Moreover our present work suggests that AXL

Moreover, our present work suggests that AXL could be a modulator of sunitinib response, at least for cell lines that present high endogenous levels of this RTK activation, since we observed an increased responsiveness to sunitinib in syk inhibitors activated with AXL receptor ligand. A role for AXL in molecular targeted therapies modulation is not new and has been related with resistance. For example, de novo activation of AXL has been associated with imatinib, lapatinib treatment and mainly with resistance to EGFR inhibitors in several tumor models [45], [46], [69], [70], [71], [72]. Interestingly, we found that AXL-silenced cells exhibited higher activation levels of the MAPK and AKT pathways and that some RTKs, such as EGFR, are overactivated upon AXL inhibition. Several preclinical and clinical studies have illustrated that deregulation of one signaling pathway can sometimes alleviate or bypass the “addiction” to another pathway [73], [74]. Thus, to overcome the activation of intracellular signaling pathways that render resistance to RTK inhibitors, several combinations of drugs are currently being tested to target both ERK/MAPK and PI3K pathways, and their use in combination with other targeted therapies holds great promise [75], [76], [77]. Our in vitro studies showed that by combining sunitinib with AKT inhibitors the lower sensitivity of AXL-knockdown cells to sunitinib could be reverted. This suggests that this therapeutic strategy may be effective in glioblastoma patients, mainly for those with low AXL activity who are less likely to be responsive to sunitinib treatment alone.
Acknowledgments This work was funded by Fundação para a Ciência e Tecnologia (FCT), Portugal (project: PTDC/SAU-TOX/114549/2009). Olga Martinho is a recipient of a Post-Doc fellowship (UMINHO/BPD/32/2013) from QREN. We would like to acknowledge Dr. Shuang-En Chuang from the National Health Research Institute, Taiwan, for providing AXL vectors, and Dr. Raquel Andrade for critical review of the manuscript.
Introduction Axl is a member of the large receptor tyrosine kinase (RTK) family of transmembrane proteins. Axl belongs to the TAM (tyro-3, Axl and Mer) subfamily of RTKs and consists of a characteristic structure including a two immunoglobin-like domains with dual fibronectin type III repeats in the extracellular component and a cytoplasmic kinase domain [1], [2]. Axl is ubiquitously expressed in a wide variety of tissues and cells including the hippocampus and cerebellum, monocytes, macrophages, platelets, endothelial cells, heart, skeletal muscle, liver, kidney, and testis [3]. Axl binds to growth arrest-specific gene 6 (Gas-6) and exerts biological effects through PI3K, STAT and NF-kB signaling pathways [1], [2], [4]. Axl overexpression and ligand-induced activation are observed in a variety of cancers including breast [5], colon [6], esophageal [7], glioma [8], liver [9], lung [10], thyroid [11], pancreatic [12] and prostate [13], [14]. Axl binding to its ligand Gas-6 promotes growth, survival, and proliferation by activation of the RAS/RAF/MAPK/ERK1/2 and PI3K signaling pathways [2]. Axl overexpression increases motility in glioblastoma cells through regulation of the actin cytoskeleton [15]. Similarly, Axl overexpression confers an invasive phenotype to breast cancer cells that could be reduced by RNA interference, suggesting that Axl plays a role in cell motility and adhesion [16]. Furthermore, Axl knockdown completely abolished the ability of cells emerging from primary tumors to colonize the lungs [16]. Supporting these preclinical studies, Axl expression correlates with adherence and metastatic potential of lung cancer cells in patients with adenocarcinoma [10]. Axl overexpression and its role in pathogenesis have been well described for pancreatic and prostate cancers [2], [12], [13], [14]. Previously we have shown that 55% of pancreatic adenocarcinomas tested were positive for Axl, and that this expression was associated with lymph node metastasis [12]. Tumors with high Axl expression were also associated with a poor survival rate compared to those with low levels of expression [12]. Genetic downregulation of Axl in pancreatic cancer cells resulted in reduced invasive and migratory capacity, demonstrating the importance of Axl in metastasis [12]. Similarly, nearly 50% of prostate tumors were shown to up-regulate Axl by 2- to 1500-fold [14]. In prostate tumors the intensity of staining for Axl increases with tumor grade, and bone metastases have elevated expression compared to the metastases to lymph nodes [13].