Loss of the EP2 prostaglandin E2 receptor in immortalized human keratinocytes results in increased invasiveness and decreased paxillin expression

A Review of Prostanoid Receptors: Expression, Characterization, Regulation, and Mechanism of Action

Prostaglandin signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain to cell survival. Disruption of normal prostanoid signaling is implicated in numerous disease states. Prostaglandin signaling is facilitated by G-protein-coupled, prostanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of prostanoid receptors with particular emphasis on human isoforms.

Topical application of a platelet activating factor receptor agonist suppresses phorbol ester-induced acute and chronic inflammation and has cancer chemopreventive activity in mouse skin

Platelet activating factor (PAF) has long been associated with acute edema and inflammatory responses. PAF acts by binding to a specific G-protein coupled receptor (PAF-R, Ptafr). However, the role of chronic PAF-R activation on sustained inflammatory responses has been largely ignored. We recently demonstrated that mice lacking the PAF-R (Ptafr-/- mice) exhibit increased cutaneous tumorigenesis in response to a two-stage chemical carcinogenesis protocol. Ptafr-/- mice also exhibited increased chronic inflammation in response to phorbol ester application. In this present study, we demonstrate that topical application of the non-hydrolysable PAF mimetic (carbamoyl-PAF (CPAF)), exerts a potent, dose-dependent, and short-lived edema response in WT mice, but not Ptafr -/- mice or mice deficient in c-Kit (c-Kit^W-sh/W-sh mice). Using an ear inflammation model, co-administration of topical CPAF treatment resulted in a paradoxical decrease in both acute ear thickness changes associated with a single PMA application, as well as the sustained inflammation associated with chronic repetitive PMA applications. Moreover, mice treated topically with CPAF also exhibited a significant reduction in chemical carcinogenesis. The ability of CPAF to suppress acute and chronic inflammatory changes in response to PMA application(s) was PAF-R dependent, as CPAF had no effect on basal or PMA-induced inflammation in Ptafr-/- mice. Moreover, c-Kit appears to be necessary for the anti-inflammatory effects of CPAF, as CPAF had no observable effect in c-Kit^W-sh/W-sh mice. These data provide additional evidence that PAF-R activation exerts complex immunomodulatory effects in a model of chronic inflammation that is relevant to neoplastic development.

The role of the EP receptors for prostaglandin E2 in skin and skin cancer

One of the most common features of exposure of skin to ultraviolet (UV) light is the induction of inflammation, a contributor to tumorigenesis, which is characterized by the synthesis of cytokines, growth factors and arachidonic acid metabolites, including the prostaglandins (PGs). Studies on the role of the PGs in non-melanoma skin cancer (NMSC) have shown that the cyclooxygenase-2 (COX-2) isoform of the cyclooxygenases is responsible for the majority of the pathological effects of PGE₂. In mouse skin models, COX-2 deficiency significantly protects against chemical carcinogen- or UV-induced NMSC while overexpression confers endogenous tumor promoting activity. Current studies are focused on identifying which of the G protein-coupled EP receptors mediate the tumor promotion/progression activities of PGE₂ and the signaling pathways involved. As reviewed here, the EP1, EP2, and EP4 receptors, but not the EP3 receptor, contribute to NMSC development, albeit through different signaling pathways and with somewhat different outcomes. The signaling pathways activated by the specific EP receptors are context specific and likely depend on the level of PGE₂ synthesis, the differential levels of expression of the different EP receptors, as well as the levels of expression of other interacting receptors. Understanding the role and mechanisms of action of the EP receptors potentially offers new targets for the prevention or therapy of NMSCs.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

PGE(2) is a UVR-inducible autocrine factor for human melanocytes that stimulates tyrosinase activation

Prostaglandins activate signalling pathways involved in growth, differentiation and apoptosis. Prostaglandin E(2) (PGE(2)) is released by keratinocytes following ultraviolet irradiation (UVR) and stimulates the formation of dendrites in melanocytes. We show that multiple irradiations of human melanocytes with UVR-activated cPLA(2), the rate-limiting enzyme in eicosanoid synthesis and stimulated PGE(2) secretion. PGE(2) increased cAMP production, tyrosinase activity and proliferation in melanocytes. PGE(2) binds to four distinct G-protein coupled receptors (EP(1-4)). We show that PGE(2) stimulates EP(4) receptor signalling in melanocytes, resulting in cAMP production. Conversely, PGE(2) also stimulated the EP(3) receptor in melanocytes, resulting in lowered basal cAMP levels. These data suggest that relative levels or activity of these receptors controls effects of PGE(2) on cAMP in melanocytes. The data are the first to identify PGE(2) as an UVR-inducible autocrine factor for melanocytes. These data also show that PGE(2) activates EP(3) and EP(4) receptor signalling, resulting in opposing effects on cAMP production, a critical signalling pathway that regulates proliferation and melanogenesis in melanocytes.

Regulation of EP receptors in non-small cell lung cancer by epigenetic modifications

BACKGROUND

Cyclooxygenase (COX)-2 is frequently overexpressed in non-small cell lung cancer (NSCLC) and results in increased levels of prostaglandin E2 (PGE(2)), an important signalling molecule implicated in tumourigenesis. PGE(2) exerts its effects through the E prostanoid (EP) receptors (EPs1-4).

METHODS

The expression and epigenetic regulation of the EPs were evaluated in a series of resected fresh frozen NSCLC tumours and cell lines.

RESULTS

EP expression was dysregulated in NSCLC being up and downregulated compared to matched control samples. For EPs1, 3 and 4 no discernible pattern emerged. EP2 mRNA however was frequently downregulated, with low levels being observed in 13/20 samples as compared to upregulation in 5/20 samples examined. In NSCLC cell lines DNA CpG methylation was found to be important for the regulation of EP3 expression, the demethylating agent decitabine upregulating expression. Histone acetylation was also found to be a critical regulator of EP expression, with the histone deacteylase inhibitors trichostatin A, phenylbutyrate and suberoylanilide hydroxamic acid inducing increased expression of EPs2-4. Direct chromatin remodelling was demonstrated at the promoters for EPs2-4.

CONCLUSIONS

These results indicate that EP expression is variably altered from tumour to tumour in NSCLC. EP2 expression appears to be predominantly downregulated and may have an important role in the pathogenesis of the disease. Epigenetic regulation of the EPs may be central to the precise role COX-2 may play in the evolution of individual tumours.

The EP1 subtype of prostaglandin E2 receptor: role in keratinocyte differentiation and expression in non-melanoma skin cancer

We have previously demonstrated that the EP1 subtype of PGE2 receptor is expressed in the differentiated compartment of normal human epidermis and is coupled to intracellular calcium mobilization. We therefore hypothesized that the EP1 receptor is coupled to keratinocyte differentiation. In in vitro studies, radioligand binding, RT-PCR, immunoblot and receptor agonist-induced second messenger studies demonstrate that the EP1 receptor is up-regulated by high cell density in human keratinocytes and this up-regulation precedes corneocyte formation. Moreover, two different EP1 receptor antagonists, SC51322 and AH6809, both inhibited corneocyte formation. SC51322 also inhibited the induction of differentiation-specific proteins, cytokeratin K10 and epidermal transglutaminase. We next examined the immunolocalization of the EP1 receptor in non-melanoma skin cancer in humans. Well-differentiated SCCs exhibited significantly greater membrane staining, while spindle cell carcinomas and BCCs had significantly decreased membrane staining compared with normal epidermis. This data supports a role for the EP1 receptor in regulating keratinocyte differentiation.

Antagonism of the prostaglandin E receptor EP4 inhibits metastasis and enhances NK function

Cyclooxygenase-2 (COX-2) is associated with aggressive breast cancers. The COX-2 product prostaglandin E(2) (PGE(2)) acts through four G-protein-coupled receptors designated EP1-4. Malignant and immortalized normal mammary epithelial cell lines express all four EP. The EP4 antagonist AH23848 reduced the ability of tumor cells to colonize the lungs or to spontaneously metastasize from the mammary gland. EP4 gene silencing by shRNA also reduced the ability of mammary tumor cells to metastasize. Metastasis inhibition was lost in mice lacking either functional Natural Killer (NK) cells or interferon-gamma. EP4 antagonism inhibited MHC class I expression resulting in enhanced ability of NK cells to lyse mammary tumor target cells. These studies support the hypothesis that EP4 receptor antagonists reduce metastatic potential by facilitating NK-mediated tumor cell killing and that therapeutic targeting of EP4 may be an alternative approach to the use of COX inhibitors to limit metastatic disease.

Differential expression of E-prostanoid receptors in human hepatocellular carcinoma

Recent studies have shown that inhibition of cyclooxygenases (e.g. COX-2) exerts antitumorigenic effects on hepatocellular carcinomas (HCCs), which are to a significant extent due to the abrogation of PGE(2) synthesis. PGE(2) acts via differentially regulated prostaglandin receptors (EP(1-4)). Our study was designed to investigate the expression pattern of EP-receptors in HCCs and to evaluate the therapeutic potential of selective EP-receptor antagonists. Using tissue microarrays including a total of 14 control livers, 17 liver cirrhoses, 22 premalignant dysplastic nodules (DNs) and 162 HCCs with different histological grades, the expression of COX-2, mPGES-1 and -2 and EP(1-4)-receptors was analyzed. Western immunoblot analyses were performed to confirm the expression in HCC cell lines. The effects of EP(1-4)-receptor antagonism on cell viability and apoptosis were investigated using MTT-assays and FACS-analyses, respectively. COX-2, mPGES-1 and -2 and EP(1-4)-receptors were expressed in all HCC tissues. COX-2 expression was highest in DNs and declined with loss of HCC-differentiation. With respect to COX-2 expression, a converse expression of EP(1-3) -receptors and mPGES-1 and -2 was found in DNs compared to HCCs. Selectively antagonizing EP(1)- and EP(3)-receptors reduced the viability of HCC cells in a dose-dependent manner, which was associated with apoptosis induction. Our results suggest a differential regulation of EP-receptor subtype expression with dedifferentiation of HCCs in which a converse expression pattern for COX-2 in comparison to EP(1-3)-receptors occurs. Of clinical interest, selectively antagonizing EP(1)- and EP(3)-receptors may provide a novel systemic therapeutic approach to the treatment of HCCs.

Thrombin inhibits migration of human hepatic myofibroblasts

Several lines of data recently pointed out a role of the serine proteinase thrombin in liver fibrogenesis, but its mechanism of action is unknown. The aim of this study was to evaluate the effect of thrombin on the migration of human liver myofibroblasts. We show here that thrombin inhibits both basal migration and platelet-derived growth factor (PDGF)-BB-induced migration of myofibroblasts. By using a thrombin antagonist, a protease-activated receptor (PAR)-1 mimetic peptide, and a PAR-1 antibody, we show that this effect is dependent on the catalytic activity of thrombin and on PAR-1 activation. Thrombin's effect on basal migration was dependent on cyclooxygenase 2 (COX-2) activation because it was blocked by the COX-2 inhibitors NS-398 and nimesulide, and pharmacological studies showed that it was relayed through prostaglandin E(2) and its EP(2) receptor. On the other hand, thrombin-induced inhibition of PDGF-BB-induced migration was not dependent on COX-2. We show that thrombin inhibits PDGF-induced Akt-1 phosphorylation. This effect was consecutive to inhibition of PDGF-beta receptor activation through active dephosphorylation. Thus thrombin, through two distinct mechanisms, inhibits both basal- and PDGF-BB-induced migration of human hepatic liver myofibroblasts. The fine tuning of myofibroblast migration may be one of the mechanisms used by thrombin to regulate liver fibrogenesis.

Deletion of Prostaglandin E2 EP2 Receptor Protects against Ultraviolet-Induced Carcinogenesis, but Increases Tumor Aggressiveness

Ultraviolet (UV) light is a complete carcinogen inducing and promoting squamous-cell carcinoma (SCC) of the skin. Recent work has shown that SCC initiation and promotion are enhanced by prostaglandin E2 (PGE2). PGE2 interacts with specific EP receptors to regulate cellular functions. Previous work from our group has shown that the prostaglandin E2 EP2 receptor is a powerful regulator of keratinocyte growth. SKH-1 hairless mice lacking the EP2 receptor were therefore studied to understand how this growth signaling pathway contributes to photocarcinogenesis. Our data indicate that UV-irradiated mice lacking EP2 receptors exhibit decreased proliferation and a poor capacity for epidermal hypertrophy in response to UV injury. In a chronic irradiation model, these animals were protected from tumor formation, developing 50% fewer tumors than wild-type controls. Despite this capacity to protect against tumorigenesis, animals lacking EP2 receptors grew tumors that were larger in size, with a more aggressive phenotype. Further study suggested that this susceptibility may be associated with synthesis of active metalloproteinase enzymes in greater quantities than keratinocytes expressing the EP2 receptor, thereby enhancing the invasive potential of EP2-/- cells.

Targeting prostaglandin E EP receptors to inhibit metastasis

It is well established that high cyclooxygenase-2 (COX-2) expression contributes to the aggressive behavior of breast and other malignancies. Due to concerns regarding the safety of long-term use of COX-2 inhibitors as well as a desire to seek more effective alternatives to prevent and treat metastatic disease, we tested the hypothesis that inhibition of downstream signaling by the COX-2 product prostaglandin E(2) (PGE(2)) would be as effective as inhibiting global prostaglandin synthesis. PGE(2) acts through four G-protein-coupled receptors designated EP1-4. Here, we summarize data from many laboratories regarding the role of individual E-series of prostaglandin (EP) receptors on cancer behavior and we discuss our own recent findings that antagonists of the PGE receptor subtype 4, EP4, inhibit experimental metastasis in a murine model of hormone-resistant, metastatic breast cancer. These initial results indicate that selective targeting of individual EP receptors should be investigated as an approach to exploit the high COX-2 activity in many epithelial malignancies.

Effects of UVB on E prostanoid receptor expression in murine skin

Prostaglandin E2 (PGE2) upregulation in response to UV light exposure is a significant factor in the development of non-melanoma skin cancer. It is known that PGE2 signals via the E prostanoid receptors, EP1-4, but the role that each receptor plays in skin carcinogenesis is unclear. Immunohistochemical analysis of EP receptor staining in unirradiated and UVB-exposed SKH-1 mouse skin demonstrated the localization of EP1 and EP2 to the plasma membrane of differentiated epidermal keratinocytes. In contrast, the EP3 receptor localized to the basal layer of the epidermis in unirradiated skin and throughout the epidermis in UVB-exposed skin. In unirradiated skin, cytoplasmic EP4 staining was seen throughout the epidermis, in dermal leukocytes, and in vascular endothelium. However, UVB exposure resulted in relocalization of the EP4 receptor to the plasma membrane of keratinocytes, with no change in the dermal staining pattern. In tumors isolated from UVB-exposed mice, EP1 and EP2 staining was detected in the more differentiated cells surrounding keratin pearls, whereas EP3 and EP4 were detectable throughout the tumors. Differential expression of the EP receptors suggests that each receptor may play a distinct role in skin tumor development.

Importance of the EP(1) receptor in cutaneous UVB-induced inflammation and tumor development

Chronic exposure to UV light, the primary cause of skin cancer, results in the induction of high levels of cyclooxygenase-2 (COX-2) expression in the skin. The involvement of COX-2 in the carcinogenesis process is mediated by its enzymatic product, prostaglandin E(2) (PGE(2)). PGE(2) has been shown to have a variety of activities that can contribute to tumor development and growth. The effects of PGE(2) on different cell types are mediated by four E prostanoid (EP) receptors, EP(1)-EP(4). While recent studies have demonstrated the importance of EP(1) in the development of colon and breast cancer, the extent of EP(1) involvement in the cutaneous photocarcinogenesis process is unknown. This study found that topical treatment with celecoxib or the specific EP(1) antagonist ONO-8713 decreased acute UVB-induced inflammation in the skin and significantly reduced the number of tumors per mouse following 25 weeks of UVB exposure and topical treatment. This study suggests that drugs designed to block EP(1) may have the potential to be used as anti-inflammatory and/or chemopreventive agents that reduce the risk of skin cancer development.

Potentiation of tumor formation by topical administration of 15-deoxy-delta12,14-prostaglandin J2 in a model of skin carcinogenesis

The effect of prostaglandins on the development of papillomas has been investigated in mice receiving prostaglandins E2 (PGE2) or the cyclopentenone 15-deoxy-delta(12,14)-PGJ2 (15dPGJ2) topically, using the 7,12-dimethylbenz[a]anthracene (DMBA)-induced tetradecanoylphorbol acetate (TPA)-promoted model of skin carcinogenesis. The presence of 15dPGJ2 during DMBA and TPA treatment inhibited apoptosis and increased the rate, number, size and vascularization of the papillomas, some of them progressing into carcinomas. Moreover, skin sections from mice treated for one week with DMBA and 15dPGJ2 showed a much reduced rate of apoptotic cells, and an enhanced expression of vascular epithelial growth factor when compared with animals receiving DMBA, with or without PGE2. The analysis of molecular events in the MCA3D keratinocyte cell line showed that 15dPGJ2 activated Ras and improved cell viability by inhibiting DMBA-dependent apoptosis. In addition to this, cell adhesion was impaired in MCA3D keratinocytes co-treated with 15dPGJ2 and DMBA, at the same time when the expression of cyclooxygenase-2 (COX-2) was observed under these conditions. These effects mediated by 15dPGJ2 might contribute to understand the role of COX-2 metabolites in carcinogenesis, leading to an increase of cell viability after mutagenic injury and therefore in the progression of tumors.

Inhibition of chemically induced skin carcinogenesis by sulindac is independent of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta)

Inhibition of cyclooxygenase-2 (COX2) by non-steroidal anti-inflammatory drugs (NSAID) is known to suppress skin carcinogenesis. It was further suggested that inhibition of COX2-derived prostaglandins by NSAIDs could reduce levels of putative endogenous ligands of peroxisome proliferator-activated receptor-beta (PPARbeta), and these ligands could potentiate tumorigenesis. However, it is currently unclear whether ligand activation of PPARbeta either inhibits or potentiates carcinogenesis. The present studies were designed to examine the mechanism of NSAID-mediated chemoprevention in skin, and, in particular, to determine the role of PPARbeta in this process. A two-stage skin carcinogenicity bioassay was performed using wild-type and PPARbeta-null mice that were fed either a control diet or one containing 0.32 g sulindac/kg diet. Significant inhibition of chemically induced skin carcinogenesis was observed in both wild-type and PPARbeta-null mice, and this was associated with a marked decrease in the concentration of skin prostaglandins including PGE(2) and PGI(2). Results from these studies demonstrate that inhibition of COX2 by dietary sulindac in mouse skin can effectively inhibit chemically induced skin carcinogenesis, and suggest that the mechanism underlying this chemopreventive effect is independent of PPARbeta. Additionally, results from these studies do not support the hypothesis that ligand activation of PPARbeta by COX-derived metabolites potentiates chemically induced skin carcinogenesis.

Prostaglandin E2 inhibits proliferation and migration of HTR-8/SVneo cells, a human trophoblast-derived cell line

Normal placentation requires a highly coordinated control of proliferation, migration and invasiveness of extravillous trophoblast cells. Since prostaglandin E2 is a major prostanoid synthesized by intrauterine tissues and highly involved in pregnancy homeostasis, we examined the possibility that it modulates extravillous trophoblast cell functions. Here, we report the presence of mRNAs for prostaglandin E2 EP2 and EP4 receptor isoforms and of proteins in both first-trimester human chorionic villi and in the human trophoblast-derived HTR-8/SVneo cells. Moreover we found that: (i) this cell line releases prostaglandin E2 and the output is enhanced by interleukin-1beta; (ii) the prostanoid consistently inhibits serum- or epidermal growth factor-induced cell proliferation and also migration. An involvement of cAMP in the prostaglandin E2 antiproliferative action is suggested by the observation that the prostanoid greatly enhances cAMP level in HTR-8/SVneo cells and that forskolin inhibits cell proliferation; moreover the administration of prostaglandin E2 plus forskolin, a condition which evokes a synergistic enhancement of cAMP, induces a major impairment of cell growth. Provided that our data are applicable to the trophoblast tissue in vivo, we suggest that prostaglandin E2 exerts an important control on extravillous trophoblast cell functions, preventing an excessive proliferation and migration.

Prostaglandin E receptor EP3 deficiency modifies tumor outcome in mouse two-stage skin carcinogenesis

We have recently shown that the prostaglandin E(2) (PGE(2)) receptor EP(3) plays an important role in suppression of colon cancer cell proliferation and that its deficiency enhances late stage colon carcinogenesis. Here we examined the effects of EP(3)-deficiency on two-stage skin carcinogenesis. 7,12-Dimethylbenz[a]anthracene (50 microg/200 microl of acetone) was thus applied to the back skin of female EP(3)-knockout and wild-type mice at 8 weeks of age, followed by treatment with 12-O-tetradecanoylphorbol-13-acetate (5 microg/200 microl of acetone) twice a week for 25 weeks. First tumor appearance was observed in EP(3)-knockout mice at week 10, which was 3 weeks later than in EP(3) wild-type mice, and multiplicity observed at week 11 was significantly lower in the EP(3)-knockout case. However, histological examination showed that the tumor incidence and multiplicity at week 25 were not significantly changed in knockout mice and wild-type mice (incidence, 19/19 versus 23/24; multiplicity, 3.58 +/- 0.51 versus 3.17 +/- 0.63, respectively). Interestingly, there were no squamous cell carcinomas (SCCs) in the EP(3)-knockout mice, while SCCs were observed in 3 out of 24 wild-type mice. Furthermore, benign keratoacanthomas only developed in EP(3)-knockout mice (6/19 versus 0/24, P < 0.01). The results suggest that PGE(2) receptor EP(3) signaling might contribute to development of SCCs in the skin.

Immunolocalization of Low-Affinity Prostaglandin E2 Receptors, EP1 and EP2, in Adult Human Epidermis

Four prostaglandin (PG)E(2) receptors have been described, termed E-series prostaglandin receptors (EP(1)-EP(4)), that can be further subclassified as low-affinity (EP(1) and EP(2)) or high-affinity (EP(3) and EP(4)) receptors. Activation of the low-affinity PGE(2) receptors is likely to be important in mediating the actions of the high levels of PGE(2) found in various pathologic processes. The pattern of expression of these receptors in epidermis, however, is unknown. We therefore examined the immunolocalization of the EP(1) and EP(2) receptors in human epidermis. The EP(1) and EP(2) receptors demonstrated both plasma membrane and perinuclear or nuclear staining within the basal and spinous layers. Within the granular layer, both receptors were expressed in the cytoplasm with a grainy or granular appearance. The major differences were that the EP(2) receptor demonstrated a zone of decreased to absent plasma membrane staining in the superficial spinous layer and only scattered cellular staining within the granular layer. In contrast, the EP(1) receptor was prominently expressed throughout the stratum granulosum and the plasma membrane staining pattern was seen throughout the spinous layer. In cultured primary human keratinocytes, we also verified the presence of functional EP(1) receptor coupled to intracellular calcium mobilization and EP(2) receptor coupled to cAMP production.

Effects of PGF2alpha on human melanocytes and regulation of the FP receptor by ultraviolet radiation

Prostaglandins are potent lipid hormones that activate multiple signaling pathways resulting in regulation of cellular growth, differentiation, and apoptosis. In the skin, prostaglandins are rapidly released by keratinocytes following ultraviolet radiation and are chronically present in inflammatory skin lesions. We have shown previously that melanocytes, which provide photoprotection to keratinocytes through the production of melanin, express several receptors for prostaglandins, including the PGE2 receptors EP1 and EP3 and the PGF2alpha receptor FP, and that PGF2alpha stimulates melanocyte dendricity. We now show that PGF2alpha stimulates the activity and expression of tyrosinase, the rate-limiting enzyme in melanin synthesis. Analysis of FP receptor regulation showed that the FP receptor is regulated by ultraviolet radiation in melanocytes in vitro and in human skin in vivo. We also show that ultraviolet irradiation stimulates production of PGF2alpha by melanocytes. These results show that PGF2alpha binding to the FP receptor activates signals that stimulate a differentiated phenotype (dendricity and pigmentation) in melanocytes. The regulation of the FP receptor and the stimulation of production of PGF2alpha in melanocytes in response to ultraviolet radiation suggest that PGF2alpha could act as an autocrine factor for melanocyte differentiation.

Proteinase-activated receptor-2 stimulates prostaglandin production in keratinocytes: analysis of prostaglandin receptors on human melanocytes and effects of PGE2 and PGF2alpha on melanocyte dendricity

Prostaglandins (PG) are key mediators of diverse functions in the skin and several reports suggest that PG mediate post-inflammatory pigmentary changes through modulation of melanocyte dendricity and melanin synthesis. The proteinase-activated receptor 2 (PAR-2) is important for skin pigmentation because activation of keratinocyte PAR-2 stimulates uptake of melanosomes through phagocytosis in a Rho-dependent manner. In this report, we show that activation of keratinocyte PAR-2 stimulates release of PGE(2) and PGF(2alpha) and that PGE(2) and PGF(2alpha) act as paracrine factors that stimulate melanocyte dendricity. We characterized the expression of the EP and FP receptors in human melanocytes and show that human melanocytes express EP1 and EP3, and the FP receptor, but not EP2 and EP4. Treatment of melanocytes with EP1 and EP3 receptor agonists resulted in increased melanocyte dendricity, indicating that both EP1 and EP3 receptor signaling contribute to PGE(2)-mediated melanocyte dendricity. Certain EP3 receptor subtypes have been shown to increase adenosine 3',5'-cyclic monophosphate (cAMP) through coupling to Gs, whereas EP1 is known to couple to Gq to activate phospholipase C with elevation in Ca(2+). The cAMP/protein kinase A system is known to modulate melanocyte dendrite formation through modulation of Rac and Rho activity. Neither PGF(2alpha) or PGE(2) elevated cAMP in human melanocytes showing that dendricity observed in response to PGE(2) and PGF(2alpha) is cAMP-independent. Our data suggest that PAR-2 mediates cutaneous pigmentation both through increased uptake of melanosomes by keratinocytes, as well as by release of PGE(2) and PGF(2alpha) that stimulate melanocyte dendricity through EP1, EP3, and FP receptors.

Expression of cyclooxygenase 2, microsomal prostaglandin E synthase 1, and EP receptors is increased in rat oesophageal squamous cell dysplasia and Barrett's metaplasia induced by duodenal contents reflux

BACKGROUND AND AIM

It is known that bile acids can induce mucosal injury, stimulate cell proliferation, and promote tumorigenesis. A large body of genetic and biochemical evidence indicate that the biosynthetic pathway of prostaglandin E2 (PGE2) may play an important role in human and rodent tumours. Therefore, we examined the expression pattern of cyclooxygenase 1 (COX-1), COX-2, and microsomal prostaglandin E synthase 1 (mPGES-1), as well as EP receptor subtypes in rat oesophageal lesions induced by duodenal contents reflux.

METHODS

Oesophagoduodenal anastomosis was performed in rats to induce duodenal contents reflux. We examined histological changes and expression of COX-1, COX-2, mPGES-1, and EP receptor subtypes in the oesophagus by immunohistochemistry and reverse transcription-polymerase chain reaction.

RESULTS

Normal control oesophageal tissues showed COX-1 expression in subepithelial stromal cells, including endothelial cells and muscular cells, and did not reveal expression of COX-2 or mPGES-1. In the case of squamous cell lesions, immunoreactivity of COX-1 was similar to that of normal lesions, and COX-2 was maximally expressed around the vascular papillae of tissues showing dysplasia and surrounding epithelial layer and basal layer. mPGES-1 was highly expressed in stromal cells with COX-2 expression. In the case of Barrett's oesophagus, COX-2 and mPGES-1 were predominantly in subepithelial stromal cells. mRNA levels of COX-2, mPGES-1, EP2, EP3, and EP4 were higher in the experimental groups than in controls.

CONCLUSIONS

We suggest that the biosynthetic pathway of PGE2 may play an important role in oesophageal squamous cell dysplasia and glandular metaplasia induced by duodenal contents reflux.

Cyclooxygenases in the skin: pharmacological and toxicological implications

Cyclooxygenase (COX), a prostaglandin-endoperoxide synthase (PTGS), catalyzes the formation of prostaglandins from arachidonic acid. Prostaglandins are lipid signaling mediators that play a central role in a broad range of diverse physiological and pathophysiological processes, including inflammation, reproduction, nocioception, and gastrointestinal protection. Inhibition of cyclooxygenase activity is the mechanism by which nonsteroidal antiinflammatory drugs (NSAIDS) exert their analgesic, antipyretic, antiinflammatory, and antithrombotic effects. COX is currently believed to exist in three isoforms. In this review, we provide a concise state-of-the-art description of the role of COX in pharmacology and toxicology of skin including its involvement in normal physiology, cutaneous inflammation, nociception, wound healing, and tumorigenesis. COX-dependent pathways influence keratinocyte differentiation, hair follicle development, and hair growth. The critical role of COX-2 in pathophysiology of skin is also addressed. COX-2 mediates inflammatory processes in skin, including inflammatory hyperalgesia and nociception, and administration of specific COX-2 inhibitors reduces edema, vascular permeability, and other markers of cutaneous inflammation. A number of studies in animal models and in humans show that COX-2 inhibitors possess cancer chemopreventive properties. Selective COX-2 inhibitors have a more favorable side-effect profile. Topical formulations of COX-2 inhibitors are being developed as a novel pharmacologic approach for the treatment of COX-2 mediated skin diseases.

Expression of Cyclooxygenase Isozymes During Morphogenesis and Cycling of Pelage Hair Follicles in Mouse Skin: Precocious Onset of the First Catagen Phase and Alopecia upon Cyclooxygenase-2 Overexpression

Cyclooxygenase (COX)-1 and -2 catalyze the key reaction in prostaglandin biosynthesis. Whereas COX-1 is found in most tissues, COX-2, with a few exceptions, is not expressed in normal tissues but becomes transiently induced in the course of inflammatory reactions. In many neoplastic epithelia, COX-2 is constitutively overexpressed. Here we show that COX isozymes are spatiotemporally expressed during morphogenesis of dorsal skin epithelium of NMRI mice. COX-1 and COX-2 mRNA and protein were detected in embryonic and postnatal epidermal tissue by RT-PCR, northern blot, and immunoblot analysis indicating that both isoforms may contribute to prostaglandin production. Being barely detectable in interfollicular epidermis and resting hair follicles of adult mice, COX-2 protein appeared in embryonic skin first in epidermal precursor cells and later on in the basal cells and the peridermal layer of the stratified epidermis. In the course of pelage hair follicle morphogenesis, COX-2 remained expressed in the basal interfollicular compartment and, in addition, became apparent in elongated hair germs and hair pegs and later on in the outer root sheath cells of the distal and proximal hair follicles as well as in basal sebaceous gland cells. During the subsequent synchronous phases of hair cycling, COX-2 expression declined in catagen, was barely detectable in telogen, and was reinduced in the basal outer root sheath and basal sebaceous gland cells of anagen hair follicles. COX-1 immunosignals were detected predominantly in the interfollicular spinous and granular layers of the developing, neonatal, and adult epidermis but not in follicular epithelial cells of developing or cycling hair follicles. Dendritic cells in the interfollicular epidermis and distal hair follicles were also COX-1-positive. Transgenic overexpression of COX-2 under the control of a keratin 5 promoter in basal cells of the interfollicular and follicular epidermis induced a precocious entry into the first catagen stage of postnatal hair follicle cycling and a subsequent disturbance of hair follicle phasing. Furthermore, transgenic mice developed an alopecia. Inhibition of transgenic COX-2 activity by feeding the specific COX-2 inhibitor valdecoxib suppressed the development of alopecia, indicating that COX-2-mediated prostaglandin synthesis is involved in hair follicle biology.

Epidermal COX-2 Induction Following Ultraviolet Irradiation: Suggested Mechanism for the Role of COX-2 Inhibition in Photoprotection

The cyclooxygenase isoforms, COX-1 and COX-2, are involved in the biosynthesis of prostaglandin E2, a major prostaglandin involved in epidermal homeostasis and repair. Cancer originating in the epidermis can develop when keratinocyte proliferation and apoptosis become dysregulated, resulting in sustained epidermal hyperplasia. COX-2 inhibitors, which demonstrate significant in vivo selectivity relative to COX-1, suppress both ultraviolet-induced epidermal tumor development and progression, suggesting that prostaglandin regulation of keratinocyte biology is involved in the pathogenesis of epidermal neoplasia. In this study, we characterized the expression of COX-1 and COX-2, as well as keratinocyte proliferation, differentiation, and apoptosis, following acute ultraviolet irradiation in the hairless SKH-1 mouse. Following acute ultraviolet exposure, COX-2 expression was predominantly induced in the basal keratinocyte layer coincident with an increase in keratinocyte proliferation and apoptosis. The role of COX-2 was further evaluated using a selective COX-2 inhibitor, SC-791, as well as the traditional nonsteroidal COX inhibitor, indomethacin. Following acute ultraviolet irradiation, inhibition of COX-2 with either inhibitor decreased epidermal keratinocyte proliferation. Likewise, keratinocyte apoptosis was increased with COX-2 inhibition, particularly in the proliferating basal keratinocyte layer. There was also a modest inhibition of keratinocyte differentiation. These data suggest that COX-2 expression is probably necessary for keratinocyte survival and proliferation occurring after acute ultraviolet irradiation. We hypothesize that selective COX-2 inhibition, as described herein, may lead to enhanced removal of ultraviolet-damaged keratinocytes, thereby decreasing malignant transformation in the epidermis.