Activation of Elk-1, an Ets transcription factor, by glucose and EGF treatment of insulinoma cells

Elk-1, a member of the ternary complex factor family of Ets domain proteins that bind serum response elements, is activated by phosphorylation in a ce ll-specific manner in response to growth factors and other agents. The purp ose of the current study was to determine whether Elk-1 activation contribu tes to glucose-/depolarization-induced Ca2+-dependent induction of immediat e early response genes in pancreatic islet beta -cells. The results of expe riments in insulinoma (MIN6) cells demonstrated that Elk-1-binding sites (E ts elements) in the Egr-1 gene promoter contribute to transcriptional activ ation of the gene. Treatment with either epidermal growth factor (EGF), a k nown inducer of beta -cell hyperplasia, glucose, or KCl-induced depolarizat ion resulted in Ser(383) phosphorylation and transcriptional activation of Elk-1 (4 +/- 0.3-, P = 0.003, 2.3 +/- 0.19-, P = 0.002, and 2.2 +/- 0.1- fo ld, P = 0.001 respectively). The depolarization response was inhibited by t he Ca2+ channel blocker verapamil and by the MEK inhibitor PD98059 (53 +/- 6 and 55 +/- 0.5%, respectively). EGF-induced activation of Elk-1 was also inhibited by PD98059 (60 +/- 5%). A dominant negative Ras produced partial inhibition (42%) of the depolarization-induced Elk-1 transcriptional activa tion. Transfection with a constitutively active Ca2+/calmodulin kinase IV p lasmid also resulted in Elk-1 transcriptional activation. Experiments with p38, phosphatidylinositol 3-kinase, and protein kinase A inhibitors indicat ed that these pathways are not involved. We conclude that Elk-1 activation contributes to glucose-/depolarization-induced Ca2+ dependent induction of immediate early growth response genes in pancreatic islet beta -cells. Furt hermore, the results demonstrated a convergence of nutrient- and growth fac tor-mediated signaling pathways on Elk-1 activation through induction of Ra s/mitogen-activated protein kinase ERK-1 and -2. The role of these pathways in the glucose-induced proliferation of islet beta -cells can now be asses sed.