Downregulation of the upstream binding factor1 by glycogen synthase kinase3beta in myeloid cells induced to differentiate.
The upstream binding factor 1 (UBF1), one of the proteins that regulate the activity of RNA polymerase I, is downregulated in 32D myeloid cells induced to differentiate into granulocytes, either by the type 1 insulin-like growth factor (IGF-1) or the granulocytic colony stimulating factor (G-CSF). Downregulation of UBF1 is largely due to protein degradation, while mRNA levels are not affected. Inhibition of UBF1 degradation by lithium chloride (LiCl)and lactacystin suggest a role of glycogen synthase kinase beta (GSK3beta) in a proteasome-dependent degradation of UBF. GSK3beta phosphorylates in vitro and in vivo the UBF protein, which has five putative motifs for phosphorylation by GSK3beta. Elimination and/or mutations of these motifs stabilize the UBF1 protein even in cells induced to differentiate. Conversely, a stably transfected, constitutively active GSK3beta accelerates the downregulation of UBF1. We show further that activation of the differentiating protein C/EPBalpha in 32D cells transformed by the oncogenic BCR/ABL protein causes downregulation of UBF1. Finally, inhibition of differentiation of myeloid cells by a dominant negative mutant of Stat3 stabilizes the UBF1 protein, while rapamycin-induced differentiation of myeloid cells downregulates UBF1 levels. Taken together, our results indicate that the induction of granulocytic differentiation in 32D murine myeloid cells causes the degradation of UBF1, via GSK3beta and the proteasome pathway.
Liu
Mingli
M
Kimmel Cancer Center, Thomas Jefferson University, 624 Bluemle, Life Sciences Building, Philadelphia, Pennsylvania 19107, USA.
Tu
Xiao
X
Ferrari-Amorotti
Giovanna
G
Calabretta
Bruno
B
Baserga
Renato
R
D000818:Animals; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000235:genetics / Q000378:metabolism; D002454:Cell Differentiation; D002460:Cell Line; D015536:Down-Regulation; D016044:Fusion Proteins, bcr-abl / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation; D038362:Glycogen Synthase Kinase 3 / Q000235:genetics / Q000494:pharmacology*; D000071679:Glycogen Synthase Kinase 3 beta; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D051379:Mice; D009154:Mutation / Q000235:genetics; D022423:Myeloid Cells / Q000166:cytology / Q000378:metabolism*; D010766:Phosphorylation; D035463:Pol1 Transcription Initiation Complex Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D011480:Protease Inhibitors / Q000494:pharmacology; D017526:Receptor, IGF Type 1 / Q000235:genetics / Q000378:metabolism; D050796:STAT3 Transcription Factor / Q000235:genetics / Q000378:metabolism; D013347:Subcellular Fractions
D022763:CCAAT-Enhancer-Binding Protein-alpha; D035463:Pol1 Transcription Initiation Complex Proteins; D011480:Protease Inhibitors; D050796:STAT3 Transcription Factor; C063844:transcription factor UBF; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1; D016044:Fusion Proteins, bcr-abl; D000071679:Glycogen Synthase Kinase 3 beta; C000605507:Gsk3b protein, mouse; D038362:Glycogen Synthase Kinase 3