Wednesday , November 22 2017
Home / Resources / Clinical Gems / International Textbook of Diabetes Mellitus, 4th Ed., Excerpt #25: Insulin Gene Expression and Biosynthesis Part 1 of 6

International Textbook of Diabetes Mellitus, 4th Ed., Excerpt #25: Insulin Gene Expression and Biosynthesis Part 1 of 6

DeFronzoCoverIntroduction

The unique property of the pancreatic β cell is its ability to secrete insulin to enable circulating glucose levels to be maintained within a narrow physiologic range, despite wide fluctuations in energy intake and expenditure. It is able to sense the glucose concentration in the extracellular milieu, and adapt its insulin secretion rate via a complex interplay between nutrients, hormones, and neuronal signals. Whereas the minute-to-minute regulation of insulin secretion occurs at the level of exocytosis of preformed insulin, adaptation to long-term changes in the environment also involves regulated changes in the transcription rate of the insulin gene, translation of the mRNA, and processing of the proinsulin molecule into fully mature insulin. These processes are coordinately regulated by glucose (Figure 6.1) under normal circumstances, and their perturbation leads to β-cell dysfunction and type 2 diabetes.

ITDMFig6.1The first section of this chapter focuses on the structure of the insulin gene, normal regulation of its transcription, and dysregulation under pathologic circumstances. In the second section, the successive steps leading from translation of the insulin mRNA molecule to the storage of mature insulin into readily releasable secretory granules are presented, as well as the metabolic regulation of these processes. Space limitations prevent us from exhaustively citing the work of all investigators who contributed to this field. The reader is encouraged to refer to the cited review articles for complete reference lists.

Insulin gene expression

Structure of the insulin gene

The insulin gene is specifically expressed in pancreatic β cells, although low levels of expression have been detected in the brain [1], the thymus [2], and in the yolk sac during fetal development [3]. The gene’s sequence is highly conserved throughout evolution, and is present as a single copy in most species, including humans, where it is located on chromosome 11 between the genes for tyrosine hydroxylase and insulin-like growth factor 2. In rodents, there are two nonallelic insulin genes (I and II), resulting from duplication of the original insulin II gene [4]. The human insulin gene contains three exons and two introns. The first intron is within the 5′-untranslated region, whereas the second intron interrupts the C-peptide coding sequence. The mature preproinsulin mRNA is 446 base pairs (bp) long.

Pancreatic β-cell-enriched gene transcription is controlled by a number of proteins, including both positive-acting islet- enriched transcription factors including Pax6, Pancreatic and duodenal homeobox-1 (Pdx-1), Neurogenic differentiation 1 (NeuroD1/Beta2), and v-maf musculoaponeurotic fibrosarcoma oncogene homolog A (MafA) [5,6], and ubiquitously distributed transcriptional activators responsive to specific signaling pathways (e.g. activating transcription factor-2 responsive to Ca+2 signaling [7]; nuclear factor of activated T cells (NFAT) responsive to Ca+2 [8]; kinases such as extracellular-regulated kinases (ERK)1/2 [9]; the transcriptional repressors CCAAT/enhancer-binding protein beta (C/EBPβ) [10] and c-Jun [11]; phosphatases (e.g. calcineurin [12]); and coactivators such as p300 [13]. Insulin gene expression is principally controlled by a highly conserved region lying approximately 340bp upstream of the transcription initiation start, termed the enhancer/promoter control region [14,15]. Consid- erable progress has been made in defining the many different cis– and trans-acting factors that ensure precise transcriptional regulation, with the focus here on describing the β-cell-enriched transcription factors most pertinent to metabolically regulated expression, specifically Pdx-1, NeuroD1/Beta2, and MafA.

Pdx-1 is a homeodomain protein that plays a major role in pancreatic β-cell development and function [16,17]. It primarily binds as a monomer to the conserved AT-rich A3 box (-201/-196 bp) and activates insulin transcription, although this protein also appears to act as a repressor in other gene contexts [18]. Pdx-1 is produced early in rodent pancreatic progenitors, and is essential to acinar, ductal, and islet endocrine cell formation [5]. Both homozygous and heterozygous mutations in the PDX-1 gene have been identified in humans, which lead, respectively, to complete agenesis of the pancreas and a form of maturity-onset diabetes in the young known as MODY 4 [5].

NeuroD1/Beta2 is a basic helix-loop-helix (bHLH) transcription factor, which binds at the conserved insulin E1 (-100/-91bp) site in a complex with ubiquitously expressed E-box proteins. NeuroD1-null mice die of severe diabetes shortly after birth due to its crucial role in β-cell formation [19]. Moreover, deletion of NeuroD1 specifically in adult β cells in vivo causes glucose intolerance and loss of expression of many genes associated with cell maturation and function [20]. Mutations in human NEUROD1 predisposes one to another form of maturity-onset diabetes in the young, MODY 6 [21], presumably because of its importance in the production and maintenance of fully functional glucose-responsive β cells.

The MafA activator is a basic leucine zipper protein, which binds as a dimer to the conserved insulin C1/RIPE3b1 (-118/-107bp) element. MafA, and the only additional islet synthesized large Maf family member, MafB, are expressed unusually late in pancreatic cell development in relation to other islet-enriched transcription factors. In rodents, MafB is principally present in developing α cells and β cells, and then becomes restricted to α cells soon after birth [22,23]. In contrast, MafA is only found in β cells, with expression first detected during the principal wave of insulin-positive cell production at embryonic day 13.5 in mice [24]. In human islets, MafB is not only present in α cells but also co-produced with MafA in β cells [25]. A novel role for MafA and MafB in β-cell maturation and function was revealed upon comparing their properties to other islet-enriched transcription factor mutant mice. While islet cell populations are either lost or re-specified in most transcription factor knockout mice [5], the principal defect in MafB-/- embryos was reduced insulin and glucagon hormone expression, with no change in endocrine cell numbers or islet cell identity [22,23]. In contrast, islet cell development was unaffected in MafA-/- [26] and MafAΔPanc [27] mice, although glucose-regulated insulin secretion was compromised in adults.

Overall, a highly sophisticated network of transcription factors provides the infrastructure for precisely regulating insulin gene transcription, which relies on the cooperative and synergistic interactions between transcription factors and recruited coactivators. Significantly, these factors are also crucial in regulating many other β-cell genes, resulting in a variety of distinct developmental and adult phenotypes in studies of total and conditional transcription factor knockout mice. In the following section, the effect of increasing glucose levels, the most physiologically impactful mediator of β-cell function on Pdx-1, NeuroD1, and MafA will be presented (Figure 6.1).

Click here to view all Chapter 6 references.