Engaging Ly‐6A/Sca‐1 triggers lipid raft‐dependent and ‐independent responses in CD4(+) T‐cell lines

INTRODUCTION: The lymphocyte antigen 6 (Ly‐6) supergene family encodes proteins of 12–14 kda in molecular mass that are either secreted or anchored to the plasma membrane through a glycosyl‐phosphatidylinisotol (GPI) lipid anchor at the carboxy‐terminus. The lipidated GPI‐anchor allows localization of Ly‐6 proteins to the 10–100 nm cholesterol‐rich nano‐domains on the membrane, also known as lipid rafts. Ly‐6A/Sca‐1, a member of Ly‐6 gene family is known to transduce signals despite the absence of transmembrane and cytoplasmic domains. It is hypothesized that the localization of Ly‐6A/Sca‐1 with in lipid rafts allows this protein to transduce signals to the cell interior. METHODS AND RESULTS: In this study, we found that cross‐linking mouse Ly‐6A/Sca‐1 protein with a monoclonal antibody results in functionally distinct responses that occur simultaneously. Ly‐6A/Sca‐1 triggered a cell stimulatory response as gauged by cytokine production with a concurrent inhibitory response as indicated by growth inhibition and apoptosis. While production of interleukin 2 (IL‐2) cytokine by CD4(+) T cell line in response to cross‐linking Ly‐6A/Sca‐1 was dependent on the integrity of lipid rafts, the observed cell death occurred independently of it. Growth inhibited CD4(+) T cells showed up‐regulated expression of the inhibitory cell cycle protein p27(kip) but not of p53. In addition, Ly‐6A/Sca‐1 induced translocation of cytochrome C to the cytoplasm along with activated caspase 3 and caspase 9, thereby suggesting an intrinsic apoptotic cell death mechanism. CONCLUSIONS: We conclude that opposing responses with differential dependence on the integrity of lipid rafts are triggered by engaging Ly‐6A/Sca‐1 protein on the membrane of transformed CD4(+) T cells.