Superactive insulins

Insight into the structural and biological relevance of the T/R transition of the N-terminus of the B-chain in human insulin

The N-terminus of the B-chain of insulin may adopt two alternative conformations designated as the T- and R-states. Despite the recent structural insight into insulin–insulin receptor (IR) complexes, the physiological relevance of the T/R transition is still unclear. Hence, this study focused on the rational design, synthesis, and characterization of human insulin analogues structurally locked in expected R- or T-states. Sites B3, B5, and B8, capable of affecting the conformation of the N-terminus of the B-chain, were subjects of rational substitutions with amino acids with specific allowed and disallowed dihedral φ and ψ main-chain angles. α-Aminoisobutyric acid was systematically incorporated into positions B3, B5, and B8 for stabilization of the R-state, and N-methylalanine and d-proline amino acids were introduced at position B8 for stabilization of the T-state. IR affinities of the analogues were compared and correlated with their T/R transition ability and analyzed against their crystal and nuclear magnetic resonance structures. Our data revealed that (i) the T-like state is indeed important for the folding efficiency of (pro)insulin, (ii) the R-state is most probably incompatible with an active form of insulin, (iii) the R-state cannot be induced or stabilized by a single substitution at a specific site, and (iv) the B1–B8 segment is capable of folding into a variety of low-affinity T-like states. Therefore, we conclude that the active conformation of the N-terminus of the B-chain must be different from the “classical” T-state and that a substantial flexibility of the B1–B8 segment, where GlyB8 plays a key role, is a crucial prerequisite for an efficient insulin–IR interaction.

Differences in bioactivity between human insulin and insulin analogues approved for therapeutic use- compilation of reports from the past 20 years

In order to provide comprehensive information on the differences in bioactivity between human insulin and insulin analogues, published in vitro comparisons of human insulin and the rapid acting analogues insulin lispro (Humalog®), insulin aspart ( NovoRapid®), insulin glulisine (Apidra®), and the slow acting analogues insulin glargine (Lantus®), and insulin detemir (Levemir®) were gathered from the past 20 years (except for receptor binding studies). A total of 50 reports were retrieved, with great heterogeneity among study methodology. However, various differences in bioactivity compared to human insulin were obvious (e.g. differences in effects on metabolism, mitogenesis, apoptosis, intracellular signalling, thrombocyte function, protein degradation). Whether or not these differences have clinical bearings (and among which patient populations) remains to be determined.

Design of an active ultrastable single-chain insulin analog: synthesis, structure, and therapeutic implications

Single-chain insulin (SCI) analogs provide insight into the inter-relation of hormone structure, function, and dynamics. Although compatible with wild-type structure, short connecting segments (<3 residues) prevent induced fit upon receptor binding and so are essentially without biological activity. Substantial but incomplete activity can be regained with increasing linker length. Here, we describe the design, structure, and function of a single-chain insulin analog (SCI-57) containing a 6-residue linker (GGGPRR). Native receptor-binding affinity (130 +/- 8% relative to the wild type) is achieved as hindrance by the linker is offset by favorable substitutions in the insulin moiety. The thermodynamic stability of SCI-57 is markedly increased (DeltaDeltaG(u) = 0.7 +/- 0.1 kcal/mol relative to the corresponding two-chain analog and 1.9 +/- 0.1 kcal/mol relative to wild-type insulin). Analysis of inter-residue nuclear Overhauser effects demonstrates that a native-like fold is maintained in solution. Surprisingly, the glycine-rich connecting segment folds against the insulin moiety: its central Pro contacts Val(A3) at the edge of the hydrophobic core, whereas the final Arg extends the A1-A8 alpha-helix. Comparison between SCI-57 and its parent two-chain analog reveals striking enhancement of multiple native-like nuclear Overhauser effects within the tethered protein. These contacts are consistent with wild-type crystal structures but are ordinarily attenuated in NMR spectra of two-chain analogs, presumably due to conformational fluctuations. Linker-specific damping of fluctuations provides evidence for the intrinsic flexibility of an insulin monomer. In addition to their biophysical interest, ultrastable SCIs may enhance the safety and efficacy of insulin replacement therapy in the developing world.

Efficient Production of Bioactive Insulin from Human Epidermal Keratinocytes and Tissue-Engineered Skin Substitutes: Implications for Treatment of Diabetes

Despite many years of research, daily insulin injections remain the gold standard for diabetes treatment. Gene therapy may provide an alternative strategy by imparting the ability to secrete insulin from an ectopic site. The epidermis is a self-renewing tissue that is easily accessible and can provide large numbers of autologous cells to generate insulin-secreting skin substitutes. Here we used a recombinant retrovirus to modify human epidermal keratinocytes with a gene encoding for human proinsulin containing the furin recognition sequences at the A-C and B-C junctions. Keratinocytes were able to process proinsulin and secrete active insulin that promoted glucose uptake. Primary epidermal cells produced higher amounts of insulin than cell lines, suggesting that insulin secretion may depend on the physiological state of the producer cells. Modified cells maintained the ability to stratify into 3-dimensional skin equivalents that expressed insulin at the basal and suprabasal layers. Modifications at the furin recognition sites did not improve proinsulin processing, but a single amino acid substitution in the proinsulin B chain enhanced C-peptide secretion from cultured cells and bioengineered skin substitutes 10- and 28-fold, respectively. These results suggest that gene-modified bioengineered skin may provide an alternative means of insulin delivery for treatment of diabetes.

A conserved histidine in insulin is required for the foldability of human proinsulin: structure and function of an ALAB5 analog

The insulins of eutherian mammals contain histidines at positions B5 and B10. The role of His(B10) is well defined: although not required in the mature hormone for receptor binding, in the islet beta cell this side chain functions in targeting proinsulin to glucose-regulated secretory granules and provides axial zincbinding sites in storage hexamers. In contrast, the role of His(B5) is less well understood. Here, we demonstrate that its substitution with Ala markedly impairs insulin chain combination in vitro and blocks the folding and secretion of human proinsulin in a transfected mammalian cell line. The structure and stability of an Ala(B5)-insulin analog were investigated in an engineered monomer (DKP-insulin). Despite its impaired foldability, the structure of the Ala(B5) analog retains a native-like T-state conformation. At the site of substitution, interchain nuclear Overhauser effects are observed between the methyl resonance of Ala(B5) and side chains in the A chain; these nuclear Overhauser effects resemble those characteristic of His(B5) in native insulin. Substantial receptor binding activity is retained (80 +/- 10% relative to the parent monomer). Although the thermodynamic stability of the Ala(B5) analog is decreased (DeltaDeltaG(u) = 1.7 +/- 0.1 kcal/mol), consistent with loss of His(B5)-related interchain packing and hydrogen bonds, control studies suggest that this decrement cannot account for its impaired foldability. We propose that nascent long-range interactions by His(B5) facilitate alignment of Cys(A7) and Cys(B7) in protein-folding intermediates; its conservation thus reflects mechanisms of oxidative folding rather than structure-function relationships in the native state.

Plasmid-electroporated primary hepatocytes acquire quasi-physiological secretion of human insulin and restore euglycemia in diabetic mice

We describe the durable correction of streptozotocin-induced murine diabetes by in vivo implantation of primary mouse hepatocytes electroporated ex vivo with a human proinsulin cDNA plasmid construct controlled by glucose and zinc regulatory elements. Transfected hepatocytes increased insulin transgene transcription and secretion within 10-20 min of exposure to 25 mM glucose or 60 microM zinc. Insulin release did not occur from secretory granules. Electroporated Rosa26 hepatocytes ( approximately 8 x 10(5) viable cells) were implanted in C57BL/6J diabetic mice in one of three sites: unresected liver, regenerating liver or mesentery. Control diabetic mice were implanted with untransfected hepatocytes. At 30 days after implantation, 8/15 control mice were alive, while 19/19 treated mice were alive. The ratio of body weight on day 30/nadir body weight was significantly higher for all treated groups compared with controls. All eight surviving control mice were hyperglycemic 30 days post-implantation, while 16/19 treated diabetic mice remained normoglycemic. Treated mice had lower mean glucose values (P< or =0.001) without fasting hypoglycemia and better glucose tolerance (P< or =0.0003) than untreated controls. All (6/6) diabetic mice implanted in regenerating liver and 71% (5/7) implanted in unresected liver were alive 77 days after implantation. Engrafted hepatocytes were identified, mainly around central veins, by staining for beta-galactosidase activity and with anti-human insulin antibody.

Insulin and analogue effects on protein degradation in different cell types. Dissociation between binding and activity

In adult animals, the major effect of insulin on protein turnover is inhibition of protein degradation. Cellular protein degradation is under the control of multiple systems, including lysosomes, proteasomes, calpains, and giant protease. Insulin has been shown to alter proteasome activity in vitro and in vivo. We examined the inhibition of protein degradation by insulin and insulin analogues (Lys(B28),Pro(B29)-insulin (LysPro), Asp(B10)-insulin (B10), and Glu(B4),Gln(B16),Phe(B17)-insulin (EQF)) in H4, HepG2, and L6 cells. These effects were compared with receptor binding. Protein degradation was examined by release of trichloroacetic acid-soluble radioactivity from cells previously labeled with [(3)H]leucine. Short- and intermediate-lived proteins were examined. H4 cells bound insulin with an EC(50) of 4.6 x 10(-9) m. LysPro was similar. The affinity of B10 was increased 2-fold; that of EQF decreased 15-fold. Protein degradation inhibition in H4 cells was highly sensitive to insulin (EC(50) = 4.2 x 10(-11) and 1.6 x 10(-10) m, short- and intermediate-lived protein degradation, respectively) and analogues. Despite similar binding, LysPro was 11- to 18-fold more potent than insulin at inhibiting protein degradation. Conversely, although EQF showed lower binding to H4 cells than insulin, its action was similar. The relative binding potencies of analogues in HepG2 cells were similar to those in H4 cells. Examination of protein degradation showed insulin, LysPro, and B10 were equivalent while EQF was less potent. L6 cells showed no difference in the binding of the analogues compared with insulin, but their effect on protein degradation was similar to that seen in HepG2 cells except B10 inhibited intermediate-lived protein degradation better than insulin. These studies illustrate the complexities of cellular protein degradation and the effects of insulin. The effect of insulin and analogues on protein degradation vary significantly in different cell types and with different experimental conditions. The differences seen in the action of the analogues cannot be attributed to binding differences. Post-receptor mechanisms, including intracellular processing and degradation, must be considered.

Insulin delivery with plasmid DNA

Success in controlling hyperglycemia in type I diabetics will require a restoration of basal insulin. To this end, three plasmid DNAs (pDNA) encoding preproinsulin were compared for constitutive expression and processing to insulin in nonendocrine cells in vitro. The pDNAs were designed to express rat proinsulin I (VR-3501), rat proinsulin I with the B10 aspartic acid point mutation (VR-3502), and a derivative of VR-3502 with a furin cleavage site added at the B-chain and C-peptide junction (VR-3503). Cells transfected with VR-3501 or VR-3502 were able to secrete only proinsulin, whereas transfection with VR-3503 yielded 30-70% mature insulin, which could be increased to >99% by cotransfection with a furin expression plasmid (VR-3505). The insulin produced was biologically active. The bilateral injection of 100 microg of VR-3502 plasmid into the tibialis anterior muscles of mice on two consecutive days yielded, on average, several hundred picograms of heterologous proinsulin per milliliter of serum. In BALB/c mice, serum proinsulin peaked 7-14 days postinjection and declined to preinjection levels by days 21-28. In athymic nude mice, serum proinsulin was sustained for at least 6 weeks. The therapeutic efficacy of delivering insulin via muscle injection of pDNA was evaluated in athymic nude mice made diabetic with the beta cell toxin streptozotocin (STZ). All animals given control DNA died within 1 week of receiving STZ while 40% of the mice coinjected with plasmids VR-3503 and VR-3505 lived through the duration of the 4-week experiment. Muscles of the surviving animals contained 17-100 ng of immune-reactive insulin (IRI), 86-94% of which was mature insulin. The results suggest that heterologous insulin made in muscle increased the survival rate. We propose that insulin plasmid expression in skeletal muscle may be a valid approach to basal insulin delivery. The feasibility of plasmid DNA-based delivery of basal insulin was investigated. An expression system consisting of pDNAs encoding a selectively mutated rat preproinsulin and mouse furin was developed and characterized in vitro and in vivo. When injected with preproinsulin pDNA, the mouse tibialis anterior muscle expressed and released proinsulin into serum at levels comparable to normal basal insulin in rodents. These heterologous proinsulin levels were sustained for several weeks in immune-compromised nondiabetic mice. Mouse muscle coinjected with a pDNA encoding the endopeptidase furin and a pDNA encoding a pre-proinsulin modified to contain two furin cleavage sites produced fully processed insulin. This muscle-made insulin appears to have contributed to the survival of mice treated with a highly diabetogenic dose of streptozotocin, a beta cell toxin. The results demonstrate that skeletal muscle is able to express and deliver therapeutic insulin from plasmid DNA.

An insulin analogue with gamma-amino butyric acid substitution for A13Leu-A14Tyr

An insulin A chain analogue, [A13-14 GABA, A21 Ala]A chain, for which the dipeptide Leu-Try at A13-A14 was substituted by a non-coded amino acid, gamma-amino butyric acid (GABA) and A21 Asn by Ala, was prepared by stepwise Fmoc solid-phase manual synthesis and then combined with the natural B chain of porcine insulin to yield an insulin analogue, [A13-14 GABA, A21Ala] porcine insulin (GABA substituted insulin). This insulin analogue still retains 50% in vivo biological activity and 59% in receptor binding capacity. It can also be crystallized. These results indicate that its overall conformation is similar to the native form and that the side chains of A13Leu and A14Tyr are not essential for insulin activity. In addition, the replacement of a normal C-N peptide bond by an unnatural C-C bond may have general meaning in structure and function studies of other proteins.

Growth promoting and metabolic activity of the human insulin analogue [GlyA21,ArgB31,ArgB32]insulin (HOE 901) in muscle cells

[GlyA21,ArgB31,ArgB32]insulin (HOE 901) represents a biosynthetic human insulin analogue that, due to its isoelectric point, precipitates at neutral tissue pH leading to a retarded absorption rate and a corresponding longer duration of action. In the present investigation we have evaluated the growth promoting and metabolic activity of this analogue in muscle tissue using exponentially growing H9c2 cardiac myoblasts and adult rat ventricular cardiomyocytes. Equilibrium binding studies of 125I-labelled IGF-I (insulin-like growth factor I) to differentiating myoblasts revealed the presence of 7 x 10(3) IGF-I receptors per cell. In contrast, no specific binding of insulin could be detected. Competition binding experiments showed a slightly higher affinity of HOE 901 for the IGF-I receptor when compared to regular human insulin with IC50 (half-inhibitory concentration) values of 70 and 101 nM, respectively. However, the supermitogenic insulin analogue [AspB10]insulin competed significantly more efficiently for IGF-I binding (IC50: 44 nM). Maximum growth promoting activity of the peptides was then determined in serum-starved myoblasts by an incubation with the peptides (5 x 10(-7) M) for 16 h in the presence of [3H]thymidine. [Asp(B10)]Insulin produced a stimulation of DNA synthesis (about 3-fold) which was comparable to the effect of IGF-I and significantly (P < 0.005) higher than the effect of HOE 901 with the latter being essentially equipotent to native insulin. Comparable results were obtained at lower concentrations of the peptides (10(-9) to 10(-8) M). Metabolic activity of HOE 901 was determined by measuring the dose-dependent stimulation of 3-O-methylglucose transport in adult cardiomyocytes. Maximum transport stimulation was identical for insulin and HOE 901 with EC50 (half-effective concentration) values of 0.7 x 10(-10) and 1.9 x 10(-9) M, respectively. We concluded that the IGF-I receptor-mediated growth promoting activity of HOE 901 in muscle cells and the maximal metabolic activity of this analogue are not different from those of native human insulin. It is suggested that differential interaction with IGF-I receptors significantly contributes to the action profile of insulin analogues.

The role of the C-terminus of the insulin B-chain in modulating structural and functional properties of the hormone

Within the scope of structure-function studies on the proteohormone insulin, the role of the C-terminal segment B26-B30 for self-association and receptor interaction was analyzed. Insulin derivatives with modifications in the region B26-B30 were synthesized by trypsin-catalyzed coupling reactions of des-(B23-B30)-insulin with synthetic peptides. The peptides were obtained by Fmoc solid-phase peptide synthesis. Insulins with multiple amino acid-->glycine substitutions were examined to distinguish between the influence of the side chains and the influence of the main chain in positions B27-B30 on the self-association of the hormone. The analogues [GlyB27,B28,B29,B30]insulin and [GlyB27,B28,B30]insulin exhibit relative receptor affinities of 80% and self-associate. The successive extension of [AlaB26]des-(B27-B30)-insulin-B26-amide (relative receptor binding 273%) with amino acids corresponding to the native sequence B27-B30 showed the influence of the length of the B-chain on receptor affinity: the extension by B27-threonine amide reduces receptor binding to 71%, all further prolongations have only small effects on the binding. The effect of the B28-side chain on main-chain conformation, self-association and receptor binding was examined with [XB28]des-(B29-B30)-insulin-B28-amides (X = Phe, Gly, D-Pro). While the glycine and D-proline analogues (relative binding 104 and 143%, respectively) retain the self-association properties typical of insulin, [PheB28]des-(B29-B30)-insulin-B28-amide (relative binding 50%) shows diminished self-association. The backbone-modified insulin derivative [SarB26]des-(B27-B30)-insulin-B26-amide (sarcosine = N-methylglycine) exhibits an unexpectedly high receptor affinity of 1100% which demonstrates that the B26-amide hydrogen of the native hormone is not important for receptor binding.

5-Hydroxytryptophan: an absorption and fluorescence probe which is a conservative replacement for [A14 tyrosine] in insulin

Use of insulin's intrinsic tyrosine absorption and fluorescence to monitor its interaction with the insulin receptor is limited because the spectral properties of the receptor tryptophan residues mask the spectral properties of the hormone tyrosine residues. We describe the synthesis of an insulin analog where A14 tyrosine is replaced by a tryptophan analog, 5-hydroxytryptophan. This insulin is spectrally enhanced since 5-hydroxytryptophan has an absorption band above 300 nm which is at lower energies than the absorption of tryptophan. Steady-state and time-resolved fluorescence parameters indicate that 5-hydroxytryptophan reports the same information about the environment of the A14 side chain as does the corresponding tryptophan-containing insulin. The synthetic hormone is a full agonist compared to porcine insulin, but has slightly reduced specific activity. Consequently, this spectrally enhanced insulin analog will be useful for hormone-receptor interaction studies since it can be observed by both absorption and fluorescence even in the presence of the tryptophan-containing receptor.

Contribution of the B16 and B26 tyrosine residues to the biological activity of insulin

We report the synthesis and biological evaluation of five insulin analogues in which one or both of the B-chain tyrosine residues have been substituted. [B16 Phe]insulin and [B16 Trp]insulin display a very modest reduction in potency (c. 65%) relative to porcine insulin; [B26 Phe]insulin is less active (30-50%), and the doubly substituted [B16 Phe, B26 Phe]insulin displays still lower potency (c. 35%). The further substitution of Asp for B10 His in [B16 Phe, B26 Phe]insulin raises its activity to approximately twofold greater than natural insulin, an increase of approximately fivefold over the parent compound. We conclude that the bulk and/or aromaticity of the amino acid residue at position B16, but not its hydrogen-bonding capacity, contributes to the biological activity of the hormone. We further conclude that hydrogen-bonding capacity or special side-chain packing characteristics are required at the B26 position for insulin to display high biological activity.

The effect of placement of tryptophan residues in selected A-chain positions on the biological profile of insulin

In continuation of our efforts to study the solution structure and conformational dynamics of insulin by time-resolved fluorescence spectroscopy, we have synthesized and examined the biological activity of five insulin analogues in which selected naturally occurring residues in the A-chain have been replaced with the strongly fluorescent tryptophan residue. The potency of these analogues was evaluated in lipogenesis assays in isolated rat adipocytes, in receptor binding assays using rat liver plasma membranes, and in two cases, in receptor binding assays using adipocytes. [A3 Trp]insulin displays a potency of 3% in receptor binding assays in both liver membranes and in adipocytes, but only 0.06% in lipogenesis assays as compared to porcine insulin. [A10 Trp]insulin displays a potency of ca. 40% and ca. 25% in rat liver receptor binding and lipogenesis assays, respectively. [A13 Trp]insulin displays a potency of ca. 39% in rat liver receptor binding assays, but only ca. 9% in receptor binding in adipocytes; in lipogenesis assays, [A13 Trp]insulin displays a potency of ca. 12%, comparable to its potency in adipocyte receptor binding assays. [A15 Trp]insulin exhibits a potency of 18% and 9% in rat liver receptor binding and lipogenesis assays, respectively. The doubly substituted analogue, [A14 Trp, A19 Trp] insulin, displays a potency of ca. 0.7% in both rat liver receptor binding assays and lipogenesis assays.(ABSTRACT TRUNCATED AT 250 WORDS)

Recombinant-DNA-derived insulin analogues as potentially useful therapeutic agents

Whilst insulin is a spectacularly successful drug for the management of diabetes mellitus, it remains difficult to mimic the physiological pattern of insulin secretion, even using the various quick-acting and insoluble formulations that are available. The introduction of recombinant-DNA technology to the manufacture of therapeutic insulin has made the rational design and production of insulin analogues with altered pharmacokinetic and pharmacological properties possible. Such analogues include 'monomeric' insulins, which do not form the insulin-zinc hexamer in solution and are absorbed more rapidly from the injection site, and long-acting insulins, which are absorbed very slowly at physiological pH. Many of these analogues are being tested clinically, and it is possible that the next generation of insulin therapy will be various combinations of rationally designed insulin analogues produced by industrial biotechnology.

The A14 position of insulin tolerates considerable structural alterations with modest effects on the biological behavior of the hormone

As part of our aim to investigate the contribution of the tyrosine residue found in the 14 position of the A-chain to the biological activity of insulin, we have synthesized six insulin analogues in which the A14 Tyr has been substituted by a variety of amino acid residues. We have selected three hydrophilic and charged residues--glutamic acid, histidine, and lysine--as well as three hydrophobic residues--cycloleucine, cyclohexylalanine, and naphthyl-(1)-alanine--to replace the A14 Tyr. All six analogues exhibit full agonist activity, reaching the same maximum stimulation of lipogenesis as is achieved with porcine insulin. The potency for five of the six analogues, [A14 Glu]-, [A14 His]-, [A14 Lys]-, [A14 cycloleucine]-, and [A14 naphthyl-(1)-alanine]-insulins in receptor binding assays ranges from 40-71% and in stimulation of lipogenesis ranges from 35-120% relative to porcine insulin. In contrast, the potency of the sixth analogue, [A14 cyclohexylalanine]insulin, in both types of assays is less than 1% of the natural hormone. The retention time on reversed-phase high-performance liquid chromatography for the first five analogues is similar to that of bovine insulin, whereas for the sixth analogue, [A14 cyclohexylalanine]insulin, it is approximately 11 min longer than that of the natural hormone. This suggests a profound change in conformation of the latter analogue. Apparently, the A14 position of insulin can tolerate a wide latitude of structural alterations without substantial decrease in potency. This suggests that the A14 position does not participate directly in insulin receptor interaction. Only when a substitution which has the potential to disrupt the conformation of the molecule is made at this position, is the affinity for the receptor, and hence the biological potency, greatly reduced.