Suramin inhibits binding of the V3 region of HIV-1 envelope glycoprotein gp120 to galactosylceramide, the receptor for HIV-1 gp120 on human colon epithelial cells.

The infection of human colonic epithelial cells HT-29 by human immunodeficiency virus type 1 (HIV-1) occurs independently of CD4, the main HIV-1 receptor expressed on lymphocytes and macrophages. Recent studies from our group have shown that HT-29 cells express the glycosphingolipid galactosylceramide (GalCer), a potential alternative receptor for the HIV-1 envelope glycoprotein gp120. The binding of recombinant gp120 to GalCer was blocked by monoclonal antibodies directed against the third variable region (V3) of gp120, suggesting that the V3 domain was implicated in GalCer recognition. In the present report, we show that suramin, a polysulfonyl naphtylurea known to inhibit retroviral reverse transcriptases in vitro, blocks HIV-1 infection in HT-29 cells. The effect is dose dependent, with a half-maximal inhibition (IC50) achieved for a suramin concentration of 54 micrograms/ml. Since [3H]suramin was not significantly internalized into HT-29 cells during our infection assay (i.e. 2 h), we have considered the possibility that the drug could act at an extracellular step of the HIV-1 cycle. Using a high performance thin layer chromatography binding assay, we show that suramin inhibits binding of HIV-1 gp120 to purified GalCer with an IC50 of 25 micrograms/ml. Suramin does not bind to GalCer, since preincubation of GalCer with suramin did not prevent the subsequent attachment of gp120. Using a solid-phase assay, we show that [3H]suramin specifically binds to recombinant gp120 and that this binding could be blocked by a monoclonal antibody specific for the conserved GPGRAF motif of the V3 domain of gp120. We also demonstrate that [3H]suramin binds to multibranched synthetic GPGRAF peptides that block HIV-1 infection in HT-29 cells. Binding of [3H]suramin to V3 peptides is specific and inhibited by unlabeled suramin (IC50 of 28 micrograms/ml). In contrast, the suramin derivative NF036, that is unable to block HIV-1 infection in HT-29 cells, does not inhibit the binding of [3H]suramin to V3 peptides. Taken together, these results suggest that suramin blocks HIV-1 infection in HT-29 cells because it binds to the V3 domain of gp120 and hence prevents the interaction between gp120 and the GalCer receptor.