Induction of adenosine 3',5'-monophosphate binding proteins by N6,O2'-dibutyryladenosine 3',5'-monophosphate in mouse neuroblastoma cells. Analysis by two-dimensional gel electrophoresis

cAMP signalling mechanisms with aging in the Ceratitis capitata brain

Aging has been associated with alterations in protein phosphorylation. This study was undertaken to examine eventual changes in cAMP-dependent protein kinase (PKA) activity and enzyme regulatory subunit levels from the dipterous Ceratitis capitata brain with postmaturational aging and senescence. PKA activity was determined in cytosolic and membrane fractions of the C. capitata brain during the adult stage of the insect lifespan. PKA activity markedly increased at the first stages of the life of the fly both in cytosol and in membranes. A lower peak of PKA activity was evident both in particulate and cytosolic fractions in the terminal phase of the life of the fly. Thus, PKA activity was significantly higher in the brain of mature flies when compared to the brain of aged flies. It is possible that increases in cAMP-dependent protein phosphorylation levels characterize the terminal aging process in the insect nervous tissue. On the other hand, levels of regulatory (R) subunit were also measured in membranes and cytosol by immunoblotting. Cytosolic regulatory subunit levels were more elevated near the terminal phase of life, whereas in membranes, regulatory subunit levels decrease in senescence in parallel with particulate PKA activity. The increased R subunit level in cytosol may reflect a cellular response mechanisms to down-regulate the kinase system in aged flies.

The regulatory subunit of cAMP-dependent protein kinase as a target for chemotherapy of cancer and other cellular dysfunctional-related diseases

Three separate experimental approaches, using site-selective cAMP analogs, antisense strategy and retroviral vector-mediated gene transfer, have provided evidence that two isoforms, the RI- and RII-regulatory subunits of cAMP-dependent protein kinase, have opposite roles in cell growth and differentiation; RI being growth stimulatory while RII is a growth-inhibitory and differentiation-inducing protein. As RI expression is enhanced during chemical or viral carcinogenesis, in human cancer cell lines and in primary human tumors, it is a target for cancer diagnosis and therapy. 8-Cl-cAMP and RI antisense oligodeoxynucleotide, those that effectively down-regulate RI alpha and up-regulate RII beta, provide new approaches toward the treatment of cancer. This approach to modulation of RI vs RII cAMP transducers may also be beneficial toward therapy of endocrine or cellular dysfunction-related diseases where abnormal signal transduction of cAMP is critically involved.

Role of site-selective cAMP analogs in the control and reversal of malignancy

Two isoforms of cAMP receptor protein, RI and RII, the regulatory subunits of cAMP-dependent protein kinase, transduce opposite signals, the RI being stimulatory and the RII being inhibitory of cell proliferation. In normal cells RI and RII exist at a specific physiological ratio whereas in cancer cells such physiological balance of these receptor proteins is disrupted. Reversal and suppression of malignancy can be achieved when the physiologic ratio of these intracellular signal transducers of cAMP is restored as shown by the use of site-selective cAMP analogs, antisense oligodeoxynucleotides or gene transfer, suggesting new approaches to cancer control.

Neurite extension and increased expression of RI cyclic AMP-binding protein in ouabain-resistant neuroblastoma mutants

Morphological and biochemical parameters of neuroblastoma differentiation were assessed in 12 clonal derivatives of the N-18 mouse neuroblastoma cell line selected for their ouabain-resistant (ouar) property. When cultured in a normal growth medium, nine of the 12 ouar cell lines exhibited a more complex pattern of neurite outgrowth than the parental N-18 cells. The morphological pattern most frequently observed with the ouar cells was the extension of several branched processes per cell. This pattern of spontaneous neurite outgrowth in the ouar cell lines can be correlated with an increase in expression of the 47,000-dalton RI cyclic AMP (cAMP)-binding protein. The growth rate, intracellular level of cAMP, and acetylcholinesterase activity of the ouar cell lines were not significantly different from those of the parental N-18 neuroblastoma cells. Treatment of the parental and ouar neuroblastoma cell lines with 1 mM N6, O2-dibutyryl cAMP promoted an elaborate pattern of neurite outgrowth and marked increases in acetylcholinesterase and RI cAMP-binding activities. The distinctive pattern of differentiation phenotype exhibited by the ouar cells and the dibutyryl cAMP-induced differentiated neuroblastoma cell suggests that these two protocols yielded different degrees of differentiation. Furthermore, our results suggest a linkage of the biochemical events underlying ouabain resistance and expression of differentiation phenotypes in the mouse neuroblastoma cells.

Specificity of the action of cAMP agonists in the induction of RI cAMP-binding protein in mouse neuroblastoma cells

Differentiation in the mouse neuroblastoma cells is induced by cAMP and is characterized by neurite extension and increased acetylcholinesterase, cAMP-phosphodiesterase, and RI cAMP-binding activities. To gain a better understanding of the regulation of expression and the possible function of the RI cAMP-binding protein in neuroblastoma cell differentiation, we evaluate the specificity of action of cAMP analogues and agents that increased intracellular cAMP concentration in the induction of the 47,000-dalton RI protein. The amount of RI in cell extracts was quantitated by the photoactivated incorporation of 8-N3-[32P]cAMP into the 47,000-dalton RI and by ELISA and Western blot techniques. Our results showed that dibutyryl cAMP, forskolin, prostaglandin E1, 3-isobutyl-1-methyl xanthine, and papavarine gave a 2- to 4-fold increase in the RI cAMP-binding protein coincident with the expression of various morphological and biochemical differentiation phenotypes in the mouse neuroblastoma cells. However, the effects of 8-bromo-cAMP were different. 8-Bromo-cAMP effectively promoted neurite extension and increased acetylcholinesterase and cAMP-phosphodiesterase activities; however, there was no concomitant increase in the RI cAMP-binding protein. The result raises interesting questions concerning the coupling of expression of the various differentiation phenotypes in the mouse neuroblastoma cells.

Induction and quantitation of the RI cAMP-binding protein in clonal mouse neuroblastoma cell lines: evidence that the increase in RI is not linked to neurite outgrowth

This is a study of the regulation of expression of the RI cAMP-binding protein in mouse neuroblastoma cells as it relates to neurotransmitter phenotype and neurite outgrowth. Dibutyryl cAMP was used to promote differentiation of the cholinergic NS-20, the adrenergic N1E-115, the neurotransmitter-inactive N-18, and the neurite-minus N1A-103 mouse neuroblastoma cells. The amount of the RI cAMP-binding protein was quantitated by photoaffinity labeling of the 47,000-dalton RI protein with 8-N3-[32P]cAMP and by Western blot, ELISA, and immunocytochemistry. Our results showed that dibutyryl cAMP induced the RI cAMP-binding protein by three to fivefold in each of the four neuroblastoma cell lines examined. The increased expression of the RI cAMP-binding protein was not linked to neurite outgrowth, a parameter of morphological differentiation in the neuroblastoma cells. Thus, the RI cAMP-binding protein can be induced in the neurite-minus N1A-103 neuroblastoma round cells; further, 8-bromo-cAMP effected neurite outgrowth without inducing the RI cAMP-binding protein in the neurite-positive cell lines. Indirect immunocytochemistry of RI showed a cytoplasmic localization with little evidence of nuclear staining. The increase in RI cAMP-binding protein coincided with an increase in the cAMP-phosphodiesterase and a decrease in cAMP-dependent phosphotransferase activity in the mouse neuroblastoma cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Site-selective cAMP analogs induce nuclear translocation of the RII cAMP receptor protein in Ha-MuSV-transformed NIH/3T3 cells

Site-selective cAMP analogs, depending on the position of their substituents on the adenine ring, selectively bind to either site 1 or site 2 of the known cAMP binding sites of protein kinase. Treatment of Harvey murine sarcoma virus-transformed NIH/3T3 cells with such site-selective analogs results in growth inhibition and phenotypic reversion, and the combination of a C-8 thio or halogen analog (site 1 selective) with an N6 analog (site 2 selective) produces a synergistic effect. We report here that the growth inhibitory effect of the analogs correlates with the nuclear translocation of the RII cAMP receptor protein, the regulatory subunit of protein kinase type II. The transformed NIH/3T3 cells contained no detectable level of RII in the nucleus, whereas nontransformed NIH/3T3 cells exhibited a high level of nuclear RII. Within 30 min after treatment of the transformed cells with the site-selective analogs, immunofluorescence against the RII protein markedly increased in the cell nucleus. The nuclear translocation of the RII cAMP receptor protein is an early event in the reverse transformation of the fibroblasts treated with site-selective cAMP analogs.

Induction of the RI cAMP-binding protein in the "neurite-minus" N1A-103 mouse neuroblastoma cell line

We examined the effects of agents/conditions that raise intracellular cAMP concentration in the induction of the 47,000-dalton RI cAMP-binding protein in the "neurite-minus" N1A-103 mouse neuroblastoma cells. The amount of RI in cell extracts was quantitated by photoaffinity labeling with 8-N3-[32P]cAMP and by ELISA and Western blot. Our results showed that treatment of the N1A-103 neuroblastoma cells with 20 microM forskolin or 1 mM dibutyryl cAMP, or reducing the serum concentration in the culture medium from 10% to 1% caused a 3-4 fold increase in the amount of RJ. Our results suggest that the induction of RJ can occur independent of morphological differentiation in the mouse neuroblastoma cells.

The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity

Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.

Isolation of a mutant LLC-PK1 cell line defective in hormonal responsiveness. A pleiotropic lesion in receptor function

A mutant LLC-PK1 cell line, M18, was isolated after a single treatment of the parent culture with N-methyl-N'-nitro-N-nitroso-guanidine. In contrast to LLC-PK1 cells, the mutant did not exhibit production of urokinase-type plasminogen activator (uPA) in response to the hormones calcitonin and vasopressin, but produced the expected levels of uPA upon stimulation by the receptor-independent adenylate cyclase activators forskolin and cholera toxin, as well as by the phosphodiesterase inhibitor isobutylmethylxanthine and the 8-bromo analogue of adenosine cyclic monophosphate, Br8cAMP. The patterns of activation of cAMP-dependent protein kinase were identical to those of uPA induction: calcitonin and vasopressin were without effect, but the response to all other agents was normal. In similar fashion, mutant cell homogenates displayed normal activation of adenylate cyclase upon treatment with sodium fluoride, forskolin, or the non-hydrolyzable GTP analogue guanosine 5'-[beta, gamma-imino]triphosphate, but were unresponsive to calcitonin or vasopressin. The ability of M18 cells to bind radioactively labelled calcitonin and vasopressin was measured. The mutant possessed less than 4% of the normal levels of the receptor binding activity for both hormones. Somatic cell hybrids formed between M18 and LLC-PK1 cells were found to retain normal hormone binding activity and responsiveness to hormones, indicating that the defect in M18 cells was recessive. M18 was concluded most probably to contain a single mutation impairing the function of two distinct polypeptide hormone receptors.

High-efficiency cloning of DNA sequences complementary to mouse neuroblastoma polyadenylated RNA

A cDNA library was efficiently synthesized from mouse neuroblastoma poly(A)+RNA. Several modifications of the oligo(dC)(dG) tailing procedure were used. After first strand synthesis, a dATP tail was added to the 3'-end of the cDNA. The second strand was primed for synthesis with oligo(dT). Blunt ends were produced on the cDNA by treatment with S1 nuclease. Size-enriched fractions of high molecular weight DNAs were obtained by passing the cDNA over a Sepharose CL-4B column. The optimal tailing time for each cDNA fraction was individually tested. Tailing reactions used terminal deoxynucleotidyl transferase and annealing reactions used a (G)-tailed Pst I cut pBR322. E. coli K12 RR1 cells were transformed and 2.5-5 X 10(6) transformants per microgram cDNA insert were obtained for each size fraction. The transformants had an average insert size of 1200 base pairs and were 98% ampicillin sensitive. Our modifications in the method for cDNA library synthesis had 3 advantages. (1) Homopolymer-primed cDNA treated with S1 nuclease allowed the blunt ends to be tailed synchronously. This allowed a higher transformation efficiency without loss of 5'-sequences. (2) Time tailing determined the most efficient tail length and optimized the transformation efficiency in each size fraction. (3) A Sephadex G-50 mini-column was used to desalt and dry nitrogen was used to concentrate the ds cDNA instead of the usual ethanol precipitation. This resulted in almost 100% recovery of synthesized products at each step of this procedure.

Selective increase of R-I subunit of cyclic AMP-dependent protein kinase in glia-rich primary cultures upon treatment with dibutyryl cyclic AMP

Levels of cyclic GMP-dependent protein kinase and of the subunits (R-I, R-II and C) of cyclic AMP-dependent protein kinase were determined in two types of neural primary cell cultures that were either treated or not treated with dibutyryl cyclic AMP. Astroglia-rich cell cultures from newborn rat brain responded to exposure to dibutyryl cyclic AMP by a 2-3-fold increase in the level of R-I subunit, as demonstrated by two radioimmunological procedures, while the levels of the other subunits (R-II and C) and of cyclic GMP-dependent protein kinase remained unaffected. In contrast, neuron-rich cell cultures from embryonic rat brain did not display such a change in the level of R-I subunit. Thus, the elevation in the level of R-I elicited by dibutyryl cyclic AMP in normal non-malignant neural cells in culture was restricted to glial rather than neuronal cells.

Spermine specifically inhibits the phosphorylation of an 11,000-dalton nuclear protein in various cultured mammalian cell lines

The effect of polyamines (putrescine, spermidine and spermine) on endogenous protein phosphorylation in mouse neuroblastoma cells was investigated by using techniques of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The results indicated that spermine at 1mM completely inhibited the phosphorylation of the 11,000-dalton and 120,000-dalton proteins in nuclear fractions. The inhibition of the phosphorylation of the 11,000-dalton but not the 120,000-dalton protein by spermine was also observed in five other cell lines examined and appeared to be a general phenomenon. The inhibitory effect of spermine on the phosphorylation of the 11,000-dalton protein was specific, other cations such as ammonium chloride, arginine, putrescine, cyclen and trien were ineffective at equal molar or much higher concentrations.

Characterization of cyclic AMP-binding proteins in rat sertoli cells using a photoaffinity ligand

Protein-bound cyclic AMP (cAMP) levels in cultured rat Sertoli cells have been determined after exposure to follicle-stimulating hormone (FSH) and agents which elevate intracellular cAMP or mimic cAMP action. Changes in the content of protein-bound cAMP were correlated with changes in receptor availability determined by measuring [3H] cAMP binding. Using the photoaffinity analog of cAMP, 8-N3 [32P] cAMP, two major cAMP-binding proteins in Sertoli cell cytosol, with molecular weights of 47 000 and 53 000 daltons, were identified as regulatory subunits of type I and type II cAMP-dependent protein kinases, respectively. Densitometric analysis of autoradiograms demonstrated differential activation of the two isozymes in response to treatment with FSH and other agents. Results of this study demonstrate the value of measuring changes in protein-bound cAMP and the utility of the photoaffinity labeling technique in correlating hormone-dependent processes in which activation of cAMP-dependent protein kinase occurs.

Activation of cyclic AMP-dependent protein kinase isoenzymes: studies using specific antisera

A novel method for rapidly determining the amount and degree of association-dissociation of the Type I and Type II cAMP-dependent protein kinases has been developed and validated. Antibodies directed against the regulatory subunits of Type I and Type II cAMP-dependent protein kinases were used. The antibodies formed complexes with holoenzymes and regulatory subunits which were precipitated by goat anti-rabbit IgG (immunoglobulin G). These complexes bound [3H]cAMP with an apparent Kb of 20 nM for protein kinase I and 80 nM for protein kinase II. Immunoprecipitated protein kinases I and II were catalytically active when incubated with cAMP, [gamma-32P]ATP, and histone H2B. When mixtures of the two kinase isoenzymes or cytosol were incubated with various amounts of [3H]cAMP and the isoenzymes were separated by precipitation with antisera specific for each isoenzyme, the amount of [3H]cAMP associated with immunoprecipitates was proportional to the concentration of [3H]cAMP. In contrast, the catalytic activity that was immunoprecipitated varied inversely with the concentration of [3H]cAMP, showing that the activation of protein kinase could be assessed by the disappearance of catalytic activity from the immunoprecipitates. In the absence of MgATP protein kinase I was activated by a 10-fold lower concentration of cAMP than protein kinase II. However, when MgATP was added to the incubation, there was no significant difference in the binding of [3H]cAMP or dissociation of catalytic subunits of the two isoenzymes. The anti-R antibodies were also used to rapidly quantitate the concentration of regulatory subunits and the relative ratio of protein kinases I and II in tissue cytosols.

Calcium/calmodulin-dependent phosphorylation of vimentin in rat sertoli cells

Ca2+-dependent protein phosphorylation and the role of calmodulin in this process was investigated in subcellular fractions of primary cultures of rat Sertoli cells. Significant Ca2+/calmodulin-dependent protein phosphorylation in Sertoli cells was restricted to the cytosol fraction. The calmodulin dependence of these effects was confirmed by using the calmodulin inhibitor trifluoperazine. One of the Ca2+/calmodulin-dependent phosphoproteins was identified as the intermediate filament protein vimentin, based on the following criteria: (i) migration pattern in two-dimensional polyacrylamide gels, (ii) Ca2+/calmodulin-dependent phosphorylation of a 58-kilodalton protein present in detergent-insoluble intermediate filament protein extract of Sertoli cells, and (iii) peptide mapping of the phosphoprotein. These data support a role for Ca2+/calmodulin-dependent protein phosphorylation in the modulation of Sertoli cell cytoskeletal components.

Determination and comparative analysis of the catalytic subunit of adenosine 3',5'-cyclic phosphate-dependent protein kinase by an enzyme-linked immunosorbent assay

A specific antiserum against bovine heart catalytic subunit was used for the determination of the catalytic subunit in an enzyme-linked immunosorbent assay. Under the conditions elaborated the assay has a lower detection limit for catalytic subunit of 0.25 pmol/ml. In crude bovine heart extracts the concentration of catalytic subunit was determined by this method to be 0.18 +/- 0.02 mumol/kg wet wt. The immunochemical comparison of various animal species and cells, including organisms like amoebae and yeast, shows the broad applicability of the assay and provides evidence that the catalytic subunit is a highly conserved molecule.

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.

Specific nuclear binding of adenosine 3',5'-monophosphate-binding protein complex with subsequent poly(A) RNA synthesis in embryonic chick cartilage

We used embryonic chick pelvic cartilage as a model to study the mechanism by which cyclic AMP increases RNA synthesis. Isolated nuclei were incubated with [32P]-8-azidoadenosine 3,5'-monophosphate ([32P]N3cAMP) with no resultant specific nuclear binding. However, in the presence of cytosol proteins, nuclear binding of [32P]N3cAMP was demonstrable that was specific, time dependent, and dependent on a heat-labile cytosol factor. The possible biological significance of the nuclear binding of the cyclic AMP-protein complex was identified by incubating isolating nuclei with either cyclic AMP or cytosol cyclic AMP-binding proteins prepared by batch elution DEAE cellulose chromatography (DEAE peak cytosol protein), or both, in the presence of cold nucleotides and [3H]uridine 5'-triphosphate. Poly(A) RNA production occurred only in nuclei incubated with cyclic AMP and the DEAE peak cytosol protein preparation. Actinomycin D inhibited the incorporation of [3H]uridine 5'-monophosphate into poly(A) RNA. The newly synthesized poly(A) RNA had a sedimentation constant of 23S. Characterization of the cytosol cyclic AMP binding proteins using [32P]N3-cAMP with photoaffinity labeling three major cAMP-binding complexes (41,000, 51,000, and 55,000 daltons). The 51,000 and 55,000 dalton cyclic AMP binding proteins were further purified by DNA-cellulose chromatography. In the presence of cyclic AMP they stimulated poly(A) RNA synthesis in isolated nuclei. The 51,000-dalton cyclic AMP-binding protein was the predominant one that bound to the nuclei. While cyclic AMP-dependent protein kinsae activity was present in the cytosol and DEAE peak cytosol proteins, it was not present in the DNA-cellulose-bound, cyclic AMP-binding proteins. We conclude that one possible mechanism by which cyclic AMP increases RNA synthesis is by complexing to a 51,000-dalton cytosol cyclic AMP-binding protein and being subsequently translocated to the nucleus, where it is specifically bound and associated with induction of poly(A) RNA synthesis.

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.