PDX1 (pancreatic and duodenal homeobox 1), also known as insulin promoter factor 1, is a transcription factor in the ParaHox gene cluster.[5] In vertebrates, Pdx1 is necessary for pancreatic development, including β-cell maturation, and duodenal differentiation. In humans this protein is encoded by the PDX1 gene, which was formerly known as IPF1.[6][7] The gene was originally identified in the clawed frog Xenopus laevis[8] and is present widely across the evolutionary diversity of bilaterian animals, although it has been lost in evolution in arthropods and nematodes.[5] Despite the gene name being Pdx1, there is no Pdx2 gene in most animals; single-copy Pdx1 orthologs have been identified in all mammals.[9] Coelacanth and cartilaginous fish are, so far, the only vertebrates shown to have two Pdx genes, Pdx1 and Pdx2.[10]
Function
Pancreatic development
In pancreatic development, Pdx1 is expressed by a population of cells in the posteriorforegut region of the definitive endoderm, and Pdx1+epithelial cells give rise to the developing pancreatic buds, and eventually, the whole of the pancreas—its exocrine, endocrine, and ductal cell populations.[11] Pancreatic Pdx1+ cells first arise at mouse embryonic day 8.5-9.0 (E8.5-9.0), and Pdx1 expression continues until E12.0-E12.5.[12] Homozygous Pdx1 knockout mice form pancreatic buds but fail to develop a pancreas,[13] and transgenic mice in which tetracycline application results in death of Pdx1+ cells are almost completely apancreatic if doxycycline (tetracycline derivative) is administered throughout the pregnancy of these transgenic mice, illustrating the necessity of Pdx1+ cells in pancreatic development.[12]
Pdx1 is accepted as the earliest marker for pancreatic differentiation, with the fates of pancreatic cells controlled by downstream transcription factors.[13] The initial pancreatic bud is composed of Pdx1+pancreatic progenitor cells that co-express Hlxb9, Hnf6, Ptf1a and NKX6-1. These cells further proliferate and branch in response to FGF-10 signaling. Afterwards, differentiation of the pancreatic cells begins; a population of cells has Notch signaling inhibited, and subsequently, expresses Ngn3. This Ngn3+ population is a transient population of pancreatic endocrine progenitors that gives rise to the α, β, Δ, PP, and ε cells of the islets of Langerhans.[12] Other cells will give rise to the exocrine and ductal pancreatic cell populations.
β-cell maturation and survival
The final stages of pancreas development involves the production of different endocrine cells, including insulin-producing β-cells and glucagon-producing α-cells. Pdx1 is necessary for β-cell maturation: developing β-cells co-express Pdx1, NKX6-1, and insulin, a process that results in the silencing of MafB and the expression of MafA, a necessary switch in maturation of β-cells.[11] At this stage of pancreas development, the experimental decrease in the expression of Pdx1 results in a production of a smaller number of β-cells and an associated increase in the number of α-cells.[14]
In the mature pancreas, Pdx1 expression seems to be required for the maintenance and survival of β-cells. For instance, experimentally reducing the level of Pdx1 expression at this stage makes β-cells produce higher amounts of glucagon,[15] suggesting that Pdx1 inhibits the conversion of β-cells into α-cells. Furthermore, Pdx1 appears to be important in mediating the effect of insulin on the apoptotic programmed cell death of β-cells: a small concentration of insulin protects β-cells from apoptosis, but not in cells where Pdx1 expression has been inhibited.[16][17]
Duodenum
Pdx1 is necessary for the development of the proximal duodenum and maintenance of the gastro-duodenal junction.[18] Duodenal enterocytes, Brunner's glands and entero-endocrine cells (including those in the gastric antrum) are dependent on Pdx1 expression. It is a ParaHox gene, which together with Sox2 and Cdx2, determines the correct cellular differentiation in the proximal gut.[18] In mature mice duodenum, several genes have been identified which are dependent on Pdx1 expression and include some affecting lipid and iron absorption.[19]
^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^ abBrooke, N. M., Garcia-Fernàndez, J., & Holland, P. W. (1998). The ParaHox gene cluster is an evolutionary sister of the Hox gene cluster. Nature, 392(6679), 920.
^"PDX1". HGNC. Archived from the original on 3 March 2016. Retrieved 22 April 2016.
^Stoffel M, Stein R, Wright CV, Espinosa R, Le Beau MM, Bell GI (July 1995). "Localization of human homeodomain transcription factor insulin promoter factor 1 (IPF1) to chromosome band 13q12.1". Genomics. 28 (1): 125–6. doi:10.1006/geno.1995.1120. PMID7590740.
^Wright, C. V., Schnegelsberg, P., & De Robertis, E. M. (1989). XlHbox 8: a novel Xenopus homeo protein restricted to a narrow band of endoderm. Development, 105(4), 787-794.
^ Gannon M, Ables ET, Crawford L, et al. pdx-1 function is specifically required in embryonic beta cells to generate appropriate numbers of endocrine cell types and maintain glucose homeostasis. Dev Biol. 2007;314(2):406-17. doi:10.1016/j.ydbio.2007.10.038
^Ahlgren U, Jonsson J, Jonsson L, Simu K, Edlund H. beta-cell-specific inactivation of the mouse Ipf1/Pdx1 gene results in loss of the beta-cell phenotype and maturity onset diabetes. Genes Dev. 1998;12(12):1763-8.
^Fujimoto, Kei, and Kenneth S. Polonsky. "Pdx1 and other factors that regulate pancreatic β‐cell survival." Diabetes, Obesity and Metabolism 11 (2009): 30-37.
^Fajans SS, Bell GI, Polonsky KS (September 2001). "Molecular mechanisms and clinical pathophysiology of maturity-onset diabetes of the young". N. Engl. J. Med. 345 (13): 971–80. doi:10.1056/NEJMra002168. PMID11575290.
Stoffel M, Stein R, Wright CV, et al. (1995). "Localization of human homeodomain transcription factor insulin promoter factor 1 (IPF1) to chromosome band 13q12.1". Genomics. 28 (1): 125–6. doi:10.1006/geno.1995.1120. PMID7590740.
Inoue H, Riggs AC, Tanizawa Y, et al. (1996). "Isolation, characterization, and chromosomal mapping of the human insulin promoter factor 1 (IPF-1) gene". Diabetes. 45 (6): 789–94. doi:10.2337/diabetes.45.6.789. PMID8635654.
Waeber G, Thompson N, Nicod P, Bonny C (1997). "Transcriptional activation of the GLUT2 gene by the IPF-1/STF-1/IDX-1 homeobox factor". Mol. Endocrinol. 10 (11): 1327–34. doi:10.1210/mend.10.11.8923459. PMID8923459.
Watada H, Kajimoto Y, Kaneto H, et al. (1997). "Involvement of the homeodomain-containing transcription factor PDX-1 in islet amyloid polypeptide gene transcription". Biochem. Biophys. Res. Commun. 229 (3): 746–51. doi:10.1006/bbrc.1996.1875. PMID8954967.
Stoffers DA, Zinkin NT, Stanojevic V, et al. (1997). "Pancreatic agenesis attributable to a single nucleotide deletion in the human IPF1 gene coding sequence". Nat. Genet. 15 (1): 106–10. doi:10.1038/ng0197-106. PMID8988180. S2CID25406130.
