Severe pulmonary arterial hypertension (PAH) is characterized by clustered proliferation of endothelial cells in the lumina of small size pulmonary arteries resulting in concentric obliteration of the lumina and formation of complex vascular structures known as plexiform lesions. mechanism(s). Notably, in systemic and pulmonary vascular endothelial cells, smooth muscle cells, and fibroblasts 2ME exerts stronger anti-mitotic effects than E2 itself. E2 and 2ME, despite having similar effects on other cardiovascular cells, have opposing effects on endothelial cells; that is, in endothelial cells, E2 is pro-mitogenic, pro-angiogenic and anti- apoptotic, whereas 2ME is antimitogenic, anti-angiogenic and pro-apoptotic. This may have significant ramifications in severe PAH that involves uncontrolled proliferation of monoclonal, apoptosis resistant endothelial cells. Based on its cellular effects, 2ME should be expected to attenuate the progression of disease and provide protection in severe PAH. In contrast, E2, due to its mitogenic, angiogenic, and anti-apoptotic effects (otherwise desirable in normal, quiescent endothelial cells), may Indocyanine green distributor even adversely affect endothelial remodeling in PAH and this may be even more significant if the E2s effects on injured endothelium are not opposed by 2ME (e.g., in the event of reduced E2 conversion to 2ME due to hypoxia, inflammation, drugs, environmental factors, or genetic polymorphism of metabolizing enzymes). This review focuses on the effects of estrogens and their metabolites on pulmonary vascular pathobiology and the development of experimental PAH, and offers potential explanation for the estrogen paradox in PAH. Furthermore, we propose that unbalanced estradiol metabolism may lead to the development of PAH. Recent animal data and studies in patients with PAH support this concept. +Table 1); Figure 1A – Oxidative metabolism of E2 mainly occurs at the C2, C4 and C16 positions and leads to formation of metabolites with different and often contrary biological effects. In both in men and women, E2 metabolism occurs primarily by oxidation and this oxidative metabolism of E2 largely determines the nature of its biological effects (44), with dozen of E2 metabolites found to be biologically active. The first step is the conversion of E2 to less estrogenic estrone (E1) by oxidation at C17 position by 17 -hydroxysteroid-dehydrogenase (17-HSD), a process that is reversible and favors formation of E1. The reverse reduction may occur, albeit more slowly. Further metabolism includes the metabolic pathways, i.e., oxidation at C16, C4 and C2 position, resulting in production of biologically active bad and good metabolites. In this regard, oxidative hydroxylation of the D ring at C16 is a major metabolic pathway, the is the dominant pathway for E2 hydroxylation (47). In contrast to C4 and C16 hydroxylation, C2 hydroxylation is a direct process that involves no unstable intermediates (48) and leads to formation of non-estrogenic metabolites. E2 is metabolized to 2-hydroxyestradiol (2HE) by CYP1A1/CYP1B1. 2HE has little estrogenic activity and is quickly cleared from the plasma (t/2=90; 49) by prompt conversion (i.e., O-methylation by catechol-O-methyltransferase) to 2-methoxyestradiol (2ME), a metabolite with no estrogenic activity (50, 51). Importantly, the CYP-450 isoforms largely Indocyanine green distributor responsible for 2-hydroxylation of E2 (i.e., CYP1A1/2 CYP1B1) are expressed in cardiovascular cells, and conversion of 2HE to 2ME takes place in both endothelial cells and VSMCs (44, 52-54). 2-Methoxyestradiol is extensively metabolized by type-2 17-hydroxysteroid dehydrogenase (17HSD-2) to 2-methoxyestrone (2ME1), a metabolite largely lacking antiproliferative activity. Importantly, over-expression and increased activity of 17HSD-2 abolish the anti-mitogenic effects of 2ME in tumor cells (55). 2ME1 has little or no antimitogenic effect in cardiovascular cells, but may be converted back to 2ME by 17HSD type-1 and thereby CD1D exhibit cardiovascular effects. All-retinoic acid (atRA) increases 17HSD type-1 expression and activity (56, 57), and would thereby be expected to increase the conversion of 2ME1 to 2ME. Our preliminary studies confirm the importance of this metabolic conversion in experimental PAH. In monocrotaline pulmonary hypertensive rats, preventive treatment with 2ME1 attenuates development of PAH, right ventricular hypertrophy, and pulmonary vascular remodeling (58; Table 3), suggesting that in vivo significant conversion of 2ME1 back to 2ME takes place in PAH (56). Furthermore, in male rats with MCT-induced PAH treated with atRA, 2ME or their combination, 2ME and atRA have synergistic effects in ameliorating PAH and vascular remodeling (59). These studies strongly suggest that 2ME-2ME1 inter-conversion takes place in vivo and that the Indocyanine green distributor 17may play a significant role in the development of PAH. Table 3 The Effects of Gender and Sex Steroid Hormones and Their Metabolites and Analogs in Experimental Pulmonary Hypertension estradiol br / treatmentReduces pulmonary vascular br / resistance to normoxia and hypoxia br.