Abstract C89: ESRP1-CD44v-xCT axis as a potential therapeutic target for the metastasis
Authors: Yoshida Go J. , Tsuchihashi Kenji , Yae Toshifumi , Nagano Osamu , Saya Hideyuki 
Published: August 2014

Metastasis of tumor cells is a multi-step phenomenon including dissociation from primary site, intravasation, long-term circulation in the blood, extravasation, and colonization in the pre-metastatic niche in the distant organs. It is quite important that cancer cells are always exposed to redox stress throughout this muti-step metastatic phenomenon. This is why metastasis occurs only when cancer cells are refractory to the high level of reactive oxygen species (ROS) in the tumor microenvironment.

CD44, one of the cancer stem cell (CSC) markers, makes diversity in isofroms due to the alternative splicing via epithelial splicing regulatory protein1 (ESRP1) or transcription factor grainyhead-like 2 (Grhl2). In the clinical perspective, a high expression level of ESRP1 or Grhl-2 in the tumor cells is generally associated with a poor prognosis in breast cancer patients. In addition, epigenetic modifications including DNA and histone methylation at metastasis-related genes have been also considered as a cause of cancer metastasis. This is why the depletion of CD44variant isoform (CD44v) is not necessarily accompanied by epithelial-mesenchymal transition (EMT), given that EMT has widely been recognized to expand the CSC population.

Besides, CD44 is not only a surface marker of CSCs but also a functional molecule; for instance, the interaction between CD44v6 and c-Met, the receptor of hepatocyte growth factor (HGF), activates the Ras-dependent survival signal of tumor cells. We have recently identified that CD44 variant 8-10 (CD44v9) stabilizes the localization of xCT glutamate-cystine transporter at the cellular membrane, thereby increasing the intracellular level of reduced glutathione (GSH). In fact, enrichment of CD44v9-positive tumor cells is frequently recognized in the metastatic foci and recalcitrant cases.

The expression level of CD44v9 is positively correlated with the Akt/PI3K/mTOR signal activity of metastatic breast tumor cells. Akt/PI3K/mTOR signal is related to cholesterol synthesis and to adaptation to the hypo-nutrition, or hypoxia. In the case of metastasis of breast cancer cells to the lungs, tumor-entrained neutrophils (TEN) accumulate in the lung to inhibit lung metastasis through the increased redox stress in the NADPH-dependent manner. Thus, the presence of xCT transporter promotes the GSH synthesis and the protection from tumor stroma-derived ROS, which plays a fundamental role in the adaptive resistance to conventional cancer therapy for the metastasis.

We have also revealed that ESRP1-dependent alternative splicing of CD44 mRNA increases both the intracellular GSH level and the metastatic potential of the breast cancer cells. We have demonstrated that depletion of ESRP1 or Grhl2 in metastatic breast cancer cells adversely affects the metastatic potential, since they fail to express CD44v9. Notably, even the tumor cells which are lack of CD44v9 are able to form the metastasis to lungs, but the ability of survival and proliferation in the microenvironment rich in redox stress are considerably lower. Akt/PI3K/mTOR activity is significantly suppressed compared with the aggressive metastatic foci mainly composed of CD44v9-positive tumor cells.

Therapies targeted to alternative splicing of CD44 as well as those that target CD44v-mediated stabilization of xCT might be expected to imapir the adaptive resistance to ROS in the metastatic stem-like cancer cells and thereby rendering them susceptible to currently available cancer treatments. Considering the high dependency to lipid synthesis In the survival and proliferation of metastatic breast cancer cells, HMG-CoA reductase inhibitors contribute to the synergistic inhibition of the metastatic exacerbation.

Citation Format: Go J. Yoshida, Kenji Tsuchihashi, Toshifumi Yae, Osamu Nagano, Hideyuki Saya. ESRP1-CD44v-xCT axis as a potential therapeutic target for the metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr C89.