Regulation of insulin secretion from novel engineered insulinoma cell lines

In the accompanying article, we describe the creation of novel cell lines derived from RIN 1046-38 rat insulinoma cells by stable transfection with combinations of genes encoding human insulin, GLUT2, and glucokinase. Herein we describe the regulation of insulin secretion and glucose metabolism in these new cell lines. A cell line (betaG I/17) expressing only the human proinsulin transgene exhibits a clear increase in basal insulin production (measured in the absence of secretagogues) relative to parental RIN 1046-38 cells. betaG I/17 cells engineered for high levels of GLUT2 expression and a twofold increase in glucokinase activity (betaG 49/206) or engineered for a 10-fold increase in glucokinase activity alone (betaG 40/110) exhibit a 66% and 80% suppression in basal insulin secretion relative to betaG I/17 cells, respectively. As a result, betaG 49/206 and betaG 40/110 cells exhibit potent insulin-secretory responses to glucose alone (6.1- and 7.6-fold, respectively) or to glucose plus isobutylmethylxanthine (10.8- and 15.1-fold, respectively) that are clearly larger than the corresponding responses of betaG I/17 or parental RIN 1046-38 cells. betaG 49/206 and betaG 40/110 cells also exhibit a rapid and sustained response to glucose plus isobutyl-methylxanthine in perifusion studies that is clearly larger in magnitude than that of the two control lines. Glucose dose-response studies show that both engineered and non-engineered lines respond maximally to submillimolar concentrations of glucose and that betaG 49/206 cells are the most sensitive to low concentrations of the hexose, consistent with their clearly elevated rate of [5-3H]glucose usage. Finally, 5-thioglucose, a potent inhibitor of low-K(m) hexokinases, most effectively normalizes glucose concentration dependence for insulin secretion in the cell line with highest glucokinase expression (betaG 40/110). We conclude that GLUT2 and/or glucokinase expression imposes tight regulation of basal insulin secretion in cell lines that overexpress human proinsulin, allowing a marked improvement in the range of secretagogue responsiveness in such cells.