Molecular identification of estrogen receptors (ERα and ERβ) and their differential expression during VTG synthesis in the liver of lizard Podarcis sicula

In non-mammalian vertebrates yolk deposition in the oocytes is a hormone-dependent, gender-specific process. Produced by the ovary under gonadotropin stimulation, Estradiol 17-β (E2) plays a key role in the liver synthesis of vitellogenin (VTG) which in turn is taken up by vitellogenic oocytes in the ovary. In many species a negative role in liver synthesis of VTG in females is also played by progesterone. Experimental administration of E2 induces the expression of the VTG silent gene also in the liver of males of all the species studied. However, the role of the two isoforms of estrogen receptors, ERα and β, in this process is still unclear.In order to elucidate what kind of ER is involved in the liver synthesis of VTG in the lizard Podarcis sicula, we obtained by means of RT-PCR two fragments of 430 bp and 130 bp from total ovarian mRNA, encoding respectively for ERα and ERβ. Expression analysis of these two specific isoforms of ERs in the liver showed that in non-breeding females, and in wildlife untreated males only ERβ is expressed. In breeding vitellogenic females and in E2-treated males both α and β receptors are expressed. Furthermore, in females experimentally treated with progesterone during the breeding period, expression of ERα disappears. Conversely, treatment of females with E2 in the non-breeding period induces expression of ERα. Immunohistochemical analysis and Western blotting showed that the presence of irVTG in liver and plasma is always parallel to hepatic expression of ERα in all the different experimental conditions. Our data strongly suggest that expression of ERα may be necessary for VTG synthesis in Podarcis. The possible modulatory role of ERβ is also discussed.