REPTILIAN (CHRYSEMYS-PICTA) HEPATIC PROGESTERONE RECEPTORS - RELATIONSHIP TO PLASMA STEROIDS AND THE VITELLOGENIC CYCLE

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.