Abstract 4889: Transforming growth factor-β signaling in breast cancer subtypes

Abstract

Abstract Transforming growth factor β (TGF-β) plays a dual role in breast cancer (BC), acting as a tumor suppressor during the early stages of tumor development and assuming a prometastatic role, enhancing invasion and metastasis during the later stages of progression. This inherent complexity of TGF-β signaling in BC prompted us to characterize the TGF-β signaling pathway as a function of BC subtype. For this purpose, we used a panel of human BC cell lines that were either estrogen receptor (ER) positive (luminal) or -negative (basal-like) and either HER2 positive or -negative. Both luminal and basal-like cell lines secreted bioactive TGF-β, as determined by activation of the PAI-1 promoter (PAI-1pr) in PAI-1pr-LUC transfected mink lung epithelial cells (TMLC). Nonetheless, we had previously found that basal-like BC cell lines typically express a TGF-β response gene signature (TBRS), while luminal cell lines do not. Consistent with this observation, treatment with exogenous TGF-β induced phosphorylation of Smad2/3 in the ER-negative but not in ER-positive cells. Furthermore, we found a striking inverse correlation between the level of ER-alpha and TGF-β receptor II (TβRII) transcripts. Whereas basal-like cell lines typically expressed high levels of TβRII mRNA, luminal lines uniformly lacked TβRII expression. These results were confirmed by multiplex qRT-PCR. It has been shown previously that treatment of ER-positive BC cell lines with estradiol suppresses their production of TGF-β, while treatment with anti-estrogens such as tamoxifen increases the level of secreted TGF-β. In fact, it has been proposed that induction of TGF-β by tamoxifen might mediate its anti-tumor effects against ER-positive BC. However, our results essentially exclude the possibility that tamoxifen-induced growth inhibition is mediated by autocrine effects of TGF-β, as the tamoxifen-responsive BC cell lines are intrinsically unresponsive to TGF-β. On the other hand, TGF-β may mediate tamoxifen-resistance, as previous studies have shown that treatment of tumor bearing mice with a TGF-β neutralizing antibody could overcome resistance to tamoxifen. Moreover, recent studies indicate that both tamoxifen-and fulvestrant-resistant MCF-7 cell variant express high levels of TGF-β pathway genes, including TβRII. Therefore, we examined the mechanism of TβRII gene silencing in luminal BC lines. Treatment of ER-positive cell lines with 5-Aza-deoxycytidine alone, Trichostatin-A alone or a combination of both, resulted in a significant induction of TβRII mRNA expression in cells treated with both agents. These findings suggest that TβRII gene silencing in luminal BC cell lines is the result of both DNA methylation and histone deacetylation. We are currently investigating the possibility that re-expression of TβRII mRNA might sensitize luminal BC cells to treatment with anti-estrogens, TGF-β antagonists or a combination of the two modalities. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4889.