Cytolytic Shiga toxins (Stx) are believed to be largely responsible for renal damage in post-diarrheal hemolytic-uremic syndrome (D + HUS). Despite the general belief that endothelial cells are the primary target of Stx, there is evidence that proximal tubules may be a site of toxin action. We hypothesized that cultured proximal tubular cells are sensitive to the cytotoxic effects of Stx. Cultured human proximal tubular cells were exposed to Stx-1 in the presence and absence of a variety of inflammatory factors likely to be elevated in the kidney or serum of patients with D + HUS. Cell survival, protein synthesis, total cell levels and synthesis of Stx receptors (GB3), and Stx binding were measured. Proximal tubules were extremely sensitive to the cytotoxic effect of Stx-1 with an LD50 at least equal to, if not less than, that seen with Vero cells. Interleukin-1 (IL-1), lipopolysaccharide (LPS), and butyrate (but not tumor necrosis factor or interleukin-6) up-regulated proximal tubule sensitivity to Stx-1. IL-1 increased Stx-1 binding, but did not alter total cell levels or synthesis of GB3, the glycosphingolipid receptor for Stx-1. In contrast, LPS and butyrate, despite increasing Stx-1 sensitivity, had no effect on Stx-1 binding. These studies indicate that proximal tubules are exquisitely sensitive to Stx-1 cytotoxicity and that inflammatory factors can increase toxin responsiveness through a variety of mechanisms. It is suggested that proximal tubules may be an important early target of Stx-1 action in D + HUS.