XIHbox 8, an endoderm-specific Xenopus homeodomain protein, is closely related to a mammalian insulin gene transcription factor

Role of transcription factors in the transdifferentiation of pancreatic islet cells

The α and β cells act in concert to maintain blood glucose. The α cells release glucagon in response to low levels of glucose to stimulate glycogenolysis in the liver. In contrast, β cells release insulin in response to elevated levels of glucose to stimulate peripheral glucose disposal. Despite these opposing roles in glucose homeostasis, α and β cells are derived from a common progenitor and share many proteins important for glucose sensing and hormone secretion. Results from recent work have underlined these similarities between the two cell types by revealing that β-to-α as well as α-to-β transdifferentiation can take place under certain experimental circumstances. These exciting findings highlight unexpected plasticity of adult islets and offer hope of novel therapeutic paths to replenish β cells in diabetes. In this review, we focus on the transcription factor networks that establish and maintain pancreatic endocrine cell identity and how they may be perturbed to facilitate transdifferentiation.

Treatment options for diabetes: potential role of stem cells

There are diseases and injuries in which a patient's cells or tissues are destroyed that can only be adequately corrected by tissue or organ transplants. Stem cells may be able to generate new tissue and even cure diseases for which there is no adequate therapy. Type 1 diabetes (T1DM), an insulin-dependent diabetes, is a chronic disease affecting genetically predisposed individuals, in which insulin-secreting beta (β)-cells within pancreatic islets of Langerhans are selectively and irreversibly destroyed by autoimmune assault. Type 2 diabetes (T2DM) is characterized by a gradual decrease in insulin sensitivity in peripheral tissues and the liver (insulin resistance), followed by a gradual decline in β-cell function and insulin secretion. Successful replacing of damaged β-cells has shown considerable potential in treating T1DM, but lack of adequate donors is a barrier. The literature suggests that embryonic and adult stem cells are promising alternatives in long-term treatment of diabetes. However, any successful strategy should address both the need for β-cell replacement and controlling the autoimmune response to cells that express insulin. This review summarizes the current knowledge of options and the potential of stem cell transplantation in diabetes treatment.

α-cell role in β-cell generation and regeneration

This review considers the role of α-cells in β-cell generation and regeneration. We present recent evidence obtained from lineage-tracing studies showing that α-cells can serve as progenitors of β-cells and present a hypothetical model how injured β-cells might activate α-cells in adult islets to promote β-cell regeneration. β-cells appear to arise by way of their trans-differentiation from undifferentiated α progenitor cells, pro-α-cells, both during embryonic development of the islets and in the adult pancreas in response to β-cell injuries. Plasticity of α-cells is endowed by the expression of the gene encoding proglucagon, a prohormone that can give rise to glucagon and glucagon-like peptides (GLPs). The production of glucagon from proglucagon is characteristic of fully-differentiated α-cells whereas GLP-1 is a product of undifferentiated α-cells. GLP-1, a cell growth and survival factor, is proposed to promote the expansion of neurogenin3-expressing, undifferentiated pro-α-cells during development. β-cells arise from pro-α-cells by a change in the relative amounts of the transcription factors Arx and Pax4, master regulators of the α- and β-cell lineages, respectively. A paracrine/autocrine model is proposed whereby injuries of β-cells in adult islets induce the production and release of factors, such as stromal cell-derived factor-1, that cause the de-differentiation of adjacent α-cells into pro-α-cells. Pro-α-cells produce GLP-1 and its receptor that renders them competent to trans-differentiate into β-cells. The trans-differentiation of pro-α-cells into β-cells provides a potentially exploitable mechanism for the regeneration of β-cells in individuals with type 1 diabetes.

A feat of metabolic proportions: Pdx1 orchestrates islet development and function in the maintenance of glucose homeostasis

Emerging evidence over the past decade indicates a central role for transcription factors in the embryonic development of pancreatic islets and the consequent maintenance of normal glucose homeostasis. Pancreatic and duodenal homeobox 1 (Pdx1) is the best studied and perhaps most important of these factors. Whereas deletion or inactivating mutations of the Pdx1 gene causes whole pancreas agenesis in both mice and humans, even haploinsufficiency of the gene or alterations in its expression in mature islet cells causes substantial impairments in glucose tolerance and the development of a late-onset form of diabetes known as maturity onset diabetes of the young. The study of Pdx1 has revealed crucial phenotypic interrelationships of the varied cell types within the pancreas, particularly as these impinge upon cellular differentiation in the embryo and neogenesis and regeneration in the adult. In this review, we describe the actions of Pdx1 in the developing and mature pancreas and attempt to unify these actions with its known roles in modulating transcriptional complex formation and chromatin structure at the molecular genetic level.

Targeted deletion of a cis-regulatory region reveals differential gene dosage requirements for Pdx1 in foregut organ differentiation and pancreas formation

Pdx1 (IPF-1 in humans, which is altered in MODY-4) is essential for pancreas development and mature beta-cell function. Pdx1 is expressed dynamically within the developing foregut, but how its expression characteristics are linked to the various steps of organ specification, differentiation, and function is unknown. Deletion of a conserved enhancer region (Area I-II-III) from Pdx1 produced a hypomorphic allele (Pdx1(DeltaI-II-III)) with altered timing and level of expression, which was studied in combination with wild-type and protein-null alleles. Lineage labeling in homozygous Area I-II-III deletion mutants (Pdx1(DeltaI-II-III/DeltaI-II-III)) revealed lack of ventral pancreatic bud specification and early-onset hypoplasia in the dorsal bud. Acinar tissue formed in the hypoplastic dorsal bud, but endocrine maturation was greatly impaired. While Pdx1(-/-) (protein-null) mice have nonpancreatic abnormalities (e.g., distorted pylorus, absent Brunner's glands), these structures formed normally in Pdx1(DeltaI-II-III/DeltaI-II-III) and Pdx1(DeltaI-II-III/-) mice. Surprisingly, heterozygous (Pdx1(+/DeltaI-II-III)) mice had abnormal islets and a more severe prediabetic condition than Pdx1(+/-) mice. These findings provide in vivo evidence of the differential requirements for the level of Pdx1 gene activity in the specification and differentiation of the various organs of the posterior foregut, as well as in pancreas and gut endocrine cell differentiation.

Transcription factors direct the development and function of pancreatic beta cells

Transcription factors orchestrate intricate pathways of cellular growth and differentiation by regulating the rate of transcription of an array of genes. Genetic and biochemical studies have begun to unravel the complex cascade of factors that controls the proliferation and differentiation of cells in the developing pancreas. The specific pathway leading to the development of the insulin-secreting beta cell has been a focus of many of these studies because an understanding of the transcription factors governing this pathway will be crucial to the engineering of new beta cells to cure diabetes. In recent years, the number of transcription factors that has been implicated in beta-cell differentiation and function has grown considerably. Here, we outline the known role of transcription factors in beta-cell development, and describe how these factors form a network of gene activation signals that mediates insulin transcription.

Initiation and early patterning of the endoderm

We review the early stages of endoderm formation in the major animal models. In Amphibia maternal molecules are important in initiating endoderm formation. This is followed by successive signaling events that establish and then pattern the endoderm. In other organisms there are differences in endodermal development, particularly in the initial, prephylotypic stages. Later many of the same key families of transcription factors and signaling cassettes are used in all animals, but more work will be needed to establish exact evolutionary homologies.

PDX:PBX complexes are required for normal proliferation of pancreatic cells during development

The homeobox factor PDX-1 is a key regulator of pancreatic morphogenesis and glucose homeostasis; targeted disruption of the PDX-1 gene leads to pancreatic agenesis in pdx-1(-/-) homozygotes. Pdx-1 heterozygotes develop normally, but they display glucose intolerance in adulthood. Like certain other homeobox proteins, PDX-1 contains a consensus FPWMK motif that promotes heterodimer formation with the ubiquitous homeodomain protein PBX. To evaluate the importance of PDX-1:PBX complexes in pancreatic morphogenesis and glucose homeostasis, we expressed either wild-type or PBX interaction defective PDX-1 transgenes under control of the PDX-1 promoter. Both wild-type and mutant PDX-1 transgenes corrected glucose intolerance in pdx-1 heterozygotes. The wild-type PDX-1 transgene rescued the development of all pancreatic lineages in pdx-1(-/-) animals, and these mice survived to adulthood. In contrast, pancreata from pdx-1(-/-) mice expressing the mutant PDX-1 transgene were hypoplastic, and these mice died within 3 weeks of birth from pancreatic insufficiency. All pancreatic cell types were observed in pdx-1(-/-) mice expressing the mutant PDX-1 transgene; but the islets were smaller, and increased numbers of islet hormone-positive cells were noted within the ductal epithelium. These results indicate that PDX-1:PBX complexes are dispensable for glucose homeostasis and for differentiation of stem cells into ductal, endocrine, and acinar lineages; but they are essential for expansion of these populations during development.

Transforming growth factor beta induction of insulin gene expression is mediated by pancreatic and duodenal homeobox gene-1 in rat insulinoma cells

Although transforming growth factor-beta (TGF-beta) stimulates pancreatic islet cells to synthesize and secret insulin, the mechanism underlying this effect is not known. To investigate this question, we examined the insulin promoter activity focusing on a transcription factor, pancreatic and duodenal homeobox gene-1 (PDX-1) that binds to the A3 element of the rat insulin promoter. Studies performed using the rat insulinoma cell line, INS-1 showed that TGF-beta stimulation of endogenous insulin mRNA expression correlated with increased activity of a reporter construct containing the insulin promoter. A potential mechanism for this increase arose from, electrophoretic mobility shift assay showing that the nuclear extract from TGF-beta treated cells contained higher levels of A3 binding activity. Western blot analysis confirmed that PDX-1 was increased in the nuclear extract from INS-1 cells treated with TGF-beta. As expected, a mutant insulin promoter that lacked the PDX-1 binding site was not stimulated by TGF-beta. In summary, the results of these studies show that TGF-beta stimulates the transcription of insulin gene and this action is mediated by the transcription factor, PDX-1.

Exploring the role of homeobox and zinc finger proteins in pancreatic cell proliferation, differentiation, and apoptosis

Transcription factors are DNA binding proteins that regulate gene expression in response to a large variety of extracellular stimuli, and thereby act as key molecular switches for controlling cell differentiation, proliferation, and apoptosis. During the last decade, a myriad of these proteins have been identified and classified into different structural families, including homeobox, zinc finger, leucine zipper, and helix-loop-helix transcription factors. Members of the homeobox and zinc finger superfamilies are among the best-characterized transcription factors known to act as potent regulators of normal development in organisms ranging from insects to humans. In addition, mutations or aberrant expression in genes encoding these proteins can result in neoplastic transformation in several different cell types, further supporting their role as "guardians" of normal cell growth and differentiation. Therefore, the purpose of this article is to review this field of research with a particular emphasis on the role of homeobox- and zinc finger-containing transcription factors in pancreatic cell growth, cell differentiation, and apoptosis. The potential participation of these proteins in neoplastic transformation is also discussed.