UVB light upregulates prostaglandin synthases and prostaglandin receptors in mouse keratinocytes.

Prostaglandins belong to a class of cyclic lipid-derived mediators synthesized from arachidonic acid via COX-1, COX-2 and various prostaglandin synthases. Members of this family include prostaglandins such as PGE(2), PGF(2alpha), PGD(2) and PGI(2) (prostacyclin) as well as thromboxane. In the present studies we analyzed the effects of UVB on prostaglandin production and prostaglandin synthase expression in primary cultures of undifferentiated and calcium-differentiated mouse keratinocytes. Both cell types were found to constitutively synthesize PGE(2), PGD(2) and the PGD(2) metabolite PGJ(2). Twenty-four hours after treatment with UVB (25 mJ/cm(2)), production of PGE(2) and PGJ(2) increased, while PGD(2) production decreased. This was associated with increased expression of COX-2 mRNA and protein. UVB (2.5-25 mJ/cm(2)) also caused marked increases in mRNA expression for the prostanoid synthases PGDS, mPGES-1, mPGES-2, PGFS and PGIS, as well as expression of receptors for PGE(2) (EP1 and EP2), PGD(2) (DP and CRTH2) and prostacyclin (IP). UVB was more effective in inducing COX-2 and DP in differentiated cells and EP1 and IP in undifferentiated cells. UVB readily activated keratinocyte PI-3-kinase (PI3K)/Akt, JNK and p38 MAP signaling pathways which are known to regulate COX-2 expression. While inhibition of PI3K suppressed UVB-induced mPGES-1 and CRTH2 expression, JNK inhibition suppressed mPGES-1, PGIS, EP2 and CRTH2, and p38 kinase inhibition only suppressed EP1 and EP2. These data indicate that UVB modulates expression of prostaglandin synthases and receptors by distinct mechanisms. Moreover, both the capacity of keratinocytes to generate prostaglandins and their ability to respond to these lipid mediators are stimulated by exposure to UVB.

Chemotherapeutic efficacy of topical celecoxib in a murine model of ultraviolet light B-induced skin cancer.

Over a million nonmelanoma skin cancer cases will be reported in the United States this year alone. Currently the primary form of treatment for these types of skin tumors is excision. However, excision of the initial lesion may not be curative because almost 50% of patients with one nonmelanoma skin cancer lesion develop another tumor within the next 5 yr at the site or adjacent to the site of excision. As with other types of epithelial based cancers, there is mounting evidence for the role of cyclooxygenase-2 (COX-2) and its products, particularly prostaglandin E(2) (PGE(2)), in the development of nonmelanoma skin cancer. To avoid the excision process, the present study was designed to evaluate the possible chemotherapeutic effect of directly treating established tumors with a topical formulation of the specific COX-2 inhibitor celecoxib. Skh/hr hairless mice were irradiated three times per wk for 16 wk to induce tumor formation. The mice were then divided into two groups and treated topically with either 500 microg celecoxib or the vehicle for 6 wk. Our results demonstrated that although topical treatment with celecoxib was not able to induce regression of established tumors, it did prevent new tumor formation after the onset of photocarcinogenesis. Although further studies are warranted, these data suggest that topical celecoxib treatment may prove to be effective in preventing the recurrence of tumors at the site of nonmelanoma skin cancer excision.

Improvement of radiotherapy or chemoradiotherapy by targeting COX-2 enzyme.

Radiation therapy has traditionally been the treatment of choice for locally or regionally advanced cancer, but its therapeutic efficacy is often hindered by limited tolerance of normal tissues and by tumor radioresistance. To improve therapeutic outcome, radiotherapy is frequently combined with chemotherapeutic drugs that are themselves cytotoxic and may sensitize cells to radiation. Solid evidence exists that administering standard chemotherapeutic agents during the course of radiotherapy (concurrent chemoradiotherapy) increases both local tumor control and patient survival in a number of cancer sites. These therapeutic improvements, however, have been achieved at the expense of considerable normal tissue toxicity. To improve chemoradiotherapy further, there have been extensive explorations of the potential of newer chemotherapeutic agents, including irinotecan (CPT-11, Camptosar) and other topoisomerase inhibitors. Preclinical studies have shown that these agents are potent radiosensitizers, providing a strong biologic rationale for using these drugs in combination with radiotherapy. These studies also generated information critical for designing effective treatment schedules in clinical settings. The therapeutic efficacy of topoisomerase inhibitor-radiation combinations is currently being tested clinically. Recent advances in molecular biology have discovered many cellular molecules, including the cyclooxygenase-2 (COX-2) enzyme, that promote tumor cell survival and are responsible for tumor resistance to cytotoxic agents, and hence may serve as potential targets for augmentation of radio (or chemo) response. COX-2 is often overexpressed in premalignant lesions and cancer, and is involved in carcinogenesis, tumor growth, and metastatic spread. Preclinical studies provided solid evidence that inhibition of this enzyme with selective COX-2 inhibitors prevents carcinogenesis, slows the growth of established tumors, and enhances tumor response to radiation without appreciably affecting normal tissue radioresponse. The mechanisms of enhancement of tumor radioresponse involve direct actions on tumor cells and indirect actions, primarily on tumor vasculature. COX-2 inhibitors also improve tumor response to chemotherapeutic agents, including irinotecan. Additional therapeutic benefit was observed for celecoxib (Celebrex), a selective COX-2 inhibitor, consisting of a strong reduction in irinotecan-induced diarrhea. Thus, selective targeting of COX-2 may potentially improve radiotherapy, chemotherapy, or chemoradiotherapy--a therapeutic strategy that is currently being tested in clinical trials.

CUGBP2 plays a critical role in apoptosis of breast cancer cells in response to genotoxic injury.

Posttranscriptional control of gene expression plays a key role in regulating gene expression in cells undergoing apoptosis. Cyclooxygenase-2 (COX-2) is a crucial enzyme in the conversion of arachidonic acid to prostaglandin E2 (PGE(2)) and is significantly upregulated in many types of adenocarcinomas. COX-2 overexpression leads to increased PGE(2) production, resulting in increased cellular proliferation. PGE(2) enhances the resistance of cells to ionizing radiation. Accordingly, understanding mechanisms regulating COX-2 expression may lead to important therapeutic advances. Besides transcriptional control, COX-2 expression is significantly regulated by mRNA stability and translation. We have previously demonstrated that RNA binding protein CUGBP2 binds AU-rich sequences to regulate COX-2 mRNA translation. In the current study, we have determined that expression of both COX-2 mRNA and CUGBP2 mRNA are induced in MCF-7 cells, a breast cancer cell line, following exposure to 12 Gy gamma-irradiation. However, only CUGBP2 protein is induced, but COX-2 protein levels were not altered. Silencer RNA (siRNA)-mediated inhibition of CUGBP2 reversed the block in COX-2 protein expression. Furthermore, MCF-7 cells underwent apoptosis in response to radiation injury, which was also reversed by CUGBP2 siRNAs. These data suggest that CUGBP2 is a critical regulator of the apoptotic response to genotoxic injury in breast cancer cells.

Fructose-1,6-diphosphate attenuates prostaglandin E2 production and cyclo-oxygenase-2 expression in UVB-irradiated HaCaT keratinocytes.

1. Fructose-1,6-diphosphate (FDP), a glycolytic metabolite, is reported to ameliorate inflammation and inhibit the nitric oxide production in murine macrophages stimulated with endotoxin. It is also reported that FDP has cytoprotective effects against hypoxia or ischaemia/reperfusion injury in brain and heart. However, underlying mechanisms of its various biological activities are not completely understood. 2. In this study, we examined the effects of FDP on UVB-induced prostaglandin production in HaCaT keratinocytes. 3. Ultraviolet B (UVB, 280-320 nm) irradiation (30 mJ cm(-2)) increased prostaglandin E(2)(PGE(2)) production, which was significantly decreased by FDP in a concentration dependent manner. NS-398, a cyclo-oxygenase-2 (COX-2) selective inhibitor completely inhibited UVB-induced PGE(2) production showing that COX-2 activity is responsible for the increase in PGE(2) production under our experimental conditions. 4. UVB irradiation increased total COX activity and COX-2 mRNA in HaCaT keratinocytes, which were significantly blocked by FDP in a concentration dependent manner. 5. N-acetylcysteine (NAC) significantly attenuated UVB-induced PGE(2) production, COX activity and COX-2 mRNA expression indicating oxidative components might contribute to these events. 6. FDP reduced UVB-induced increase in cellular reactive oxygen species (ROS) level although it did not show direct radical scavenging effect in the experiment using 1,1-diphenyl-2picrylhydrazil (DPPH). FDP preserved the cellular antioxidant capacity including catalase activity and GSH content after irradiation. 7. Our data obtained hitherto suggest that FDP may have a protective role in UVB-injured keratinocyte by attenuating PGE(2) production and COX-2 expression, which are possibly through blocking intracellular ROS accumulation.

Cyclooxygenase-2 modulates brain inflammation-related gene expression in central nervous system radiation injury.

Although the contribution of cyclooxygenase-2 (COX-2) to peripheral inflammation is well documented, little is known about its role in brain inflammation. For this purpose we studied COX-2 expression in the mouse brain following ionizing radiation in vivo, as well as in murine glial cell cultures in vitro. The possible role of COX-2 in modulating brain inflammation was examined utilizing NS-398, a COX-2 selective inhibitor. Our results indicate that COX-2 is significantly induced in astrocyte and microglial cultures by radiation injury as well as in brain. Increased levels of prostaglandin E(2) in irradiated brain were reduced by NS-398. Moreover, NS-398 administration significantly attenuated levels of induction for the majority of inflammatory mediators examined, including TNFalpha, IL-1beta, IL-6, iNOS, ICAM-1, and MMP-9. In contrast, the chemokines MIP-2 and MCP-1 showed enhanced levels of induction following NS-398 administration. These results indicate that COX-2 modulates the inflammatory response in brain following radiation injury, and suggest the use of COX-2 selective inhibitors for the management of CNS inflammation.

Biological predictors of response to radiotherapy in head and neck cancer: recent advances and emerging perspectives.

The study of new biological parameters has received considerable attention in radiotherapy during the last decade due to their potential value in predicting treatment response in squamous cell carcinoma of the head and neck (SCC-HN) and the foreseen possibility of selecting altered fractionation radiotherapy for the individual patient. Although there are established clinical parameters in SCC-HN patients that relate to radiation response (extent of disease, hemoglobin level), recent advances with direct measurement of tumor oxygenation, inherent radiosensitivity and proliferation rate have increased the promise of individualization of treatment strategy according to these radiobiologically based parameters. Molecular research has now identified a host of new biological parameters with potential predictive utility; oncogenes, tumor suppressor genes, cell-cycle control genes, apoptosis genes and angiogenesis genes have been extensively studied and correlated with radiation response. Moreover, study of the epidermal growth factor receptor signal-transduction system as a possible response modulator has recently fostered molecular strategies which employ blockade of the receptor to down-regulate tumor growth. This article briefly reviews and analyzes the main controversial issues and drawbacks that hinder the general use of biological parameters for predicting tumor response to radiotherapy. It highlights the future perspectives of radiotherapy predictive assay research and the need to shift from single-parameter analysis to multiparametric studies which take into account several potential predictors that together are involved in different biological and clinical pathways.

Radiation induces upregulation of cyclooxygenase-2 (COX-2) protein in PC-3 cells.

PURPOSE: To investigate the impact of gamma-irradiation on cyclooxygenase-2 (COX-2) expression and its enzymatic activity in PC-3 cells. Cell cycle redistribution, viability, and apoptosis were quantitated in control and irradiated cells with or without the COX-2 inhibitor NS-398. METHODS AND MATERIALS: Western blot analysis was used to assess COX-2 protein expression. Prostaglandin (PGE(2)) was measured after addition of arachidonic acid (AA) using a Monoclonal Immunoassay Kit. Cell cycle and apoptosis were assessed using flow cytometry. RESULTS: We observed a dose-dependent increase in COX-2 of 37.0%, 79.7%, and 97.5% following irradiation with 5, 10, and 15 Gy, respectively. The PGE(2) level of irradiated cells was higher than in controls (1512 +/- 157.5 vs. 973.7 +/- 54.2 rhog PGE(2)/mL; p < 0.005, n = 4) while cells irradiated in the presence of NS-398 had reduced PGE(2) levels (218.8 +/- 80.1 rhog PGE(2)/mL; p < 0.005; n = 4). We found no differences in cell cycle distribution or apoptosis between cells irradiated in the presence or absence of NS-398. CONCLUSIONS: COX-2 protein is upregulated and enzymatically active after irradiation, resulting in elevated levels of PGE(2). This effect can be suppressed by NS-398, which has clinical implications for therapies combining COX-2 inhibitors with radiation therapy.

Crypt stem cell survival in the mouse intestinal epithelium is regulated by prostaglandins synthesized through cyclooxygenase-1.

Prostaglandins (PGs) are important mediators of epithelial integrity and function in the gastrointestinal tract. Relatively little is known, however, about the mechanism by which PGs affect stem cells in the intestine during normal epithelial turnover, or during wound repair. PGs are synthesized from arachidonate by either of two cyclooxygenases, cyclooxygenase-1 (Cox-1) or cyclooxygenase-2 (Cox-2), which are present in a wide variety of mamalian cells. Cox-1 is thought to be a constitutively expressed enzyme, and the expression of Cox-2 is inducible by cytokines or other stimuli in a variety of cell types. We investigated the role of PGs in mouse intestinal stem cell survival and proliferation following radiation injury. The number of surviving crypt stem cells was determined 3.5 d after irradiation by the microcolony assay. Radiation injury induced a dose-dependent decrease in the number of surviving crypts. Indomethacin, an inhibitor of Cox-1 and Cox-2, further reduced the number of surviving crypts in irradiated mice. The indomethacin dose response for inhibition of PGE2 production and reduction of crypt survival were similar. DimethylPGE2 reversed the indomethacin-induced decrease in crypt survival. Selective Cox-2 inhibitors had no effect on crypt survival. PGE2, Cox-1 mRNA, and Cox-1 protein levels all increase in the 3 d after irradiation. Immunohistochemistry for Cox-1 demonstrated localization in epithelial cells of the crypt in the unirradiated mouse, and in the regenerating crypt epithelium in the irradiated mouse. We conclude that radiation injury results in increased Cox-1 levels in crypt stem cells and their progeny, and that PGE2 produced through Cox-1 promotes crypt stem cell survival and proliferation.

Light induces peroxidation in retina by activating prostaglandin G/H synthase.

Prostaglandin G/H synthase (PGHS) has been shown to generate peroxides to a significant extent in the retina and absorbs light at the lower end of the visible spectrum. We postulated that PGHS could be an important initial source of peroxidation in the retina exposed to light, which would in turn alter retinal function. Exposure of pig eyes (in vivo) to light (350 fc/3770 lx) caused after 3 h a 50% increase and by 5 h a 30% decrease in a- and b-wave amplitudes of the electroretinogram (ERG) which were comparable at 380-650 nm and 380-440 nm but were not observed at wavelengths > 450 nm. These effects of light were prevented by free radical scavengers (dimethylthiourea and high-dose allopurinol) and PGHS inhibitors (naproxen and diclofenac), but stable analogs of prostaglandins did not affect the ERG. Both increases and subsequent decreases in ERG wave amplitudes following light exposure in vivo were associated with increases in retinal prostaglandin and malondialdehyde (peroxidation product) levels, which were inhibited by the nonselective PGHS blockers, naproxen and diclofenac. Similar observations were made in vitro on isolated porcine eyecups as well as on retinal membranes exposed to light (250 fc/ 2700 lx) 380-650 nm and 380-440 nm but not at > 500 nm. Both PGHS-1 and PGHS-2 contributed equivalently to light-induced prostaglandin synthesis, as shown after selective PGHS-2 blockers, but mRNA expression of PGHS-1 and 2 was not affected by light. Finally, light stimulated activities of pure PGHS-1 and PGHS-2 isozymes, and these were also shown to produce superoxide radical (detected with fluorogenic spin trap, proxyl fluorescamine). Taken together, data suggest that PGHS- (1 and 2) is activated by short wavelength visible light, and in the retina is an important source of reactive oxygen species which in turn alter retinal electrophysiological function. PGHS thus seems a likely chromophore in setting forth photic-induced retinal injury. Findings provide an explanation for increased sensitivity of the retina to visible light predominantly at the far blue range of its spectrum.

Radiation reduces cyclooxygenase activity in cultured human endothelial cells at low doses.

The effect of radiation on prostaglandin (PG) production was investigated in cultured human umbilical vein endothelial cells. It was found that 48 hours after irradiation the endothelial cell capacity to synthesize prostacyclin (PGI2), Prostaglandin E2 (PGE2) and prostaglandin F2 alpha (PGF2 alpha) from exogenous arachidonic acid (AA) was strongly reduced in a radiation dose-dependent way, with 50% of the maximal inhibition at approximately 2 Gy. By incubating the endothelial cells between 24 and 48 hours after irradiation with 50 U/ml interleukin-2 (IL-2), which is known to selectively stimulate de novo synthesis of cyclooxygenase, the PGI2 synthesis from exogenous AA was nearly completely restored. Basal PGI2 release was not influenced by radiation (up to 25 Gy), nor was there increased cell damage as measured by LDH release during 72 hours after irradiation compared with controls. Clonogenic cell survival after irradiation showed a typical exponential radiation dose-response curve with a fairly broad initial shoulder. The data presented in this study suggest that the reduction of endothelial PGI2 synthesis after low doses of radiation is primarily due to a reduction in the activity of the enzyme cyclooxygenase.

Target size analysis of prostaglandin endoperoxide synthase. Radiation inactivation of both cyclooxygenase and peroxidase correlated with the monomer of 72 kDa.

To determine the size of the functional catalytic unit of prostaglandin endoperoxide (prostaglandin H) synthase, radiation inactivation experiments were performed. Both microsomes from ovine seminal vesicles and purified enzyme were irradiated with 10 MeV electrons. The enzymic activities of prostaglandin H synthase, cyclooxygenase and peroxidase, showed mono-exponential inactivation curves dependent on radiation dose, indicating molecular masses of approximately 72 kDa. The enzyme in microsomes, in its native environment, as well as in its purified state after solubilisation with nonionic detergent showed identical molecular masses. The results clearly demonstrate that the monomer of the enzyme with an apparent molecular mass of 72 kDa (SDS/PAGE) is the functional unit for catalysis of both activities. Hence the two active sites of cyclooxygenase and peroxidase reside on the same polypeptide chain.

Thromboxane increase in irradiated animals is caused by stimulation of cyclooxygenase activity.

The irradiation of whole body of rabbits with a dose of 6.0 Gy causes an increase in thromboxane synthesis from exogenous arachidonic acid. The uptake of [14C]arachidonic acid and the total amount of radioactivity released during collagen stimulated aggregation of platelets are not changed following the exposure of animals. The irradiation changes the relation between released arachidonic acid and synthesized thromboxane. The amount of 12-hydroxyeicosatetraenoic acid remains unchanged. The results indicate that the increase in thromboxane synthesis is not associated with the activation of phospholipase but is caused by stimulation of cyclooxygenase activity.

Regional release of cyclooxygenase products after radiation exposure of the rat.

The present study evaluated the regional release of cyclooxygenase products 4 h following 20 Gy gamma irradiation. Thoracic shielding reduced the radiation-induced increase in immunoreactive thromboxane B2 (iTxB2) excretion to control levels while abdominal shielding partially attenuated the altered excretion of this cyclooxygenase product. To assess the role the kidneys play in the radiation-induced increase in iTxB2 excretion, an in situ isolated perfused rat kidney model was developed. The excretion rate of iTxB2 from irradiated isolated perfused kidneys was not significantly different from sham-irradiated perfused kidneys. Radiation exposure did alter renal cyclooxygenase product release in that the excretion of immunoreactive prostaglandin E2 (iPG2) and immunoreactive 6-keto-PGF1 alpha was significantly increased (P less than 0.05) in irradiated isolated perfused kidneys. These data show that radiation-induced increases in iTxB2 excretion are primarily due to altered extrarenal synthesis and/or metabolism of this arachidonate metabolite.