Steroid hormones are novel nucleoside transport inhibitors by competition with nucleosides for their transporters

Regulation of Hypoxic-Adenosinergic Signaling by Estrogen: Implications for Microvascular Injury

Loss of estrogen, as occurs with normal aging, leads to increased inflammation, pathologic angiogenesis, impaired mitochondrial function, and microvascular disease. While the influence of estrogens on purinergic pathways is largely unknown, extracellular adenosine, generated at high levels by CD39 and CD73, is known to be anti-inflammatory in the vasculature. To further define the cellular mechanisms necessary for vascular protection, we investigated how estrogen modulates hypoxic-adenosinergic vascular signaling responses and angiogenesis. Expression of estrogen receptors, purinergic mediators inclusive of adenosine, adenosine deaminase (ADA), and ATP were measured in human endothelial cells. Standard tube formation and wound healing assays were performed to assess angiogenesis in vitro. The impacts on purinergic responses in vivo were modeled using cardiac tissue from ovariectomized mice. CD39 and estrogen receptor alpha (ERα) levels were markedly increased in presence of estradiol (E2). Suppression of ERα resulted in decreased CD39 expression. Expression of ENT1 was decreased in an ER-dependent manner. Extracellular ATP and ADA activity levels decreased following E2 exposure while levels of adenosine increased. Phosphorylation of ERK1/2 increased following E2 treatment and was attenuated by blocking adenosine receptor (AR) and ER activity. Estradiol boosted angiogenesis, while inhibition of estrogen decreased tube formation in vitro. Expression of CD39 and phospho-ERK1/2 decreased in cardiac tissues from ovariectomized mice, whereas ENT1 expression increased with expected decreases in blood adenosine levels. Estradiol-induced upregulation of CD39 substantially increases adenosine availability, while augmenting vascular protective signaling responses. Control of CD39 by ERα follows on transcriptional regulation. These data suggest novel therapeutic avenues to explore in the amelioration of post-menopausal cardiovascular disease, by modulation of adenosinergic mechanisms.

Progestins as Anticancer Drugs and Chemosensitizers, New Targets and Applications

Progesterone and its synthetic analogues, progestins, participate in the regulation of cell differentiation, proliferation and cell cycle progression. Progestins are usually applied for contraception, maintenance of pregnancy, and hormone replacement therapy. Recently, their effectiveness in the treatment of hormone-sensitive tumors was revealed. According to current data, the anticancer activity of progestins is mainly mediated by their cytotoxic and chemosensitizing influence on different cancer cells. In connection with the detection of previously unknown targets of the progestin action, which include the membrane-associated progesterone receptor (PR), non-specific transporters related to the multidrug resistance (MDR) and mitochondrial permeability transition pore (MPTP), and checkpoints of different signaling pathways, new aspects of their application have emerged. It is likely that the favorable influence of progestins is predominantly associated with the modulation of expression and activity of MDR-related proteins, the inhibition of survival signaling pathways, especially TGF-β and Wnt/β-catenin pathways, which activate the proliferation and promote MDR in cancer cells, and the facilitation of mitochondrial-dependent apoptosis. Biological effects of progestins are mediated by the inhibition of these signaling pathways, as well as the direct interaction with the nucleotide-binding domain of ABC-transporters and mitochondrial adenylate translocase as an MPTP component. In these ways, progestins can restore the proliferative balance, the ability for apoptosis, and chemosensitivity to drugs, which is especially important for hormone-dependent tumors associated with estrogen stress, epithelial-to-mesenchymal transition, and drug resistance.

Contribution of CNT1 and ENT1 to ribavirin uptake in human hepatocytes

The objective of this study was to investigate the contributions of a sodium-dependent concentrative nucleoside transporter (CNT) 1 and an equilibrative nucleoside transporter (ENT) 1 to ribavirin uptake in human hepatocytes. The initial studies in oocytes expressing CNT1 and ENT1 showed increases in ribavirin uptake, indicating that ribavirin was a substrate for both CNT1 and ENT1. The CNT1- and ENT1-mediated ribavirin uptake showed concentration dependency with the following kinetics parameters: Km 26.3 μM and Vmax 426.2 fmol/min/oocyte for CNT1; Km 70.5 μM and Vmax 134.3 fmol/min/oocyte for ENT1. Ribavirin uptake clearance in six human hepatocytes ranged from 21.3 to 300.7 μL/min. Estimation of the contributions of CNT1 and ENT1 to the hepatic uptake of ribavirin by using a relative activity factor method indicated that the relative contribution of ENT1 to the ribavirin uptake was 82.8 ± 3.9%. Real-time polymerase chain reaction analysis of CNT1 and ENT1 expressions in the hepatocytes showed that ENT1 mRNA expression was closely correlated with ribavirin uptake (R = 0.95, P = 0.003) while CNT1 was not. The findings indicated that ENT1 was the major transporter controlling the hepatic uptake of ribavirin.