Blockade of maitotoxin-induced endothelial cell lysis by glycine and L-alanine

The maitotoxin (MTX)-induced cell death cascade in bovine aortic endothelial cells (BAECs) is a model for oncotic/necrotic cell death. The cascade is initiated by an increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)), which is followed by the biphasic uptake of vital dyes. The initial phase of dye entry reflects activation of large pores and correlates with surface membrane bleb formation; the second phase reflects cell lysis. In the present study, the effect of the cytoprotective amino acid glycine was examined. Glycine had no effect on MTX-induced change in [Ca(2+)](i) or on the first phase of vital dye uptake but produced a concentration-dependent (EC(50) approximately 1 mM) inhibition of the second phase of dye uptake. No cytoprotective effect was observed with l-valine, l-proline, or d-alanine, whereas l-alanine was equieffective to glycine. Furthermore, glycine had no effect on MTX-induced bleb formation. To test the hypothesis that glycine specifically blocks formation of a lytic "pore," the loss of fluorescence from BAECs transiently expressing GFP and concatemers of GFP ranging in size from 27 to 162 kDa was examined using time-lapse videomicroscopy. MTX-induced loss of GFP was rapid, correlated with the second phase of dye uptake, and was relatively independent of molecular size. The MTX-induced loss of GFP from BAECs was completely blocked by glycine. The data suggest that the second "lytic" phase of MTX-induced endothelial cell death reflects formation of a novel permeability pathway that allows macromolecules such as GFP or LDH to escape, yet can be prevented by the cytoprotective agents glycine and l-alanine.