INHIBITION OF ENDOSOMAL ACIDIFICATION IN NORMAL-CELLS MIMICS THE DERANGEMENTS OF CELLULAR INSULIN AND INSULIN-RECEPTOR METABOLISM OBSERVED IN NON-INSULIN-DEPENDENT DIABETES-MELLITUS

Dissociation of the insulin-insulin receptor complex plays a crucial r ole in the processing of both insulin and the insulin receptor, and th e acidification of endocytic vesicles may be the mechanism by which in ternalized insulin is dissociated from its receptor and properly sorte d and processed. Internalized insulin-insulin receptor complexes are a bnormally processed in cells from patients with non-insulin-dependent diabetes mellitus (NIDDM). Accordingly, to further investigate the mec hanisms of the derangements observed in NIDDM cells, we examined the e ffects of the ionophore monensin, which inhibits endosomal acidificati on, on the cellular processing of insulin and insulin receptor in mono cytes from control subjects (n = 12) and NIDDM patients (n = 14). This study confirms that monocytes from NIDDM patients, compared with cell s from normal controls, had reduced binding (P < .01), internalization (P < .01), and degradation (P < .01) of insulin. In addition, the rel ease of intracellular radioactivity was slower (P < .01), and recyclin g of the insulin receptor was inhibited (P <.01). Moreover, these defe cts were associated with a significant (P < .01) decrease of dissociat ion of the internalized insulin-insulin receptor complex. In cells fro m normal controls, incubation with monensin decreased insulin blinding (P < .01), but not insulin internalization. High-performance liquid c hromatography (HPLC) analysis of intracellular radioactivity showed th at after monensin intracellular intact insulin significantly increased (P < .01), thus suggesting a decrease of intracellular insulin degrad ation. Moreover, insulin receptor recycling was completely disrupted. All of the!se derangements were associated with a significant decrease (P < .01) of dissociation of insulin-insulin receptor complexes. On t he contrary, in diabetic monocytes, monensin had no significant additi onal effect on NIDDM-linked alterations. Comparison of the results obt ained in cells from NIDDM patients to those found in monensin-treated normal cells demonstrates that NIDDM and monensin gave rise to a super imposable impairment of dissociation of the intracellular insulin-insu lin receptor complex, associated with similar abnormal sorting and pro cessing of insulin and its receptor. The only defect present in NIDDM cells but not in monensin-treated cells is the decrease of insulin int ernalization, which thus seems independent of the action of monensin o n the processing of internalized insulin-insulin receptor complex. The se results suggest that the impairment of dissociation of the insulin- insulin receptor complex may play a crucial role in the subsequent alt ered processing of insulin and insulin receptor. Moreover, they raise the question as to a possible similar alteration of the same intracell ular mechanism by NIDDM and monensin, and point out that the derangeme nts found in cells from NIDDM patients could be localized within the e ndosomal apparatus and consist mainly of a defective acidification of its interior. Copyright (C) 1997 by W.B. Saunders Company.