Glucagon-like peptide-1 (GLP-1) enhances insulin secretion and synthesis. It also regulates the insulin, glucokinase, and GLUT2 genes. It mediates increases in glucose-stimulated insulin secretion by activating adenylyl cyclase and elevating free cytosolic calcium levels in the beta-cell. In addition, GLP-1 has been shown, both in vitro and in vivo, to be involved in regulation of the transcription factor, pancreatic duodenal homeobox-1 protein (PDX-1), by increasing its total protein levels, its translocation to the nucleus and its binding and resultant increase in activity of the insulin gene promoter in beta-cells of the pancreas. Here we have investigated the role of protein kinase A (PKA) in these processes in RIN 1046-38 cells. Three separate inhibitors of PKA, and a cAMP antagonist, inhibited the effects of GLP-1 on PDX-1. Furthermore, forskolin, (which stimulates adenylyl cyclase and insulin secretion), and 8-Bromo-cAMP, (an analog of cAMP which also stimulates insulin secretion), mimicked the effects of GLP-1 on PDX-1. These effects were also prevented by PKA inhibitors. Glucose-mediated increases in nuclear translocation of PDX-1 were not prevented by PKA inhibitors. Our results suggest that regulation of PDX-1 by GLP-1 occurs through activation of adenylyl cyclase and the resultant increase in intracellular cAMP, in turn, activates PKA, which ultimately leads to increases in PDX-1 protein levels and translocation of the protein to the nuclei of beta-cells.

Glucagon-like peptide-1 (GLP-1) enhances insulin biosynthesis and secretion as well as transcription of the insulin, GLUT2 and glucokinase genes. The latter are also regulated by the PDX-1 homeoprotein. We investigated the possibility that GLP-1 may be having its long-term pleiotropic effects through a hitherto unknown regulation of PDX-1. We found that PDX-1 mRNA level was significantly increased (p<0.01) after 2 hours and insulin mRNA level was subsequently increased (p<0.01) after 3 hours of treatment with GLP-1 (10 nM) in RIN 1046-38 insulinoma cells. Under these experimental conditions, there was also a 1.6-fold increase in the expression of PDX-1 protein in whole cell and nuclear extracts. Overexpression of PDX-1 in these cells confirmed the finding of the wild type cells such that GLP-1 induced a 2-fold increase in whole cell extracts and a 3-fold increase in nuclear extracts of PDX-1 protein levels. The results of electrophoretic mobility shift experiments showed that PDX-1 protein binding to the Al element of the rat insulin II promoter was also increased 2 h post treatment with GLP-1. In summary, we have uncovered a previously unknown aspect to the regulation of PDX-1 in beta cells. This has important implications in the physiology of adult beta cells and the treatment of type 2 diabetes mellitus with GLP-1 or its analogs.

Ligand binding to the angiotensin II (Ang II) AT1 receptor on vascular smooth muscle cells (VSMCs) activates the Janus-activated kinase (JAK)/signal transducers and activators of transcription (STAT) pathway. We have shown previously that the JAK2 tyrosine kinase and the Src family p59 Fyn tyrosine kinase are required for Ang II-induced STAT1 tyrosine phosphorylation in VSMCs. The mitogen-activated protein kinase phosphatase, MKP-1, is required for STAT1 tyrosine dephosphorylation. In the present study, using specific enzyme inhibitors and antisense oligonucleotides, we show that Ang II-induced tyrosine phosphorylation and nuclear translocation of STAT3 in VSMCs is mediated by p60 c-Src, whereas tyrosine dephosphorylation is mediated by calcineurin. Calcineurin is activated in response to Ang II stimulation of VSMCs and is translocated to the nucleus. In addition, we show that Ang II-induced serine phosphorylation of STAT3 in VSMCs is mediated by mitogen-activated protein kinase and that dephosphorylation is mediated by protein phosphatase 2A (PP2A). PP2A translocates to the nucleus in response to Ang II stimulation of VSMCs and forms a complex with STAT3 in an Ang II-dependent manner.

In this article, we show that glucagon-like peptide 1 (GLP-1) can induce AR42J cells to differentiate into insulin, pancreatic polypeptide, and glucagon-positive cells. In their natural state, these cells, which are derived from a chemically induced pancreatic tumor, possess exocrine and neuroendocrine properties but are negative for islet hormones and their mRNAs. We found that when these cells were exposed to GLP-1 (1 or 10 nmol), a peptide normally released from the gut in response to food and a modulator of insulin release, intracellular cAMP levels were increased, and proliferation of cells was increased for the first 24 h, followed by inhibition. Up to 50% of the cells became positive for islet hormones. The mRNAs for glucose transporter 2 and glucokinase were detected in the GLP-1-treated cells. Insulin was detected by radioimmunoassay (RIA) in the medium of GLP-1-treated cells, and the cells were capable of releasing insulin in a glucose-mediated fashion. Exendin-4, an analog of GLP-1, in some critical experiments performed in a similar manner to GLP-1, with the exception of it being 10-fold more potent. We therefore propose that GLP-1 and exendin-4 are capable of causing pancreatic precursor cells to differentiate into islet cells.