Initiation factor eIF2-independent mode of c-Src mRNA translation occurs via an internal ribosome entry site.
Overexpression and activation of the c-Src protein have been linked to the development of a wide variety of cancers. The molecular mechanism(s) of c-Src overexpression in cancer cells is not clear. We report here an internal ribosome entry site (IRES) in the c-Src mRNA that is constituted by both 5'-noncoding and -coding regions. The inhibition of cap-dependent translation by m(7)GDP in the cell-free translation system or induction of endoplasmic reticulum stress in hepatoma-derived cells resulted in stimulation of the c-Src IRES activities. Sucrose density gradient analyses revealed formation of a stable binary complex between the c-Src IRES and purified HeLa 40 S ribosomal subunit in the absence of initiation factors. We further demonstrate eIF2-independent assembly of 80 S initiation complex on the c-Src IRES. These features of the c-Src IRES appear to be reminiscent of that of hepatitis C virus-like IRESs and translation initiation in prokaryotes. Transfection studies and genetic analysis revealed that the c-Src IRES permitted initiation at the authentic AUG351, which is also used for conventional translation initiation of the c-Src mRNA. Our studies unveiled a novel regulatory mechanism of c-Src synthesis mediated by an IRES element, which exhibits enhanced activity during cellular stress and is likely to cause c-Src overexpression during oncogenesis and metastasis.
Allam
Heba
H
Department of Biological Sciences, University of Denver, Denver, Colorado 80208-0183, USA.
Ali
Naushad
N
D020121:5' Untranslated Regions; D006528:Carcinoma, Hepatocellular / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D002471:Cell Transformation, Neoplastic / Q000235:genetics / Q000378:metabolism*; D002474:Cell-Free System / Q000378:metabolism; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism*; D016391:Genes, src; D006367:HeLa Cells; D006801:Humans; D009362:Neoplasm Metastasis; D010442:Peptide Chain Initiation, Translational; D012315:RNA Caps / Q000235:genetics / Q000378:metabolism; D054682:Ribosome Subunits, Small, Eukaryotic / Q000235:genetics / Q000378:metabolism*; D013312:Stress, Physiological
D020121:5' Untranslated Regions; D015852:Eukaryotic Initiation Factor-2; D012315:RNA Caps