title: Id3 upregulates BrdU incorporation associated with a DNA damage response, not replication, in human pancreatic β-cells.
abstract: Elucidating mechanisms of cell cycle control in normally quiescent human pancreatic β-cells has the potential to impact regeneration strategies for diabetes. Previously we demonstrated that Id3, a repressor of basic Helix-Loop-Helix (bHLH) proteins, was sufficient to induce cell cycle entry in pancreatic duct cells, which are closely related to β-cells developmentally. We hypothesized that Id3 might similarly induce cell cycle entry in primary human β-cells. To test this directly, adult human β-cells were transduced with adenovirus expressing Id3. Consistent with a replicative response, β-cells exhibited BrdU incorporation. Further, Id3 potently repressed expression of the cyclin dependent kinase inhibitor p57 (Kip2 ) , a gene which is also silenced in a rare β-cell hyperproliferative disorder in infants. Surprisingly however, BrdU positive β-cells did not express the proliferation markers Ki67 and pHH3. Instead, BrdU uptake reflected a DNA damage response, as manifested by hydroxyurea incorporation, γH2AX expression, and 53BP1 subcellular relocalization. The uncoupling of BrdU uptake from replication raises a cautionary note about interpreting studies relying solely upon BrdU incorporation as evidence of β-cell proliferation. The data also establish that loss of p57 (Kip2) is not sufficient to induce cell cycle entry in adult β-cells. Moreover, the differential responses to Id3 between duct and β-cells reveal that β-cells possess intrinsic resistance to cell cycle entry not common to all quiescent epithelial cells in the adult human pancreas. The data provide a much needed comparative model for investigating the molecular basis for this resistance in order to develop a strategy for improving replication competence in β-cells.
authors.affiliation: Sanford Children's Health Research Center, Sanford-Burnham Institute for Medical Research, La Jolla, CA, USA.
authors.forename: Seung-Hee
authors.initials: SH
authors.lastname: Lee
authors.forename: Ergeng
authors.initials: E
authors.lastname: Hao
authors.forename: Fred
authors.initials: F
authors.lastname: Levine
authors.forename: Pamela
authors.initials: P
authors.lastname: Itkin-Ansari
mesh_terms: D000818:Animals; D001973:Bromodeoxyuridine / Q000378:metabolism*; D002453:Cell Cycle / Q000235:genetics / Q000502:physiology; D048708:Cell Growth Processes / Q000235:genetics / Q000502:physiology; D050761:Cyclin-Dependent Kinase Inhibitor p57 / Q000037:antagonists & inhibitors / Q000096:biosynthesis* / Q000235:genetics; D004249:DNA Damage; D005786:Gene Expression Regulation; D006657:Histones / Q000378:metabolism; D006801:Humans; D007150:Immunohistochemistry; D051796:Inhibitor of Differentiation Proteins / Q000235:genetics / Q000378:metabolism*; D050417:Insulin-Secreting Cells / Q000166:cytology / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D019394:Ki-67 Antigen / Q000378:metabolism; D008297:Male; D051379:Mice; D008813:Mice, Inbred ICR; D014162:Transfection; D000071857:Tumor Suppressor p53-Binding Protein 1; D015854:Up-Regulation
chemical_list: D050761:Cyclin-Dependent Kinase Inhibitor p57; C468783:H2AX protein, human; D006657:Histones; D051796:Inhibitor of Differentiation Proteins; D047908:Intracellular Signaling Peptides and Proteins; D019394:Ki-67 Antigen; C498301:TP53BP1 protein, human; D000071857:Tumor Suppressor p53-Binding Protein 1; C067679:Idb3 protein, mouse; D001973:Bromodeoxyuridine