Glucocorticoids and cyclic AMP synergistically regulate the abundance of preproenkephalin messenger RNA in neuroblastoma-glioma hybrid cells

Effects of immobilization stress on neurochemical markers in the motivational system of the male rat

Mesolimbic regions involved in motivated behavior are altered in animals undergoing repeated exposure to social stress. Here we test the hypothesis that other forms of persistent stress would also influence these same endpoints. Adult male Sprague-Dawley rats were exposed to immobilization stress either once (2 h) or repeatedly (2 hx10 days) and brains were harvested immediately after the last immobilization. A trio of indirect markers associated with dopaminergic activity was measured including dopamine transporter (DAT) and dopamine D2 receptor subtype (D2r) ligand levels as well as mRNA levels of the endogenous opioid enkephalin (ENK-mRNA). A single 2-h session of immobilization stress produced an increase in striatal ENK-mRNA levels and DAT ligand binding compared with group-housed controls. In animals undergoing repeated immobilization stress and singly housed post-stress, we found a significant reversal in the direction of ENK-mRNA levels and DAT binding in the striatum, in addition to an increase in D2r-binding density in the shell of the nucleus accumbens compared with single-stress-exposed rats. In another experiment using the same stress paradigm but allowing pair-housing post-stress, we found no alteration of ENK-mRNA but significant increases in DAT and D2r binding in the dorsal striatum. A major difference between single and group housing is the habituation of the corticosterone (CORT) stress response over 10-day stress in group-housed rats. The present results parallel previous findings by our laboratory that repeated stress results in a relative reduction of ENK-mRNA levels and increased D2r-binding density in the striatum of rats. Furthermore, our data are consistent with the hypothesis that chronic stress induces an allostatic attenuation of the mesolimbic dopaminergic system in animals that do not habituate to the stressor, possibly due in part to persistent CORT elevations.

The role of delta opioid receptors in the anxiolytic actions of benzodiazepines

The anxiolytic effects of benzodiazepines appear to involve opioid processes in the amygdala. In previous experiments, overexpression of enkephalin in the amygdala enhanced the anxiolytic actions of the benzodiazepine agonist diazepam in the elevated plus maze. The effects of systemically administered diazepam are also blocked by injections of naltrexone into the central nucleus of the amygdala. The current studies investigated the role of delta opioid receptors in the anxiety-related effects of diazepam. Three days following bilateral stereotaxic injections of viral vectors containing cDNA encoding proenkephalin or beta-galactosidase (control vector), the delta opioid receptor antagonist naltrindole (10 mg/kg, s.c.) attenuated the enhanced anxiolytic effects of 1-2 mg/kg diazepam in rats overexpressing preproenkephalin in the amygdala. Despite this effect, naltrindole failed to attenuate the anxiolytic action of higher diazepam doses (3 mg/kg) in animals with normal amygdalar enkephalin expression. Similarly, the mu opioid receptor antagonist, beta-funaltrexamine (20 mg/kg, s.c.), had no effect on the anxiolytic effect of diazepam alone. These data support a role for delta opioid receptors in the opioid-enhanced anxiolytic effects of diazepam.

Increases in preproenkephalin mRNA levels in the Syrian hamster: The influence of glucocorticoids is dependent on age and tissue

In adult hamsters, basal proenkephalin (Penk) gene expression in adrenals is independent of glucocorticoids and glucocorticoid receptor blockade, by RU 486, increases striatal preproenkephalin (PPenk) mRNA levels. However, glucocorticoids maintain both basal and induced Penk gene expression in rat adrenal (medulla) and striatum. This suggests species and tissue-specific differences in Penk gene regulation. Since studies show temporal coordination in Penk gene expression in developing hamster adrenal and striatum, we tested the hypothesis that increasing PPenk mRNA levels are dependent, while basal levels are independent of glucocorticoids in developing hamsters. To facilitate this study, we examined the influence of glucocorticoids on the temporal increases in developing hamster PPenk mRNA observed in adrenals between postnatal days 0 and 4 and in striatum between postnatal days 12 and 48. PPenk mRNA levels were determined in hamster pups after treatment with increasing doses of metyrapone (an 11beta hydroxylase inhibitor) or with the glucocorticoid receptor antagonist RU 486 +/- metyrapone between postnatal days 2 and 4. Levels were also determined 36 days after hypophysectomy at age 16-17 days. Although plasma glucocorticoid levels and/or the influence from glucocorticoids were reduced, only developmental increases in PPenk mRNA are influenced by glucocorticoids in hamster adrenals, while basal adrenal mRNA levels are unchanged. However, pituitary influence on striatal PPenk mRNA levels appears complex and may involve steroid and/or non-steroid factors. These results suggest that glucocorticoids regulate hamster Penk gene expression via a mechanism that varies with age and tissue and functions during the induction of the Penk gene and not to maintain basal gene expression. Possible mechanisms and species variation are discussed.

Dexamethasone blocks the rapid biological effects of 17beta-estradiol in the rat uterus without antagonizing its global genomic actions

Estrogens and glucocorticoids have opposing effects on the female reproductive tract, but the molecular basis for this antagonism is poorly understood. We therefore examined the biological and transcriptional programs induced by estrogens and glucocorticoids in the uterus of immature female rats. Estradiol 17beta (E2) rapidly induced morphological changes reminiscent of an acute inflammatory response, including infiltration of eosinophils, edema in the stroma and myometrium, and a decrease in the height of luminal epithelial cells, whereas dexamethasone (Dex) only altered stromal cell morphology. When coadministered with E2, Dex completely blocked the proinflammatory effects of E2. Surprisingly, examination of E2 and Dex effects on gene expression using cDNA microarrays and real-time PCR revealed that these hormones had similar effects on the expression of many genes and that very few genes displayed antagonistic regulation. Together, these results indicate strong discord between the early biologic and genomic actions of estrogens and glucocorticoids and highlight a complex regulatory role for glucocorticoids and GR in the mammalian uterus.

The comparative analysis of proenkephalin mRNA expression induced by cholera toxin and pertussis toxin in primary cultured rat cortical astrocytes

In rat astrocytes, incubation with cholera toxin (CTX; 0.1 microg/ml) for 8 h increased proenkephalin (proENK) mRNA level (10-fold), which was further increased by dexamethasone (DEX; 1 microM) (2.2-fold as much as CTX alone). Although pertussis toxin (PTX; 0.1 microg/ml) did not affect the basal proENK mRNA level, DEX significantly increased proENK mRNA level in PTX-treated cells (6-fold). The inhibition of protein synthesis by cycloheximide (CHX; 15 microM) also increased proENK mRNA level in PTX-treated cells (5.2-fold), but not in CTX-stimulated cells. The treatment with CTX, but not PTX, increased c-Fos and Fra-2 protein levels as well as AP-1, CRE, or ENKCRE-2 DNA binding activity, but neither toxin affected Fra-1, c-Jun, JunB, and JunD protein levels. CHX significantly attenuated CTX-induced increase of c-Fos or Fra-2 protein level and AP-1, CRE, or ENKCRE-2 DNA binding activity, although CHX alone did not affect the basal AP-1, CRE, and ENKCRE-2 DNA binding activities. Phosphorylated CREB level was increased by both CTX and PTX, although the magnitude of phosphorylation of CREB by PTX was much less than that by CTX. In addition, CHX further or persistently increased PTX- or CTX-induced phosphorylated CREB levels in parallel with increases in proENK mRNA. However, DEX did not alter the basal or stimulated phosphorylated-CREB level. These results suggest that the elevation of phosphorylation of CREB rather than AP-1 level may be involved in CTX-induced and CHX-dependent-PTX-induced increase of proENK mRNA level. In addition, AP-1 expression or CREB phosphorylation appears not to be involved the potentiative action of DEX on proENK mRNA expression in CTX- and PTX-treated astrocytes.

The differential molecular mechanisms underlying proenkephalin mRNA expression induced by forskolin and phorbol-12-myristic-13-acetate in primary cultured astrocytes

In rat astrocytes, forskolin (FSK; 5 microM) and phorbol-12-myristic-13-acetate (PMA; 2.5 microM) increase the proenkephalin (proENK) mRNA level via different pathways. FSK-induced proENK mRNA expression is independent of protein de novo synthesis, and well correlated with CREB phosphorylation. This is in contrast to PMA-induced proENK mRNA expression that is dependent on protein de novo synthesis and is well correlated with the increase of AP-1 DNA binding activity rather than CREB phosphorylation. Differential regulation of AP-1 proteins by PMA and FSK was also observed. While c-Fos, Fra-2 and JunB were increased in response to either stimuli, only Fra-1, c-Jun and JunD were increased by PMA. The combined treatment with FSK and PMA additively increased the proENK mRNA level, which was correlated with AP-1 or ENKCRE-2 DNA binding activity, and CREB phosphorylation. Dexamethasone (DEX; 1 microM) further enhanced FSK- or PMA-induced proENK mRNA expression, which was not correlated with the activation of AP-1 expression and CREB phosphorylation, suggesting that synergistic interaction of glucocorticoid with PKA or PKC pathway for the regulation of proENK mRNA expression appears to be mediated by other pathways rather than CREB and AP-1 families.

Interleukin-6 regulation of kappa opioid receptor gene expression in primary sertoli cells

Three classes of opioid receptors--mu, delta, and kappa--mediate physiological and pharmacological functions of the endogenous opioid peptides and exogenous opioid compounds in the central nervous system (CNS), as well as in peripheral tissues including the immune system. Using reverse transcriptase polymerase chain reaction (RT-PCR) analysis, we show that freshly isolated and highly purified somatic (Sertoli and Leydig) and specific germ (spermatogonia, pachytene spermatocytes, round, and elongating spermatids) cells of the rat testis differentially express the mRNAs for these opioid receptor genes. Furthermore, to identify a functional mechanism for cytokine regulation of testicular opioid receptor gene expression, we employed primary Sertoli cells as a model system. In a semiquantitative PCR analysis using the S16 ribosomal RNA gene as an internal control, we show that interleukin-6 reduces kappa opioid receptor mRNA levels from 6 to 24 h of treatment in primary Sertoli cells. This regulation requires new RNA and protein synthesis and is partially mediated by the protein kinase A pathway. These findings are consistent with a role for the cytokine and opioid signaling pathways in Sertoli cellular function and the interaction that exists between the opioid and the immune systems in the CNS.

Preproenkephalin mRNA levels are regulated by acute stress and estrogen stimulation

Enkephalins facilitate female reproductive behavior. Within the limbic system and hypothalamus, estrogen induced the expression of preproenkephalin (PPE) mRNA. Estrogen injection caused a biphasic increase in the PPE mRNA levels within the ventromedial hypothalamic nucleus and posterodorsal medial amygdala. The first peak of PPE mRNA levels occurred within an hour, and the second 24 to 48 h after subcutaneous injection of estrogen. The present studies indicated that the rapid first peak of PPE mRNA expression was stress induced, whereas the second peak was estrogen induced. In the posterodorsal medial amygdala but not in the ventromedial hypothalamic nucleus, the antiestrogen, tamoxifen, did not inhibit the first peak, but blocked the second peak of PPE mRNA expression. Subcutaneous oil injection induced a 1-h peak of PPE mRNA levels but not a 24-h peak. Peak levels of plasma corticosterone were coincident with peak PPE mRNA levels. Adrenalectomy plus a constant, low level of corticosterone eliminated the injection-induced increase of corticosterone levels and the subsequent increase in PPE mRNA expression in the ventromedial hypothalamic nucleus and posterodorsal medial amygdala. The present results indicate that both stress steroids and estrogen positively regulate PPE mRNA levels in the ventromedial hypothalamic nucleus and posterodorsal medial amygdala. These results are consistent with the hypothesis that acute, mild stress may contribute to the activation of circuits that facilitate reproductive behavior in the female.

Down-regulation of delta opioid receptor mRNA by an anabolic steroid in neuronal hybrid cells

Nandrolone, an anabolic androgenic steroid, reduced delta opioid receptor (DOR) mRNA and the number of DOR binding sites in two neuronal hybrid cell lines: NG 108-15 and the GT1-1 cells. Both cell lines express DOR but only GT1-1 cells express androgen receptors (AR). DOR mRNA levels were maximally decreased by approximately 45% in NG 108-15 cells and by approximately 38% in GT1-1 cells exposed for 24 h to 10(-6) M nandrolone. This action was partly due to a decrease in the rate of transcription of DOR mRNA and was not blocked by the androgen antagonist flutamide. Flutamide antagonized the repression of AR mRNA induced by nandrolone. The synthetic glucocorticoid dexamethasone (10(-6) M) did not modify DOR steady-state transcripts in either cell line. These results suggest that nandrolone presumably regulate DOR mRNA levels through mechanisms independent of the androgen and glucocorticoid receptors.

Descending modulation of opioid-containing nociceptive neurons in rats with peripheral inflammation and hyperalgesia

Inflammation and hyperalgesia induce a dramatic up-regulation of opioid messenger RNA and peptide levels in nociceptive neurons of the spinal dorsal horn. Descending axons modulate nociceptive transmission at the spinal level during inflammatory pain, and may play a role in the development of persistent pain. The role of descending bulbospinal pathways in opioid-containing nociceptive neurons was examined. Removal of descending inputs to the spinal cord was performed by complete spinal transection at the midthoracic level. Seven days after spinal transection, rats received a unilateral hindpaw injection of complete Freund's adjuvant, a noxious stimulus that produces inflammation and hyperalgesia. Tissues from the L4 and L5 segments of the spinal cord were removed and analysed by northern blotting and immunocytochemistry. Spinal transection resulted in a further increase in both dynorphin and enkephalin messenger RNA content following complete Freund's adjuvant injection. There was a similar distribution and number of dynorphin-immunoreactive cells in transected rats compared to rats which received sham surgery. These data suggest that increased dynorphin messenger RNA ipsilateral to inflammation, in rats without descending axons, was due to increased expression within the same cells and not to recruitment of additional dynorphin-expressing cells. This reflects a greater dynamic response of nociceptive neurons to noxious stimuli in the absence of descending modulation. Therefore, the net effect of descending afferents on spinal nociceptive circuits may be to reduce the response of opioid-containing neurons to noxious stimulation from the periphery.

Multiple pathways regulate the expression of genes encoding sodium channel subunits in developing neurons

In primary cultures of fetal neurons, activation of sodium channels with either alpha-scorpion toxin or veratridine caused a rapid and persistent decrease of mRNAs encoding beta2 and different sodium channel alpha mRNAs. In contrast, beta1 subunit mRNA was up-regulated by sodium channel activation. This phenomenon was calcium-independent. The effects of activating toxins on mRNAs of different sodium channel subunits were mimicked by membrane depolarization. An important aspect of this study was the demonstration that cAMP also caused rapid reduction of alphaI, alphaII and alphaIII mRNA levels whereas beta1 subunit mRNA was up regulated and beta2 subunit mRNA was not affected. Sodium channel activation by veratridine was shown to increase cAMP immunoreactivity in cultured neurons, but alphaII mRNA down-regulation induced by activating toxins was not reversed by protein kinase A antagonists, indicating that this phenomenon is not protein kinase A dependent. The effects of cAMP and membrane depolarisation were antagonized by the PKA inhibitor H89. These results are indicative of the existence of multiple and independent regulatory pathways modulating the expression of sodium channel genes in the developing central nervous system.

Regulation of gene expression by corticoid hormones in the brain and spinal cord

Glucocorticoids (GC) and mineralocorticoids (MC) have profound regulatory effects upon the central nervous system (CNS). Hormonal regulation affects several molecules essential to CNS function. First, evidences are presented that mRNA expression of the alpha3 and beta1-subunits of the Na,K-ATPase are increased by GC and physiological doses of MC in a region-dependent manner. Instead, high MC doses reduce the beta1 isoform and enzyme activity in amygdaloid and hypothalamic nuclei, an effect which may be related to MC control of salt appetite. The alpha3-subunit mRNA of the Na,K-ATPase is also stimulated by GC in motoneurons of the injured spinal cord, suggesting a role for the enzyme in GC neuroprotection. Second, we provide evidences for hormonal effects on the expression of mRNA for the neuropeptide arginine vasopressin (AVP). Our data show that GC inhibition of AVP mRNA levels in the paraventricular nucleus is sex-hormone dependent. This sexual dimorphism may explain sex differences in the hypothalamic-pituitary-adrenal axis function between female and male rats. Third, steroid effects on the astrocyte marker glial fibrillary acidic protein (GFAP) points to a complex regulatory mechanism. In an animal model of neurodegeneration (the Wobbler mouse) showing pronounced astrogliosis of the spinal cord, in vivo GC treatment down-regulated GFAP immunoreactivity, whereas the membrane-active steroid antioxidant U-74389F up-regulated this protein. It is likely that variations in GFAP protein expression affect spinal cord neurodegeneration in Wobbler mice. Fourth, an interaction between neurotrophins and GC is shown in the injured rat spinal cord. In this model, intensive GC treatment increases immunoreactive low affinity nerve growth factor (NGF) receptor in motoneuron processes. Because GC also increases immunoreactive NGF, this mechanism would support trophism and regeneration in damaged tissues. In conclusion, evidences show that some molecules regulated by adrenal steroids in neurons and glial cells are not only involved in physiological control, but additionally, may play important roles in neuropathology.

The stimulation of rat astrocytes with phorbol-12-myristate-13-acetate increases the proenkephalin mRNA: involvement of proto-oncogenes

The effect of phorbol-12-myristate-13-acetate (PMA) on the regulation of proenkephalin (proENK) mRNA level, ENKCRE-2 or AP-1 DNA binding activity, and the mRNA and protein levels of proto-oncogenes (c-fos, fra-1, and c-jun) in primary cultured rat astrocytes were studied. The proENK mRNA level was elevated at 4 h after the treatment of PMA (2.5 microM) without altering the intracellular proENK protein level, and this increase was attenuated by pre-treatment with cycloheximide (CHX; 15 microM), a protein synthesis inhibitor. Both AP-1 and ENKCRE-2 DNA binding activities were markedly increased at 1-4 h by PMA treatment and these PMA-induced responses were inhibited by pre-treatment with CHX, showing that the increase of proENK mRNA level was well correlated with the AP-1 and ENKCRE-2 DNA binding activities. In contrast, although the phospho-CREBP level was also increased by PMA at 0.5-1 h, the pre-treatment with CHX further increased the PMA-induced phospho-CREBP level. In addition, PMA caused the induction of c-fos, c-jun and fra-1 mRNA level and, especially, PMA-induced increase of fra-1 mRNA level was further enhanced by CHX treatment at 4 h. Furthermore, western immunoblot assay showed that PMA caused induction of c-Fos, Fra-1, and c-Jun protein levels. PMA-induced increases of proto-oncoproteins levels were also inhibited by CHX treatment. The results suggest that newly synthesized AP-1 proteins, such as c-Fos, Fra-1, and c-Jun may play important roles in the regulation of PMA-induced proENK gene expression in cultured rat astrocytes. Phospho-CREB protein appears not to be involved in the regulation of PMA-induced proENK gene expression.

Activation of protein kinase A prevents the ethanol-induced up-regulation of delta-opioid receptor mRNA in NG108-15 cells

We have used a sensitive solution hybridization assay with a riboprobe transcribed from the coding sequence of the delta-opioid receptor gene (DOR) to study the up-regulation of the DOR mRNA by ethanol in NG108-15 cells. Exposure of the cells to compounds that increase cAMP levels (forskolin, forskolin + IBMX, or dibutyryl cAMP) resulted in the attenuation of ethanol-induced up-regulation of DOR mRNA. The inactive analogue of forskolin, 1,9-dideoxy forskolin had no effect. Northern blot analysis of RNA extracts from ethanol-, forskolin- or ethanol + forskolin-treated cells showed proportional changes in each of the multiple DOR mRNA bands, so that no difference was observed in the fraction of the total hybridization signal produced by each band of the DOR mRNA. In the absence of ethanol, forskolin or dibutyryl cAMP reduced the basal levels of DOR mRNA. The cAMP analogue (Rp)-cAMPS, a protein kinase A (PKA) inhibitor, increased DOR mRNA levels. However, the combination of (Rp)-cAMPS and ethanol did not further increase DOR mRNA levels compared to ethanol or (Rp)-cAMPS alone. Signaling through cAMP and PKA down-regulates DOR mRNA levels. The ethanol-induced increase in DOR mRNA levels in NG108-15 cells appears to be mediated via a reduction of PKA.

Dexamethasone represses phorbol ester-, forskolin-, and calcium-stimulated expression of a preproenkephalin A promoter-chloramphenicol acetyltransferase gene via a receptor-mediated mechanism

CV-1 cells were stably transfected with a preproenkephalin A (PPE) promoter-chloramphenicol acetyltransferase (CAT) reporter plasmid containing -176 to +171 bp of the human PPE gene. Low levels of CAT were expressed constitutively. The reporter enzyme activity was induced by treatment of the cells for 6 h with drugs that increased intracellular cAMP (forskolin and 8-bromo-cAMP), intracellular calcium (A23187), or protein kinase C activity (tetradecanoyl phorbol-4-acetate, TPA) in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine. Co-administration of dexamethasone reduced the magnitude of phorbol ester-stimulated CAT activity by about 50%, while there were smaller but not significant effects on forskolin- or A23187-stimulated expression of this reporter construct. In transient transfections which included the PPE-CAT reporter gene and a glucocorticoid receptor expression plasmid, dexamethasone significantly reduced stimulated expression of the reporter by TPA, forskolin, and A23187. The effect was observed with 10(-8)-10(-6) M dexamethasone and was blocked by the presence of the glucocorticoid antagonist RU486, suggesting that the effect of dexamethasone was mediated by the glucocorticoid receptor. The promoter region contained in this construct lacks a classical glucocorticoid response element or known negative elements; thus, dexamethasone may reduce stimulated expression of the PPE promoter via indirect effects.

Molecular mechanisms underlying the regulation of proenkephalin gene expression in cultured spinal cord cells

The regulation of proenkephalin (proENK) mRNA levels by cAMP and protein kinase C (PKC) pathways was studied in cultured rat spinal cord cells in the present study. Spinal cord cells were cultured from 14 day (E 14) embryos of Sprague-Dawley rats. After 7 days in vitro, the spinal cord cells were incubated with either forskolin (5 microM) or phorbol-13-myristate acetate (PMA; 2.5 microM) for 1, 3, 6, 9, 12 or 24 h and total RNA and proteins were isolated for Northern and Western blot analyses. The proENK mRNA level began to increase within an hour, then reached and remained at a peak 3-12 h after stimulation by both forskolin and PMA. The increased proENK mRNA level in forskolin-treated cells was slightly decreased 24 h after the stimulation, whereas the level of proENK mRNA returned to basal levels in PMA-treated cells. A Western blot assay revealed that the intracellular level of proENK protein was not changed by treatment with either forskolin or PMA. Pretreatment of cells with cycloheximide (a protein synthesis inhibitor; 10 microM) did not affect the forskolin- or PMA-induced increase of proENK mRNA. However, pretreatment with nimodipine (an L-type Ca2+ channel blocker; 2 microM), omega-conotoxin (an N-type Ca2+ channel blocker; 1 microM), calmidazolium (a calmodulin antagonist; 1 microM) or KN-62 (a Ca2+/calmodulin-dependent protein kinase II inhibitor; 5 microM) attenuated the forskolin- or PMA-induced increase of proENK mRNA levels. Dexamethasone (1 microM) did not affect the forskolin-induced increase of proENK mRNA levels. Our results suggest that the elevation of proENK mRNA levels in the spinal cord is regulated by both cAMP and PKC pathways. Calcium influx through both L- and N-type calcium channels, calmodulin and Ca2+/calmodulin-dependent protein kinase II appear to be involved in the increase of proENK mRNA levels induced by either forskolin or PMA. Furthermore, ongoing protein synthesis is not required for forskolin- or PMA-induced alterations in proENK mRNA.

Regulation of the expression of the proenkephalin gene in cultured meningeal fibroblasts: opposite effects of alpha 1- and beta 2-adrenoceptors

Meningeal fibroblasts express the proenkephalin gene during embryonal development but terminate the expression shortly before birth. When brought into primary culture at postnatal day 1, the fibroblasts again express the gene. Activation of protein kinase A reduces this expression and thus may contribute to its prenatal termination. Since the noradrenergic innervation of the meninges begins around the time of birth, it was investigated in the present study, how adrenergic agonists affected the levels of proenkephalin mRNA in cultured fibroblasts. The beta 2-adrenoceptor agonists salbutamol and procaterol increased the levels of endogenous cAMP and diminished the concentration of proenkephalin mRNA indicating that the cultured fibroblasts possessed this beta-subtype. In contrast, noradrenaline increased the level of proenkephalin mRNA in a concentration-dependent manner. This effect was independent of endogenous cAMP and was mediated by alpha 1-adrenoceptors. The data indicate that the noradrenergic innervation of the meninges at the time of birth is not responsible for the termination of the proenkephalin gene expression.

Molecular cloning and characterization of a novel form of neuropeptide gene as a developmentally regulated molecule

To examine the molecular basis controlling neuronal differentiation, subtraction library construction and differential screening were used to identify cDNAs whose mRNA levels are regulated in mouse NS20Y cells by dibutyryl cyclic AMP treatment. One of them, N27K, whose mRNA increases transiently during both neuronal differentiation in NS20Y cells and development in mouse brain. The deduced amino acid sequence of N27K comprises 212 amino acid residues and is a novel form of a precursor protein for a new neuropeptide nociceptin/orphanin FQ, which we independently cloned as N23K. That is, the putative protein encoded by N27K is 25 amino acids longer than that encoded by N23K. Using an antibody against a C-terminal peptide of the N27K protein that recognizes a 27-kDa protein in Western blot analysis, a punctate structure in the perinuclear region and areas near the tip of neurites is visualized in neurally differentiating NS20Y cells. The time of maximal expression correlates with periods of neurite extension, and expression decreases as the neuritic network develops. Immunohistochemistry of tissue sections of the mouse central nervous system revealed that reactivity for the anti-N27K protein antibody can detected in early generated neurons at embryonic day 14, in virtually all immature neurons at postnatal day 1, and in subsets of neurons of discrete brain regions such as the hypothalamus and spinal cord in adults. This remarkable redistribution suggests that N27K may be involved in a process in neurite outgrowth and nervous system development.

Evidence for the involvement of 5-lipoxygenase products in the regulation of the expression of the proenkephalin gene in cultured astroglial cells

Cultured astroglial cells secrete eicosanoids which are produced by the cyclooxygenase and lipoxygenases. These cells also transcribe the proenkephalin gene. In the present study, it was investigated whether agents which inhibit the metabolism of arachidonic acid affect the basal and stimulated expression of the gene. Tetradecanoyl phorbol acetate (TPA; 1-1000 nmol/l) increases the concentration of proenkephalin mRNA in these cells by activating protein kinase C. The enhancement in proenkephalin mRNA caused by TPA (10 nmol/l) was not affected by the cyclooxygenase inhibitor indomethacin (5 mumol/l). However, nordihydroguaiaretic acid, which blocks cyclooxygenase and lipoxygenases, potentiated the effect of TPA on proenkephalin mRNA, when used at concentrations of 0.5-50 mumol/l. Two selective inhibitors of 5-lipoxygenase, i.e. MK886 (5 mumol/l) and BAY X1005 (1 mumol/l), also enhanced the effect of TPA (10 nmol/l) without affecting the basal expression of the gene. When added to the incubation medium, leukotriene E4 (10-1000 nmol/l) diminished in a dose-dependent manner the basal and TPA-induced expression of the proenkephalin gene. It is concluded that in astroglial cells derived from cortex of new-born rats products of 5-lipoxygenase can diminish the action of protein kinase C on the proenkephalin gene.

Rat meningeal fibroblasts in primary culture express the proenkephalin gene

When brought into primary culture, rat meningeal fibroblasts contained proenkephalin-mRNA detected with Northern blot hybridization. In contrast, splenic fibroblasts did not express the gene under the same culture conditions. In situ hybridization showed that the meningeal fibroblasts did not uniformly express the gene: groups of positive cells were surrounded by cells with low or no proenkephalin-mRNA. Some fibroblasts which contained the mRNA species took up bromo-deoxyuridine indicating that the expression of the gene also occurred in proliferating cells, but was not restricted to this group. In chromaffin and astroglial cells, activation of protein kinase A or C with 8 Br.cAMP or O-tetradecanoyl 13-phorbolacetate, respectively, increases the expression of the gene. In meningeal fibroblasts, however, both agents reduced the levels of proenkephalin-mRNA. In the case of 8Br.cAMP, this effect was blocked by the protein synthesis inhibitor cycloheximide indicating that a newly-synthesized protein was involved. Cultured meningeal fibroblasts appear to be useful for studies on the cell specificity of the expression of this peptide gene as well on its regulation.

The transcriptional regulation of the preproenkephalin gene

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Interaction of protein kinases A and C in their effects on the proenkephalin gene in astroglial cells

In several cell types, the expression of the proenkephalin (PEnk) gene is enhanced after activation of protein kinase A. In the present study, astroglial cells cultured from rat cortex were used to investigate whether protein kinases A and C can act in a synergistic manner on the endogenous proenkephalin gene. The activator of protein kinase C tetradecanoylphorbolacetate (0.001-1 microM) increased the level of proenkephalin-mRNA in a concentration dependent manner. When used together with the phosphodiesterase inhibitor Rolipram (1 microM), the effect of tetradecanoylphorbolacetate (0.01 microM) was potentiated. 8-Bromoadenosine 3',5'-cyclic monophosphate (0.01-1 mM) also enhanced the expression of the proenkephalin gene. When used together with tetradecanoylphorbolacetate (0.01 and 0.1 microM), respectively, both agents had additive effects. Inhibition of protein synthesis with cycloheximide (35 microM) significantly changed the effects of both agents. While the effect of 8Br.cAMP (1 mM) on PEnk-mRNA was enhanced, that of tetradecanoylphorbolacetate (0.1 microM) was abolished. The results provide evidence for a synergistic effect of protein kinase A and C on the expression of the proenkephalin gene in astroglial cells. However, the protein kinases seem to act via different transcription factors on the expression of the proenkephalin gene.

Opioids in neural and nonneural tissues

The endogenous opioid peptides (EOP) are grouped in three families, each deriving from the posttranslational processing of a distinct precursor molecule and exhibiting high affinity for a specific opioid receptor. The genes of EOPs are expressed in a wide variety of sites, including many nerve, neurosecretory, and endocrine cells. In reviewing the vast literature on this subject, a few patterns begin to emerge. First, the distribution of EOPs in tissues appears to be a distinct characteristic of each family of opioids. Second, the EOP producing cells can be grouped into two broad categories: those expressing only one and those expressing multiple EOP genes. Most EOP-producing nerve and neurosecretory cells fall into the first category, that is, they express one EOP gene, whereas most nonneural cells fall into the second category, that is, they express multiple EOP genes. Third, it appears that there is a relationship between opioids, proliferation rate, and state of differentiation of cells, since it has been shown that (a) mitogenic factors may change the EOP profile of a cell, and that (b) opioids may inhibit the proliferation rate of normal or neoplastic cells. The physiologic implication of these observations is briefly discussed.

Preproenkephalin RNA increases in the hypothalamus of rats stressed by social deprivation

1. Pharmacological evidence indicates that stress induced by brief (14 to 20-day) social deprivation in the rat is associated with an activation of the central preproenkephalin (ENK) opioid system. This study examines the neurochemical evidence that substantiates such an activation. 2. Using a specific ENK complementary DNA probe, ENK RNA levels were measured by dot blot and Northern blot analyses in different brain areas of socially deprived rats. Immunoreactivity to met-enkephalin-derived peptides was also evaluated by radioimmunoassay in the same brain regions. 3. Brief social deprivation increased the levels of ENK RNA and enkephalin immunoreactivity in whole hypothalamus. 4. Our data suggest that this type of stress appears to be associated to an induction of ENK gene transcription in hypothalamus.

Regulation of prodynorphin gene expression in the hippocampus by glucocorticoids

The regulation of prodynorphin gene expression by glucocorticoids in the hippocampus was examined in rats that were adrenalectomized (ADX) either 7, 30, 60 and 90 days prior to sacrifice. Peptide levels in the hippocampus of ADX rats were determined by radioimmunoassay and immunocytochemistry. Prodynorphin (PDYN) mRNA was measured by Northern blot analysis and in situ hybridization. A time-dependent decrease in dynorphin A(1-8)(DYN) levels in the hippocampus (18% at 7 days; 44% at 30 days; 58% at 60 days) of ADX rats was found, which was accompanied by a comparable decrease in the abundance of PDYN mRNA. An in situ hybridization analysis revealed that both the number of positively hybridized cells and the number of silver grains per cell were decreased in the dentate gyrus after ADX. The administration of dexamethasone after surgery reversed the peptide and mRNA attenuation induced by ADX. ADX had no effect on the expression of proenkephalin mRNA or [Met5]-enkephalin immunoreactivity in the hippocampus. Examination of thionin-counterstained tissue showed that the dentate granule cell layer was intact. The decrement of DYN expression in this system is proposed to have resulted from the removal of glucocorticoid input and not dentate granule cell loss. This study provides the strong evidence for a differential susceptibility of these two opioid peptides in the hippocampus to the removal of glucocorticoids. In addition, these data provide support for a potentially selective, glucocorticoid-permissive component in PDYN gene expression.

Glucocorticoid regulation of preproenkephalin gene expression in the rat forebrain

The effects of glucocorticoids on the levels of preproenkephalin (PPE) mRNA in the rat forebrain were analyzed with in situ hybridization and dot blots. In adrenally-intact rats, high levels of PPE mRNA, as assessed by in situ hybridization, were localized in the caudate-putamen, nucleus accumbens, central amygdala, and ventrolateral ventromedial hypothalamus (VMHVL), and low levels in the hippocampus. After adrenalectomy, the density of PPE mRNA-positive cells and the level of PPE mRNA/cell were decreased in all regions except the hippocampus. Acute treatment with corticosterone (CORT) in adrenalectomized rats increased the level of PPE mRNA/cell in the caudate-putamen and VMHVL. In intact rats, chronic treatment with CORT increased the density of PPE mRNA-positive cells in the caudate-putamen and hippocampus, and the level of PPE mRNA/cell in the caudate-putamen and nucleus accumbens. The effect of chronic CORT treatment on PPE mRNA in the striatum, amygdala, hippocampus and mediobasal hypothalamus was assessed with dot blots. Chronic CORT treatment increased PPE mRNA levels in the caudate-putamen and hippocampus. There was a good correlation between results on the effect of chronic CORT treatment on PPE mRNA levels in intact rats, obtained from dot blots and in situ hybridization. Results from this study suggest that glucocorticoids are required for the maintenance of basal PPE mRNA levels in most regions of the rat forebrain. There is, however, considerable regional heterogeneity in the effect of glucocorticoid treatment on PPE mRNA levels in adrenalectomized and intact rats. Increased PPE mRNA levels in response to high circulating levels of glucocorticoids, e.g. in stress, may have important pathophysiological consequences.

Inhibition of anti-CD3 monoclonal antibody-induced T-cell proliferation by dexamethasone, isoproterenol, or prostaglandin E2 either alone or in combination

1. The purpose of these studies was to investigate the modulation of the proliferation of human T cells obtained from peripheral blood by dexamethasone (DEX), isoproterenol (ISO), and prostaglandin E2 (PGE2). The former two substances interact with T cells via the glucocorticoid and beta-adrenergic receptors respectively. When occupied by their natural ligands, glucocorticosteroids and catecholamines, these receptors have a role in modulating T-cell function during stress. During the inflammatory response increased levels of PGE2 bind to their receptors on T cells and thus alter responsiveness. Proliferation of T cells was induced by immobilized anti-CD3 monoclonal antibody (mAb) in the presence or absence of an additional costimulatory signal delivered by anti-CD28 mAb. 2. Various physiologic concentrations of DEX, ISO, or PGE2 were added at the time of initiation of the cultures and subsequent proliferation of the unstimulated T cells was determined. The results demonstrate that physiologic concentrations of all three of these agents inhibit the anti-CD3 mAb-induced proliferation of T cells. 3. Although DEX and PGE2 were equipotent in suppressing T-cell proliferation, ISO was much less effective. 4. Because concomitant elevations in the peripheral levels of these substances may occur, experiments were performed to determine the T-cell inhibitory effects of DEX together with either PGE2 or ISO. Synergistic suppression of T-cell proliferation was observed when various concentrations of DEX and PGE2, but not DEX and ISO, were added to cultures. This synergistic suppression could not be explained by an increase in cAMP accumulation in T cells stimulated with DEX and PGE2. 5. Finally, the addition of anti-CD28 mAb to anti-CD3 mAb-stimulated T cells overcame much of the suppression of proliferation induced by PGE2 or ISO but less so than that induced by DEX.

Primary sequence of -1436 to +53 bp of the rat preproenkephalin gene putative Z-DNA and regulatory motifs

We report novel sequence data extending -1436 bases 5' of the rat proenkephalin gene start site known as E4. We noted an interesting stretch of 58 bases of alternating pyrimidines that lies immediately adjacent to 71 bases of an alternating purine-pyrimidine Z-DNA-like sequence that lies between -694 bp and -566 bp. Multiple sequence homologies to putative cis-acting regulatory factor binding sites were identified by a computer aided sequence search.

Transactivation of the proenkephalin gene promoter by the Tax1 protein of human T-cell lymphotropic virus type I

Human T-cell lymphotropic virus type I (HTLV-I), an etiologic agent for adult T-cell leukemia, is strongly associated with certain neurological diseases. The HTLV-I genome encodes a protein, Tax1, that transactivates viral gene transcription. CD4-positive T helper lymphocytes express the proenkephalin gene, and enkephalins have been implicated as neuroimmunomodulators. We have investigated the effect of Tax1 on the proenkephalin gene promoter in C6 rat glioma cells and demonstrated its transactivation. Analysis using 5' deletion mutants of the promoter region showed that sequences upstream of base pair -190 are necessary for maximal transactivation. Forskolin, a cAMP modulator, synergistically increased Tax1-mediated transactivation of the proenkephalin promoter. Neither Tax1 transactivation alone nor Tax1/cAMP synergism exclusively involved cAMP-responsive elements. Endogenous proenkephalin gene expression increased in Tax1-expressing C6 cells. Since HTLV-I infects lymphocytes, which express proenkephalin mRNA, Tax1 transregulation of proenkephalin expression may provide bidirectional communication between the nervous and immune systems in HTLV-I-related diseases.

Posttranslational processing of proenkephalin in SK-N-MC cells: evidence for phosphorylation

SK-N-MC cells have recently been shown to be a rich source of proenkephalin and/or the proenkephalin-derived peptide, peptide B. We have investigated the synthesis and the posttranslational processing of proenkephalin in these cells. SK-N-MC cells retain very little of the proenkephalin synthesized; greater than 99% of the immunoreactive enkephalin synthesized within a 48-h period is secreted into the medium rather than contained intracellularly. When medium samples were subjected to gel filtration and assayed for the various enkephalins present within proenkephalin, only two major molecular-weight classes of peptides, with molecular weights and immunoreactive profiles consistent with those of proenkephalin and the 3.6-kDa carboxyl-terminal fragment peptide B, were observed. The proenkephalin-like peptide present in medium samples was shown by western blot procedures to consist of a 32-kDa protein with a slight amount of a higher-molecular-weight immunoreactive component above it. Only proenkephalin-sized peptides were present within cell extracts. Radiolabeled proenkephalin added to cell cultures was also cleaved to products similarly sized to those found in medium extracts; radiolabeled proenkephalin incubated in the absence of cells was not cleaved. Cleavage of exogenous proenkephalin thus probably at least partially occurs following secretion. Cell radiolabeling experiments with [32P]orthophosphate demonstrated that SK-N-MC proenkephalin is phosphorylated. Microheterogeneity of proenkephalin was also observed using isoelectric focusing coupled with western blotting. Our results suggest that the SK-N-MC cell line represents a useful model to study the earliest steps of the posttranslational processing of human proenkephalin in a neuronal cell type.

Region-specific regulation of preproenkephalin mRNA in cultured astrocytes

Regulation of preproenkephalin (PPE) mRNA was examined in astrocytes cultured from several regions of the neonatal rat brain. Astrocytes from these regions expressed differing levels of PPE mRNA, with higher levels in astrocytes from the hypothalamus followed by frontal cortex and striatum. Further, PPE mRNA was regulated differently in hypothalamic than in striatal glia. Treatment of striatal astrocytes with the beta-adrenergic agonist, isoproterenol, or with agents which directly increased intracellular cAMP (forskolin or 8-bromo-cAMP) elevated levels of PPE mRNA. By contrast, none of these treatments altered levels of PPE mRNA in hypothalamic astrocytes despite increasing cAMP levels 60-fold. These observations indicate that there is striking regional heterogeneity in the expression and regulation of PPE mRNA by astrocytes, suggesting that proenkephalin or its derived peptides help to mediate region-specific brain functions.

Expression of preprodynorphin in human small cell lung carcinoma cell lines

The expression of preprodynorphin has been studied using the Northern blot technique. Ten human cell lines, six small cell lung carcinoma (SCLC), one large cell carcinoma (LCC), two neuroblastoma and one lymphoblast-like cell line, were screened with a preprodynorphin cRNA-probe. Tryptic digestion followed by radioimmunoassay for Leu-enkephalin-Arg6 was used to detect possible translation of the preprodynorphin transcript. Of the ten cell lines investigated we found that all expressed preprodynorphin-mRNA to various degrees, and that this transcript is also translated. Two of the cell lines, neuroblastoma SK-N-MC and SCLC H69, also expressed preproenkephalin-mRNA. This set of cell lines provides a useful model of human origin in which the regulation of the preprodynorphin gene and the posttranslational processing of its products can be studied and compared.

Circadian regulation of the biosynthesis of cardiac Met-enkephalin and precursors in normotensive and spontaneously hypertensive rats

Preproenkephalin A mRNA (ppEnk mRNA) and immunoreactive Met-enkephalin (ir-Met-Enk) were measured in the heart of 4, 8 and 16 week-old normotensive Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. WKY rats displayed a small decrease in their cardiac concentration of free (1.3 to 1.0 pmol/g) and cryptic (enzyme processed: 5.3 to 3.7 pmol/g) ir-Met-Enk with aging while the abundance of ppEnk mRNA increased by 3.2 fold between 4 and 16 week-old animals. Similar decreases in free (1.5 to 1.0 pmol/g) and cryptic (5.6 to 4.2 pmol/g) ir-Met-Enk levels were observed in SHR with aging but the rise in the level of ppEnk mRNA was much more pronounced reaching at 16 week-old levels of 7.3 times higher than at 4 week-old and 4.3 times higher than in age-matched WKY. The lack of correlation between the concentration of free and cryptic ir-Met-Enk and the abundance of ppEnk mRNA led us to measure the level of peptides in the heart of 16 week-old animals sacrificed at 4 hr intervals over a 24 hr period. SHR rats displayed circadian variations in their heart content of free and cryptic ir-Met-Enk and increased levels (1.6 fold) of cryptic peptide as compared with WKY at the beginnings of light (6 hr) and dark (18 and 22 hr) periods, suggesting the occurrence of cyclic and transitory upregulation of cDNA transcription and/or derepression of mRNA translation.

Transcription of the rat and mouse proenkephalin genes is initiated at distinct sites in spermatogenic and somatic cells

During spermatogenesis, several genes are expressed in a germ cell-specific manner. Previous studies have demonstrated that rat and mouse spermatogenic cells produce a 1,700-nucleotide proenkephalin RNA, while somatic cells that express the proenkephalin gene contain a 1,450-nucleotide transcript. Using cDNA cloning, RNA protection, and primer extension analyses, we showed that transcription of the rat and mouse spermatogenic-cell RNAs is initiated downstream from the proenkephalin somatic promoter in the first somatic intron (intron As). In both species, the germ cell cap site region consists of multiple start sites distributed over a length of approximately 30 base pairs. Within rat and mouse intron As, the region upstream of the germ cell cap sites is GC rich and lacks TATA sequences. A consensus binding site for the transcription factor SP1 was identified in intron As downstream of the proenkephalin germ cell cap site region. These features are characteristic of several previously described promoters that lack TATA sequences. Homologies were also identified between the proenkephalin and rat cytochrome c spermatogenic-cell promoters, including the absence of a TATA box, a multiple start site region, and several common sequences. This promoter motif thus may be shared with other genes expressed in male germ cells.

Transcription of the rat and mouse proenkephalin genes is initiated at distinct sites in spermatogenic and somatic cells

During spermatogenesis, several genes are expressed in a germ cell-specific manner. Previous studies have demonstrated that rat and mouse spermatogenic cells produce a 1,700-nucleotide proenkephalin RNA, while somatic cells that express the proenkephalin gene contain a 1,450-nucleotide transcript. Using cDNA cloning, RNA protection, and primer extension analyses, we showed that transcription of the rat and mouse spermatogenic-cell RNAs is initiated downstream from the proenkephalin somatic promoter in the first somatic intron (intron As). In both species, the germ cell cap site region consists of multiple start sites distributed over a length of approximately 30 base pairs. Within rat and mouse intron As, the region upstream of the germ cell cap sites is GC rich and lacks TATA sequences. A consensus binding site for the transcription factor SP1 was identified in intron As downstream of the proenkephalin germ cell cap site region. These features are characteristic of several previously described promoters that lack TATA sequences. Homologies were also identified between the proenkephalin and rat cytochrome c spermatogenic-cell promoters, including the absence of a TATA box, a multiple start site region, and several common sequences. This promoter motif thus may be shared with other genes expressed in male germ cells.

Isolation and characterization of glucocorticoid- and cyclic AMP-induced genes in T lymphocytes

Glucocorticoids and cyclic AMP exert dramatic effects on the proliferation and viability of murine T lymphocytes through unknown mechanisms. To identify gene products which might be involved in glucocorticoid-induced responses in lymphoid cells, we constructed a lambda cDNA library prepared from murine thymoma WEHI-7TG cells treated for 5 h with glucocorticoids and forskolin. The library was screened with a subtracted cDNA probe enriched for sequences induced by the two drugs, and cDNA clones representing 11 different inducible genes were isolated. The pattern of expression in BALB/c mouse tissues was examined for each cDNA clone. We have identified two clones that hybridized to mRNAs detected exclusively in the thymus. Other clones were identified that demonstrated tissue-specific gene expression in heart, brain, brain and thymus, or lymphoid tissue (spleen and thymus). The kinetics of induction by dexamethasone and forskolin were examined for each gene. The majority of the cDNA clones hybridized to mRNAs that were regulated by glucocorticoids and forskolin, two were regulated only by glucocorticoids, and three hybridized to mRNAs that required both drugs for induction. Inhibition of protein synthesis by cycloheximide resulted in the induction of all mRNAs that were inducible by glucocorticoids. Preliminary sequence analysis of four of the 11 cDNAs suggests that two cDNAs represent previously undescribed genes while two others correspond to the mouse VL30 retrovirus-like element and the mouse homolog of chondroitin sulfate proteoglycan core protein.

Isolation and characterization of glucocorticoid- and cyclic AMP-induced genes in T lymphocytes

Glucocorticoids and cyclic AMP exert dramatic effects on the proliferation and viability of murine T lymphocytes through unknown mechanisms. To identify gene products which might be involved in glucocorticoid-induced responses in lymphoid cells, we constructed a lambda cDNA library prepared from murine thymoma WEHI-7TG cells treated for 5 h with glucocorticoids and forskolin. The library was screened with a subtracted cDNA probe enriched for sequences induced by the two drugs, and cDNA clones representing 11 different inducible genes were isolated. The pattern of expression in BALB/c mouse tissues was examined for each cDNA clone. We have identified two clones that hybridized to mRNAs detected exclusively in the thymus. Other clones were identified that demonstrated tissue-specific gene expression in heart, brain, brain and thymus, or lymphoid tissue (spleen and thymus). The kinetics of induction by dexamethasone and forskolin were examined for each gene. The majority of the cDNA clones hybridized to mRNAs that were regulated by glucocorticoids and forskolin, two were regulated only by glucocorticoids, and three hybridized to mRNAs that required both drugs for induction. Inhibition of protein synthesis by cycloheximide resulted in the induction of all mRNAs that were inducible by glucocorticoids. Preliminary sequence analysis of four of the 11 cDNAs suggests that two cDNAs represent previously undescribed genes while two others correspond to the mouse VL30 retrovirus-like element and the mouse homolog of chondroitin sulfate proteoglycan core protein.

Phorbol ester regulates the abundance of enkephalin precursor mRNA but not of amyloid beta-protein precursor mRNA in rat testicular peritubular cells

Cultured peritubular cells prepared from the testes of 20-day-old rats contained both preproenkephalin (A) mRNA (1.5 kb) and amyloid beta-protein precursor mRNA (3.6 and 2.8 kb). The phorbol ester TPA and forskolin (an adenylate cyclase activator) increased the preproenkephalin mRNA abundance to 9.0 and 5.8 times the control, respectively. TPA alone had no effect on the intracellular cAMP level. A combination of TPA and forskolin elicited a synergistic increase in the ppEnk mRNA abundance over 30-fold. Dexamethasone potentiated the effect of forskolin but not of TPA. These results suggest that TPA regulates the preproenkephalin mRNA abundance through a cAMP-independent pathway. In contrast, TPA, forskolin, and dexamethasone showed little or no effect on the abundance of amyloid beta-protein precursor mRNA.

Endogenous opioids and neural cancer: an immunoelectron microscopic study

[Met5]-enkephalin, an endogenous opioid peptide derived from proenkephalin A, participates in tumorigenic events by serving as a natural trophic factor that inhibits cell replication. In order to understand how endogenous opioids function in modulating neoplasia, the present study examined the fine structural association of enkephalin with the cellular components of a tumor cell. Immunoelectron microscopic studies were undertaken using antibodies recognizing [Met5]-enkephalin-like substances, and murine S20Y neuroblastoma cells that are known to be responsive to endogenous opioid modulation. Enkephalin was found throughout the cell body and process. Immunoreactivity was associated with the plasma membrane, outer nuclear envelope, and a variety of organelles. With the exception of aggregates of immunoreactivity subjacent to the inner nuclear envelope, the nucleus was not reactive. These results establish that growth-related enkephalins are localized discretely within neuroblastoma cells. Since neuroblastoma cells produce and secrete enkephalins, and enkephalins interact with receptors to mediate actions on cell replication, this study examined enkephalins involved in two different patterns of traffic; further work will be needed to examine each aspect.

Modulation of proenkephalin A gene expression by cyclic AMP

Regulation of proenkephalin A expression was studied in the human neuroblastoma SK-N-MC cell line with respect to mRNA-level, translation, posttranslational processing of the prohormone and secretion of the processed products into the culture medium. Cells were treated with either norepinephrine (NE), dexamethasone (DEX), dibutyryl-3',5'-cyclic AMP (dbcAMP) or the combination of NE and DEX. In an additional investigation, proenkephalin A mRNA levels were determined after 9 h of treatment with dbcAMP, NE, isoproterenol, NE + propranolol and dbcAMP + DEX. NE or dbcAMP for 1-48 h transiently elevated proenkephalin A mRNA 1.5-4.5 times compared to control. The effect of NE was partially blocked by the beta-adrenoceptor antagonist propranolol and was reproduced by the beta-adrenoceptor agonist isoproterenol, suggesting involvement of the beta-adrenoceptor. DEX alone had no significant effect. However it markedly antagonized the effect of NE but not that of dbcAMP suggesting an action on the beta-adrenoceptor. The intracellular content of Met-enkephalin-Arg6,Phe7 immunoreactivity was increased during drug treatment in parallel with changes in proenkephalin A mRNA. DEX gave no effect. No significant change in the ratio of low versus high molecular weight immunoreactive material could be detected in the cell extracts as determined at different time points. Secretion of immunoreactivity into the culture medium increased 5-fold after 18 h of treatment with NE, whereas dbcAMP gave a 2-fold increase. The proportion of low-molecular weight secreted material increased markedly. DEX alone did not induce any change but inhibited the effect of NE. Apparently, regulation of gene expression, prohormone processing and secretion are coordinated by a cAMP-dependent mechanism.

Morphine activation of c-fos expression in rat brain

The post-receptor mechanism of opiate action has been studied by examining the activation by morphine of the proto-oncogene c-fos and its encoded nucleoprotein pp55c-fos (FOS) in rat caudate-putamen, which is rich in the mu-type opiate receptor. Following an acute morphine treatment, c-fos mRNA levels in rat caudate-putamen were increased to maximum (420% of control level) at 45 minutes and returned to control levels at 90 minutes. This induction was completely abolished by naloxone, a morphine antagonist. Fos protein, detected by immunocytochemistry, was also increased 3 hours after morphine injection, in the caudate-putamen, but not in the olfactory tubercle, which does not have the mu-type opiate receptor. Upon activation of opiate receptors by morphine, the c-fos gene is activated and Fos protein may act as a signal transducer uniquely involved in the mechanism of opiate addiction at the level of gene regulation.

Enhanced oxygen metabolism of peritoneal macrophages in the presence of murine neuroblastoma cells is partly caused by enkephalins

A bioluminescent technique was used to show that murine neuroblastoma (NB) cells or cell-free extracts (H variant) were able to enhance the release of reactive oxygen intermediates (ROI) from peritoneal macrophages in vitro. L-variant NB cells were ineffective. Physiological concentrations of met-enkephalin produced the same effect in vitro but not leu-enkephalin. When both H- and L-variant cells, or their extracts, were incubated together with macrophages, ROI production was not increased. Similar findings were detectable when met- and leu-enkephalin were cultured together with macrophages. In vivo, preliminary studies gave the same results. The concentration rate of met- to leu-enkephalin was higher in H-variant than in L-variant NB cells. We conclude from our results that met-enkephalin can enhance the release of ROI from peritoneal macrophages. The difference in the effects produced by the H and L variants is due to differences in the concentrations of enkephalins released.

Interleukin 1 stimulates its own receptor expression on human fibroblasts through the endogenous production of prostaglandin(s)

The regulation of interleukin 1 receptor (IL 1R) expression on human dermal fibroblasts was investigated. On exposure to IL 1 for 3 h at 37 degrees C, the capacity of fibroblasts to bind 125I-labeled human recombinant IL 1 alpha (125I-IL 1 alpha) was reduced by 75%. The IL 1 binding capability of the fibroblasts was restored to control levels by 16 h after removal of unbound IL 1, and then increased to about twofold over that of control cells by 48 h. This later enhancement of IL 1 receptor expression after IL 1 treatment was abolished by indomethacin. Addition of exogenous (PGE1 and PGE2, also analogues of AMP, or forskolin increased the specific binding of 125I-IL 1 alpha to fibroblasts. Scatchard analysis indicated that PGE2 increased the number of IL 1R from approximately 1.6 X 10(3) to 5.4 X 10(3) per cell without change in the binding affinity. These data suggest that the later IL 1-induced up-regulation of IL 1R is mediated by IL 1 stimulation of endogenous prostaglandin production. The combination of PGE2 and prednisolone increased the number of IL 1R on fibroblasts in an additive manner.

Expression of the enkephalin precursor gene in rat Sertoli cells. Regulation by follicle-stimulating hormone

Searching for somatic cells expressing the preproenkephalin (A) gene in the testis, we have isolated Sertoli cells from the testes of 20-day-old rats. Cultured Sertoli cells contained a single species (about 1.5 kb) of preproenkephalin mRNA, and follicle-stimulating hormone (FSH) transiently increased the mRNA abundance to a maximum (about 30 molecules per cell) at 12 h. Various compounds that activate the cyclic AMP system in Sertoli cells similarly increased the abundance of preproenkephalin mRNA. Moreover, FSH increased intracellular Met-enkephalin immunoreactive peptides in Sertoli cells. Thus, the preproenkephalin gene expression in Sertoli cells is positively regulated by FSH through the cyclic AMP system.

Regulation of neuropeptide gene expression by steroid hormones

Steroid hormones modify several brain functions, at least in part by altering expression of particular genes. Of interest are those genes that are involved in cell-cell communication in the brain, for instance neuropeptide genes and genes that code for enzymes involved in synthesis of neurotransmitters. Steroid regulation of mRNA levels for several genes has been reported, including the genes coding for the neuropeptides vasopressin, corticotropin releasing factor, luteinizing hormone-releasing factor, pro-opiomelanocortin; somatostatin, preproenkephalin, and the enzyme tyrosine hydroxylase. Steroid control of releasing factor genes is consistent with classical neuroendocrine concepts of negative feedback. Steroid-induced plasticity of gene expression is sometimes in evidence, with the presence or absence of a particular steroid inducing expression of a neuropeptide gene in neurons that under other conditions do not express the gene. As a means of gaining some insight into the mechanism of action of steroid hormones, several groups have determined some of the neuropeptide profiles of neurons that contain receptors for steroid hormones. Marked heterogeneity is found, in that often only a subpopulation of phenotypically-similar neurons, even within a single brain area, contains receptors for a given steroid.

Regulation of fibronectin biosynthesis by dexamethasone, transforming growth factor beta, and cAMP in human cell lines

The regulation of fibronectin (FN) biosynthesis by dexamethasone (a synthetic glucocorticoid), forskolin (an activator of adenylate cyclase), and transforming growth factor beta (TGF-beta) was examined in six human cell lines. Dexamethasone treatment produced the largest increase in FN biosynthesis in the fibrosarcoma cell line, HT-1080 (approximately 45-fold). This seems to result from a dexamethasone-mediated increase in FN mRNA stability which increases the message half-life from approximately 11 to 26 h. The relative instability of FN mRNA in the fibrosarcoma (t1/2 11 h) compared to normal fibroblasts (70 h) appears to result from the particular transformed phenotype of the HT-1080 cells. Forskolin and TGF-beta increase the rate of FN gene transcription in most of the cell lines. These effects (four- to six-fold) occur rapidly and do not require protein synthesis in the responsive cell lines which include normal fibroblasts. However, in the fibrosarcoma (HT-1080), a surprisingly large induction (20-30-fold) is observed and this induction is different from that in the normal fibroblasts and the other cell lines in that both protein synthesis and a lag period are required. Synergism is seen with dexamethasone and either forskolin or TGF-beta in HT-1080 cells increasing the rate of FN biosynthesis approximately 200-fold to a level similar to normal fibroblasts. This seems to result from a combination of FN mRNA stabilization (dexamethasone) and increased transcription (forskolin and TGF-beta).