Polyamine requirements in nicotinamide-stimulated beta-cell differentiation in fetal porcine islet-like cell clusters

The impact of nicotinamide on regulation of cell replication and hormone production by fetal porcine islet-like cell clusters (ICCs) containing a low fraction of beta-cells was investigated. For this purpose, ICCs were produced in tissue culture for 4 days in the presence or absence of nicotinamide (10 mM). ICCs formed in the presence of nicotinamide showed increased rates of (pro)insulin biosynthesis, contained more insulin, and displayed elevated contents of polyamines when compared with untreated ICCs. Immunocytochemical analyses of autoradiographed ICCs disclosed that nicotinamide expanded the relative size of the beta-cell population, and that the vitamin increased DNA synthesis of non-beta-cells only. The possibility that the effects of nicotinamide were conveyed by the increased polyamine content was explored by giving nicotinamide together with inhibitors of rate-limiting enzymes of polyamine biosynthesis, a maneuver that precluded the increases in polyamines of whole ICCs. Although this treatment prevented the increases in beta-cell population size, insulin production, and insulin content evoked by nicotinamide, the elevated cell replication nonetheless persisted. We conclude that the stimulatory effects of nicotinamide on insulin production and content by fetal porcine ICCs result from neoformation of beta-cells through differentiation. Since these effects were prevented by blocking the increased polyamine content, it is suggested that a stimulated synthesis of polyamines may be an important event in mediating the differentiating action of nicotinamide.