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Galectin-3 Regulates a Molecular Switch from N-Ras to K-Ras Usage in Human Breast Carcinoma Cells

¹ Department of Neurobiochemistry, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv, Israel and ² Tumor Progression and Metastasis Program, Karmanos Cancer Institute, School of Medicine, Wayne State University, Detroit, Michigan

Requests for reprints: Yoel Kloog, Department of Neurobiochemistry, George S. Wise Faculty of Life Sciences, Tel-Aviv University, 69978 Tel-Aviv, Israel. Phone: 972-3-640-9699; Fax: 972-3-640-7643; E-mail: yoelk{at}tauex.tau.ac.il.

Galectin-3 (Gal-3), a pleiotropic carbohydrate-binding protein, is a selective binding partner of activated K-Ras-GTP. Because both proteins are antiapoptotic and associated with cancer progression, we questioned the possible functional role of Gal-3 in K-Ras activation. We found that overexpression of Gal-3 in human breast cancer cells (BT-549/Gal-3) coincided with a significant increase in wild-type (wt) K-Ras-GTP coupled with loss in wt N-Ras-GTP, whereas the nononcogenic Gal-3 mutant proteins [Gal-3(S6E) and Gal-3(G182A)] failed to induce the Ras isoform switch. Only wt Gal-3 protein coimmunoprecipitated and colocalized with oncogenic K-Ras, resulting in its activation with radical alterations in Ras signaling pathway, whereby the activation of AKT and Ral was suppressed and shifted to the activation of extracellular signal-regulated kinase (ERK). Specific inhibitors for Ras or mitogen-activated protein/ERK kinase (farnesylthiosalicylic acid and UO126, respectively) inhibited Gal-3–mediated apoptotic resistance and anchorage-independent growth functions. In conclusion, this study shows that Gal-3 confers on BT-549 human breast carcinoma cells several oncogenic functions by binding to and activation of wt K-Ras, suggesting that some of the molecular functions of Gal-3 are, at least in part, a result of K-Ras activation.

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