ENHANCED TRANSLATIONAL EFFICIENCY OF A NOVEL TRANSFORMING GROWTH FACTOR-BETA-3 MESSENGER-RNA IN HUMAN BREAST-CANCER CELLS

The mRNA for transforming growth factor beta3 (TGF-beta3) includes a long (1.1-kb) 5' noncoding region which exerts a potent inhibitory effect on translational efficiency. We now report that many human breast cancer cell lines (T47-D, SK-BR-3, ZR-75-1, and BT-474) express two mRNA species for TGF-beta3: the 3.5-kb transcript previously described as the only TGF-beta3 mRNA species in cells and a novel 2.6-kb transcript which lacks -870 nucleotides from the 5' noncoding region. The 5' end of the shorter transcript was sequenced, establishing it to be a 5' truncation of the full-length TGF-beta3 transcript. Estradiol decreased mRNA levels of both TGF-beta3 mRNA transcripts to an equivalent degree in estrogen receptor-positive cells. In contrast, the synthetic progestin gestodene altered the relative abundance of the two transcripts, preferentially diminishing the expression of the 2.6-kb transcript. The potential for enhanced mRNA translation attributable to the shorter 5' noncoding region was evaluated by transfection of cells with chimeric plasmid constructs in which the transcription unit consisted of coding sequence for chloramphenicol acetyltransferase downstream of the 5' noncoding sequence from TGF-beta3. The translational efficiency of chloramphenicol acetyltransferase-encoding mRNA containing the shorter 5' noncoding region of the 2.6-kb TGF-beta3 transcript was approximately seven times greater than with the full-length 5' noncoding region of TGF-beta3. Polysome analysis of TGF-beta3 mRNA in SK-BR-3 cells supported the hypothesis that the 2.6-kb transcript was more actively engaged in translation.