POU Domain Transcription Factor brain 4 Confers Pancreatic α-Cell-Specific Expression of the Proglucagon Gene Through Interaction with a Novel Proximal Promoter G1 Element

Abstract

The proglucagon gene is expressed in a highly restricted tissue-specific manner in the ␣ cells of the pancreatic islet, the hypothalamus, and the small and large intestines. Proglucagon is processed to glucagon and glucagon-like peptides GLP-1 and -2. Glucagon is expressed in ␣ cells and regulates glucose homeostasis. GLP-1 is implicated in the control of insulin secretion, food intake, and satiety signaling, and GLP-2 is implicated in regulating small-bowel growth. Cell-specific expression of the proglucagon gene is mediated by proteins that interact with the proximal G1 promoter element which contains several AT-rich domains with binding sites for homeodomain transcription factors. In an attempt to identify major homeodomain proteins involved in pancreatic ␣-cell-specific proglucagon expression, we found that the POU domain transcription factor brain 4 is abundantly expressed in proglucagon-producing islet cell lines and rat pancreatic islets. In the latter, brain 4 and glucagon immunoreactivity colocalize in the outer mantle of islets. Electrophoretic mobility shift assays with specific antisera identify brain 4 as a major constituent of nuclear proteins of glucagonproducing cells that bind to the G1 element of the proglucagon gene proximal promoter. Transcriptional transactivation experiments reveal that brain 4 is a major regulator of proglucagon gene expression by its interaction with the G1 element. The finding that a neuronal transcription factor is involved in glucagon gene transcription may explain the presence of proglucagon in certain areas of the brain as well as in pancreatic ␣ cells. Further, this finding supports the idea that the neuronal properties of endodermis-derived endocrine pancreatic cells may find their basis in regulation of gene expression by neuronal transcription factors.

Protein expression changes in a cell system of beta-cell maturation reflect an acquired sensitivity to IL-1?

“…It is generally believed that the endocrine, exocrine and ductal cell types are derived from endodermal cells [5]. Several studies have shown that the endocrine stem cell require several specific transcription factors to mature into single-hormone-expressing cells [6,7,8,9]. The pancreatic duodenum homeobox 1 (PDX-1) has been shown to be important both for pancreas development [26] and subsequent maturation of the endocrine cells [27], whereas later in the mature islets PDX-1 expression is restricted to the insulin-producing beta cells [8].…”

“…The pancreatic duodenum homeobox 1 (PDX-1) has been shown to be important both for pancreas development [26] and subsequent maturation of the endocrine cells [27], whereas later in the mature islets PDX-1 expression is restricted to the insulin-producing beta cells [8]. Another transcription factor, Nkx-6.1, is important for maturation of the insulin-producing beta cells [8], whereas Brain-4 is important for maturation of the glucagon-producing α-cells [7]. Previous analysis of mRNA gene expression profiles by semi-quantitative multiplex RT-PCR of the two NHI-phenotypes used in this study, grown under the same conditions, showed that several genes associated with late stages of betacell maturation, such as PDX-1 were expressed in both phenotypes, whereas Nkx-6.1 was restricted to the beta-cell phenotype [20].…”

“…During development of the pancreas, all four endocrine cell-types (α, β, δ and pp cells) are believed to arise from the same stem cell [5] and this specialization is dependent upon activation of different transcription factors [6,7,8,9]. Since sensitivity to cytokines, free radicals and toxic chemicals is exquisite for the beta cell, this could reflect an acquired trait during the maturation of stem cells into mature insulin producing beta cells.…”

Interaction of Maf Transcription Factors with Pax-6 Results in Synergistic Activation of the Glucagon Promoter

“…The ␣-cell-specific glucagon gene expression in pancreas is conferred by the proximal G1 promoter element (11). The G1 element contains two AT-rich sequences that are recognized by the homeodomain containing Cdx-2/3, Pax-6, and brain-4 (12)(13)(14). Pax-6 and Cdx-2/3 have been shown to bind directly to each other and to transactivate synergistically the glucagon gene via their interaction with the G1 element (14).…”

Transcriptional regulation of α‐cell differentiation

“…Pax6 binds to and transactivates the G3A element of the preproglucagon promoter [57]. Therefore, Pax6 plays a crucial but not indispensable role in the development and hormone production of endocrine cells, including α-cells.Brain4 (Brn4), a POU domain transcription factor, is activated in first-wave glucagon-expressing cells around E10 [56,59]. Around E19.5, its expression is restricted to the mantle of the islet [56], and in the adult rat pancreas it is only Neurogenic differentiation factor 1 (NeuroD1)-deficient mice exhibit severe diabetes with hyperglycaemia and ketonuria as early as postnatal day 2, and die perinatally [61].…”