The regulatory and catalytic subunits of cAMP-dependent protein kinases are associated with transcriptionally active chromatin during changes in gene expression

Molecular cloning, chromosomal localization, and cell cycle-dependent subcellular distribution of the A-kinase anchoring protein, AKAP95

The cyclic AMP-dependent protein kinase (PKA) type II is directed to different subcellular loci through interaction of the RII subunits with A-kinase anchoring proteins (AKAPs). A full-length human clone encoding AKAP95 was identified and sequenced, and revealed a 692-amino acid open reading frame that was 89% homologous to the rat AKAP95 (V. M. Coghlan, L. K. Langeberg, A. Fernandez, N. J. Lamb, and J. D. Scott (1994) J. Biol. Chem. 269, 7658-7665). The gene encoding AKAP95 was mapped to human chromosome 19p13.1-q12 using somatic cell hybrids and PCR. A fragment covering amino acids 414-692 of human AKAP95 was expressed in Escherichia coli and shown to bind RIIalpha. Competition with a peptide covering the RII-binding domain of AKAP Ht31 abolished RIIalpha binding to AKAP95. Immunofluorescence studies in quiescent human Hs-68 fibroblasts showed a nuclear localization of AKAP95, whereas RIIalpha was excluded from the nucleus. In contrast, during mitosis AKAP95 staining was markedly changed and appeared to be excluded from the condensed chromatin and localized outside the metaphase plate. Furthermore, the subcellular localizations of AKAP95 and RIIalpha overlapped in metaphase but started to segregate in anaphase and were again separated as AKAP95 reentered the nucleus in telophase. Finally, RIIalpha was coimmunoprecipitated with AKAP95 from HeLa cells arrested in mitosis, but not from interphase HeLa cells, demonstrating a physical association between these two molecules during mitosis. The results show a distinct redistribution of AKAP95 during mitosis, suggesting that the interaction between AKAP95 and RIIalpha may be cell cycle-dependent.

Cyclic AMP-mediated inhibition of transcription of the malic enzyme gene in chick embryo hepatocytes in culture. Characterization of a cis-acting element far upstream of the promoter

Glucagon, acting via cAMP, inhibits transcription of the malic enzyme gene in chick embryo hepatocytes. In transiently transfected hepatocytes, fragments from the 5'-flanking DNA of the malic enzyme gene confer cAMP responsiveness to linked reporter genes. The major inhibitory cAMP response element at -3180/-3174 base pairs (bp) is similar to the consensus binding site for AP1. DNA fragments from -3134/-3115, -1713/-944, and -413/-147 bp also contain inhibitory cAMP response elements. The negative action of cAMP is mimicked by overexpression of the catalytic subunit of protein kinase A, inhibited by overexpression of a specific inhibitor of protein kinase A, and inhibited by overexpression of the T3 receptor; these results indicate involvement of the classical eukaryotic pathway for cAMP action and suggest interaction between the T3 and cAMP pathways. Sequence-specific complexes form between nuclear proteins and a DNA fragment containing -3192/-3158 bp of 5'-flanking DNA. In nuclear extracts prepared from cells treated with chlorophenylthio-cyclic AMP and T3, the complexes have different masses than those formed with extracts from cells treated with T3 alone. Antibodies to c-Fos or ATF-2 inhibit formation of the complex formed by proteins from cells treated with chlorophenylthio-cyclic AMP and T3 but not by those from cells treated with T3 alone. These results suggest an important role for c-Fos and ATF-2 in glucagon-mediated inhibition of transcription of the malic enzyme gene.

The nuclear matrix: a structural milieu for genomic function

While significant progress has been made in elucidating molecular properties of specific genes and their regulation, our understanding of how the whole genome is coordinated has lagged behind. To understand how the genome functions as a coordinated whole, we must understand how the nucleus is put together and functions as a whole. An important step in that direction occurred with the isolation and characterization of the nuclear matrix. Aside from the plethora of functional properties associated with these isolated nuclear structures, they have enabled the first direct examination and molecular cloning of specific nuclear matrix proteins. The isolated nuclear matrix can be used for providing an in vitro model for understanding nuclear matrix organization in whole cells. Recent development of high-resolution and three-dimensional approaches for visualizing domains of genomic organization and function in situ has provided corroborative evidence for the nuclear matrix as the site of organization for replication, transcription, and post-transcriptional processing. As more is learned about these in situ functional sites, appropriate experiments could be designed to test molecular mechanisms with the in vitro nuclear matrix systems. This is illustrated in this chapter by the studies of nuclear matrix-associated DNA replication which have evolved from biochemical studies of in vitro nuclear matrix systems toward three-dimensional computer image analysis of replication sites for individual genes.

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.

Alterations in the cAMP signal transduction pathway in mouse lung tumorigenesis

Alterations in the cAMP signal transduction pathway are associated with mouse lung neoplasia, cAMP effects are mediated by activating cAMP-dependent protein kinase isozymes, PKA I and PKA II. E9, a tumorigenic cell line, exhibited decreased PKA I levels compared to C10 cells, a nontumorigenic cell line of similar epithelial origin. Western immunoblots of PKA subunit proteins demonstrated low concentrations of both the catalytic (C) and regulatory (RI) PKA I subunits. Although RII (regulatory subunit of PKA II) concentrations were similar in both cell lines, RII from E9 cells was more highly phosphorylated than in C10 cells. RII phosphorylation status regulates cAMP activation of PKA II. Northern-blot analysis of mRNA content indicated diminished expression of both C and RI mRNA in E9 relative to C10 cells. Several endogenous PKA substrate proteins present in C10 cells were minimally phosphorylated by PKA in E9 cells. Forskolin, which raises cellular cAMP content, increased phosphorylation of a protein doublet in intact C10 cells, but not in E9 cells. Decreased PKA I expression and alterations in RII phosphorylation in lung neoplasia may contribute to anomalous regulation by cAMP, thereby diminishing cAMP-mediated growth inhibitory effects.

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.

Site-selective 8-Cl-cAMP which causes growth inhibition and differentiation increases DNA (CRE)-binding activity in cancer cells

Control mechanisms of normal differentiation are disrupted in cancer cells but can be restored by treatment with site-selective cAMP analogs. The cellular events associated with such changes entail compartmental redistribution of the cAMP-dependent protein kinase type II regulatory subunit, RII beta. The results of this study indicate that the molecular mechanisms of action involve changes in specific DNA-binding activity of putative transcription factors. Gel retardation analyses revealed that nuclear extracts from cells of various human cancer cell lines [colon cancer (LS-174T), gastric cancer (TMK-1), and leukemia (K-562)] and rodent pheochromocytoma (PC12) show a concentration-dependent increase in binding activity to a synthetic DNA that contained the cAMP-responsive element 5'-TGACGTCA-3' after treatment with 8-Cl-cAMP. Such an increase in cAMP-responsive element binding activity was not observed in the 8-C1-cAMP-unresponsive MKN-1 gastric cancer cells. These findings indicate that the antitumor activity of site-selective cAMP analogs may reside in the induction of transcription factors that restore normal gene regulation in cancer cells.

Regulation of tumor necrosis factor expression in a macrophage-like cell line by lipopolysaccharide and cyclic AMP

Bacterial lipopolysaccharide (LPS, 1 microgram/ml) induced the rapid production of tumor necrosis factor (TNF-alpha) mRNA in the RAW264 macrophage-like cell line. TNF-alpha mRNA peaked within 45 min of LPS treatment and remained high for greater than 3 hr. Transcription of TNF-alpha mRNA was increased within 15 min of LPS treatment. The quantity of TNF-alpha mRNA in LPS-stimulated cells was reduced to basal levels by treatment with cAMP, cAMP analogs, or agents which raise intracellular cAMP. This was not a general effect on all mRNA levels as the expression of a second gene, ornithine decarboxylase, was enhanced by cAMP treatment. cAMP did not have an effect on the stability of TNF-alpha mRNA. This is in contrast to the protein synthesis inhibitor, cycloheximide, which leads to a stabilization of TNF-alpha mRNA. Our results suggest that the primary regulation of tumor necrosis factor by cAMP and LPS occurs at the transcriptional level.

Activity-dependent regulation of gene expression in muscle and neuronal cells

In both the central and the peripheral nervous systems, impulse activity regulates the expression of a vast number of genes that code for synaptic proteins, including neuropeptides, enzymes involved in neurotransmitter biosynthesis and degradation, and membrane receptors. In recent years, the mechanisms involved in these regulations became amenable to investigation by the methods of recombinant DNA technology. The first part of this review focuses on the activity-dependent control of nicotinic acetylcholine receptor biosynthesis in vertebrate muscle, a model case for the regulation of synaptic protein biosynthesis at the postsynaptic level. The second part summarizes some examples of neuronal proteins whose biosynthesis is under the control of transsynaptic impulse activity. The first, second, and third intracellular messengers involved in membrane-to-gene signaling are discussed, as are possible posttranscriptional control mechanisms. Finally, models are proposed for a role of neuronal activity in the genesis and stabilization of the synapse.

Cyclic AMP-dependent protein kinase phosphorylates and inactivates the yeast transcriptional activator ADR1

It has been proposed in several eukaryotic systems that the regulation of gene transcription involves phosphorylation of specific transcription factors. We report here that the yeast transcriptional activator ADR1 is phosphorylated in vitro by cyclic AMP-dependent protein kinase and that mutations which enhance the ability of ADR1 to activate ADH2 expression decrease ADR1 phosphorylation. We also show that increased kinase activity in vivo inhibits ADH2 expression in an ADR1 allele-specific manner. Our data suggest that glucose repression of ADH2 is in part mediated through a cAMP-dependent phosphorylation-inactivation of the ADR1 regulatory protein.

Temporal relationship between nerve-stump-length-dependent changes in the autophosphorylation of a cyclic AMP-dependent protein kinase and the acetylcholine receptor content in skeletal muscle

The acetylcholine receptor (AChR) content and the autophosphorylation of the regulatory subunit of cyclic AMP-dependent protein kinase type II (R-II) were evaluated in rats soleus muscles at 24, 30 and 66 hr after surgical denervation by cutting the nerve at a short distance (short-nerve-stump) and at a long distance (long-nerve-stump) from the muscle. AChR content was based on the specific binding of [125I]alpha-bungarotoxin (BUTX); changes in the autophosphorylation of R-II were based upon the predominant in vitro 32P-phosphorylation of a 56-Kd soluble protein in cytosolic fractions of solei. The AChR content and the 32P-autophosphorylation of R-II were increased in samples from short-nerve-stump solei, but not from long-nerve-stump solei, after a denervation-time of 30 hr. This nerve-stump-length dependency indicates that the two denervation effects are not related to the immediate halt of impulse-evoked muscle contractility. Furthermore, the results show that alterations in the 32P-autophosphorylation of R-II occurred before, as well as whenever, increases in the AChR content were found. Speculatively, this temporal relationship may be significant with respect to the potential role of R-II in gene expression.