Dibutyryl cAMP-induced protein changes in differentiating mouse neuroblastoma cells

Expression of calmodulin-dependent phosphodiesterase, calmodulin-dependent protein phosphatase, and other calmodulin-binding proteins in human SMS-KCNR neuroblastoma cells

Calmodulin (CaM)-dependent enzymes, such as CaM-dependent phosphodiesterase (CaM-PDE), CaM-dependent protein phosphatase (CN), and CaM-dependent protein kinase II (CaM kinase II), are found in high concentrations in differentiated mammalian neurons. In order to determine whether neuroblastoma cells express these CaM-dependent enzymes as a consequence of cellular differentiation, a series of experiments was performed on human SMS-KCNR neuroblastoma cells; these cells morphologically differentiate in response to retinoic acid and phorbol esters [12-O-tetradecanoylphorbol 13-acetate (TPA)]. Using biotinylated CaM overlay procedures, immunoblotting, and protein phosphorylation assays, we found that SMS-KCNR cells expressed CN and CaM-PDE, but did not appear to have other neuronal CaM-binding proteins. Exposure to retinoic acid, TPA, or conditioned media from human HTB-14 glioma cells did not markedly alter the expression of CaM-binding proteins; 21-day treatment with retinoic acid, however, did induce expression of novel CaM-binding proteins of 74 and 76 kilodaltons. Using affinity-purified polyclonal antibodies, CaM-PDE immunoreactivity was detected as a 75-kilodalton peptide in undifferentiated cells, but as a 61-kilodalton peptide in differentiated cells. CaM kinase II activity and subunit autophosphorylation was not evident in either undifferentiated or neurite-bearing cells; however, CaM-dependent phosphatase activity was seen. Immunoblot analysis with affinity-purified antibodies against CN indicated that this enzyme was present in SMS-KCNR cells regardless of their state of differentiation. Although SMS-KCNR cells did not show a complete pattern of neuronal CaM-binding proteins, particularly because CaM kinase II activity was lacking, they may be useful models for examination of CaM-PDE and CN expression. It is possible that CaM-dependent enzymes can be used as sensitive markers for terminal neuronal differentiation.

Development of sodium channel protein during chemically induced differentiation of neuroblastoma cells

We have previously shown that the [3H]saxitoxin binding site of the sodium channel is expressed independently of the [125I]scorpion toxin binding site in chick muscle cultures and in rat brain. In the present work, we studied the development of the sodium channel protein during chemically induced differentiation of N1E-115 neuroblastoma cells, using [3H]saxitoxin binding, [125I]scorpion toxin binding, and 22Na uptake techniques. When grown in their normal culture medium, these cells are mostly undifferentiated, bind 90 +/- 10 fmol of [3H]saxitoxin/mg of protein and 112 +/- 14 fmol of [125I]scorpion toxin/mg protein, and, when stimulated with scorpion toxin and batrachotoxin, take up 70 +/- 5 nmol of 22Na/min/mg of protein. Cells treated with dimethyl sulfoxide (DMSO) or hexamethylene-bis-acetamide (HMBA) differentiate morphologically within 3 days. At this time, the [3H]saxitoxin binding, the [125I]scorpion toxin binding, and the 22Na uptake values are not very different from those of undifferentiated cells. With subsequent time in DMSO or HMBA, these values continue to increase, a result indicating that the main period of sodium channel expression occurs well after the cells have assumed the morphologically differentiated state. The data indicate that the expression of sodium channels and morphological differentiation are independently regulated neuronal properties, that the attainment of morphological differentiation is necessary but not in itself sufficient for full expression of the sodium channel proteins, and that, in contrast to the chick muscle cultures and rat brain, the [3H]saxitoxin site and [125I]scorpion toxin site appear to be coregulated in N1E-115 cells.

Changes in synthesis of concanavalin A binding proteins during nerve cell differentiation

Changes in the synthesis of concanavalin A binding proteins were analyzed at various times during nerve cell differentiation. Both mouse neuroblastoma cells as well as primary cells isolated from chicken dorsal root ganglia were used in these studies. Conditions were optimized for the biochemical differentiation of these cells, which were pulsed with radioactive methionine at several times during differentiation. Soluble extracts were prepared and optimum conditions for the isolation of those proteins which bind the lectin concanavalin A were determined. Incorporation of radioactivity into specific concanavalin A binding proteins was analyzed by two-dimensional gel electrophoresis and by computerized densitometry of the autoradiographs. Our results show that 10 concanavalin binding proteins are made only in differentiating neuroblastoma cells, and that twelve such proteins are made only in differentiating dorsal root ganglia cells. In addition, the synthesis of several other concanavalin A binding proteins increases dramatically during nerve cell differentiation. This class of proteins is now being further characterized by biochemical and immunological techniques.

Cyclic nucleotides affect growth and differentiation of cultured rat embryonic shields

The modified organ culture of rat embryonic shields provides favorable conditions during 2 weeks for the differentiation of main tissue types. Since the terminal differentiation in explants is inferior to that obtained in the homografts of the same shields under the kidney capsule, we tried to improve the culture medium by adding some known regulatory molecules: db-cAMP, db-cGMP, ATP, AMP, and butyric acid. These agents were added to the liquid medium in the concentration of 1 mM. In the first part of the study the explants were fixed and weighed after 8 or 14 days in vitro culture, and histological sections were examined. When the explants were treated with db-cAMP during the second week of culture, the skeletal muscle appeared more frequently in the treated series than in controls, and the weight of the treated explants was sometimes increased when compared with the control series. The db-cGMP had no effect on differentiation, but stimulated the growth of the explants when applied during the first week of culture. On the contrary, the db-cAMP when added during the first week, severely impeded the growth of explants. Other agents seem to be ineffective. In the second part, the content of cAMP and cGMP was measured in normal explants. The radioimmunoassay showed the same content of cAMP and cGMP during the entire culture period. In the third part of our study the incorporation of tritiated uridine and tritiated thymidine was measured during the second week of culture after the addition of db-cAMP. During the first days of treatment with db-cAMP the uptake of tritiated uridine and thymidine was inhibited, whereas on the seventh day the uptake was similar to that of the control. We can conclude that both cyclic nucleotides have a visible effect on growth whereas only cAMP has a positive impact on the differentiation of myotubes in cultured rat embryonic shields.

Modulation of synapse formation by cyclic adenosine monophosphate

Synapses between neuroblastoma-hybrid cells and myotubes exhibit a high degree of plasticity. Increase of cyclic adenosine monophosphate (AMP) levels of the hybrid cells for several days results in the appearance of functional voltage-sensitive Ca2+ channels, which are required for evoked secretion of acetylcholine. The results show that cyclic AMP regulates synaptogenesis by regulating the expression of voltage-sensitive Ca2+ channels, and suggest that cyclic AMP affects posttranslational modifications of some glycoproteins and cellular levels of certain proteins.

Protein synthesis by astrocytes in primary cultures

Protein synthesis, measured as leucine incorporation into acid-precipitable proteins, was determined in astrocytes in primary cultures obtained from the cerebral hemispheres of newborn mice. As can be expected for eucaryotic, ribosomal protein synthesis, the incorporation was almost completely inhibited by cycloheximide (0.01 mM), but unaffected by chloramphenicol (0.03 mM). The rate of synthesis, measured during exposure to a high (0.8 mM) concentration of leucine was 5.4 nmol/hr/mg protein in mature (i.e., at least 4-week-old) cultures. This value is at least twice as high as the protein synthesis rates reported for the adult brain in vivo, suggesting that a very considerable part of the protein synthesis in the adult brain may take place in astrocytes. The molecular weight distribution of the synthesized proteins was determined by polyacrylamide gel electrophoresis, demonstrating synthesis of at least 50 different polypeptides, ranging in molecular weight between 190,000 and 27,000 daltons. The pattern of the synthesized proteins underwent considerable alteration with age in young cultures in which the total content of protein was still increasing, but it was remarkably stable after the age of two weeks. Exposure to dibutyryl cyclic AMP, which is known to alter morphology, content of glial fibrillary acidic protein (GFA), and activities of certain enzymes in the cultures in the cultured astrocytes, caused marked alterations in the pattern of the synthesized proteins.

Cytochemical localization of adenylate-cyclase in embryonic mouse molars

The cytochemical method of HOWELL and WHITFIELD (1972) and the revised cytochemical medium of KEMPEN et al. (1978) has been used to investigate the localization of adenylate-cyclase activity in 18, 19 and 20 day old first embryonic mandibular molars. Membrane associated precipitate granules were observed in preameloblasts and odontoblasts localized in the area where preodontoblasts become post-mitotic. Heat inactived teeth and the samples incubated in media without substrate were negative for membrane bound lead deposits. A correlation may exist between high adenylate-cyclase activity and the terminal differentiation of odontoblasts.

Changes in RNA synthesis and messenger RNA content in the cerebellum of rats with graft versus host disease

Cerebellar RNA accumulation, synthesis, and functional capacity was studied in 14-day-old F1 hybrid rats subjected to neonatally induced graft versus host disease (GVHD). There was a decrease in RNA synthetic rate as measured by the uptake of labeled precursors into RNA. The decrease in total cerebellar RNA synthesis was reflected both in a reduced amount of Nissl substance, visible in cresyl violet-stained 10-micron-thick sections of cerebella, and in the total amount of cytoplasmic RNA isolated from individual cerebella from diseased animals compared with control littermates. Analysis of the RNA translational capacity in wheat germ protein synthesizing systems showed that RNA from experimental animals was also biologically less active. Qualitative differences between protein populations in control and diseased animals were analyzed by two-dimensional gel electrophoresis. There were few alterations in the steady state levels of cerebellar protein. However, two-dimensional gel electrophoresis of the peptides synthesized in vitro by RNA from control and diseased animals showed that there were several changes in the relative abundance of some mRNAs between the two RNA populations. These data show that the cerebellar RNA from rats with GVHD differs both qualitatively and quantitatively from that of controls.

Phosphorylation of endogenous proteins by adenosine 3':5'-monophosphate-dependent protein kinase in mouse neuroblastoma cells

Increased intracellular adenosine 3':5'-monophosphate (cAMP) levels and activation of cAMP-dependent protein kinases (ATP:protein phosphotransferase, EC 2.7.1.37) in vivo were correlated in mouse neuroblastoma cells grown in the presence of 1 mM-6N,O2'-dibutyryl 3':5'-monophosphate (Bt2cAMP). The time course for activation showed that cAMP-dependent protein kinases were activated by 30 min. A heat-stable inhibitor protein inhibited a majority of activated cAMP-dependent protein kinase. Activation of cAMP-dependent protein kinase caused additional phosphorylation of proteins when compared with untreated control cells, as demonstrated by endogenous phosphorylation of proteins in vitro using [gamma-32P]ATP and analysis by two-dimensional polyacrylamide gel electrophoresis. The phosphorylation data show selective phosphorylation of specific proteins by cAMP-independent and cAMP-dependent protein kinase. Among the proteins in the postmitochondrial supernatant fraction phosphorylated by cAMP-dependent protein kinases, two proteins with a molecular weight of 43,000 were heavily phosphorylated. It is suggested that phosphorylation of cellular proteins by cAMP-dependent protein kinases might be involved in the cAMP-modulated biochemical changes in neuroblastoma cells.

Relative increase in polysomal mRNA for R1 cAMP-binding protein in neuroblastoma cells treated with 1,N6-dibutyryl-adenosine 3',-5'-phosphate

Polysomal RNAs were isolated from control neuroblastoma cells and those treated with 1,N6-dibutyrl-adenosine 3',5'-phosphate (Bt2cAMP) and translated in wheat germ lysates. Comparison of proteins synthesized in vitro on two-dimensional gel electrophoretograms showed that there was a specific induction in the synthesis of a protein, Mr 48000, by the polysomal RNAs from Bt2cAMP-treated cells. This protein was identified as the R1 cAMP-binding protein by its coelectrophoresis with unlabelled binding protein and by its specific retention on 8-(6-aminohexylamino)-adenosine 3',5'-phosphate linked to Sepharose. Quantification of the proteins synthesized in vitro with subsaturating inputs of polysomal RNAs showed that there was a 1.4--1.7-fold increase in the synthesis of the R1 cAMP-binding protein by polysomal RNAs isolated from Bt2cAMP-treated cells. There was a similar increase when purified polyadenylated mRNA populations were compared. showing there was no change in the ratio of adenylated to nonadenylated mRNAs in the induced mRNA population. There was no corresponding increase in the synthesis of the R2 cAMP-binding protein although the relative synthesis of several other proteins was also increased and the synthesis of actin and the alpha and beta-tubulin subunits was decreased. The increased levels of the R1 cAMP-binding protein found in Bt2cAMP-treated neuroblastoma cells are therefore partly caused by a specific accumulation of its mRNA on polysomes. The mRNA content of the cytoplasmic messenger ribonucleoprotein (mRNP) population of control cells was insufficient to account for this increase by a translocation of R1 mRNA from the mRNP to the polysome fraction in Bt2cAMP-treated cells. The increase in polysomal R1 mRNA is therefore caused by its increased transcription of post-transcriptional processing or its decreased rate of degradation in Bt2cAMP-treated cells. Although the R1 and R2 binding proteins have identical molecular weights and similar pI values, the specific induction of the mRNA for R1 cAMP-binding protein and the differential distribution of the R1 and R2 mRNAs between the polysomal and messenger ribonucleoprotein compartments show that these two cAMP-binding proteins are encoded by different mRNA populations.

Phosphorus 31 NMR of neuroblastoma clonal lines. Effect of cell confluency state and dibutyryl cyclic AMP

Phosphorus nuclear magnetic resonance is used to study changes in the levels of the major phosphate-containing intermediary metabolites concomitant with induced cellular differentiation in the N-18 and C-46 neuroblastoma clonal lines. The study reveals differences between the 31P-NMR profiles of the two clonal lines and also striking differences attendant to dibutyryl cAMP-mediated morphological differentiation in the N-18 clone. Phosphorus-31 NMR would appear to provide a new technique with which to study genetic differentiation.

Complexity of polysomal polyadenylated RNA in undifferentiated and differentiated neuroblastoma cells

Using cDNA probes, we have analysed the sequence complexity and the frequency distribution of the polysomal poly(A)-containing RNA from neuroblastoma cells at two different developmental states: either as round, immature neuroblasts, or as differentiated cells exhibiting the morphological properties of mature neurons. The total complexities measured for mRNA from undifferentiated and differentiated cells are identical and correspond to approximately 7000 average-sized sequences of 1750 nucleotides distributed in the same three abundance classes. We have determined the homology between the mRNA populations corresponding to the two developmental states by heterologous cross-hybridization: all the sequence from differentiated cells are present in the polysomes of undifferentiated cells. Conversely, the mRNA from differentiated cells fails to hybridize with about 15% of hybridizable cDNA corresponding to undifferentiated cells. This difference probably results from the disappearance of some mRNA species and may be related to the terminal differentiation of neuroblastoma cells.

Selective changes in the phosphorylation of endogenous proteins in subcellular fractions from cyclic AMP-induced differentiated neuroblastoma cells

The endogenous phosphorylation of specific proteins was studied in subcellular fractions from proliferating and cAMP-induced differentiated neuroblastoma cells. Fractions containing nuclear, membrane-bound, and cytosolic proteins were incubated with [gamma-32P]ATP, in the presence and absence of added cyclic nucleotides. Phosphate incorporation into specific proteins was determined by slab-gel electrophoresis of sodium dodecyl sulfate-solubilized reaction products. Cytosol fractions from differentiated cells demonstrated a twofold increase in cAMP-dependent phosphorylation of a specific protein with apparent mol wt of 59,000 daltons and a comparable decrease in cAMP-independent phosphorylation of another protein (97,000). The nuclear fraction of differentiated cells showed an increase in the cAMP-independent phosphorylation of two nonhistone proteins (110,000 and 102,000). Membrane fractions from differentiated cells exhibited a differential decrease in endogenous phosphorylation of specific proteins. Selective alterations in the phosphorylation of specific proteins in various subcellular components may be important biochemical events associated with the increased levels of differentiated functions in neuroblastoma cells in culture.

Induction of cyclic AMP-binding proteins by dibutyryl cyclic AMP in mouse neuroblastoma cells

Mouse neuroblastoma cells grown in the presence of 1 mM N6,O2'-dibutyryl-cyclic AMP showed a 3-fold increase in cyclic AMP-binding proteins. The role of dibutyryl cyclic AMP in the introduction of cyclic AMP-binding proteins in these cells has been studied. Induced cyclic AMP-binding proteins were observed in the cytoplasm 15 h after dibutyryl cyclic AMP treatment. The increase in cyclic AMP-binding proteins required RNA and protein synthesis. It is suggested that the 15-h lag occurs at the post-transcriptional and/or translational level. Cyclic AMP-binding proteins are found in both soluble and particulate cell fractions. Dibutyryl cyclic AMP increased binding proteins in both fractions. The control and dibutyryl cyclic AMP-induced binding proteins showed similar affinity for cyclic AMP. The data indicate that dibutyryl cyclic AMP caused the following sequential events: a 12-fold increase in cyclic AMP levels; a 40% increase in phosphodiesterase activity; and a 300% (3-fold) increase in cyclic AMP-binding proteins. It is suggested that the differentiation of mouse neuroblastoma cells involves increased levels of cyclic AMP and cyclic AMP-binding proteins.