title: Iron chelation and regulation of the cell cycle: 2 mechanisms of posttranscriptional regulation of the universal cyclin-dependent kinase inhibitor p21CIP1/WAF1 by iron depletion.
abstract: Iron (Fe) plays a critical role in proliferation, and Fe deficiency results in G(1)/S arrest and apoptosis. However, the precise role of Fe in cell-cycle control remains unclear. We observed that Fe depletion increased the mRNA of the universal cyclin-dependent kinase inhibitor, p21(CIP1/WAF1), while its protein level was not elevated. This observation is unique to the G(1)/S arrest seen after Fe deprivation, as increased p21(CIP1/WAF1) mRNA and protein are usually found when arrest is induced by other stimuli. In this study, we examined the posttranscriptional regulation of p21(CIP1/WAF1) after Fe depletion and demonstrated that its down-regulation was due to 2 mechanisms: (1) inhibited translocation of p21(CIP1/WAF1) mRNA from the nucleus to cytosolic translational machinery; and (2) induction of ubiquitin-independent proteasomal degradation. Iron chelation significantly (P < .01) decreased p21(CIP1/WAF1) protein half-life from 61 (+/- 4 minutes; n = 3) to 28 (+/- 9 minutes, n = 3). Proteasomal inhibitors rescued the chelator-mediated decrease in p21(CIP1/WAF1) protein, while lysosomotropic agents were not effective. In Fe-replete cells, p21(CIP1/WAF1) was degraded in an ubiquitin-dependent manner, while after Fe depletion, ubiquitin-independent proteasomal degradation occurred. These results are important for considering the mechanism of Fe depletion-mediated cell-cycle arrest and apoptosis and the efficacy of chelators as antitumor agents.
authors.affiliation: Iron Metabolism and Chelation Program, Department of Pathology, University of Sydney, New South Wales, Australia.
authors.forename: Dong
authors.initials: D
authors.lastname: Fu
authors.forename: Des R
authors.initials: DR
authors.lastname: Richardson
mesh_terms: D001943:Breast Neoplasms; D002453:Cell Cycle / Q000502:physiology*; D045744:Cell Line, Tumor; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics / Q000378:metabolism*; D003676:Deferoxamine / Q000494:pharmacology; D005260:Female; D005786:Gene Expression Regulation; D006801:Humans; D007502:Iron Chelating Agents / Q000494:pharmacology*; D021381:Protein Transport; D012313:RNA / Q000235:genetics / Q000302:isolation & purification; D012323:RNA Processing, Post-Transcriptional; D012333:RNA, Messenger / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D050759:Cyclin-Dependent Kinase Inhibitor p21; D007502:Iron Chelating Agents; D012333:RNA, Messenger; D012313:RNA; D003676:Deferoxamine