The roles of cAMP and cAMP-dependent protein kinase in the regulation of adrenocortical functions: analysis using DNA-mediated gene transfer

DNA-mediated gene transfer was used to evaluate the cause and effect relationship between mutations in cAMP-dependent protein kinase activity and cellular resistance of adrenocortical tumor cells to ACTH and cAMP. Protein kinase defective, Kin 8 adrenocortical tumor cells were transformed with genomic DNA from an ACTH- and cAMP-responsive adrenocortical cell line and screened for the recovery of morphological responses to the cAMP analog 8-bromo-cAMP (8BrcAMP). 8BrcAMP-responsive transformants were recovered with a frequency of approximately 0.5 per 10(3) transformation-competent cells. These transformants recovered the ability to round up in the presence of ACTH and were able to respond to both ACTH and 8BrcAMP with increased steroidogenesis. They also recovered cAMP-dependent protein kinase activity. The transformants, however, were unstable and concomitantly lost cAMP-dependent protein kinase activity and steroidogenic and morphological responses to ACTH and 8BrcAMP. These observations suggest that a single gene, probably the gene encoding the regulatory subunit of cAMP-dependent protein kinase, is responsible for the resistance of the Kin 8 mutant to ACTH and cAMP.     

Recovery of cyclic nucleotide regulation in protein-kinase-defective adrenal cells through somatic cell fusion

A mutant cell line (designated Kin-8), isolated from the Y1 mouse adrenocortical tumor cell line on the basis of its resistance to growth-inhibition by 8-bromoadenosine 3', 5'-monophosphate (8BrcAMP), was resistant to the steroidogenic effects of the cyclic nucleotide analog and did not round up in the presence of 8BrcAMP as did responsive Y1 adrenal cells. In Kin-8 cells, the mutation to cyclic nucleotide resistance was associated with a defective type 1 cAMP-dependent protein kinase activity, suggesting an obligatory role for the enzyme in the regulation of these adrenal functions. In this study, the Kin-8 mutant was fused with a rat glioma cell line (C6) in order to analyze the genetic behavior of the protein kinase mutation in somatic cell hybrids. The growth of C6 glial cells was inhibited by 8BrcAMP and its cAMP-dependent protein kinase responded normally to cAMP. In addition, C6 cells had no capacity for steroidogenesis nor did they round up when treated with 8BrcAMP. In Kin-8 X C6 hybrids, the protein kinase mutation seemed to behave recessively. The growth of hybrid cells was inhibited by 8BrcAMP and the protein kinase responded to cAMP over a normal range. Kin-8 X C6 hybrids, when treated with 8BrcAMP, exhibited steroidogenic activities which were greater than the activity measured in either fusion partner and which had lower ED50 values for 8BrcAMP. In addition, Kin-8 X C6 hybrids rounded up in the presence of 8BrcAMP, a morphologic change unlike that seen with either fusion partner. The effects of 8BrcAMP on the steroidogenic activity and morphology of Kin-8 X C6 hybrids was reminiscent of the effects of the cyclic nucleotide on cAMP-responsive, parental Y1 adrenal cells. These results suggest that cell fusion provided a normal cAMP-dependent protein kinase for Kin-8 cells and led to the recovery of a cAMP-responsive adrenal phenotype. type. These results provide additional evidence in support of an obligatory role for cAMP-dependent protein kinase in the regulation of adrenal steroidogenesis, cell division, and cell shape.     

Plasminogen activator regulation in osteoblasts: parathyroid hormone inhibition of type-1 plasminogen activator inhibitor and its mRNA.

In order to determine the mechanism by which parathyroid hormone (PTH) stimulates plasminogen activator (PA) activity in rat osteoblasts, we investigated the effect of human PTH(1-34) [hPTH(1-34)] on the synthesis of mRNAs for tissue-type PA (tPA), urokinase-type PA (uPA), and PA inhibitor-1 (PAI-1), and on release of PA activity and PAI-1 protein in both normal rat calvarial osteoblasts and UMR 106-01 osteogenic sarcoma cells. hPTH(1-34) (0.25-25 nM) decreased PAI-1 mRNA and protein, and increased PA activity in both cell types in a dose-dependent manner with ED50 of about 1 nM for both responses. Forskolin and isobutylmethylxanthine also stimulated PA activity and decreased PAI-1 protein and mRNA in both cell types. hPTH(1-34) did not show any consistent effect on tPA and uPA mRNA in calvarial osteoblasts, but a modest (two-fold) increase of both mRNAs was observed in UMR 106-01 cells treated with 25 nM hPTH(1-34). However, when protein synthesis was inhibited with 100 microM cycloheximide, the increase of tPA and uPA mRNA by hPTH(1-34) was enhanced in UMR 106-01 cells and became evident in calvarial osteoblasts. Fibrin autography also revealed that hPTH(1-34) increases tPA and uPA activity, especially after cycloheximide treatment in UMR 106-01 cells. These results strongly suggest that PTH increases PA activity predominantly by decreasing PAI-1 protein production through a cyclic adenosine monophosphate (cAMP)-dependent mechanism in rat osteoblasts. The reduction of PAI-1 protein by PTH results in enhanced action of both tPA and uPA, and would contribute to the specific roles of these PAs in bone.     

Mechanisms of cAMP-mediated gene induction: examination of renal epithelial cell mutants affected in the catalytic subunit of the cAMP-dependent protein kinase

The precise mechanistic role of the cAMP-dependent protein kinase (cAMP-PK) in cAMP-mediated gene induction remains unclear. Renal epithelial cell mutants were compared to the LLC-PK1 parental cell line for induction of the cAMP-responsive urokinase-type plasminogen activator (uPA) gene, as quantitated by the technique of mRNA solution hybridization. The FIB4 and FIB6 mutants, which possess less than 10% parental cAMP-PK catalytic (C) subunit activity, showed markedly diminished uPA mRNA induction in response to agents elevating intracellular cAMP such as the cAMP analogue 8-bromo-cAMP and the adenylate cyclase-stimulating hormones vasopressin and calcitonin. In contrast, the mutant cells responded to a similar or greater extent than the parental cells in terms of uPA mRNA induction following treatment with the Ca2+/phospholipid-dependent protein kinase activator phorbol 12-myristate 13-acetate (PMA). Elevation of intracellular cAMP was found to induce a translocation of the cAMP-PK C subunit from the perinuclear Golgi region to the nucleus in both parental and mutant cell lines, as shown by immunocytochemical techniques. Results argue for the role of the cAMP-PK C subunit activity and possibly nuclear translocation of the C subunit in cAMP-mediated uPA induction, which is mechanistically distinct from the PMA-stimulated response.     

LLC-PK1 cell mutants in cAMP metabolism respond normally to phorbol esters

Mutants of the pig kidney cell line, LLC-PK1, affected in cAMP metabolism, were examined for cAMP-dependent protein kinase (cAMP-PK) activity and for cAMP-mediated induction of urokinase-type plasminogen activator (uPA). The FIB4 and FIB6 mutant cell lines possessed about 10% parental levels of cAMP-PK activity and concomitantly reduced uPA production (10-20% parental) in response to calcitonin, forskolin and 8-bromo cAMP. The FIB1, FIB2 and FIB5 mutant cell lines had about 70% parental levels of cAMP-PK and the synthesis of uPA was 40-60% parental. Thus, cAMP-mediated induction of uPA showed a dependence on the absolute levels of cAMP-PK. However, uPA synthesis in response to phorbol-12-myristate-13-acetate by all of the mutants was similar to parental, which indicates that enzyme induction mediated by phorbol esters does not involve cAMP or cAMP-PK.     

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.     

cAMP inhibits CSF-1-stimulated tyrosine phosphorylation but augments CSF-1R-mediated macrophage differentiation and ERK activation

Macrophage colony stimulating factor (M-CSF) or CSF-1 controls the development of the macrophage lineage through its receptor tyrosine kinase, c-Fms. cAMP has been shown to influence proliferation and differentiation in many cell types, including macrophages. In addition, modulation of cellular ERK activity often occurs when cAMP levels are raised. We have shown previously that agents that increase cellular cAMP inhibited CSF-1-dependent proliferation in murine bone marrow-derived macrophages (BMM) which was associated with an enhanced extracellular signal-regulated kinase (ERK) activity. We report here that increasing cAMP levels, by addition of either 8-bromo cAMP (8BrcAMP) or prostaglandin E(1) (PGE1), can induce macrophage differentiation in M1 myeloid cells engineered to express the CSF-1 receptor (M1/WT cells) and can potentiate CSF-1-induced differentiation in the same cells. The enhanced CSF-1-dependent differentiation induced by raising cAMP levels correlated with enhanced ERK activity. Thus, elevated cAMP can promote either CSF-1-induced differentiation or inhibit CSF-1-induced proliferation depending on the cellular context. The mitogen-activated protein kinase/extracellular signal-related protein kinase kinase (MEK) inhibitor, PD98059, inhibited both the cAMP- and the CSF-1R-dependent macrophage differentiation of M1/WT cells suggesting that ERK activity might be important for differentiation in the M1/WT cells. Surprisingly, addition of 8BrcAMP or PGE1 to either CSF-1-treated M1/WT or BMM cells suppressed the CSF-1R-dependent tyrosine phosphorylation of cellular substrates, including that of the CSF-1R itself. It appears that there are at least two CSF-1-dependent pathway(s), one MEK/ERK dependent pathway and another controlling the bulk of the tyrosine phosphorylation, and that cAMP can modulate signalling through both of these pathways.     

Pathway of urokinase-type plasminogen activator induction in the T47D and LLC-PK1 cell lines

Induction of urokinase-type plasminogen activator (uPA) in response to either reagents activating cAMP-dependent protein kinase (cAMP-PK) or the calcium ion phospholipid-dependent kinase (C-kinase) was compared in the LLC-PK1 and T47D cell lines. The two cell lines exhibited quantitatively different responses to calcitonin, to the phosphodiesterase inhibitor isobutylmethylxanthine, and to the adenylate cyclase activator forskolin. Both showed activation of cAMP-PK in response to all these reagents, with T47D cells displaying a greater extent of activation. T47D cells, however, failed to produce uPA in response to calcitonin, forskolin, or the cAMP analog 8-bromo-cAMP, whereas LLC-PK1 cells produced high levels of uPA in response to all these agents. Both cell lines responded to phorbol esters in terms of uPA induction, though to differing extents. Phorbol myristate acetate (PMA) was shown conclusively not to activate cAMP-PK in either cell line, even at concentrations 10-fold higher than those promoting maximal uPA induction. It was concluded that phorbol ester-mediated induction of uPA does not involve cAMP or cAMP-PK activation. These results are discussed in relation to proposed models concerning the role of cAMP-PK in uPA induction.     

Activation of cAMP-dependent protein kinase alters the chromatin structure of the urokinase-type plasminogen activator gene promoter.

In LLC-PK1 cells, the urokinase-type plasminogen activator (uPA) gene is induced by two of the major signal transduction pathways, the protein kinase C (PKC) and the cAMP-dependent protein kinase (PKA) pathways. We have analyzed the chromatin structure of 26 kb of the uPA gene locus and have shown that PKA activation but not PKC activation induce major chromatin structural alterations in the uPA gene promoter. In uninduced cells, several DNase I hypersensitive (HS) sites were detected in the 5' and 3' flanking regions but not in the transcribed region. Two of the sites correspond to previously characterized regulatory sites: a cAMP responsive site at nucleotide position -3500 with respect to the initiation site, and the PEA3/AP1 site at -2100 that mediates PKC activation. After the activation of PKA but not PKC, a strong HS site was induced at -2600. Functional analysis of this region revealed cAMP responsive activity. Chromatin structural alterations again brought about specifically by PKA but not by PKC were were also detected in the upstream of the promoter by topoisomerase I cleavage site analysis, with two prominent sites appearing at -2800 and -3300. These results suggest that the strong cAMP induction of the uPA gene requires structural alterations that permit cooperative interactions between the multiple cAMP responsive sites.     

PAI-1 inhibits urokinase-induced chemotaxis by internalizing the urokinase receptor

PAI-1 (plasminogen activator inhibitor-1) binds the urokinase-type plasminogen activator (uPA) and causes its degradation via its receptor uPAR and low-density lipoprotein receptor-related protein (LRP). While both uPA and PAI-1 are chemoattractants, we find that a preformed uPA-PAI-1 complex has no chemotactic activity and that PAI-1 inhibits uPA-induced chemotaxis. The inhibitory effect of PAI-1 on uPA-dependent chemotaxis is reversed when uPAR internalization is inhibited by the 39 kDa receptor-associated protein or by anti-LRP antibodies. Under the same conditions, the uPA-PAI-1 complex is turned into a chemoattractant causing cytoskeleton reorganization and extracellular-regulated kinase/mitogen-activated protein kinases activation. Thus, uPAR internalization by PAI-1 regulates cell migration.     

cAMP analogues downregulate the expression of granulocyte macrophage colony-stimulating factor (GM-CSF) in human bone marrow stromal cells in vitro.

The stimulation of granulocyte macrophage-colony stimulating factor (GM-CSF) by interleukin-1 (IL-1) has been shown to be counteracted in different mesenchymal cell systems by cyclic adenosine monophosphate (cAMP) agonists. The aim of this study was the evaluation of different cAMP agonists on GM-CSF expression in human bone marrow stromal cells. Incubation of secondary haematopoietic progenitor cell deprived human stromal cell cultures with IL-1 or TNF-alpha induced GM-CSF protein expression in culture supernatants and GM-CSF-mRNA in adherent stromal cells. The coincubation with 8-bromo-cAMP (8BrcAMP), a water soluble cAMP analogue, inhibited this GM-CSF stimulation at the protein and the mRNA level. This effect was dose dependent with a maximal inhibition of about 65% occurring at a 8BrcAMP concentration of 0.75 mM. In addition to 8BrcAMP, other cAMP agonists such as dibutyryl-cAMP, forskolin, pertussis toxin, or prostaglandin E2 (PGE2) had the same inhibitory effect on GM-CSF stimulation by IL-1. Coincubation with the cyclooxygenase inhibitor indomethacin had no significant influence on GM-CSF expression in stromal cells. Our results provide evidence that the previously described inhibitory effect of cAMP agonist PGE2 on haematopoietic progenitor cells in vivo is, at least in part, mediated by modulating the expression of GM-CSF in bone marrow stromal cells.     

Regulation of plasminogen activators in human thyroid follicular cells and their relationship to differentiated function

Human thyroid cells in culture take up and organify (125)I when cultured in TSH (acting through cAMP) and insulin. They also secrete urokinase (uPA) and tissue-type (tPA) plasminogen activators (5-100 IU/10(6)cells/day). TSH and insulin both decreased secreted PA activity (PAA), uPA and tPA protein and their mRNAs. Autocrine fibroblast growth factor increased secreted PAA and inhibited thyroid cell (125)I uptake. Epidermal growth factor (EGF) and the protein kinase C (PKC) activator, TPA significantly increased PAA and inhibited thyroid differentiated function, (TPA > EGF). For TPA, effects were rapid, increased PAA secretion and decreased (125)I uptake being seen at 4 h whereas for EGF, a 24 h incubation was required. qRT-PCR showed significantly increased mRNA expression of uPA with lesser effects on tPA. Aprotinin, which inhibits PAA, increased (125)I uptake but did not abrogate the effects of TPA and EGF. The MEKK inhibitor, PD98059 partially reversed the effects of EGF and TPA on PAA, and largely reversed the effects of EGF but not TPA on differentiated function. PKC inhibitors bisindoylmaleimide 1, and the specific PKCbeta inhibitor, LY379196 completely reversed the effects of TPA on (125)I uptake and PAA whereas EGF effects were unaffected. TPA inhibited follicle formation and this effect was blocked by LY379196 but not PD98059. We conclude that in thyroid cells, MAPK activation inversely correlates with (125)I uptake and directly correlates with PA expression, in contrast to the effects of cAMP. TPA effects on iodide metabolism, dissolution of follicles and uPA synthesis are mediated predominantly through PKCbeta whereas EGF exerts its effects through MAPK but not PKCbeta.