Agonism, Estrogen receptor leads to Increase, Vitellogenin synthesis in liver

Original text - unknown contributor

High degree of plausibility in fathead minnow, zebrafish and other cyprinid species.

Added by C. Baettig June 24, 2024

In egg laying vertebrates such as fish vitellogenin (VTG) synthesis occurs in the female liver after activation of estrogen receptors (ERs), including ERα and ERβ isoforms, by endogenous steroids and a variety of exogenous chemicals that bind to ERs (e.g., Brock & Shapiro, 1983; Denslow et al., 1999; Miracle et al., 2006). In mature female fish VTG is incorporated into growing oocytes by the ovary and is converted into yolk protein. However, neither adult male fish nor juvenile fish normally produce VTG, but the hepatic ER is present in males, as are the genes that encode for vtg expression and can therefore be induced by exogenous compounds (Heppell et al., 1995).

Agonism of the ER is expected to increase vtg transcription and translation and under high estrogen stimulation the fold increase of vtg transcripts increases by orders of magnitude (Brock & Shapiro, 1983). As such, induction of VTG levels in male fish has been used extensively as a biomarker of estrogen exposure (Wheeler et al., 2005).

Original text - unknown contributor

A wide range of studies using adult fish show that induction of plasma vitellogenin (VTG) occurs within 21 days in vivo aquatic exposure to estrogen receptor agonists (eg 17beta-estradiol and 4-tert pentylphenol) as shown during the successful validation of the OECD Test Guideline 229 and related protocols. A smaller number of experiment studies with fish have shown that within the OECD Test Guideline 2010, larval fish can also show induction of whole body VTG levels within 21 days aquatic exposure to estrogen receptor agonists.

Added by C. Baettig June 24, 2024

There are numerous publications supporting this relationship including multiple review articles (e.g., Matozzo et al., 2008; Palmer & Selcer, 1996; Verderame & Scudiero, 2017). A few specific examples are listed below.

• Estradiol and diarylpropionitrile (DPN), an ERβ selective agonist, induced a dose-dependent increase in VTG synthesis in rainbow trout hepatocytes (Leaños-Castañeda & Van Der Kraak, 2007).
• DPN has also been shown to increase ERα and vtg expression and synthesis post-injection in Mozambique tilapia in vivo (Davis et al., 2010).
• A study focusing on benzophenone derivatives found that BP1 (2,4-dihydroxybenzophenone), BP2 (2,2′,4,4′-tetrahydroxybenzophenone), and THB (2,4,4′-trihydroxybenzophenone) were human ERα (hERα) and hERβ and rainbow trout ERα (rtERα) and rtERβ agonists. To investigate ER activation profiles of the derivatives in vitro tests, i.e., competitive binding, reporter gene based assays, vitellogenin (Vtg) induction in isolated rainbow trout hepatocytes, and proliferation based assays were completed. hERβ was more strongly activated, which is an inverse finding to natural ligand 17β-estradiol (E2) where hERα is more strongly activated. BPs were more active in rtERα than in hERα assays. Significant VTG induction was detected in hERα, hERβ, rtERα, and rtERβ cultures (Molina-Molina et al., 2008).
• Tollefsen et al. (2003) looked at multiple endogenous (e.g., estrone (E1), estradiol (E2), and estriol (E3)) and exogenous estrogens (e.g., ethynyloestradiol (EE2), diethylstilbestrol (DES), genistein, zearalenone, bisphenol A) and found they induced dose-dependent VTG synthesis in Atlantic salmon hepatocytes.
• Shen et al. (2021) used in silico methods to screen 1056 pesticides for potential agonistic activity. They found 72 pesticides to be potential ER agonists, 14 of which have been previously reported as ER agonists. To test whether these pesticides were ER agonists, 10 were selected from the list, three that were previously reported as ER agonists and seven previously unreported as ER agonists. They found all 10 pesticides exhibited ERα agonistic activity in human or zebrafish cells and of the 10, seven also induced vtg1 and vtg2 mRNA in zebrafish.
• Xu et al (2020) also showed increase in plasma VTG following exposure to aryloxy-phenoxypropionate (APP) herbicides, after measuring the binding patterns of quizalofop-P-ethyl (QPE), clodinafop-propargyl (CP) and haloxyfop-P (HP) with ERα.
• In male fathead minnows exposed to E2 and 1H,1H,10H,10H-perfluorodecane-1,10-diol (FC-10 diol) for 21 days expression of hepatic esr1 and vtg were both significantly increased (Ankley et al. in prep).
• In male fathead minnows exposed to methoxychlor, a weak estrogen agonist, there was a clear induction of VTG (Ankley et al. 2001). In the same study exposure to methyltestosterone, a synthetic androgen, caused a significant induction of VTG in both male and female fathead minnows. This level of induction in female fathead minnows resulted in a dose-dependent increase in VTG, to concentrations approximately 10-fold higher than those observed in control fish. These funding were likely due to the conversion of methyltestosterone to methylestradiol (Hornung et al., 2004).

Original text - unknown contributor

There are generally few inconsistencies for experimental studies using model fish species dervied from pathogen-free laboratory cultures. However, there can some uncertainties where wild fish have been used for experimental purposes.

Added by C. Baettig June 24, 2024

• Some uncertainty remains regarding which ER subtypes regulate vtg gene expression in the liver of fish. In general, the literature suggests a close interplay between ER subtypes, primarily ERα and Erβ, in the regulation of vitellogenesis. Consequently, at present, the key event relationship is generalized to impacts on all ER subtypes, even though it remains possible that impacts on a particular sub-type may drive the effect on vitellogenin transcription and translation.
• Using selective agonists and antagonists for ERα and ERβ, it was concluded that ERβ was primarily responsible for inducing vitellogenin production in rainbow trout and that compounds exhibiting ERα selectivity would not be detected using a vitellogenin ELISA bioassay (Leaños-Castañeda & Van Der Kraak, 2007). However, a subsequent study conducted in tilapia concluded that agonistic and antagonistic characteristics of mammalian, isoform-specific ER agonists and antagonists, cannot be reliably extrapolated to piscine ERs (Davis et al., 2010).
• Based on RNA interference knock-down experiments Nelson and Habibi (2010) proposed a model in which all ER subtypes are involved in E2-mediated vitellogenesis, with ERβ isoforms stimulating expression of both vitellogenin and ERα gene expression, and ERα helping to drive vitellogenesis, particularly as it becomes more abundant following sensitization.