Targeted Knockdown of the RNA-binding Protein CRD-BP Promotes Cell Proliferation via an Insulin-like Growth Factor II-dependent Pathway in Human K562 Leukemia Cells

Abstract

The c-myc mRNA coding region determinant-binding protein (CRD-BP) was first identified as a masking protein that stabilizes c-myc mRNA in a cell-free mRNA degradation system. Thus, CRD-BP is thought to promote cell proliferation by maintaining c-Myc at critical levels. CRD-BP also appears to be an oncofetal protein, based upon its expression during mammalian development and in some tumors. By using K562 leukemia cells as a model, we show that CRD-BP gene silencing by RNA interference significantly promoted proliferation, indicating an inhibitory effect of CRD-BP on proliferation. Unexpectedly, CRD-BP knockdown had no discernible effect on c-myc mRNA levels. CRD-BP is also known as insulin-like growth factor II (IGF-II) mRNA-binding protein-1. It has been reported to repress translation of a luciferase reporter mRNA containing an IGF-II 5 -untranslated region known as leader 3 but not one containing IGF-II leader 4. CRD-BP knockdown markedly increased IGF-II mRNA and protein levels but did not alter translation of luciferase reporter mRNAs containing 5 -untranslated regions consisting of either IGF-II leader 3 or leader 4. Addition of antibody against IGF-II to cell cultures inhibited the proliferative effect of CRD-BP knockdown, suggesting that regulation of IGF-II gene expression, rather than c-myc mRNA levels, mediates the proliferative effect of CRD-BP knockdown. Thus, we have identified a dominant function for CRD-BP in cell proliferation of human K562 cells, involving a possible IGF-II-dependent mechanism that appears independent of its ability to serve as a c-myc mRNA masking protein.

Expression of the RNA-binding protein IMP1 correlates with poor prognosis in ovarian carcinoma

“…Previous studies revealed that IMP1 knockdown in mammary adenocarcinoma-derived MCF-7 cells resulted in reduced proliferation correlating with decreased steady state levels of c-MYC (Ioannidis et al, 2005). In contrast, the knockdown of IMP1 caused increased proliferation correlating with elevated IGF-II protein levels in K562 leukemia cells (Liao et al, 2004). In agreement with studies performed in MCF-7 cells, IMP1 knockdown in ovarian carcinoma-derived ES-2 induced decreased proliferation starting 72 h after the transfection of siRNAs.…”

IGF2BP1 overexpression causes fetal-like hemoglobin expression patterns in cultured human adult erythroblasts

“…S3). Of note, manipulation of IGF2BP1 in K562 erythroleukemia cells similarly had only minor effects on those genes (26). Following these initial studies, the potential for IGF2BP1-OE to regulate the expression levels of the let-7 miRNAs or LIN28 in adult CD34(+) erythroblasts via a feedback loop was investigated.…”

Regulation of local expression of cell adhesion and motility-related mRNAs in breast cancer cells by IMP1/ZBP1

“…Re-activation or elevated expression of the ZBP1 gene has been detected in mammalian tumors of different origins and has been considered a feature marker of clinical samples (Gu et al, 2004;Ioannidis et al, 2005;Yisraeli, 2005). A number of studies have revealed the positive or negative involvement of ZBP1 in tumorigenesis and tumor progression, including cancer cell proliferation, invasion and metastasis (Liao et al, 2004;Ross et al, 2001;Tessier et al, 2004;Wang et al, 2004). Substantial evidence implicates the role of ZBP1 in breast cancer invasiveness.…”