Cyclooxygenase-deficient mice. A summary of their characteristics and susceptibilities to inflammation and carcinogenesis

Cyclooxygenase-2 expression in murine and human nonmelanoma skin cancers: implications for therapeutic approaches

Inflammatory stimuli result in the production of cutaneous eicosanoids, which are known to contribute to the process of tumor promotion. Cyclooxygenase (COX), the rate-limiting enzyme for the production of prostaglandins (PG) from arachidonic acid, exists in at least two isoforms, COX-1 and COX-2. COX-1 is constitutively expressed in most tissues and plays various physiological roles, whereas increased COX-2 expression is known to occur in several types of epithelial neoplasms. Enhanced PG synthesis is a potential contributing factor in UVB-induced nonmelanoma skin cancers (NMSC). Increased COX-2 staining occurs in murine skin neoplasms after chronic exposure to carcinogenic doses of UVB. In this study, immunohistochemical and Western blot analyses were employed to assess longitudinally COX-2 expression in a standard mouse UVB complete carcinogenesis protocol and in human basal cell carcinomas (BCC) and squamous cell carcinomas (SCC). During UVB irradiation of mice, COX-2 expression consistently increased in the hyperplastic skin, the benign papillomas and the SCC. COX-2 expression was also increased in human actinic keratoses, SCC and BCC as well as in murine SCC and BCC. The pattern of COX-2 expression was quite variable, occurring in a patchy distribution in some lesions with staining confined mainly to suprabasal cell layers. In general, COX-2 expression progressively became more extensive in benign papillomas and well-differentiated murine SCC. The staining was predominantly cytoplasmic and perinuclear in some focal areas in tissue stroma around both murine and human tumors. Western blot analysis confirmed negative COX-2 expression in normal skin, whereas acute UVB exposure resulted in increased enzyme expression, which continued to increase in developing papillomas and SCC. Because of the evidence indicating a pathogenic role for eicosanoids in murine and human skin neoplasms, we performed studies to assess the anti-inflammatory and anticarcinogenic effects of green tea extracts, which are potent antioxidants. Acute exposure of the human skin to UVB (minimum erythema dose x 4) caused a transient enhancement of the COX-2 expression, which reverted to baseline within hours; however, in murine skin the expression persisted for several days. Pretreatment with the topically applied green tea extract (1 mg/cm2) largely abrogated the acute COX-2 response to UVB in mice or humans. In summary, enhanced COX-2 expression serves as a marker of epidermal UVB exposure for murine and human NMSC. These results suggest that COX-2 inhibitors could have potent anticarcinogenic effects in UVB-induced skin cancer.

Prostaglandins as modulators of immunity

Prostaglandins are potent lipid molecules that affect key aspects of immunity. The original view of prostaglandins was that they were simply immunoinhibitory. This review focuses on recent findings concerning prostaglandin E2 (PGE2) and the PGD2 metabolite 15-deoxy-Delta(12,14)-PGJ2, and their divergent roles in immune regulation. We will highlight how these two seminal prostaglandins regulate immunity and inflammation, and play an emerging role in cancer progression. Understanding the diverse activities of these prostaglandins is crucial for the development of new therapies aimed at immune modulation.

Involvement of peripheral cyclooxygenase-1 and cyclooxygenase-2 in inflammatory pain

Pain-induced functional impairment in the rat (PIFIR) is a model of inflammatory and arthritic pain similar to that of clinical gout. Nociception is induced by the intra-articular injection of uric acid into the right hind limb, inducing its dysfunction. Animals then receive analgesic drugs and the recovery of functionality over time is assessed as an expression of antinociception. We have examined the role of peripheral prostaglandins synthesized by cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) in inflammatory pain using the PIFIR model. Rofecoxib (a selective COX-2 inhibitor) and SC-560 (a selective COX-1 inhibitor) both produced dose-dependent effects. When the inhibitors were administered before uric acid, they showed similar potency, but the antinociceptive efficacy of SC-560 was lower than rofecoxib; the best antinociceptive effects were obtained with the dose of 100 microg/articulation of each inhibitor (pre-treatment). In post-treatment (inhibitors administered after the uric acid), rofecoxib showed the least antinociceptive effect and SC-560 was more potent than rofecoxib. The inhibition of both COX-1 and COX-2 produced a more profound analgesic effect than the inhibition of either COX-1 or COX-2 alone. The present data support the idea that both COX isoforms contribute to the development and maintenance of local inflammatory nociception. Thus, it could be expected that inhibition of both COX-1 and COX-2 is required for non-steroidal anti-inflammatory drugs (NSAID)-induced antinociception in the rat. These findings suggest that the therapeutic effects of NSAIDs may involve, at least in part, inhibition of COX-1 and COX-2.

Absence of nuclear factor kappaB inhibition by NSAIDs in hepatocytes

Stimulation of fetal hepatocytes with proinflammatory cytokines and lipopolysaccharide promotes the expression of cyclooxygenase-2 (COX-2) and nitric oxide synthase-2 (NOS-2), whereas the hepatoma cell line HepG2 exhibits a behavior similar to that described for adult hepatocytes and only expresses NOS-2. The effect of nonsteroidal anti-inflammatory drugs (NSAIDs) on the inflammatory onset was analyzed in these cells since in addition to the inhibition of cyclooxygenase activity, these drugs interfere with other signaling pathways related with the inflammatory response. Inhibition of nuclear factor kappaB (NF-kappaB) activation by aspirin and salicylate has been described in many cells. However, incubation of hepatic cells with salicylate, aspirin, indomethacin, ibuprofen, or 5,5-dimethyl-3(3-fluorophenyl)-4-(4-methylsulfonyl)phenyl-2(5H)-furanone (DFU), a fluorinated derivative of rofecoxib, failed to impair IkappaB kinase activity, the processing of NF-kappaB, and the expression of NF-kappaB-dependent genes, such as NOS-2. Moreover, selective COX-2 inhibitors did not promote apoptosis in hepatocytes under inflammatory conditions, suggesting that prostaglandins are not required to maintain cell viability. In conclusion, these data indicate that hepatocytes are not sensitive to NF-kappaB inhibition by NSAIDs and that these drugs, especially the COX-2 selective inhibitors, do not alter cell viability.

The tissue-specific, compensatory expression of cyclooxygenase-1 and -2 in transgenic mice

Prostaglandins are essential regulators of tissue homeostasis, reproduction and inflammation. We have recently shown that cells derived from cyclooxygenase (COX)-deficient mice express higher, compensatory levels of the remaining COX isozyme [Kirtikara et al., J. Exp. Med., 187, 517 (1998)]. To assess this compensatory expression phenomenon in vivo, we quantified COX-1 and COX-2 mRNA levels in various organs of COX-1- and COX-2-ablated mice using a reverse transcriptase-polymerase chain reaction (RT-PCR) method. We found that COX-1 and COX-2 mRNAs in the brains of COX-ablated mice were elevated > 2-fold compared with wild-type (WT) animals. COX-2 mRNA was enhanced approximately 2-fold in the kidneys and stomachs of COX-1-deficient mice while COX-1 expression remained unchanged. Conversely, the livers of COX-2-deficient mice expressed 15-fold higher COX-1 mRNA levels, while hepatic COX-2 mRNA levels were not significantly altered in the COX-1-ablated mice. Steady state levels of COX-1 and COX-2 mRNAs in the hearts, lungs and spleens of WT, COX-1- and COX-2-deficient mice were indistinguishable from each other. Peritoneal macrophages isolated from COX-1- and COX-2-ablated mice also expressed significantly higher steady-state levels of cytoplasmic phospholipase A2 and 5-lipooxygenase mRNAs suggesting a global upregulation of eicosanoid biosynthetic pathways in COX-deficient mice. These data suggest that expression of both COX-1 and COX-2 can be re-programmed to compensate for the lack of both alleles of the alternate COX gene in transgenic mice.

Increased susceptibility to ischemic brain injury in cyclooxygenase-1-deficient mice

Cyclooxygenase-1 (COX-1), a rate-limiting enzyme in the synthesis of prostanoids, is involved in selected vasodilatatory responses of the cerebral circulation. Cyclooxygenase-1-null mice were used to determine whether COX-1 influences cerebral ischemic damage. The middle cerebral artery was occluded in COX-1 -/- and +/+ mice (n = 9/group), and lesion volume was determined in thionin-stained sections 24 or 96 hours later. Middle cerebral artery occlusion produced larger infarcts in COX-1 -/- mice, both at 24 (35 +/- 17%; P < 0.05) and 96 hours (41 +/- 16%; P < 0.05) after ischemia. The enlargement was not due to increased susceptibility to glutamate excitotoxicity, because microinjection of N-methyl-D-aspartate or kainate in the parietal cortex produced comparable lesions in COX-1 +/+ and -/- mice ( P > 0.05; n = 8/group). To examine the contribution of hemodynamic factors to the enlargement of the infarct, cerebral blood flow was monitored by laser-Doppler flowmetry in the ischemic territory (n = 6/group). Although the reduction in cerebral blood flow was comparable in the ischemic core ( P > 0.05), at the periphery of the ischemic territory the reduction was greater in COX-1 -/- mice (-58 +/- 4%) than in COX-1 +/+ mice (-34 +/- 5%; P < 0.05). It is concluded that mice lacking COX-1 are more susceptible to focal cerebral ischemia, an effect that can be attributed to a more severe cerebral blood flow reduction in vulnerable regions at the periphery of the ischemic territory. Thus, the vascular effects of COX-1 may contribute to maintain cerebral blood flow in the postischemic brain and, as such, play a protective role in ischemic brain injury.

Is COX-2 inhibition a panacea for cancer prevention?

The epidemiologic evidence and rodent studies suggest strongly that nonselective inhibitors of cyclooxygenase (COX) enzymes such as aspirin, inhibiting both COX-1 and COX-2 isoforms, reduce the incidence of and mortality from intestinal tumors. Genetically manipulated animals show that both Cox-1 and Cox-2 disruptions decrease the tumor yield, both in genetically predisposed and in carcinogen-treated mice. The mechanisms by which COX-1 and COX-2 deficiency decrease tumorigenesis are still unknown. Cox-2 overexpression increased the tumor yield in mammary glands of the multiparous, but not virginal female transgenic mice using the murine mammary tumor virus promoter. The Cox-2 protein was strongly induced during pregnancy and lactation. These data suggest that Cox-2 overexpression may be an important target for cancer chemoprevention. This finding was supported by the observed cancer-preventive effects of the COX-2-specific inhibitors in humans and in rodents. However, based on the available data, we cannot totally attribute the cancer preventive effects of nonsteroidal antiinflammatory drugs (NSAIDs) to COX-2 alone-even COX-1 may have an important role in cancer prevention as suggested by the Cox-1-deficient Min mice. It is likely that COX-1 plays a more important role in NSAID-induced toxicity in humans, such as in gastric ulcer formation-but inhibition of COX-2 may not be without toxic manifestations either, as suggested by the poor survival of the Cox-2-nulled mice. Combinations of COX-2 inhibitors with other agents that target other pathways in carcinogenesis may be a more efficacious and a less toxic strategy in cancer chemoprevention.

Inflammation, carcinogenesis and cancer

To fulfill their role in host-defense, granulocytes secrete chemically reactive oxidants, radicals, and electrophilic mediators. While this is an effective way to eradicate pathogenic microbes or parasites, it inevitably exposes epithelium and connective tissue to certain endogenous genotoxic agents. In ordinary circumstances, cells have adequate mechanisms to reduce the genotoxic burden imposed by these agents to a negligible level. However, inflammation persisting for a decade eventually elevates the risk of cancer sufficiently that it is discernible in case control epidemiological studies. Advances in our understanding of tumor suppressors and inflammatory mediators offer an opportunity to assess the molecular and cellular models used to guide laboratory investigations of this phenomenon. Disappointing results from recent clinical trials with anti-oxidant interventions raise questions about the risks from specific endogenous agents such as hydrogen peroxide and oxy radicals. Simultaneously, the results from the anti-oxidant trials draw attention to an alternate hypothesis, favoring epigenetic inactivation of key tumor suppressors, such as p53, and the consequent liability this places on genomic integrity.

Differences in the detection of cyclo-oxygenase 1 and 2 proteins in symptomatic and asymptomatic carotid plaques

BACKGROUND

The expression of cyclo-oxygenase (COX) 1 and 2 has been demonstrated in atherosclerotic arteries. In the present study this was correlated with symptoms arising from a carotid plaque.

METHODS

Carotid plaques from 12 asymptomatic patients were compared with 11 plaques from patients who had had neurological symptoms within the preceding 30 days. Sections were stained with haematoxylin and eosin, elastin van Gieson and goat antihuman antibodies to COX-1 and COX-2. Plaque morphology was correlated with neurological symptoms. The area with positive COX-1 and COX-2 staining was measured by computerized planimetry in entire cross-sections and in specific areas of the plaque.

RESULTS

There was a significant association between cap thinning and plaque rupture with symptoms (P = 0.003). The percentage area of positive staining in entire cross-sections for both COX-1 and COX-2 was significantly greater in symptomatic plaques (P = 0.001 and 0.0004 respectively). Staining in symptomatic plaques was significantly greater in the cap (COX-1: P = 0.001; COX-2: P = 0.0001) and shoulder (COX-1: P = 0.008; COX-2: P = 0.007) regions of the plaque. COX-1 expression in the sclerotic area was not increased (P = 0.15) although COX-2 staining was significantly greater (P = 0.04).

CONCLUSION

Both COX-1 and COX-2 detection was increased in symptomatic plaques. COX may contribute to plaque rupture and the onset of symptoms.

Immunohistochemical expression of cyclooxygenase-2 in skin cancers

BACKGROUND

Cyclooxygenase (COX), also known as prostaglandin endoperoxide synthase, catalyses the conversion of arachidonic acid to prostanoids. There are two different isoforms of COX, referred to as COX-1 and COX-2. Overexpression of COX-2 has been demonstrated in various neoplasms, such as experimentally promoted tumors, gastrointestinal cancers and breast tumors.

METHODS

In this study, we used immunohistochemistry to investigate COX-2 expression in a series of basal cell epitheliomas (BCE), Bowen's disease, squamous cell carcinomas (SCC) and metastatic tumors of the skin.

RESULTS

Four of 16 BCE showed a positive reaction for COX-2 and the adenoid type of BCE was the most strongly positive. In Bowen's disease, the extent of positive staining for COX-2 was even higher than that in BCE. Eleven of 15 SCC showed a positive reaction for COX-2 and the pattern of staining was heterogeneous with more intense staining in the center of the tumor nests. In metastatic tumors, the percentage of COX-2-positive tumor cells and the intensity of their staining was low compared with Bowen's disease and SCC.

CONCLUSIONS

These results indicate that the intensity of COX-2 staining and its heterogeneous distribution are related to the degree of cellular differentiation and the various phenotypes of tumor cells, but the extent of COX-2 staining did not correlate with the degree of malignancy.

Overexpression of cyclooxygenase-2 is sufficient to induce tumorigenesis in transgenic mice

The cyclooxygenase (COX)-2 gene encodes an inducible prostaglandin synthase enzyme that is overexpressed in adenocarcinomas and other tumors. Deletion of the murine Cox-2 gene in Min mice reduced the incidence of intestinal tumors, suggesting that it is required for tumorigenesis. However, it is not known if overexpression of Cox-2 is sufficient to induce tumorigenic transformation. We have derived transgenic mice that overexpress the human COX-2 gene in the mammary glands using the murine mammary tumor virus promoter. The human Cox-2 mRNA and protein are expressed in mammary glands of female transgenic mice and were strongly induced during pregnancy and lactation. Female virgin Cox-2 transgenic mice showed precocious lobuloalveolar differentiation and enhanced expression of the beta-casein gene, which was inhibited by the Cox inhibitor indomethacin. Mammary gland involution was delayed in Cox-2 transgenic mice with a decrease in apoptotic index of mammary epithelial cells. Multiparous but not virgin females exhibited a greatly exaggerated incidence of focal mammary gland hyperplasia, dysplasia, and transformation into metastatic tumors. Cox-2-induced tumor tissue expressed reduced levels of the proapoptotic proteins Bax and Bcl-x(L) and an increase in the anti-apoptotic protein Bcl-2, suggesting that decreased apoptosis of mammary epithelial cells contributes to tumorigenesis. These data indicate that enhanced Cox-2 expression is sufficient to induce mammary gland tumorigenesis. Therefore, inhibition of Cox-2 may represent a mechanism-based chemopreventive approach for carcinogenesis.

Evidence for a direct role of cyclo-oxygenase 2 in implant wear debris-induced osteolysis

Aseptic loosening is a major complication of prosthetic joint surgery and is manifested as chronic inflammation, pain, and osteolysis at the bone implant interface. The osteolysis is believed to be driven by a host inflammatory response to wear debris generated from the implant. In our current study, we use a selective inhibitor (celecoxib) of cyclo-oxygenase 2 (COX-2) and mice that lack either COX-1 (COX-1-/-) or COX-2 (COX-2-/-) to show that COX-2, but not COX-1, plays an important role in wear debris-induced osteolysis. Titanium (Ti) wear debris was implanted surgically onto the calvaria of the mice. An intense inflammatory reaction and extensive bone resorption, which closely resembles that observed in patients with aseptic loosening, developed within 10 days of implantation in wild-type and COX-1-/- mice. COX-2 and prostaglandin E2 (PGE2) production increased in the calvaria and inflammatory tissue overlying it after Ti implantation. Celecoxib (25 mg/kg per day) significantly reduced the inflammation, the local PGE2 production, and osteolysis. In comparison with wild-type and COX-1-/- mice, COX-2-/- mice implanted with Ti had a significantly reduced calvarial bone resorption response, independent of the inflammatory response, and significantly fewer osteoclasts were formed from cultures of their bone marrow cells. These results provide direct evidence that COX-2 is an important mediator of wear debris-induced osteolysis and suggests that COX-2 inhibitors are potential therapeutic agents for the prevention of wear debris-induced osteolysis.

Role of ETB and B2 receptors in the ex vivo platelet inhibitory properties of endothelin and bradykinin in the mouse

1. We have developed a model to study the inhibitory properties of endogenous autacoids triggered by systemically-administered vasoactive peptides, on platelet aggregation ex vivo in the mouse. 2. Adenosine diphosphate (ADP) (0.5-10 microM) induces a concentration-dependent aggregation of platelet-rich plasma derived from C57BL/6 mice. Intravenously-administered endothelin-1 (0.01-1 nmolx kg(-1)), the selective ETB agonist, IRL-1620 (0.0 -1 nmol x kg(-1)) or bradykinin ( 1-100 nmol x kg(-1)) significantly reduced in a dose-dependent fashion the ADP-induced platelet aggregation. 3. The non-selective cyclo-oxygenase (COX) inhibitor, indomethacin, a selective COX-2 inhibitor NS-398 or the prostacyclin synthase inhibitor, tranylcypromine (10 mg x kg(-1)), markedly reduced the inhibitory properties of endothelin-1, whereas only a combination of both indomethacin, NS-398 or tranylcypromine and L-NAME (10 mg x kg(-1)) were required to abolish the response to bradykinin. 4. An ETB-selective antagonist (BQ-788) or knockout of the B2 receptor gene (in B2 knockout mice) abolishes the platelet inhibitory properties of endothelin-1 and bradykinin, respectively. 5. Our results suggest that intravenously-administered endothelin-1 and bradykinin, through ETB and B2 receptor activation, respectively, inhibit platelet aggregation ex vivo in the mouse. The inhibitory properties of endothelin-1 require the activation of COX-2 and the subsequent generation of prostacyclin. In addition to the two previously mentioned factors, nitric oxide is required for the anti-aggregatory effects of bradykinin.

Reduced susceptibility to ischemic brain injury and N-methyl-D-aspartate-mediated neurotoxicity in cyclooxygenase-2-deficient mice

Cyclooxygenase-2 (COX-2), a prostanoid-synthesizing enzyme that contributes to the toxicity associated with inflammation, has recently emerged as a promising therapeutic target for several illnesses, ranging from osteoarthritis to Alzheimer's disease. Although COX-2 has also been linked to ischemic stroke, its role in the mechanisms of ischemic brain injury remains controversial. We demonstrate that COX-2-deficient mice have a significant reduction in the brain injury produced by occlusion of the middle cerebral artery. The protection can be attributed to attenuation of glutamate neurotoxicity, a critical factor in the initiation of ischemic brain injury, and to abrogation of the deleterious effects of postischemic inflammation, a process contributing to the secondary progression of the damage. Thus, COX-2 is involved in pathogenic events occurring in both the early and late stages of cerebral ischemia and may be a valuable therapeutic target for treatment of human stroke.