Physiological Roles of ERM Proteins and Transcriptional Regulators in Supporting Membrane Expression of Efflux Transporters as Factors of Drug Resistance in Cancer

Abstract

One factor contributing to the malignancy of cancer cells is the acquisition of drug resistance during chemotherapy via increased expression of efflux transporters, such as P-glycoprotein (P-gp), multidrug resistance-associated proteins (MRPs), and breast cancer resistance protein (BCRP). These transporters operate at the cell membrane, and are anchored in place by the scaffold proteins ezrin (Ezr), radixin (Rdx), and moesin (Msn) (ERM proteins), which regulate their functional activity. The identity of the regulatory scaffold protein(s) differs depending upon the transporter, and also upon the tissue in which it is expressed, even for the same transporter. Another factor contributing to malignancy is metastatic ability. Epithelial-mesenchymal transition (EMT) is the first step in the conversion of primary epithelial cells into mesenchymal cells that can be transported to other organs via the blood. The SNAI family of transcriptional regulators triggers EMT, and SNAI expression is used is an indicator of malignancy. Furthermore, EMT has been suggested to be involved in drug resistance, since drug excretion from cancer cells is promoted during EMT. We showed recently that ERM proteins are induced by a member of the SNAI family, Snail. Here, we first review recent progress in research on the relationship between efflux transporters and scaffold proteins, including the question of tissue specificity. In the second part, we review the relationship between ERM scaffold proteins and the transcriptional regulatory factors that induce their expression.

Keywords: ERM proteins; P-glycoprotein; SNAI family; efflux transporters; epithelial–mesenchymal transition; membrane expression; transcriptional regulators.

Figure 1

Snail-induced Epithelial-to-mesenchymal transition enhances P-gp-mediated multidrug resistance in HCC827 Cells. (a–d) The mRNA expression levels of epithelial markers E-cadherin, occludin, and claudin-1 were reduced in Snail-overexpressing cells. On the other hand, the mRNA expression levels of mesenchymal markers vimentin were increased in Snail-overexpressing cells relative to Mock cells, respectively. (e–g) The mRNA expression level of Msn was significantly increased in Snail-overexpressing HCC827 cells compared to Mock cells, but the expression levels of Ezr and Rdx were unchanged. (h) The mRNA expression level of MDR1 was decreased in Snail-overexpressing HCC827 cells compared to Mock cells. (i) There was no significant difference in P-gp protein expression levels between 2 groups. (j) The efflux rate of Rho123 was increased in Snail-overexpressing cells compared to Mock cells. Moreover, Rho123 efflux was inhibited by coincubation with P-gp inhibitor elacridar(Elc). (k) In Snail-overexpressing cells, the efflux rate of 10 mM Rho 123 was significantly increased compared to Mock cells, and this increase was significantly suppressed by knockdown of Msn. *: p < 0.05, **: p < 0.01, N.S.: not significant (Student’s t-test (a–i) and Holm test (j,k)).

Figure 2

Presumed mechanisms of P-glycoprotein (P-gp)-mediated drug resistance during Snail-induce epithelial–mesenchymal transition (EMT).