Case Western Reserve University USA.
Title: An atypical human endogenous retrovirus, ERV3 env, induces human chorionic gonadotropin (β-hCG) in a model of placental trophoblast.
Neal S. Rote completed his Ph.D. at Temple University School of Medicine and postdoctoral studies at Heidelberg University and UCLA School of Medicine. He is the William Weir, M.D. Professor of Reproductive Biology and Professor of Pathology at Case Western Reserve University School of Medicine and Academic Vice Chair and Director of Research in the Department of Obstetrics and Gynecology, University Hospitals Case Medical Center, Cleveland, OH. He has published more than 110 papers in reproductive biology and 75 chapters and books, been NIH-funded for 32 years, and served on many NIH review committees.
Normal human placentation requires differentiation of specialized cells (villous cytotrophoblast into syncytiotrophoblast); principally characterized by intercellular fusion and production of the hormone chorionic gonadotropin (hCG). Concurrently trophoblasts express cellular genes of apparent retroviral origin (endogenous retroviruses; ERV), including endogenous retroviral element ERV3 env. We reported that, unlike other retroviral env regions that encode fusion proteins, ERV3 env regulated induction of the β subunit of hCG (β-hCG). The biological relevance of ERV3 env was questioned in a report of adults with homozygous stop mutations leading to a “natural knockout” of ERV3 env although a truncated (p25) SU protein was produced that lacked the biologically active regions typically attributed to exogenous or endogenous retroviral Env proteins. The p25 region was never tested for capacity to induce β-hCG. ERV3 env contains two proposed translational start sites at nt 595 and nt 715. We cloned and inserted the entire ERV3 env open reading frame (ERV3), the SU region, and the p25 region, beginning at both proposed start sites, in stable expression vectors into BeWo cells (a model for villous cytotrophoblast differentiation when treated with forskolin), quantified levels of intracellular β-hCG and actin by western blot analysis, and data expressed as means (SD) of the ratio of β-hCG to actin of three independent experiments. β-hCG was not detectable in untreated BeWo or those transfected with vector alone (negative controls) and maximally expressed in forskolin-treated cells (positive control; 1.83 + 0.83). Transfection with ERV3, ERV3 SU, and p25 induced significantly (P<0.01) greater levels of β-hCG expression. BeWo cells were also stably transfected with vectors expressing siRNAs targeted to regions near the start sites, and the cell lines treated with forskolin or vehicle alone for 24, 48, and 72 hr. Transfection with si670, targeted to nt 670-688, between the proposed start sites, completely prevented induction of β-hCG by forskolin at all time points. Thus, ERV3 env is an atypical retroviral element with a unique trophoblast hormone regulatory site in the SU region and in the p25 truncated protein expressed in individuals with described homozygous stop mutations.
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