Biomedical subjects

Christian Klausen

Publications and source records attributed to Christian Klausen.

5 recordsLinked to original sources

PubMed2005

PKC and ERK are differentially involved in gonadotropin-releasing hormone-induced growth hormone gene expression in the goldfish pituitary.

Gonadotropin-releasing hormone (GnRH) is produced by the hypothalamus and stimulates the synthesis and secretion of gonadotropin hormones. In addition, GnRH also stimulates the production and secretion of growth hormone (GH) in some fish species and in humans with certain clinical disorders. In the goldfish pituitary, GH secretion and gene expression are regulated by two endogenous forms of GnRH known as salmon GnRH and chicken GnRH-II. It is well established that PKC mediates GnRH-stimulated GH secretion in the goldfish pituitary. In contrast, the signal transduction of GnRH-induced GH gene expression has not been elucidated in any model system. In this study, we demonstrate, for the first time, the presence of novel and atypical PKC isoforms in the pituitary of a fish. Moreover, our results indicate that conventional PKC alpha is present selectively in GH-producing cells. Treatment of primary cultures of dispersed goldfish pituitary cells with PKC activators (phorbol ester or diacylglycerol analog) did not affect basal or GnRH-induced GH mRNA levels, and two different inhibitors of PKC (calphostin C and GF109203X) did not reduce the effects of GnRH on GH gene expression. Together, these results suggest that, in contrast to secretion, conventional and novel PKCs are not involved in GnRH-stimulated increases in GH mRNA levels in the goldfish pituitary. Instead, PD98059 inhibited GnRH-induced GH gene expression, suggesting that the ERK signaling pathway is involved. The results presented here provide novel insights into the functional specificity of GnRH-induced signaling and the regulation of GH gene expression.

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PubMed2005

Role of PKC in the regulation of gonadotropin subunit mRNA levels: interaction with two native forms of gonadotropin-releasing hormone.

Gonadotropin-releasing hormone (GnRH) is an important regulator of reproduction in all vertebrates through its actions on the production and secretion of pituitary gonadotropin hormones (GtHs). Most vertebrate species express at least two GnRHs, including one form, designated chicken (c)GnRH-II or type II GnRH, which has been well conserved throughout evolution. The goldfish brain and pituitary contain salmon GnRH and cGnRH-II. In goldfish, GnRH-induced luteinizing hormone (LH) secretion involves PKC; however, whether PKC mediates GnRH stimulation of GtH subunit mRNA levels is unknown. In this study, we used inhibitors and activators of PKC to examine its possible involvement in GnRH-induced increases in GtH-alpha, follicle-stimulating hormone (FSH)-beta and LH-beta mRNA levels in primary cultures of dispersed goldfish pituitary cells. Treatment with PKC inhibitors calphostin C and GF109203X unmasked a basal repression of GtH subunit mRNA levels by PKC; both inhibitors increased GtH subunit mRNA levels in a dose-dependent manner. PKC activators, 12-O-tetradecanoylphorbol 13-acetate (TPA), and 1,2-dioctanoyl-sn-glycerol, stimulated GtH subunit mRNA levels, whereas an inactive phorbol ester (4-alpha-TPA) was without effect. Thus, a dual, inhibitory and stimulatory, influence for PKC in the regulation of GtH subunit mRNA levels is suggested. In contrast, PKC inhibitor- and activator-induced effects were, for the most part, additive to those of GnRH, suggesting that conventional and novel PKCs are unlikely to be involved in GnRH-stimulated increases in GtH subunit mRNA levels. Our data illustrate major differences in the signal transduction of GnRH effects on GtH secretion and gene expression in the goldfish pituitary.

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PubMed2002

Multiplicity of gonadotropin-releasing hormone signaling: a comparative perspective.

GnRH regulation of GtH synthesis and release involves PKC- and Ca(2+)-dependent pathways. There are differential signaling mechanisms in different cells, tissues and species. Signaling mechanisms involved in GnRH-mediated GtH release appear to be more conserved compared to that of GnRH-induced GtH gene expression. This may in part be due to different 5' regulatory regions on the GtH-subunit genes. Cell type specific expression of various signaling and/or exocytotic components may also be responsible for the observed differences in signaling between gonadotropes and somatotropes in the goldfish and tilapia pituitaries. However, this can not explain the observed differences in post receptor mechanisms for sGnRH and cGnRH-II in gonadotropes which is more likely to result from the existence of GnRH receptor subtypes. Support for this hypothesis is also provided by observations on mechanisms of autocrine/paracrine regulation of ovarian function by sGnRH and cGnRH-II in the goldfish ovary in which GnRH antagonists only block GnRH stimulation of oocyte meiosis and do not affect inhibitory effects of sGnRH. It should be easier to explain observed variations concerning GnRH-induced responses as more information becomes available on different types of GnRH receptors, and their distribution and function in mammals and non-mammalian vertebrates.

Animals↗

PubMed2002

Time- and dose-related effects of gonadotropin-releasing hormone on growth hormone and gonadotropin subunit gene expression in the goldfish pituitary.

The goldfish brain contains two molecular forms of gonadotropin-releasing hormone (GnRH): salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II). In a preliminary report, we demonstrated the stimulation of gonadotropin hormone (GtH) subunit and growth hormone (GH) mRNA levels by a single dose of GnRH at a single time point in the goldfish pituitary. Here we extend the work and demonstrate time- and dose-related effects of sGnRH and cGnRH-II on GtH subunit and GH gene expression in vivo and in vitro. The present study demonstrates important differences between the time- and dose-related effects of sGnRH and cGnRH-II on GtH subunit and GH mRNA levels. Using primary cultures of dispersed pituitary cells, the minimal effective dose of cGnRH-II required to stimulate GtH subunit mRNA levels was found to be 10-fold lower than that of sGnRH. In addition, the magnitudes of the increases in GtH subunit and GH mRNA levels stimulated by cGnRH-II were found to be higher than the sGnRH-induced responses. However, no significant difference was observed between sGnRH and cGnRH-II-induced responses in vivo. Time-related studies also revealed significant differences between sGnRH- and cGnRH-II-induced production of GtH subunit and GH mRNA in the goldfish pituitary. In general, the present study provides novel information on time- and dose-related effects of sGnRH and cGnRH-II on GtH subunit and GH mRNA levels and provides a framework for further investigation of GnRH mechanisms of action in the goldfish pituitary.

Animals↗

PubMed2002

Function-specific calcium stores selectively regulate growth hormone secretion, storage, and mRNA level.

Ca(+) stores may regulate multiple components of the secretory pathway. We examined the roles of biochemically independent intracellular Ca(2+) stores on acute and long-term growth hormone (GH) release, storage, and mRNA levels in goldfish somatotropes. Thapsigargin-evoked intracellular Ca(2+) concentration ([Ca(2+)](i)) signal amplitude was similar to the Ca(2+)-mobilizing agonist gonadotropin-releasing hormone, but thapsigargin (2 microM) did not acutely increase GH release, suggesting uncoupling between [Ca(2+)](i) and exocytosis. However, 2 microM thapsigargin affected long-term secretory function. Thapsigargin-treated cells displayed a steady secretion of GH (2, 12, and 24 h), which decreased GH content (12 and 24 h), but not GH mRNA/production (24 h). In contrast to the results with thapsigargin, activating the ryanodine (Ry) receptor (RyR) with 1 nM Ry transiently increased GH release (2 h). Prolonged activation of RyR (24 h) reduced GH release, contents and apparent production, without changing GH mRNA levels. Inhibiting RyR with 10 microM Ry increased GH mRNA levels, production, and storage (2 h). Increasing [Ca(2+)](i) independently of Ca(2+) stores with the use of 30 mM KCl decreased GH mRNA. Collectively, these results suggest that parts of the secretory pathway can be controlled independently by function-specific Ca(2+) stores.

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