Hypoxia-inducible factor 1 controls the expression of the uncoordinated-5-B receptor, but not of netrin-1, in first trimester human placenta.

Uncoordinated-5 homologs 1-4 (UNC5H1-4) transmembrane netrin receptors are reported to control a number of cellular processes, including axonal guidance, angiogenesis and cell proliferation. These receptors are known as "dependence receptors" because they are able to induce apoptosis in the absence of their ligand, netrin. We have recently reported the localization of netrin-1 and its uncoordinated-5-B (UNC5B) receptor in both villous and extravillous cytotrophoblasts in the human placenta. However, the roles that netrin-1 and UNC5B play in the development of the placenta, as well as the regulation of their expression during the early stages of placental development, remain unexplored. Placental explants were used to demonstrate a proliferative effect of netrin-1 on cytotrophoblasts, as assessed by Ki67 staining. Primary cytotrophoblasts collected at different gestational ages during the first trimester of pregnancy indicated that netrin-1 mRNA expression decreased after 6 weeks of gestation (wg), whereas UNC5B expression increased gradually up to 13-14 wg. The BeWo cell line was used to evaluate the effect of hypoxia on the expression of netrin-1 and UNC5B. Primary cytotrophoblast and BeWo cells cultured under hypoxic conditions exhibited a decrease in the expression of UNC5B both at the mRNA and protein levels; in contrast, hypoxia induced no change in the levels of netrin-1. When hypoxia-inducible factor 1α (HIF-1α) was knocked down by siRNA, we found a significant increase in UNC5B expression, indicating that the HIF-1 pathway is involved in hypoxia-induced UNC5B transcriptional down-regulation. Altogether, these results demonstrate the role of netrin-1 as a new mitogenic factor for cytotrophoblastic cells, report the pattern of expression of netrin-1 and its receptor, UNC5B, in the human placenta during the first trimester of pregnancy, and bring insights into the direct control of the expression of UNC5B by HIF-1.