Characterization of transforming growth factor‐β growth regulatory effects and receptors on bovine mammary cells

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Abstract
Transforming growth factor‐β (TGF‐β) has been shown to inhibit mammary morphogenesis, growth, and differentiation in murine studies. We have characterized TGF‐β receptors and their autoregulation, and the growth response to TGF‐β1 and TGF‐β2 in cultured bovine mammary epithelium (MAC‐T) and fibroblasts. Affinity labelling studies revealed that fibroblast and epithelial cells contained type I, II, and III (betaglycan) receptors, with the type III receptor being the predominant binding component. On both fibroblasts and epithelial cells, TGF‐β1 and TGF‐β2 had equal binding affinities for the type I and II receptors, but TGF‐β2 had a higher affinity for the type III receptor. Also, preincubation of MAC‐T cells with 50 pM TGF‐β1 or TGF‐β2 markedly downregulated TGF‐β receptors. Proliferative response was measured using both total DNA and 3H‐thymidine incorporation. Both TGF‐β isoforms were effective in inhibiting proliferation of MAC‐T cells and fibroblasts. Inhibition of proliferation was not altered following immortalization of fibroblasts with SV‐40 Large‐T‐antigen (LT), even when the cells acquired a transformed phenotype. Inhibition of proliferation was not a result of cytotoxicity, as TGF‐β at concentrations 1,000‐fold higher than ED50 levels did not increase cell death. Moreover, the inhibition was reversible as shown by return of cellular proliferation to control levels following TGF‐β removal. Although growth inhibition was not transient as culture of MAC‐T cells in TGF‐β resulted in sustained inhibition of proliferation for at least 144 h.