Objective: The authors determined the molecular mechanisms for the failure of transforming growth factor-beta (TGF-beta) to inhibit the growth of SW1116 and SW48 colon cancer cell lines.
Background: Transforming growth factor-beta 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-beta appears to arrest proliferation of enterocytes as they leave the intestinal crypt and move to the villus tip. Transforming growth factor-beta actions are mediated by binding to heteromeric complexes of type I and type II TGF-beta receptors. Loss of TGF-beta responsiveness may contribute to uncontrolled cell growth and cancer.
Methods: The effects of TGF-beta 1 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-beta on the expression of c-myc, TGF-alpha, and TGF-beta in SW48 cells, SW1116 cells, and CCL-64 cells (c-myc only) were measured by Northern blot analysis. Expression of TGF-beta 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-beta 1 (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-beta 1 (12 ng/mL) for 4 hours did not substantially alter the mRNA levels of c-myc, TGF-alpha, and TGF-beta 1 in either colon tumor cell line, although levels of c-myc mRNA in CCL-64 cells were reduced by TGF-beta 1 treatment. Competitive displacement of [125I]-TGF-beta 1 binding detected high levels (16,500 TGF-beta receptors per cell) of specific, high-affinity (200 pmol/L half-displacement) TGF-beta receptors on CCL-64 cells. In marked contrast, very low levels of TGF-beta 1 binding to SW1116 cells (250 receptors per cell) and SW48 cells (260 receptors per cell) were detected. Autoradiograms of CCL-64 cells affinity labelled with [125I]TGF-beta 1 revealed the presence of type I, type II, and type III TGF-beta receptors. No TGF-beta receptors were identified on SW1116 cells, and only very low levels of the nonsignaling type III TGF-beta receptors were detected on SW48 cells. Reverse transcriptase-polymerase chain reaction amplification detected mRNAs for type I, type II, and type III TGF-beta receptors in CCL-64 cells. SW48 cells, and SW1116 cells.
Conclusions: These results suggest that the lack of growth inhibition by TGF-beta in SW48 and SW1116 colon cancer cells may be caused by a lack of expression of functional TGF-beta receptors.