Na:K:2Cl Cotransporter (NKCC) of Intestinal Epithelial Cells: SURFACE EXPRESSION IN RESPONSE TO cAMP

Abstract

During intestinal chloride secretion, epithelial uptake of salts is accomplished largely by a bumetanide-sensitive Na:K:2Cl cotransporter designated here as NKCC. Using monoclonal antibodies directed against NKCC from the human crypt epithelial cell line, T84, we define its surface localization as a function of cotransporter activation. Immunoelectron microscopy, confocal localization, and selective surface biotinylation studies revealed that the 195-kDa NKCC protein is polarized to the basolateral domain. Following immunoprecipitation, several polypeptides coprecipitated with the 195-kDa cotransporter including two prominent proteins of molecular mass 160 and 130 kDa. Immunoblotting with three distinct anti-NKCC monoclonal antibodies in conjunction with deglycosylation experiments suggested that the 160- and 130-kDa bands represented novel proteins unrelated to the cotransporter. Stimulation of T84 monolayers with cAMP agonists, a condition which elicits chloride secretion and leads to microfilament-dependent NKCC activation, did not significantly increase the number of bumetanide-binding sites and only marginally increased surface expression of the 195-kDa cotransporter available for surface biotinylation. In contrast, cAMP agonist stimulation increased the surface expression of the coprecipitating 160- and 130-kDa proteins approximately 6-fold. The increase in surface 160- and 130-kDa proteins was attenuated by phalloidin preloading the cells, a condition which also prevents activation of NKCC without influencing the activity of other membrane transporters participating in chloride secretion. These studies define the polarized distribution of the NKCC protein on intestinal epithelia, indicate that NKCC may be associated with two other previously unidentified membrane proteins and such association is influenced by the F-actin cytoskeleton.