Involvement of CaMK II in the Genistein-Induced Potentiation of Insulin Secretion.

Lee, Soo Jin; Jang, Hyun Ju; Kim, Hyo-E; Choi, Sung-E; Park, Tae Jin; Jo, Young Eun; Kim, Yen Kyung; Jung, Sun Hye; Kim, Hae Jin; Kim, Dae Jung; Lee, Kwan Woo; Kang, Yup

Genistein, an isoflavonoid phytoestrogen isolated from soybean seed, possesses tyrosine kinase inhibition activity and potentiates insulin secretion. Although the potentiation effect is glucose-dependent, the cellular mechanism involved in the potentiation effect remains unclear. First, we investigated the genistein-induced potentiation of insulin secretion by other insulinotropic agents in INS-1 cells. Genistein also potentiated the insulin secretion triggered by nutrients [leucine/glutamine (Leu/GIn) or pyruvate],but not the effect of calcium influx enhancers (tolbutamide, KCI, or Bay K8644) on secretion, suggesting that genistein augments amplification signals in the secretion process. In addition, genistein potentiated glimepiride-induced insulin secretion, suggesting that glimepiride elicits other signals, as well as a calcium influx. Second, we investigated the involvement of PKA, PKC, and CaMK II in amplifying the genistein-induced potentiation. Studies using pharmacological inhibitors showed that a CaMK II inhibitor reduced the genistein-induced potentiation effect, while PKA and PKC inhibitors did not. Furthermore, immunoblotting studies demonstrated that genistein significantly enhanced the phospbo-CaMK II level elicited by Leu/GIn and glimepiride. Third, to determine whether calcium is involved in the genistein-induced enhancement of CaMK II phosphorylation, we measured the intracellular calcium level. Leu/Gin and glimepiride increased the intracellular calcium level and genistein augmented the Leu/GIn- and glimepirideinduced calcium increase. Last, to determine whether the potentiation effect of genistein is due to tyrosine kinase inhibition activity, we investigated the potentiation effect of another tyrosine kinase inhibitor (tyrostatin 25). Tyrostatin 25 potentiated insulin secretion triggered by Leu/Gin, but it did not potentiate the secretion triggered by glimepiride, suggesting that the potentiation effect of genistein is involved elsewhere in addition to tyrosine kinase inhibition activity. In conclusion, our findings suggest that genistein potentiates insulin secretion via the calcium-mediated augmentation of CaMK II activation.

PHYTOESTROGENS; PROTEIN-tyrosine kinases; INSULIN; SECRETION; PROTEIN kinases; CALCIUM in the body; PHOSPHORYLATION

Diabetes, 2007, Vol 56, pA432

0012-1797

Academic Journal