Uncoupling between the insulin-receptor cycle and the cellular degradation of the hormone in cultured foetal hepatocytes. Effect of drugs and temperature that inhibit insulin degradation

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 role in the processing of both insulin and the insulin receptor, and the acidification of endocytic vesicles may be the mechanism by which internalized insulin is dissociated from its receptor and properly sorted and processed. Internalized insulin-insulin receptor complexes are abnormally processed in cells from patients with non-insulin-dependent diabetes mellitus (NIDDM). Accordingly, to further investigate the mechanisms of the derangements observed in NIDDM cells, we examined the effects of the ionophore monensin, which inhibits endosomal acidification, on the cellular processing of insulin and insulin receptor in monocytes from control subjects (n = 12) and NIDDM patients (n = 14). This study confirms that monocytes from NIDDM patients, compared with cells from normal controls, had reduced binding (P < .01), internalization (P < .01), and degradation (P < .01) of insulin. In addition, the release of intracellular radioactivity was slower (P < .01), and recycling of the insulin receptor was inhibited (P < .01). Moreover, these defects were associated with a significant (P < .01) decrease of dissociation of the internalized insulin-insulin receptor complex. In cells from normal controls, incubation with monensin decreased insulin binding (P < .01), but not insulin internalization. High-performance liquid chromatography (HPLC) analysis of intracellular radioactivity showed that after monensin intracellular intact insulin significantly increased (P < .01), thus suggesting a decrease of intracellular insulin degradation. Moreover, insulin receptor recycling was completely disrupted. All of these derangements were associated with a significant decrease (P < .01) of dissociation of insulin-insulin receptor complexes. On the contrary, in diabetic monocytes, monensin had no significant additional effect on NIDDM-linked alterations. Comparison of the results obtained in cells from NIDDM patients to those found in monensin-treated normal cells demonstrates that NIDDM and monensin gave rise to a superimposable impairment of dissociation of the intracellular insulin-insulin receptor complex, associated with similar abnormal sorting and processing of insulin and its receptor. The only defect present in NIDDM cells but not in monensin-treated cells is the decrease of insulin internalization, which thus seems independent of the action of monensin on the processing of internalized insulin-insulin receptor complex. These results suggest that the impairment of dissociation of the insulin-insulin receptor complex may play a crucial role in the subsequent altered processing of insulin and insulin receptor. Moreover, they raise the question as to a possible similar alteration of the same intracellular mechanism by NIDDM and monensin, and point out that the derangements found in cells from NIDDM patients could be localized within the endosomal apparatus and consist mainly of a defective acidification of its interior.

Sequential insulin degradation in cultured fetal hepatocytes in relation to chloroquine-dependent events

Insulin cellular degradation was studied in cultured 18-day-old fetal rat hepatocytes in the presence and absence of insulin degradation inhibitors that decrease the glycogenic response to insulin. After cell incubation with 3 nM [125I]A14 or -B26 insulin, hormone degradation products associated with cells or present in the medium were analyzed by high-performance liquid chromatography. Within cells, four components containing intact [125I]A14 insulin A-chain and part of the B-chain (A1-A4, according to increasing retention times) were found together with two [125I]B26 insulin B-chain COOH-terminal fragments (B1 and B2). Medium degradation intermediates comprised B1 and B2 but not A1-A4. Cellular insulin fragments A3 and B2 exhibited a maximal transient accumulation after 2 min, whereas the others increased progressively to plateau after 10 min. Chloroquine inhibited the formation of A1, A2, and B1 by 70-80%, whereas that of A3, A4, and B2 was not significantly affected. N-ethylmaleimide and bacitracin, two inhibitors of insulin-degrading enzyme (IDE), decreased the formation of chloroquine-dependent cellular peptides. Thus cell-associated insulin degradation implied primarily two cleavages in B-chain near the COOH-terminus, the one sensitive to chloroquine and IDE inhibitors occurring after endosomal segregation of insulin and its receptor.

Role of Hsp70 synthesis in the fate of the insulin-receptor complex after heat shock in cultured fetal hepatocytes

The influence of a mild heat shock on the fate of the insulin-receptor complex was studied in cultured fetal rat hepatocytes whose insulin glycogenic response is sensitive to heat [Zachayus and Plas (1995): J Cell Physiol 162:330-340]. After exposure from 15 min to 2 hr at 42.5 degrees C, the amount of (125)1-insulin associated with cells at 37 degrees C was progressively decreased (by 35% after 1 hr), while the release of (125)1-insulin degradation products into the medium was also inhibited (by 75%), more than expected from the decrease in insulin binding. Heat shock did not affect the insulin-induced internalization of cell surface insulin receptors but progressively suppressed the recycling at 37 degrees C of receptors previously internalized at 42.5 degrees C in the presence of insulin. When compared to the inhibitory effects of chloroquine on insulin degradation and insulin receptor recycling, which were immediate (within 15 min), those of heat shock developed within 1 hr of heating. The protein level of insulin receptors was not modified after heat shock and during recovery at 37 degrees C, while that of Hsp72/73 exhibited a transitory accumulation inversely correlated with variations in insulin binding, as assayed by Western immunoblotting from whole cell extracts. Coimmunoprecipitation experiments revealed a heat shock-stimulated association of Hsp72/73 with the insulin receptor. Affinity labeling showed an interaction between (125)1-insulin and Hsp72/73 in control cells, which was inhibited by heat shock. These results suggest that increased Hsp72/73 synthesis interfered with insulin degradation and prevented the recycling of the insulin receptor and its further thermal damage via a possible chaperone-like action in fetal hepatocytes submitted to heat stress.

Effects of modified insulin B21-B26 fragments on glycogenesis and on insulin-receptor complex fate in cultured fetal hepatocytes

Biological activity and interference with insulin receptor complex fate of two modified sequences of insulin B21-B26, beta-Ala-Arg-Gly-Phe-Phe-Tyr-NH2 (DP-432) and beta-Ala-Arg-Pro-Phe-Phe-Tyr-NH2 (DP-640), were studied in cultured 18-day-old fetal rat hepatocytes known to respond to insulin by an acute stimulation of glycogenesis. The two derivatives stimulated [14C]glucose incorporation into glycogen in the absence of insulin independently of the deprivation of serum in the medium. The maximal effect of 3 mM DP-640 after 2 h, more pronounced than with 3 mM DP-432, was of the same order as that obtained with 10 nM insulin alone (stimulation index: 4.7 +/- 0.7, 2.5 +/- 0.2 and 3.6 +/- 0.9, n = 4, with DP-640, DP-432 and insulin, respectively) whereas insulin B-chain decreased glycogen labeling. Simultaneous addition of derivatives and insulin at maximal concentrations produced nearly additive effects. DP-640, as well as DP-432, increased the amount of [125I](A14) or (B26) human insulin associated with cells at 37 degrees C and inhibited intracellular insulin degradation with differences depending on the kind of insulin isomer and derivative, while the rapid insulin receptor cycle was not affected. Thus, the two derivatives specifically modified the cellular processing of insulin in cultured fetal hepatocytes, and exerted an insulin-like effect on glycogenesis clearly enhanced through modification of DP-432 by substitution of glycine for proline (DP-640).