TGF-BETA REGULATES EXPRESSION OF TENASCIN ALTERNATIVE-SPLICING ISOFORMS IN FETAL-RAT LUNG

Two distinct mRNA-splice isoforms of tenascin (TN) are expressed diffe rentially during rat lung development. The unique temporal expression pattern of two TN isoforms suggests the expression of tenascin is stri ctly developmentally regulated in rat lung tissue. We investigated mol ecular mechanisms which modulate alternative-splicing expression of TN in lung development. The effect of transforming growth factor-beta 1 (TGF-beta 1) on regulation of expression of TN isoforms was examined b y in vitro lung explant culture. Immunoblotting with anti-TN antibody detected two TN polypeptides in rat lung explant culture, the larger [ relative molecular weight (M(r)) 230, TN230] polypeptide and the small er (M(r) 180, TN180). TGF-beta 1 markedly induced the TN180 isoform an d caused only a moderate increase of the TN230 isoform. The effects of TGF-beta 1 were shown to be dose dependent over a physiological range of TGF-beta 1 protein concentration. The induction of TN isoform bios ynthesis by TGF-beta 1 was detected 12 h after addition of the growth factor, and the effects endured for up to 48 h at a dose of 5 ng/ml. B y reverse transcriptase-polymerase chain reaction through amplificatio n of the entire fibronectin type III (FN-III) splicing domain, two dis tinct TN isoforms were detected in total RNA isolated from gestational day 21 rat lung explant culture treated with TGF-beta 1 and from post natal day 8 rat lung. The larger isoform contained five FN-III alterna tive splicing repeats [1,420 base pairs (bp)], but the shorter splicin g isoform lacked four FN-III alternative splicing repeats (340 bp). Th e results confirm that regulation of the TN isoform expression in feta l rat lung occurred at the mRNA level. They further indicate that two protein subunits, TN180 and TN230, were derived from two mRNA-splicing variants: one with 7.3 kb containing five FN-III alternative splicing repeats, and one with 6.4 kb containing one FN-III alternative-splici ng repeat.