Regulation of Insulinoma Cell Proliferation and Insulin Accumulation by Peptides and Second Messengers

Abstract

The regulation of clonal rat insulinoma (RINm5F) cell roliferation and hormone capable of inducing differentiation, i.e. decreased growth and increased insulin accumulation, by the tumor cells. In particular, interest was focused on the role of a number of peptides as well as pharmacological robes modulating various signal proliferation and insulin accumulation. Growth hormone stimulated insulin accumulation and inhibited DNA synthesis, whereas galanin and insulin-like growth factor I caused a moderate suppression of insulin accumulation but did not affect proliferation, while epidermal growth factor, transforming growth factor i3, platelet-derived growth factor, acidic and basic fibroblast growth factor, bradykinin and somatostatin were virtually inactive on all parameters tested. Exo enous a significant decrease in both l3-cell mitogenesis and insulin accumulation, effects that were mediated through nitric oxide eneration. The vitamin A derivative release but did not affect mitogenesis. By contrast, y-tocopherol was inactive on both these parameters. The a-adrenergic agonist clonidine evoked a slight inhibition of serum-stimulated DNA synthesis, without influencing insulin accumulation, whereas phenylephrine did not affect any of these parameters. Carbamylcholine increased insulin accumulation, but not cell proliferation, whereas the adenylyl cyclase activator forskolin suppressed mito enesis but did not affect insulin accumulation. Inhibition of protein kinase C! with staurosporine or prolonged treatment with phorbol ester suppressed DNA s nthesis, as did the tyrosine kinase with glibenclamide enhanced DNA synthesis, while openin o these channels with diazoxide suppressed cell growth. Conversely, preventing Fa2+ influx by the Ca2+ channel antagonist D-600, chelating intracellular Ca2+ by fura-2 AM or inhibiting the Ca2+ /calmodulin-dependent protein kinase by calmidazol resulted in a decreased DNA synthesis. On the other hand, uncontrolled influx or mobilization of Ca2+ by ionomycin or tha sigar in resulted in an arrested DNA synthesis. accumulation can be moddated by various peptidergic and pharmacological agents regulating certain signal transduc tion However, mitogenesis in the insulinoma cells seeming1 is contro led in a vastly different manner in studK , 1.e. that rowth hormone, contrarily to its effect on normal kcells, suppresses insu inoma cel F growth, merits further elucidation of the underlying mechanisms. Possibly the hormone might become of utility in a clinical setting in the treatment of patients with insulin-producing tumors.