Type 1 diabetes is a metabolic disorder caused by the autoimmune destruction of insulin-producing pancreatic -cells, which results in a lack of insulin;1 therefore, diabetic patients must take daily insulin injections. However, multiple daily insulin injections are cumbersome and do not perfectly mimic -cell function, which can result in the development of chronic complications such as renal failure, retinopathy, neuropathy, and heart disease. Considerable research has been directed into developing more efficient methods to deliver physiological levels of insulin that will result in the tight regulation of blood glucose levels.
Islet transplantation is one possible approach to replace conventional exogenous insulin therapy. However, the scarcity of islets from cadaveric donors and autoimmune attack of transplanted islets are limitations of this approach, motivating research into other sources of insulin-producing cells for replacement therapy for type 1 diabetes. Several cell sources have been investigated for the development of surrogate -cells, such as hepatocytes2,34,5,6 and neuroendocrine cells7 genetically engineered to produce insulin, but no satisfactory results have been obtained because these cells lack critical -cell characteristics.8 For example, hepatocytes express glucose transporter 2 (GLUT2) and glucokinase (GK), which are required for glucose sensing, but do not have secretory vesicles to control insulin release quickly in response to glucose. Neuroendocrine cells possess secretory vesicles but do not have glucose-sensing systems.
Enteroendocrine cells are hormone-producing epithelial cells located in the gut lining.9 K cells are one subpopulation of enteroendocrine cells and are located mainly in the stomach, duodenum, and jejunum. They secrete glucose-dependent insulinotropic peptide (GIP) in a glucose-dependent manner, which acts on pancreatic -cells to stimulate the rapid release of insulin after a meal.10,11,12 Secretion of GIP is very similar to that of insulin after oral glucose ingestion in humans.13 In addition, K cells contain the prohormone convertases PC2 and PC1/3, which are needed to process proinsulin to insulin.14,15 It was recently reported that transgenic mice that express human insulin under the control of the GIP promoter are resistant to streptozotocin (STZ)-induced diabetes and that insulin secretion in these mice was glucose-responsive.14 These findings encouraged us to investigate the use of K cells as a target for insulin gene therapy. In this study, we generated a stable enteroendocrine cell line expressing insulin under the control of the GIP promoter and showed that these cells produced insulin in a glucose-dependent manner. Transplantation of these cells into STZ-induced diabetic non-obese diabetic/severe combined immunodeficiency (NOD.scid) mice decreased blood glucose levels to the normal range and cleared glucose appropriately after exogenous glucose loading.