Biological effects of encapsulated insulin on transfected Chinese hamster ovary cells

Oral administration of insulin incorporated into the wall of isobutylcyanoacrylate nanocapsule to diabetic rats induces a long-lasting normalization of their fasting glycaemia. In this study, we examined the biological action of encapsulated insulin on DNA and glycogen syntheses in Chinese hamster ovary cells transfected with the human insulin receptor gene. In the 10(-11) mol/l-10(-9) mol/l concentration range, encapsulated insulin elicited responses comparable to those induced by native insulin: at 10(-9) mol/l, the rates of glycogen and DNA synthesis were enhanced by factors 3 and 2.5, respectively. Encapsulated insulin at 10(-7) mol/l evoked receptor desensitization although it did not induce receptor down-regulation and did not alter receptor recycling for up to 6 h. Chloroquine decreased the action of native insulin on glycogen synthesis, but did not affect the dose-response characteristics of encapsulated insulin. Acid-washing of the cells after 1 h of stimulation decreased maximal insulin responsiveness and provoked a dose response curve for encapsulated insulin similar to that of the native hormone. Direct measurement of effective insulin binding activity showed that encapsulated insulin (at 10(-8) and 10(-7) mol/l) was withdrawn from the incubation medium 5-8 times less efficiently than native insulin. These data are in agreement with previous results showing that the polymeric wall protects encapsulated insulin from degradation. Persistence of intact encapsulated insulin inside and outside the cell may result in modifying signalling events and thus be responsible for the observed cellular desensitization.