Reduction of translation initiation factor 4E decreases the malignancy of ras ‐transformed cloned rat embryo fibroblasts

Abstract

Expression of the T24^ras oncogene induces malignancy (tumor growth, invasion and metastasis) in cloned rat embryo fibroblasts (CREF T24). In CREF T24, the rate of phosphorylation of eukaryotic translation initiation factor 4E (elF-4E) is increased, resulting in increased protein synthesis rates. We have recently shown that reducing the protein levels of elF-4E in CREF T24 (AS4E line) markedly decreases soft-agar colonization, increases tumor latency periods and increases tumor doubling times without significantly altering monolayer growth. In this study, cells with reduced elF-4E had delayed and reduced invasiveness and decreased experimental metastasis. Furthermore, reduced elF-4E levels correlated with decreased expression of the metastasis-associated 92-kDa collagenase type-IV and exon-6 variants of the CD44 adhesion molecule [CD44(6v)]. Reduced elF-4E levels correlated inversely with increased levels of the putative metastasis-suppressor protein nm23. Cell lines established from AS4E tumors and lung metastases exhibited increased levels of elF-4E protein and protein synthesis rates compared to the AS4E line. Tumor-derived AS4E had the shortened tumor latency periods of CREF T24 but displayed the slow tumor-growth rates of AS4E. Tumor-derived AS4E exhibited the metastatic capacity of CREF T24 controls. Furthermore, tumor- and lung-nodule-derived AS4E expressed levels of CD44 (6v) and the 92-kDa collagenase type IV comparable to CREF T24 and displayed reduced levels of nm23 relative to AS4E. These results demonstrate that elF-4E is an important effector molecule involved in oncogenic p2l^ras-induced malignant transformation.