Silencing overexpression of FXYD3 protein in breast cancer cells amplifies effects of doxorubicin and γ-radiation on Na⁺/K⁺-ATPase and cell survival

FXYD3, also known as mammary tumor protein 8, is overexpressed in several common cancers, including in many breast cancers. We examined if such overexpression might protect Na⁺/K⁺-ATPase and cancer cells against the high levels of oxidative stress characteristic of many tumors and often induced by cancer treatments. We measured FXYD3 expression, Na⁺/K⁺-ATPase activity and glutathionylation of the β1 subunit of Na⁺/K⁺-ATPase, a reversible oxidative modification that inhibits the ATPase, in MCF-7 and MDA-MB-468 cells. Expression of FXYD3 was suppressed by transfection with FXYD3 siRNA. A colorimetric end-point assay was used to estimate cell viability. Apoptosis was estimated by caspase 3/7 (DEVDase) activation using a Caspase fluorogenic substrate kit. Expression of FXYD3 in MCF-7 breast cancer cells was ~eightfold and ~twofold higher than in non-cancer MCF-10A cells and MDA-MB-468 cancer cells, respectively. A ~50 % reduction in FXYD3 expression increased glutathionylation of the β1 Na⁺/K⁺-ATPase subunit and reduced Na⁺/K⁺-ATPase activity by ~50 %, consistent with the role of FXYD3 to facilitate reversal of glutathionylation of the β1 subunit of Na⁺/K⁺-ATPase and glutathionylation-induced inhibition of Na⁺/K⁺-ATPase. Treatment of MCF-7 and MDA-MB- 468 cells with doxorubicin or γ-radiation decreased cell viability and induced apoptosis. The treatments upregulated FXYD3 expression in MCF-7 but not in MDA-MB-468 cells and suppression of FXYD3 in MCF-7 but not in MDA-MB-468 cells amplified effects of treatments on Na⁺/K⁺-ATPase activity and treatment-induced cell death and apoptosis. Overexpression of FXYD3 may be a marker of resistance to cancer treatments and a potentially important therapeutic target.