CCR Translations Tumor Endothelial Cells Join the Resistance

In this issue of Clinical Cancer Research, Xiong and colleagues report that isolated tumor-specific endothelial cells (TECs) from human hepatocellular carcinoma (HCC) are less sensitive to cytotoxic and antiangiogenic drugs when compared to their normal counterparts in vitro (1). The results of this study are in good accord with the growing body of in vivo evidence and clinical data suggesting that, in contrast to dogma, TECs may acquire drug resistance or be less sensitive to antiangiogenic strategies compared to normal endothelial cells (NECs). Due to genomic instability and a high mutation rate, tumor cells are mutable and may become drug resistant over time. It was therefore proposed over 30 years ago by Folkman that targeting the tumor-associated endothelium, which provides blood and nutrients to growing tumor cells, could be an alternative strategy for eliminating solid tumors (2). Almost a half century of angiogenesis research has produced several groundbreaking antiangiogenic therapies that are now used for treating some cancers and other angiogenesis-dependent diseases, including age-related macular degeneration (3). As the prototype of successful bench-to-bedside investigation, the anti-VEGF (vascular endothelial growth factor) monoclonal antibody Bevacizumab (Avastin) is approved by the FDA for treating colon, breast, and lung cancers in combination with chemotherapy. When Folkman postulated that tumors could be shrunk by targeting the blood vessels feeding them, it was assumed that the tumor endothelium was “normal” and was unlikely to evade antiangiogenic therapies. However, a number of studies have now documented changes at the morphologic and molecular levels in TECs from a variety of tumors (4), and clinical studies seem to support the possibility that TECs may become refractory to antiangiogenic therapy over time (particularly to anti-VEGF therapies) (ref. 5). Perhaps more disconcerting, some antiangiogenic therapies have recently been reported to unexpectedly facilitate metastasis in preclinical studies (6, 7). Why are antiangiogenic therapies not producing the sustained antitumor benefit as hoped? A two-tiered model of resistance to antiangiogenic therapies was recently put forth (8). First, TECs may develop “evasive” resistance by adapting to a specific angiogenesis inhibitor, for example by upregulating compensatory cellular survival pathways in response to anti-VEGF treatment. Second, inherent differences in TECs compared to their normal counterparts might impinge on the effectiveness of antiangiogenic therapies. These inherent differences may come about due to acquired alterations, perhaps as TECs evolve in the face of microenvironmental stress created by the growing mass of tumor cells. It has been challenging to address specific questions about TEC biology because these cells are difficult to isolate and culture. But as Xiong and colleagues have done, one approach is to use antibody-coupled magnetic beads to isolate ECs from collagenase-digested tumors and counterpart tissues (Fig. 1). Using this technique, Xiong and colleagues identified inherent differences in TECs from HCC compared to their counterparts from normal liver. The authors determined that compared to NECs, TECs isolated fromHCCwere less sensitive to adriamycin, 5-fluoruracil, and Sorafenib (an inhibitor of VEGFR-2, PDGFR, and c-Kit) when cultured and treated with each drug ex vivo. Because drug resistance usually implies activation of compensatory pathways after inhibition of a specific pathway, these findings do not necessarily mean that TECs have developed drug resistance by the textbook definition; instead, the author's results suggest that TECs are inherently less sensitive to these drugs evenwithout prior treatment. Therefore, whatever changes in TECs that resulted in their decreased sensitivity were already present. Our laboratory (9) and others (10) have shown similar differences in drug sensitivity when TECs and other tumor stromal cells were compared to their normal counterparts in vitro. It may be that common pathways (e.g., p53) known to mediate cellular responses to chemotherapies are defective both in the tumor stromal cells and in the tumor cells themselves (11). Sorafenib is currently approved by the FDA for the treatment of HCC, and, if feasible, it would be informative if the authors were to isolate TECs from patients with HCC after Sorafenib treatment. In that way, whether TECs from these patients acquire “evasive” resistance, perhaps by upregulating compensatory cellular survival pathways, could be determined. Authors' Affiliations: Vascular Biology Program, Departments of Surgery and Pathology, Harvard Medical School, Boston, Massachusetts Received 4/30/09; accepted 5/6/09; published OnlineFirst 7/28/09. Requests for reprints: Andrew C. Dudley, Children's Hospital, 300 Longwood Avenue, Boston, MA 02115. Phone: 617-919-2166; Fax: 617-7300233; E-mail: [email protected]. F 2009 American Association for Cancer Research. doi:10.1158/1078-0432.CCR-09-0902