More than 650,000 people die each year from breast cancer, making it a particularly significant disease worldwide. The development of about 70 percent of breast tumors depends on steroid hormones, namely estrogens. Estrogens trigger the signaling pathways that support tumor growth and progression. Hydroxytamoxifen halting estrogen-induced tumor growth is among the most effective drugs in current anticancer therapy. The purpose of this work was to investigate approaches to overcome breast cancer cell resistance to hydroxytamoxifen (HT).
Cells with resistance to antiestrogen hydroxytamoxifen were obtained by long-term incubation of parental MCF7 cells with this drug. Estrogen receptor α expression was analyzed by immunoblotting. The resistant MCF7/HT cells were found not to lose ERα expression. These cells were found to have fairly high ERα activity, as assessed by reporter gene assays. Akt kinase belongs to the PI3K/Akt/mTOR signaling pathway, its increased activity was detected in resistant cells. Three types of Akt inhibitors were evaluated, including Akt inhibitor IV (6-(2-benzothiazolyl)-1-ethyl-2-[2-(methylphenylamino)ethenyl]-3-phenyl-1H-benzimidazolium, monoiodide), 10-DEBC (2-chloro-N,N-diethyl-10H-phenoxazine-10-butanamine, monohydrochloride), and luminespib (HSP90 inhibitor, 5-[2,4-dihydroxy-5-(1-methylethyl)phenyl]-N-ethyl-4-[4-(4-morpholinylmethyl)phenyl]-3-isoxazolecarboxamide). All three compounds showed high antiproliferative activity against hydroxytamoxifen-resistant cells. The IC50 value of 10-DEBC was 4.2 µM, when Akt inhibitor IV was more active with IC50 value of 390 nM. The HSP90 inhibitor luminespib, which reduces Akt expression, showed the highest activity against parental and hydroxytamoxifen-resistant breast cancer cells, with an IC50 value of 14 and 18 nM respectively. Thus, the obtained hydroxytamoxifen-resistant cells were found to partially retain hormone signaling. Increased Akt activity was observed in resistant cells, which associated with their sensitivity to selective Akt inhibitors. The best effects were discovered for HSP90-Akt blocker luminespib with IC50 value of about 20 nM.