Diabetes mellitus is a metabolic disorder associated with abnormally high blood glucose levels in patients. Untreated or improperly treated diabetes can result in acute complications that can lead to death or serious long-term complications. Therefore, several approaches have been considered to develop effective and convenient treatments for diabetes, such as gene therapy [1
]. In the last decade, several gene therapy strategies have been developed to regulate physiologic blood glucose levels in patients with diabetes. One option that has been studied is to replace pancreatic β-cells with engineered non-pancreatic β-cells to generate potential target cells for gene therapy. These surrogate β-cells were able to synthesize and secrete active insulin; however, insulin secretion was not regulated by glucose, unlike endogenous β-cells [4
]. Recent studies using enteroendocrine cells have shown the good potential for using these cells for diabetes gene therapy. This is primarily because of their ability to respond dynamically to a nutrient stimulus.
Enteroendocrine cells are located in the intestinal mucosa and secrete incretin hormones such as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1). GIP and GLP-1 are naturally secreted from gut K and L-cells, respectively. K-cells are mainly scattered in the duodenum whereas L-cells are predominantly found in the distal ileum. Both GIP and GLP-1 are rapidly secreted following food intake and return to the basal level following their degradation into inactive forms [8
]. Many signaling factors including nutrients, neural elements, and hormones are capable of regulating the secretion of incretins although nutrients in the lumen strongly stimulate GIP and GLP-1 secretion [10
]. It was reported that GIP and GLP-1 hormones induce insulin secretion from pancreatic β-cells to control blood glucose homeostasis. These two hormones are also involved in many other metabolic pathways. For example, GIP promotes β-cell proliferation, whereas GLP-1 potently inhibited glucagon secretion and induces gastric emptying and satiety [12
Some properties of enteroendocrine cells make them appealing candidates for surrogate β-cells [13
]. First, K and L-cells express prohormone convertases 2 and 3, which are required to process proinsulin to mature insulin [14
]. Second, the expression of glucokinase and glucose transporter II in K and L-cells provides a glucose sensitive system similar to that of pancreatic β-cells [15
]. Third, K and L-cells have cell-specific promoters like GIP and GLP-1, respectively. Finally, K and L-cells are easily accessible. Some research groups have studied insulin expression under the control of a GIP promoter and reported that K-cells secrete insulin in response to different nutrient stimuli at different levels in vitro
and in vivo
]. In other investigations, engineered L-cells were used as the host cells for insulin expression and viral promoters were used to express the insulin gene [19
]. By contrast, the proglucagon promoter was used in our previous study, in which we examined the expression of insulin in L-cells [22
]. These observations using K and L-cells raised a critical question concerning which intestinal cells should be used in the context of diabetes gene therapy for effective regulation of glucose homeostasis.
Therefore, the aim of this study was to evaluate and compare insulin expression efficiency in engineered K and L-cells maintained in identical conditions. To achieve this aim, pure K and L-cells were isolated from a heterogeneous population of intestinal cells (STC-1 cells) as basic models for gene expression studies. Then, we examined insulin expression in response to stimulation by glucose and meat hydrolysate (MH) in both cell types. The results of the expression studies provide useful information on the competency of both intestinal cell lines for synthesizing and secreting insulin. We found that meat hydrolysate is a potent stimulator of insulin expression in both types. Interestingly, we found no significant difference in insulin expression between the two cell types. Taken together, our results provide evidence to show that gut K and L-cells can be used to study gene expression or establish cell-based therapies. Our assessment of insulin gene expression in these cells also indicates that K and L-cells are potential candidate hosts for gene therapeutic treatment of diabetes or related disorders.