CD46/CD3 costimulation induces morphological changes of human T cells and activation of Vav, Rac, and extracellular signal-regulated kinase mitogen-activated protein kinase.

Efficient T cell activation requires at least two signals, one mediated by the engagement of the TCR-CD3 complex and another one mediated by a costimulatory molecule. We recently showed that CD46, a complement regulatory receptor for C3b as well as a receptor for several pathogens, could act as a potent costimulatory molecule for human T cells, highly promoting T cell proliferation. Indeed, we show in this study that CD46/CD3 costimulation induces a synergistic activation of extracellular signal-related kinase mitogen-activated protein kinase. Furthermore, whereas T lymphocytes primarily circulate within the bloodstream, activation may induce their migration toward secondary lymphoid organs or other tissues to encounter APCs or target cells. In this study, we show that CD46/CD3 costimulation also induces drastic morphological changes of primary human T cells, as well as actin relocalization. Moreover, we show that the GTP/GDP exchange factor Vav is phosphorylated upon CD46 stimulation alone, and that CD46/CD3 costimulation induces a synergistic increase of Vav phosphorylation. These results prompted us to investigate whether CD46/CD3 costimulation induced the activation of GTPases from the Rho family. Indeed, we report that the small GTPase Rac is also activated upon CD46/CD3 costimulation, whereas no change of Rho and Cdc42 activity could be detected. Therefore, CD46 costimulation profoundly affects T cell behavior, and these results provide important data concerning the biology of primary human T cells.

Noninterference of cytochrome P4501A2 in the cytotoxicity of tacrine using genetically engineered V79 Chinese hamster cells for stable expression of the human or rat isoform and two human hepatocyte cell lines.

Tacrine (THA) is the only drug currently approved for the treatment of Alzheimer's disease. A common side effect of this drug in humans is major hepatotoxicity. THA-induced toxicity may be related to a metabolic pathway implicating cytochrome P450 1A2 (CYP1A2). The purpose of this study was to clarify the role of the metabolic conversion of THA by CYP1A2 in the cytotoxicity of THA. The cytotoxicity of THA was evaluated in two human hepatocyte cell lines, HepG2 and Chang liver, and on the V79 Chinese hamster cell line, which does not express cytochrome P450 activity, and its variants, genetically engineered for expression of human or rat CYP1A2. Cells expressing human CYP1A2 metabolized THA to form its 1-OH derivative (Vmax = 9.36 +/- 0.57 pmol min(-1) mg(-1) total protein), whereas no metabolism was observed with the nonexpressing parental cells. In all cell lines, THA induced a marked decrease in cell viability and a strong inhibition of RNA and protein synthesis. However, these cytotoxic effects did not differ in parental V79 cells and variant cells expressing human or rat CYP1A2. The IC50 were tenfold higher for cell viability than for RNA and protein inhibition after 3 hr of incubation but were similar after 24 hr (P < 0.0001), indicating that this early inhibition was not a transient effect and could lead to cell death. These results strongly suggest that THA-induced cytotoxicity is not mediated by CYP1A2.