CB2 receptors represent an alternative site of action of endocannabinoids that opens the possibility of nonpsychotropic therapeutic interventions using enhanced endocannabinoid levels in localized brain areas.

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Cyclooxygenase (COX; prostaglandin G/H synthase, EC 1.14.99.1) catalyzes the first two steps in the biosynthesis of prostaglandins (PGs). The two COX isoforms COX-1 and COX-2 are the targets of the widely used nonsteroidal anti-inflammatory drugs, indicating a role for these enzymes in pain, fever, inflammation, and tumorigenesis. The ubiquitous constitutive expression of COX-1 and inducible expression of COX-2 have led to the widely held belief that COX-1 produces homeostatic PGs, while PGs produced by COX-2 are primarily pathophysiological. However, recent discoveries call this paradigm into question and reveal as yet underappreciated functions for both enzymes. This review focuses on some of these new insights.-Rouzer, C. A., and L. J. Marnett. Cyclooxygenases: structural and functional insights. J. Lipid Res. 2009. 50: S29-S34. Supplementary key words prostaglandinThe cyclooxygenase isoforms (COX-1 and COX-2) are among the most thoroughly studied and best understood mammalian oxygenases. Possessing two separate but linked active sites, the COXs catalyze the bis-dioxygenation and subsequent reduction of arachidonic acid (AA) to prostaglandin (PG)G 2 and PGH 2 (Fig. 1A). The mechanism of oxygenation has been well characterized through kinetics, mutagenesis, and X-ray crystallography (1-3). PGH 2 is subject to metabolism by downstream enzymes yielding the family of PGs, each member of which exerts a range of physiologic effects through specific G-protein-coupled receptors (Fig. 1B) (4, 5). The discovery that the COXs are the target of the nonsteroidal anti-inflammatory drugs (NSAIDs), which play a primary therapeutic role in the treatment of pain, fever, and inflammation (6), promulgated the first wave of experimentation on the constitutively expressed COX-1 during the 1970s and 1980s. Then, just as interest began to wane, the discovery of the inducible isoform, COX-2, rekindled a massive new effort that ultimately led to new insights about both isoforms. A search of PubMed over the past 2 years indicates that there have been over 70 review articles containing "cyclooxygenase" in their title, leading one to question the need for yet another. However, despite the overwhelming mass of data available on these enzymes, recent discoveries suggest that some original assumptions concerning their roles in physiology and pathophysiology require reexamination. This review will emphasize these issues. STRUCTURE OF THE COX ENZYMESHuman COX-1 and COX-2 are homodimers of 576 and 581 amino acids, respectively. Both enzymes contain three high mannose oligosaccharides, one of which facilitates protein folding. A fourth oligosaccharide, present only in COX-2, regulates its degradation. Considering the 60% identity in sequence between COX-1 and COX-2, it is not surprising that their three-dimensional structures are nearly superimposable. Each subunit of the dimer consists of three domains, the epidermal growth factor domain (residues 34-72), the membrane binding domain (residues 73-116), and the catalytic domain comprisi...

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Cyclooxygenases (COX) play an important role in lipid signaling by oxygenating arachidonic acid to endoperoxide precursors of prostaglandins and thromboxane. Two cyclooxygenases exist which differ in tissue distribution and regulation but otherwise carry out identical chemical functions. The neutral arachidonate derivative, 2-arachidonylglycerol (2-AG), is one of two described endocannabinoids and appears to be a ligand for both the central (CB1) and peripheral (CB2) cannabinoid receptors. Here we report that 2-AG is a substrate for COX-2 and that it is metabolized as effectively as arachidonic acid. COX-2-mediated 2-AG oxygenation provides the novel lipid, prostaglandin H 2 glycerol ester (PGH 2 -G), in vitro and in cultured macrophages. PGH 2 -G produced by macrophages is a substrate for cellular PGD synthase, affording PGD 2 -G. Pharmacological studies reveal that macrophage production of PGD 2 -G from endogenous sources of 2-AG is calcium-dependent and mediated by diacylglycerol lipase and COX-2. These results identify a distinct function for COX-2 in endocannabinoid metabolism and in the generation of a new family of prostaglandins derived from diacylglycerol and 2-AG. Cyclooxygenase (COX 1; prostaglandin endoperoxide synthase, EC 1.14.99.1) catalyzes the bis-dioxygenation of arachidonic acid, generating prostaglandin (PG) H 2 , the precursor to a diverse family of lipid mediators including PGs, thromboxane, and prostacyclin (1). The discovery of a second COX isoform, COX-2, has provided important insights into the molecular basis of inflammation, hyperalgesia, and cancer and has established a novel pharmacological target for their treatment (2-4). The major functional differences between COX-1 and COX-2 are believed to be related to their differential regulation and tissue distribution (5). COX-1 is a constitutive enzyme, whereas COX-2 is inducible and highly regulated by a range of agonists (6, 7). COX-1 activity accounts for PG and thromboxane production in gastric mucosa, kidney, and platelets (3). COX-2 activity is primarily responsible for PG biosynthesis in the central nervous system and inflammatory cells (8 -10). These observations suggest that COX-1 and COX-2 serve different physiological and pathophysiological functions.The possibility that COX-2 has distinct biochemical functions has not been explored extensively. There are conserved structural differences between the active sites of COX-1 and COX-2 which have been exploited in the development of selective COX-2 inhibitors. It is possible that these or other structural differences may have evolved to support separate biochemical functions for the two COX isoforms. The first indication of a separate biochemical function for COX-2 was provided by the observation that it selectively oxygenates the neutral ethanolamide derivative of arachidonic acid, anandamide (11). Anandamide and 2-arachidonylglycerol (2-AG) are endogenous ligands for the cannabinoid receptors that bind ⌬ 9 -tetrahydrocannabinol and mediate its pharmacological effects (12). The ca...

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