Responsiveness to Transforming Growth Factor-β (TGF-β)-Mediated Growth Inhibition Is a Function of Membrane-Bound TGF-β Type II Receptor in Human Breast Cancer Cells

Transforming growth factor-β (TGF-β) is a potent inhibitor of growth and proliferation of breast epithelial cells, and loss of sensitivity to its effects has been associated with malignant transformation and tumorigenesis. The biological effects of TGF-β are mediated by the TGF-β receptor complex, a multimer composed of TGF-β receptor type I (TβR-I) and TGF-β receptor type II (TβR-II) subunits. Evidence suggests that loss of expression of TβR-II is implicated in the loss of sensitivity of tumorigenic breast cell lines to TGF-β-mediated growth inhibition. A panel of human breast cell lines, including the immortalized MCF-10F and tumorigenic MCF-7, ZR75-1, BT474, T47-D, MDA-MB231, BT20, and SKBR-3 cell lines, was characterized for responsiveness to TGF-β-induced G1 growth arrest. Only the nontumorigenic MCF-10F and the tumorigenic MDA-MB231 cell lines demonstrated a significant inhibitory response to TGF-β1 and a significant binding of 125I-labeled TGF-β ligand. While expression of TβR-I mRNA was similar across the panel of cell lines, TβR-II mRNA expression was decreased significantly in all seven tumorigenic cell lines in comparison with the nontumorigenic MCF-10F cell line. When total cellular protein was fractionated by centrifugation, TβR-I protein was observed in both the cytosolic and membrane fractions at similar levels in all cell lines; however, TβR-II protein was present in the cytosolic fraction in all cell lines, but was observed in the membrane fraction of only the TGF-β-responsive MCF-10F and MDA-MB231 cells. Thus, lack of membrane-bound TβR-II protein appears to be an important determinant of resistance to TGF-β-mediated growth inhibition in this group of breast cell lines.