Stimulation of pancreatic β-cells with glucose activates the protein kinases B-Raf and extracellular signal-regulated protein kinase that participate in glucose sensing. Inhibition of both kinases results in impairment of glucose-regulated gene transcription. To analyze the signaling pathway controlled by B-Raf, we expressed a conditionally active form of B-Raf in INS-1 insulinoma cells. Here, we show that stimulation of B-Raf strongly activated the transcription factor AP-1 which is accompanied by increased c-Jun and c-Fos promoter activities, an upregulation of c-Jun and c-Fos biosynthesis, and elevated transcriptional activation potentials of c-Jun and c-Fos. Mutational analysis identified the AP-1 sites within the c-Jun promoter and the serum response element (SRE) within the c-Fos promoter as the essential genetic elements connecting B-Raf stimulation with AP-1 activation. In line with this, the transcriptional activation potential of the SRE-binding protein Elk-1 was increased following B-Raf activation. The signal pathway from B-Raf to AP-1 required the activation of c-Jun. We identified the cyclin D1 gene as a delayed response gene for AP-1 following stimulation of B-Raf in insulinoma cells. Moreover, MAP kinase phosphatase-1 and the Ca²⁺/calmodulin-dependent protein phosphatase calcineurin were identified to function as shut-off-devices for the signaling cascade connecting B-Raf stimulation with the activation of AP-1. The fact that stimulation with glucose, activation of L-type voltage-gated Ca²⁺ channels, and stimulation of B-Raf all trigger an activation of AP-1 indicates that AP-1 is a point of convergence of signaling pathways in β-cell.

用葡萄糖刺激胰岛β细胞激活参与葡萄糖感测的蛋白激酶B-Raf和细胞外信号调节蛋白激酶。两种激酶的抑制均导致葡萄糖调节的基因转录受损。为了分析B-Raf控制的信号通路，我们在INS-1胰岛素瘤细胞中表达了B-Raf的有条件活性形式。在这里，我们表明，B-Raf的刺激强烈激活了转录因子AP-1，并伴随着c-Jun和c-Fos启动子活性的增加，c-Jun和c-Fos生物合成的上调以及转录激活电位的升高c-Jun和c-Fos。突变分析确定c-Jun启动子内的AP-1位点和c-Fos启动子内的血清反应元件（SRE）是将B-Raf刺激与AP-1激活相联系的基本遗传元件。与此相符，B-Raf激活后，SRE结合蛋白Elk-1的转录激活潜能增加。从B-Raf到AP-1的信号通路需要激活c-Jun。我们确定细胞周期蛋白D1基因为胰岛素瘤细胞中B-Raf刺激后AP-1的延迟反应基因。此外，MAP激酶磷酸酶-1和钙 我们确定细胞周期蛋白D1基因为胰岛素瘤细胞中B-Raf刺激后AP-1的延迟反应基因。此外，MAP激酶磷酸酶-1和钙 我们确定细胞周期蛋白D1基因为胰岛素瘤细胞中B-Raf刺激后AP-1的延迟反应基因。此外，MAP激酶磷酸酶-1和钙^{2 +} /钙调蛋白依赖性蛋白磷酸酶钙调神经磷酸酶被确定为关闭信号传导级联的装置，该信号级联将B-Raf刺激与AP-1的激活联系起来。葡萄糖刺激，L型电压门控Ca ²⁺通道激活和B-Raf刺激均触发AP-1激活这一事实表明，AP-1是β-信号传导途径的汇合点。细胞。