Induction of insulin and islet amyloid polypeptide production in pancreatic islet glucagonoma cells by insulin promoter factor 1.

Abstract

Insulin promoter factor 1 (IPF1), a member of the homeodomain protein family, serves an early role in pancreas formation, as evidenced by the lack of pancreas formation in mice carrying a targeted disruption of the IPF1 gene [Jonsson, J., Carlsson, L., Edlund, T. & Edlund, H. (1994) Nature (London) 371,[606][607][608][609]. In adults, IPF1 expression is restricted to the a-cells in the islets of Langerhans. We report here that IPF1 induces expression of a subset of 8-cell-specific genes (insulin and islet amyloid polypeptide) when ectopically expressed in clones of transformed pancreatic islet a-cells. In contrast, expression of IPF1 in rat embryo fibroblasts factor failed to induce insulin and islet amyloid polypeptide expression. This is most likely due to the lack of at least one other essential insulin gene transcription factor, the basic helix-loop-helix protein Beta2/NeuroD, which is expressed in both a-and P-cells. We conclude that IPF1 is a potent transcriptional activator of endogenous insulin genes in non-8 islet cells, which suggests an important role of IPF1 in a-cell maturation.Insulin promoter factor 1 (IPF1) is expressed in precursor cells during pancreas ontogeny (1, 2), and expression is required for pancreas formation (3,4). During ontogeny, IPF1 expression becomes restricted to the nuclei of the insulin-producing pancreatic islet (3-cells, suggesting that maintenance of IPF1 expression is necessary for the differentiation islet p3-cells from an IPF1-positive precursor common to all islet cells (2, 5). This restricted expression profile within the islets is reflected in the transplantable rat pancreatic insulinoma (IN) and glucagonoma (AN), which show substantial similarity to the mature islet f3-and a-cells, respectively (6-8). Thus, the AN is lacking IPF1 expression, as is the normal a-cell, and was recently found to be similar to normal a-cells in its expression of glucokinase as well as of the glucose-regulated insulinotropic peptide and glucagon-like peptide 1 receptors (9, 10). In vitro IPF1 binds to multiple sites in the insulin promoter and activates insulin gene reporter constructs when cotransfected into cell lines (1,5,11,12). This activity is dependent on cooperation between IPF1 and insulin enhancer factor-1 (IEF-1; refs. 5 and 12), a heterodimer composed of Beta2/ NeuroD, which is present in both a-and (3-cells, and ubiquitous class A helix-loop-helix proteins, such as Betal/rat E-box binding protein (REB; ref. 13) and products of the E2A gene (E47, E12, and ITF-1; refs. 14-19). In addition to IPF1 and IEF-1 binding sites, transcriptional regulation of the insulin gene requires a number of other cis-elements to which factors not yet cloned are binding (20)(21)(22)(23). To address whether IPF1 could activate transcription of the otherwise silent insulin genes in islet cells lacking IPF1 but expressing at least a subset of the other insulin gene transcription factors, a cDNA encoding rat IPF1 (24) under transcriptional control of the cytomegalovirus promoter was stably tra...

IB1 Reduces Cytokine-induced Apoptosis of Insulin-secreting Cells

“…The MSL AN697C1 subclone gave rise to two derived cell lines, namely the glucagon-secreting AN-glu, and after stable transfection with the transcription factor pancreatic duodenal homeobox factor-1, the insulin secreting ANins (27). Despite having similar rates of NO synthesis, we found that the AN-ins cells were more susceptible to apoptosis elicited by IL-1␤.…”

Experimental Models of Transcription Factor-Associated Maturity-Onset Diabetes of the Young

“…The in vivo study on mice with ␤-cell-specific deletion of PDX-1 (36) and our in vitro work on INS-1 cells expressing DN-Pdx-1 (31) indicate that PDX-1 promotes the ␤-cell phenotype by maintaining the expression of ␤-cell-specific genes while suppressing glucagon levels. Suppression of glucagon expression by PDX-1 requires synergism with other ␤-cell-specific transcription factors because ectopic expression of PDX-1 in glucagonoma cells failed to eliminate glucagon (40). We also showed that maximum induction of Brain-4, an ␣-cell-specific transcription factor, initiated detectable levels of glucagon but did not attenuate the expression of ␤-cell-specific genes in INS-1 cells (31).…”

Transcriptional networks controlling pancreatic development and beta cell function

“…A similar expression profile results from dominant negative suppression of Pdx1 in INS1 beta cells [85]. In vitro binding and transient transfection studies suggest that Pdx1 interacts directly with the 5′ flanking regions of these four genes [86,87,88,89], while ChIP experiments have confirmed that Pdx1 binds in vivo to the insulin and IAPP genes in cultured beta cell lines [90,91].…”