Ammonia effects on pyruvate/lactate production in astrocytes--interaction with glutamate

Ammonia exerts a multitude of metabolic and non-metabolic effects on brain tissue. In the present communication we have investigated its effect on lactate production rates, pyruvate production rates and pyruvate/lactate ratios in mouse cerebrocortical astrocytes and neurons in primary cultures. No effects were found in neurons. All three parameters were affected by ammonia in astrocytes, but less potently and to a smaller degree in cells that had been treated with dibutyryl cyclic AMP (morphologically differentiated cells) than in untreated cells (morphologically undifferentiated cells). In the differentiated cells ammonia had virtually no effect up to a concentration of 1.0 mM, but at 3.0 mM it increased lactate production and decreased pyruvate/lactate ratio significantly. In the undifferentiated cells ammonia greatly increased lactate accumulation (by 80% at 3.0 mM) and it inhibited pyruvate accumulation (by 40% at 3.0 mM). It thereby reduced the pyruvate/lactate ratio progressively within the entire range 0.1-3.0 mM ammonia. In support of the hypothesis that the ammonia-induced reduction of pyruvate/lactate ratio is secondary to depletion of cellular glutamate by formation of glutamine (and glutathione) and a resulting interruption of the malate-aspartate shuttle (MAS), the addition of glutamate to the incubation medium significantly diminished the ammonia-induced reduction of pyruvate/lactate ratio, whereas it had no effect on the increased lactate production. It is discussed that MAS interruption may have additional consequences in astrocytes.

Characterization of cAMP-dependent proteolysis of GATA-6

Cyclic AMP-dependent proteolysis of GATA-6(Delta50) was characterized using inhibitors for intracellular signaling pathways. Among these kinase inhibitors, only H-89 and K252a inhibited the proteolysis induced by dbcAMP, a membrane permeable cAMP analogue, others such as PD98059, SB203580, calphostine C, PP1, and KN-93 did not do so. These results suggest that A-kinase, but not C-kinase, MEK, P38 MAP-kinases or Src kinase, could participate in the observed phenomenon. We further demonstrated that an inhibitor for ubiquitin isopeptidase (Delta12-PGJ2) inhibited the degradation of GATA-6(Delta50) in the presence of dbcAMP, suggesting that the cAMP-dependent proteolysis could be mediated through the ubiquitin-proteasome pathway, although proteasome activity did not change significantly during dbcAMP treatment. The full-length GATA-6 was also responsive to the induced degradation. Furthermore, mutation of a potential phosphorylation site (Ser-290-->Ala) for A- and C-kinases, and deletion of the PEST sequence of GATA-6 did not abolish the degradation. All these results suggest that cellular factor(s) may play a crucial role in mediating the activation of the cAMP-dependent process.

Cyclic AMP-mediated regulation of transcription factor Lot1 expression in cerebellar granule cells

Lot1, a zinc finger transcription factor acting as a tumor suppressor gene on tumoral cells, is highly expressed during brain development. In developing rat cerebellum, Lot1 expression is high in cerebellar granule cells (CGC), a neuronal population undergoing postnatal neurogenesis. The time course of Lot1 cerebellar expression closely matches the expression of pituitary adenylate cyclase-activating polypeptide (PACAP) receptors coupled to adenylyl cyclase. The aim of this study was to ascertain whether Lot1 expression is regulated by cAMP-dependent pathways and to identify mechanisms of Lot1 activation in CGC cultures. Our results show that Lot1 expression in CGC is cAMP-dependent, as treatments with either forskolin or PACAP-38 induced an increase in its expression at both the mRNA and protein levels. This effect on Lot1 expression was mimicked by dibutyryl cAMP and suppressed by protein kinase A and MEK inhibitors. In parallel, we found that treatments with forskolin and PACAP-38 in precursor CGC inhibited bromodeoxyuridine incorporation by 25 and 35%, respectively, indicating a negative effect on neuronal precursor proliferation. Luciferase reporter analysis and mutagenesis of the Lot1 promoter region indicated a crucial role of the AP1-binding site (located at -268 bp) in cAMP-induced Lot1 transcription. In addition, cotransfection experiments indicated that the c-Fos/c-Jun heterodimer is responsible for cAMP-dependent Lot1 transcriptional activation. In conclusion, our data demonstrate that, in CGC, Lot1 is under the transcriptional control of cAMP through an AP1 site regulated by the c-Fos/c-Jun heterodimer and suggest that this gene may be an important element of the cAMP-mediated pathway that regulates neuronal proliferation through the protein kinase A-MEK signaling cascade.

Aryl hydrocarbon receptor activation by cAMP vs. dioxin: divergent signaling pathways

Even before the first vertebrates appeared on our planet, the aryl hydrocarbon receptor (AHR) gene was present to carry out one or more critical life functions. The vertebrate AHR then evolved to take on functions of detecting and responding to certain classes of environmental toxicants. These environmental pollutants include polycyclic aromatic hydrocarbons (e.g., benzo[a]pyrene), polyhalogenated hydrocarbons, dibenzofurans, and the most potent small-molecular-weight toxicant known, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin). After binding of these ligands, the activated AHR translocates rapidly from the cytosol to the nucleus, where it forms a heterodimer with aryl hydrocarbon nuclear translocator, causing cellular responses that lead to toxicity, carcinogenesis, and teratogenesis. The nuclear form of the activated AHR/aryl hydrocarbon nuclear translocator complex is responsible for alterations in immune, endocrine, reproductive, developmental, cardiovascular, and central nervous system functions whose mechanisms remain poorly understood. Here, we show that the second messenger, cAMP (an endogenous mediator of hormones, neurotransmitters, and prostaglandins), activates the AHR, moving the receptor to the nucleus in some ways that are similar to and in other ways fundamentally different from AHR activation by dioxin. We suggest that this cAMP-mediated activation may reflect the true endogenous function of AHR; disruption of the cAMP-mediated activation by dioxin, binding chronically to the AHR for days, weeks, or months, might be pivotal in the mechanism of dioxin toxicity. Understanding this endogenous activation of the AHR by cAMP may help in developing methods to counteract the toxicity caused by numerous environmental and food-borne toxic chemicals that act via the AHR.

Positive inotropism in mammalian skeletal muscle in vitro during and after fatigue

We tested the hypothesis that positive inotropic factors decrease fatigue and improve recovery from fatigue in mammalian skeletal muscle in vitro. To induce fatigue, we stimulated mouse soleus and extensor digitorum longus (EDL) to perform isometric tetanic contractions (50 impulses x s(-1) for 0.5 s) at 6 contractions x min(-1) for 60 min in soleus and 3 contractions x min(-1) for 20 min in EDL. Muscles were submerged in Krebs-Henseleit bicarbonate solution (Krebs) at 27 degrees C gassed with 95% nitrogen - 5% carbon dioxide (anoxia). Before and for 67 min after the fatigue period, muscles contracted at 0.6 contractions x min(-1) in 95% oxygen - 5% carbon dioxide (hyperoxia). We added a permeable cAMP analog (N6, 2'-O-dibutyryladenosine 3':5'-cyclic monophosphate at 10(-3) mol x L(-1) (dcAMP)), caffeine (2 x 10(-3) mol x L(-1), or Krebs as vehicle control at 25 min before, during, or at the end of the fatigue period. In soleus and EDL, both challenges added before fatigue significantly increased developed force but only caffeine increased developed force when added during the fatigue period. At the end of fatigue, the decrease in force in challenged muscles was equal to or greater than in controls so that the force remaining was the same or less than in controls. EDL challenged with dcAMP or caffeine at any time recovered more force than controls. In soleus, caffeine improved recovery except when added before fatigue. With dcAMP added to soleus, recovery was better after challenges at 10 min and the end of the fatigue period. Thus, increased intracellular concentrations of cAMP and (or) Ca2+ did not decrease fatigue in either muscle but improved recovery from fatigue in EDL and, in some conditions, in soleus.

Progesterone inhibits protein kinase A (PKA) in Xenopus oocytes: demonstration of endogenous PKA activities using an expressed substrate

3'-5' cyclic adenosine monophosphate (cAMP)-dependent protein kinase, PKA, is thought to be a key enzyme that controls prophase arrest in vertebrate oocytes. It has long been established that overexpression of the catalytic subunit of PKA inhibits hormone-induced frog oocyte maturation whereas overexpression of the regulatory subunits induces hormone-independent oocyte maturation. However, the activities of endogenous oocyte PKA, or its regulation by the maturation-inducing hormone progesterone, have never been directly demonstrated in frog oocytes. We have developed a novel expressed substrate for PKA in live oocytes by constructing a fusion protein containing an N-terminal myristylation sequence (derived from the Src tyrosine kinase) followed by an antigenic epitope tag and a substrate motif (the C-terminal cytoplasmic domain of beta2 adrenergic receptor). Following mRNA injection, the phosphorylation status of the substrate was determined by two-dimensional electrophoresis followed by epitope immunoblotting, or alternatively by SDS-PAGE followed by immunoblotting using antibodies specifically recognizing the PKA-phosphorylated form of the substrate. In prophase oocytes, the expressed protein, myr-HA-beta2AR-C, was fully phosphorylated on a single PKA site (Ser346 of human beta2 adrenergic receptor). Within one hour of the addition of progesterone, the PKA site became mostly dephosphorylated. No re-phosphorylation of the PKA site, and therefore no reactivation of PKA, was observed throughout the entire maturation process. To demonstrate the generality of this PKA substrate, we analyzed its phosphorylation status in COS-7 cells following transfection. We show that dibutyryl cAMP rapidly stimulates phosphorylation of the PKA site. These results represent the first biochemical demonstration of regulation of endogenous Xenopus oocyte PKA by progesterone. Furthermore, myr-HA-beta2AR-C should be widely adaptable as an in vivo PKA activity indicator.

Isolation of CHO-K1 clones defective in cAMP-dependent proteolysis, as determined by the stability of exogenously expressed GATA-6

Degradation of the GATA-6(Delta50) protein expressed in a CHO-K1 clone (tc1-17a) is stimulated in the presence of dbcAMP through proteasome without new protein synthesis [FEBS Lett. 408 (1997) 301], whereas the intrinsic GC-box-binding protein was stable. To examine the cellular mechanism responsible for this specific degradation of GATA-6(Delta50), we initially introduced the blasticidin-S deaminase gene carrying a promoter with GATA motifs that are recognized by GATA-6. The resulting cell line (tc2G2) grew in the presence of blasticidin S. However, the presence of both blasticidin S and dbcAMP was lethal due to degradation of GATA-6. Cells resistant to such lethality were isolated by chemical mutagenesis. The GATA-6(Delta50) in these resistant cells was stable in the presence of dbcAMP in contrast to that in the parent tc2G2 cells, as determined by gel-mobility shift analysis and Western blotting. These clones could be beneficial for identification and characterization of the components participating in the signaling pathway for both protein degradation and cAMP-dependent biological processes.

Modulation of glycogen phosphorylase activity affects 5-phosphoribosyl-1-pyrophosphate availability in rat hepatocyte cultures

The effect of modulation of the rate of glycogenolysis on the availability of 5-phosphoribosyl-1-pyrophosphate (PRPP) was investigated in rat hepatocyte cultures. Dibutyryl cyclic AMP (dbcAMP), forskolin and glucagon, activating glycogen phosphorylase through activation of protein kinase A (PKA), were found to raise PRPP availability by 44%-56%. Arg-vasopressin and phenylephrine, activating glycogen phosphorylase through the phosphoinositide cascade, did not affect PRPP availability. dbcAMP, but not phenylephrine, increased the degradation of pre labeled glycogen by 57%. Caffeine and CP-91149, inhibitors of glycogen phosphorylase, decreased PRPP availability by 33% and 43%, respectively. The finding that induction of glycogenolysis enhances, and inhibition of glycogenolysis decelerates PRPP generation suggests that glycogenolysis is a major contributor to PRPP generation in liver tissue in the basal (postabsorptive) state.

Adenylyl cyclase-cAMP system inhibits thyroid hormone-stimulated osteocalcin synthesis in osteoblasts

It is generally recognized that thyroid hormone modulates osteoblast cell function. We have previously shown that triiodothyronine (T(3)) activates p38 mitogen-activated protein (MAP) kinase, resulting in the synthesis of osteocalcin in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the effect of the adenylyl cyclase-cAMP system on thyroid hormone-stimulated osteocalcin synthesis in these cells. Dibutyryl-cAMP (DBcAMP) reduced the osteocalcin synthesis stimulated by T(3). Forskolin and cholera toxin suppressed the osteocalcin synthesis while dideoxyforskolin, a forskolin derivative that does not activate adenylyl cyclase, had little effect on the synthesis. KT5720, a selective inhibitor of protein kinase A, reversed the inhibitory effect of forskolin or DBcAMP. DBcAMP and forskolin markedly reduced the phosphorylation of p38 MAP stimulated by T(3). Pituitary adenylate cyclase-activating polypeptide (PACAP) significantly inhibited the T(3)-stimulated osteocalcin synthesis. These results strongly suggest that the adenylyl cyclase-cAMP system has an inhibitory role in thyroid hormone-stimulated osteocalcin synthesis via suppression of p38 MAP kinase activation in osteoblasts.

Modulation of the steroidogenic activity of luteinizing hormone by insulin and insulin-like growth factor-I through interaction with the cAMP-dependent protein kinase signaling pathway in the trout ovary

In the salmonid ovary, luteinizing hormone (LH) is the major gonadotropic hormone stimulating the production of steroids during the periovulatory period and its effects are mediated by the cAMP-dependent protein kinase (PKA) signaling pathway. We have previously shown that the in vitro steroidogenic activity of LH in the salmonid ovary is inhibited by insulin-like growth factor I (IGF-I) which, like insulin, has specific receptors in both theca and granulosa layers. In the present study, we have investigated the modulatory effects of insulin on salmon LH (sLH)-stimulated steroid production in preovulatory theca layers of brown trout (Salmo trutta) and the effects of both insulin and IGF-I on the sLH-stimulated cAMP/PKA signaling pathway. Our results show that insulin, like IGF-I, blocked the stimulatory effects of sLH, dibutyryl cAMP and IBMX on testosterone (T) production but not those of sLH on cAMP production. Furthermore, insulin and IGF-I blocked the activation of PKA induced by sLH and these effects were correlated with changes in the total protein content of the catalytic (C) and type II regulatory (RII) subunits of PKA. Interestingly, insulin and IGF-I had different effects on total PKA subunit content since insulin potentiated the sLH-stimulated increase in RII subunit content whereas IGF-I blocked the sLH-stimulated increase in total C subunit content. The effects of insulin and IGF-I in trout theca layers appeared to be mediated by the mitogen-activated protein kinase (MAPK) signaling pathway because inhibition of extracellular signal-regulated kinase 1/2(ERK1/2) activity completely blocked the inhibitory effects of insulin and IGF-I on the sLH-stimulated production of T and because insulin and IGF-I increased the total protein content of ERK1/2 in trout theca layers. Therefore, our results suggest that insulin and IGF-I, probably through the MAPK pathway, block the action of sLH in trout theca layers by modulating the cAMP/PKA signaling pathway.

Sphingosine kinase 1 is involved in dibutyryl cyclic AMP-induced granulocytic differentiation through the upregulation of extracellular signal-regulated kinase, but not p38 MAP kinase, in HL60 cells

The role of sphingosine kinase (SPHK) in the dibutyryl cyclic AMP (dbcAMP)-induced granulocytic differentiation of HL60 cells was investigated. During differentiation, SPHK activity was increased, as were mRNA and protein levels of SPHK1, but not of SPHK2. Pretreatment of HL60 cells with N,N-dimethylsphingosine (DMS), a potent SPHK inhibitor, completely blocked dbcAMP-induced differentiation. The phosphorylation of mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase 1/2 (ERK1/2), and p38 MAPK was also increased during dbcAMP-induced differentiation. Pretreatment of HL60 cells with the MEK inhibitor, U0126, but not the p38 MAPK inhibitor, SB203580, completely suppressed dbcAMP-induced ERK1/2 activation and granulocytic differentiation, but did not affect the increase in SPHK activity. DMS inhibited dbcAMP-induced ERK1/2 activation, but had little effect on p38 MAPK activation. DMS had no effect on the dbcAMP-induced membrane translocation of protein kinase C (PKC) isozymes, and PKC inhibitors had no significant effect on ERK activation. The overexpression of wild-type SPHK1, but not dominant negative SPHK1, resulted in high basal levels of ERK1/2 phosphorylation and stimulated granulocytic differentiation in HL60 cells. These data show that SPHK1 participates in the dbcAMP-induced differentiation of HL60 cells by activating the MEK/ERK pathway.

Tumor necrosis factor-alpha (TNF alpha) inhibits progesterone and estradiol-17beta production from cultured granulosa cells: presence of TNFalpha receptors in bovine granulosa and theca cells

The aim of this study was to investigate whether functional tumor necrosis factor-alpha (TNFalpha) receptors are present in the granulosa cells and the cells of theca interna (theca cells), obtained from bovine follicles classified into one of three groups. Each group was defined as either small vesicular ovarian follicles (small follicles; 3-5 mm in diameter), preovulatory mature ovarian follicles (preovulatory follicles) or atretic follicles (12-18 mm) according to gross examination of the corpus luteum in the epsilateral or contralateral ovary and the uterus (size, color, consistency and mucus), and the ratio of progesterone (P(4)) and estradiol-17beta (E(2)) concentrations in follicular fluid. A Scatchard analysis showed the presence of a high-affinity binding site on both granulosa and theca cells from all follicles examined (dissociation constant: 4.7 +/- 0.15 to 6.9 +/- 1.40 nM). Moreover, TNFalpha receptor concentrations in granulosa and theca cells obtained from atretic follicles were significantly higher than those in the cells from preovulatory follicles (P<0.05). Exposure of cultured granulosa cells from small antral follicles to recombinant human TNFalpha (rhTNFalpha; 0.06-6 nM) inhibited E(2) secretion in a dose-dependent fashion (P<0.01), but did not affect P(4) secretion. In addition, rhTNFalpha inhibited follicle stimulating hormone-, forskolin- or dibutylyl cyclic AMP-induced P(4) and E(2) secretion by the cells (P<0.01). These results indicate the presence of functional TNFalpha receptors in bovine granulosa and theca cells in small, preovulatory and atretic follicles, and suggest that TNFalpha plays a role in regulating their secretory function.

Nitric oxide-an endogenous inhibitor of gastric acid secretion in isolated human gastric glands

BACKGROUND

Endothelial nitric oxide synthase (eNOS) has previously been detected in the glandular part of the human gastric mucosa. Furthermore, nitric oxide (NO) has been shown to influence gastric secretion in various animal models. The present study was conducted to investigate the influence of exogenously and endogenously derived NO on histamine- and cAMP-stimulated gastric acid secretion in isolated human oxyntic glands.

METHODS

Oxyntic glands were isolated from human gastric biopsies and were subsequently pre-treated with NO donors and nitric oxide synthase inhibitors and then exposed to histamine or dibutyryl-cAMP (db-cAMP). The secretory response of the glands was determined as accumulation of [14C]aminopyrine.

RESULTS

The histamine- or db-cAMP-induced acid secretion was attenuated by L-arginine, a known source of endogenous NO, and also by the NO-donors sodium nitroprusside (SNP) and S-nitroso-N-acetyl-penicillamine (SNAP). Pre-treatment with either of the NOS inhibitors NG-nitro-L-arginine methyl ester (L-NAME) or NG-nitro-L-arginine (L-NNA) enhanced the secretory response.

CONCLUSION

Our results show that NO inhibits gastric acid secretion in isolated human gastric glands, and that there is endogenous formation of NO within the glandular epithelium in the vicinity of the parietal cells.

cAMP and Schwann cells promote axonal growth and functional recovery after spinal cord injury

Central neurons regenerate axons if a permissive environment is provided; after spinal cord injury, however, inhibitory molecules are present that make the local environment nonpermissive. A promising new strategy for inducing neurons to overcome inhibitory signals is to activate cAMP signaling. Here we show that cAMP levels fall in the rostral spinal cord, sensorimotor cortex and brainstem after spinal cord contusion. Inhibition of cAMP hydrolysis by the phosphodiesterase IV inhibitor rolipram prevents this decrease and when combined with Schwann cell grafts promotes significant supraspinal and proprioceptive axon sparing and myelination. Furthermore, combining rolipram with an injection of db-cAMP near the graft not only prevents the drop in cAMP levels but increases them above those in uninjured controls. This further enhances axonal sparing and myelination, promotes growth of serotonergic fibers into and beyond grafts, and significantly improves locomotion. These findings show that cAMP levels are key for protection, growth and myelination of injured CNS axons in vivo and recovery of function.

cAMP-induced astrocytic differentiation of C6 glioma cells is mediated by autocrine interleukin-6

Elevation in the level of intracellular cAMP is known to induce the astrocytic differentiation of C6 glioma cells by unknown mechanisms. In this report, we show that cAMP-induced autocrine interleukin 6 (IL-6) promoted astrocytic differentiation of C6 cells. Treatment of cells with N(6),2'-O-dibutyryl cAMP (Bt(2)AMP) and theophylline caused the delayed phosphorylation of signal transducer and activator of transcription 3 (STAT3), as well as the expression of an astrocyte marker, glial fibrillary acidic protein (GFAP). Overexpression of the dominant-negative form of STAT3 leads to the suppression of GFAP promoter activity, suggesting that STAT3 activity was essential for cAMP-induced GFAP promoter activation. On the other hand, the IL-6 gene was quickly induced by Bt(2)AMP/theophylline, and subsequent IL-6 protein secretion was stimulated. In addition, recombinant IL-6 induced GFAP expression and STAT3 phosphorylation. Most importantly, treatment with IL-6-neutralizing antibody dramatically reduced the cAMP-induced GFAP expression and STAT3 phosphorylation and reversed the cellular morphological changes that had been caused by Bt(2)AMP/theophylline. Taken together, these results indicated that Bt(2)AMP/theophylline lead to delayed STAT3 activation via autocrine IL-6. These processes subsequently led to the induction of GFAP. IL-6 secretion is thus thought to be a key event in controlling the astrocytic differentiation of C6 cells.

Experimental modulation of cell-cell adhesion, invasiveness and differentiation in trophoblast cells

The establishment of pregnancy in the human decisively depends on the competence of the early trophoblast to interact during implantation with (1). the uterine epithelium and subsequently (2). with the endometrial stroma and blood vessels. In the interaction with uterine epithelium cell-to-cell adhesion appears to be a critical element, involving initially (and astonishingly) apical cell poles of both epithelia. The subsequent invasion of the stroma includes both adhesive interactions with and degradation of extracellular matrix. How these different processes are regulated in detail remains largely unknown. While the invasiveness of the trophoblast is known to be regulated in local and temporal terms it has remained unclear so far whether trophoblast adhesiveness to cells and/or matrix is subject to a coupled regulation or whether both properties involve different, maybe sequentially effective, control mechanisms. It is also not known how the regulation of these activities is related to the differentiation pathways leading to the formation of noninvasive villous trophoblast serving endocrine as well as nutritive functions. This communication reviews experiments using normal cytotrophoblast cells isolated from first trimester or term placentae as well as malignant trophoblast (choriocarcinoma) cells treated with a panel of compounds known to modulate cell differentiation [retinoic acid, methotrexate, dibutyryl-cAMP, phorbol-(12-myristoyl-13-acetyl)-diester]. Parameters indicative of trophoblast differentiation [in particular chorionic gonadotrophin (hCG) secretion] as well as adhesion to uterine epithelial cells and invasion into extracellular matrix in vitro were monitored. While expression of differentiation parameters was increased by all drug treatments, adhesion to uterine epithelial cells in vitro was reduced. Modulation of invasiveness, however, followed a different pattern: while it was reduced in normal trophoblast cells it was even increased in choriocarcinoma cells with various substances. The response of cells with respect to production of extracellular matrix proteins or matrix-degrading proteinases showed a complex pattern that again lacked a stringent correlation with hCG production and adhesion, and in addition also with invasive behavior. These results suggest that adhesiveness of trophoblast to uterine epithelial cells and invasiveness into the uterine stroma (extracellular matrix) are subject to different control mechanisms. They support the view that trophoblast-endometrium interactions involve a cascade of various adhesion and migration processes whose cellular and molecular basis is complex but accessible to experimental investigation using a variety of available in vitro systems.

Characterization of the expression and regulation of genes necessary for myo-inositol biosynthesis and transport in the seminiferous epithelium

In many mammals, the concentration of myo-inositol in the fluid of the seminiferous tubules is dramatically higher than levels found in serum. Two enzymes involved in myo-inositol synthesis: myo-inositol-1-phosphate synthase (ISYNA1) and myo-inositol monophosphatase-1 (IMPA1), are known to have high activity in the testes. ISYNA1 is an isomerase that catalyzes the conversion of glucose-6-phoshate to myo-inositol-1-phosphate. IMPA1 then hydrolyzes the phosphate group to produce myo-inositol. Although no physiological role for the high concentration of myo-inositol has yet to be elucidated, it has been suggested that it could be involved in osmoregulation. Previous research on these enzymes in the testis has focused on enzyme activity. The objective of this study was to evaluate the expression of these genes and the myo-inositol transporter, Slc5a3, within the testis. Using Northern blot analyses, we found that all three genes, Impa1, Isyna1, and Slc5a3 are expressed in Sertoli cells. Isyna1 is highly expressed in two types of germ cells, pachytene spermatocytes and round spermatids. IMPA1 was expressed in round spermatids. Slc5a3 expression is upregulated when Sertoli cells are treated with 0.1 mM dibutyryl cAMP. When Sertoli cells were cultured in a hypertonic medium, there was an increase in the expression of Isyna1 and Slc5a3. We postulate that this upregulation is a result of the capability of the Sertoli cell to sense and then react to a change in osmolarity by increasing the transport and production of the osmolyte myo-inositol.

Flagellar adhesion between mating type plus and mating type minus gametes activates a flagellar protein-tyrosine kinase during fertilization in Chlamydomonas

When Chlamydomonas gametes of opposite mating type are mixed together, flagellar adhesion through sex-specific adhesion molecules triggers a transient elevation of intracellular cAMP, leading to gamete activation in preparation for cell-cell fusion and zygote formation. Here, we have identified a protein-tyrosine kinase (PTK) activity that is stimulated by flagellar adhesion. We determined that the protein-tyrosine kinase inhibitor genistein inhibited fertilization, and that fertilization was rescued by dibutyryl cAMP, indicating that the genistein-sensitive step was upstream of the increase in cAMP. Incubation with ATP of flagella isolated from non-adhering and adhering gametes followed by SDS-PAGE and immunoblotting with anti-phosphotyrosine antibodies showed that adhesion activated a flagellar PTK that phosphorylated a 105-kDa flagellar protein. Assays using an exogenous protein-tyrosine kinase substrate confirmed that the activated PTK could be detected only in flagella isolated from adhering gametes. Our results indicate that stimulation of the PTK is a very early event during fertilization. Activation of the PTK was blocked when gametes underwent flagellar adhesion in the presence of the protein kinase inhibitor staurosporine, but not in the presence of the cyclic nucleotide-dependent protein kinase inhibitor, H8, which (unlike staurosporine) does not block the increases in cAMP. In addition, incubation of gametes of a single mating type in dibutyryl cAMP failed to activate the PTK. Finally, flagella adhesion between plus and minus fla10-1 gametes, which have a temperature-sensitive lesion in the microtubule motor protein kinesin-II, failed to activate the PTK at elevated temperatures. Our results show that kinesin-II is essential for coupling flagellar adhesion to activation of a flagellar PTK and cAMP generation during fertilization in Chlamydomonas.

P-locus is a target for the melanogenic effects of MC-1R signaling: a possible control point for facultative pigmentation

Melanocortin receptor type 1 (MC-1R) is an important control point for ultraviolet ray (UVR)-induced tanning response in the skin. In this study, we show that p-locus is a downstream target for MC-1R signaling. The expression of p-locus was up-regulated by alpha-MSH as well as db-cAMP, a synthetic analogue of cAMP that mimics activation of MC-1R. Furthermore, p-locus transcript abundance was significantly increased in epidermal melanocytes of white skin with facultative (UVR-induced) pigmentation. Because p-locus product is essential for pigmentation and also has been shown to be highly polymorphic in human population, we propose that the pigmentary response to the melanocortin peptides/UVR would be affected not only by MC-1R mutations but also by the functionality of p-locus product. These factors together could account for the many different levels of tanning ability seen in the white population.

Expression of adrenomedullin and its receptor by chondrocyte phenotype cells

For clarifying a process of de-differentiation in culturing chondrocytes, the present study was undertaken to investigate the secretion of adrenomedullin (AM) by chondrocyte phenotype cells and whether or not AM effects this proliferation in a cAMP-dependent fashion. Chondrocyte phenotype cells expressed AM and the AM receptor, and secreted high concentration of AM into the culture medium. When added to cultures, AM increased the intracellular cAMP level and decreased the number of these cells in a similar concentration-dependent fashion. Addition of forskolin and dibutyryl-cAMP caused a significant decrease in the number of these cells. Furthermore, the effect of AM was inhibited by a cAMP-dependent protein kinase A inhibitor (H89). The present findings indicate that AM has an autocrine/paracrine type of anti-proliferative effect on these cells mediated via a cAMP-dependent pathway and raise the possibility that AM plays a role in the local modulation of a process of de-differentiation by culturing chondrocyte phenotype cells.

Effect of drugs affecting synthesis or degradation of connexin on onset or completion of ethylene glycol induced inhibition of intercellular gap junctional communication

The role of a connexin synthesis and degradation in the onset or completion of the ethylene glycol-induced inhibition of the gap junctional intercellular communication (GJIC) in V79-4 Chinese hamster cell line was studied as a model of an interaction between the cells and a potential tumor promoter. GJIC was assessed on two levels: on the cytophysiological level - the dye coupling method, and on the immunocytochemical level - the immunolabeling of connexin43. The specific activator of connexin synthesis - Dibutyryl cAMP made the onset of the EG-induced inhibition of GJIC slower, but its effect was only temporary. On the other hand it also speeded up the re-establishment of standard values of GJIC after the removal of EG. Although the non-specific inhibitor of protein degradation via proteasomes - leupeptin increased the amount of connexin plaques on cell membranes, its effect on GJIC remained insignificant. The non-specific inhibitors of transcription - actinomycin D and translation - cycloheximide significantly inhibited the re-establishment of the standard values of GJIC after the removal of EG. The results indicate that although the storage of connexins in Golghi complex probably plays the principal role in the control of the gap junctional communication, the extensive changes in GJIC activity depend on the de novo synthesis of connexin per se.

Effect of reduced maternal inspired oxygen on hepatic glucose metabolism in the rat fetus

Perturbations in glucose metabolism in the fetus and in the neonate are a consistent finding in several different animal models of intrauterine growth retardation (IUGR) as well as in humans. Studies in rats who have undergone IUGR have shown decreased hepatic glycogen stores in the fetus and delayed induction of cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) at birth. Hepatic transcription factors CCAAT enhancer binding protein (C/EBP)alpha and C/EBPbeta and the increase in cyclic AMP at birth have been implicated in the initial appearance of PEPCK-C. We have examined the effect of IUGR induced by reduced maternal inspired oxygen (fractional inspired oxygen concentration 0.14) on a) the expression of genes for hepatic C/EBPalpha, C/EBPbeta, PEPCK-C and glycogen synthase; and b) transcription of the genes for C/EBPbeta and PEPCK-C by dibutyryl cyclic AMP in the fetus. Three days (d 18-21) of decrease in maternal inspired oxygen resulted in lower maternal arterial PO(2) and a lower birth weight of the pups (p < 0.01). Fetuses that underwent IUGR had significantly lower concentrations of plasma glucose, hepatic glycogen, and glycogen synthase mRNA and a higher hepatic lactate:pyruvate ratio. They also had lower levels of hepatic PEPCK-C mRNA at birth. The concentration of hepatic mRNA for C/EBPalpha and C/EBPbeta as well as the transcription factors themselves were not affected by the decreased maternal inspired oxygen. Fetal injection of dibutyryl cyclic AMP after 24 h of decreased maternal inspired oxygen (d 18-19) had no effect on the expression of C/EBPbeta. However, it resulted in an attenuated induction of PEPCK-C in the fetuses with IUGR. We speculate that a decrease in maternal inspired oxygen induced certain mediators, either in the mother or in the placenta, that caused lower fetal glucose concentration and affected the transcription of genes involved in fetal hepatic glucose metabolism. IUGR, as a result of decreased fractional inspired oxygen concentration may also be the consequence of pH-mediated changes in uterine blood flow. However, these remain to be examined in this experimental model.

The intracellular pH-regulatory HCO3-/Cl- exchanger in the mouse oocyte is inactivated during first meiotic metaphase and reactivated after egg activation via the MAP kinase pathway

The HCO(3)(-)/Cl(-) exchanger is quiescent in the unfertilized mouse egg but is highly active in regulating intracellular pH in the early embryo and required for normal development. We show here that the HCO(3)(-)/Cl(-) exchanger is active in first meiotic prophase (GV) oocyte but inactivated during meiotic metaphase before the MI to MII transition. Reactivation does not occur until the activated egg enters interphase. A quiescent HCO(3)(-)/Cl(-) exchanger is not simply a general feature of metaphase, because activity did not decrease during first mitotic metaphase. Inactivation of the HCO(3)(-)/Cl(-) exchanger during MI coincided with the activation of MAP kinase (MAPK), whereas its reactivation coincided with the loss of MAPK activity after egg activation. Maintaining high MAPK activity after egg activation prevented the normal reactivation of the HCO(3)(-)/Cl(-) exchanger. Inactivating MAPK in unfertilized MII eggs resulted in HCO(3)(-)/Cl(-) exchanger activation. Preventing MAPK activation during first meiotic metaphase prevented the inactivation of HCO(3)(-)/Cl(-) exchange. Conversely, activating MAPK in the GV oocyte resulted in inactivation of HCO(3)(-)/Cl(-) exchange. These results imply that the HCO(3)(-)/Cl(-) exchanger in mouse oocytes is negatively regulated by MAPK. Thus, suppression of pH-regulatory mechanisms during meiosis is a novel function of MAPK and cytostatic factor activity in the oocyte.

Interleukin-6 and cAMP induce stromal cell-derived factor-1 chemotaxis in astroglia by up-regulating CXCR4 cell surface expression. Implications for brain inflammation

The chemokine stromal cell-derived factor-1 (SDF-1) and its receptor CXCR4 control the migration of neurons and microglial cells in the central nervous system. Although functional CXCR4 is also expressed by astroglia, recent studies have failed to observe a chemotactic response of these cells to SDF-1. Here, we demonstrate that SDF-1-dependent chemotaxis can be induced by treating cultured cortical astroglia with either dibutyryl cyclic AMP (dbcAMP; 10(-4) m) or interleukin-6 (IL-6; 10 ng/ml). Flow cytometric analysis revealed that both the dbcAMP- and IL-6-induced onset of SDF-1-dependent chemotaxis of astroglia are due to the increased cell surface expression of CXCR4. In addition, dbcAMP and IL-6 also increased CXCR4 transcript levels, further suggesting that both treatments primarily affect CXCR4 surface expression in astroglia by stimulation of gene expression. Moreover, unlike the case with IL-6 and dbcAMP, which allowed for an optimal chemotactic response to SDF-1 only after 48 h, a similar chemotactic response, associated with an increase in CXCR4 cell surface expression, already occurred after 24 h when astroglial cultures were maintained with medium conditioned by IL-6- or dbcAMP-pretreated astrocytes, indicating that the stimulatory effects of IL-6 and cAMP on CXCR4 cell surface expression involve a secondary mechanism. The findings that elevated extracellular levels of IL-6 or factors positively coupled to cAMP result in increased CXCR4 cell surface expression levels and subsequent SDF-1-dependent chemotaxis in central nervous system astrocytes point to a crucial role of this chemokine during reactive gliosis and human immunodeficiency virus-mediated dementia.

The role of cAMP-dependent signaling in receptor-recognized forms of alpha 2-macroglobulin-induced cellular proliferation

Ligation of alpha(2)-macroglobulin receptors by receptor-recognized forms of alpha(2)-macroglobulin (alpha(2)M*) activates various signaling cascades and promotes cell proliferation. It also elevates cAMP in murine peritoneal macrophages. We now report that a significant elevation of cAMP-response element-binding protein (CREB) occurs in alpha(2)M*-stimulated cells, and this effect is potentiated by isobutylmethylxanthine, dibutyryl-cAMP, or forskolin. An alpha(2)M* concentration-dependent rapid increase in phosphorylated CREB at Ser(133) also occurred, a necessary event in its activation. Inhibition of Ca(2+)/calmodulin kinase, protein kinases A and C, tyrosine kinases, ribosomal S6 kinase, farnesyl transferase, extracellular signal-regulated kinases 1/2, phosphatidylinositol 3-kinase, or p38 mitogen-activated protein kinase markedly reduce alpha(2)M*-induced phosphorylation of CREB, indicating a role for the p21(ras)-dependent and phosphatidylinositol 3-kinase signaling pathways in regulating CREB activation by alpha(2)M*. Finally, silencing the CREB gene by transfecting cells with a homologous gene sequence double-stranded RNA drastically reduced the expression of CREB and blocked the ability of alpha(2)M* to promote macrophage cell division. We conclude that cAMP-dependent signal transduction as well as other signaling cascades are essential for alpha(2)M*-induced cell proliferation.