title: Polyamine-modulated c-Myc expression in normal intestinal epithelial cells regulates p21Cip1 transcription through a proximal promoter region.
abstract: Maintenance of intestinal mucosal epithelial integrity requires cellular polyamines that regulate expression of various genes involved in cell proliferation, growth arrest and apoptosis. Our previous studies have shown that polyamines are essential for expression of the c-myc gene and that polyamine-induced c-Myc plays a critical role in stimulation of normal IEC (intestinal epithelial cell) proliferation, but the exact downstream targets of induced c-Myc are still unclear. The p21Cip1 protein is a major player in cell cycle control, which is primarily regulated at the transcriptional level. The current study was designed to determine whether induced c-Myc stimulates normal IEC proliferation by repressing p21Cip1 transcription following up-regulation of polyamines. Overexpression of the ODC (ornithine decarboxylase) gene increased levels of cellular polyamines, induced c-Myc expression and inhibited p21Cip1 transcription, as indicated by repression of p21Cip1 promoter activity and a decrease in p21Cip1 protein levels. In contrast, depletion of cellular polyamines by inhibiting ODC enzyme activity with alpha-difluoromethylornithine decreased c-Myc, but increased p21Cip1 transcription. Ectopic expression of wild-type c-myc not only inhibited basal levels of p21Cip1 transcription in control cells, but also prevented increased p21Cip1 in polyamine-deficient cells. Experiments using different p21Cip1 promoter mutants showed that transcriptional repression of p21Cip1 by c-Myc was mediated through Miz-1- and Sp1-binding sites within the proximal region of the p21Cip1 promoter in normal IECs. These findings confirm that p21Cip1 is one of the direct mediators of induced c-Myc following increased polyamines and that p21Cip1 repression by c-Myc is implicated in stimulation of normal IEC proliferation.
authors.affiliation: Cell Biology Group, Department of Surgery, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
authors.forename: Lan
authors.initials: L
authors.lastname: Liu
authors.forename: Xin
authors.initials: X
authors.lastname: Guo
authors.forename: Jaladanki N
authors.initials: JN
authors.lastname: Rao
authors.forename: Tongtong
authors.initials: T
authors.lastname: Zou
authors.forename: Bernard S
authors.initials: BS
authors.lastname: Marasa
authors.forename: Jie
authors.initials: J
authors.lastname: Chen
authors.forename: Jose
authors.initials: J
authors.lastname: Greenspon
authors.forename: Robert A
authors.initials: RA
authors.lastname: Casero
authors.forename: Jian-Ying
authors.initials: JY
authors.lastname: Wang
mesh_terms: D000818:Animals; D001665:Binding Sites; D002460:Cell Line; D049109:Cell Proliferation; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004847:Epithelial Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D005786:Gene Expression Regulation; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D007422:Intestines / Q000166:cytology / Q000187:drug effects; D009154:Mutation / Q000235:genetics; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism; D009955:Ornithine Decarboxylase / Q000378:metabolism; D065108:Ornithine Decarboxylase Inhibitors; D011073:Polyamines / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D050828:Protein Inhibitors of Activated STAT; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D014158:Transcription, Genetic / Q000235:genetics*; D044767:Ubiquitin-Protein Ligases
chemical_list: D050759:Cyclin-Dependent Kinase Inhibitor p21; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D009687:Nuclear Proteins; D065108:Ornithine Decarboxylase Inhibitors; D011073:Polyamines; D050828:Protein Inhibitors of Activated STAT; D016271:Proto-Oncogene Proteins c-myc; D044767:Ubiquitin-Protein Ligases; D009955:Ornithine Decarboxylase; C484534:Pias2 protein, rat