Interruption of Escherichia coli heat-stable enterotoxin-induced guanylyl cyclase signaling and associated chloride current in human intestinal cells by 2-chloroadenosine

Diarrhea induced by Escherichia coli heat-stable enterotoxin (ST(a)) is mediated by a receptor guanylyl cyclase cascade. The present study establishes that an intracellular nucleotide-dependent pathway disrupts toxin-induced cyclic GMP (cGMP) production and the associated chloride (Cl^- ) flux that underlie intestinal secretion. Incubation of Caco 2 human intestinal epithelial cells with the nucleoside analog 2-chloroadenosine (2ClAdo) resulted in a concentration- and time-dependent inhibition of toxin- induced cGMP production. Inhibition of cGMP production correlated with the metabolic conversion of 2ClAdo to 2-chloroadenosine triphosphate. The effect of 2ClAdo did not reflect activation of adenosine receptors, inhibition of adenosine deaminase, or modification of the binding or distribution of ST(a) receptors. Guanylyl cyclase activity in membranes prepared from 2ClAdo- treated cells was inhibited, in contrast to membranes from cells not exposed to 2ClAdo, demonstrating that inhibition of guanylyl cyclase C (GCC) was mediated by a noncompetitive mechanism. Treatment of Caco 2 cells with 2ClAdo also prevented ST(a)-induced Cl^- current. Application of 8-bromo-cGMP, the cell-permeant analog of cGMP, to 2ClAdo-treated cells reconstituted the Cl^- current, demonstrating that inhibition of Cl^- flux reflected selective disruption of ligand stimulation of GCC rather than the chloride channel itself. Thus, the components required for adenine nucleotide inhibition of GCC signaling are present in intact mammalian cells, establishing the utility of this pathway to elucidate the mechanisms regulating ST-dependent guanylyl cyclase signaling and intestinal fluid homeostasis. In addition, these data suggest that the adenine nucleotide inhibitory pathway may be a novel target to develop antisecretory therapy for enterotoxigenic diarrhea.