Point mutation of the autophosphorylation site or in the nuclear location signal causes protein kinase A RII beta regulatory subunit to lose its ability to revert transformed fibroblasts

cAMP-PKA/EPAC signaling and cancer: the interplay in tumor microenvironment

Cancer is a complex disease resulting from abnormal cell growth that is induced by a number of genetic and environmental factors. The tumor microenvironment (TME), which involves extracellular matrix, cancer-associated fibroblasts (CAF), tumor-infiltrating immune cells and angiogenesis, plays a critical role in tumor progression. Cyclic adenosine monophosphate (cAMP) is a second messenger that has pleiotropic effects on the TME. The downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), exchange protein activated by cAMP (EPAC) and ion channels. While cAMP can activate PKA or EPAC and promote cancer cell growth, it can also inhibit cell proliferation and survival in context- and cancer type-dependent manner. Tumor-associated stromal cells, such as CAF and immune cells, can release cytokines and growth factors that either stimulate or inhibit cAMP production within the TME. Recent studies have shown that targeting cAMP signaling in the TME has therapeutic benefits in cancer. Small-molecule agents that inhibit adenylate cyclase and PKA have been shown to inhibit tumor growth. In addition, cAMP-elevating agents, such as forskolin, can not only induce cancer cell death, but also directly inhibit cell proliferation in some cancer types. In this review, we summarize current understanding of cAMP signaling in cancer biology and immunology and discuss the basis for its context-dependent dual role in oncogenesis. Understanding the precise mechanisms by which cAMP and the TME interact in cancer will be critical for the development of effective therapies. Future studies aimed at investigating the cAMP-cancer axis and its regulation in the TME may provide new insights into the underlying mechanisms of tumorigenesis and lead to the development of novel therapeutic strategies.

PKA Cα subunit mutation triggers caspase-dependent RIIβ subunit degradation via Ser114 phosphorylation

Mutations in the PRKACA gene are the most frequent cause of cortisol-producing adrenocortical adenomas leading to Cushing's syndrome. PRKACA encodes for the catalytic subunit α of protein kinase A (PKA). We already showed that PRKACA mutations lead to impairment of regulatory (R) subunit binding. Furthermore, PRKACA mutations are associated with reduced RIIβ protein levels; however, the mechanisms leading to reduced RIIβ levels are presently unknown. Here, we investigate the effects of the most frequent PRKACA mutation, L206R, on regulatory subunit stability. We find that Ser¹¹⁴ phosphorylation of RIIβ is required for its degradation, mediated by caspase 16. Last, we show that the resulting reduction in RIIβ protein levels leads to increased cortisol secretion in adrenocortical cells. These findings reveal the molecular mechanisms and pathophysiological relevance of the R subunit degradation caused by PRKACA mutations, adding another dimension to the deregulation of PKA signaling caused by PRKACA mutations in adrenal Cushing's syndrome.

PDE10 inhibition increases GluA1 and CREB phosphorylation and improves spatial and recognition memories in a Huntington's disease mouse model

Huntington's disease (HD) causes motor disturbances, preceded by cognitive impairment, in patients and mouse models. We showed that increased hippocampal cAMP-dependent protein kinase (PKA) signaling disrupts recognition and spatial memories in R6 HD mouse models. However, unchanged levels of hippocampal phosphorylated (p) cAMP-responsive element-binding protein (CREB) suggested unaltered nuclear PKA activity in R6 mice. Here, we extend this finding by showing that nuclear pPKA catalytic subunit (Thr197) and pPKA substrate levels were unaltered in the hippocampus of R6/1 mice. Phosphodiesterases (PDEs) play an important role in the regulation of PKA activity. PDE10A, a cAMP/cGMP dual-substrate PDE, was reported to be restricted to the nuclear region in nonstriatal neurons. Using cell fractionation we confirmed that PDE10A was enriched in nuclear fractions, both in wild-type and R6/1 mice hippocampus, without differences in its levels or intracellular distribution between genotypes. We next investigated whether inhibition of PDE10 with papaverine could improve cognitive function in HD mice. Papaverine treatment improved spatial and object recognition memories in R6/1 mice, and significantly increased pGluA1 and pCREB levels in R6/1 mice hippocampus. Papaverine likely acted through the activation of the PKA pathway as the phosphorylation level of distinct cGMP-dependent kinase (cGK) substrates was not modified in either genotype. Moreover, hippocampal cAMP, but not cGMP, levels were increased after acute papaverine injection. Our results show that inhibition of PDE10 improves cognition in R6 mice, at least in part through increased GluA1 and CREB phosphorylation. Thus, PDE10 might be a good therapeutic target to improve cognitive impairment in HD.

mTORC1 inhibition via rapamycin promotes triacylglycerol lipolysis and release of free fatty acids in 3T3-L1 adipocytes

Signaling by mTOR complex 1 (mTORC1) promotes anabolic cellular processes in response to growth factors, nutrients, and hormonal cues. Numerous clinical trials employing the mTORC1 inhibitor rapamycin (aka sirolimus) to immuno-suppress patients following organ transplantation have documented the development of hypertriglyceridemia and elevated serum free fatty acids (FFA). We therefore investigated the cellular role of mTORC1 in control of triacylglycerol (TAG) metabolism using cultured murine 3T3-L1 adipocytes. We found that treatment of adipocytes with rapamycin reduced insulin-stimulated TAG storage ~50%. To determine whether rapamycin reduces TAG storage by upregulating lipolytic rate, we treated adipocytes in the absence and presence of rapamycin and isoproterenol, a β2-adrenergic agonist that activates the cAMP/protein kinase A (PKA) pathway to promote lipolysis. We found that rapamycin augmented isoproterenol-induced lipolysis without altering cAMP levels. Rapamycin enhanced the isoproterenol-stimulated phosphorylation of hormone sensitive lipase (HSL) on Ser-563 (a PKA site), but had no effect on the phosphorylation of HSL S565 (an AMPK site). Additionally, rapamycin did not affect the isoproterenol-mediated phosphorylation of perilipin, a protein that coats the lipid droplet to initiate lipolysis upon phosphorylation by PKA. These data demonstrate that inhibition of mTORC1 signaling synergizes with the β-adrenergic-cAMP/PKA pathway to augment phosphorylation of HSL to promote hormone-induced lipolysis. Moreover, they reveal a novel metabolic function for mTORC1; mTORC1 signaling suppresses lipolysis, thus augmenting TAG storage.

Detection of protein biomarkers using high-throughput immunoblotting following focal ischemic or penetrating ballistic-like brain injuries in rats

PRIMARY OBJECTIVE

Recent efforts have been aimed at developing a panel of protein biomarkers for the diagnosis/prognosis of the neurological damage associated with acute brain injury.

METHODS AND PROCEDURES

This study utilized high-throughput immunoblotting (HTPI) technology to compare changes between two animal models of acute brain injury: penetrating ballistic-like brain injury (PBBI) which mimics the injury created by a gunshot wound and transient middle cerebral artery occlusion (MCAo) which is a model of stroke. Brain and blood were collected at 24-hours post-injury.

MAIN OUTCOMES AND RESULTS

This study identified the changes in 18 proteins following PBBI and 17 proteins following MCAo out of a total of 998 screened proteins. Distinct differences were observed between the two models: five proteins were up- or down-regulated in both models, 23 proteins changed in only one model and one protein was differentially expressed. Western blots were used to verify HTPI results for selected proteins with measurable changes observed in both blood and brain for the proteins STAT3, Tau, PKA RII beta, 14-3-3 epsilon and p43/EMAPII.

CONCLUSIONS

These results suggest distinct post-injury protein profiles between brain injury types (traumatic vs. ischemic) that will facilitate strategies aimed at the differential diagnosis and prognosis of acute brain injury.

Rap1 and p38 MAPK mediate 8-chloro-cAMP-induced growth inhibition in mouse fibroblast DT cells

8-Cl-cAMP, which is known to induce differentiation, growth inhibition, and apoptosis in various cancer cells, has been investigated as a putative anti-cancer drug. Previously, we reported that 8-Cl-cAMP and its metabolite 8-Cl-adenosine induce growth inhibition and apoptosis through p38 mitogen-activated protein kinase (MAPK) activation. To further investigate the signal mechanisms that regulate the cellular effects of 8-Cl-cAMP, we focused on a small GTPase Rap1 that is known to be involved in growth inhibition and reverse-transformation. 8-Cl-cAMP and 8-Cl-adenosine could increase Rap1 activity, which was blocked by ABT702-an adenosine kinase inhibitor. This suggests that 8-Cl-cAMP-induced Rap1 activation is also dependent on the metabolic degradation of 8-Cl-cAMP. Overexpression of a constitutively active mutant form of Rap1 (Rap1V12) attenuated cellular growth and soft-agar colony formation, which was basically the same effect as that observed with the 8-Cl-cAMP treatment. Furthermore, the Rap1V12 transfectant showed a high level of p38 MAPK activation. However, 8-Cl-cAMP-induced Rap1 activation was not diminished by SB203580, a p38 MAPK inhibitor, suggesting that Rap1 activation might act upstream of p38 MAPK activation during 8-Cl-cAMP-induced growth inhibition.

Phosphoproteomic analysis of synaptosomes from human cerebral cortex

Protein phosphorylation is a crucial post-translational modification mechanism in the regulation of synaptic organization and function. Here, we analyzed synaptosome fractions from human cerebral cortex obtained during therapeutic surgery. To minimize changes in the phosphorylation state of proteins, the tissue was homogenized within two minutes of excision. Synaptosomal proteins were digested with trypsin and phosphopeptides were isolated by immobilized metal affinity chromatography and analyzed by liquid chromatography and tandem mass spectrometry. The method allowed the detection of residues on synaptic proteins that were presumably phosphorylated in the intact cell, including synapsin 1, syntaxin 1, and SNIP, PSD-93, NCAM, GABA-B receptor, chaperone molecules, and protein kinases. Some of the residues identified are the same or homologous to sites that had been previously described to be phosphorylated in mammals whereas others appear to be novel sites which, to our knowledge, have not been reported previously. The study shows that new phosphoproteomic strategies can be used to analyze subcellular fractions from small amounts of tissue for the identification of phosphorylated residues for research and potentially for diagnostic purposes.

Protein kinase A isozyme switching: eliciting differential cAMP signaling and tumor reversion

The cAMP-dependent protein kinase types I (PKA-I) and II (PKA-II), composed of identical catalytic (C) subunits but distinct regulatory (R) subunits (RI versus RII), are expressed in a balance of cell growth and differentiation. Distortion of this balance may underlie tumorigenesis and tumor growth. Here, we used PC3M prostate carcinoma cells as a model to overexpress wild type and mutant R and C subunit genes and examined the effects of differential expression of these genes on tumor growth. Only the RIIbeta and mutant RIalpha-P (a functional mimic of RIIbeta) transfectants exhibited growth inhibition in vitro, reverted phenotype, and apoptosis, and inhibited in vivo tumor growth. DNA microarrays demonstrated that RIIbeta and RIalpha-P overexpression upregulated a cluster of differentiation genes, while downregulating transformation and proliferation signatures. Overexpression of RIalpha and Calpha, which upregulated PKA-I, elicited the expression signatures opposite that elicited by RIIbeta overexpression. Total colocalization of Calpha and RIIbeta seen by confocal microscopy in the RIIbeta cell nucleus supports the opposed genomic regulation demonstrated between Calpha and RIIbeta cells. Differential expression of PKA R subunits may therefore serve as a tumor-target-based gene therapy for PC3M prostate and other cancers.

Minireview: PRKAR1A: normal and abnormal functions

The type 1alpha regulatory subunit (RIalpha) of cAMP-dependent protein kinase (PKA) (coded by the PRKAR1A gene) is the main component of type I PKA, which regulates most of the serine-threonine kinase activity catalyzed by the PKA holoenzyme in response to cAMP. Carney complex (CNC), or the complex of spotty skin pigmentation, myxomas, and endocrine overactivity, is a multiple endocrine (and not only) neoplasia syndrome that is due to PRKAR1A-inactivating mutations. The R1alpha protein and PRKAR1A mRNA have been found to be up-regulated in a series of cell lines and human and rodent neoplasms, suggesting this molecule's involvement in tumorigenesis and its potential role in cell cycle regulation, growth, and/or proliferation. Alterations in PKA activity elicit a variety of effects depending on the tissue, developmental stage, degree of differentiation, and cAMP levels. In addition, RIalpha may have functions independent of PKA. The presence of inactivating germline mutations and the loss of its wild-type allele in some CNC lesions indicate that PRKAR1A might function as a tumor suppressor gene in these tissues, but could PRKAR1A be a classic tumor suppressor gene? Probably not, and this review explains why.

A mouse model for Carney complex

Mice with complete inactivation of the type Ialpha regulatory subunit (RIalpha) of cyclic (c) AMP-dependent protein kinase (PKA) (coded by the Prkar1a gene) die early in embryonic life. To bypass the early embryonic lethality of Prkar1a-/- mice, we established transgenic mice carrying an antisense transgene for Prkar1a exon 2 (X2AS) under the control of a tetracycline-responsive promoter. Mice developed thyroid follicular hyperplasia and adenomas, adrenocortical hyperplasia, and other features reminiscent of PPNAD, and histiocytic and epithelial hyperplasias, lymphomas, and other mesenchymal tumors. This mouse provides a useful tool for the investigation of cAMP, RIalpha, and PKA functions and confirms Prkar1a's critical role in tumorigenesis in endocrine and other tissues.

Down-Regulation of IL-2 Production in T Lymphocytes by Phosphorylated Protein Kinase A-RIIβ

The beta isoform of the type II regulatory subunit (RIIbeta) of protein kinase A suppresses CREB transcriptional activity and c-Fos production in T cells following activation via the TCR. Because CREB is an integral nuclear transcription factor for IL-2 production by T cells, we tested the hypothesis that RIIbeta down-regulates IL-2 expression and IL-2 production in T cells. Stable transfection of RIIbeta in Jurkat T cells led to an approximately 90% reduction in IL-2 mRNA and IL-2 protein following T cell activation. The inhibition of IL-2 production was associated with phosphorylation of the RIIbeta subunit at serine 114 (pRIIbeta) and localization of pRIIbeta in intranuclear clusters. A serine 114 phosphorylation-defective mutant, RIIbeta(S114A), did not form these intranuclear clusters as well as wild-type RIIbeta, and did not inhibit IL-2 mRNA and protein synthesis, indicating that serine 114 phosphorylation is required for both nuclear localization and down-regulation of IL-2 production by RIIbeta. In contrast to its effect on IL-2, RIIbeta induced constitutive up-regulation of CD154 mRNA and cell surface expression. Thus, pRIIbeta differentially regulates gene expression following T cell activation. Unexpectedly, we also found that stable overexpression of another protein kinase A regulatory subunit, RIalpha, had the opposite effect on IL-2 expression, causing a 3- to 4-fold increase in IL-2 production following stimulation. In summary, our data demonstrate a novel mechanism by which serine 114 phosphorylation and nuclear localization of RIIbeta controls the regulation of gene expression in T cells.

Antisense protein kinase A RI alpha-induced tumor reversion: portrait of a microarray

Antisense oligonucleotides can selectively block disease-causing genes due to the specificity of the Watson-Crick base-pairing mechanism of action. A genome-wide view of antisense technology is illustrated via protein kinase A RI alpha antisense. Complementary DNA microarray analysis of the RI alpha antisense-induced expression profile shows the up- and down-regulation of clusters of coordinately expressed genes that define the molecular portrait of a reverted tumor cell phenotype. This global view broadens the horizons of antisense technology; it advances the promise of antisense beyond a single target gene to the whole cell and the whole organism. Along with recent rapid advances in oligonucleotide technologies-including new chemical and biological understanding of more sophisticated nucleic acid drugs-oligonucleotide-based gene silencing offers not only an exquisitely specific genetic tool for exploring basic science but also an exciting possibility for treating and preventing cancer and other diseases.

cAMP/PKA-mediated regulation of erythropoiesis

The role of cyclic AMP (cAMP) as second messenger in erythropoiesis has been suggested in the early 1980s. However, careful analysis showed that cAMP is not generated in direct response to the main erythropoiesis-controlling cytokines such as erythropoietin (Epo). As a result, cAMP disappeared from the central stage in research of erythropoiesis. Instead, other signal transduction pathways, including the Ras/extracellular regulated kinase (ERK)-pathway, the phosphatidylinositol 3-kinase (P13K) and the signal transducer and activator of transcription (STAT5)-pathways, have been found and explored. In concert, these signaling pathways control the transcriptional machinery of erythroid cells. Although cAMP is not directly generated in response to Epo stimulation, it has recently been demonstrated that increased cAMP-levels and in particular the cAMP-dependent protein kinase A (PKA) can modulate erythroid signal transduction pathways. In some cases, like the ERK-signaling pathway, PKA affects signal transduction by regulating the balance between specific phosphatases and kinases. In other cases, such as the STAT5 pathway, PKA enhances Epo signaling by inducing recruitment of additional co-regulators of transcription. In addition to STAT5, PKA also activates other transcription factors that are required for erythroid gene expression. This review discusses the impact of cAMP/PKA on Epo-mediated signaling pathways and summarizes the role of cAMP in malignant erythropoiesis.

Protein kinase A regulatory subunit type II beta directly interacts with and suppresses CREB transcriptional activity in activated T cells

Levels of the type IIbeta regulatory subunit (RIIbeta) of protein kinase A are abnormally high in the nuclei of T cells of some subjects with the autoimmune disorder systemic lupus erythematosus (SLE). However, the role of nuclear RIIbeta in the regulation of T cell function is unknown. Based on previous studies demonstrating that nuclear protein kinase A-RII subunits can modify cAMP response element (CRE)-dependent transcription, we tested the hypothesis that nuclear RIIbeta can alter CRE-directed gene expression in T cells through interaction with the nuclear transcription factor CRE-binding protein CREB. To test this hypothesis, we used the RIIbeta-deficient S49 and the Jurkat T cell lines. In both cell lines, transient transfection of RIIbeta resulted in nuclear localization of a portion of the ectopically expressed RIIbeta. In vitro and in vivo analyses revealed a novel, specific interaction between RIIbeta and CREB that mapped to the N-terminal 135 aa of RIIbeta. In functional studies, RIIbeta inhibited the transcriptional activity of a GAL4-CREB fusion protein by 67% in Jurkat T cells following activation with anti-CD3 and anti-CD28 mAbs. Importantly, deletion of the CREB-binding region of RIIbeta completely abrogated inhibition. Additionally, RIIbeta suppressed CRE-directed reporter gene expression and substantially reduced induction of promoter activity and endogenous protein levels of the CREB-dependent gene, c-fos, in activated T cells. We conclude that nuclear RIIbeta can act as a repressor of CREB transcriptional activity in T cells, providing a potential functional significance for aberrant levels of nuclear RIIbeta in systemic lupus erythematosus T cells.

Organization, sequence, and phylogenetic analysis of the ribosomal protein S3 gene from Drosophila virilis

Ribosomal protein S3 (RPS3) is a multifunctional ribosomal protein: it is a structural and functional component of the ribosome, and also a DNA repair enzyme involved in the DNA base excision repair pathway. Here we cloned and characterized the genomic organization of the ribosomal protein S3 gene (RpS3) homolog in Drosophila virilis. We then compared gene structure and protein sequences of RpS3 from vertebrates, invertebrates, and plants. These comparisons revealed that RpS3 genes from plants to mammals have highly conserved coding and amino acid sequences, and also protein size. Further comparisons of the protein sequences show that important domains are well conserved in both localization and sequence. In contrast, comparison of gene size and organization reveals differing patterns and levels of conservation. Whereas invertebrate RpS3 genes are small in size and gene organization is variable (from zero to four introns), vertebrates have a considerably larger (but variable) gene size and a uniform gene organization. The larger gene size in vertebrates is due to increased number and expansion of introns. Although the plant RpS3 genes are relatively small ( approximately 1.8 kb), their organization resembles that seen in vertebrates. The high conservation through different phyla may suggest that RPS3 might be under great functional constraints, both in its capacity as a component of the ribosome and as a component of a DNA repair system. Finally, electrophoretic mobility shift assays indicate that an upstream element binds a nuclear protein(s).

Dissecting the circuitry of protein kinase A and cAMP signaling in cancer genesis: antisense, microarray, gene overexpression, and transcription factor decoy

Expression of the RI alpha subunit of the cAMP-dependent protein kinase type I (PKA-I) is enhanced in human cancer cell lines, in primary tumors, in transformed cells, and in cells upon stimulation of growth. Signaling via the cAMP pathway may be complex, and the biological effects of the pathway in normal cells may depend upon the physiological state of the cells. However, results of different experimental approaches such as antisense exposure, 8-Cl-cAMP treatment, and gene overexpression have shown that the inhibition of RI alpha/PKA-I exerts antitumor activity in a wide variety of tumor-derived cell lines examined in vitro and in vivo. cDNA microarrays have further shown that in a sequence-specific manner, RI alpha antisense induces alterations in the gene expression profile of cancer cells and tumors. The cluster of genes that define the "proliferation-transformation" signature are down-regulated, and those that define the "differentiation-reverse transformation" signature are up-regulated in antisense-treated cancer cells and tumors, but not in host livers, exhibiting the molecular portrait of the reverted (flat) phenotype of tumor cells. These results reveal a remarkable cellular regulation, elicited by the antisense RI alpha, superimposed on the regulation arising from the Watson-Crick base-pairing mechanism of action. Importantly, the blockade of both the PKA and PKC signaling pathways achieved with the CRE-transcription factor decoy inhibits tumor cell growth without harming normal cell growth. Thus, a complex circuitry of cAMP signaling comprises cAMP growth regulatory function, and deregulation of the effector molecule by this circuitry may underlie cancer genesis and tumor progression.

cAMP-dependent protein kinase types I and II differentially regulate cAMP response element-mediated gene expression: implications for neuronal responses to ethanol

We have shown that ethanol induces translocation of cAMP-dependent protein kinase (PKA) to the nucleus, cAMP response element-binding protein (CREB) phosphorylation, and cAMP response element-mediated gene transcription in NG108-15 cells. However, little is known about which PKA types regulate this process. We show here that under basal conditions NG108-15 cells contain type I PKA (CbetaRIbeta) primarily in cytosol and type II PKA (CalphaRIIbeta) in the particulate and nuclear fractions. Antagonists of both type I and type II PKA inhibit forskolin- and ethanol-induced cAMP response element-mediated gene transcription. However, only the type II PKA antagonist inhibits forskolin-induced Calpha and ethanol-induced Calpha and RIIbeta translocation to the nucleus and CREB phosphorylation; the type I antagonist is without effect. Our data suggest that forskolin- and ethanol-induced CREB phosphorylation and gene activation are differentially mediated by the two types of PKA. We propose that type II PKA is translocated and activated in the nucleus and induces CREB phosphorylation that is necessary but not sufficient for gene transcription. By contrast, type I PKA is activated in the cytoplasm, turning on a downstream pathway that activates other transcription cofactors that interact with phosphorylated CREB to induce gene transcription.

Nuclear translocation of the catalytic subunit of protein kinase A induced by an antisense oligonucleotide directed against the RIalpha regulatory subunit

The regulatory (R) subunits of cAMP-dependent protein kinase (PKA) are implicated in the regulation of cell proliferation and differentiation. There are two isoforms of PKA that are distinguished by two types of R subunit, RI and RII. Evidence suggests that RI is associated with proliferation and RII is associated with cell differentiation. Previous work in this laboratory has demonstrated that depletion of the RIalpha subunit by treatment with an antisense oligonucleotide (ODN) induces differentiation in leukemia cells and growth arrest and apoptosis in epithelial cancer cells. Using the prostate cancer cell line PC3M as a model system, we have developed a cell line that overexpresses a retroviral vector construct containing the RIalpha antisense gene. This cell line has been characterized and the effectiveness of the construct determined. In the work presented here, we demonstrate by immunocytochemistry that treatment with RIalpha antisense ODN induces translocation of the Calpha subunit of PKA to the nucleus of PC3M prostate cancer cells. The translocation of Calpha triggered by exogenous antisense ODN treatment mirrors that observed in cells endogenously overexpressing the antisense gene. Triggering the nuclear translocation of the Calpha subunit of PKA in the cell may be an important mechanism of action of RIalpha antisense that regulates cell growth independent of adenylate cyclase and cellular cAMP levels. The nuclear localization of the Calpha subunit of PKA may be an essential step in revealing the mechanism whereby this critical kinase regulates cell growth.

8-Cl-cAMP induces cell cycle-specific apoptosis in human cancer cells

8-Cl-cyclic adenosine monophosphate (8-Cl-cAMP) has been known to induce growth inhibition and differentiation in a variety of cancer cells by differential modulation of protein kinase A isozymes. To understand the anticancer activity of 8-Cl-cAMP further, we investigated the effect of 8-Cl-cAMP on apoptosis in human cancer cells. Most of the tested human cancer cells exhibited apoptosis upon treatment with 8-Cl-cAMP, albeit with different sensitivity. Among them, SH-SY5Y neuroblastoma cells and HL60 leukemic cells showed the most extensive apoptosis. The effect of 8-Cl-cAMP was not reproduced by other cAMP analogues or cAMP-elevating agents, showing that the effect of 8-Cl-cAMP was not caused by simple activation of protein kinase A (PKA). However, competition experiments showed that the binding of 8-Cl-cAMP to the cAMP receptor was essential for the induction of apoptosis. After the treatment of 8-Cl-cAMP, cells initially accumulated at the S and G2/M phases of the cell cycle and then apoptosis began to occur among the population of cells at the S/G2/M cell cycle phases, indicating that the 8-Cl-cAMP-induced apoptosis is closely related to cell cycle control. In support of this assumption, 8-Cl-cAMP-induced apoptosis was blocked by concomitant treatment with mimosine, which blocks the cell cycle at early S phase. Interestingly, 8-Cl-cAMP did not induce apoptosis in primary cultured normal cells and non-transformed cell lines, showing that 8-Cl-cAMP-induced apoptosis is specific to transformed cells. Taken together, our results show that the induction of apoptosis is one of the mechanisms through which 8-Cl-cAMP exerts anticancer activity.

Differential binding of cAMP-dependent protein kinase regulatory subunit isoforms Ialpha and IIbeta to the catalytic subunit

Limited trypsin digestion of type I cAMP-dependent protein kinase holoenzyme results in a proteolytic-resistant Delta(1-72) regulatory subunit core, indicating that interaction between the regulatory and catalytic subunits extends beyond the autoinhibitory site in the R subunit at the NH(2) terminus. Sequence alignment of the two R subunit isoforms, RI and RII, reveals a significantly sequence diversity at this specific region. To determine whether this sequence diversity is functionally important for interaction with the catalytic subunit, specific mutations, R133A and D328A, are introduced into sites adjacent to the active site cleft in the catalytic subunit. While replacing Arg(133) with Ala decreases binding affinity for RII, interaction between the catalytic subunit and RI is not affected. In contrast, mutant C(D328A) showed a decrease in affinity for binding RI while maintaining similar affinities for RII as compared with the wild-type catalytic subunit. These results suggest that sequence immediately NH(2)-terminal to the consensus inhibition site in RI and RII interacts with different sites at the proximal region of the active site cleft in the catalytic subunit. These isoform-specific differences would dictate a significantly different domain organization in the type I and type II holoenzymes.

Association of deficient type II protein kinase A activity with aberrant nuclear translocation of the RII beta subunit in systemic lupus erythematosus T lymphocytes

Systemic lupus erythematosus (SLE) is an autoimmune disorder of indeterminate etiology characterized by abnormal T cell signal transduction and altered T cell effector functions. We have previously observed a profound deficiency of total protein kinase A (PKA) phosphotransferase activity in SLE T cells. Here we examined whether reduced total PKA activity in SLE T cells is in part the result of deficient type II PKA (PKA-II) isozyme activity. The mean PKA-II activity in SLE T cells was 61% of normal control T cells. The prevalence of deficient PKA-II activity in 35 SLE subjects was 37%. Deficient isozyme activity was persistent over time and was unrelated to SLE disease activity. Reduced PKA-II activity was associated with spontaneous dissociation of the cytosolic RIIbeta2C2 holoenzyme and translocation of the regulatory (RIIbeta) subunit from the cytosol to the nucleus. Confocal immunofluorescence microscopy revealed that the RIIbeta subunit was present in approximately 60% of SLE T cell nuclei compared with only 2-3% of normal and disease controls. Quantification of nuclear RIIbeta subunit protein content by immunoprecipitation and immunoblotting demonstrated a 54% increase over normal T cell nuclei. Moreover, the RIIbeta subunit was retained in SLE T cell nuclei, failed to relocate to the cytosol, and was associated with a persistent deficiency of PKA-II activity. In conclusion, we describe a novel mechanism of deficient PKA-II isozyme activity due to aberrant nuclear translocation of the RIIbeta subunit and its retention in the nucleus in SLE T cells. Deficient PKA-II activity may contribute to impaired signaling in SLE T cells.

Dual anticancer activity of 8-Cl-cAMP: inhibition of cell proliferation and induction of apoptotic cell death

8-Cl-cAMP induces apoptotic cell death in human cancer cells. To look at this more closely, we examined the changes in the levels of Bcl-2 family proteins during 8-Cl-cAMP-induced apoptosis of SH-SY5Y human neuroblastoma cells. Following the treatment with 8-Cl-cAMP, Bcl-2 was transiently down-regulated and Bad was increased continuously up to day 5. In addition, overexpression of Bcl-2 efficiently blocked the 8-Cl-cAMP-induced apoptosis, suggesting Bcl-2 family proteins may be involved in the 8-Cl-cAMP-induced apoptosis. The contribution of the apoptotic cell death and the inhibition of cell proliferation in the 8-Cl-cAMP-induced growth inhibition was closely monitored in the Bcl-2-overexpressing cells. Though the apoptosis was reduced significantly, no significant difference was observed in the inhibition of cell proliferation up to day 2 of 8-Cl-cAMP treatment. These results suggest that 8-Cl-cAMP exerts anticancer activity by two distinct mechanisms, i.e. , through the inhibition of cell proliferation as well as the induction of apoptosis. Supporting this notion was the observations that (1) suppression of apoptosis by zVAD did not abrogate 8-Cl-cAMP-induced inhibition of cell proliferation, and (2) 8-Cl-cAMP did not show additive inhibition of cell proliferation in RIIbeta-overexpressing cells.

Extracellular protein kinase A as a cancer biomarker: its expression by tumor cells and reversal by a myristate-lacking Calpha and RIIbeta subunit overexpression

Overexpression of cAMP-dependent protein kinase (PKA) type I isozyme is associated with cell proliferation and neoplastic transformation. The presence of PKA on the external surface of LS-174T human colon carcinoma cells has been shown. Here, we show that cancer cells of various cell types excrete PKA into the conditioned medium. This extracellular PKA (ECPKA) is present in active, free catalytic subunit (C subunit) form, and its activity is specifically inhibited by PKA inhibitory protein, PKI. Overexpression of the Calpha or RIalpha subunit gene of PKA in an expression vector, which up-regulates intracellular PKA type I, markedly up-regulates ECPKA expression. In contrast, overexpression of the RIIbeta subunit, which eliminates PKA type I, up-regulates PKA type II, and reverts the transformed phenotype, down-regulates ECPKA. A mutation in the Calpha gene that prevents myristylation allows the intracellular PKA up-regulation but blocks the ECPKA increase, suggesting that the NH(2)-terminal myristyl group of Calpha is required for the ECPKA expression. In serum of cancer patients, the ECPKA expression is up-regulated 10-fold as compared with normal serum. These results indicate that the ECPKA expression is an ordered cellular response of a living cell to actively exclude excess intracellular PKA molecules from the cell. This phenomenon is up-regulated in tumor cells and has an inverse relationship with the hormone dependency of breast cancer. Thus, the extracellular PKA may serve as a potential diagnostic and prognostic marker for cancer.

8-Cl-cAMP antagonizes mitogen-activated protein kinase activation and cell growth stimulation induced by epidermal growth factor

The growth factor-activated mitogenic pathways are often disregulated in tumour cells and, therefore, they can provide specific molecular targets for novel anti-tumour approaches. 8-Chloro-cAMP (8-Cl-cAMP), a synthetic cAMP analogue, is a novel anti-tumour agent that has recently undergone clinical evaluation. We investigated the effects of 8-Cl-cAMP on the epidermal growth factor (EGF)/EGF receptor (EGF-R) signalling in human epidermoid cancer KB cells, which are responsive to the mitogenic stimulus of EGF. We found that the growth-promoting activity of EGF was completely abolished when EGF treatment was performed in combination with 8-Cl-cAMP. The inhibition of the EGF-induced proliferation by 8-Cl-cAMP was paralleled by the blockade of the EGF-stimulated activation of mitogen-activated protein kinases (MAPK), ERK-1 and ERK-2. Conversely, we found an increase of EGF-R expression and EGF-R tyrosine phosphorylation when KB cells were growth inhibited by 8-Cl-cAMP. Moreover, the activity of Raf-1 and MEK-1 protein kinases, the activators upstream MAPK in the phosphorylation cascade induced by EGF, was not modified in 8-Cl-cAMP-treated cells. We concluded that the impairment of KB cell response to EGF, induced by 8-Cl-cAMP, resides in the specific inhibition of MAPK/ERKs activity while the function of the upstream elements in the EGF-R signalling is preserved.

Protein kinase A-Ialpha subunit-directed antisense inhibition of ovarian cancer cell growth: crosstalk with tyrosine kinase signaling pathway

Expression of the RIalpha subunit of cAMP-dependent protein kinase type I is increased in human cancers in which an autocrine pathway for epidermal growth factor-related growth factors is activated. We have investigated the effect of sequence-specific inhibition of RIalpha gene expression on ovarian cancer cell growth. We report that RIalpha antisense treatment results in a reduction in RIalpha expression and protein kinase A type I, and inhibition of cell growth. The growth inhibition was accompanied by changes in cell morphology and appearance of apoptotic nuclei. In addition, EGF receptor, c-erbB-2 and c-erbB-3 levels were reduced, and the basal and EGF-stimulated mitogen-activated protein kinase activities were reduced. Protein kinase A type I and EGF receptor levels were also reduced in cells overexpressing EGF receptor antisense cDNA. These results suggest that the antisense depletion of RIalpha leads to blockade of both the serine-threonine kinase and the tyrosine kinase signaling pathways resulting in arrest of ovarian cancer cell growth.

Ala99ser mutation in RI alpha regulatory subunit of protein kinase A causes reduced kinase activation by cAMP and arrest of hormone-dependent breast cancer cell growth

Expression of the RIalpha regulatory subunit of protein kinase A type I is increased in human cancer cell lines, in primary tumors, in cells after transformation, and in cells upon stimulation of growth. Ala99 (the pseudophosphorylation site) of human RIalpha was replaced with Ser (RIalpha-p) for the structure-function analysis of RIalpha. MCF-7 hormone-dependent breast cancer cells were transfected with an expression vector for the wild-type RIalpha or mutant RIalpha-p. Overexpression of RIalpha-P resulted in suppression of protein kinase A type II, the isozyme of type I kinase, production of kinase exhibiting reduced cAMP activation, and inhibition of cell growth showing an increase in G0/G1 phase of the cell cycle and apoptosis. The wild-type RIalpha overexpression had no effect on protein kinase A isozyme distribution or cell growth. Overexpression of protein kinase A type II regulatory subunit, RIIbeta, suppressed RIalpha and protein kinase A type I and inhibited cell growth. These results show that the growth of hormone-dependent breast cancer cells is dependent on the functional protein kinase A type I.

The RIIbeta regulatory subunit of protein kinase A binds to cAMP response element: an alternative cAMP signaling pathway

cAMP, through the activation of cAMP-dependent protein kinase (PKA), is involved in transcriptional regulation. In eukaryotic cells, cAMP is not considered to alter the binding affinity of CREB/ATF to cAMP-responsive element (CRE) but to induce serine phosphorylation and consequent increase in transcriptional activity. In contrast, in prokaryotic cells, cAMP enhances the DNA binding of the catabolite repressor protein to regulate the transcription of several operons. The structural similarity of the cAMP binding sites in catabolite repressor protein and regulatory subunit of PKA type II (RII) suggested the possibility of a similar role for RII in eukaryotic gene regulation. Herein we report that RIIbeta subunit of PKA is a transcription factor capable of interacting physically and functionally with a CRE. In contrast to CREB/ATF, the binding of RIIbeta to a CRE was enhanced by cAMP, and in addition, RIIbeta exhibited transcriptional activity as a Gal4-RIIbeta fusion protein. These experiments identify RIIbeta as a component of an alternative pathway for regulation of CRE-directed transcription in eukaryotic cells.

Type II protein kinase A up-regulation is sufficient to induce growth inhibition in SK-N-SH human neuroblastoma cells

We have previously reported that overexpression of RII beta subunit of protein kinase A, which markedly reduces RI alpha protein, induces growth inhibition in SK-N-SH human neuroblastoma cells. To determine whether the reduction of RI alpha or protein kinase A isozyme type I is essential in the growth inhibition of SK-N-SH cells, we overexpressed RI alpha in sense and antisense orientation. Type I protein kinase A activity was increased in the RI alpha-overexpressing cells and was decreased in the RI alpha antisense-expressing cells. However, the changes in type I protein kinase A activities did not affect cell growth. Overexpression of RII beta or C alpha increased type II protein kinase A and inhibited cell growth in both cell lines regardless of the type I protein kinase A level. These results indicate that type II protein kinase A is the main effector in the cAMP-mediated growth regulation of SK-N-SH human neuroblastoma cells.

Adrenomedullin expression in human tumor cell lines. Its potential role as an autocrine growth factor

Although adrenomedullin (AM) previously has been identified in human tumors, its role has remained elusive. Analysis by reverse transcriptase-polymerase chain reaction (RT-PCR) revealed AM mRNA in 18 of 20 human normal tissues representing major organs, and 55 of 58 (95%) malignant cell lines. Western blot and high performance liquid chromatography analysis showed immunoreactive AM species of 18, 14, and 6 kDa that are consistent with the precursor, intermediate product, and active peptide, respectively. Immunohistochemistry and in situ RT-PCR performed on paraffin-embedded tumor cell lines of various tissue origins exhibited AM cytoplasmic staining. Neutralizing monoclonal antibody to AM inhibits tumor cell growth in a concentration-dependent manner, an effect that was reversed with the addition of exogenous AM. Responding tumor cells were shown to have approximately 50,000 AM receptors per cell by Scatchard analysis with 125I-AM and expressed AM receptor mRNA by RT-PCR. Our data showed 36 of 48 (75%) tumor cell lines expressed AM receptor mRNA by RT-PCR assessment, all of them also expressed AM. In the presence of AM, cAMP levels were shown to increase in tumor cells. Our collective data demonstrate that AM and AM receptor are expressed in numerous human cancer cell lines of diverse origin and constitute a potential autocrine growth mechanism that could drive neoplastic proliferation.