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

J Cell Physiol. 2008 Oct;217(1):172-83. doi: 10.1002/jcp.21486.

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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • 5' Untranslated Regions / genetics
  • Blotting, Northern
  • Blotting, Western
  • Breast Neoplasms / genetics*
  • Cell Line, Tumor
  • Female
  • Gene Expression Regulation*
  • Humans
  • Protein Biosynthesis*
  • RNA, Messenger
  • RNA, Small Interfering
  • RNA-Binding Proteins / genetics*
  • Receptor, IGF Type 1 / genetics*
  • Reverse Transcriptase Polymerase Chain Reaction
  • Transfection

Substances

  • 5' Untranslated Regions
  • RNA, Messenger
  • RNA, Small Interfering
  • RNA-Binding Proteins
  • Receptor, IGF Type 1