Tumor Endothelial Cells with Distinct Patterns of TGFß-Driven Endothelial-to-Mesenchymal Transition.

ABSTRACT

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 TGFß stimulation. Although some TECs strikingly upregulate α 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 TGFß treatment. Compared with NECs, SMA(+) TECs were 40% less motile in wound-healing assays and formed more stable vascular-like networks in vitro when challenged with TGFß. Lineage tracing using ZsGreen(Cdh5-Cre) reporter mice confirmed that only a fraction of vessels in breast tumors contain SMA(+) TECs, suggesting that not all endothelial cells (EC) respond identically to TGFß in vivo. Indeed, examination of 84 TGFß-regulated target genes revealed entirely different genetic signatures in TGFß-stimulated NEC and TEC cultures. Finally, we found that basic FGF (bFGF) exerts potent inhibitory effects on many TGFß-regulated genes but operates in tandem with TGFß to upregulate others. ECs challenged with TGFß secrete bFGF, which blocks SMA expression in secondary cultures, suggesting a cell-autonomous or lateral-inhibitory mechanism for impeding mesenchymal differentiation. Together, our results suggest that TGFß-driven EndMT produces a spectrum of EC phenotypes with different functions that could underlie the plasticity and heterogeneity of the tumor vasculature.