Intracellular targeting of the endoplasmic reticulum nuclear envelope by retrograde transport may determine cell hypersensitivity to verotoxin via globotriaosyl ceramide fatty acid isoform traffic

The pentameric B subunit of verotoxin (VT) mediates the attachment to cell surface globotriaosyl ceramide (Gb(3)) to facilitate receptor-mediated endo cytosis of the toxin. In highly toxin-sensitive tumor cells, the holotoxin and VT1 B subunit is targeted intracellularly to elements of the endoplasmi c reticulum (ER)/nuclear membrane. In less sensitive cells, the toxin is ta rgeted to components of the Golgi apparatus. We have studied two cell syste ms: the induced VT hypersensitivity of human astrocytoma cell lines culture d in the presence of sodium butyrate (compared to sodium propionate and cap ronate) and the increased VT sensitivity of multiple drug-resistant mutants as compared to parental human ovarian carcinoma cells. In both cases, a di fference in the intracellular retrograde transport of the receptor-bound in ternalized toxin to the ER/nuclear envelope, as opposed to the Colgi, corre lated with a >1,000-fold increase in cell sensitivity to VT. This change in intracellular routing may be due to sorting of Gb(3) fatty acid isoforms, since nuclear targeting was found in turn to correlate with the preferentia l synthesis of Gb(3) containing shorter chain (primarily C1 6) fatty acid s pecies. We propose that the isoform-dependent traffic of Gb(3) from the cel l surface to the ER/nuclear membrane provides a new signal transduction pat hway for Gb(3) binding proteins. J Cell Physiol 177:646-660, 1998. (C) 1998 Wiley-Liss, Inc.