Ionomycin inhibits thyrotropin-releasing hormone-induced translocation of protein kinase C in GH4C1 pituitary cells

Pituitary expression of protein kinase C isotypes during early development

Protein kinase C (PKC) is a critical regulator of signal transduction and cell function in many tissues, including pituitary. Although PKC influences pituitary hormone secretion in adults, its role in determining characteristic perinatal patterns of hormone secretion and synthesis is not known, and the expression of major PKC isotypes in perinatal pituitary is poorly defined. We therefore determined the developmental, cell-specific expression of the major PKC isotypes, using Western analysis and double label immunohistochemistry, in pituitaries of perinatal and mature rats. Expression of specific PKC isotypes was strikingly age-dependent. Pituitary expression of PKC alpha was particularly high in neonates and declined significantly with age, with levels in adult rats approximately half those of neonates as assessed by Western analysis. Similarly, immunohistochemistry indicated that PKC alpha was less abundant in adult than in neonatal pituitaries; the most intensely staining cells of both age groups were identified as somatotrophs and gonadotrophs. In contrast to PKC alpha, pituitary expression of PKC epsilon increased approximately two-fold with advancing age as assessed by Western analysis; this age-dependent pattern was confirmed by immunohistochemistry. Perinatal pituitaries expressed PKC epsilon in some somatotrophs and in all gonadotrophs, whereas PKC epsilon expression was limited to gonadotrophs in the mature pituitary. Pituitary expression of PKC betaII, delta, and zeta did not differ with age, and PKC gamma was not detected in pituitaries of any age group. These results indicate that expression of PKC isotypes within the pituitary is developmentally regulated in a cell-specific and isotype-specific manner, and are consistent with the concept that PKC contributes to the regulation of pituitary function during early development.

Protein kinase C and growth regulation of pituitary adenomas

The present study was undertaken to explore the role of the Protein Kinase C (PKC) signal transduction system in growth regulation of pituitary adenomas. Primary tumor cultures were plated from fresh surgical tumor specimens. The PKC inhibitors Staurosporine and Tamoxifen were added at varying dosages to the cell cultures. Measurements of cell proliferation were performed by [3H]-thymidine uptake and the [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H tetrazolium bromide] (MTT) assay. After a 48 h treatment period, both [3H]-thymidine uptake and absorbance on the MTT assay decreased in a dose-related manner in both the staurosporine and tamoxifen-treated cultures (IC50 of 10 nM and 30 microM respectively). Direct measurement of PKC activity using an in vitro assay revealed very high activity (range of 1465-5708 pmol/min/mg protein; within the range previously published for malignant glioma specimens) in 12 frozen specimens of pituitary adenomas (9 nonfunctional adenomas, 1 prolactinoma, 1 gonadotrophin-secreting and 1 corticotroph-secreting adenoma). In contrast, PKC activity measured in normal adenohypophysis was comparatively very low. These data indicate that pituitary adenoma cells display high PKC activity and are sensitive to growth inhibition by PKC inhibitors. These data suggest a role for the PKC system in regulating pituitary tumor growth, which may have implications for future therapy of these tumors.

Simultaneous measurement of [Ca2+]i and secretion-coupled membrane turnover, by single cell fluorescence microscopy

Thyrotropin releasing hormone (TRH), which stimulates prolactin secretion, increases the fluorescence of cultured bovine anterior pituitary (bAP) cells in the presence of the non-permeant membrane indicator dye FM 1-43 [Stafford SJV. Shorte SL. Schofield JG. (1993) Use of a fluorescent dye to measure secretion from intact bovine anterior pituitary cells. Biosci. Rep., 13, 9-17]. FM 1-43 is non-fluorescent in aqueous solution but becomes fluorescent when incorporated into the plasma membrane. The membrane area accessible to FM 1-43 dye, and therefore cell fluorescence, increases during exocytosis as secretory granules fuse with the plasma membrane, and endocytosis as vesicles formed at the plasma-membrane fuse with intracellular organelle membranes. We have here measured changes in FM 1-43 uptake and the intracellular calcium concentration ([Ca2+]i) concurrently in the same cells on exposure to TRH, phorbol myristate acetate (PMA) or NH4Cl. TRH (0.1-10 microM) caused a transient increase in [Ca2+]i in 70-90% of bAP cells and in 60-90% of the responding cells also caused a sustained increased FM 1-43 fluorescence. TRH increased [Ca2+]i but did not affect FM 1-43 fluorescence in GH3 rat pituitary cells, probably because they contain too few secretory granules to give a detectable increase. The dopamine D2-receptor agonist quinpirole (10 microM) had little effect on the TRH-induced [Ca2+]i rise in bAP cells, but abolished the increase in FM 1-43 fluorescence. The phorbol ester PMA (0.3-3 microM) caused a small, transient increase in [Ca2+]i followed by a fall to levels lower than original resting levels in 40-60% of bAP cells and increased FM 1-43 uptake in cells showing these changes. Extracellular NH4Cl, which mobilises calcium from an ionomycin-insensitive calcium store, caused a transient [Ca2+]i increase in over 90% of the bAP-cells and increased FM 1-43 uptake in a subpopulation (> 50%) of these. The Na+/H+ ionophore monensin prevented the increase in FM 1-43 fluorescence but not the [Ca2+]i rise induced by TRH, prevented the increases in both FM 1-43 fluorescence and [Ca2+]i caused by NH4Cl, and reduced the number of cells showing a rise in FM 1-43 fluorescence in response to PMA from 64% to 34%. The Ca(2+)-ATPase inhibitor thapsigargin reduced the number of bAP cells displaying TRH-induced increases in [Ca2+]i and membrane-turnover from 74% to 18%, but did not affect the changes in [Ca2+]i or FM 1-43 fluorescence caused by PMA or NH4Cl. We discuss the relationships between the secretogogue-induced increases in FM 1-43 fluorescence and changes in intracellular [Ca2+]i under these conditions.

The effect of Ca2+ on the translocation of protein kinase C in bovine tracheal smooth muscle

The role of calcium in protein kinase C redistribution was studied in bovine tracheal smooth muscle preparations contracted by methacholine. Previous results have shown that, in the presence of normal extracellular Ca2+, 10 microM methacholine produced a sustained contraction and a sustained translocation of protein kinase C from the cytosol to the membrane. In the present study, when tissues were preincubated in Ca(2+)-free buffer containing 1 mM EGTA, methacholine produced a rapid but transient elevation in membrane-associated protein kinase C activity which was detected at 30 s and had returned to basal within 20 min. The redistribution of protein kinase C from the cytosol to the membrane induced by 1 microM methacholine in normal Ca2+ was reversed by removal of the extracellular Ca2+ and addition of 2 mM EGTA during agonist stimulation. Removal of the Ca2+ caused approximately 50% relaxation after 10 min. Verapamil (30 microM) partially reversed the methacholine-induced protein kinase C redistribution and caused approximately 40% relaxation after 15 min. Sodium nitroprusside (10 microM) caused a rapid relaxation and complete reversal of the protein kinase C redistribution induced by methacholine. High K+ (60 mM) also induced a sustained contraction and redistribution of protein kinase C from the cytosol to the membrane. Suitable antagonists were added to the bathing medium to block the effects of endogenous mediators which could be released by KCl-induced depolarization. Thus, translocation of protein kinase C is obtained in the absence of receptor activation.(ABSTRACT TRUNCATED AT 250 WORDS)

A morphometric and computer-assisted three-dimensional reconstruction study of neural tube formation in chick embryos

The origin of the driving forces for neural tube formation remains uncertain but is currently thought to involve the participation of microfilament bundles situated in the apical ends of neuroepithelial cells. In the work presented here, we show how morphometric measurements that map local variations in the apical geometry of neuroepithelial cells (especially apical constriction) can provide information on the distribution of motive forces within the neuroepithelium during neural tube formation. When used in combination with computer-assisted, three-dimensional reconstruction, it becomes possible to analyze the morphometric data from a dynamic, three-dimensional perspective. As an example application of this method, we have used morphometry to evaluate the effects of ionomycin on the developing neuroepithelium. Treatment of early (stages 6-8) chick embryos with 5 microM ionomycin was found to cause rapid bending of the neuroepithelium within 1 min of exposure and a dramatic acceleration of the normal sequence of neural tube formation. Electron microscopy and morphometry revealed that this acceleration was coincident with a marked increase in the local degree of apical constriction of neuroepithelial cells, presumably a consequence of enhanced contractile activity of apical microfilament bundles. This work shows that transient elevation of free calcium levels can accelerate the usual sequential phases of NT formation. The rapidity of the response (hours of normal development reduced to minutes), increased prominence of apical microfilament bundles, and the enhanced degree of apical constriction strongly support a direct causal role for apical microfilament bundles and apical constriction of neuroepithelial cells in bending of the neuroepithelium.

Up-modulation of phorbol dibutyrate receptors by carbachol and arachidonic acid in rat prostatic epithelial cells

Phorbol dibutyrate (PDBu) binding to rat prostatic epithelial cells has been measured as an indirect determination of protein kinase C in this cell system. Analysis of [3H]PDBu binding using competitive displacement demonstrated a single class of PDBu receptors with a Kd = 141 nM and a binding capacity of 4.8 pmol PDBu bound/mg cell protein. Raising cytosolic Ca2+ levels by redistribution of intracellular Ca2+ after cell treatment with carbachol or arachidonic acid (which also affects the bulk biophysical properties of the cell membrane) resulted in up-regulation of the available number of PDBu receptors. These results appear to be a consequence of PKC translocation from the cytosolic compartment to the plasma membrane after a cytosolic Ca2+ increase, confirming previous results in other cell systems.

Structural and functional diversities of a family of signal transducing protein kinases, protein kinase C family; two distinct classes of PKC, conventional cPKC and novel nPKC

Recent molecular cloning and biochemical experiments on the nature of protein kinase C (PKC) have revealed the existence of two distinct classes of phorbol ester (and diacylglycerol) receptor/protein kinase, conventional PKC (cPKC) and novel PKC (nPKC). Each of these classes contains multiple related molecules expressed in tissues and cells in a type-specific manner. Although nPKC does not show the typical PKC activity ascribable to conventional PKCs and thus was neglected in earlier studies, several lines of evidence suggest that nPKCs are involved in a variety of cell responses to physiological stimuli and phorbol esters. It is possible that in some cases nPKC is the major mediator of the so-called PKC-activators, such as phorbol esters, mezerein, and bryostatins. In addition to the clear difference between cPKC and nPKC, functional diversity among conventional PKCs has also been demonstrated; PKC gamma differs in its competence to mediate the signal toward transcriptional activation through TPA-responsive cis-acting elements from cPKC alpha and nPKC epsilon. The differences between cPKC and nPKC and among the individual members of each of these two classes, and their specific pattern of distribution in tissues and cells, provide a rationale by which to explain the specificity and diversity of cellular responses to external stimuli generating DAG and to phorbol esters. The results presented here also provide a means to dissect the complex signaling pathway in cells and to analyze the molecular basis underlying the signal transduction processes mediated by this family of protein kinases.

Two Ca2+ transport systems are distinguished on the basis of their Mg2+ dependency in a post-nuclear particulate fraction of bovine adrenal cortex

Inositol 1,4,5-trisphosphate (InsP3) is a second messenger responsible for Ca2+ release from an internal store whose nature and location remains undefined. To get more information on this intracellular Ca2+ store, a post-nuclear particulate fraction was prepared from bovine adrenal cortex and its Ca2+ uptake and release activities were monitored with the fluorescent indicator Fura-2. In the presence of Mg2+ (2 mM), the particulate preparation showed high ATP-dependent Ca2+ sequestering activity and decreased the ambient Ca2+ concentration to about 150 nM. In the absence of Mg2+, Ca2+ was still sequestered but less efficiently, reaching a level around 170 nM. In the presence of Mg2+, the Ca2+ released by a maximal dose of InsP3 (2 microM) was completely resequestered whereas in the absence of Mg2+, no resequestration occurred even after complete degradation of InsP3. The use of selective agents such as oligomycin, saponin, ionomycin and biliary salts indicated that Ca2+ was stored in three different pools which are distinct from the mitochondria and from inside-out membrane vesicles. Our data also indicate that InsP3 releases Ca2+ from a pool which is filled up by a Mg2(+) -dependent Ca2+ ATPase.

Possible role of Ca2(+)-independent protein kinase C isozyme, nPKC epsilon, in thyrotropin-releasing hormone-stimulated signal transduction: differential down-regulation of nPKC epsilon in GH4C1 cells

Protein kinase C (PKC) molecular species of GH4C1 cells were analyzed after separation by hydroxyapatite column chromatography. A novel Ca2(+)-independent PKC, nPKC epsilon, was identified together with two conventional Ca2(+)-dependent PKCs, PKC alpha and beta II by analysis of kinase and phorbol ester-binding activities, immunoblotting using isozyme-specific antibodies, and Northern blotting. These PKCs are down-regulated differently when cells are stimulated by outer stimuli; phorbol esters deplete PKC beta II and nPKC epsilon from the cells more rapidly than PKC alpha, whereas thyrotropin-releasing hormone (TRH) at 200 nM depletes nPKC epsilon exclusively with a time course similar to that induced by phorbol esters. However, translocation of PKC alpha and beta II to the membranes is elicited by both TRH and phorbol esters. These results suggest that TRH and phorbol ester activate PKC alpha and beta II differently but that nPKC epsilon is stimulated similarly by both stimuli. Thus, in GH4C1 cells, Ca2(+)-independent nPKC epsilon may play a crucial role distinct from that mediated by Ca2(+)-dependent PKC alpha and beta II in a cellular response elicited by both TRH and phorbol esters.

Protein kinase C in insulin releasing cells. Putative role in stimulus secretion coupling

The evidence for the involvement of protein kinase C (PKC) in insulin secretion stimulated by glucose and Ca2(+)-mobilizing receptor agonists has been reviewed. Results of phorbol ester binding to intact cells and the measurements of the proportion of PKC associated with the membrane after cell fractionation are presented. Glucose stimulation leads to increased phorbol ester binding without causing membrane insertion of the enzyme which, however, occurs with receptor agonists. It is suggested that the rise in cytosolic Ca2+ in response to glucose favours the apposition of PKC to the membrane whereas intercalation of the enzyme requires phospholipase C-mediated generation of diacylglycerol. It is possible that this effect of glucose on PKC, although not involved in the initiation of secretion, could explain the potentiation of insulin release observed in the presence of the receptor agonists.

The mechanisms by which vasoactive intestinal peptide (VIP) and thyrotropin releasing hormone (TRH) stimulate prolactin release from pituitary cells

The effect of vasoactive intestinal peptide (VIP) on prolactin (PRL) secretion from pituitary cells is reviewed and compared to the effect of thyrotropin releasing hormone (TRH). These two peptides induced different secretion profiles from parafused lactotrophs in culture. TRH was found to increase PRL secretion within 4 s and induced a biphasic secretion pattern, while VIP induced a monophasic secretion pattern after a lag time of 45-60 s. The secretion profiles are compared to changes in adenylate cyclase activity, production of inositol polyphosphates, changes in intracellular calcium concentrations and changes in electrophysiological properties of the cell membrane.

Determination of the phase behaviour of phosphatidylethanolamine admixed with other lipids and the effects of calcium chloride: implications for protein kinase C regulation

The phase behaviour of 1-palmitoyl-2-oleoylphosphatidylethanolamine (POPE) was studied by differential scanning calorimetry and 31P-NMR spectroscopy. Modulation of the phase behaviour of POPE by 1-palmitoyl-2-oleoylphosphatidylserine (POPS). 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), 1,2-di-olein (DOG), CaCl2, MgCl2, and combinations of these substances was studied. The bilayer-forming lipids, POPS and POPC, raise the bilayer-to-hexagonal phase-transition temperature of POPE. The POPC has a greater effect than POPS, probably because the former lipid is more miscible with POPE. Addition of 10 mM CaCl2 has little effect on the phase-transitions of POPE/POPC mixtures, but it greatly decreases the effectiveness of POPS in raising the bilayer-to-hexagonal phase-transition temperature of POPE. The effectiveness of DOG in lowering the phase-transition temperature of POPE is also greatly reduced in the presence of 10 mM CaCl2. This phenomenon may play a role in the negative feedback regulation of protein kinase C.

TPA (12-O-tetradecanoylphorbol-13-acetate) enhances the central hypoglycemic action of thyrotropin-releasing hormone in mice

This study examined the effect of the calcium- and phospholipid-dependent protein kinase C (PKC) activator, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on the plasma glucose responses to central thyrotropin-releasing hormone (TRH) injection in mice in order to evaluate the involvement of PKC in the mechanism of TRH action in the central nervous system (CNS). TRH (0.1-10 micrograms), as well as the neuroactive TRH analogs, CG 3509, CG 3703, DN 1417, RX 77368, [Nva2]-TRH, KPC-TRH, and TRH-Gly (0.1-10 micrograms), injected centrally in normoglycemic mice reduced the circulating glucose levels in a dose-dependent manner. TPA (0.1-1 microgram), administered centrally together with TRH (1 microgram) or the TRH analogs strongly enhanced the hypoglycemic response. Similar doses of TPA had no effect on plasma glucose when administered alone or together with TRH analogs devoid of central hypoglycemic action, i.e. [Glu1]-TRH, [Phe2]-TRH, and [Gly3]-TRH (1 microgram). Central injection of a TPA analog lacking PKC-stimulating activity, 4-alpha-phorbol (0.1-1 microgram) had no effect on the hypoglycemic response to coadministered TRH. These results, demonstrating a specific effect of TPA in enhancing the hypoglycemic response to central TRH or its neuroactive, though not inactive, analogs are consistent with a possible role for PKC in the mechanism of TRH action in the CNS.

Inhibition of protein kinase C activity by the Leishmania donovani lipophosphoglycan

Purified lipophosphoglycan from Leishmania donovani was found to inhibit the activity of protein kinase C isolated from rat brain. Protein kinase C inhibition by lipophosphoglycan was continuous for 30 minutes. The glycoconjugate was a competitive inhibitor with respect to diolein, a noncompetitive inhibitor with respect to phosphatidylserine, and had no significant effect on protein kinase M and protein kinase A. A possible physiological role of lipophosphoglycan as a negative effector of protein kinase C is suggested.