title: Inhibition of macrophage phagocytosis by methylation inhibitors. Lack of correlation of protein carboxymethylation and phospholipid methylation with phagocytosis.
abstract: Adenosine (Ado), deoxyadenosine (dAdo), and adenine arabinoside (AraA) inhibit the phagocytosis of IgG-coated erythrocytes and zymosan by resident and thioglycollate-elicited macrophages (thio-macrophages) in a dose-dependent and reversible manner. 3-Deazaadenosine (3cAdo) and adenine (Ade) also inhibit the phagocytosis by resident macrophages. Homocysteine thiolactonate (Hcy) potentiates the inhibition by Ado and 3cAdo while erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) potentiates the inhibition by Ado, dAdo and AraA. This inhibition has a very rapid onset and the drugs do not interfere with the binding of IgG-coated erythrocytes to macrophages. The combination of Ado, Hcy and EHNA does not appreciably affect the intracellular level of ATP and S-adenosyl-L-methionine (AdoMet) in thio-macrophages but causes accumulations of Ado and S-adenosyl-L-homocysteine (AdoHcy) up to 135 and 145 nmol/mg of protein, respectively. During phagocytosis reversal, Ado is metabolized within 15 min while AdoHcy decreases log-arithmically with a half-life of 50 min. Carboxymethylation and phospholipid methylation, however, resume about 60-90 min after phagocytosis has recovered, and thus cannot function as transmembrane signals for phagocytosis. Other evidence showing the lack of correlation between phagocytosis and carboxymethylation inhibition include 1) Ado + Hcy inhibit carboxymethylation much better than Ado + EHNA (91 versus 75%) in thio-macrophage, but the two combinations show comparable phagocytosis inhibition potency; 2) Ado + Hcy inhibit carboxymethylation almost as well as Ado + Hcy + EHNA, but the latter is a much more effective drug combination for phagocytosis inhibition; 3) Ade and 3cAdo, although inhibiting resident macrophage phagocytosis as well as Ado + EHNA + Hcy, are much weaker carboxymethylation inhibitors; 4) dAdo and AraA potently inhibit phagocytosis but not carboxymethylation. The difference in the apparent methylation levels is not due to changes in the specific activities of AdoMet, which decrease with a half-life of 88 min. Interestingly, after the initial lag phase of about 90 min after the initiation of inhibition reversal, carboxymethylation and phagocytosis increase in parallel. In a log-log plot of carboxymethylation, phospholipid methylation, or phagocytosis versus the intracellular AdoHcy accumulation, a linear relationship is obtained. It is possible that AdoHcy accumulation is responsible for phagocytosis inhibition but inhibits by a mechanism other than interfering with protein and lipid methylations.
mesh_terms: D000225:Adenine / Q000031:analogs & derivatives / Q000494:pharmacology; D000241:Adenosine / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002478:Cells, Cultured; D003839:Deoxyadenosines / Q000494:pharmacology; D006710:Homocysteine / Q000031:analogs & derivatives / Q000494:pharmacology; D007700:Kinetics; D008264:Macrophages / Q000187:drug effects / Q000502:physiology*; D008745:Methylation; D051379:Mice; D010587:Phagocytosis / Q000187:drug effects*; D010743:Phospholipids / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism*; D014740:Vidarabine / Q000494:pharmacology
chemical_list: D003839:Deoxyadenosines; D010743:Phospholipids; D011506:Proteins; D006710:Homocysteine; C010192:9-(2-hydroxy-3-nonyl)adenine; D000255:Adenosine Triphosphate; C007957:homocysteine thiolactone; D014740:Vidarabine; D000225:Adenine; D000241:Adenosine