Stimulation of insulin release is uncoupled from insulin biosynthesis in tumoral RINm5F cells

RINm5F cells, an insulin-secreting subclone of a rat insulinoma cell line, were incubated in serum-free medium up to 24 hours in the presence or absence of glucagonlike peptide-1(7-36)amide in various concentrations, 3-isobutyl-1 methylxanthine (1 mM), choleratoxin (10 nM), carbachol (1 mM), and potassium (40 mM). Insulin release and biosynthesis were measured by the immunoreactive insulin content of the cells and the medium. Steady-state levels of insulin-specific mRNA were determined by Northern and slot blot analysis. Short-term insulin release is significantly stimulated by all secretagogues tested. A significant increase of insulin biosynthesis by any of the various secretagogues was not detectable on the peptide and mRNA level. Sodium butyrate (1 mM), a differentiating agent, increased insulin-specific mRNA levels in RINm5F cells after 72 hours. In conclusion, substances known to stimulate short-term insulin release in RINm5F cells do not stimulate insulin biosynthesis, indicating an uncoupling of insulin secretion and biosynthesis in these transformed beta cells.