Targeting MET and AXL overcomes resistance to sunitinib therapy in renal cell carcinoma.

Antiangiogenic therapy resistance occurs frequently in patients with metastatic renal cell carcinoma (RCC). The purpose of this study was to understand the mechanism of resistance to sunitinib, an antiangiogenic small molecule, and to exploit this mechanism therapeutically. We hypothesized that sunitinib-induced upregulation of the prometastatic MET and AXL receptors is associated with resistance to sunitinib and with more aggressive tumor behavior. In the present study, tissue microarrays containing sunitinib-treated and untreated RCC tissues were stained with MET and AXL antibodies. The low malignant RCC cell line 786-O was chronically treated with sunitinib and assayed for AXL, MET, epithelial-mesenchymal transition (EMT) protein expression and activation. Co-culture experiments were used to examine the effect of sunitinib pretreatment on endothelial cell growth. The effects of AXL and MET were evaluated in various cell-based models by short hairpin RNA or inhibition by cabozantinib, the multi-tyrosine kinases inhibitor that targets vascular endothelial growth factor receptor, MET and AXL. Xenograft mouse models tested the ability of cabozantinib to rescue sunitinib resistance. We demonstrated that increased AXL and MET expression was associated with inferior clinical outcome in patients. Chronic sunitinib treatment of RCC cell lines activated both AXL and MET, induced EMT-associated gene expression changes, including upregulation of Snail and ß-catenin, and increased cell migration and invasion. Pretreatment with sunitinib enhanced angiogenesis in 786-0/human umbilical vein endothelial cell co-culture models. The suppression of AXL or MET expression and the inhibition of AXL and MET activation using cabozantinib both impaired chronic sunitinib treatment-induced prometastatic behavior in cell culture and rescued acquired resistance to sunitinib in xenograft models. In summary, chronic sunitinib treatment induces the activation of AXL and MET signaling and promotes prometastatic behavior and angiogenesis. The inhibition of AXL and MET activity may overcome resistance induced by prolonged sunitinib therapy in metastatic RCC.

CTGF is a therapeutic target for metastatic melanoma.

Metastatic melanoma remains a devastating disease with a 5-year survival rate of less than five percent. Despite recent advances in targeted therapies for melanoma, only a small percentage of melanoma patients experience durable remissions. Therefore, it is critical to identify new therapies for the treatment of advanced melanoma. Here, we define connective tissue growth factor (CTGF) as a therapeutic target for metastatic melanoma. Clinically, CTGF expression correlates with tumor progression and is strongly induced by hypoxia through HIF-1 and HIF-2-dependent mechanisms. Genetic inhibition of CTGF in human melanoma cells is sufficient to significantly reduce orthotopic tumor growth, as well as metastatic tumor growth in the lung of severe combined immunodeficient (SCID) mice. Mechanistically, inhibition of CTGF decreased invasion and migration associated with reduced matrix metalloproteinase-9 expression. Most importantly, the anti-CTGF antibody, FG-3019, had a profound inhibitory effect on the progression of established metastatic melanoma. These results offer the first preclinical validation of anti-CTGF therapy for the treatment of advanced melanoma and underscore the importance of tumor hypoxia in melanoma progression.

The role of hypoxia-inducible factors in tumorigenesis.

Hypoxia-inducible factors (HIFs) are essential mediators of the cellular oxygen-signaling pathway. They are heterodimeric transcription factors consisting of an oxygen-sensitive alpha subunit (HIF-alpha) and a constitutive beta subunit (HIF-beta) that facilitate both oxygen delivery and adaptation to oxygen deprivation by regulating the expression of genes that control glucose uptake, metabolism, angiogenesis, erythropoiesis, cell proliferation, and apoptosis. In most experimental models, the HIF pathway is a positive regulator of tumor growth as its inhibition often results in tumor suppression. In clinical samples, HIF is found elevated and correlates with poor patient prognosis in a variety of cancers. In summary, HIF regulates multiple aspects of tumorigenesis, including angiogenesis, proliferation, metabolism, metastasis, differentiation, and response to radiation therapy, making it a critical regulator of the malignant phenotype.