Regulation and function of AP-1 in insulinoma cells and pancreatic ß-cells.

Cav1.2 L-type voltage-gated Ca2+ channels play a central role in pancreatic ß-cells by integrating extracellular signals with intracellular signaling events leading to insulin secretion and altered gene transcription. Here, we investigated the intracellular signaling pathway following stimulation of Cav1.2 Ca2+ channels and addressed the function of the transcription factor activator protein-1 (AP-1) in pancreatic ß-cells of transgenic mice. Stimulation of Cav1.2 Ca2+ channels activates AP-1 in insulinoma cells. Pharmacological and genetic experiments identified c-Jun N-terminal protein kinase as a signal transducer connecting Cav1.2 Ca2+ channel activation with gene transcription. Moreover, the basic region-leucine zipper proteins ATF2 and c-Jun or c-Jun-related proteins were involved in stimulus-transcription coupling. We addressed the functions of AP-1 in pancreatic ß-cells analyzing a newly generated transgenic mouse model. These transgenic mice expressed A-Fos, a mutant of c-Fos that attenuates DNA binding of c-Fos dimerization partners. In insulinoma cells, A-Fos completely blocked AP-1 activation following stimulation of Cav1.2 Ca2+ channels. The analysis of transgenic A-Fos-expressing mice revealed that the animals displayed impaired glucose tolerance. Thus, we show here for the first time that AP-1 controls an important function of pancreatic ß-cells in vivo, the regulation of glucose homeostasis.

Zn2+ ions inhibit gene transcription following stimulation of the Ca2+ channels Cav1.2 and TRPM3.

Zinc, a trace element, is necessary for the correct structure and function of many proteins. Therefore, Zn2+ has to be taken up by the cells, using specific Zn2+ transporters or Ca2+ channels. In this study, we have focused on two Ca2+ channels, the L-type voltage-gated Cav1.2 channel and the transient receptor potential channel TRPM3. Stimulation of either channel induces an intracellular signaling cascade leading to the activation of the transcription factor AP-1. The influx of Ca2+ ions into the cytoplasm is essential for this activity. We asked whether extracellular Zn2+ ions affect Cav1.2 or TRPM3-induced gene transcription following stimulation of the channels. The results show that extracellular Zn2+ ions reduced the activation of AP-1 by more than 80% following stimulation of either voltage-gated Cav1.2 channels or TRPM3 channels. Experiments performed with cells maintained in Ca2+-free medium revealed that Zn2+ ions cannot replace Ca2+ ions in inducing gene transcription via stimulation of Cav1.2 and TRPM3 channels. Re-addition of Ca2+ ions to the cell culture medium, however, restored the ability of these Ca2+ channels to induce a signaling cascade leading to the activation of AP-1. Secretory cells, including neurons and pancreatic ß-cells, release Zn2+ ions during exocytosis. We propose that the released Zn2+ ions function as a negative feedback loop for stimulus-induced exocytosis by inhibiting Ca2+ channel signaling.

Ternary complex factor regulates pancreatic islet size and blood glucose homeostasis in transgenic mice.

A hallmark of diabetes mellitus is the inability of pancreatic ß-cells to secrete sufficient amounts of insulin for maintaining normoglycemia. The formation of smaller islets may underlie the development of a diabetic phenotype, as a decreased ß-cell mass will produce an insufficient amount of insulin. For a pharmacological intervention it is crucial to identify the proteins determining ß-cell mass. Here, we identified the ternary complex factor (TCF) Elk-1 as a regulator of the size of pancreatic islets. Elk-1 mediates, together with a dimer of the serum-response factor (SRF), serum response element-regulated gene transcription. Elk-1 is activated in glucose-treated pancreatic ß-cells but the biological functions of this protein in ß-cells are so far unknown. Elk-1 and homologous TCF proteins are expressed in islets and insulinoma cells. Gene targeting experiments revealed that the TCF proteins show redundant activities. To solve the problem of functional redundancy of these homologous proteins, we generated conditional transgenic mice expressing a dominant-negative mutant of Elk-1 in pancreatic ß-cells. The mutant competes with the wild-type TCFs for DNA and SRF-binding. Expression of the Elk-1 mutant in pancreatic ß-cells resulted in the generation of significantly smaller islets and increased caspase-3 activity, indicating that apoptosis was responsible for the reduction of the pancreatic islet size. Glucose tolerance tests revealed that transgenic mice expressing the dominant-negative mutant of Elk-1 in pancreatic ß-cells displayed impaired glucose tolerance. Thus, we show here for the first time that TCF controls important functions of pancreatic ß-cells in vivo. Elk-1 may be considered as a new therapeutic target for the treatment of diabetes.

Pharmacological inhibition of TRPM8-induced gene transcription.

Transient receptor potential melastatin-8 (TRPM8) channels are activated by cold temperature, menthol and icilin, leading to cold sensation. TRPM8 activation is connected with various diseases, indicating that a specific pharmacological antagonist, allowing nongenetic channel suppression, would be a valuable tool for therapy and basic research. Here, we assessed the biological activity and specificity of various TRPM8 inhibitors following stimulation of TRPM8 channels with either icilin or menthol. Recently, we showed that icilin strikingly upregulates the transcriptional activity of AP-1. By measuring AP-1 activity, we assessed which compound interrupted the TRPM8-induced intracellular signaling cascade from the plasma membrane to the nucleus. We tested the specificity of various TRPM8 inhibitors by analyzing AP-1 activation following stimulation of TRPM3 and TRPV1 channels, L-type voltage-gated Ca2+ channels, and Gαq-coupled receptors, either in the presence or absence of a particular TRPM8 inhibitor. The results show that the TRPM8 inhibitors BCTC, RQ-00203078, TC-1 2014, 2-APB, and clotrimazole blocked TRPM8-mediated activation of AP-1. However, only the compound RQ-00203078 showed TRPM8-specificity, while the other compounds function as broad-spectrum Ca2+ channel inhibitors. In addition, we show that progesterone interfered with TRPM8-induced activation of AP-1, as previously shown for TRPM3 and TRPC6 channels. TRPM8-induced transcriptional activation of AP-1 was additionally blocked by the compound PD98059, indicating that extracellular signal-regulated protein kinase-1/2 is essential to couple TRPM8 stimulation with transcriptional activation of AP-1. Moreover, an influx of Ca2+-ions is essential to induce the intracellular signaling cascade leading to the activation of AP-1.

Calcineurin controls gene transcription following stimulation of a Gαq-coupled designer receptor.

Stimulation of Gaq-coupled receptors triggers the activation of gene transcription via a rise of intracellular Ca2+. To investigate the role of the Ca2+/calmodulin-dependent phosphatase calcineurin in regulating transcription following Gαq-coupled receptor stimulation, we used a gain-of-function approach and expressed ΔCnA, a constitutively active mutant of calcineurin A. Furthermore, we expressed hM3Dq, a designer receptor that is specifically coupled to Gαq and can be activated by the pharmacological compound clozapine-N-oxide. Stimulation of hM3Dq or expression of ΔCnA induced transcription of a reporter gene controlled by the calcineurin substrate nuclear factor of activated T cells (NFAT), suggesting that calcineurin increased NFAT-regulated gene transcription. In contrast, expression of ΔCnA attenuated hM3Dq-induced biosynthesis of the transcription factors c-Fos and Egr-1 and reduced both c-Fos and Egr-1 promoter activities. A dissection of the c-Fos and Egr-1 promoters revealed that calcineurin inhibited serum response element-mediated transcription. In particular, the expression of ΔCnA reduced the transcriptional activity of the ternary complex factor Elk-1 following stimulation of hM3Dq receptors. Furthermore, ΔCnA reduced the transcriptional activity of the transcription factor CREB and thus attenuated transcription mediated by the cAMP response element. In summary, we show that calcineurin functions as a positive and negative modulator of gene transcription.

Stimulation of B-Raf increases c-Jun and c-Fos expression and upregulates AP-1-regulated gene transcription in insulinoma cells.

Stimulation of pancreatic ß-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 ß-cell.