Transport of putrescine in the isolated rabbit intestine

The transepithelial fluxes of putrescine were studied in sections of the three segments of rabbit intestine mounted in an Ussing chamber. The ileum exhibited the highest mucosal-to-serosal (Jms) and serosal-to-mucosal (Jsm) unidirectional fluxes of 1 mumol/l [3H]putrescine. Putrescine net flux (Jnet = Jms - Jsm) was deduced to be positive through the duodenum (Jnet = 53.40 +/- 14.30 pmol h-1 cm-2), not significantly different from zero through the jejunum (Jnet = 8.90 +/- 19.20 pmol h-1 cm-2) and negative through the ileum (Jnet = -34.30 +/- 13.80 pmol h-1 cm-2). Increasing putrescine concentration up to 10 mmol/l led to an increase in Jms, Jsm and Jnet without affecting the transport polarity in the ileum. The tissue retention of putrescine after 100 min was higher by the serosal side than by the mucosal side of the ileum. In parallel experiments, isolated pieces of ileum accumulated putrescine to a five- to sixfold concentration gradient by a ouabain-inhibitable mechanism. In contrast with arginine and in order of decreasing potency, putrescine, cystamine (a transglutaminase inhibitor), spermidine and spermine (1 mmol/l) reduced both unidirectional fluxes of putrescine across the ileum in the Ussing chamber. The latter effectors, except spermine, and N,N-dimethylcasein (1 mg/ml) led to an important, if not complete, suppression of putrescine secretion by the ileum, while the calmodulin antagonist melittin (0.3 micrograms/ml) reversed the polarity of polyamine transport, suggesting the involvement of transglutaminase in putrescine transport. We conclude that the heterogeneous pattern of putrescine transport along the small-intestinal epithelium constitutes an important feature of the regulation of polyamine concentrations in this tissue.

Pertussis toxin provides evidence for two independent signalling pathways leading to the activation of the nerve growth factor gene

Increased expression of the nerve growth factor (NGF) gene may be obtained by treating L929 fibroblasts with serum, phorbol 12-myristate 13-acetate (PMA), or 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). The possible involvement of GTP-binding proteins (G proteins) in these regulatory events was monitored by exposing the cells to pertussis toxin (PT), a compound known to inactivate several types of G proteins by ADP ribosylation. Measurements of the pool of NGF mRNA by Northern blot analysis, and quantification of the factor secreted by the cells with a double-site ELISA assay, indicate that pretreatment with PT decreases by about 60% the effect of serum on the levels of NGF transcript and secreted factor. This effect is accompanied by a corresponding decrease of the expression of c-fos gene, which takes place soon after the addition of serum to the cells. In contrast, PT had no effect on the basal level of NGF mRNA found in cells maintained in serum-free medium or in cells stimulated with PMA or 1,25-(OH)2D3. These results indicate that some serum factor(s) acts via plasma membrane receptors able to interact with PT-sensitive G proteins to modulate NGF gene expression. In contrast, 1,25-(OH)2D3 appears to mediate its action through a different signalling pathway, which is likely to require its cytosolic receptor, and is independent of PT-sensitive G protein and c-fos induction.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of nerve growth factor synthesis in primary glial cells by phorbol 12-myristate 13-acetate: role of protein kinase C

Phorbol 12-myristate 13-acetate (PMA) induces a dramatic production of nerve growth factor (NGF) in primary cultures of newborn mouse astrocytes maintained in a serum-free medium. This stimulation is dose-dependent and a maximal effect on the levels of cell-secreted factor was observed at a concentration of 10 nM. At this concentration, the promoting effect of PMA appears much more important than that elicited by 10% fetal calf serum (FCS) under the same culture conditions. PMA acts primarily on the accumulation of NGF mRNA, which was detected by northern blot analysis after 6 h of treatment. This accumulation may be totally or partially prevented when PMA-treated glial cells are concomitantly exposed to the protein kinase inhibitors H-7, H-9, and to a lesser degree, HA-1004. The known specificity of these inhibitors agrees with the possibility that protein kinase C (PKC), which constitutes so far the sole known target of PMA, represents a key element involved in the stimulation of NGF gene. The role of PKC is further supported by the observation that alpha phorbol didecanoate, which has no activity on PKC, is depleted of effect on the synthesis of NGF. Likewise, 1,2-dioctanoylglycerol (1,2-DOG) has a weak, but significant promoting action on the production of NGF, unlike the 1,3-isomer which is not active on PKC. Finally, a treatment of 15 min with 100 nM PMA is sufficient to stimulate the cells, suggesting that the activation phase of PKC, rather than its down regulation, constitutes an important trigger leading to an increased expression of the NGF gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone promotes transient nerve growth factor synthesis in rat cerebellar neuroblasts

Primary cultures of cerebellum from 5-day-old rats indicated that proliferating neuroblasts synthesize and release nerve growth factor (NGF). Since NGF promotes DNA synthesis in these cells, our findings demonstrate that the early developing cerebellum is a suitable physiological model for studying the autocrine mitogenic action of NGF. Thyroid deficiency led to a greater reduction in the NGF content of the cerebellum than of the olfactory bulbs or hippocampus. Cerebellar NGF mRNA was also very sensitive to hormone deprivation. Physiological amounts of thyroid hormone stimulated both the mitotic activity and NGF production of cultured cerebellar neuroblasts. A lack of thyroid hormone is known to markedly alter cell formation in the cerebellum where postnatal neurogenesis is highly significant, in contrast to the olfactory bulbs and hippocampus. Taken together, these results suggest that the hormonal control of cell formation in the cerebellum is, at least partly, mediated by the autocrine mitogenic action of NGF. The thyroid hormone could temporally regulate the transient NGF synthesis by cerebellar neuroblasts directly and/or through its ontogenetic action, and hence all the NGF-dependent trophic effects.

MC903, an analogue of 1,25-dihydroxyvitamin D3, increases the synthesis of nerve growth factor

The effect of MC903, an analogue of 1,25-dihydroxyvitamin D3, on the expression of the nerve growth factor (NGF) gene has been studied in L cells. MC903 induces an increase in both NGF mRNA and protein with a time course similar to that obtained with 1,25-dihydroxyvitamin D3. This finding points to the potential importance of 1,25-dihydroxyvitamin D3 derivatives in the treatment of NGF-sensitive disorders.

Antagonistic effects of dexamethasone and 1,25-dihydroxyvitamin D3 on the synthesis of nerve growth factor

Dexamethasone is known to decrease the pool of nerve growth factor (NGF) mRNA in various experimental systems. The negative regulatory effect of the glucocorticoid was first observed in mouse fibroblast-like L929 cells, and was subsequently reported to take place in many experimental systems, including in vivo following sciatic nerve injury. Conversely, another steroid hormone, 1,25-dihydroxy-vitamin D3 (1,25-(OH)2D3) was recently reported to promote NGF synthesis in mouse L929 cells. The present work was undertaken to investigate the effect of the concomitant addition of both steroids to L929 cells. Measurements of NGF mRNA and assays of the mature protein secreted by the cells provide evidence that the negative regulation exerted by dexamethasone may be counteracted in a dose-dependent manner by the positive action of 1,25-(OH)2D3, and vice versa. Therefore, the expression of the NGF gene can be regulated in a subtle way by the balance between the two steroids. It may be expected on the basis of these observations that in tissues that are responsive to both hormones, administration of 1,25-(OH)2D3 should be able to reverse the down-regulation of NGF synthesis elicited by glucocorticoids.

Nerve growth factor-induced neuronal differentiation is accompanied by differential splicing of beta-amyloid precursor mRNAs in the PC12 cell line

The effect of the neurotrophic factor nerve growth factor (NGF) on the expression of the beta-amyloid gene has been studied in the clonal nerve cell line PC12. The neuronal differentiation of PC12 cells in the presence of NGF was accompanied by a shift in the ratio of beta-amyloid precursor protein (APP) transcripts. In particular there was reduced expression of the transcript coding for the longest precursor form (APP770) and a concomitant increase in the shortest (APP695) transcript following NGF treatment.

1,25-Dihydroxyvitamin D3 is a potent inducer of nerve growth factor synthesis

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3), a metabolically active form of vitamin D, is shown to increase in a dose-dependent manner the cellular pool of NGF mRNA in murine L-929 fibroblasts cultured in a serum-free medium. This effect can be detected as early as 3 hours after 1,25-(OH)2D3 addition and persists for at least 28 hours. It is accompanied by an enhancement of the amount of NGF protein secreted in the culture medium. Since the proto-oncogene c-fos appears involved in the regulation of the NGF gene (Mocchetti et al.: Proceedings of the National Academy of Sciences of the United States of America 86: 3871-895, 1989; Hengerer et al: Proceedings of the National Academy of Sciences of the United States of America 87:3899-3903, 1990), the effect of 1,25-(OH)2D3 on c-fos expression was analysed and compared to that elicited by other inducers of the NGF gene, serum (Wion et al: FEBS Letters 189:37-41, 1985) and phorbol 12-myristate 13-acetate (PMA) (Wion et al: FEBS Letters 262:42-44, 1990). Addition of serum or PMA to L-929 cells was rapidly followed by a transient activation of the c-fos gene. In contrast, c-fos transcripts remained undetected in the presence of 1,25-(OH)2D3. The failure to find any evidence of c-fos expression suggests that 1,25-(OH)2D3 could enhance the pool of NGF mRNA by a mechanism independent of the c-fos pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effect of sunflower seed oil on hepatic and renal D-amino acid oxidase in the rat

1. The specific activity of hepatic and renal peroxisomal D-amino acid oxidase (D-AAOX) was measured in rats fed diets containing various quantities of vegetable oil. 2. Increasing the amount of dietary sunflower seed oil (SSO) from 10 to 25% (w/w) reduced the specific activity of hepatic D-AAOX by up to 30% after 10 days. 3. In both tissues, the enzyme activity was moderately decreased during the first two-day period after administration of the 25% SSO diet was begun. Unlike hepatic D-AAOX, renal D-AAOX returned to its baseline level in the kidney after the third day. 4. In contrast to SSO, hydrogenated coconut oil (HCO) did not evoke alterations of D-AAOX activity. 5. The activity levels of another peroxisomal enzyme, L-2-hydroxy acid oxidase (L-HAOX), in the liver of rats fed the high-SSO diet vs those fed the control diet were similar. 6. The subcellular distribution of D-AAOX and L-HAOX was not altered in the liver of rats fed the 25% SSO diet during the 10-day period.

Kinetics of the inhibition of hog kidney D-amino acid oxidase by short-, medium- and long-chain fatty acids

Various fatty acids were studied in vitro as inhibitors of pure hog kidney D-amino acid oxidase by means of a spectrophotometric peroxidase-coupling method using D-methionine as a substrate. All the fatty acids tested behaved as substrate-competitive inhibitors of the enzyme. The affinity of the saturated aliphatic acids for D-amino acid oxidase decreased from pentanoate (5:0; Ki = 220 microM) to laurate (12:0; Ki = 675 microM), then rose to a maximum with stearate (18:0; Ki = 36 microM), suggesting the presence of a site in the active center of the enzyme that accepts long-chain fatty acid alkyl groups. Unsaturation did not further increase the affinity of the fatty acid for this binding site.

Oxidation of the supplemental methionine source L-2-hydroxy-4-methylthiobutanoic acid by pure L-2-hydroxy acid oxidase from chicken liver

The peroxisomal enzyme L-2-hydroxy acid oxidase A (EC 1.1.3.1) was isolated from chicken liver to better evaluate its part in the utilization of the L isomer of supplemental DL-hydroxy-4-methylthiobutanoic acid by birds fed diets containing the methionine hydroxy analogue. The 650-fold purified enzyme, a 169 kDa protein composed of four apparently identical subunits, exhibited a specific activity of 1.3 mumol glycolate oxidized.min-1.mg protein-1. Glycolate (Km = 0.10 mmol/L) was actually a better substrate than L-2-hydroxyisocaproate (Km = 0.63 mmol/L), L-2-hydroxy-4-methylthiobutanoate (Km = 1.73 mmol/L) and L-lactate (Km = 10.13 mmol/L). Under all substrate concentrations tested, the enzyme activity toward L-2-hydroxyisocaproate and L-2-hydroxy-4-methylthiobutanoate was 55 and 17%, respectively, of that toward glycolate. Although the highly purified enzyme was unable to oxidize D-lactate, D-methionine, L-methionine, L-mandelate and beta-phenyl-L-lactate, the latter two aromatic substrates were significantly oxidized by the first ammonium sulfate precipitate obtained during the isolation procedure, supposedly because of the presence of L-2-hydroxy acid oxidase isozyme B. Because the hepatic tissue concentration of glycolate, the physiological substrate for the enzyme, was rather low (10 mumol/L) as compared to the concentration of the methionine hydroxy analogue, one can expect that the conversion of L-2-hydroxy-4-methylthiobutanoate to 2-keto-4-methylthiobutanoate prior to L-methionine formation might proceed at a substantial rate in chickens fed the supplemental methionine source.

Nerve growth factor (NGF), proNGF and NGF receptor-like immunoreactivity in BB rat retina

Nerve growth factor (NGF) and NGF precursor (proNGF)-like material were localized immunohistochemically in diabetic and non-diabetic adult rat retina using affinity-purified immunoglobulins to mature NGF and to synthetic peptides that reproduce sequences of the proNGF. Immunoreactivity for NGF and proNGF was detected in retinal ganglion cells, Müller cells, retinal pigment epithelium and areas adjacent to the internal limiting membrane. Immunolocalization of the NGF receptor using a monoclonal antibody (MC192) revealed positive staining of retinal pigment epithelium and Müller cells but not of ganglion cells. The staining intensity and distribution of NGF, proNGF, and NGF receptor-like immunoreactivity were similar in retinas of diabetic rats. These results suggest that NGF is synthesized endogenously in the retina by both neuronal and non-neuronal cell constituents, and that diabetes has no detectable effect on NGF/NGF receptor distribution and possibly on NGF biosynthesis.

Purification and some characteristics of chicken liver L-2-hydroxyacid oxidase A

The isozyme A of L-2-hydroxyacid oxidase is a peroxisomal flavoenzyme that catalyzes the oxidation of short-chain aliphatic L-2-hydroxyacids in many tissues of higher organisms. A new purification procedure allowed us to obtain a 1400-fold purified enzyme from chicken liver. The N-terminal amino acid of the polypeptide chain was found to be blocked as that of spinach glycolate oxidase, contrastingly with that of rat kidney isozyme B. Its amino acid composition was comparable to that of other known L-2-hydroxyacid oxidases. Despite different substrate specificity, some immunological identity was observed between chicken liver L-2-hydroxyacid isozyme A and rat kidney isozyme B.

Phorbol 12-myristate 13-acetate (PMA) increases the expression of the nerve growth factor (NGF) gene in mouse L-929 fibroblasts

The rise of the NGF mRNA pool which takes place following exposure of L-929 fibroblasts to serum was prevented in the presence of 5 microM K-252a, a compound which inhibits several species of protein kinase activities. To characterize further this phenomenon, L-929 cells growing in a serum-free medium were exposed to cyclic nucleotide analogs, to a divalent cation ionophore or to the phorbol ester PMA. Only this latter compound induced an enhancement of the NGF mRNA pool, suggesting an involvement of protein kinase C in the upregulation of the NGF transcripts. The effects of PMA or serum also require a synthesis of protein since the level of NGF transcripts remained stable in the presence of cycloheximide.

Neuronal localization of the nerve growth factor precursor-like immunoreactivity in the rat brain

The distribution of the nerve growth factor precursor(proNGF)-like immunoreactivity was examined in the adult rat brain with affinity-purified antisera directed against synthetic peptides that reproduce sequences of the precursor protein. Immunoreactivity was localized in defined areas of the neocortex, hippocampal formation, thalamus/hypothalamus, striatum, olfactory bulb, pons and spinal cord, which are regions previously reported to contain NGF mRNA. Interestingly, immunoreactivity was also observed in the septum and diagonal band of Broca known to contain very low NGF mRNA levels. Using immunohistochemical methods combined with the retrograde transport of a protein gold complex we demonstrate that proNGF-like immunoreactivity is localized within neuronal cell bodies, in the cortex, hippocampus and septum. These results suggest that the immunological approach may not only identify NGF-synthesizing cells, but also cells which may accumulate proNGF or some cleavage products by an uptake mechanism.

Secretion of nerve growth factor in cultures of glial cells and neurons derived from different regions of the mouse brain

The regional ability of central neurons and glial cells to produce nerve growth factor (NGF) was studied in vitro. NGF secretion was compared in cultures of perinatal astrocytes or embryonic neurons that were derived from various mouse brain structures. No regional differences were detected among cultures of post-natal day 2 glial cells of hippocampal, cortical, striatal, or mesencephalic origin. In all cases, levels of NGF released by the cells were very similar. They were closely correlated to the growth rate as shown by the fact that exponentially growing cells produced relatively more factor than did confluent cells, a finding in agreement with previous observations. Unlike growth-phase cells, primary astrocytes immediately plated at high cell density did not secrete any assayable factor before the 7th day of culture. Levels of NGF found during the following days remained low. In contrast, striking differences were observed among cultures of embryonic neurons. NGF was found in relatively large amounts in cultures of embryonic day 17 or 19 striatal neurons, whereas media conditioned by neurons from the mesencephalon, cortex, or septum contained much less factor. Amounts of NGF assayed in cultures of hippocampal neurons varied with the time of sampling of this brain structure. Levels of factor were significantly higher in media conditioned by embryonic day 19 neurons than in media of embryonic day 17 neurons. However, amounts of NGF found in supernatants of hippocampal neurons remained smaller than those present in cultures of striatal nerve cells. Altogether, the results suggest that, in addition to astrocytes, central neurons may also synthesize and secrete NGF in vitro and that this phenomenum is dependent on both the origin and the developmental stage of the neuronal population.

Oxidation of methionine and 2-hydroxy 4-methylthiobutanoic acid stereoisomers in chicken tissues

Oxidation of DL-2-hydroxy 4-methylthiobutanoic acid (DL-HMB), DL-methionine (DL-MET) and L-methionine (L-MET) in chicken tissue homogenates was compared using 1-14C-labelled tracers. The pattern of oxidation of the substrates was similar at both low (0.7 mM) and high (20 mM) concentrations. The rate of conversion to 2-keto 4-methylthiobutanoic acid (KMB) was highest for DL-MET and lowest for L-MET in kidney, liver and intestinal mucosa. In breast muscle, rates for DL-MET and L-MET were similar at 0.7 mM, but DL-HMB showed the highest rate at 20 mM. Kidney contained the highest specific activity for oxidation of all three substrates. Raising the pH of liver and kidney homogenates from 7.5 to 8.6 increased the oxidation of DL-MET, exclusively. Experiments with inhibitors of D-2-hydroxy acid dehydrogenase (EC 1.1.99.6) and L-2-hydroxy acid oxidase (EC 1.1.3.15) suggested that D- and L-HMB were stereospecifically oxidized by the enzymes. KMB stimulated L-MET oxidation in kidney yet inhibited L-MET oxidation in liver homogenates. The effect of KMB on DL-MET and DL-HMB oxidation also varied between tissues. Amino-oxyacetate inhibited L-MET oxidation completely and DL-MET and DL-HMB oxidation almost completely in both kidney and liver. L-Cycloserine was less potent than amino-oxyacetate and decreased L-MET oxidation more in kidney than in liver. It can be calculated from the results that, at low substrate concentrations, the liver contributes principally to the whole body oxidation of both DL-HMB and DL-MET. At high (greater than physiological) concentrations, DL-HMB would be oxidized principally in skeletal muscle. At all concentrations, L-MET would be converted to KMB mainly in the muscle.

Levels of nerve growth factor secreted by rat primary fibroblasts and iris transplants are influenced by serum and glucocorticoids

Previous work performed with mouse fibroblast-like L cells has shown that the level of expression of NGF gene is modulated in these transformed cells by the composition of the growth medium. Glucocorticoids were found to exert a down-regulation on NGF production, while serum stimulated the synthesis of the factor. The contrasting effects of serum and dexamethasone were further investigated in cultures of primary rat fibroblasts or in iris transplants. ELISA assays of NGF released by fibroblasts or by transplanted irides showed that both experimental systems responded to dexamethasone by a 4-5-fold decrease of the amounts of secreted factor. Half-maximal effect took place at a concentration of 3-5 X 10(-9) M, a value close to the dissociation constant of the glucocorticoid receptor in fibroblasts. The glucocorticoid did not influence the secretion of macromolecules. Assays of NGF mRNA performed at a concentration of 10(-7) M dexamethasone indicated that the steroid decreased the pool of NGF transcripts in either experimental systems. In contrast to dexamethasone, serum induced a 4-fold enhancement of the amounts of factor secreted by fibroblasts. This effect was reproduced with serum that was previously heat-treated at mild acidic pH, or with a macromolecular fraction of this heat-treated serum which contains an effector promoting NGF synthesis in L cells. The fact that promotion of NGF synthesis takes place in primary cells raises the possibility that this process may also occur in vivo, for instance following disruption of vasculature, as a part of a wound mechanism. Data collected with iris transplants provide some support to this interpretation.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipopolysaccharide-stimulated rat brain macrophages release NGF in vitro

Amoeboid microglial cells purified from primary cultures of embryonic rat brains and stimulated with bacterial lipopolysaccharides released high amounts of nerve growth factor. This finding suggests that brain macrophages play a major neurotrophic role during development, in adulthood, and in the regenerative events that follow localized lesions.

Na+-independent and nonstereospecific transport of 2-hydroxy 4-methylthiobutanoic acid by brush border membrane vesicles from chick small intestine

1. The transport of L- and DL-2-hydroxy 4-methylthiobutanoic acid (HMB), the methionine hydroxy analogue, by brush border membrane vesicles (BBMV) from chick small intestine was the sum of a saturable Michaelian component and a diffusive term. 2. Unlike that of L- and DL-MET, uptake was Na+-independent and electroneutral. 3. The inhibition of L-HMB transport by L-lactate, a structural analogue, and D-HMB as well, was of the competitive type. 4. Preloading of BBMV with D-HMB but not with L-lactate or L-MET trans-stimulated the influx of labelled L-HMB. 5. HMB uptake by rat and chick intestinal BBMV exhibited similar characteristics but the chick nonstereospecific transport system appeared to be unable to carry out L-lactate translocation.