Synergistic role for pituitary growth hormone in the regulation of hepatic estrogen and progesterone receptors and vitellogenesis in female freshwater turtles, Chrysemys picta.
Previous studies using the freshwater turtle Chrysemys picta have demonstrated that estradiol, progesterone (P), testosterone (T), and growth hormone (GH) regulate hepatic vitellogenin synthesis, suggesting a multihormonal regulation of vitellogenesis in the turtle. In this study we further investigated the interaction between estradiol-17beta (E) and growth hormone in the regulation of vitellogenin (vtg) in hypophysectomized post-reproductive female turtles (C. picta). Northern blot analysis was used to monitor the changes in vtg mRNA, ER mRNA, and PR mRNA expression; Western blot to determine changes in PR isoform expression and a homologous ELISA for measurement of plasma vtg. Compared to sham-operated controls, hypophysectomy did not reduce the hepatic levels of any parameters below the seasonal norm. Changes in these parameters in hypophysectomized animals after administration of GH alone, estrogen alone, or GH in combination with estrogen were well correlated. The effect of estrogen alone was greater than that of GH, and elevated all end-points analyzed. With the exception of plasma vitellogenin, the effect of GH plus estrogen was significantly greater than either hormone alone. In contrast to changes in ER mRNA, vtg mRNA, and vtg protein, the effect of estrogen and GH plus estrogen treatments on PRA mRNA and PRC mRNA, although significant, was relatively modest. However, changes in PRA and PRB protein were large (5- to 10-fold), and of similar magnitude to the changes in ER mRNA, vtg mRNA, and plasma vtg. Further, PRA and PRB protein levels appeared to be differentially affected. Thus, in sham and normal animals, only PRB was detected, and the levels were similar. After GH administration, PRB increased 4- to 5-fold, and PRA became detectable. Estrogen increased both isoforms of PR equally (approximately 6-fold), and some synergism was apparent when the two hormones were administered together, PR levels being the highest in this group (about an 8-fold increase). The results suggest that although estrogen is the primary regulatory factor involved in activation of vitellogenin synthesis, basal levels of all primary components of signaling pathways involved in vitellogenesis measured here may be maintained in the absence of either estrogen or GH, and that GH alone can activate transcription of some of these important transcription factors.
Lora
Noemi Custodia
NC
Department of Biology, Boston University, Boston, MA 02215, USA.
Novillo
Apolonia
A
Callard
Ian P
IP
D000818:Animals; D004797:Enzyme-Linked Immunosorbent Assay; D004958:Estradiol / Q000494:pharmacology*; D005260:Female; D013006:Growth Hormone / Q000494:pharmacology*; D008099:Liver / Q000502:physiology; D010902:Pituitary Gland / Q000502:physiology; D012333:RNA, Messenger / Q000032:analysis / Q000096:biosynthesis; D011960:Receptors, Estrogen / Q000096:biosynthesis* / Q000378:metabolism; D011980:Receptors, Progesterone / Q000378:metabolism*; D015398:Signal Transduction; D014158:Transcription, Genetic; D014426:Turtles / Q000502:physiology*; D014818:Vitellogenesis / Q000502:physiology*
D012333:RNA, Messenger; D011960:Receptors, Estrogen; D011980:Receptors, Progesterone; D004958:Estradiol; D013006:Growth Hormone