Regulation of the apoptosis-inducing kinase DRAK2 by cyclooxygenase-2 in colorectal cancer.

BACKGROUND: Cyclooxygenase-2 (COX-2) is over-expressed in colorectal cancer (CRC), rendering tumour cells resistant to apoptosis. Selective COX-2 inhibition is effective in CRC prevention, although having adverse cardiovascular effects, thus focus has shifted to downstream pathways. METHODS: Microarray experiments identified genes regulated by COX-2 in HCA7 CRC cells. In vitro and in vivo regulation of DRAK2 (DAP kinase-related apoptosis-inducing kinase 2 or STK17beta, an apoptosis-inducing kinase) by COX-2 was validated by qRT-PCR. RESULTS: Inhibition of COX-2 induced apoptosis and enhanced DRAK2 expression in HCA7 cells (4.4-fold increase at 4 h by qRT-PCR, P=0.001), an effect prevented by co-administration of PGE(2). DRAK2 levels were suppressed in a panel of human colorectal tumours (n=10) compared to normal mucosa, and showed inverse correlation with COX-2 expression (R=-0.68, R2=0.46, P=0.03). Administration of the selective COX-2 inhibitor rofecoxib to patients with CRC (n=5) induced DRAK2 expression in tumours (2.5-fold increase, P=0.01). In vitro silencing of DRAK2 by RNAi enhanced CRC cell survival following COX-2 inhibitor treatment. CONCLUSION: DRAK2 is a serine-threonine kinase implicated in the regulation of apoptosis and is negatively regulated by COX-2 in vitro and in vivo, suggesting a novel mechanism for the effect of COX-2 on cancer cell survival.

Glycosylation of the human prostacyclin receptor: role in ligand binding and signal transduction.

Prostacyclin, a potent vasodilator and inhibitor of platelet aggregation, acts through a cell-surface G protein-coupled receptor [prostacyclin (IP)]. The human (h) IP contains two consensus sites for N-linked glycosylation (N(7) and N(78)). However, the role of glycosylation is unknown. Mutant receptors (N(7)-Q(7),N(78)-Q(78) and N(7),N(78)-Q(7),Q(78)) were generated by replacing N(7) and/or N(78) with Q's. Receptor glycosylation was similar in the wild-type and N(7)-Q(7) and was inhibited with tunicamycin. N(78)-Q(78) and N(7),N(78)-Q(7),Q(78) demonstrated little or no glycosylation. Membrane localization was reduced for each mutant concomitant with impaired glycosylation. Partial localization to the plasma membrane allowed direct examination of the effect of glycosylation on IP function. High-affinity binding to N(7)-Q(7) was similar (K(d) = 21.7 +/- 1.7 nM, n = 4) to that of the wild-type receptor (K(d) = 24.3 +/- 3.6 nM, n = 4), despite a reduced value for B(max) (0.35 +/- 0.03 fmol/mg of protein versus 3.34 +/- 0.52 fmol/mg of protein, n = 4). Binding to N(78)-Q(78) (B(max) = 0.27 +/- 0.03 fmol/mg of protein, n = 3; K(d) = 149.1 +/- 11.1, n = 3) and N(7),N(78)-Q(7),Q(78) (no specific binding) was further impaired. Agonist-induced adenylyl cyclase activation was reduced in N(7)-Q(7) cells, whereas N(78)-Q(78) cells responded only to high concentrations of iloprost and N(7),N(78)-Q(7),Q(78) were unresponsive. Inositol phosphate generation was evident only with the wild-type. Only the wild-type and N(7)-Q(7) receptors underwent agonist-induced sequestration. Our findings demonstrate greater glycosylation at N(78) compared with N(7). The extent of N-linked glycosylation of hIP may be important for membrane localization, ligand binding, and signal transduction.

Internalization and sequestration of the human prostacyclin receptor.

Prostacyclin (PGI(2)), the major product of cyclooxygenase in macrovascular endothelium, mediates its biological effects through its cell surface G protein-coupled receptor, the IP. PKC-mediated phosphorylation of human (h) IP is a critical determinant of agonist-induced desensitization (Smyth, E. M., Hong Li, W., and FitzGerald, G. A. (1998) J. Biol. Chem. 273, 23258-23266). The regulatory events that follow desensitization are unclear. We have examined agonist-induced sequestration of hIP. Human IP, tagged at the N terminus with hemagglutinin (HA) and fused at the C terminus to the green fluorescent protein (GFP), was coupled to increased cAMP (EC(50) = 0.39 +/- 0.09 nm) and inositol phosphate (EC(50) = 86. 6 +/- 18.3 nm) generation when overexpressed in HEK 293 cells. Iloprost-induced sequestration of HAhIP-GFP, followed in real time by confocal microscopy, was partially colocalized to clathrin-coated vesicles. Iloprost induced a time- and concentration-dependent loss of cell surface HA, indicating receptor internalization, which was prevented by inhibitors of clathrin-mediated trafficking and partially reduced by cotransfection of cells with a dynamin dominant negative mutant. Sequestration (EC(50) = 27.6 +/- 5.7 nm) was evident at those concentrations of iloprost that induce PKC-dependent desensitization. Neither the PKC inhibitor GF109203X nor mutation of Ser-328, the site for PKC phosphorylation, altered receptor sequestration indicating that, unlike desensitization, internalization is PKC-independent. Deletion of the C terminus prevented iloprost-induced internalization, demonstrating the critical nature of this region for sequestration. Internalization was unaltered by cotransfection of cells with G protein-coupled receptor kinases (GRK)-2, -3, -5, -6, arrestin-2, or an arrestin-2 dominant negative mutant, indicating that GRKs and arrestins do not play a role in hIP trafficking. The hIP is sequestered in response to agonist activation via a PKC-independent pathway that is distinct from desensitization. Trafficking is dependent on determinants located in the C terminus, is GRK/arrestin-independent, and proceeds in part via a dynamin-dependent clathrin-coated vesicular endocytotic pathway although other dynamin-independent pathways may also be involved.

Insight into prostaglandin, leukotriene, and other eicosanoid functions using mice with targeted gene disruptions.

In recent years, there has been an exponential increase in the number of targeted gene disruptions performed in mice. At least 18 different gene knockouts have now been reported that have direct relevance to eicosanoid biology. These include genes that influence substrate availability (phospholipases), metabolism to eicosanoids (e.g., prostaglandin H synthases, lipoxygenases), and eicosanoid action (e.g., receptors for various prostaglandins). This minireview will outline the phenotype of these knockout mice and what has been learned about eicosanoid functions through use of this novel methodology.