Antagonistic effects of a covalently dimerized insulin derivative on insulin receptors in 3T3-L1 adipocytes

Targeting the insulin receptor with hormone and peptide dimers

Insulin is a key hormone involved in the regulation of overall energetic homeostasis of the organism. The dimeric character of the receptor for insulin evokes ideas about its activation or inhibition with peptide dimers that could either trigger or block the structural transition of the insulin receptor, leading to its activation. Herewith, we present the chemical engineering and biological characterization of several series of insulin dimers or dimers of specific peptides that should be able to bind receptors for insulin or insulin growth factor 1. The hormones or peptides in the dimers were interconnected with different linkers, consisting of triazole moieties and 3, 6, 8, 11, or 23 polyethylene glycol units. The prepared dimers were weaker in binding to insulin receptors than human insulin. However, some of the insulin dimers showed preferential binding specificity toward the isoform A of the insulin receptor, and the insulin dimers also stimulated the insulin receptor more strongly than would be consistent with their binding affinities. Our results suggest that designing insulin dimers may be a promising strategy for modulating the ability of the hormone to activate the receptor or to alter its specificity toward insulin receptor isoforms.

Recombinant Insulin-Like Growth Factor 1 Dimers: Receptor Binding Affinities and Activation Abilities

Insulin-like growth factor 1 (IGF-1) and its IGF-1 receptor (IGF-1R) belong to an important biological system that is involved in the regulation of normal growth, but that has also been recognized as playing a role in cancer. IGF-1R antagonists could be interesting for the testing of their potential antiproliferative properties as an alternative to IGF-1R tyrosine-kinase inhibitors or anti-IGF-1R monoclonal antibodies. In this study, we were inspired by the successful development of insulin dimers capable of antagonizing insulin effects on the insulin receptor (IR) by simultaneous binding to two separated binding sites and by blocking structural rearrangement of the IR. We designed and produced in Escherichia coli three different IGF-1 dimers in which IGF-1 monomers are interlinked through their N- and C-termini, with linkers having 8, 15 or 25 amino acids. We found that the recombinant products were susceptible to the formation of misfolded or reduced variants, but that some of them were able to bind IGF-1R in low nanomolar affinities and all of them activate IGF-1R proportionally to their binding affinities. Overall, our work can be considered as a pilot study that, although it did not lead to the discovery of new IGF-1R antagonists, explored the possibility of recombinant production of IGF-1 dimers and led to the preparation of active compounds. This work could inspire further studies dealing, for example, with the preparation of IGF-1 conjugates with specific proteins for the study of the hormone and its receptor or for therapeutic applications.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s10989-023-10499-1.

Discovery of Insulin Receptor Partial Agonists MK-5160 and MK-1092 as Novel Basal Insulins with Potential to Improve Therapeutic Index

We have identified a series of novel insulin receptor partial agonists (IRPAs) with a potential to mitigate the risk of hypoglycemia associated with the use of insulin as an antidiabetic treatment. These molecules were designed as dimers of native insulin connected via chemical linkers of variable lengths with optional capping groups at the N-terminals of insulin chains. Depending on the structure, the maximal activation level (%Max) varied in the range of ∼20-70% of native insulin, and EC₅₀ values remained in sub-nM range. Studies in minipig and dog demonstrated that IRPAs had sufficient efficacy to normalize plasma glucose levels in diabetes, while providing reduction of hypoglycemia risk. IRPAs had a prolonged duration of action, potentially making them suitable for once-daily dosing. Two lead compounds with %Max values of 30 and 40% relative to native insulin were selected for follow up studies in the clinic.

Controlled intramolecular antagonism as a regulator of insulin receptor maximal activity

In the treatment of insulin-dependent diabetes the risk of a fatal insulin overdose is a persistent fear to most patients. In order to potentially reduce the risk of overdose, we report the design, synthesis, and biochemical characterization of a set of insulin analogs designed to be fractionally reduced in maximal agonism at the insulin receptor isoforms. These analogs consist of native insulin that is site-specifically conjugated to a peptide-based insulin receptor antagonist. The structural refinement of the antagonist once conjugated to insulin provided a set of partial agonists exhibiting between 25 and 70% of the maximal agonism of native insulin at the two insulin receptor isoforms, with only slight differences in inherent potency. These rationally-designed partial agonists provide an approach to interrogate whether control of maximal activity can provide glycemic control with reduced hypoglycemic risk.

Deciphering Brain Insulin Receptor and Insulin-Like Growth Factor 1 Receptor Signalling

Insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R) are highly conserved receptor tyrosine kinases that share signalling proteins and are ubiquitously expressed in the brain. Central application of insulin or IGF1 exerts several similar physiological outcomes, varying in strength, whereas disruption of the corresponding receptors in the brain leads to remarkably different effects on brain size and physiology, thus highlighting the unique effects of the corresponding hormone receptors. Central insulin/IGF1 resistance impacts upon various levels of the IR/IGF1R signalling pathways and is a feature of the metabolic syndrome and neurodegenerative diseases such as Alzheimer's disease. The intricacy of brain insulin and IGF1 signalling represents a challenge for the identification of specific IR and IGF1R signalling differences in pathophysiological conditions. The present perspective sheds light on signalling differences and methodologies for specifically deciphering brain IR and IGF1R signalling.

Agonism and antagonism at the insulin receptor

Insulin can trigger metabolic as well as mitogenic effects, the latter being pharmaceutically undesirable. An understanding of the structure/function relationships between insulin receptor (IR) binding and mitogenic/metabolic signalling would greatly facilitate the preclinical development of new insulin analogues. The occurrence of ligand agonism and antagonism is well described for G protein-coupled receptors (GPCRs) and other receptors but in general, with the exception of antibodies, not for receptor tyrosine kinases (RTKs). In the case of the IR, no natural ligand or insulin analogue has been shown to exhibit antagonistic properties, with the exception of a crosslinked insulin dimer (B29-B’29). However, synthetic monomeric or dimeric peptides targeting sites 1 or 2 of the IR were shown to be either agonists or antagonists. We found here that the S961 peptide, previously described to be an IR antagonist, exhibited partial agonistic effects in the 1–10 nM range, showing altogether a bell-shaped dose-response curve. Intriguingly, the agonistic effects of S961 were seen only on mitogenic endpoints (³H-thymidine incorporation), and not on metabolic endpoints (¹⁴C-glucose incorporation in adipocytes and muscle cells). The agonistic effects of S961 were observed in 3 independent cell lines, with complete concordance between mitogenicity (³H-thymidine incorporation) and phosphorylation of the IR and Akt. Together with the B29-B’29 crosslinked dimer, S961 is a rare example of a mixed agonist/antagonist for the human IR. A plausible mechanistic explanation based on the bivalent crosslinking model of IR activation is proposed.

Glycoinsulins: Dendritic Sialyloligosaccharide-Displaying Insulins Showing a Prolonged Blood-Sugar-Lowering Activity

Mono-, di-, and trisialyloligosaccharides were introduced to mutant insulins through enzymatic reactions. Sugar chains were sialylated by alpha2,6-sialyltransferase (alpha2,6-SiaT) via an accessible glutamine residue at the N-terminus of the B-chain attached by transglutaminase (TGase). Sia2,6-di-LacNAc-Ins(B-F1Q) and Sia2,6-tri-LacNAc-Ins(B-F1Q), displaying two and three sialyl-N-acetyllactosamines, respectively, were administered to hyperglycemic mice. Both branched glycoinsulins showed prolonged glucose-lowering effects compared to native or lactose-carrying insulins, showing that sialic acid is important in obtaining a prolonged effect. Sia2,6-tri-LacNAc-Ins(B-F1Q), in particular, induced a significant delay in the recovery of glucose levels.

Structure-activity relationship of covalently dimerized insulin derivatives: correlation of partial agonist efficacy with cross-linkage at lysine B29

The effects of 7 covalently dimerized insulin derivatives on glucose transport in differentiated 3T3-L1 cells were investigated. Symmetric cross-linkage at lysine B29 with a bridge of 2 (oxalyl), 8 (suberoyl) or 12 (dodecanedioyl) carbon atoms produced derivatives with essentially unaltered receptor binding affinity but largely reduced intrinsic activity. Regardless of the chain length, these derivatives inhibited the effect of submaximal insulin concentrations. Insulin derivatives cross-linked at phenylalanine B1 or asymmetrically at B1/B29 were full agonists of the insulin receptor. When lysine B29 was cross-linked with the inactive desoctapeptide(B23-B30)insulin at phenylalanine B1, the intrinsic activity of the resulting dimer was lower than that of insulin, but higher than that of the symmetric B29-dimers. It is concluded that linkage at the B29-lysines, and not at the B1-phenylalanine, leads to partial agonism of dimerized insulin derivatives, regardless of the length of the crosslinker.

The signaling potential of the receptors for insulin and insulin-like growth factor I (IGF-I) in 3T3-L1 adipocytes: comparison of glucose transport activity, induction of oncogene c-fos, glucose transporter mRNA, and DNA-synthesis

The receptors for insulin and insulin-like growth factor I (IGF-I) have in common a high sequence homology and diverse overlapping functions, (e.g., the stimulation of acute metabolic events and the induction of cell growth.). In the present study, we have compared the potential of insulin and IGF-I receptors in stimulating glucose transport activity, glucose transporter gene expression, DNA-synthesis, and expression of proto-oncogene c-fos in 3T3-L1 adipocytes which express high levels of both receptors. Binding of both hormones to their own receptors was highly specific as compared with binding to the respective other receptor (insulin receptor: KD = 3.6 nM, KI of IGF-I greater than 500 nM; IGF-I receptor, KD = 1.1 nM, KI of insulin = 191 nM). Induction of proto-oncogene c-fos mRNA by insulin and IGF-I paralleled their respective receptor occupancy and was thus induced by both hormones via their own receptor (EC50 of insulin, 3.7; IGF-I, 3.9 nM). Similarly, both insulin and IGF-I increased DNA synthesis (EC50 of insulin, 5.8 nM; IGF-I, 4.0 nM), glucose transport activity (EC50 of insulin, 1.7 nM; IGF-I, 1.4 nM), and glucose transporter (GLUT4) mRNA levels in concentrations corresponding with their respective receptor occupancy. These data indicate that in 3T3-L1 cells the alpha-subunits of insulin and IGF-I receptors have an equal potential to stimulate a metabolic and a mitogenic response.