Control of JunB and Extracellular Matrix Protein Expression by Transforming Growth Factor-β1 Is Independent of Simian Virus 40 T Antigen-Sensitive Growth-Inhibitory Events

Abstract

Treatment of Mv1Lu mink lung epithelial cells with transforming growth factor-β1 (TGF-β1) prevents phosphorylation of the retinoblastoma susceptibility gene product, RB, in late G₁ phase of the cell cycle, which is thought to retain RB in a growth-suppressive state. This effect is paralleled by cell cycle arrest in late G₁ (M. Laiho, J. A. DeCaprio, J. W. Ludlow, D. M. Livingston, and J. Massague, Cell 62:175-185,1990). Arrest can be prevented by expression of simian virus 40 T antigen, which binds to underphosphorylated RB, presumably blocking its growth-suppressive activity. The response of cells to TGF-β1, however, is complex and includes changes in the levels of expression of genes encoding nuclear transcription factors and extracellular matrix components. To define the relationships among various components of the TGF-β1 response, we have investigated the effect of TGF-pl on cells whose growth-inhibitory response to this factor is prevented by T antigen. TGF-β1 adution to exponentially growing MvlLu cells increased the levels of junB mRNA and of three extracellular matrix proteins: plasminogen activator inhibitor-1, fibronectin, and thrombospondin. Kinetically, the effects. on junB and plasminogen activator inhibitor-1 expression occurred faster (half-maximal at 1 to 2 h) than the effects on fibronectin and thrombospondin expression (half-maximal at 6 to 10 h). These effects either preceded or overlapped, respectively, the withdrawal of MvlLu cells from the cell cycle. Expression of a transfected T-antigen gene in MvlLu cells, however, did not prevent any of these responses to TGF-β1. The results indicate that TGF-β1-stimulated expression of junB and extracellular matrix proteins in MvlLu cells can occur independently of the T-antigen-sensitive events that lead to growth arrest.