Pituitary Dopamine D1 Receptor and Growth Hormone Gene Expression in Chinese Grass Carp PDF Download

Author: Xinyan Wang
Publisher: Open Dissertation Press
ISBN: 9781374674707
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Languages : en
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This dissertation, "Pituitary Dopamine D1 Receptor and Growth Hormone Gene Expression in Chinese Grass Carp" by Xinyan, Wang, 汪新艷, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled PITUITARY DOPAMINE D1 RECEPTOR AND GROWTH HORMONE GENE EXPRESSION IN CHINESE GRASS CARP Submitted by Wang Xinyan for the degree of Doctor of Philosophy at The University of Hong Kong in April 2007 In fish models, dopamine (DA) serves as a potent stimulator for growth hormone (GH) release by direct action at the pituitary level via activation of DA D1 receptors. Although multiple subtypes of D1 receptors have been reported, the form(s) of D1 receptor expressed in the fish pituitary responsible for DA-stimulated GH release has not been clarified. Since previous studies have focused mainly on GH secretion, the mechanisms for DA D1 regulation of GH gene expression are largely unknown. In this study, using grass carp (Ctenopharyngodon idellus) as an animal model, the structural identity of the D1 receptor expressed in the carp pituitary was established by molecular cloning. Sequence alignment and phylogenetic analysis have revealed that the receptor is a member of the D1A receptor subfamily. Functional expression studies also confirmed that the receptor binding properties and signaling coupling via cAMP production by this grass carp D1A receptor were highly comparable to that of mammalian D1 receptors. This newly cloned receptor was found to be a single copy gene in the grass carp genome and expressed predominantly in the pituitary. At the pituitary level, D1A receptor expression could be detected in the somatotrophs and lactotrophs, but not in gonadotrophs. In carp pituitary cells, D1A receptor transcript levels could be elevated by GH treatment, whereas the opposite effect was noted by removing endogenous GH using immunoneutralization. In contrast to GH treatment, luteinizing hormone (LH) or its functional analog hCG was found to suppress D1A receptor mRNA expression. Similarly, the differential actions of GH and hCG were also observed in grass carp somatotrophs. In this case, GH-induced D1A receptor gene expression could be blocked by simultaneous treatment with hCG, suggesting that D1A receptor expression in carp somatotrophs is regulated by local interactions between GH and LH at the pituitary level. To further examine the functional role of D1 receptors in GH regulation, the mechanisms for DA D1 stimulation of GH gene expression were also elucidated by direct measurement of "steady-state" GH mRNA level, GH transcript stability, and GH primary transcript production in grass carp pituitary cells and indirect measurement of GH promoter activity in GH cells with stable expression of grass carp D1A receptors. In these studies, DA D1 stimulation was effective in increasing GH mRNA and GH primary transcript expression without affecting the half-life of GH transcripts. These stimulatory effects also occurred with parallel increase in GH promoter activity, suggesting that GH gene transcription can be activated through pituitary D1A receptors by actions acting at the promoter level. Using pharmacological and molecular manipulations perturbing individual signaling steps, MAPK MAPK activation of MAPK (P and P ) and PI3K/Akt cascades coupled to the 42/44 38 AC/cAMP/ PKA pathway was shown to be the key mechanisms mediating DA D1 stimulation of GH gene expression. Activation of these signaling events also induced subsequent phosphorylation of the transcription factor CREB to trigger GH gene transcription at the promoter level.