ADAM15 overexpression in NIH3T3 cells enhances cell-cell interactions

ADAM15 is a member of the family of metalloprotease-disintegrins that have been shown to interact with integrins in an RGD- and non-RGD-dependent manner. In the present study, we examined the effects of ADAM15 overexpression on cell-matrix and cell-cell interactions in NIH3T3 cells. Tetracycline-regulated ADAM15 overexpression in NIH3T3 cells leads to an inhibition of migration on a fibronectin-coated filter in a Boyden chamber assay and in a scratch wound model. The effects of ADAM15 overexpression on cell migration are not due to changes in matrix attachment or to the lack of extracellular signal-regulated kinase signaling response to PDGF or fibronectin. However, a decrease in monolayer permeability with ADAM15 overexpression and altered cell morphology suggest a possible increase in cell-cell interaction. Analysis of adhesion of NIH3T3 cells to a polyclonal population of cells retrovirally transduced to overexpress ADAM15 demonstrates a 45% increase in cell adhesion, compared with enhanced green fluorescent protein-expressing control cells. In addition, we demonstrate localization of HA-epitope-tagged ADAM15 to cell-cell contacts in an epithelial cell line that forms extensive cell-cell contact structures. Thus, overexpression of ADAM15 in NIH3T3 cells appears to enhance cell-cell interactions, as suggested by decreased cell migration, altered cell morphology at the wound edge, decreased monolayer permeability, and increased cell adhesion to monolayers of cells expressing ADAM15 by retroviral transduction.

Tumor necrosis factor-alpha-converting enzyme (ADAM17) mediates the cleavage and shedding of fractalkine (CX3CL1)

Fractalkine (CX3CL1) is an unusual member of the chemokine family that is synthesized with its chemokine domain at the end of a mucin-rich, transmembrane stalk. This membrane-bound localization allows fractalkine to function as an adhesion molecule for cells bearing its receptor, CX3CR1. In addition, fractalkine can be proteolytically released from the cell surface, generating a soluble molecule that functions as a chemoattractant similar to the other members of the chemokine family. In this study, we have examined the mechanisms that regulate the conversion between these two functionally distinct forms of fractalkine. We demonstrate that under normal conditions fractalkine is synthesized as an intracellular precursor that is rapidly transported to the cell surface where it becomes a target for metalloproteinase-dependent cleavage that causes the release of a fragment containing the majority of the fractalkine extracellular domain. We show that the cleavage of fractalkine can be markedly enhanced by stimulating cells with phorbol 12-myristate 13-acetate (PMA), and we identify tumor necrosis factor-alpha converting enzyme (TACE; ADAM17) as the protease responsible for this PMA-induced fractalkine release. In addition, we provide data showing that TACE-mediated fractalkine cleavage occurs at a site distinct from the dibasic juxtamembrane motif that had been suggested previously based on protein sequence homologies. The identification of TACE as a major protease responsible for the conversion of fractalkine from a membrane-bound adhesion molecule to a soluble chemoattractant will provide new information for understanding the physiological function of this chemokine.

xIAP induces cell-cycle arrest and activates nuclear factor-kappaB : new survival pathways disabled by caspase-mediated cleavage during apoptosis of human endothelial cells

Survival of human vascular endothelial cells depends on their ability to activate the transcription factor nuclear factor-kappaB (NF-kappaB), a regulator of antiapoptotic genes, such as the X chromosome-linked inhibitor of apoptosis protein (xIAP). In the present study, we demonstrated expression of xIAP in the endothelial lining of normal human arteries and veins and elevated levels in highly malignant human endothelial tumors. Using retroviral infection of human endothelial cells, we identified two novel survival mechanisms mediated by xIAP in endothelial cells. First, xIAP can activate the transcription factor NF-kappaB, a known survival factor for human endothelial cells. This positive feedback loop induced by xIAP is mediated via phosphorylation and sustained degradation of inhibitor (I) kappaBalpha. Second, xIAP can inhibit cell proliferation via downregulation of cyclins A and D1 and induction of the cyclin-dependent kinase inhibitors p21(Cip1/Waf1) and p27(Kip1). Cleavage of xIAP by caspases during endothelial cell apoptosis disables both of these biological functions of xIAP. Thus, caspase-mediated cleavage of xIAP interrupts a positive regulatory cytoprotective loop between NF-kappaB and xIAP and increases the vulnerability of the cell to apoptosis by releasing it from an xIAP-mediated quiescent state.

Stereoselective metabolism of enflurane by human liver cytochrome P450 2E1

The toxicity of the chiral fluorinated volatile anesthetics halothane, enflurane, and isoflurane is closely related to their metabolism by hepatic cytochrome P450. Although individual anesthetic enantiomers have been shown to exhibit a difference in anesthetic efficacy, metabolism of anesthetic enantiomers has not been reported. Human liver enflurane metabolism to difluoromethoxydifluoroacetic acid (DFMDFA) and inorganic fluoride is catalyzed in vivo and in vitro by cytochrome P450 2E1. The purpose of this investigation was to characterize enflurane racemate and enantiomer metabolism to test the hypothesis that fluorinated ether anesthetic metabolism by cytochrome P450 2E1 exhibits substrate stereoselectivity. Enflurane metabolism by microsomes from three human livers and by microsomes containing cDNA-expressed human P450 2E1 was measured at saturating enflurane concentrations. DFMDFA was quantitated with gas chromatography-mass spectrometry by detection of the ethanolamide derivative. In microsomes from all three livers, (R)-enflurane metabolism was significantly greater than that of (S)-enflurane, whereas rates of racemic enflurane metabolism were generally between those seen for the R- and S-enantiomers. The ratio of (R)-enflurane to (S)-enflurane metabolism in the three livers studied was 2.1:1, 1.9:1, and 1.4:1. (R)-, (S)-, and racemic enflurane were all metabolized by expressed P450 2E1. The ratio of (R)-enflurane to (S)-enflurane metabolism was 1.9:1. The metabolic enantiomeric selectivity of human liver P450 2E1 for (R)-enflurane suggests that enflurane metabolism by P450 2E1 occurs by direct substrate oxidation, rather than indirectly through the generation of a P450-dependent reactive oxygen species, and supports the hypothesis that the P450 2E1 active site is somewhat restrictive and capable of stereochemical discrimination.