Colon Cancer Cells That Are Not Growth Inhibited by TGF-β Lack Functional Type I and Type II TGF-β Receptors

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Abstract
Objective The authors determined the molecular mechanisms for the failure of transforming growth factor-beta (TGF-β) to inhibit the growth of SW1116 and SW48 colon cancer cell lines. Background Transforming growth factor-β is a bifunctional regulator of cell growth that typically stimulates proliferation of mesenchymal cells, but inhibits proliferation of normal epithelial cells. In the colon, TGF-β appears to arrest proliferation of enterocytes as they leave the intestinal crypt and move to the villus tip. Transforming growth factor-β actions are mediated by binding to heteromeric complexes of type I and type II TGF-β receptors. Loss of TGF-β responsiveness may contribute to uncontrolled cell growth and cancer. Methods The effects of TGF-β1 on DNA synthesis were measured by incorporation of tritiated thymidine into DNA of cultures of moderately differentiated adenocarcinoma (SW48) and poorly differentiated adenocarcinoma (SW1116) colon cell lines and a mink lung epithelial cell line (CCL-64). The effects of TGF-β on the expression of c-myc, TGF-α, and TGF-β in SW48 cells, SW1116 cells, and CCL-64 cells (c-myc only) were measured by Northern blot analysis. Expression of TGF-β receptors in the cell lines was measured using competitive binding assays, receptor affinity labelling techniques, and reverse transcriptase-polymerase chain reaction. Results Incubation with TGF-β1 (50 ng/mL) did not decrease serum-stimulated uptake of [3H]-thymidine into actively growing cultures of SW48 or SW1116 cells, but suppressed DNA synthesis of actively growing CCL-64 cells by 90%. Similarly, incubation with TGF-β1 (12 ng/mL) for 4 hours did not substantially alter the mRNA levels of c-myc, TGF-α, and TGF-β1 in either colon tumor cell line, although levels of c-myc mRNA in CCL-64 cells were reduced by TGF-β1 treatment.