gamma-Tocopherol partially protects insulin-secreting cells against functional inhibition by nitric oxide

Preceding the onset of type 1 diabetes mellitus, pancreatic islets are infi ltrated by macrophages secreting interleukin-1 beta (IL-1 beta) which induc es beta -cell apoptosis and exerts inhibitory actions on islet beta -cell i nsulin secretion. IL-1 beta seems to act chiefly through induction of nitri c oxide (NO) synthesis, Hence, IL-1 beta and NO have been implicated as key effector molecules in type 1 diabetes mellitus. In this paper, the influen ce of endogenously produced and exogenously delivered NO on the regulation of cell proliferation, cell viability and discrete parts of the stimulus-se cretion coupling in insulin-secreting RINm5F cells was investigated. Becaus e vitamin E may delay diabetes onset in animal models, we also investigated whether tocopherols may protect beta -cells from the suppressive actions o f IL-1 and NO in vitro. To this end, the impact of NO on insulin secretory responses to activation of phospholipase C (by carbamylcholine), protein ki nase C (by phorbol ester), adenylyl cyclase (by forskolin), and Ca2+ influx through voltage-activated Ca2+ channels (by K+-induced depolarization) was monitored in culture after treatment with IL-1 beta or by co-incubation wi th the NO donor spermine-NONOate. It was found that cell proliferation, via bility, insulin production and the stimulation of insulin release evoked by carbamylcholine and phorbol ester were impeded by IL-1 beta or spermine-NO NOate, whereas the hormone output by the other secretagogues was not altere d by NO. Pretreatment with gamma -tocopherol (but not gamma -tocopherol) af forded a partial protection against the inhibitory effects of NO, whereas s pecifically inhibiting inducible NO synthase with N-nitro-L-arginine comple tely reversed the IL-1 beta effects. In contrast, inhibiting guanylyl cycla se with ODQ (1H-[1,2,4]oxadiazolo[4,3-alpha ]quinoxaline-1-one) or blocking low voltage-activated Ca2+ channels with NiCl2 failed to influence the act ions of NO. In conclusion, our data show that NO inhibits growth and insuli n secretion in RINm5F cells, and that gamma -tocopherol may partially preve nt this, The results suggest that phospholipase C or protein kinase C may b e targeted by NO, In contrast, cGMP or low voltage-activated Ca2+ channels appear not to mediate the toxicity of NO in these cells. These adverse effe cts of NO on the beta -cell, and the protection by gamma -tocopherol, may b e of importance for the development of the impaired insulin secretion chara cterizing type 1 diabetes mellitus, and offer possibilities for interventio n in this process. (C) 2000 Academic Press.