Endothelin-1 gene expression and regulation of endothelin mRNA and protein biosynthesis in avascular human amnion: Potential source of amniotic fluid endothelin

We demonstrated previously that preproendothelin mRNA is present in avascular human amnion tissue and in human amnion cells maintained in primary monolayer culture. In this investigation we sought to identify the specific endothelin (ET) gene that is expressed in amnion and to determine whether endothelin is produced by amnion. Using oligonucleotides specific for ET-1, ET-2, and ET-3 mRNA, we identified preproET-1 mRNA in human amnion tissue. By radioimmunoassay of ET we found that human amnion tissue explants and amnion cells in culture secrete immunoreactive ET into the medium. PreproET mRNA levels and immunoreactive ET production by human amnion cells in monolayer culture are increased in response to treatment with agents that are known to be present in human amniotic fluid, i.e. epidermal growth factor, interleukin-1, and tumor necrosis factor-α. We found that the level of preproET mRNA in amnion cells was low compared with that in human umbilical endothelial cells; treatment with cycloheximide together with a stimulus of ET-1 gene transcription led to a striking increase in the level of preproET mRNA in amnion cells compared with a much weaker response in endothelial cells. These findings suggest that protein synthesis-dependent mechanisms may be of great importance in maintaining low levels of preproET mRNA in amnion tissue and in regulating the amount of preproET mRNA in amnion exposed to stimuli of ET-1 transcription. In addition, we demonstrated that immunoreactive ET is present in human amniotic fluid at the midtrimester of pregnancy and at term. Thus, it is likely that the avascular fetal amnion is one tissue site of origin of ET in amniotic fluid during human pregnancy.