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 l ong (1.1-kb) 5' noncoding region which exerts a potent inhibitory effe ct on translational efficiency. We now report that many human breast c ancer cell lines (T47-D, SK-BR-3, ZR-75-1, and BT-474) express two mRN A 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 transcrip t which lacks -870 nucleotides from the 5' noncoding region. The 5' en d of the shorter transcript was sequenced, establishing it to be a 5' truncation of the full-length TGF-beta3 transcript. Estradiol decrease d mRNA levels of both TGF-beta3 mRNA transcripts to an equivalent degr ee in estrogen receptor-positive cells. In contrast, the synthetic pro gestin 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 shorte r 5' noncoding region was evaluated by transfection of cells with chim eric plasmid constructs in which the transcription unit consisted of c oding sequence for chloramphenicol acetyltransferase downstream of the 5' noncoding sequence from TGF-beta3. The translational efficiency of chloramphenicol acetyltransferase-encoding mRNA containing the shorte r 5' noncoding region of the 2.6-kb TGF-beta3 transcript was approxima tely 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 su pported the hypothesis that the 2.6-kb transcript was more actively en gaged in translation.