Galectin-1 induced activation of the mitochondrial apoptotic pathway: evidence for a connection between death-receptor and mitochondrial pathways in human Jurkat T lymphocytes

Galectin-1 (gal-1) triggers T cell death by several distinct intracellular pathways including the activation of the death-receptor pathway. The aim of this study was to investigate whether gal-1 induced activation of the death-receptor pathway in Jurkat T lymphocytes mediates apoptosis via the mitochondrial pathway linked by truncated Bid (tBid). We demonstrate that gal-1 induced proteolytic cleavage of the death agonist Bid, a member of the Bcl-2/Bcl-xL family and a substrate of activated caspase-8, was inhibited by caspase-8 inhibitor II (Z-IETD-FMK). Downstream of Bid, gal-1 stimulated mitochondrial cytochrome c release as well as the activation and proteolytic processing of initiator procaspase-9 were effectively decreased by caspase-8 inhibitor II. Blocking of gal-1 induced cleavage of effector procaspase-3 by caspase-8 inhibitor II as well as by caspase-9 inhibitors I (Z-LEHD-FMK) and III (Ac-LEHD-CMK) indicates that receptor and mitochondrial pathways converged in procaspase-3 activation and contribute to proteolytic processing of effector procaspase-6 and -7. Western blot analyses and immunofluorescence staining revealed that exposure of Jurkat T cells to gal-1 resulted in the cleavage of the DNA-repair enzyme poly (ADP-ribose) polymerase, cytoskeletal alpha-fodrin, and nuclear lamin A as substrates of activated caspases. Our data demonstrate that Bid provides a connection between the death receptor and the mitochondrial pathway of gal-1 induced apoptosis in human Jurkat T lymphocytes.