Hepatic steroid receptors and plasma steroid hormone binding proteins in non-mammalian models
Embargo Date
Indefinite
OA Version
Citation
Abstract
A comparison of the mechanisms of hormonal regulation of vitellogenesis in oviparous versus viviparous reptiles is important to understand the evolution of viviparity and the sex- differentiated hepatic functions in vertebrates. In the present study, hepatic steroid receptors and plasma steroid hormone binding proteins (SHBPs) were investigated in an oviparous turtle, Chrysemys picta, and a viviparous watersnake, Nerodia. In both snakes and turtles, hepatic estrogen receptors in cytosol and nuclear salt extracts exhibited high affinity, low capacity, steroid and target tissue specificity. In contrast to the turtle estrogen receptors, snake estrogen receptors had an unusually high stereospecificity and did not bind to calf thymus DNA-cellulose . However, seasonal changes in hepatic estrogen receptor occured in both species; vitellogenic animals had higher levels of estrogen receptor and a greater percentage of total receptor in the nuclear fraction. In contrast, an SHBP characterized in snake plasma showed no vitellogenesis-related changes in binding capacity, although a decrease in SHBP level occured in mid-late pregnancy. Data from multiple competition studies, Scatchard analyses, sucrose gradient ultacentrifugation and gel electrophoresis suggests that there is a single, medium-high affinity plasma SHBP in this species.
Estrogen treatment of reproductively inactive turtles increased liver nuclear estrogen receptors. Hypophysectomy decreased nuclear receptor levels in both control and estrogen injected animals and restoration was achieved by ovine growth hormone (oGH) treatment. Thus, pituitary modulation of hepatic estrogen sensitivity may be due to a synergistic pituitary factor similar to oGH acting via the estrogen receptor system.
Progesterone receptors were also characterized in turtle liver cytosol and this was the first demonstration of hepatic progesterone receptors in any animal. Hepatic progesterone receptors exhibited high affinity, limited capacity and steroid specificity. Two high affinity subunits, A and B, were separated by DEAE anion-exchange chromatography. After photoaffinity labeling with 3 H-R5020, 50Spolyacrylamide gel electrophoresis revealed MWs of 95 KDa and 123 KDa for A and B respectively. The demonstration of a liver progesterone receptor suggests that progesterone inhibition of hepatic vitellogenesis may be a direct effect.
Description
Dissertation (Ph.D.)--Boston University
License
PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.