H7-resistant protein kinase C substrates in two-dimensional gels of proestrous rat anterior pituitary gland

The hypothalamic-pituitary-ovarian axis and manipulations of the oestrous cycle in the brushtail possum

The main purpose of this review is to provide a comprehensive update on what is known about the regulatory mechanisms of the hypothalamic-pituitary-ovarian axis in the brushtail possum, and to report on the outcomes of attempts made to manipulate by hormonal means, these processes in the possum. Over the last 15 years, several unique features of possum reproductive physiology have been discovered. These include an extended follicular phase despite elevated concentrations of FSH during the luteal phase, and early expression of LH receptors on granulosa cells of small antral follicles, suggesting a different mechanism for the selection of a dominant follicle. The use of routine synchronisation protocols that are effective in eutherians has failed to be effective in possums, and so the ability to reliably synchronise oestrus in this species remains a challenge.

Gene expression and secretion of LH and FSH in relation to gene expression of GnRH receptors in the brushtail possum (Trichosurus vulpecula) demonstrates highly conserved mechanisms

In eutherian mammals, the gonadotrophins (LH and FSH) are synthesized and stored in gonadotroph cells under the regulation of multiple mechanisms including GnRH. Very little is known about the regulation of gonadotrophin secretion and storage in pituitary glands of marsupials. This study revealed, using quantitative PCR and heterologous RIA techniques, thatLHBmRNA expression levels remained constant over the oestrous cycle, regardless of the presence of a preovulatory LH surge, which is characteristic of a hormone secreted under regulation. Our sampling regime was unable to detect pulses of LH during the follicular phase, althoughGNRHRmRNA levels had increased at this time. Pulses of LH were, however, detected in the luteal phase of cycling females, in anoestrus females and in males. There was a positive correlation between gene expression ofFSHBand plasma levels of FSH at different stages of the oestrous cycle and no pulses of FSH were detected at any time; all characteristics of a hormone secreted via the constitutive pathway. Usingin situhybridisation and immunohistochemistry methods, we determined that mRNA expression ofLHBandFSHB, and protein storage of gonadotrophins exhibited a similar pattern of localisation within the pituitary gland. Additionally, sexual dimorphism of gonadotroph populations was evident. In summary, these findings are similar to that reported in eutherians and considering that marsupial evolution diverged from eutherians over 100 million years ago suggests that the regulation of gonadotrophins is highly conserved indeed.

The Role of Protein Kinase C in Regulation of TCDD-Mediated CYP1A1 Gene Expression

Cytochrome P450 1A1 (CYP1A1) is induced by halogenated and polycyclic aromatic hydrocarbons following activation of the aryl hydrocarbon receptor (AhR). Protein kinase C (PKC) has been implicated in the regulation of this response. In tissue culture, induction of PKC activity with phorbol esters synergizes the actions of TCDD-induced CYP1A1, while PKC inhibitors block induction of CYP1A1 by TCDD. Here, the actions of specific PKC inhibitors on CYP1A1 induction were examined using a HepG2 human cell line (TV101L) that carries a stably integrated firefly luciferase gene under control of the human CYP1A1 promoter (-1612/+293). TV101 cells were treated with TCDD and either the kinase inhibitor staurosporine or one of the PKC inhibitors GF109203X, Gö6983, or Gö6976. Aryl hydrocarbon receptor-dependent activation of CYP1A1-luciferase and cellular PKC activity were measured. TCDD treatment induced CYP1A1-luciferase activity in an AhR-dependent manner, as determined by binding of nuclear AhR to xenobiotic response elements (XREs). Dose-dependent inhibition of PKC activity by staurosporine was concordant with inhibition of TCDD-induced CYP1A1-luciferase activity. However, the PKC inhibitors GF109203X, Gö6983, and Gö6976 blocked PKC activity at concentrations independent of those necessary to block TCDD induction of CYP1A1-luciferase activity. For all inhibitors, reduction in CYP1A1-luciferase activity was independent of AhR activation, as determined by electrophoretic mobility shift analysis of TCDD-activated nuclear AhR. The specific PKC inhibitors did not significantly alter cytosolic or nuclear levels of AhR protein, whether alone or in combination with TCDD. These results suggested that PKC was not the sole factor responsible for regulation of CYP1A1.

No Evidence for Pituitary Priming to Gonadotropin-Releasing Hormone in Relation to Luteinizing Hormone (LH) Secretion Prior to the Preovulatory LH Surge in Ewes1

The purpose of this study was to determine the occurrence of and the regulatory mechanisms involved in priming of the pituitary to GnRH before the preovulatory LH surge in sheep. Experiment 1: Forty-two ewes had progestagen devices removed after 14 days and were assigned to luteal (Lut) or follicular (Foll) groups. Fifteen days later, blood sampling was initiated either immediately or 36 h after induced luteolysis in groups Lut and Foll, respectively. After 4 h, ewes were administered either saline (n = 5) or 250 ng (n = 8) or 10 microg (n = 8) of GnRH. Five ewes per treatment group were killed 1 h later, while remaining animals were blood sampled for a further 7 h. Experiment 2: Eighteen ewes were allocated to Lut and Foll groups (described above). Blood samples were collected from 2 h before GnRH (10 microg) treatment until 7 h after. Despite up-regulated GnRH-R mRNA levels in Foll ewes, pituitary content and plasma levels of LH and LHbeta mRNA levels were similar between groups. Mean FSHbeta mRNA and plasma FSH levels were elevated in Lut ewes but declined after GnRH treatment. Inversely, plasma estradiol and inhibin-A concentrations were higher in Foll ewes and declined after GnRH treatment. Fewer LH(+ve)/secretogranin II(-ve) (SgII(-ve)) granules were present in gonadotropes of Foll ewes, coincident with increased basal LH levels. Fewer smaller sized granules were present after GnRH treatment. In conclusion, there was no evidence of self-priming before onset of the preovulatory LH surge. Constitutive release of LH(+ve)/SgII(-ve) granules may maintain basal LH levels while smaller sized, presumably mature granules may be preferentially released after GnRH stimulation.

Extracellular calcium influx stimulates metalloproteinase cleavage and secretion of heparin-binding EGF-like growth factor independently of protein kinase C

The phorbol ester, tetradecanoyl-phorbol 13-acetate (TPA), stimulates rapid proteolytic processing of the transmembrane, pro- form of heparin-binding epidermal growth factor-like growth factor (HB-EGF) at cell surfaces, suggesting the involvement of protein kinase C (PKC) isoforms in the HB-EGF secretion mechanism. To test this possibility, we expressed a chimeric protein, consisting of proHB-EGF fused to placental alkaline phosphatase (AP) near the amino terminus of processed HB-EGF, in NbMC-2 prostate epithelial cells. The proHB-EGF-AP chimera localized to plasma membranes and functioned as a diphtheria toxin receptor. Secreted HB-EGF-AP bound to heparin and exhibited potent growth factor activity. The presence of the AP moiety allowed highly quantitative measurements of cleavage-secretion responses of proHB-EGF to extracellular stimuli. As expected, rapid secretion of HB-EGF-AP was induced in a time- and dose-dependent manner by TPA. However, this was also observed with the Ca2+ ionophore, ionomycin, suggesting the involvement of extracellular Ca2+ ions in the secretion mechanism. Ionomycin-induced secretion was inhibited by extracellular calcium chelation but not by the PKC inhibitors, GF109203X, staurosporine, or chelerythrine. The TPA-mediated secretion effect was inhibited by staurosporine, GF109203X, and by pretreatment with TPA, but not by calcium chelation. A small secretion response was induced by thapsigargin, which releases Ca2+ from intracellular stores, but this was completely eliminated by extracellular calcium chelation. Ionomycin- and TPA-induced HB-EGF-AP secretion was not dependent on the presence of the proHB-EGF cytoplasmic domain and was specifically inhibited by the metalloproteinase inhibitors 1,10-phenanthroline and tissue inhibitor of metalloproteinase-1 (TIMP-1). These data demonstrate that extracellular Ca2+ influx activates a membrane-associated metalloproteinase to process proHB-EGF by a pathway that does not require PKC.

1-O-Octadecyl-2-O-methylglycerophosphocholine inhibits protein kinase C-dependent phosphorylation of endogenous proteins in MCF-7 cells

Studies with leukaemic cells, based primarily on in vitro assays, have suggested that antitumour ether lipids have only a moderate effect on protein kinase C (PKC) activity, and, furthermore, inhibition of PKC is unlikely to be involved in the mechanism of inhibition of cell proliferation by these compounds. To determine if this is also the case for epithelial cancer cells, we examined the effect of 1-O-octadecyl-2-O-methylglycerophosphocholine (ET18-OCH3) on PKC-induced phosphorylation of endogenous proteins in MCF-7 cells under incubation conditions where the drug inhibited cell proliferation. As expected, stimulation of quiescent 32P-labelled MCF-7 cells with 1 microM PMA resulted in the phosphorylation of a number of proteins. The PMA-induced phosphorylation of the proteins was abolished by preincubation of the cells with Ro 31-8220 (5 microM) for 20 min, or 10 microg/ml ET18-OCH3 for 3 h before stimulation with PMA. Thus under incubation conditions where ET18-OCH3 inhibited the proliferation of MCF-7 cells, the ether lipid potently inhibited the activity of PKC in intact cells. This inhibition was unlikely to be due to the effect of the compound on PKC translocation since there was little effect of ET18-OCH3 on the translocation of the alpha, gamma and epsilon species of PKC. These results suggest that a role for the inhibition of PKC activity by ET18-OCH3 in the mechanism of inhibition of cell proliferation by ET18-OCH3 cannot yet be discounted in epithelial cancer cells. In addition, we also observed that ET18-OCH3 enhanced the phosphorylation of selected proteins under basal unstimulated conditions. Although some of these proteins were also observed to be phosphorylated in response to PMA stimulation, the phosphorylation induced by ET18-OCH3 was not inhibited by Ro 31-8220, indicating that this was not mediated by PKC.

The phosphatidylinositol 3-kinase inhibitor, wortmannin, inhibits insulin-induced activation of phosphatidylcholine hydrolysis and associated protein kinase C translocation in rat adipocytes

We questioned whether phosphatidylinositol 3-kinase (PI 3-kinase) and protein kinase C (PKC) function as interrelated signalling mechanisms during insulin action in rat adipocytes. Insulin rapidly activated a phospholipase D that hydrolyses phosphatidylcholine (PC), and this activation was accompanied by increases in diacylglycerol and translocative activation of PKC-alpha and PKC-beta in the plasma membrane. Wortmannin, an apparently specific PI 3-kinase inhibitor, inhibited insulin-stimulated, phospholipase D-dependent PC hydrolysis and subsequent translocation of PKC-alpha and PKC-beta to the plasma membrane. Wortmannin did not inhibit PKC directly in vitro, or the PKC-dependent effects of phorbol esters on glucose transport in intact adipocytes. The PKC inhibitor RO 31-8220 did not inhibit PI 3-kinase directly or its activation in situ by insulin, but inhibited both insulin-stimulated and phorbol ester-stimulated glucose transport. Our findings suggest that insulin acts through PI 3-kinase to activate a PC-specific phospholipase D and causes the translocative activation of PKC-alpha and PKC-beta in plasma membranes of rat adipocytes.