Cell type-dependent and -independent control of HER-2/neu translation

Overexpression of the HER-2 oncogene occurs in a variety of human tumors, i ncluding 25-30% of breast carcinomas, and has been associated with an adver se prognosis. Amplification of the HER-2 gene is frequently detected in tum ors, but by itself may not fully account for HER-2 overexpression since tra nscriptional and posttranscriptional mechanisms also regulate HER-2 protein synthesis. Our studies reveal that the efficiency of HER-2 translation dif fers between primary and transformed cells. In primary human fibroblasts an d human mammary epithelial cells, the HER-2 mRNA is associated with monosom e and small polysome fractions. In contrast, in BT474 and MCF-7 human breas t cancer cell lines and in COS-7 cells the mRNA co-sedimented with larger p olysomes, indicating that it is more efficiently translated in these transf ormed cells. Northern analysis revealed no detectable mRNA size difference, and nuclease S1 protection and sequence analyses showed no differences bet ween the HER-2 transcript leader in primary cells compared to transformed h uman cells, The transcript leader in all cell types contains a short upstre am open reading frame that is also conserved in other mammalian species. Tr ansient transfection assays revealed that the HER-2 transcript leader repre ssed downstream translation approximately five-fold in both primary and tra nsformed cells and mutation of the upstream initiation codon alleviated mos t of the inhibitory effect. These results indicate that HER2 expression is translationally controlled both by a short upstream open reading frame that represses HER-2 translation in a cell type-independent manner, and by a di stinct cell type-dependent mechanism that increases translational efficienc y of HER-2 in transformed cells. (C) 1999 Elsevier Science Ltd. All rights reserved.