Activation of mitogenic pathways and sensitization to estrogen-induced apoptosis: two independent characteristics of tamoxifen-resistant breast cancer cells?

Abstract

Paradoxical induction of apoptosis by estrogen has been described previously for estrogen-deprived and antiestrogen-resistant breast cancer cells. In this study we analyzed the possible interrelations between cell sensitization to estrogen apoptotic action and cell ability to (anti)estrogen-independent growth. Using tamoxifen-resistant sublines derived from the parent MCF-7 breast cancer cells by long-term tamoxifen treatment we demonstrated that resistant cells are characterized by increased level of EGF receptor and unexpected increase of VEGF receptor 2 (Flk-1/KDR) and its specific ligand, VEGF-A. The importance of the VEGF signaling in the autocrine regulation of cell growth was indicated by the ability of VEGF inhibitor, soluble fragment of Flt-1/Fc chimera, to suppress the phosphorylation of MAP kinases as well as to inhibit the estrogen-independent growth of MCF-7 cells. Sensitization of tamoxifen-resistant cells to estrogen-induced apoptosis required the additional continuous cultivation in steroid-depleted medium and did not depend on the activity of both EGF and VEGF pathways. Finally, we showed that treatment of the cells with 17beta-estradiol (10(-9) M) resulted in a marked increase in p53 level both in the resistant cells undergoing apoptosis and in the parent MCF-7 cells insensitive to apoptotic estrogen action. These data provide an important support for the existence of a disbalance between pro- and anti-apoptotic machinery in the resistant breast cancer cells that forms independently of the acquired ability to estrogen-independent growth.

The phenomenon of acquired resistance to metformin in breast cancer cells: The interaction of growth pathways and estrogen receptor signaling

“…Lysates for Western blots of AMPK and phosphoAMPK were prepared in the buffer containing 50 mM Tris‐HCl, pH 7.5, 0.5% Igepal CA‐630, 150 mM NaCl, 1mM EDTA, 1mM DTT, 1mM PMSF, 0.1 mM Na‐orthovanadate and aprotinin, leupeptin, pepstatin (1 µg/mL of each) without sonication as described earlier in Ref. . Protein content was determined by Bradford method.…”

Enhancement of vascular endothelial growth factor–mediated angiogenesis in tamoxifen-resistant breast cancer cells: role of Pin1 overexpression

“…VEGF secretion by cancer cells is essential for angiogenesis during carcinogenesis (9,10) and also plays an essential role in transducing mitogenic signals (11). The expression level and the signaling activity of VEGF receptor 2 (Flk-1) are enhanced in TAMresistant breast cancer cells (12). Moreover, overexpression of VEGF in estrogen-dependent MCF-7 breast cancer cells abolishes their estrogen-dependent growth phenotype and stimulates estrogen-independent growth in ovariectomized mice (13).…”

The relationships between snail1 and estrogen receptor signaling in breast cancer cells

“…The most frequent mechanism of hormonal resistance is based on the activation of hormone-independent mitogenic signaling, including receptor tyrosine kinases, cell cycle-regulating proteins, PI3K/Akt cascade, etc. [Gnatyshak and Dryzhak, 1991;Dowsett, 2001;Weinberg et al, 2005;Scherbakov et al, 2006;Steelman et al, 2011].…”