G. Giannoukos et Ip. Callard, REPTILIAN (CHRYSEMYS-PICTA) HEPATIC PROGESTERONE RECEPTORS - RELATIONSHIP TO PLASMA STEROIDS AND THE VITELLOGENIC CYCLE, Journal of steroid biochemistry and molecular biology, 55(1), 1995, pp. 93-106
In non-mammals, estrogen-induced yolk precursors produced by the adult
female liver are the main nutritional source for development. Evidenc
e exists that progesterone exerts counter-regulatory effects on estrog
en-induced vitellogenesis, and we have used the turtle model (Chrysemy
s picta) to study changes in hepatic progesterone receptor during the
vitellogenic cycle. Using radioligand methods, we show that high and l
ower affinity binding sites are present in the cytosolic but not nucle
ar extracts. The lower affinity site is detectable at all times of the
year; the high affinity site is mainly observed during non-vitellogen
ic periods and does not correlate with plasma estrogen. DNA-cellulose
chromatography shows that PR-A is present in spring, summer, and winte
r, and that PR-B is down-regulated except in animals which recently la
id eggs. Western blots confirm the presence of PR in all months, but P
R-A (88 kDa) is the dominant isoform. PR-B (125 kDa) is well correlate
d with the luteal phase, winter and fall. Immunocytochemical studies s
how that PR is nuclear in location, and nuclear heat shock protein 90
(hsp90) is present. Competitive binding studies of both sites reveal t
hat progesterone is the most effective ligand for both, followed by pr
egnenolone, deoxycorticosterone, and R5020. RU 486 does not bind to th
e high affinity site but binds moderately well to the lower affinity s
ite. This study suggests that progesterone receptor isoforms are diffe
rentially expressed and may be involved as transcriptional regulators
of hepatic function outside the periods of active vitellogenesis in th
e turtle.