Stable expression and visualization of Mat-8 (FXYD-3) tagged with a fluorescent protein in Chinese hamster ovary (CHO)-K1 cells

Displacement of Native FXYD Protein From Na+/K+-ATPase With Novel FXYD Peptide Derivatives: Effects on Doxorubicin Cytotoxicity

The seven mammalian FXYD proteins associate closely with α/β heterodimers of Na⁺/K⁺-ATPase. Most of them protect the β1 subunit against glutathionylation, an oxidative modification that destabilizes the heterodimer and inhibits Na⁺/K⁺-ATPase activity. A specific cysteine (Cys) residue of FXYD proteins is critical for such protection. One of the FXYD proteins, FXYD3, confers treatment resistance when overexpressed in cancer cells. We developed two FXYD3 peptide derivatives. FXYD3-pep CKCK retained the Cys residue that can undergo glutathionylation and that is critical for protecting the β1 subunit against glutathionylation. FXYD3-pep SKSK had all Cys residues mutated to Serine (Ser). The chemotherapeutic doxorubicin induces oxidative stress, and suppression of FXYD3 with siRNA in pancreatic- and breast cancer cells that strongly express FXYD3 increased doxorubicin-induced cytotoxicity. Exposing cells to FXYD3-pep SKSK decreased co-immunoprecipitation of FXYD3 with the α1 Na⁺/K⁺-ATPase subunit. FXYD3-pep SKSK reproduced the increase in doxorubicin-induced cytotoxicity seen after FXYD3 siRNA transfection in pancreatic- and breast cancer cells that overexpressed FXYD3, while FXYD3-pep CKCK boosted the native protein’s protection against doxorubicin. Neither peptide affected doxorubicin’s cytotoxicity on cells with no or low FXYD3 expression. Fluorescently labeled FXYD3-pep SKSK was detected in a perinuclear distribution in the cells overexpressing FXYD3, and plasmalemmal Na⁺/K⁺-ATPase turnover could not be implicated in the increased sensitivity to doxorubicin that FXYD3-pep SKSK caused. FXYD peptide derivatives allow rapid elimination or amplification of native FXYD protein function. Here, their effects implicate the Cys residue that is critical for countering β1 subunit glutathionylation in the augmentation of cytotoxicity with siRNA-induced downregulation of FXYD3.

Overexpression of FXYD-3 is involved in the tumorigenesis and development of esophageal squamous cell carcinoma

Objective. To investigate the association of FXYD-3 expression with clinicopathological variables and PINCH in patients with ESCC. Patients and Methods. Expression of FXYD-3 protein was immunohistochemically examined in normal esophageal mucous (n = 20) and ESCC (n = 64). Results. Expression of FXYD-3 in the cytoplasm markedly increased from normal esophageal epithelial cells to primary ESCC (P = 0.001). The expression of FXYD-3 was correlated with TNM stages and depth of tumor invasion. Furthermore, the cases with lymph node metastasis tended to show a higher frequency of positive expression than those without metastasis (P = 0.086), and FXYD-3 expression tended to be positively related to the expression of PINCH (P = 0.063). Moreover, the cases positive for both proteins had the highest frequency of lymph node metastasis (P = 0.001). However, FXYD-3 expression was not correlated with patient's gender (P = 0.847), age (P = 0.876), tumor location (P = 0.279), size (P = 0.771), grade of differentiation (P = 0.279), and survival (P = 0.113). Conclusion. Overexpression of FXYD-3 in the cytoplasm may play an important role in the tumorigenesis and development in the human ESCC, particularly in combination with PINCH expression.

Down-regulation of FXYD3 expression in human lung cancers: its mechanism and potential role in carcinogenesis

FXYD3 is a FXYD-containing Na,K-ATPase ion channel regulator first identified as a protein overexpressed in murine breast tumors initiated by oncogenic ras or neu. However, our preliminary study revealed that FXYD3 expression was down-regulated in oncogenic KRAS-transduced airway epithelial cells. This contradiction led us to investigate the role of FXYD3 in carcinogenesis of the lung. FXYD3 mRNA and protein levels were lower in most of the lung cancer cell lines than in either the noncancerous lung tissue or airway epithelial cells. Protein levels were also lower in a considerable proportion of primary lung cancers than in nontumoral airway epithelia; FXYD3 expression levels decreased in parallel with the dedifferentiation process. Also, a somatic point mutation, g55c (D19H), was found in one cell line. Forced expression of the wild-type FXYD3, but not the mutant, restored the well-demarcated distribution of cortical actin in cancer cells that had lost FXYD3 expression, suggesting FXYD3 plays a role in the maintenance of cytoskeletal integrity. However, no association between FXYD3 expression and its promoter's methylation status was observed. Therefore, inactivation of FXYD3 through a gene mutation or unknown mechanism could be one cause of the atypical shapes of cancer cells and play a potential role in the progression of lung cancer.

Interaction of Mat-8 (FXYD-3) with Na+/K+-ATPase in Colorectal Cancer Cells

Mat-8 was fused with a Myc-tag or green fluorescent protein at its carboxyl terminus, and then expressed in Chinese hamster ovary K1 cells. Determination of the cellular localization of the tagged proteins suggested that they were localized on the intracellular membrane, being not only detected around the nuclear envelope but also partly overlapping with markers for endosomes and Golgi bodies. However, Mat-8 with the Myc-tag was detected on the plasma membrane as well as the intracellular membrane, when it was expressed in colorectal cancer cells. The membrane fraction of the cancer cells was solubilized and immuno-precipitated with an antibody for the Myc-tag. Western-blotting analysis demonstrated that the Na+/K+-ATPase alpha subunit was present in the precipitate. Furthermore, the immuno-precipitate obtained with an antibody for the Na+/K+-ATPase alpha subunit reacted with that for the Myc-tag. These results suggested that Mat-8 could be associated with Na+/K+-ATPase similar to other FXYD family members. The Gly41-->Arg mutation in the transmembrane region of Mat-8 inhibited its association with the Na+/K+-ATPase alpha subunit and localization on the plasma membrane, whereas the Cys44-->Ala or Cys49-->Ala substitution did not. Thus the conserved Gly41 residue in the transmembrane domain could be indispensable for localization of Mat-8 on the cell surface.