Inhibition of protein kinase C activity in cultured pituitary cells attenuates both cyclic AMP-independent and -dependent secretion of ACTH

Corticotropin-releasing hormone activates protein kinase C in an isoenzyme-specific manner

Protein kinase C (PKC) has recently emerged as mediator of corticotropin-releasing hormone (CRH) effects. Aim of the present study was to study the effects of CRH on each PKC isoenzyme. As a model we have used the PC12 rat pheochromocytoma cell line, expressing the CRH type 1 receptor (CRHR1). Our data were as follows: (a) CRH-induced rapid phosphorylation of conventional PKCalpha and PKCbeta, accompanied by parallel increase of their concentration within nucleus. (b) CRH suppressed the phosphorylation of novel PKCdelta and PKCtheta;, which remained in the cytosol. (c) CRH-induced transient phosphorylation of atypical PKClambda and had no effect on PKCmu. (d) The effect of CRH on each PKC isoenzyme was blocked by a CRHR1 antagonist. (e) Blockade of conventional PKC phosphorylation inhibited CRH-induced calcium ion mobilization from intracellular stores as well as the CRH-induced apoptosis and Fas ligand production. In conclusion, our findings suggest that CRH via its CRHR1 receptor differentially regulates PKC-isoenzyme phosphorylation, an apparently physiologically relevant effect since blockade of conventional PKC phosphorylation abolished the biological effect of CRH.

Plasma alpha-MSH and acetylated beta-endorphin levels following stress vary according to CRH sensitivity of the pituitary melanotropes in common carp, Cyprinus carpio

Pituitary melanotropes release alpha-melanocyte-stimulating hormone (alpha-MSH) and acetylated beta-endorphin (NAc beta-end) during stress responses. However, effects of stressors on plasma concentrations of these hormones are highly inconsistent among fish species. Here, we show that also within a species, the common carp (Cyprinus carpio), fish sometimes respond with elevated alpha-MSH and NAc beta-end plasma levels, and at other times not. The origin of this variable response was investigated by (1) studying the effects of corticotropin-releasing hormone (CRH) on alpha-MSH and NAc beta-end release in vitro, (2) establishing where in the second messenger pathway coupled to CRH receptors melanotrope responsiveness is determined, and (3) testing modulatory actions of other hypothalamic factors (here opioid beta-endorphin). Melanotropes were in a high or low responsive state to CRH in vitro, which was especially evident when tissue was tested from fish kept at higher ambient water temperatures, and this correlates with the variability in alpha-MSH and NAc beta-end responses in vivo. Relative rates of alpha-MSH and NAc beta-end release following stimulation with CRH in vitro match plasma level changes in vivo, and this indicates that the CRH pathway does act in vivo. cAMP did not stimulate melanotropes in the low responsive state to release hormones in vitro. Thus, the mechanism that determines the cell status, occurs downstream of cAMP accumulation. Opioid beta-endorphin differentially modulated the actions of CRH, as NAc beta-end, but not alpha-MSH, release was inhibited. This response was not observed in the stress paradigms studied. We conclude that the variation in alpha-MSH and NAc beta-end stress responses in vivo correlates with many CRH responses in vitro; whether a cell is in a high or low responsive state to CRH is determined downstream of accumulation of the second messenger. We propose that melanotropes have to be in the high responsive state to be activated by CRH during stress in carp and other teleosts.

Involvement of NO in the endothelium-independent relaxing effects of N(omega)-hydroxy-L-arginine and other compounds bearing a C=NOH function in the rat aorta

The mechanisms of vasorelaxation elicited by N(omega)-hydroxy-L-arginine (L-NOHA) and other compounds bearing a C=NOH function and the structural determinants governing this effect were investigated in rat aorta. L-NOHA, formamidoxime, five aromatic monosubstituted amidoximes, and one aromatic monosubstituted ketoxime elicited relaxation in endothelium-denuded rings. N-Hydroxyguanidine and substituted N-hydroxyguanidines were markedly less active. Relaxations induced by L-NOHA and by the most active studied compound, 4-chlorobenzamidoxime (ClBZA), were unmodified by the presence of endothelium. In endothelium-denuded rings, they were blunted by the NO scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (300 microM) and by the inhibitor of guanylyl-cyclase activation 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one (1 microM). In addition, L-NOHA- and ClBZA both caused cGMP accumulation. L-Arginine, but not D-arginine (1 mM), antagonized the effect of L-NOHA but not ClBZA. Both L-NOHA- and ClBZA-induced relaxations were inhibited by the NAD(P)H-dependent enzymes inhibitor diphenyliodonium (30 microM) and the NAD(P)H-dependent reductases inhibitor 7-ethoxyresorufin (10 microM), but they were unmodified by the cytochrome P450 (P450) inhibitor proadifen (10 microM) and by the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 300 microM). These results show that L-NOHA and other compounds with a C=NOH function can cause endothelium-independent relaxation in the rat aorta. They suggest that activation of guanylyl cyclase and NO formation is implicated in relaxation and that a 7-ethoxyresorufin-sensitive NAD(P)H-dependent pathway is involved. On one hand, L-NOHA and amidoximes may be useful tools for characterizing this pathway in blood vessels and, on the other, may offer a novel approach for treating vascular diseases with impaired endothelial NO activity.

Nitric oxide synthase inhibition promotes carcinogen-induced preneoplastic changes in the colon of rats

l-Arginine is metabolized either to polyamines through arginase and ornithine decarboxylase (ODC) activities or to citrulline and nitric oxide (NO, nitrogen monoxide) through the NO synthase (NOS) pathway. Polyamine levels and ODC activity are high in tumor cells. The aim of this study was to test whether N(G)-nitro-l-arginine methyl ester (l-NAME), an inhibitor of NOS, modulates colon carcinogenesis. Adult male Wistar rats were treated with azoxymethane (AOM, 15 mg/kg ip), a chemical carcinogen, once a week for 2 weeks. One week after the second injection the rats were randomly divided into two groups. One group (n = 8) received l-NAME (10 mg/kg body wt/day) in drinking water. The control group (n = 8) received tap water. After 5 weeks, the rats receiving l-NAME showed enhanced mean basal arterial blood pressure, decreased heart rate, and a significant decrease of the cGMP content in the colonic mucosa. In both groups, AOM induced the formation of colonic aberrant crypt foci (ACF). In l-NAME-treated rats, the number of ACF was higher than in controls by 47%. ODC activity was enhanced by 11-fold. S-Adenosyl-methionine-decarboxylase activity and putrescine concentration were significantly increased in the colonic mucosa of l-NAME-treated rats. The data suggest that l-NAME promotes carcinogen-induced preneoplastic changes in the colon by inhibiting NOS activity and by stimulating polyamine biosynthesis.

alpha-adrenergic stimulation of cytosolic Ca2+ oscillations and exocytosis in identified rat corticotrophs

1. The patch clamp technique was used in conjunction with a fluorescent Ca2+ indicator (indo-1, or indo-1FF) to measure simultaneously cytosolic Ca2+ concentration ([Ca2+]i), ionic current and changes in membrane capacitance in single rat corticotrophs identified with the reverse haemolytic plaque assay. 2. Application of the adrenocorticotropin (ACTH) secretagogue noradrenaline (NA; norepinephrine), triggered [Ca2+]i oscillation in corticotrophs via alpha-adrenergic receptors and the guanosine trisphosphate (GTP) binding protein-coupled phosphoinositide pathway. 3. Simultaneous measurement of [Ca2+]i and capacitance shows that exocytosis was triggered during the first cycle of NA-induced [Ca2+]i oscillation and the mean increase in cell membrane surface area was 1.4 +/- 0.3 % (n = 6). 4. When Ca2+ was directly released from the inositol 1,4, 5 trisphosphate (IP3)-sensitive store via flash photolysis of caged IP3, the mean increase in cell surface area was 1.5 +/- 0.5 % (n = 6). Thus, NA-stimulated ACTH secretion in rat corticotrophs is closely coupled to intracellular Ca2+ release. 5. Large and rapid elevation of [Ca2+]i (>15 microM) via flash photolysis of caged Ca2+ triggered two phases of exocytosis: a rapid exocytic burst that was complete in approximately 100 ms and a slow burst that continued for many seconds. 6. The rapid exocytic burst reflected the exhaustion of a pool of readily releasable granules and, on average, increased the cell surface by 2.8 +/- 0.1 % (n = 14). 7. We suggest that the relatively weak exocytic response in corticotrophs during intracellular Ca2+ release may be partially attributed to a smaller pool of readily releasable granules.

Possible mechanism of nitric oxide production from N(G)-hydroxy-L-arginine or hydroxylamine by superoxide ion

It has been speculated the N(G)-hydroxy-L-arginine (OH-L-Arg), which is an intermediate in NO production from L-arginine, may be converted to NO by superoxide ion. However, there is still no direct evidence for this conversion. In the present study this was investigated using superoxide ion generated either in acellular or cellular systems. It was found that OH-L-Arg and hydroxylamine were converted to nitrite and nitrate apparently via NO by superoxide ion in aqueous solution. Arginine remained unaffected. These changes were observed during reaction of chemical substances as well as in a biological system (zymosan-activated macrophages in culture). Superoxide dismutase prevented this transformation. OH-L-Arg was also spontaneously hydrolysed to hydroxylamine and L-citrulline, however this occurred at pH > 9 only. Activated microsomes (containing different isoforms of cytochrome P450) were unable to replace NO-synthase in its ability to produce OH-L-Arg from L-arginine. These data support the hypothesis that a pathway alterative to the well-known synthesis of NO by NO-synthase via OH-L-Arg exists. This pathway may involve the production of OH-L-Arg by NO-synthase and decomposition of OH-L-Arg to NO by the action of superoxide ion. Alternatively, hydrolysis of OH-L-Arg to hydroxylamine may occur followed by its oxidation to NO, again by superoxide ion.

Prophylactic administration of L-arginine improves the intestinal barrier function after mesenteric ischaemia

BACKGROUND

Ischaemia/reperfusion (I/R) of the intestine causes mucosal injury associated with a high death rate in rats.

AIM

To investigate whether nitric oxide (NO) might be implicated in the recovery of the intestinal mucosa after ischaemic insult.

METHODS

Wistar rats were subjected to mesenteric artery occlusion for 90 minutes. The animals were given either L-arginine, the substrate of NO synthase, or molsidomine, a NO donor. The controls received casein hydrolysate. The compounds were administered by gavage 19, 16, and 1.5 hours before ischaemia. Mucosal barrier permeability and cGMP content were determined 24 hours after ischaemia.

RESULTS

Survival after I/R was 50% in the control group. Animals treated with L-arginine or molsidomine exhibited a higher survival rate (70% and 83% respectively). Mucosal barrier permeability was decreased in rats receiving L-arginine or molsidomine compared with controls (4.0 (0.9) and 2.6 (0.6) v 11.2 (1.6) 14C-PEG pmol/segment, p < 0.05). Increased cGMP content was seen in the mucosa of the L-arginine group.

CONCLUSION

The findings suggest that pretreatment with L-arginine or molsidomine ameliorates survival after intestinal I/R and improves mucosal barrier function.

Effects of corticotropin-releasing hormone, lysine vasopressin, oxytocin, and angiotensin II on adrenocorticotropin secretion from porcine anterior pituitary cells

The aim of this study was to determine the ability of corticotropin-releasing hormone (CRH), lysine vasopressin (LVP), oxytocin (OT), and angiotensin II (AII) to stimulate adrenocorticotropin (ACTH) secretion from porcine anterior pituitary (AP) cells in vitro and to evaluate the role of protein kinase C (PKC) in the interaction between CRH and LVP. In this study, porcine AP cells were enzymatically and mechanically dispersed, cultured (150,000 cells/well) for 4 d, and then challenged with doses of various neuropeptides for 3 hr. CRH (10(-7)-10(-10) M) was the most potent of the peptides tested in stimulating ACTH release from porcine AP cells. In fact, none of the other peptides consistently affected ACTH concentrations relative to basal levels. However, LVP potentiated CRH action, even though by itself, it failed to stimulate ACTH production. Neither OT or AII potentiated CRH-stimulated ACTH release from porcine AP cells. To determine whether the inter-action between CRH and LVP was regulated partially by the protein Kinase C (PKC) pathway, we challenged AP cells in a 30-min incubation with 10(-7) M staurosporine (ST), a treatment predicted to decrease PKC activity. Then, cells were washed and challenged with 10(-9) M LVP, 10(-9) M CRH, and 10(-9) M CRH + LVP. Treatment with ST decreased (P < 0.05) CRH + LVP-stimulated ACTH release. To further demonstrate an interaction between protein kinase A (PKA) and PKC transduction pathways in the observed synergism between CRH and LVP to enhance ACTH secretion, we also challenged AP cells with 10(-7) M phorbol 12, 13-myristate acetate (PMA) and 5 microM forskolin (FOR) for 3 hr. This treatment was predicted to enhance PKA and PKC activities, respectively, and thereby enhance ACTH concentrations. Challenging cells with FOR + PMA enhanced (P < 0.001) ACTH release above basal concentrations, but more important, it increased (P < 0.001) ACTH concentration above that elicited by either drug given alone. Taken together, our in vitro studies support the conclusion that CRH is the principal regulator of ACTH secretion in the pig. In contrast to the results in most other species evaluated, vasopressin alone did not affect ACTH release. However, LVP can enhance the effectiveness of CRH in releasing ACTH, and this enhancement appears to rely, at least in part, on the activation of the PKC signal transduction pathway.

Pituitary adenylate cyclase-activating polypeptide induces expression of corticosteroid-binding globulin in cultured fetal hepatocytes: synergy with tri-iodothyronine

The purpose of the present study was to determine whether functional receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) are expressed in cultured rat fetal hepatocytes and eventually play a role in regulating gene expression of corticosteroid-binding globulin (CBG). We found PACAP38 and PACAP27 to elevate cAMP levels in hepatocytes in a dose-dependent manner, with a plateau being achieved at 10 nM and EC50 values of about 0.5-1 nM. PACAP failed to alter the turnover of inositol phosphates, whereas PACAP and VIP stimulated cAMP accumulation in an equipotent manner, suggesting the presence in these cells of type II receptor isoforms. As revealed by measurements of both CBG mRNA levels and concentrations of binding sites, long-term treatment of fetal cells with 10 nM PACAP, although resulting in partial desensitization of peptide-induced cAMP accumulation, caused a significant 3-fold elevation in CBG synthesis. This stimulatory influence of PACAP was mimicked by the cell permeant N6,2'-O-dibutyryladenosine 3',5'-phosphate (dbcAMP). Treatment of hepatocytes with tri-iodothyronine (T3) enhanced CBG expression and, most interestingly, appeared to synergize with PACAP to elicit a 2-3-fold amplification of CBG synthesis. This study thus provides first evidence for the up-regulation by PACAP and cAMP of CBG expression in fetal hepatocytes and for T3's playing a synergistic role in enhancing PACAP-induced synthesis of the binder.

Multifactorial regulation of pituitary adenylate cyclase-activating polypeptide (PACAP)-induced production of cyclic AMP in ATT-20 corticotrophs: major involvement of Rolipram-sensitive and insensitive phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) appear to play a major role in the modulation of cellular accumulations of cAMP/cGMP and hence the magnitude of the cell response to a hormone signal. These enzymes are present in cells as multiple isoforms and lie under control of various protein kinases. Because PACAP, unlike corticotropin-releasing factor (CRF), may stimulate a dual signalling pathway in pituitary cells (activating both adenylyl cyclase and phospholipase C), we used AtT-20 corticotrophs and primary cultures of rat pituitary cells to study the effect and possible differential influence of these peptides on cAMP formation. Time-course analysis indicated that, both in the absence and the presence of Rolipram (a selective type IV PDE inhibitor), PACAP stimulated a rapid and short-lived accumulation of cAMP in tumor corticotrophs, while in the presence of the non-selective inhibitor IBMX, the peptide produced a sustained high plateau level of second messenger (10 times the level generated with Rolipram at 20 min). On the contrary, when exposed to CRF, cAMP production augmented in parallel, irrespective of whether Rolipram or IBMX were present. The differential effects of the PDE inhibitors were seen with PACAP concentrations ranging from 0.1 to 100 nM, and could also be demonstrated in primary cultures of pituitary cells. Co-incubation of AtT-20 cells with Rolipram along with inhibitors of type I (but not of type III) PDEs, enhanced cAMP formation elicited by PACAP to a level significantly higher than that induced by CRF.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of protein phosphatases by okadaic acid and calyculin-A differentially modulates hormonal- and forskolin-stimulated formation of cyclic AMP in AtT-20 corticotrophs: effect of pituitary adenylate activating polypeptide and corticotropin-releasing factor

The effect of phosphatase inhibitors okadaic acid and calyculin-A on cAMP formation and adrenocorticotropic hormone (ACTH) secretion in AtT-20 corticotrophs was investigated. Both okadaic acid and calyculin-A inhibited dose-dependently the accumulation of cAMP in cells stimulated with pituitary adenylate cyclase activating factor (PACAP) and corticotropin-relating hormone (CRF). While in the case of okadaic acid the half-maximum inhibiting concentration was similar for both peptides (IC50 = 4 x 10(-7) M), it appeared that calyculin-A was about one order of magnitude more efficient in inhibiting the effect of PACAP than that of CRF (IC50 = 3.8 x 10(-9) M vs 2.0 x 10(-8) M, respectively). Importantly, the inhibitors blocked the activation by cholera toxin (which acts on Gs-like proteins) of cAMP formation, but failed to alter the effect of forskolin (which bypasses the receptor-G protein complex and activates adenylyl cyclase directly). Treatment of cells with calyculin-A significantly dampened adenylyl cyclase activity in cell membrane fraction, though to a lesser extent than it blocked cAMP formation in the whole cell. Both okadaic acid and calyculin-A inhibited CRF- and PACAP-induced secretion of ACTH. Our data hint that in AtT-20 corticotrophs, inhibition of phosphatases by modulating the state of phosphorylation of the receptor-G proteins complexes for CRF and PACAP, regulates cAMP formation and ACTH secretion.

Vasopressin, unlike phorbol ester, fails to synergistically interact with pituitary adenylate cyclase activating polypeptide (PACAP) in stimulating cyclic AMP formation and ACTH secretion in cultured anterior pituitary cells

In an attempt to determine if PACAP synergistically interacts with vasopressin (VP) and protein kinase C (PKC) to enhance cyclic AMP formation and adrenocorticotrophic hormone (ACTH) secretion, the effects of PACAP, either alone or together with VP and the phorbol ester phorbol 12-myristate 13-acetate (PMA) were examined in primary cultures of rat anterior pituitary cells. VP failed to potentiate the stimulatory effect of PACAP on cyclic AMP formation, while it dramatically enhanced the effect of corticotropin-releasing factor (CRF). However, activation of PKC upon exposure of cells to PMA amplified cyclic AMP production induced by both peptides, though in the case of PACAP, contrary to that of CRF, potentiation was markedly dependent on the blockade of phosphodiesterase (PDE) activity, for it was undetectable in the absence of the inhibitor Rolipram. Depletion of PKC by long-term treatment of pituitary cells with PMA abolished the synergistic influence of PMA. There was no significant effect of PACAP, either alone or together with PMA, on ACTH secretion, while PMA enhanced peptide secretion elicited by CRF. The data show that in anterior pituitary cells cyclic AMP accumulation induced by PACAP and CRF was differentially modulated by PKC and PDE activities and that the potentiation of PACAP-stimulated cyclic AMP accumulation by PMA was not reflected by parallel increment of ACTH secretion.

Pituitary adenylate cyclase polypeptide (PACAP) stimulates cyclic AMP formation in pituitary fibroblasts and 3T3 tumor fibroblasts: lack of enhancement by protein kinase C activation

A number of neuropeptides were shown to produce potent mitogenic effects on Swiss 3T3 fibroblasts by activating the phospholipase C pathway. Here we provide evidence for the activation by PACAP of the adenylate cyclase pathway in 3T3, as well as in non-tumoral pituitary fibroblasts, similarly to what was seen in pituitary endocrine cells. In these cells, PACAP triggered elevation of both intracellular and extracellular contents of cAMP and the effect was time- and dose-dependent, with half-maximal stimulations being induced with about 0.1 nM. Following activation of protein kinase C (PKC) by the phorbol ester phorbol 12-myristate 13-acetate (PMA), PACAP-induced cAMP production was amplified in pituitary endocrine cells, but was either unchanged or dampened in 3T3 and pituitary fibroblasts, respectively. Pretreatment of cells with pertussis toxin (PT) failed to change the effect of PMA on PACAP-stimulated adenylate cyclase activity, irrespective of the cell type being used. However, PT dramatically reduced the potentiation by PMA of cAMP production enhanced by forskolin in 3T3 cells. These results provide new evidence pointing to the presence in fibroblasts of receptors for PACAP, coupled to cAMP production, which may play a role in the modulation of the mitogenic signal. They also indicate that, compared with pituitary endocrine cells, PKC activation in fibroblasts differentially affected PACAP-induced cAMP formation and that these effects were unaltered upon inhibition by PT of Gi-like proteins.