Microenvironment and Immunology Tumor Endothelial Cells with Distinct Patterns of TGFb-Driven Endothelial-to-Mesenchymal Transition

Endothelial-to-mesenchymal transition (EndMT) occurs during development and underlies the pathophysiology of multiple diseases. In tumors, unscheduled EndMT generates cancer-associated myofibroblasts that fuel inflammation and fibrosis, and may contribute to vascular dysfunction that promotes tumor progression. We report that freshly isolated subpopulations of tumor-specific endothelial cells (TEC) from a spontaneous mammary tumor model undergo distinct forms of EndMT in response to TGFb stimulation. Although some TECs strikingly upregulate a smooth muscle actin (SMA), a principal marker of EndMT and activated myofibroblasts, counterpart normal mammary gland endothelial cells (NEC) showed little change in SMA expression after TGFb treatment. Compared with NECs, SMAþ TECs were 40% lessmotile in wound-healing assays and formedmore stable vascular-like networks in vitro when challenged with TGFb. Lineage tracing using ZsGreen reporter mice confirmed that only a fraction of vessels in breast tumors contain SMAþ TECs, suggesting that not all endothelial cells (EC) respond identically to TGFb in vivo. Indeed, examination of 84 TGFb-regulated target genes revealed entirely different genetic signatures in TGFb-stimulated NEC and TEC cultures. Finally, we found that basic FGF (bFGF) exerts potent inhibitory effects on many TGFb-regulated genes but operates in tandemwith TGFb to upregulate others. ECs challengedwith TGFb secrete bFGF,which blocks SMA expression in secondary cultures, suggesting a cell-autonomous or lateralinhibitory mechanism for impeding mesenchymal differentiation. Together, our results suggest that TGFb-driven EndMT produces a spectrum of EC phenotypes with different functions that could underlie the plasticity and heterogeneity of the tumor vasculature. Cancer Res; 75(7); 1244–54. 2015 AACR.