Estrogen-induced Ets-1 promotes capillary formation in an in vitro tumor angiogenesis model

We employed an in vitro angiogenesis model that simulates the in vivo milieu for tumor capillary formation to study the direct effects of estrogen. 17beta-estradiol (E2) treatment significantly stimulated capillary sprouting within 8 h in co-cultures of rat aortic endothelial cells (RAECs) and mouse mammary tumor cells. Co-cultures treated with either progesterone (P4) or E2+P4 showed minimal endothelial cell (EC) sprouting when compared to E2 treated cultures. Treatment with the E2 agonist ICI 182,780 dramatically inhibited capillary formation, demonstrating E2-specificity. Within hours, of E2 treatment ECs isolated from tumor cell/EC co-cultures demonstrated a statistically significant increase in both mRNA and protein levels of the transcription factor Ets-1. We observed increased matrix metalloproteinase (MMP) and decreased tissue inhibitor of metalloproteinase (TIMP) mRNA levels in these ECs following E2 treatment. Ets-1 upregulates expression of the vascular endothelial growth factor (VEGF) receptor, Flt-1 and we detected increased Flt-1 mRNA levels in ECs co-cultured with tumor cells following E2 treatment. Expression of Ets-1 contributes to destabilization of a quiescent EC phenotype in favor of an invasive angiogenic one, in part, by increasing expression of MMPs and integrin molecules that favor migration and invasion. Transfection of ECs with Ets-1 antisense prior to co-culture with E2 resulted in a 95% inhibition in capillary formation. We demonstrate here, for the first time that nanomolar concentrations of E2 directly and rapidly induced new capillary formation in a mammary tumor/EC co-culture system and suggest that this response may be mediated, in part, by an E2-induced increase in Ets-1 expression.