INS-1 cells secrete not only insulin but also glucagon

The rat insulinoma cell line INS-1 is the most commonly used clonal cell model in pancreatic beta-cell research. In contrast to freshly prepared islets of Langerhans as an integrative cell model of the endocrine pancreas, INS-1 cells are believed to reflect the behaviour of isolated beta-cells. We examined as to how INS-1 cells comply with this notion with respect to their endocrine activity.

Over a time course of 2.5 hours, INS-1 cells (passage 45–55) were exposed to steadily increasing glucose concentrations between 1.4 mM and 22.4 mM. Every thirty minutes glucose concentration was doubled and aliquots were taken to determine glucagon and corresponding immunoreactive insulin. Finally, cells were lysed and hormone and protein content were measured (n=5). Preproglucagon-mRNA was determined by real time LightCycler rT-PCR. Corresponding experiments with freshly prepared rat pancreatic islets were performed (n=3). The cumulative release of glucagon in INS-1 cells was 0.54±0.06 pmol per mg protein, representing 39±4% of the secretory output from pan-creatic islets (1.42±0.06 pmol per mg protein). Cumulative insulin secretion of INS-1 cells increased glucose-dependently, amounting to 70±11 pmol per mg protein (97±17% of that from corresponding islets). Cellular content of glucagon of INS-1 cells was 7±1 pmol per mg lysate protein which was only 1.3±0.1% of that from islets (487±65 pmol per mg protein). Similarly, the preproglucagon-mRNA content of INS-1 cells (standardized for GAPDH) accounted for 3.0±0.4% of that from islets.

INS-1 cells secrete substantial amounts of glucagon. Preproglucagon mRNA and intracellular stores of glucagon protein are less than in pancreatic islets. This may reflect differences in the intracellular metabolism and / or secretory pathway. Nonetheless, it is obvious that INS-1 do not represent an exclusively insulin producing beta-cell line.