Effects of adenosine analogues on basal, prostaglandin E1- and forskolin-stimulated cyclic AMP formation in intact neuroblastoma cells

Differential modulation of AP-1- and CRE-driven transcription by cannabinoid agonists emphasizes functional selectivity at the CB1 receptor

BACKGROUND AND PURPOSE

Long-term adaptations to pharmacological stimuli frequently originate from modulation of complex intracellular signalling pathways. We previously reported that HU210 and CP55940, two CB1 cannabinoid receptor agonists, induced opposite effects on TH expression. Herein, we characterized their influence on cAMP response element (CRE) and activator protein 1 (AP-1)-mediated regulation of gene transcription.

EXPERIMENTAL APPROACH

The activity of the agonists was examined on transfected N1E-115 cells in which expression of the luciferase reporter gene was controlled by transcription promoters consisting of repeats of either CRE or AP-1 elements. In addition, the implication of classical signalling pathways was investigated using a variety of kinase inhibitors.

KEY RESULTS

Consistent with the CB1-mediated reduction of cAMP accumulation, both ligands decreased CRE-driven luciferase expression with similar potencies. HU210 also exhibited a concentration-dependent reduction of luciferase activity in cells engineered to examine AP-1-controlled transcription, whereas such response was not obtained with CP55940. Responses were all inhibited by SR141716A and were modified in Pertussis toxin-treated cells, suggesting agonist-selective regulations of distinct Gi/o-dependent mechanisms through CB1 receptor activation. Finally, PKC inhibitors efficiently inhibited the paradoxical effect of HU210 on AP-1-mediated transcription, indicating selective regulation of PKC-dependent responses.

CONCLUSIONS AND IMPLICATIONS

Together, our results demonstrate that two cannabinoid ligands, commonly used as reference agonists acting on the same receptor with similar affinities, differentially modulate gene transcription through distinct controls of AP-1. This could reflect activation of distinct subsets of Gi/o-proteins, supporting the concept of functional selectivity at CB1 receptors.

The contribution of serine residues 1588 and 1755 to phosphorylation of the type I inositol 1,4,5-trisphosphate receptor by PKA and PKG

Type I inositol 1,4,5-trisphosphate receptors can be phosphorylated by cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG). To define the site-specificity of these events we analyzed the phosphorylation of mutant receptors expressed in intact cells. These studies showed that S(1588) and S(1755), the serine residues within kinase consensus sequences, are equally sensitive to PKA, that phosphorylation events at these sites are independent of each other, and that PKG predominantly phosphorylates S(1588). These findings provide the basis for understanding the functional consequences of type I inositol 1,4,5-trisphosphate receptor phosphorylation.

Extracellular adenosine-induced apoptosis in mouse neuroblastoma cells: studies on involvement of adenosine receptors and adenosine uptake

The induction of apoptosis by adenosine was studied in the mouse neuroblastoma cell line N1E-115. Apoptosis was characterized by fluorescence and electron microscopy, fluorescence-activated cell sorter (FACS) analysis, and caspase activity assays. A sixteen-hour exposure to 100 microM of adenosine led to chromatin condensation and caspase activation. However, selective agonists for all four adenosine receptors were ineffective. Caspase activation could be blocked partially by an inhibitor of the nucleoside transporter, dipyridamole, and completely by uridine, a competing substrate for adenosine transport. 2'-Deoxycoformycin, an inhibitor of adenosine deaminase, enhanced caspase activation by adenosine but had no effect by itself. Caspase activation could be blocked by 5'-amino-5'-deoxyadenosine, which inhibits the phosphorylation of adenosine by adenosine kinase. These results indicate that adenosine receptors are not involved in adenosine-induced apoptosis in N1E-115 cells, but that uptake of adenosine and its subsequent phosphorylation is required.

Further studies of the mechanism(s) of polyunsaturated-fatty-acid-mediated increases in intracellular cAMP formation in N1E-115 neuroblastoma cells

Following earlier observations that increasing the polyunsaturated fatty-acid (PUFA) content of N1E-115 neuroblastoma cells elevated basal and adenosine (Ado)-stimulated intracellular cyclic AMP (cAMP) formation, we carried out studies to determine the mechanism(s) by which PUFA exerted their modulatory effects. Basal increases in cAMP in the PUFA-enriched (PUFA+) cells were evident with short (60 sec) exposure to a phosphodiesterase inhibitor (Ro 20-1724), and increased to a maximum at 20 min; they were not observed in the absence of Ro 20-1724. Forskolin-stimulated cAMP formation in the presence of the Ro compound was 2- to 3-fold higher in the PUFA+ cells. Basal elevations in cAMP were reduced by approximately 70% by exposing the PUFA+ cells to Ado deaminase (ADA) or to an Ado antagonist, and were further increased by inhibiting ADA, which suggested that they could be producing endogenous Ado that activated stimulatory Ado receptors. However, this did not appear to involve PUFA-mediated stimulation of 5'-nucleotidase activity or inhibition of [3H]Ado uptake. Overall, the results of this study indicated that multiple mechanisms are involved in PUFA modulation of cAMP formation.

Non-eicosanoid functions of essential fatty acids: regulation of adenosine-related functions in cultured neuroblastoma cells

Studies have demonstrated that augmenting the omega 6 polyunsaturated-fatty-acid (PUFA) content of N1E-115 neuroblastoma cells by media supplementation with linoleic acid results in greater than or equal to 2-fold increases in basal levels of intracellular cyclic AMP (cAMP). Data suggested some involvement of increased production of adenosine from endogenous metabolites; however, increases in adenosine were not related to increased activity of 5'-nucleotidase or decreased uptake of extracellular adenosine. PUFA-dependent elevations in basal cAMP were evident within 1 min of exposure to a phosphodiesterase inhibitor; this phenomenon did not appear to be due to PUFA-dependent changes in Ca2+ uptake or to increases in sensitivity of adenylate cyclase to Ca2+. Forskolin-stimulated cAMP formation was 3-fold higher in PUFA-enriched cells than in control cells, which suggested a direct effect on the functioning of the catalytic unit. Linoleic acid supplementation resulted in a 2-fold increase in the maximum amounts of cAMP produced in response to the stable adenosine analogue, 5'-N'ethylcarboxy-amidoadenosine (NECA). The altered stimulatory response did not involve eicosanoid formation, but may have been related to an increase in the number of stimulatory adenosine receptors, as judged by binding of [3H]NECA. These studies indicate that membrane PUFA modulate adenosine-related functions in neuroblastoma cells, and suggest that a complex series of mechanisms is involved in this regulation.

Adenosine-dependent regulation of cyclic AMP accumulation in primary cultures of rat astrocytes and neurons

The regulation of intracellular cyclic AMP (cAMP) formation by adenosine (Ado) and its analogues has been examined in primary cultures of rat-brain astrocytes and neurons. In the presence of the phosphodiesterase inhibitor, Ro 20-1724, basal levels of cAMP ranged from 40-120 pmol/mg protein in both cell types. Levels were not altered by treating the cells with Ado deaminase, which suggested that they did not produce appreciable amounts of endogenous Ado under standard culture conditions. In the astrocytes, microM quantities of agonists increased cAMP up to 30-fold higher than basal values; the relative potencies were typical of an A2 Ado receptor (NECA greater than Ado greater than R-PIA). Neuron-enriched cultures exhibited a maximum fourfold increase in cAMP in response to NECA; this was decreased a further eightfold when the cultures had prolonged exposure to the antimitotic agent, c-Ara, to eliminate greater than 98% of the nonneuronal cells. Low (nM) amounts of the Ado agonists inhibited cAMP formation in both cell types. In the astrocytes, the order of potency of inhibition of isoproterenol-stimulated cAMP formation was typical of an A1 receptor (R-PIA greater than Ado greater than NECA); maximum inhibition was 55-65%. Isoproterenol did not increase cAMP in the neuronal cultures. However, forskolin-stimulated formation was effectively (approximately 50%) inhibited by A1 Ado agonists; inhibition was not affected by prolonged treatment with c-Ara. From this study we tentatively concluded that rat astrocytes and neurons both contain inhibitory A1 Ado receptors, but that the stimulatory "A2" subtype is localized mainly on astrocytes.