Inhibition of hepatoma cell growth by analogs of adenosine and cyclic AMP and the influence of enzymes in mammalian sera

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

N6‐isopentenyladenosine arrests tumor cell proliferation by inhibiting farnesyl diphosphate synthase and protein prenylation

The physiological effects of a variety of N6-substituted adenine and adenosine derivatives called cytokinins have been documented in plants, but information on their occurrence and function in other biological system is limited. Here we investigated the anti-proliferative effect of N6-isopentenyladenosine (i6A), an adenosine and isoprenoid derivative, in a thyroid cell system, FRTL-5 wild-type, and K-ras transformed KiMol cells. Addition of i6A to FRTL-5 cells caused a dose-dependent arrest of the G0-G1 cell phase transition associated with a reduction of cells in the S phase that was much more evident in KiMol cells. I6A arrested tumor cell proliferation by inhibiting farnesyl diphosphate synthase (FPPS) and protein prenylation. Indeed the addition of farnesol reversed these effects and i6A affected protein prenylation, in particular lamin B processing. I6A effect was not mediated by the adenosine receptor but was due to a direct modulation of FPPS enzyme activity as a result of its uptake inside the cells. I6A inhibited FPPS activity more efficaciously in KiMol cells than in normal FRTL-5. Moreover, the i6A anti-proliferative effect was evaluated in vivo in a nude mouse xenograft model, where KiMol cells were implanted subcutaneously. Mice treated with i6A showed a drastic reduction in tumor volume. Our findings indicate that this isoprenoid end product might be used for antineoplastic therapy, an application emulating that of the lovastatin and/or farnesyl-transferase inhibitors

Growth-related expression of the ectonucleotide pyrophosphatase PC-1 in rat liver

Plasma cell differentiation antigen-1 (PC-1) is a 5'-ectonucleotide pyrophosphatase that has been implicated in various processes including insulin- and nucleotide-mediated signaling and cell growth. We show here that the expression of both PC-1 mRNA and protein in rat liver and in hepatoma cells is strictly growth-related. Thus, the level of PC-1 in FAO hepatoma cells increased with the cell density. PC-1 was not expressed in the neonatal rat liver, but gradually appeared in the first weeks of age, to reach adult levels around the weaning period. Furthermore, PC-1 protein and mRNA largely disappeared from the liver within 24 hours following a hepatectomy of 70%, but re-appeared in the later phases (3-15 days) of the ensuing regeneration period. An equally rapid loss of PC-1 protein and mRNA could also be provoked in normal livers by the administration of the translational inhibitor, cycloheximide, but the transcriptional inhibitors, actinomycin D and alpha-amanitin, did not show these effects. Nuclear run-on assays revealed that the loss of PC-1 mRNA after hepatectomy or after the administration of cycloheximide was not caused by a decreased transcription of the PC-1 gene, suggesting that the level of PC-1 is controlled by an mRNA-stabilizing protein that is lost after hepatectomy and has a high turnover.

8-bromo-cAMP and 8-CPT-cAMP increase the density of beta-adrenoceptors in hepatocytes by a mechanism not mimicking the effect of cAMP

Addition of 8-bromo-adenosine 3',5'-cyclic monophosphate (8-bromo-cAMP) or 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (8-CPT-cAMP) to hepatocytes at the time of plating enhanced the acquisition of beta-adrenoceptors that occurs spontaneously upon culturing as primary monolayers. This effect was partially suppressed by the phosphodiesterase inhibitor isobutyl methylxanthine, and was mimicked by 8-bromo-AMP, 8-bromo-adenosine, and the adenosine kinase inhibitor 5'-amino-5'-deoxyadenosine. Agents that elevated the intracellular level of cAMP, such as glucagon and forskolin, and Sp-8-bromo-adenosine 3',5'-monophosphorothioate (Sp-8-bromo-cAMPS), a cAMP analogue that is resistant towards metabolic breakdown, did not significantly enhance beta-adrenoceptor expression when used alone, but glucagon enhanced the effect of 8-bromo-adenosine. 8-bromo-cAMP and 8-bromo-adenosine decreased cellular ATP-levels. These observations suggest that the enhanced beta-adrenoceptor acquisition was mediated mainly through the action of metabolites of 8-bromo-cAMP and 8-CPT-cAMP, although there may be a cAMP-mediated component in the effect. Several mechanisms, including depletion of ATP, are probably involved, and might affect beta-adrenoceptor degradation.

Effects of adenine nucleotides on the proliferation of aortic endothelial cells

The effects of adenine nucleotides and adenosine on DNA synthesis and cell growth have been studied in bovine aortic endothelial cells (BAECs). ATP produced a small but significant (+44%) increase of the fraction of BAECs whose nuclei are labeled by [3H]thymidine. This mitogenic effect was mimicked by ADP, the phosphorothioate analogues ATP gamma S and ADP beta S, and the nonhydrolyzable analogue adenosine 5'-(beta, gamma-imido)triphosphate (APPNP), whereas adenosine 5'-(alpha, beta-methylene)triphosphate (APCPP), a selective agonist of P2x-purinoceptors, had no effect at 10 microM and a small one at 100 microM; this profile is consistent with the involvement of P2y-receptors. Adenosine induced a mitogenic response of a magnitude similar to that of ATP. This effect was not reproduced by R-phenylisopropyl adenosine, by 5'-N-ethylcarboxamide adenosine, or by 2',5'-dideoxyadenosine, selective ligands of the A1- and A2-receptors and the P site, respectively, nor was it inhibited by 8-phenyltheophylline, an antagonist of both A1- and A2-receptors. The mechanism of this adenosine action thus remains unclear. ATP and ATP gamma S did not enhance the proliferation of BAECs cultured in the presence of fetal calf serum concentrations ranging from 0.5% to 10%. They inhibited the growth-promoting effect of basic fibroblast growth factor; among the various nucleotides tested, APCPP was the least effective to reproduce the action of ATP, suggesting the possible involvement of P2y-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

The induction, desensitization and de-induction of tyrosine aminotransferase by 8-bromo-cyclic AMP in rat hepatoma cells

Addition of 1-3 mM-8-bromo-cyclic AMP to monolayer cultures of H-4 rat hepatoma cells resulted in a rapid but short-lived increase in tyrosine aminotransferase (EC 2.6.1.5) activity. The transient nature of this induction is due to desensitization to 8-bromo-cyclic AMP. Throughout this time course of induction and desensitization, removal of 8-bromo-cyclic AMP resulted in a rapid and significant decrease in tyrosine aminotransferase activity, a process referred to as 'de-induction' in this study. We showed that the changes in tyrosine aminotransferase activity in its induction, desensitization and de-induction by 8-bromo-cyclic AMP were directly attributable to changes in the synthesis rate of the protein, and the amount of translatable and hybridizable mRNA encoding for tyrosine aminotransferase (mRNATAT). We further showed that this desensitization was specific to cyclic AMP. First, only active analogues of cyclic AMP and agents which increased cellular concentrations of cyclic AMP elicited this desensitization. Second, the desensitized cells were refractory only to the effects of 8-bromo-cyclic AMP; dexamethasone and insulin induced the tyrosine aminotransferase activity in the 8-bromo-cyclic AMP-desensitized cells in a manner similar to that of the controls. Studies on the metabolism of 8-bromo-cyclic AMP suggest that neither its degradation nor the accumulation of its primary metabolite, 8-bromoadenosine, played a significant role in modulating the expression of tyrosine aminotransferase during the time course of action of 8-bromo-cyclic AMP. These results provide evidence for a specific pretranslational mode of action of cyclic AMP in the control of tyrosine aminotransferase expression in its desensitization and de-induction, in addition to the early phase of induction.

Effect of cyclic AMP on cellular contractility and DNA synthesis in chorioretinal fibroblasts maintained in collagen matrices

Dibutyryl cyclic AMP (db-cAMP), theophylline and forskolin were found to be potent inhibitors of DNA synthesis and cell contractility in chorioretinal fibroblasts maintained in 3-dimensional collagen matrices. Dose-response curves were constructed for the inhibitory action of these agents on both cellular parameters and their interrelationship examined by regression analysis. The results obtained indicate that these parameters were equally inhibited by cAMP with the exception of high concentrations of db-cAMP (greater than 10(-3) M) where a greater effect on DNA synthesis than cell contractility was observed which was attributed to additional inhibition of DNA synthesis by db-cAMP degradation products. It is proposed from the present results and those of other investigators that cAMP regulates non-transformed fibroblast proliferation and contraction through a common regulatory mechanism possibly involving cAMP-dependent protein kinases and calcium ions.