Increased levels of the translation initiation factor eIF4E in differentiating epithelial lung tumor cell lines

Abstract

Rates of eukaryotic protein synthesis and proliferation are dependent upon the availability of eIF4F, the cap-binding translation initiation complex that guides the ribosome onto the mRNA. One possible rate-limiting factor in eIF4F complex formation is the availability of eIF4E, which interacts specifically with the mRNA cap structure. As such, it has a potential role in the selective translation of growth-related mRNAs, with overexpression of eIF4E resulting in aberrant cell growth and transformation. A number of studies suggest that eIF4E may play a role in cellular differentiation as well as proliferation. We have previously reported that post-transcriptional regulation is involved in the induction of keratins in epithelial lung tumor cell lines exposed to the differentiation-modulating agent, bromo-deoxyuridine (BrdU). Here, we demonstrate that these BrdU-treated lung cells express elevated levels of eIF4E protein and enhanced phosphorylation of eIF4E. Overexpression of eIF4E by cDNA transfection in the poorly differentiated, keratin-negative human lung cell line, DLKP, was found to promote a flattened, more epithelial appearance to these cells, coupled with the induction of simple keratins (keratins 8 and 18). In contrast, levels of eIF4E expression were found to decrease during BrdU-induced differentiation of the leukemic cell line, HL-60, suggesting that there are cell-type differences in the response to BrdU and in the requirement for eIF4E during differentiation.