Background: By using a novel experimental system for the study of reactive fibroblast differentiation at the molecular level, we describe the induction of the alpha-isoform of smooth muscle actin by transforming growth factor-beta (TGF-beta 1) in human normal breast gland fibroblasts.
Experimental design: The experimental system allowed fibroblasts to plate and remain quiescent and nonreactive but at the same time stay sensitive to environmental cues for more than 2 weeks after explantation. Most important, the biological activity of growth factors and cytokines could be studied without cells entering the cell cycle, thus serving as a model for stromal reactions with little cell turnover.
Results: By use of double-labeling immunocytochemistry, isoelectric focusing, two-dimensional gel electrophoresis, and fluorography, evidence is presented that the effect of TGF-beta 1 is dose-dependent with a maximal response above 80 pg/ml and a course of 6 days. No other growth factor/cytokine tested (platelet-derived growth factor, interleukin-1, insulin-like growth factor-1, acidic fibroblast growth factor, basic fibroblast growth factor, epidermal growth factor, and interferon-gamma) could induce alpha-smooth muscle actin on their own or potentiate the effect of TGF-beta 1. In an inhibitory assay, only basic fibroblast growth factor was found to prevent the action of TGF-beta 1. The relative contribution of TGF-beta 1-like activity to carcinoma cell induced alpha-smooth muscle actin in fibroblasts was deciphered using TGF-beta neutralizing antibodies and medium conditioned by two different breast carcinoma cell lines. Conditioned medium elicited a fibroblast response indistinguishable from that obtained with exogenously added TGF-beta 1, and indeed neutralization attributed the entire response to the sole effect of secreted TGF-beta 1-like activity.
Conclusions: These results provide a strategy for the molecular characterization of epithelial-stromal interaction in breast neoplasia.