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Tumors are composed of cancer cells but also a larger number of diverse stromal cells in the tumor microenvironment. Stromal cells provide essential supports to tumor pathophysiology but the distinct characteristics of their signaling networks are not usually considered in developing drugs to target tumors. This oversight potentially confounds proof-of-concept studies and increases drug development risks. Here, we show in established murine and human models of breast cancer how differential regulation of Akt by the small GTPase RhoB in cancer cells or stromal endothelial cells determines their dormancy versus outgrowth when angiogenesis becomes critical. In cancer cells in vitro or in vivo, RhoB functions as a tumor suppressor that restricts EGF receptor (EGFR) cell surface occupancy as well as Akt signaling. However, after activation of the angiogenic switch, RhoB functions as a tumor promoter by sustaining endothelial Akt signaling, growth, and survival of stromal endothelial cells that mediate tumor neoangiogenesis. Altogether, the positive impact of RhoB on angiogenesis and progression supercedes its negative impact in cancer cells themselves. Our findings elucidate the dominant positive role of RhoB in cancer. More generally, they illustrate how differential gene function effects on signaling pathways in the tumor stromal component can complicate the challenge of developing therapeutics to target cancer pathophysiology.

Chronic activation of Akt signaling in the endothelium recapitulates the salient features of a tumor vasculature and can be inhibited by rapamycin, an inhibitor of mammalian target of rapamycin. This led to the hypothesis that the antitumor efficacy of rapamycin may be partially dependent on its ability to inhibit endothelial Akt signaling, making rapamycin an antiangiogenic agent and endothelial Akt pathway inhibitor. Dose-response studies with rapamycin showed that primary human endothelial cells and fibroblasts had a bimodal Akt response with effective reductions in phosphorylated Akt (pAkt) achieved at 10 ng/mL. In contrast, rapamycin increased pAkt levels in tumor cell lines. When tumor-bearing mice were treated with rapamycin doses comparable to those used clinically in transplant patients, we observed strong inhibition of mammary tumor growth. To test whether Akt activation in the endothelium was rate-limiting for this antitumor response, we engineered mouse mammary tumor virus-polyoma virus middle T antigen mice with endothelial cell-specific expression of constitutively activated Akt. We observed that the antitumor efficacy of rapamycin was reduced in the presence of elevated endothelial Akt activation. Just as we observed in MCF7 cells in vitro, rapamycin doses that were antiangiogenic resulted in increased pAkt levels in total mouse mammary tumor virus-polyoma virus middle T antigen tumor lysates, suggesting that tumor cells had an opposite Akt response following mammalian target of rapamycin inhibition compared with tumor endothelial cells. Together, these data support the hypothesis that endothelial Akt signaling in the tumor vasculature is an important target of the novel anticancer drug rapamycin.

Endothelial cells in growing tumors express activated Akt, which when modeled by transgenic endothelial expression of myrAkt1 was sufficient to recapitulate the abnormal structural and functional features of tumor blood vessels in nontumor tissues. Sustained endothelial Akt activation caused increased blood vessel size and generalized edema from chronic vascular permeability, while acute permeability in response to VEGF-A was unaffected. These changes were reversible, demonstrating an ongoing requirement for Akt signaling for the maintenance of these phenotypes. Furthermore, rapamycin inhibited endothelial Akt signaling, vascular changes from myrAkt1, tumor growth, and tumor vascular permeability. Akt signaling in the tumor vascular stroma was sensitive to rapamycin, suggesting that rapamycin may affect tumor growth in part by acting as a vascular Akt inhibitor.