Depletion of the Poly(C)-binding Proteins αCP1 and αCP2 from K562 Cells Leads to p53-independent Induction of Cyclin-dependent Kinase Inhibitor (CDKN1A) and G1 Arrest

The alpha-globin poly(C)-binding proteins (alpha CPs) comprise an abundant and widely expressed set of K-homolog domain RNA-binding proteins. alpha CPs regulate the expression of a number of cellular and viral mRNAs at the levels of splicing, stability, and translation. Previous surveys have identified 160 mRNAs that are bound by alpha CP in the human hematopoietic cell line, K562. To explore the functions of these alpha CP/mRNA interactions, we identified mRNAs whose levels are altered in K562 cells acutely depleted of the two major alpha CP proteins, alpha CP1 and alpha CP2. Microarray analysis identified 27 mRNAs that are down-regulated and 14 mRNAs that are up-regulated in the alpha CP1/2-codepleted cells. This alpha CP1/2 co-depletion was also noted to inhibit cell proliferation and trigger a G(1) cell cycle arrest. Targeted analysis of genes involved in cell cycle control revealed a marked increase in p21(WAF) mRNA and protein. Analysis of mRNP complexes in K562 cells demonstrates in vivo association of p21WAF mRNA with alpha CP1 and alpha CP2. In vitro binding assays indicate that a 127-nucleotide region of the 3'-untranslated region of p21WAF interacts with both alpha CP1 and alpha CP2, and co-depletion of alpha CP1/2 results in a marked increase in p21WAF mRNA half-life. p21WAF induction and G1 arrest in the alpha CP1/2-co-depleted cells occur in the absence of p53 and are not observed in cells depleted of the individual alpha CP isoforms. The apparent redundancy in the actions of alpha CP1 and alpha CP2 upon p21WAF expression correlates with a parallel redundancy in their effects on cell cycle control. These data reveal a pivotal role for alpha CP1 and alpha CP2 in a p53-independent pathway of p21WAF control and cell cycle progression.