Galactosylceramide and transmembrane signalling in enterocytes: Calcium response induced by HIV-1 surface-envelope glycoprotein gp120

Intracellular Ca2+ concentrations ([Ca2+](i)) were measured in single-cultured human epithelial intestinal HT-29-D4 cells by digital microscopy using the Ca2+-sensitive fluorescent dye Fura-2. Exposure of these cells to HIV-1 surface-envelope glycoprotein gp120, or to a soluble form of its precursor (gp160), resulted in a significant, dose-dependent rise in [Ca2+](i). gp120 or gp160 specifically abrogated the [Ca2+](i) response to the neuropeptide agonist neurotensin, which is a stimulator of chloride secretion via inositol trisphosphate-mediated Ca2+ mobilization. By contrast, upon exposure to neurotensin gp120 failed to show any increase in [Ca2+](i) within the same cells, suggesting that both neurotensin and gp120 stimulate a common pathway of [Ca2+](i) mobilization, gp120-/gp160-induced [Ca2+](i) responses were abolished by preincubation with neutralizing antibodies directed against the third variable domain of gp120. These antibodies inhibited the binding of gp120/gp160 to galactosylceramide (GalCer), the alternative HIV-1 receptor in HT-29-D4 cells. Furthermore, HT-29-D4 cells displayed an important increase in [Ca2+](i) to anti-GalCer mAb alone, which rendered the cells insensitive to gp120. By contrast, HT-29-D4 cells became insensitive to anti-GalCer mAb after exposure to gp120. These data indicate that HIV-1 may directly alter enterocytic functions through interaction with the GalCer receptor.