Rapalogs can promote cancer cell stemness in vitro in a Galectin-1 and H-ras-dependent manner

// Itziar M.D. Posada 1, * , Benoit Lectez 1, * , Mukund Sharma 1 , Christina Oetken-Lindholm 1 , Laxman Yetukuri 1, 3 , Yong Zhou 2 , Tero Aittokallio 3, 4 and Daniel Abankwa 1 1 Turku Center for Biotechnology, Abo Akademi University, Tykistokatu 6B, Turku, Finland 2 Department of Integrative Biology and Pharmacology, McGovern Medical School, University of Texas Health Science Center at Houston, Houston, Texas, United States of America 3 Institute for Molecular Medicine Finland, FIMM, University of Helsinki, Helsinki, Finland 4 Department of Mathematics and Statistics, University of Turku, Turku, Finland * These authors have contributed equally to this work Correspondence to: Daniel Abankwa, email: daniel.abankwa@btk.fi Keywords: mTORC1, Ras, rapamycin, galectin, cancer stem cells Received: January 31, 2017      Accepted: April 22, 2017      Published: May 11, 2017 ABSTRACT Currently several combination treatments of mTor- and Ras-pathway inhibitors are being tested in cancer therapy. While multiple feedback loops render these central signaling pathways robust, they complicate drug targeting. Here, we describe a novel H-ras specific feedback, which leads to an inadvertent rapalog induced activation of tumorigenicity in Ras transformed cells. We find that rapalogs specifically increase nanoscale clustering (nanoclustering) of oncogenic H-ras but not K-ras on the plasma membrane. This increases H-ras signaling output, promotes mammosphere numbers in a H-ras-dependent manner and tumor growth in ovo . Surprisingly, also other FKBP12 binders, but not mTor-inhibitors, robustly decrease FKBP12 levels after prolonged (>2 days) exposure. This leads to an upregulation of the nanocluster scaffold galectin-1 (Gal-1), which is responsible for the rapamycin-induced increase in H-ras nanoclustering and signaling output. We provide evidence that Gal-1 promotes stemness features in tumorigenic cells. Therefore, it may be necessary to block inadvertent induction of stemness traits in H-ras transformed cells by specific Gal-1 inhibitors that abrogate its effect on H-ras nanocluster. On a more general level, our findings may add an important mechanistic explanation to the pleiotropic physiological effects that are observed with rapalogs.