) J. Biol. Chem. 276, 18075-18081). The stimulatory effects of PGE 2 were dependent upon the activation of the phosphatidylinositol 3-kinase/Akt pathway. However, the exact signaling cascade responsible for phosphatidylinositol 3-kinase/Akt activation by PGE 2 remains poorly defined. In the present study, we demonstrate that the PGE 2 -induced migration and invasion occurs via rapid transactivation and phosphorylation of the epidermal growth factor receptor (EGFR). Within minutes following treatment, PGE 2 induces the activation of Akt. This effect was completely abolished by EGFR-specific tyrosine kinase inhibitors providing evidence for the role of the EGFR in this response. The rapid transactivation of the EGFR occurs via an intracellular Src-mediated event but not through the release of an extracellular epidermal growth factor-like ligand. EGFR transactivation was also observed in vivo by the direct comparison of normal and malignant human colorectal samples. These results suggest that in developing colonic carcinomas, the early effects of cyclooxygenase-2-derived PGE 2 are in part mediated by the EGFR, and this transactivation is responsible for subsequent downstream effects including the stimulation of cell migration and invasion.

Prostaglandin E2 (PGE2) can stimulate tumor progression by modulating several proneoplastic pathways, including proliferation, angiogenesis, cell migration, invasion, and apoptosis. Although steady-state tissue levels of PGE2 stem from relative rates of biosynthesis and breakdown, most reports examining PGE2 have focused solely on the cyclooxygenase-dependent formation of this bioactive lipid. Enzymatic degradation of PGE2 involves the NAD+-dependent 15-hydroxyprostaglandin dehydrogenase (15-PGDH). The present study examined a range of normal tissues in the human and mouse and found high levels of 15-PGDH in the large intestine. By contrast, the expression of 15-PGDH is decreased in several colorectal carcinoma cell lines and in other human malignancies such as breast and lung carcinomas. Consistent with these findings, we observe diminished 15-Pgdh expression in ApcMin+/- mouse adenomas. Enzymatic activity of 15-PGDH correlates with expression levels and the genetic disruption of 15-Pgdh completely blocks production of the urinary PGE2 metabolite. Finally, 15-PGDH expression and activity are significantly down-regulated in human colorectal carcinomas relative to matched normal tissue. In summary, these results suggest a novel tumor suppressive role for 15-PGDH due to loss of expression during colorectal tumor progression.

G protein-coupled receptor ligand-dependent transactivation of growth factor receptors has been implicated in human cancer cell proliferation, migration, and cell survival. For example, prostaglandin E 2 (PGE2)-induced transactivation of the EGF receptor (EGFR) in colorectal carcinoma cells is mediated by means of a c-Src-dependent mechanism and regulates cell proliferation and migration. Recent evidence indicates that ␤-arrestin 1 may act as an important mediator in G protein-coupled receptor-induced activation of c-Src. Whether ␤-arrestin 1 serves a functional role in these events is, however, unknown. We investigated the effects of PGE 2 on colorectal cancer cells expressing WT and mutant ␤-arrestin 1. Here we report that PGE 2 induces the association of a prostaglandin E receptor 4͞␤-arrestin 1͞c-Src signaling complex resulting in the transactivation of the EGFR and downstream Akt (PKB) signaling. The interaction of ␤-arrestin 1 and c-Src is critical for the regulation of colorectal carcinoma cell migration in vitro as well as metastatic spread of disease to the liver in vivo. These results show that the prostaglandin E͞␤-arrestin 1͞c-Src signaling complex is a crucial step in PGE2-mediated transactivation of the EGFR and may play a pivotal role in tumor metastasis. Furthermore, our data implicate a functional role for ␤-arrestin 1 as a mediator of cellular migration and metastasis. metastasis ͉ prostaglandin E2 ͉ c-Src ͉ EGF receptor ͉ prostaglandin E receptor G protein-coupled receptors (GPCRs) comprise the largest known family of plasma membrane receptors and consist of a seven-transmembrane-spanning region f lanked by an extracellular N terminus and an intracellular C terminus. Upon ligand binding, these receptors couple to heterotrimeric G proteins (G␣-and G␤␥-subunits) and catalyze the exchange of GDP for GTP, thus initiating a multitude of signaling events into the cell. These include the classical activation of phosholipases (phosholipases A, C, and D), protein kinases (PKA and PKC), and lipid kinases (phosphatidylinositol 3-kinase) as well as increased intracellular calcium levels. The desensitization of GPCRs occurs through a multistep process. GPCR kinases are recruited to the receptor by liberated ␤␥-subunits and phosphorylate the receptor on the cytoplasmic tail and intracellular loops. This phosphorylation event triggers the association of arrestin, which then traffics the receptors to clathrin-coated pits for endocytosis (1).Prostaglandins (PG) are important bioactive lipids which exert their effects through the activation of specific GPCRs as well as members of the peroxisome proliferator-activated receptor family. For example, PGE 2 is the ligand for four prostaglandin E (EP) receptor isoforms termed EP1, EP2, EP3, and EP4. Stimulation of these receptors elicits different intracellular responses (2). The stimulation of the EP1 receptor induces an increase in intracellular calcium by means of the activation of phospholipase C. EP2 and EP4 receptors couple to G␣s proteins, which generate incre...

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A large body of clinical, genetic, and biochemical evidence indicates that cyclooxygenase-2 (COX-2), a key enzyme for prostanoid biosynthesis, contributes to the promotion of colorectal cancer. COX-2-derived prostaglandin E 2 (PGE 2 ) is the most abundant prostaglandin found in several gastrointestinal malignancies. Although PGE 2 enhances intestinal adenoma growth in Apc min mice, the mechanism(s) by which it accelerates tumor growth is not completely understood. Here we investigated how PGE 2 promotes intestinal tumor growth and the signaling pathways responsible for its effects. We observed that PGE 2 treatment leads to increased epithelial cell proliferation and induces COX-2 expression in intestinal adenomas. Furthermore, we show that PGE 2 regulation of COX-2 expression is mediated by activation of a Ras-mitogen-activated protein kinase signaling cascade. One intriguing finding is that COX-2-derived PGE 2 mimics the effects of constitutively active Ras through a self amplifying loop that allows for a distinct growth advantage. (Cancer Res 2005; 65(5): 1822-9)