Expression of normal and novel glucokinase mRNAs in anterior pituitary and islet cells

The glucose-phosphorylating enzyme glucokinase likely plays an important role in regulating glucose-stimulated insulin secretion from the islets of Langerhans and has previously been thought to be expressed only in that tissue and in liver. In this study, we demonstrate high levels of glucokinase mRNA in the anterior pituitary cell line AtT20_ins, which has been engineered to secrete correctly processed insulin, as well as in primary anterior pituitary tissue. Unlike islet or liver cells, expression of glucokinase mRNA in anterior pituitary cells was not accompanied by expression of the high K_m glucose transporter (GLUT-2) mRNA. The glucokinase transcript in anterior pituitary cells was similar in size to islet glucokinase mRNA, which has a unique, elongated 5′-end relative to the liver glucokinase message. Amplification and sequence analysis of the glucokinase mRNA expressed in islets, RIN1046-38 cells, and anterior pituitary cells confirmed that the glucokinase transcripts in these cell types contain the same 5′-sequence. In addition, a novel alternative transcript was identified that contains a 52-nucleotide deletion and that predicts a 58-amino acid peptide as a result of a frame shift. Both the deleted and undeleted transcripts were found in islets, RIN cells, and AtT20_ins cells, whereas only the deleted product was identified in primary anterior pituitary tissue. An antibody prepared against a peptide found at the N terminus of the islet isoform of glucokinase easily detected a protein with a size predicted by the undeleted transcript in extracts prepared from islets, RIN1046-38 cells, and AtT20_ins cells. Since both the glucokinase protein and mRNA are naturally expressed in AtT20_ins and RIN1046-38 cells, we compared the effect of varying concentrations of glucose on insulin secretion from the two lines. Insulin secretion from RIN1046-38 cells was stimulated by glucose in a dose-dependent manner over the range 0-2.5 mM, where it reached a maximum. AtT20_ins cells, in contrast, exhibited no response to glucose at any concentration tested, despite the fact that insulin secretion from both cell lines was stimulated by incubation with dibutyryl cAMP. We conclude that glucokinase expression in AtT20_ins cells may be necessary, but is not sufficient to confer glucose-stimulated insulin secretion.