Rationale: Epidemiologic studies have identified an associate between iron deficiency (ID) and the use of oral contraceptives (CC) and ischemic stroke (IS). To date, however, the underlying mechanism remains poorly understood. Both ID and CC have been demonstrated to up-regulate the level and iron-binding ability of transferrin, with our recent study showing that this up-regulation can induce hypercoagulability by potentiating FXIIa/thrombin and blocking antithrombin-coagulation proteases interactions. Objective: To investigate whether transferrin mediates IS associated with ID or CC and the underlying mechanisms. Methods and Results: Transferrin levels were assayed in the plasma of IS patients with a history of iron-deficiency anemia (IDA), IDA patients, venous thromboembolism patients using CC, and ID mice, and in the cerebrospinal fluid of some IS patients. Effects of ID and estrogen administration on transferrin expression and coagulability and the underlying mechanisms were studied in vivo and in vitro. High levels of transferrin and transferrin-thrombin/FXIIa complexes were found in patients and ID mice. Both ID and estrogen up-regulated transferrin through hypoxia and estrogen response elements located in the transferrin gene enhancer and promoter regions, respectively. In addition, ID, administration of exogenous transferrin or estrogen, and transferrin overexpression promoted platelet-based thrombin generation and hypercoagulability, and thus aggravated IS. In contrast, anti-transferrin antibodies, transferrin knockdown, and peptide inhibitors of transferrin-thrombin/FXIIa interaction exerted anti-IS effects in vivo. Conclusions: Our findings revealed that certain factors (i.e., ID and CC) up-regulating transferrin are risk factors of thromboembolic diseases decipher a previously unrecognized mechanistic association among ID, CC and IS and provide a novel strategy for the development of anti-IS medicine by interfering with transferrin-thrombin/FXIIa interactions.

Angiotensin-converting enzyme 2 (ACE2) has been suggested as a receptor for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) entry to cause coronavirus disease 2019 (COVID-19). However, no ACE2 inhibitors have shown definite beneficiaries for COVID-19 patients, applying the presence of another receptor for SARS-CoV-2 entry. Here we show that ACE2 knockout dose not completely block virus entry, while TfR directly interacts with virus Spike protein to mediate virus entry and SARS-CoV-2 can infect mice with over-expressed humanized transferrin receptor (TfR) and without humanized ACE2. TfR-virus co-localization is found both on the membranes and in the cytoplasma, suggesting SARS-CoV-2 transporting by TfR, the iron-transporting receptor shuttling between cell membranes and cytoplasma. Interfering TfR-Spike interaction blocks virus entry to exert significant anti-viral effects. Anti-TfR antibody (EC50 16.6 nM) shows promising anti-viral effects in mouse model. Collectively, this report indicates that TfR is another receptor for SARS-CoV-2 entry and a promising anti-COVID-19 target.