The loss of hormonal dependency of breast tumor cells is often accompanied with the appearance of epithelial-mesenchymal transition (EMT) features and increase in cell metastasis and invasiveness. The central role in the EMT belongs to transcription factors Snail responded for the decrease in E-cadherin expression and cell contacts, stimulation of cell mobility and invasiveness. Aim was to study the relationships between estrogen receptor machinery and Snail1 signaling, and mechanism of Snail1 regulation in hormone-resistant breast cancer cells. The experiments were performed on the estrogen-dependent MCF-7 breast cancer cells, estrogen-hyposensitive MCF-7/LS subline generated through long-term cultivation of the parental cells in steroid-free medium, and ER-negative estrogen-resistant HBL-100 cells. Snail1, estrogen receptor, p65 NF-κB, E-cadherin levels were analyzed by Western blot. We found that decrease in the estrogen dependency is correlated with increase in Snail1 expression and activity, we demonstrated the Snail1 involvement in the negative regulation of ER, and showed that Snail1 inhibition partially restores the sensitivity of the estrogen-hyposensitive cells to antiestrogen tamoxifen. Furthermore, NF-κB was found to serve as a positive regulator of Snail1 in breast cancer cells, and simultaneous inhibition of NF-κB and Snail1 resulted in additional increase in cell response to tamoxifen. In general, the results obtained demonstrate the phenomenon of Snail1 activation in the hormone-resistant breast cancer cells, and show that Snail1 and NF-κB may serve as an important targets in the treatment of breast cancer, both estrogen-dependent and estrogen-independent tumors.

No abstract

Phosphatidylinositol-3 kinase (PI3K) belongs to one of the most important cellular proteins involved in the transmission of anti-apoptotic signal and regulation of survival pathways in tumor cells. Earlier we have found that prolonged treatment of melanoma cells with dexamethasone results in formation of a cell subline which was resistant to growth inhibitory dexamethasone action. We showed that constitutive activation of PI3K can be considered as one of the factors that regulate cell resistance to dexamethasone. Here we demonstrate that increased level of PI3K protein in dexamethasone-resistant cells correlates with partial decrease in expression of down-stream target of PI3K--protein kinase B (PKB). Study of the cell's sensitivity to various damaging agents showed that the cells after prolonged dexamethasone treatment are characterized by increased level of the resistance to both hormonal drugs and hypoxia, and at the same time with high sensitivity to ultraviolet (UV) radiation or anti-tumor agents such as adriamycin. As revealed, hypoxic conditions or short-term dexamethasone treatment of the resistant cells lead to a substantial increase in the PKB level, whereas neither UV radiation nor adriamycin affects the PKB level in these cells. We demonstrate that long-term dexamethasone treatment of melanoma cells results in the accumulation of the active form of mitogen-transducing signaling protein STAT3 (Signal Transducer and Activator of Transcription-3), which also contributes to inducing the melanoma cell's resistance to antiproliferative action of dexamethasone. We suggest that decreased level of PKB in combination with an activation of PI3K/STAT3 signaling in the melanoma cells after prolonged dexamethasone treatment may be one of the mechanisms of different sensitivity of these cells to hormonal drugs and damaging agents. The model of the progression of hormonal resistance of in vitro cultured tumor cells is presented.

No abstract

The progression of breast cancer cells to estrogen-independent growth may be accompanied with the paradoxical cell sensitization to estrogen apoptotic action; however, the mechanism of this phenomenon is still unclear. In the present study, we have shown that the sensitization of hormone-resistant breast cancer cells to estrogen apoptotic action is accompanied with the gradual NF-kappaB suppression. Using the chemical inhibitors of NF-kappaB as well as the dominant-negative NF-kappaB constructs, we have proved the sufficiency of NF-kappaB inhibition for the sensitization of the resistant cells to estrogen apoptosis. Estradiol treatment results in the additional suppression of NF-kappaB, demonstrating the possible NF-kappaB involvement in the regulation of cell response to estrogens. Totally, the results presented suggest that the constitutive NF-kappaB suppression in the estrogen-independent cells may be considered as one of the factors resulting in a imbalance between pro- and anti-apoptotic pathways and enhancement in estrogen apoptotic action in the cells.