Alterations in RNA‐binding activities of IRES‐regulatory proteins as a mechanism for physiological variability and pathological dysregulation of IGF‐IR translational control in human breast tumor cells

Abstract

The type I insulin-like growth factor receptor (IGF-IR) is integrally involved in the control of cellular proliferation and survival. An internal ribosomal entry site (IRES) within the 1,038 nucleotide 5'-untranslated region of the human IGF-IR mRNA helps to provide the tight control of IGF-IR expression necessary for maintenance of normal cellular and tissue homeostasis. The IRES maps to a discrete sequence of 85 nucleotides positioned just upstream of the IGF-IR initiation codon, allowing the ribosome to bypass the highly structured regions of the 5'-UTR as well as the upstream open reading frame. The authenticity of the IGF-IR IRES has been confirmed by its sensitivity to deletion of the promoter from a bicistronic reporter construct, and its resistance in a monocistronic reporter construct to co-expression of a viral 2A protease. We previously characterized HuR as a potent repressor of IGF-IR translation. Here we demonstrate that hnRNP C competes with HuR for binding the IGF-IR 5'-UTR and enhances IRES-mediated translation initiation in a concentration-dependent manner. We observed changes in binding of hnRNP C versus HuR to the IGF-IR 5'-UTR in response to physiological alterations in cellular environment or proliferative status. Furthermore, we have found distinct alterations in the pattern of protein binding to the IGF-IR 5'-UTR in human breast tumor cells in which IGF-IR IRES activity and relative translational efficiency are aberrantly increased. These results suggest that dysregulation of the IGF-IR IRES through changes in the activities of RNA-binding translation-regulatory proteins could be responsible for IGF-IR overexpression in a proportion of human breast tumors.

Thiazolidinediones Up-regulate Insulin-like Growth Factor-1 Receptor via a Peroxisome Proliferator-activated Receptor γ-Independent Pathway

“…The IRES activity in the 5Ј untranslated region of the IGF-1R mRNA was first described by our group in COS-1 cells and rat fibroblasts (32), and it has since been described in other cell types (44,45). Thus far, polypyrimidine tract-binding protein (32), HuR (45), and heterogeneous nuclear RNP C1/C2 (44) have been reported to associate with the 5Ј portion of the IGF-1R mRNA thereby regulating IRES-dependent translation initiation.…”

“…Thus far, polypyrimidine tract-binding protein (32), HuR (45), and heterogeneous nuclear RNP C1/C2 (44) have been reported to associate with the 5Ј portion of the IGF-1R mRNA thereby regulating IRES-dependent translation initiation. To our knowledge, this is the first report describing potential IRES-mediated translation initiation of IGF-1R in vascular SMCs.…”

MicroRNA-214 Reduces Insulin-like Growth Factor-1 (IGF-1) Receptor Expression and Downstream mTORC1 Signaling in Renal Carcinoma Cells

“…The long 5ЈUTR of IGF-1R is a target of the RNA-binding protein HuR, which delays cap-dependent translation and inhibits the internal ribosome entry site-containing IGF-1R translational block (63,64). In contrast, heterogeneous nuclear ribonucleoprotein C has been shown to interact with IGF-1R 5ЈUTR at the site of HuR binding to promote internal ribosome entry site-mediated translation of IGF-1R (65). Recently, four miRNAs, miR-7, miR-192, miR-215, and miR-145, have been identified, which repress the expression of IGF-1R by post-transcriptional mechanism in tongue squamous cell carcinoma, multiple myeloma, and VHL-deficient RCC (66 -68).…”

High expression stability of microtubule affinity regulating kinase 3 (MARK3) makes it a reliable reference gene

“…These suggest that genes with their resulting protein products that are involved in translation of mRNA are more likely to be constitutively expressed at a constant level as varying availability of proteins in the translational process may result in variability in translational efficiency (22). Increase in translational inefficiency has been implicated in the molecular aging process (23), suggesting that variability in translational efficiency is not desirable and should be minimised.…”