Apobec-1 transcription in rat colon cancer: Decreased apobec-1 protein production through alterations in polysome distribution and mRNA translation associated with upstream AUGs

Shrikant Anant, Debnath Mukhopadhyay, Ken Ichi Hirano, Thomas A. Brasitus, Nicholas O. Davidson

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

Apobec-1 catalyzes C to U editing of apolipoprotein B (apoB) mRNA in the mammalian intestine. Rat apobec-1 is transcribed from three distinct promoters, which contain distinct 5′ untranslated regions (5′UTRs) accompanied by variable numbers of in-frame upstream AUGs (uAUGs). We have observed a shift in apobec-1 promoter usage in an experimental model of colon carcinogenesis, resulting in transcripts loaded with 5′AUGs. In colon cancer, apobec-1 protein levels decreased by 90% in the cancer tissue as compared to normal tissue, suggesting an inhibitory effect of the 5′UTR on apobec-1 translation. We investigated the effects of these different 5′UTRs by site-directed mutagenesis coupled with in vitro translation studies. These studies established that the uAUGs within the 5′UTR of the alternative transcripts inhibit apobec-1 translation. This effect was independent of the length of the 5′UTR. Further analysis demonstrated that these uAUGs altered the polysome distribution, shifting the mRNA towards a denser, post-polyribosomal fraction. These findings were confirmed in transient transfection studies in vivo using HepG2 cells, where functional expression of apobec-1 was restored by mutagenesis of the uAUGs. Taken together, these data imply that rat apobec-1 gene expression is downregulated through alternative promoter usage. This dominant translational control of apobec-1 gene expression is most plausibly exerted through uAUGs.

Original languageEnglish
Pages (from-to)54-62
Number of pages9
JournalBiochimica et Biophysica Acta - Gene Structure and Expression
Volume1575
Issue number1-3
DOIs
StatePublished - May 3 2002

Keywords

  • 5′ untranslated
  • Apolipoprotein B
  • Polysome distribution
  • RNA editing
  • Upstream AUG

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