Hiromichi Fujino

Prostaglandin E2 Induced Functional Expression of Early Growth Response Factor-1 by EP4, but Not EP2, Prostanoid Receptors via the Phosphatidylinositol 3-Kinase and Extracellular Signal-regulated Kinases

Prostaglandin E2(PGE2) mediates its physiological effects by interactions with a subfamily of G-protein-coupled receptors known as EP receptors. These receptors consist of four primary subtypes named EP1, EP2, EP3, and EP4. The EP2 and EP4 subtypes are known to couple to Gαs and stimulate intracellular cyclic 3,5- adenosine monophosphate formation, whereas the EP1 and EP3 receptors are known to couple to Gαq and Gαi, respectively. Recently we found that EP2 and EP4 receptors can activate T-cell factor signaling; however, EP2 receptors did this primarily through a cAMP-dependent protein kinase-dependent pathway, whereas EP4 receptors primarily utilized a phosphatidylinositol 3-kinase (PI3K)-dependent pathway (Fujino, H., West, K. A., and Regan, J. W. (2002) J. Biol. Chem. 277, 2614–2619). We now report that PGE2 stimulation of EP4 receptors, but not EP2 receptors, leads to phosphorylation of the extracellular signal-regulated kinases (ERKs) through a PI3K-dependent mechanism. Furthermore, this activation of PI3K/ERK signaling by the EP4 receptors induces the functional expression of early growth response factor-1 (EGR-1). Under the same conditions induction of EGR-1 protein expression was not observed following PGE2 stimulation of EP2 receptors. These findings point to important differences in the signaling potential of the EP2 and EP4 receptors, which could be significant with respect to the potential involvement of EP4 receptors in inflammation and cancer.

Activation of FP Prostanoid Receptor Isoforms Leads to Rho-mediated Changes in Cell Morphology and in the Cell Cytoskeleton

Prostaglandin F2α(PGF2α) exerts its biological effects by binding to and activating FP prostanoid receptors. These receptors, which include two isoforms, the FPA and FPB, have been cloned from a number of species and are members of the superfamily of G-protein-coupled receptors. Previous studies have shown that the activation of FP receptors leads to phosphatidylinositol hydrolysis, intracellular calcium release, and activation of protein kinase C. Here, we demonstrate that PGF2α treatment of 293-EBNA (Epstein-Barr nuclear antigen) cells that have been stably transfected with either the FPA or FPB receptor isoforms leads to changes in cell morphology and in the cell cytoskeleton. Specifically, cells treated with PGF2α show retraction of filopodia and become rounded, and actin stress fibers are formed. Pretreatment of the cells with bisindolylmaleimide I, a protein kinase C inhibitor, has no effect on the PGF2α-induced changes in cell morphology, although it does block the effects of phorbol myristate acetate on cell morphology. On the other hand, the PGF2α-induced changes in cell morphology and formation of actin stress fibers can be blocked by pretreatment of the cells with C3 exoenzyme, a specific inhibitor of the small G-protein, Rho. Consistent with FP receptor induced formation of actin stress fibers and focal adhesions, FPA receptor activation also leads to rapid (within two minutes) tyrosine phosphorylation of p125 focal adhesion kinase (FAK) which can be blocked by pretreating the cells with C3 exoenzyme. Taken together, these results suggest that the FP receptor isoforms are coupled to at least two second messenger pathways, one pathway associated with protein kinase C activation, and the other with activation of Rho.

EP 4 prostanoid receptor coupling to a pertussis toxin- sensitive inhibitory G protein

The EP2 and EP4 prostanoid receptor subtypes are G-protein-coupled receptors for prostaglandin E2 (PGE2). Both receptor subtypes are known to couple to the stimulatory guanine nucleotide binding protein (Gαs) and, after stimulation with PGE2, can increase the formation of intracellular cAMP. In addition, PGE2 stimulation of the EP4 receptor can activate phosphatidylinositol 3-kinase (PI3K) leading to phosphorylation of the extracellular signal-regulated kinases (ERKs) and induction of early growth response factor-1 (EGR-1) (J Biol Chem 278: 12151–12156, 2003). We now report that the PGE2-mediated phosphorylation of the ERKs and induction of EGR-1 can be blocked by pretreatment of EP4-expressing cells with pertussis toxin (PTX). Furthermore, pretreatment with PTX increased the amount of PGE2-stimulated intracellular cAMP formation in EP4-expressing cells but not in EP2-expressing cells. These data indicate that the EP4 prostanoid receptor subtype, but not the EP2, couples to a PTX-sensitive inhibitory G-protein (Gαi) that can inhibit cAMP-dependent signaling and activate PI3K/ERK-dependent signaling.

Prostanoid receptors and phosphatidylinositol 3-kinase: a pathway to cancer?

Abstract The significance of receptor heterogeneity has, in many cases, been unclear, particularly in the case of closely related receptor subtypes that are activated by the same endogenous ligands and appear to signal through the same second messenger pathways. In this article, recent studies of the EP and FP prostanoid receptors are reviewed, showing that receptor subtypes previously thought to activate the same signaling pathways, in fact, differ through novel interactions with phosphatidylinositol 3-kinase and activation of nuclear signaling pathways. These findings might be applicable to other families of G-protein-coupled receptors and have implications in cancer and other diseases.

Delayed reversal of shape change in cells expressing FP(B) prostanoid receptors. Possible role of receptor resensitization.

Prostaglandin F2α(PGF2α) receptors are G-protein-coupled receptors consisting of two alternative mRNA splice variants, named FPA and FPB. As compared with the FPA isoform, the FPB isoform lacks the last 46 amino acids of the carboxyl terminus and, therefore, represents a truncated version of the FPA. We recently found (Pierce, K. L., Fujino, H., Srinivasan, D., and Regan, J. W. (1999)J. Biol. Chem. 274, 35944–35949) that stimulation of both isoforms with PGF2α leads to activation of a Rho signaling pathway, resulting in tyrosine phosphorylation of p125 focal adhesion kinase, formation of actin stress fibers, and cell rounding. Although the activation of Rho and subsequent cell rounding occur at a similar rate for both isoforms, we now report that following the removal of PGF2α the reversal of cell rounding is much slower for cells expressing the FPB isoform as compared with the FPA isoform. Thus, in HEK-293 cells that stably express the FPA isoform, the reversal of cell rounding appears to be complete after 1 h, whereas for FPB-expressing cells there is essentially no reversal even after 2 h. Similarly, the disappearance of stress fibers and dephosphorylation of p125 focal adhesion kinase following removal of agonist are much slower in FPB-expressing cells than in FPA-expressing cells. The mechanism of this differential reversal appears to involve a difference in receptor resensitization following the removal of agonist. Based upon whole cell radioligand binding, agonist-induced stimulation of inositol phosphate formation, and mobilization of intracellular Ca2+, the FPBisoform resensitizes more slowly than the FPA isoform. These findings suggest that the carboxyl terminus of the FPA is critical for resensitization and that the slower resensitization of the FPB isoform leads to prolonged signaling. This differential signaling distinguishes the FPA and FPB receptor isoforms and could be important toward understanding the physiological actions of PGF2α.

Prostaglandin E2 regulates cellular migration via induction of vascular endothelial growth factor receptor-1 in HCA-7 human colon cancer cells

An important event in the development of tumors is angiogenesis, or the formation of new blood vessels. Angiogenesis is also known to be involved in tumor cell metastasis and is dependent upon the activity of the vascular endothelial growth factor (VEGF) signaling pathway. Studies of mice in which the EP3 prostanoid receptors have been genetically deleted have shown a role for these receptors in cancer growth and angiogenesis. In the present study, human colon cancer HCA-7 cells were used as a model system to understand the potential role of EP3 receptors in tumor cell migration. We now show that stimulation of HCA-7 cells with PGE₂ enhanced the up-regulation of VEGF receptor-1 (VEGFR-1) expression by a mechanism involving EP3 receptor-mediated activation of phosphatidylinositol 3-kinase and the extracellular signal-regulated kinases. Moreover, the PGE₂ stimulated increase in VEGFR-1 expression was accompanied by an increase in the cellular migration of HCA-7 cells. Given the known involvement of VEGFR-1 in cellular migration, our results suggest that EP3 receptors may contribute to tumor cell metastasis by increasing cellular migration through the up-regulation of VEGFR-1 signaling.

Effects of ceramide, ceramidase inhibition and expression of ceramide kinase on cytosolic phospholipase A2α; additional role of ceramide-1-phosphate in phosphorylation and Ca2+ signaling

Ceramide and the metabolites including ceramide-1-phosphate (C1P) and sphingosine are reported to regulate the release of arachidonic acid (AA) and/or phospholipase A(2) (PLA(2)) activity in many cell types including lymphocytes. Recent studies established that C1P, a product of ceramide kinase, interacts directly with Ca(2+) binding regions in the C2 domain of alpha type cytosolic PLA(2) (cPLA(2)alpha), leading to translocation of the enzyme from the cytosol to the perinuclear region in cells. However, a precise mechanism for C1P-induced activation of cPLA(2)alpha has not been well elucidated; such as the phosphorylation signal caused by the extracellular signal-regulated kinases (ERK1/2) pathway, a downstream of the protein kinase C activation with 4beta-phorbol myristate acetate (PMA), is required or not. In the present study, we showed that the increase in intracellular ceramide levels (exogenously added cell permeable ceramides and an inhibition of ceramidase by (1S,2R)-D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol and the increase in C1P formation by transfection with the vector for human ceramide kinase significantly enhanced the Ca(2+) ionophore (A23187) -induced release of AA via cPLA(2)alphas activation in CHO cells. Ceramides did not show additional effects on the release from the cells treated with the inhibitor of ceramidase. Ceramides and C2-C1P neither had effect on the intracellular mobilization of Ca(2+) nor the phosphorylation of cPLA(2)alpha in cells. A23187/PMA-induced release of AA was enhanced by ceramides and C2-C1P and by expression of ceramide kinase. Our findings suggest that C1P is a stimulatory factor on cPLA(2)alpha that is independent of the Ca(2+) signal and the PKC-ERK-mediated phosphorylation signal.

Cellular conditioning and activation of β-catenin signaling by the FPB prostanoid receptor

FP prostanoid receptors have been identified as two isoforms named FPA and FPB. We have shown that the FPB isoform, but not the FPA, activates β-catenin-mediated transcription. We now report that the mechanism of this FPB-specific activation of β-catenin signaling occurs in two steps. The first is a conditioning step that involves an agonist-independent association of the FPBreceptor with phosphatidylinositol 3-kinase followed by constitutive internalization of a receptor complex containing E-cadherin and β-catenin. This constitutive internalization conditions the cell for subsequent β-catenin signaling by increasing the cellular content of cytosolic β-catenin. The second step involves agonist-dependent activation of Rho followed by cell rounding. Because of the conditioning step, this agonist-dependent step results in a stabilization of β-catenin and activation of transcription. Although stimulation of the FPA isoform activates Rho and induces cellular shape change, it does not activate β-catenin signaling, because the FPA does not undergo constitutive internalization and does not condition the cell for β-catenin signaling. The cellular conditioning described here for the FPB illustrates the potential of the receptor to alter the signaling environment of a cell even in the absence of agonist and has general significance for understanding G-protein-coupled receptor signaling.

Cytotoxicity induced by inhibition of thioredoxin reductases via multiple signaling pathways: Role of cytosolic phospholipase A2α-dependent and -independent release of arachidonic acid

The thioredoxin (Trx) system, comprising Trx, the selenoprotein thioredoxin reductase (TrxR), and NADPH, functions as an antioxidant system. Trx has various biological activities including growth control and anti‐apoptotic properties, and the Trx system offers a target for the development of drugs to treat and/or prevent cancer. We evaluated the role of TrxR inhibition in the release of arachidonic acid (AA), cell toxicity, and intracellular signaling pathways in L929 mouse fibrosarcoma cells. Treatment with 1‐chloro‐2,4‐dinitrobenzene (DNCB, an inhibitor of TrxR) under conditions involving limited inhibition of TrxR activity in cells, released AA before causing cytotoxicity. Treatment with an inhibitor of p38 kinase, a downstream enzyme of the apoptosis signal‐regulating kinase 1 pathway, and pyrrophenone (an inhibitor of α‐type cytosolic phospholipase A2, cPLA2α) partially but significantly decreased the DNCB‐induced release of AA and cell death. The responses were much weaker in cPLA2α knockdown L929 cells. Exogenously added AA showed cytotoxicity. DNCB increased intracellular reactive oxygen species (ROS) levels, and butylated hydroxyanisole (an antioxidant) reduced DNCB‐induced ROS formation and cell toxicity but not the phosphorylation of p38 kinase and release of AA. Auranofin, another inhibitor of TrxR having a different formula, released AA resulting in toxicity in L929 cells. DNCB caused the release of AA and cytotoxicity in A549 human lung carcinoma cells, and caused p38 kinase‐dependent toxicity in PC12 rat pheochromocytoma cells. Our data suggest that a dysfunctional Trx system triggers multiple signaling pathways, and that the AA released by cPLA2α‐dependent and ‐independent pathways is important to cytotoxicity. J. Cell. Physiol. 219: 606–616, 2009.

Prostaglandin F2α stimulation of cyclooxygenase-2 promoter activity by the FPB prostanoid receptor

We have recently shown that the FP(B) prostanoid receptor activates beta-catenin signaling through the activation of Rho in human embryo kidney (HEK)-293 cells stably expressing the FP(B) receptors. We now report that the FP(B) receptor can stimulate cyclooxygenase-2 promoter activity and may, therefore, regulate the expression of cyclooxygenase-2. This stimulation of cyclooxygenase-2 promoter activity is blocked by pretreatment with an inhibitor of Rho, but not with an inhibitor of protein kinase C (PKC). Potential up regulation of cyclooxygenase-2 expression by the FP(B) receptor would establish a positive feedback loop that would drive beta-catenin signaling and could be involved in cancer.