EPIDERMAL GROWTH-FACTOR UP-REGULATES SODIUM-GLUCOSE COTRANSPORT IN ENTEROCYTE MODELS IN THE PRESENCE OF CHOLERA-TOXIN

Background: Sodium-glucose cotransport by enterocytes is key to the su ccessful implementation of oral rehydration in diarrhea. Confluent, di fferentiated Caco-2 cells have enterocyte-like characteristics. We hav e previously shown that short-term incubation of isolated rat jejunal enterocytes with epidermal growth factor (EGF) results in the up-relat ion of sodium-glucose cotransport. The aim of this study was to examin e the effect of EGF on Caco-2 cells in the presence of cholera toxin. Methods: Caco-2 cells grown on tissue culture dishes were used for glu cose and sodium uptake studies and cells were grown on polycarbonate m embranes for transport examinations. Effects of EGF on the kinetic par ameters of sodium-glucose cotransporter, thymidine transport, and on t he activity of Na+/K+-4TPase were examined. The efficacy of basolatera l vs apical EGF on sodium and glucose transport was compared after inc ubation of the monolayers with 10 nmol/L of cholera toxin. Results: EG F increased both glucose and sodium uptake and transport, and we obser ved a simultaneous increase in the activity of Na+/K+-adenosine tripho sphatase (ATPase.). Kinetic studies performed on brush-border membrane vesicles prepared horn EGF-incubated confluent monolayers and on inta ct cells showed an increase in the maximum velocity but not the Michae lis constant, suggesting increased availability of transporters rather than conformational change. This effect was seen within minutes in bo th of the two putative transporters, high-affinity low-capacity and lo w-affinity, high-capacity There was no acute effect on thymidine uptak e. Studies in the presence of cholera toxin demonstrated a significant up-regulation in sodium-glucose cotransport when EGF was applied from the basolateral side; the increase was smaller but significant with a pical application. Conclusions: Differentiated Caco-2 cells have two k inetically distinct sodium-glucose cotransporters. Short-term incubati on of Caco-2 cells with EGF resulted in an up-regulation of sodium-glu cose cotransport and subsequent increase in Na+/K+-ATPase activity. Th e effect of basolaterally applied EGF was more significant with or wit hout incubation with cholera toxin. The early effect of EGF on glucose and sodium cotransport may have important therapeutic implications in diarrhea and dehydration states. The in vitro model described here us es a homogeneous cell population and provides a versatile system for u ptake and transport studies.