ROLE OF TYROSINE KINASE IN INSULIN RELEASE IN AN INSULIN-SECRETING CELL-LINE (INS-1)

Tyrosine kinases are involved in cell signalling of growth factors suc h as insulin and insulin-like growth factor (IGF-I) and others. Insuli n and IGF-I receptors which possibly feedback on insulin release are e stablished in insulin-secreting cells. The role of tyrosine kinase in insulin secretion is controversial. Both the tyrosine kinase inhibitor s tyrphostin 25 (TYR) and genistein (GEN), but not its structurally si milar albeit biologically inactive analogue daidzein, increase insulin release at 16.7 mM glucose in INS-1 cells, an insulin secreting cell line. Tyrosine kinase activity is inhibited by GEN, but not daidzein. The inhibitory effects of either insulin or IGF-I on insulin release a re abolished by 10(-4) M GEN but not by daidzein indicating an involve ment of tyrosine kinase in the inhibitory effect of both insulin and I GF-I on insulin release. Since GEN was argued not to be specific for t yrosine kinase, several second messengers were investigated. cAMP is n ot influenced. The insulinotropic effect of acutely administered TPA i s not influenced by GEN while in protein kinase C (PKC)-downregulated cells the insulinotropic effect of GEN is preserved: both indicate no involvement of PKC in GEN effect. Since pertussis toxin (PT) pretreatm ent has no effect on the inhibitory effects of IGF-I on insulin releas e, a PT-sensitive G-protein is not Likely to be involved. The data ind icate that tyrosine kinase is involved in the inhibitory effects of in sulin and IGF on insulin release in INS-1 cells, possibly mediating th e negative feedback effect.