Nuclear translocation of cAMP-dependent protein kinase

Threat bias in mice with inactivating mutations of Prkar1a

Anxiety disorders are associated with abnormalities in the neural processing of threat-related stimuli. However, the neurobiological mechanisms underlying threat bias in anxiety are not well understood. We recently reported that a Prkar1a heterozygote (Prkar1a(+/-)) mouse with haploinsufficiency for the main regulatory subunit (R1α) of protein kinase A (PKA) exhibits an anxiety-like phenotype associated with increased cAMP signaling in the amygdala. Prkar1a(+/-) mice provide a novel model to test the direct effect of altered PKA expression and subsequent anxiety-like behavioral phenotype on the response to threat. We hypothesized that Prkar1a(+/-)mice would exhibit a bias in threat detection since increased amygdala activity during emotional stimuli is associated with a maladaptive response. We measured behavior and PKA activity in brain areas after exposure to predator or control odor exposure in male Prkar1a(+/-) and wild-type (WT) littermates. Indeed, there were significant differences in the behavioral response to threat detection; WT mice showed the expected response of decrease in exploratory behavior during predator vs. control odor exposure, while Prkar1a(+/-) mice did not alter their behavior between conditions. Basal and total PKA activity was independently associated with genotype, with an interaction between genotype and threat condition. Prkar1a(+/-) mice had higher PKA activity in amygdala and ventromedial hypothalamus in response to predator odor. In contrast, WT mice had higher PKA activity in amygdala and orbitofrontal cortex after exposure to control odor. Dysregulated PKA activity in the amygdala-prefrontal cortex circuitry in Prkar1a(+/-) mice is associated with behavioral phenotype of anxiety and a bias for threat. This is likely related to a failure to inhibit the amydgala response, which is an effect of the genotype. These results suggest that the alteration in PKA signaling in Prkar1a(+/-) mice is not ubiquitous in the brain; tissue-specific effects of the cAMP/PKA pathway are related to threat detection and fear sensitization.

Protein kinase A: regulation and receptor-mediated delivery of antisense oligonucleotides and cytotoxic drugs

Protein kinases help regulate eukaryotic cell division. We investigated the regulation of cAMP-dependent protein kinase A (PKA) and casein kinase (CK) type I activity in normal cells and in cancer. To assess this activity in biopsies we suggest a new parameter--the ratio of CK activity and total PKA activity divided by cAMP concentration: CK/PKA/cAMP. In 98 samples of colon mucosa in normal, inflamed, polyp, and adenocarcinoma cells, we found this parameter to be fairly constant in normal conditions and increased 10-fold in colon cancer; the ratio does not depend on the place of biopsy or the patient's age or sex. Experiments with model systems of concanavalin A-stimulated lymphocytes and regenerating rat liver showed that in normal cell proliferation the parameter increases 2-3-fold, as compared to a 30-fold increase in cancer. Unlike normal cells, malignant cells show CK activation and decrease of cAMP; therefore, PKA activity decreases. This suggests a correlation of CK and PKA activity and significant damage to their regulation at pathological changes of tissue proliferation. To further study concerted CK and PKA regulation we used monoclonal antibodies (mAbs) against cAMP-dependent protein kinase regulatory subunit RKII beta. We produced 11 antibodies in three groups: inhibiting, which block cAMP binding with RII beta and inhibit holoenzyme formation (RS6); activating, which enhance cAMP binding and do not affect holoenzyme formation (RS28); and neutral (RS17). To investigate mAb influence on protein kinase regulation in live cells we permeabilized pheochromocytoma PC12 by digitonin. When used at 5-microM concentration for 5 min, digitonin allowed us to deliver mAb into PC12 cells at 30-34-nM concentration, leaving 68-75% viable cells. Protein kinase activity was measured within 0.5 and 4 h after incorporation of mAbs into cells. After 30 min incorporation, mAb RS6 blocked PKA activation in PC12 cells under the influence of cAMP; other mAbs showed no effect. mAb RS6 caused a 4-fold increase of free C subunit activity 4 h after incorporation. mAb RS38 decreased R2C2 activity and did not influence C subunit activity. The change of free C subunit activity caused by mAb incorporation was followed by a synchronized, well-balanced change of CK type I activity, which suggests a correlation between the two phosphorylation systems of cell proteins.

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.

Modulation of nuclear cyclic AMP-dependent protein kinase in dibutyryl cyclic AMP-treated rat H4IIE hepatoma cells

Biochemical and immunochemical studies were undertaken to quantify the effects of cyclic AMP on cyclic AMP-dependent protein kinase subunit levels in nuclei of H4IIE hepatoma cells. Dibutyryl cyclic AMP (10 microM) caused a significant biphasic (10 and 120 min after stimulation) increase in total nuclear protein kinase activity. The increase observed 10 min after dibutyryl cyclic AMP stimulation was primarily due to an approx. 3-fold increase of catalytic (C) subunit activity, whereas the change observed 120 min after stimulation consisted of an increase in both C subunit and cyclic AMP-independent protein kinase activities. Analysis of nuclear protein extracts by photoaffinity labelling with 8-azido cyclic [32P]AMP identified only the type II regulatory subunit (RII), but not the type I regulatory subunit (RI). Analysis of nuclear RII variants by two-dimensional gel electrophoresis demonstrated that dibutyryl cyclic AMP caused the appearance of two RII variant forms which were not present in the nuclei of unstimulated cells. Using affinity-purified polyclonal antibodies and immunoblotting procedures, we identified an approx. 2-fold increase in the RII and C subunits in nuclear extracts of dibutyryl cyclic AMP-treated hepatoma cells. Finally, the RI, RII and C subunits were quantified by an e.l.i.s.a. which indicated that dibutyryl cyclic AMP increased nuclear RII and C subunits levels biphasically, reaching peak values 10 and 120 min after the initial stimulation. Nuclear RI subunit levels were not affected. These results provide qualitative as well as quantitative evidence for a modulation by cyclic AMP of the nuclear RII and C subunit levels in rat H4IIE hepatoma cells, and indicate a relatively rapid but temporarily limited dibutyryl cyclic AMP-induced translocation of the RII and C subunits to nuclear sites.

Site-selective cyclic AMP analogs as new biological tools in growth control, differentiation, and proto-oncogene regulation

The physiologic role of cyclic adenosine monophosphate (cAMP) in the growth control of a spectrum of human cancer lines, including leukemic lines, and v-rasH oncogene-transformed NIH/3T3 cells is demonstrated by the use of site-selective cAMP analogs. These cAMP analogs, which can select either of the two known cAMP binding sites of the cAMP receptor protein, induce potent growth inhibition, phenotypic change, and differentiation (leukemic cells) of cancer cells at micromolar concentrations with no sign of cytotoxicity. The growth inhibition parallels selective modulation of cAMP-dependent protein kinase isozymes, type I versus type II, and suppression of cellular proto-oncogene expression. Site-selective cAMP analogs thus provide new biological tools for investigating cell proliferation and differentiation and also for the improved management of human cancers.

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.

Nuclear events during early chondrogenesis: phosphorylation of the precartilage 35.5-kDa domain-specific chromatin protein and its regulation by cyclic AMP

During chondrogenesis in vivo and in vitro, a family of nonhistone proteins (Mr 35,500), designated PCP 35.5, is lost from the nuclei of precartilage mesenchyme cells. A basic subcomponent of this family, designated PCP 35.5b, is phosphorylated during the first few hours of chondrogenesis in vitro by a phosphorylating system whose activity is enhanced 12- to 15-fold by exposure of differentiating precartilage cells to dibutyryl cyclic AMP. This phosphorylating system is present in isolated precartilage cell nuclei, where it retains its dependence on cyclic AMP and its specificity for PCP 35.5b. Assays for nuclear cyclic AMP inhibitable protein phosphatase activity capable of dephosphorylating PCP 35.5b were negative, indicating that the system responsible for phosphorylating this protein is a cyclic AMP-dependent protein kinase. Chromatin fractionation studies indicate that PCP 35.5b is localized at sites previously shown to be closely associated with DNase I-sensitive domains of precartilage cell chromatin. These studies define PCP 35.5b as a strategically located component of precartilage cell chromatin which is the major or sole chromatin target of cyclic AMP-dependent phosphorylation during chondrogenesis. This chromatin modification occurs prior to overt cartilage differentiation and may therefore play a regulatory role in the acquisition of the cartilage cell phenotype.

Calcium, cyclic AMP and protein kinase C--partners in mitogenesis

Evidence is steadily mounting that the proto-oncogenes, whose products organize and start the programs that drive normal eukaryotic cells through their chromosome replication/mitosis cycles, are transiently stimulated by sequential signals from a multi-purpose, receptor-operated mechanism (consisting of internal surges of Ca2+ and bursts of protein kinase C activity resulting from phosphatidylinositol 4,5-bisphosphate breakdown and the opening of membrane Ca2+ channels induced by receptor-associated tyrosine-protein kinase activity) and bursts of cyclic AMP-dependent kinase activity. The bypassing or subversion of the receptor-operated Ca2+/phospholipid breakdown/protein kinase C signalling mechanism is probably the basis of the freeing of cell proliferation from external controls that characterizes all neoplastic transformations.

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 cDNA clones coding for the catalytic subunit of mouse cAMP-dependent protein kinase

mRNA coding for the catalytic (C) subunit of cAMP-dependent protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37) was partially purified from bovine testis by polysome immunoadsorption and oligo(dT)-chromatography. This enriched mRNA preparation was used to prepare and differentially screen a cDNA library. One of the selected cDNA clones was shown to hybrid-select mRNA coding for a 40-kDa protein that was specifically precipitated with antibodies to the C subunit. This bovine cDNA clone was then used to isolate a series of mouse cDNA clones that are complementary to the entire mouse C subunit mRNA. The mouse clones code for a protein of 351 amino acids that shows 98% homology to the bovine C subunit and hybridize to a single mRNA of 2.4 kilobases in mouse heart and brain. Southern blot analysis of total genomic DNA suggests that there is a single mouse gene coding for the C subunit. mRNA levels for both the C subunit and the type I regulatory subunit in various mouse tissues and cell lines were quantitated and compared by using single-stranded RNA probes prepared with SP6 polymerase.

[Mode of action of cyclic amp in prokaryotes and eukaryotes, CAP and cAMP-dependent protein kinases]

cAMP is an ubiquitous compound which is involved in the regulation of many biological processes. In bacteria such as E. coli, cAMP mediates the activation of catabolic operons via the CAP protein. The CAP-cAMP complex, whose tridimensional structure has recently been established, binds to the promoter regions of catabolic operons at a specific site, and activates their transcription by inducing RNA polymerase to bind and initiate transcription at the correct site. Various phenomenons including protein-protein interactions or CAP-induced DNA bending or kinking could be involved in the process of forming the open transcription complex. In eukaryotes, cAMP activates cAMP dependent protein kinases which covalently modify proteins by phosphorylation on serine or threonine residues. The catalytically inactive holoenzyme is generally a tetramer containing two regulatory subunits, each capable of binding two molecules of cAMP, and two catalytic subunits. In mammalian cells, two types of cAMP dependent protein kinases (I and II) can be distinguished on the basis of their regulatory subunits; their relative proportion varies from tissue to tissue. Binding of cAMP to the regulatory subunits induces the dissociation of the holoenzyme and releases the free and active catalytic subunits. Phosphorylation of proteins occurs at sequences containing two basic residues in the vicinity of the phosphorylated serine or threonine. A heat-stable protein, present in most eukaryotic cells, specifically interacts with the catalytic subunit and inhibits its activity. The amino-acid sequence of cAMP dependent protein kinases has recently been determined. It is interesting to note that the domains responsible for cAMP binding by the regulatory subunits of mammalian cAMP dependent protein kinases and CAP share important sequence homologies. The same phenomenon is observed concerning the domain responsible for ATP binding to the catalytic subunit of cAMP dependent protein kinases and that of tyrosine-specific protein kinases from oncoviruses. Other eukaryotic proteins such as S-adenosyl-L-homocysteine (SAH) hydrolase are also capable of binding cAMP. The latter is involved in the regulation of S-adenosyl-L-methionine dependent methylations, and its activity could be affected by cAMP. Besides its role as an effector of enzymatic activity via phosphorylation, such as in the regulation of glycogen metabolism, cAMP has recently been shown to activate the transcription of a number of eukaryotic genes. This process probably also involves protein phosphorylation, but its precise mechanism remains to be understood.

Differentiation-associated increase of cAMP-dependent type II protein kinase in a murine preadipose cell line (ST 13)

The activity of cAMP-dependent protein kinase and cAMP binding activity were studied during the differentiation of ST 13 murine preadipocytes into adipocytes. We found that both activities were marginally detectable in preadipose cells and increased remarkably when the cells were induced to differentiate, preceding by several days the morphological adipose conversion. The increased cAMP-dependent protein kinase was identified as type II enzyme by means of DEAE-Sephacel chromatography and by photoaffinity labeling with 8-azido[3H]cAMP. We further showed that the increase of protein kinase activity was specific to cell differentiation with the aid of modulators of the adipose conversion (insulin, fetal bovine serum, retinoic acid and 5-bromodeoxy-uridine). We propose that the increased expression of type II cAMP-dependent protein kinase would be a biochemical index of differentiation in ST 13 preadipocytes.

The intranuclear location of a herpes simplex virus DNA-binding protein is determined by the status of viral DNA replication

The herpes simplex viral DNA-binding protein, ICP8, is targeted to two different locations in the cell nucleus as part of its maturation pathway. Prior to viral DNA synthesis ICP8 was found at discrete pre-replicative sites throughout the nucleus, where it exhibited a high salt-labile association with the nuclear matrix. During viral DNA replication ICP8 was localized in randomly distributed replication compartments, where it is bound to viral DNA. Initiation of viral DNA replication caused the protein to move from the prereplicative sites to the replication compartments, while inhibition of replication caused movement in the opposite direction. In cells where viral DNA synthesis was proceeding, a minor population of ICP8 may also have been associated with the prereplicative sites. The prereplicative sites may serve as a nuclear reservoir for ICP8 not bound to replicating or progeny DNA.

Ornithine decarboxylase may be a multifunctional protein

Ornithine decarboxylase may undergo posttranslational modifications which alter its function. Both transamidation of glutamine residues in the enzyme catalyzed by TGase and phosphorylation of serine and threonine residues catalyzed by a polyamine-stimulated protein kinase have been demonstrated. Data are presented which suggest that these modifications result in translocation of the modified protein to the nucleolus where it regulates the activity of RNA polymerase I to transcribe rDNA, the only active nucleolar genes. Transamidation of specific proteins with primary amines catalyzed by intracellular TGase may be an important posttranslational modification, capable of altering genetic transcription. The rapid half-life of ODC (10-15 min) may be related to rapid posttranslational modification with loss of enzymatic activity rather than to protein degradation.