SODIUM-BUTYRATE INDUCES NEUROENDOCRINE CYTODIFFERENTIATION IN THE INSULINOMA CELL-LINE RINM5F

The differentiating agent sodium butyrate inhibits proliferation and s timulates cell-specific hormone expression in rat insulinoma cells. In this study, we investigated the effect of sodium butyrate on neuroend ocrine cytodifferentiation in the rat insulinoma subclone, RINm5F. The cells were cultured with 0.5, 1, or 1.5 mM sodium butyrate for up to 72 h. Ultrastructurally, cells cultured with 1 mM sodium butyrate reve aled a more differentiated appearance with an induction of cellular co mpartments involved in regulated insulin secretion. Morphometric analy sis showed a significant elevation of neuroendocrine granule density. The total area of the specific granules was increased after incubation with 1 mM sodium butyrate for 48 and 72 h. Proliferation of RINm5F ce lls was inhibited by sodium butyrate in a dose-dependant manner. DNA p roduction ceased completely within 24 h at 1.5 mM sodium butyrate. Thi s concentration of sodium butyrate increased the cellular insulin cont ent 8.9-fold and the insulin production 2-fold after 72 h. The insulin release was reduced from 79 +/- 3.5% in controls to 37 +/- 5.6% of to tal in a 24-h incubation period after 3 days of culture with 1.5 mM so dium butyrate. Insulin mRNA levels increased to a maximum of 324% comp ared with controls after 48 h of culture with 1.5 mM sodium butyrate. Chromogranin A mRNA levels increased to a similar extent (368 +/- 26%) , whereas sodium butyrate did not stimulate the expression of synaptop hysin, a major membrane component of small neuroendocrine vesicles. In conclusion, our data suggest the selective induction of neuroendocrin e cytodifferentiation by sodium butyrate in RINm5F cells. These cells obviously offer a suitable model to study the regulation of neuroendoc rine differentiation and neuroendocrine granule biosynthesis.