Relationship between the affinity and proteolysis of the insulin receptor. Evidence that higher affinity receptors are preferentially degraded

An evaluation of the cross-linking model for the interaction of insulin with its receptor

The cross-linking model for insulin receptor interactions, in which a single insulin molecule may form a cross-link between an insulin receptor's alpha-subunits, has been expressed as a formal compartmental model and subjected to a systematic analysis, examining a number of predictions that have been made for this model. The kinetic parameters for the model were obtained by matching data from insulin receptor equilibrium binding studies and rates of formation of the insulin receptor complex. This analytical study has allowed a clear description of the kinetics of the ligand receptor complexes involved in such a mechanism. We conclude that the cross-linking model accounts for the anomaly of the 10-fold concentration difference in high- and low-affinity binding sites found when insulin binding is analyzed by conventional means. However, the phenomenon of acceleration of dissociation of labeled ligand by unlabeled ligand cannot be accounted for as an intrinsic part of the model. We suggest that this phenomenon arises from the destabilization of cross-link formation when a second insulin molecule binds.

Combination of insulinomimetic agents H2O2 and vanadate enhances insulin receptor mediated tyrosine phosphorylation of IRS-1 leading to IRS-1 association with the phosphatidylinositol 3-kinase

To analyze the mechanism of action of the insulinomimetic agents H2O2, vanadate, and pervanadate (H2O2 and vanadate), CHO cells or CHO cells that overexpress wild-type or mutant insulin receptor and/or the insulin receptor substrate (IRS-1) were used. H2O2 or vanadate treatment alone had little or no effect on tyrosine phosphorylation of cellular proteins; however, pervanadate treatment dramatically enhanced tyrosine phosphorylation of a number of proteins including the insulin receptor and IRS-1. However, the insulin receptor and IRS-1 coimmunoprecipitate from insulin-treated but not from pervanadate-treated cells. Pervanadate-induced tyrosine phosphorylation of the insulin receptor led to an increase in insulin receptor tyrosine kinase activity toward IRS-1 in vivo and IRS-1 peptides in vitro equal to that induced by insulin treatment. Pervanadate-enhanced phosphorylation of IRS-1 led to a fifteenfold increase in IRS-1-associated phosphatidylinositol (PtdIns) 3-kinase activity. However, insulin receptor-associated PtdIns 3-kinase activity from pervanadate-treated cells was not detectable, while insulin receptor-associated PtdIns 3-kinase activity from insulin-treated cells was 20% of the IRS-1-associated activity. Thus, pervanadate but not H2O2 or vanadate alone under these conditions mimics many of insulin actions, but pervanadate treatment does not induce insulin receptor/IRS-1 association.

A single-site allosteric model of intracellular androgen-receptor interaction

We present a single-site, two-state model for analyzing the effect of time and ligand concentration on the extent and character of 5 alpha-dihydrotestosterone, methyltrienolone or mibolerone binding to the specific androgen receptor within cultured human genital skin fibroblasts. The model has three basic attributes: formation of the initial low-affinity androgen-receptor complex, and its transformation to a higher affinity state are irreversible, first-order processes; and receptors released from complexes in each state not only differ from each other and from their pre-liganded progenitor, but can also reassociate with androgen to yield complexes in their respective parental states. The rate constants of dissociation and apparent equilibrium binding constants of the two affinity states were determined for each of the three androgens within normal cells and those of a transformation-defective mutant. When these values are combined with estimates of the rate constants at which the complexes are formed and transformed, the model accurately simulates time-dependent changes in the slopes and character of experimental Scatchard plots. It can also generate Scatchard plots that are concave, convex or sigmoidal simply by making sequential changes in its formation or transformation constants. Thus, our model can explain complex ligand-receptor binding kinetics that have heretofore been interpreted according to alternate models of transformation and binding-site multiplicity with or without properties of cooperativity, and it supports the notion that receptor recycling involves intermediate receptor states.

Distinct receptors for insulin-like growth factor I in rat renal glomeruli and tubules

Purified preparations of renal glomeruli and tubules were obtained by a procedure involving perfusion of rat kidneys with magnetic iron oxide particles to selectively separate the iron-containing glomeruli from the nonmagnetic tubules. Detergent-soluble extracts of both renal glomerular and tubular membranes showed high-affinity, specific binding of 125I-labeled insulin-like growth factor I (125I-IGF-I), whereas degradation of this peptide hormone was minimal during a 90-min incubation at 22 degrees C in the presence of 2.5 mM EDTA and 5 mM N-ethylmaleimide. The affinity of these receptors for IGF-I appeared identical in the two types of renal tissue, since 50% inhibition of 125I-IGF-I binding to both glomerular and tubular tissue occurred in the presence of approximately 3 x 10(-9) M unlabeled IGF-I. In contrast, insulin was much less effective at blocking 125I-IGF-I binding to either tissue, with 1 x 10(-6) M insulin required to produce 50% inhibition of binding. Relative to 125I-IGF-I binding, 125I-insulin binding to glomerular and tubular tissue was significantly lower per milligram protein. 125I-IGF-I was specifically cross-linked to a glomerular receptor subunit that migrated as two discrete bands with relative molecular weight (Mr) of 140,000-150,000 on sodium dodecyl sulfate polyacrylamide gels in the presence of 40 mM dithiothreitol. In contrast, 125I-IGF-I was cross-linked to a tubular receptor subunit that migrated as two discrete bands but at a slightly different position, with Mr of 120,000-140,000.(ABSTRACT TRUNCATED AT 250 WORDS)