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 such as insulin and insulin-like growth factor (IGF-I) and others. Insulin and IGF-I receptors which possibly feedback on insulin release are established in insulin-secreting cells. The role of tyrosine kinase in insulin secretion is controversial. Both the tyrosine kinase inhibitors tyrphostin 25 (TYR) and genistein (GEN), but not its structurally similar 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 diadzein. The inhibitory effects of either insulin or IGF-I on insulin release are abolished by 10(-4) M GEN but not by daidzein indicating an involvement of tyrosine kinase in the inhibitory effect of both insulin and IGF-I on insulin release. Since GEN was argued not to be specific for tyrosine kinase, several second messengers were investigated. cAMP is not influenced. The insulinotropic effect of acutely administered TPA is 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) pretreatment has no effect on the inhibitory effects of IGF-I on insulin release, a PT-sensitive G-protein is not likely to be involved. The data indicate that tyrosine kinase is involved in the inhibitory effects of insulin and IGF on insulin release in INS-1 cells, possibly mediating the negative feedback effect.