Characterization of viral insulins reveals white adipose tissue-specific effects in mice

Abstract

Objective Members of the insulin/insulin-like growth factor (IGF) superfamily are well conserved across the evolutionary tree. We recently showed that four viruses in the Iridoviridae family possess genes that encode proteins highly homologous to human insulin/IGF-1. Using chemically synthesized single-chain (sc), i.e., IGF-1-like, forms of the viral insulin/IGF-1-like peptides (VILPs), we previously showed that they can stimulate human receptors. Because these peptides possess potential cleavage sites to form double chain (dc), i.e., more insulin-like, VILPs, in this study, we have characterized dc forms of VILPs for Grouper iridovirus (GIV), Singapore grouper iridovirus (SGIV) and Lymphocystis disease virus-1 (LCDV-1) for the first time. Methods The dcVILPs were chemically synthesized. Using murine fibroblast cell lines overexpressing insulin receptor (IR-A or IR-B) or IGF1R, we first determined the binding affinity of dcVILPs to the receptors and characterized post-receptor signaling. Further, we used C57BL/6J mice to study the effect of dcVILPs on lowering blood glucose. We designed a 3-h dcVILP in vivo infusion experiment to determine the glucose uptake in different tissues. Results GIV and SGIV dcVILPs bind to both isoforms of human insulin receptor (IR-A and IR-B) and to the IGF1R, and for the latter, show higher affinity than human insulin. These dcVILPs stimulate IR and IGF1R phosphorylation and post-receptor signaling in vitro and in vivo. Both GIV and SGIV dcVILPs stimulate glucose uptake in mice. In vivo infusion experiments revealed that while insulin (0.015 nmol/kg/min) and GIV dcVILP (0.75 nmol/kg/min) stimulated a comparable glucose uptake in heart and skeletal muscle and brown adipose tissue, GIV dcVILP stimulated 2-fold higher glucose uptake in white adipose tissue (WAT) compared to insulin. This was associated with increased Akt phosphorylation and glucose transporter type 4 (GLUT4) gene expression compared to insulin in WAT. Conclusions Our results show that GIV and SGIV dcVILPs are active members of the insulin superfamily with unique characteristics. Elucidating the mechanism of tissue specificity for GIV dcVILP will help us to better understand insulin action, design new analogs that specifically target the tissues and provide new insights into their potential role in disease.

A viral insulin-like peptide is a natural competitive antagonist of the human IGF-1 receptor

“…This change in relative activity at the insulin and IGF-1 receptors is directionally comparable to that reported for human IGF-1 [ 4 , 27 ]. The insulin receptor potency of the two-chain analogs of GIV-VILP and SGIV-VILP has been separately observed to approach the IR activity of IGF-1 [ 34 ]. However, LCDV1-VILP is different from the two other viral insulin-like peptides.…”

“…The two-chain form of LCDV1-VILP is distinctly different and displays agonism at the IGF-1R, indicating that the C-chain is important for antagonism. It is also a low potency agonist at both insulin receptor isoforms, something that has not been reported when these peptides were prepared by a different procedure and studied at lower concentrations [ 34 ]. Through synthetic shuffling of the A, B, and C chains of LCDV1-VILP and IGF-1, the central importance of the B-chain to the differences in LCDV1-VILP and IGF-1 bioactivities was identified.…”

Evolution of the Insulin Gene: Changes in Gene Number, Sequence, and Processing

“…However, like the incompletely processed vertebrate insulin sequences described above, the VILPs share the conserved cysteine residues involved in disulphide bridging and are requisite to the maintain the 3D structure (51). Some VILPs have been shown to bind to insulin receptors and regulate glucose metabolism in mice (51,52), indicating that it may function in its proinsulin form, and that they might have a pathophysiological role beneficial to the viruses. Insulin-like molecules are used as toxins by cone snails (53), thus the use of an insulin-like peptide by a virus in pathophysiology should not be a surprise.…”

Viruses and Metabolism: The Effects of Viral Infections and Viral Insulins on Host Metabolism

“…In our recent study, we focused on the characterization of chemically synthesized dc versions of GIV, SGIV, and LCDV-1 VILPs ( 104 ). We showed that GIV and SGIV dcVILPs bind and stimulate both human IR and IGF-1R in vitro and in vivo.…”

“…We showed that GIV dcVILP stimulated significantly higher (approximately two fold) glucose uptake in white adipose tissue (WAT) compared to insulin. However, no differences were observed in the other insulin-sensitive tissues (skeletal muscle, brown adipose tissue, and heart) ( 104 ). This effect in WAT was associated with higher AKT phosphorylation and GLUT4 gene expression upon GIV dcVILP stimulation compared to insulin.…”