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.
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.
Meng
Zheng
Z
Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, Birmingham, Alabama, USA.
Jackson
Nateka L
NL
Choi
Hyoungsoo
H
King
Peter H
PH
Emanuel
Peter D
PD
Blume
Scott W
SW
D020121:5' Untranslated Regions / Q000235:genetics; D015152:Blotting, Northern; D015153:Blotting, Western; D001943:Breast Neoplasms / Q000235:genetics*; D045744:Cell Line, Tumor; D005260:Female; D005786:Gene Expression Regulation; D006801:Humans; D014176:Protein Biosynthesis; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D016601:RNA-Binding Proteins / Q000235:genetics*; D017526:Receptor, IGF Type 1 / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014162:Transfection
D020121:5' Untranslated Regions; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D016601:RNA-Binding Proteins; D017526:Receptor, IGF Type 1