BACKGROUND - Overexpression of transforming growth factor (TGF)-beta has been reported in human breast carcinomas resistant to antiestrogen tamoxifen, but the role of TGF-beta in this resistant phenotype is unclear. We investigated whether inhibition of TGF-beta2, which is overexpressed in LCC2 tamoxifen-resistant human breast cancer cells, could modify antiestrogen resistance.
METHODS - TGF-beta2 expression was evaluated in LCC2 cells and tamoxifen-sensitive LCC1 cells by northern blot analysis. Secreted TGF-beta activity was quantified by use of an 125I-TGF-beta competitive radioreceptor assay. Sensitivity to tamoxifen was measured in a soft agarose colony-forming assay and in a xenograft model in nude and beige/nude mice. Natural killer (NK) cell cytotoxicity was measured by 51Cr release from LCC1 and LCC2 cell targets coincubated with human peripheral blood mononuclear cells. Decrease in TGF-beta2 expression in LCC2 cells was achieved by treatment with antisense oligodeoxynucleotides and confirmed by TGF-beta2 immunoblot analysis.
RESULTS AND CONCLUSIONS - The proliferative response of LCC2 cells to tamoxifen in vitro was not altered by TGF-beta neutralizing antibodies. However, established LCC2 tumors in nude mice treated with tamoxifen plus TGF-beta antibodies failed to grow, whereas tumors treated with tamoxifen plus a control antibody continued to proliferate. This reversal of tamoxifen resistance by TGF-beta antibodies did not occur in beige/nude mice, which lack NK-cell function, suggesting that immune mechanisms may be involved in the antitumor effects of tamoxifen. Antisense TGF-beta2 oligodeoxynucleotides enhanced the NK sensitivity of LCC2 cells in the presence of tamoxifen. Finally, LCC1 tumors were markedly more sensitive to tamoxifen in NK-active than in NK-deficient mice.
IMPLICATIONS - These data suggest that host NK function mediates, in part, the antitumor effect of tamoxifen and that TGF-beta2 may abrogate this mechanism, thus contributing to tamoxifen resistance.