Delivery of DNA encoding exendin-4 linked with secretion signal peptide for the protection of Ins-1beta cells from apoptosis under hypoxia

Abstract

INTRODUCTION

Islet transplantation is a promising strategy for treating diabetes. However, problems such as a shortage of transplantable islet supply, immune rejection after transplantation, and ischemic apoptosis of islet cells have limited the clinical application of islet transplantation. Glucagon like peptide-1 (GLP-1) is an insulinotrophic protein (1). Also, GLP-1 inhibited pancreatic islet cell death in islet transplantation. GLP-1 has a very short half-life in serum, since it is degraded rapidly by dipeptidyl peptidase. An analog, exendin-4, has been shown to last longer than GLP-1. In this research, we constructed an exendin-4 expression plasmid, pβ-Ex-4. To increase the secretion of exendin-4, a secretion signal peptide (SP) was inserted into the expression plasmid, resulting in the construction of pβ-SP-Ex-4. To evaluate β cell protection under hypoxia, the exendin-4 expression plasmids were transfected into Ins-1 β cells using high molecular polyethylenimine (25 kDa, PEI25k) and the cells were incubated under hypoxic condition.

EXPERIMENTAL METHODS

Exendin-4 cDNA was synthesized chemically and inserted into the pβ vector at the EcoRI and XbaI sites. The DNA fragment encoding the exendin-4 secretion SP was synthesized chemically and inserted into pβ-Ex-4 at the KpnI and EcoRI sites (Fig. 1) to create pβ-SP-Ex-4. Restriction enzyme studies and direct sequencing confirmed correct construction of the plasmids. The plasmids were trasnfected into Ins-1 beta cells and the transfected cells were incubated at the desired concentration of oxygen (normoxia, 20% oxygen; hypoxia, 1% oxygen) for 20 hrs. After the incubation, EIA(Enzyme Immunoassay), MTT assay and caspase-3 assay and were performed.

RESULTS AND DISCUSSION

Exendin-4 cDNA was synthesized chemically and inserted into the pβ vector, resulting in construction of pβ-Ex-4. The DNA fragment encoding the exendin-4 secretion SP was synthesized chemically and inserted into pβ-Ex-4 at the (Fig. 1). To identify the effect of the SP on the cytoprotection under hypoxic condition, pβ-Ex-4 and pβ-SP-Ex-4 were transfected into Ins-1 β cells. The cells were incubated under normoxia or hypoxia. After the incubation, exendin-4 expression was measured by EIA. In the cells transfected with pβ-SP-Ex-4, the exendin-4 level in the medium was higher than that in the cells transfected with pβ?Ex-4, regardless of oxygen concentration.

CONCLUSIONS

The results suggest that the facilitated exendin-4 secretion through the incorporation of a secretion signal peptide may be useful to enhance the therapeutic effect and provide a useful strategy for β cell transplantation therapy.