Biological action and fate of photoaffinity-labelled insulin-receptor complexes

Covalent linking of two photoactivatable insulin derivatives, B2-(2-nitro,4-azidophenylacetyl)-des-PheB1-insulin and B29-(2-nitro,4-azidophenylacetyl)-insulin to viable rat adipocytes gives a system, which contains a fixed stoichiometry between hormone and receptor. The biological signal of prolonged lipogenesis has been used to study several aspects of insulin binding and action: the role of the site of the crosslink between insulin and receptor, recognition of bound photoinsulin by anti-insulin antibodies, the half-life of the biologically active complex, the pH-dependence of the biological signal, and the possible role of internalization. Furthermore, the effect of trypsin on the insulin receptor, as well as the insulin-receptor complex, has been investigated and a refined model of the receptor is presented.

A shortened insulin with full in vitro potency

Des[(B26-30)-pentapeptide]insulin-B25-amide was prepared from protected des-[(B23-30)-octapeptide]insulin (pig) and H-Gly-Phe-Phe-NH2 by trypsin-mediated semisynthesis in a yield of 9% (based on insulin). The analogue was characterized with respect to chemistry, biological function and CD spectroscopy. While des[(B26-30)-pentapeptide]insulin with free carboxylate group exhibited a typical insulin activity of only 25% in vitro, des[(B26-30)-pentapeptide]insulinamide was fully active. Therefore des[(B26-30)-pentapeptide]insulin meets all structural and dynamic requirements for recognition and binding of the receptor as well as exertion of the biological effect, provided that the negative charge in the hydrophobic environment of PheB25 is neutralized.

Biological potency of covalently linked insulin-receptor in rat adipocytes. Comparison with the potency of reversible complexes

Irradiation of photoreactive insulin derivatives in the presence of isolated rat adipocytes produces a prolonged stimulation of lipogenesis in the cells even after exogenous and reversibly bound derivative has been removed by extensive washing. The quantitative nature of this response has now been studied using 125I-B2(2-nitro-4-azidophenylacetyl)des-PheB1-insulin. This derivative possesses nearly full biological potency and binding affinity prior to irradiation. After covalent linkage to adipocytes the efficacy of the derivative is reduced to 25 +/- 4% of the reversibly bound derivative, viz. 4-times as much needs to be covalently associated as reversibly bound to induce the same level of stimulation of lipogenesis. This reduced relative molar potency is due to a reduced ability of specific covalent insulin-receptor complexes to trigger a response.

Biological activity of covalently-linked insulin-receptor complexes in rat adipocytes. Effect of pH

The relative molar potencies of covalently and reversibly-bound insulin-receptor complexes were studied as a function of pH. The insulin derivatives used were 125I-B2 (2-nitro-4-azidophenylacetyl)des-PheB1-insulin and 125I-B29(2-nitro-4-azidophenylacetyl)des-PheB1-insulin. The potencies of both types of reversible complexes were effectively identical and constant between pH 7 and 8. The relative potency of the covalent B2-complex increased from 25 to 75%, and of the covalent B29 complex from 30 to nearly 100%. This indicates that the covalently linked partners in the complex are able to flex about the cross-linkages. Variations in the potency are due to variations in the number of correctly associated, reversibly or covalently bound insulin-receptor complexes. The form of the pH dependance suggests that an ionizable group, possibly an amino group, must be deprotonated to allow effective interaction.

Internalized insulin receptors are recycled to the cell surface in rat hepatocytes

We have followed the fate of cell surface insulin receptors in isolated rat hepatocytes by both a biochemical and a morphological approach. Hepatocytes were labeled with the photoreactive and biologically active 125I-labeled insulin analogue, [2-nitro-4-azidophenylacetylB2]des-PheB1-insulin, under conditions that allow for minimal internalization (2 hr at 15 degrees C). Analysis of the cell-associated radioactivity by NaDodSO4/polyacrylamide gel electrophoresis under reducing conditions followed by autoradiography revealed the specific labeling of a major insulin receptor subunit with Mr 130,000 and a minor degradation product with Mr 125,000. When the cells were exposed at 15 degrees C to trypsin at the end of the association period, these two bands were no longer observed, indicating that the labeled receptors were at the cell surface. This trypsin sensitivity of the receptor disappeared within 30-60 min of incubation of the cells at 37 degrees C, reflecting the internalization of the hormone-receptor complexes. Over the subsequent 4 hr of incubation, this was followed by a progressive reappearance of the receptor complexes at the cell surface, as indicated by the recovery of trypsin sensitivity of the labeled insulin receptors. An identical (both chronologically and quantitatively) journey of the insulin receptors was observed when the labeled material was studied by quantitative electron microscopic autoradiography. Thus, when the cells were incubated at 37 degrees C there was a rapid decrease (30-60 min) in the percentage of autoradiographic grains associated with the plasma membrane, followed by a progressive increase in this percentage over the subsequent 4 hr of incubation. In conclusion, using a biochemical and morphological approach to trace the photoaffinity-labeled insulin receptor, we have shown that the internalized hormone-receptor complex is recycled back to the cell surface.

Internalization and molecular processing of insulin receptors in isolated rat adipocytes

The cellular fate of insulin receptors in isolated rat adipocytes was studied by using a biologically active photosensitive insulin derivative, B2(2-nitro-4-azidophenylacetyl)-des-PheB1-insulin (NAPA-DP-insulin), to photoaffinity label the insulin receptors. Insulin receptors specifically labeled with 125I-labeled NAPA-DP-insulin were identified by NaDodSO4/polyacrylamide gel electrophoresis and autoradiography. Under nonreducing conditions, specific bands of Mr 330,000, 295,000, and 260,000 were identified; under disulfide reducing conditions, these were converted into Mr 125,000 and 90,000 subunits. When cells labeled at 16 degrees C were immediately trypsinized, all of the receptor bands were degraded into lower molecular weight fragments, indicating that the labeled receptors were all on the cell surface. However, when the labeled cells were incubated at 37 degrees C for 1 hr prior to trypsin exposure, approximately equal to 30% of the receptors were found to be trypsin insensitive, indicating that this fraction was translocated intracellularly. Processing of the insulin receptors appeared to occur; incubation at 37 degrees C (but not at 16 degrees C) resulted in generation of a Mr 115,000 component from the Mr 125,000 subunit as well as in the disappearance of the Mr 330,000 and 295,000 species. Inclusion of chloroquine during photoaffinity labeling at 16 degrees C and during the subsequent incubation at 37 degrees C showed that this agent (i) increased the trypsin-insensitive (intracellular) receptor pool, (ii) blocked conversion of the Mr 125,000 subunit into the Mr 115,000 component, and (iii) prevented the disappearance of the Mr 330,000 and 295,000 species. These studies show that insulin-receptor complexes are internalized and processed intracellularly at a chloroquine-sensitive site(s).

Cross-linked [DAlaA1]Insulins. Evidence for a change in the conformation of the insulin monomer at its receptor

[(Boc-DAla)2A1,B1]Insulin was produced by reaction of insulin with phenylisothiocyanate, protection of the B29-epsilon-amino function with the Msc residue, Edman degradation, reaction of A2 and B2 with Boc-DAla-ONSu, and removal of the Msc group. This derivative was used to prepare [DAla2A1,B1]insulin, a B29-B29' dimer, and N alpha A1, N epsilon B29-diaminosuberoyl-[DAla2 A1,B1]-insulin. These derivatives had lipogenic potencies similar to those already reported for analogous derivatives of native insulin (87%, 2.8%, and 8.7% respectively). This suggests that the low activities observed for A1-B29 cross-linked insulins is not due to perturbation of the N-terminus of the A chain. It is argued that the hindrance of a slight conformational adjustment involving the C-terminus of the B chain by the cross-link is a more likely explanation.

Biochemical and morphological evidence that the insulin receptor is internalized with insulin in hepatocytes

There is morphological and biochemical evidence that insulin is internalized in hepatocytes. The present study was designed to investigate the fate of the insulin receptor itself, subsequently to the initial binding step of the hormone to the hepatocyte plasma membrane. The insulin receptor was labeled with a 125I-photoreactive insulin analogue (B2[2-nitro,4-azidophenylacetyl]des-PheB1-insulin). This photoprobe was covalently coupled to the receptor by UV irradiation of hepatocytes after an initial binding step of 2-4 h at 15 degrees C. At this temperature, only limited (approximately 20%) internalization of the ligand occurred. In a second step, hepatocytes were resuspended in insulin-free buffer and further incubated for 2-4 h at 37 degrees C. After h at 37 degrees C, no significant radioactivity could be detected in non-UV-irradiated cells, whereas 12-15 % of the radioactivity initially bound remained associated to UV-irradiated cells. Morphological analysis after electron microscopy revealed that approximately 70% of this radioactivity was internalized and preferentially associated with lysosomal structures. SDS PAGE analysis under reducing conditions revealed that most of the radioactivity was associated with a 130,000-dalton band, previously identified as the major subunit of the insulin receptor in a variety of tissues. Internalization of the labeled insulin-receptor complex at the end of the 37 degrees C incubation was further demonstrated by its inaccessibility to trypsin. Conversely, at the end of the association step, the receptor (also characterized as a predominant 130,000-dalton species) was localized on the cell surface since it was cleaved by trypsin. We conclude that in hepatocytes the insulin receptor is internalized with insulin.

[LeuB24]- and [LeuB25]insulins are not antagonists of lipogenesis in adipocytes

Semisynthetic human [LeuB24]-and [LeuB25]insulins were investigated to determine whether they show antagonistic properties towards insulin-stimulated lipogenesis in isolated fat cells. In contrast to other reports, we could detect only an additive agonistic effect when constant concentrations (e.g. 0.3 ng/ml) of the analogues were mixed with varying concentrations of insulin, or when constant concentrations (e.g. 0.3 ng/ml, 0.6 ng/ml) of insulin were mixed with varying concentrations of the analogues. Similar results were obtained with mixtures of insulin and NA2-acetyl- or NA2-propionyl-des-GlyA1-insulins. These results do not support the contention that a diabetic state could be caused by either of these mutant human insulins.

Covalent linking of photoreactive insulin to adipocytes produces a prolonged signal

The first step of insulin's many cellular functions is specific binding to receptors on the plasma membrane of target cells. The subsequent molecular basis of insulin action, particularly the coupling mechanism(s) involved in transmitting the biological message, remains largely unknown. Our approach to the problem centres on the application of a series of well characterized photo-insulins carrying an aryl-azido or nitro-aryl-azido group in positions A1, B1, B2 or B29 (refs 2--4). Specific binding to membrane components could be demonstrated with B1-, B29- (ref. 5) and A1-photo-insulins as well as a B2-derivative. We now report that lipogenesis is increased to, and maintained at, near-maximal levels for several hours after photoinduced covalent binding of B2- (2-nitro,4-azidophenylacetyl)-des-PheB1-insulin (Napa-DP-insulin) to living adipocytes.

Life stresses and depression in the menopause

Research on the climacteric has largely concentrated on relationships between ovarian insufficiency, oestrogen deficiency, and climacteric symptoms. Little attention has been paid to those who have no symptoms. It is proposed, in addition to these relationships, that the life events of this period are significantly involved; and that resultant stress may contribute to oestrogen deficiency as opposed to physiologically normal postmenopausal oestrogen levels. In this preliminary study, two groups of women under conditions of relatively low stress and high stress were tested over a period of 15 mth. They were rated on the Hamilton Rating Scale for depression and anxiety and results were compared with the corresponding total urinary oestrogen output. The results, which suggest a significant relationship between stress related to depression and oestrogen levels in the menopause, are discussed and compared with differing life circumstances of the subjects.

The re-estimation of absorbed doses of less than 1 rad measured with lithium fluoride thermoluminescent dosemeters

The technique of ultraviolet light bleaching for the re-estimation of the absorbed dose of radiation received by lithium fluoride thermoluminescence dosemeters has been extended to the extruded ribbon and PTFE disc form of dosemeter. The threshold of detection for the re-estimation of absorbed dose was found to be 1-1 rad for the PTFE disc dosemeters and 2-8 rad for the extruded ribbon dosemeters. A further study of this technique has revealed that by raising the temperature of the dosemeter during bleaching the threshold for re-estimated absorbed doses may be substantially reduced. The optimum temperature was found to be 80 degrees C and the practical threshold for the re-estimation of absorbed dose using a PTFE disc dosemeter was determined as 750 mrad.

Sites of Localization and Synthesis of Ss Protein in Mice

Ss, a serum protein whose structure and level in serum are determined by genes of the mouse H-2 complex, has been localized to a variety of cell types, using immunofluorescence and radiolabel techniques. Peritoneal macrophages and liver parenchymal cells appear to synthesize Ss for export to serum. These cells stain in the cytoplasm for Ss and release labeled Ss into the medium of cultures fed 14C amino acids. Other cultured cells, fibroblasts of Ss high strains, bear the protein only on their membranes. The membrane-bound form of Ss has at least one antigenic determinant masked; this determinant is exposed when membrane-as-sociated Ss is released into culture medium. At least in some cells then, Ss, like the other products of H-2 region genes, is membrane associated.