Atriopeptins and nitroprusside provoke opposite changes in cGMP and cAMP levels in human macrophages

Modulation of nitric oxide synthase activity in macrophages

L-Arginine is converted to the highly reactive and unstable nitric oxide (NO) and L-citrulline by an enzyme named nitric oxide synthase (NOS). NO decomposes into other nitrogen oxides such as nitrite (NO₂^-) and nitrate (NO₂^-), and in the presence of superoxide anion to the potent oxidizing agent peroxynitrite (ONOO⁻). Activated rodent macrophages are capable of expressing an inducible form of this enzyme (iNOS) in response to appropriate stimuli, i.e., lipopolysaccharide (LPS) and interferon-γ (IFNγ). Other cytokines can modulate the induction of NO biosynthesis in macrophages. NO is a major effector molecule of the anti-microbial and cytotoxic activity of rodent macrophages against certain micro-organisms and tumour cells, respectively. The NO synthesizing pathway has been demonstrated in human monocytes and other cells, but its role in host defence seems to be accessory. A delicate functional balance between microbial stimuli, host-derived cytokines and hormones in the microenvironment regulates iNOS expression. This review will focus mainly on the known and proposed mechanisms of the regulation of iNOS induction, and on agents that can modulate NO release once the active enzyme has been expressed in the macrophage.

Expression of natriuretic peptide receptor mRNA and functional response to atrial natriuretic peptide and C-type natriuretic peptide in rainbow trout (Oncorhynchus mykiss) head kidney leucocytes

The stimulatory effect of vasomodulatory natriuretic peptide hormones on macrophages and peripheral blood leucocytes in mammals is well-established. However, the relationship in lower vertebrates has not been characterised. Expression of atrial natriuretic peptide, ventricular natriuretic peptide and C-type natriuretic peptide-1, and the guanylyl cyclase-linked (GC) natriuretic peptide receptor-A and -B-type receptors (NPR-A and NPR-B, respectively) was determined by PCR from the mRNA of rainbow trout head kidney leucocytes yielding gene fragments with 100% homology to the same respective natriuretic peptide and NPR-A and -B sequences obtained from other rainbow trout tissues. A mixed population of isolated rainbow trout head kidney leucocytes was stimulated in vitro with trout atrial natriuretic peptide (specific NPR-A agonist) and trout C-type natriuretic peptide (NPR-A and -B agonist) as well as the cGMP agonist 8-bromo-cGMP or the GC inhibitor 8-bromo-phenyl-eutheno-cGMP. Respiratory burst was stimulated by trout atrial natriuretic peptide, trout C-type natriuretic peptide-1 and 8-bromo-cGMP in a dose dependant manner with the highest activity as a result of stimulation with trout C-type natriuretic peptide-1 in excess of that achieved by phorbol myristate acetate (PMA). Equimolar concentrations of the inhibitor, inhibited the respiratory burst caused by the natriuretic peptides and 8-bromo-cGMP. The natriuretic peptide receptors on rainbow trout head kidney leucocytes appear to have a stimulatory function with regard to respiratory burst that is activated through a cGMP second messenger pathway and the natriuretic peptides expressed in the head kidney leucocytes may well act in a paracrine/autocrine manner.

PDE1B2 regulates cGMP and a subset of the phenotypic characteristics acquired upon macrophage differentiation from a monocyte

Monocyte-to-macrophage differentiation with the cytokine granulocyte-macrophage colony-stimulating factor induces expression of the cyclic nucleotide phosphodiesterase PDE1B2. However, what role PDE1B2 plays in macrophage biology has not been elucidated. We have addressed this question by inhibiting PDE1B2 induction by using RNA interference. Using a retrovirus-based system, we created HL-60 stable cell lines that express a short-hairpin RNA targeting PDE1B2. HL-60 cells treated with phorbol-12-myristate-13-acetate differentiate to a macrophage-like phenotype and up-regulate PDE1B2. However, expression of PDE1B2 short hairpin RNA effectively suppresses PDE1B2 mRNA, protein, and activity up-regulation. Using the HL-60 PDE1B2 knockdown cells and agonists for either adenylyl or guanylyl cyclase, it was found that PDE1B2 predominantly regulates cGMP and plays a lesser role in cAMP regulation in response to cyclase agonists. Furthermore, in intact HL-60 cells, PDE1B2 activity can be regulated by changes in Ca+2 levels. Inhibiting PDE1B2 up-regulation does not prevent HL-60 cell differentiation, because several markers of macrophage differentiation are unaffected. However, suppression of PDE1B2 expression alters some aspects of the macrophage-like phenotype, because cell spreading, phagocytic ability, and CD11b expression are augmented. The cAMP analog 8-Bromo-cAMP reverses the changes caused by PDE1B2 knockdown. Also, PDE1B2 knockdown cells have lower basal levels of cAMP and alterations in the phosphorylation state of several probable PKA substrate proteins. Thus, the effects of PDE1B2 on differentiation may ultimately be mediated through decreased cAMP. In conclusion, PDE1B2 regulates a subset of phenotypic changes that occur upon phorbol-12-myristate-13-acetate-induced differentiation and likely also plays a role in differentiated macrophages by regulating agonist-stimulated cGMP levels.

Effects of atrial natriuretic peptide on phagocytosis and respiratory burst in murine macrophages

Atrial natriuretic peptide (ANP) is known to affect cardiovascular physiology displaying both hormone- and neurotransmitter characteristics. However, there is increasing evidence that ANP possesses additional biological activities referring to the immune system. To further strengthen this hypothesis the effect of ANP on two major functions of macrophages, i.e., phagocytosis and respiratory burst was tested. Both parameters were analyzed by flow cytometry employing bone marrow derived macrophages and the murine macrophage cell line J774. In both cell types preincubation with ANP dose dependently (10(-10)-10(-7) M) increases ingestion of opsonized fluorescent latex particles. The respiratory burst activity was monitored by oxidation of dihydrorhodamine-123 in cells stimulated either with phorbol-myristate (PMA, 10 ng/ml) or formyl-Met-Leu-Phe (fMLP, 1 microM). In both cases preexposure of cells to ANP (10(-8)-10(-6) M) for 2 h enhances reactive oxygen production. The data demonstrate an influence of ANP on important defense mechanisms of macrophages and thus extend the knowledge regarding the pharmacological profile of this natriuretic peptide.

Atrial natriuretic peptide inhibits nitric oxide synthesis in mouse macrophages

The atrial natriuretic peptide (ANP) affects cardiovascular physiology, and, as has been suggested more recently, exerts immunomodulatory activities. In this context, we examined the effect of ANP on nitric oxide (NO) synthesis in murine bone marrow derived macrophages as well as in peritoneal macrophages. Cultured macrophages were stimulated with lipopolysaccharides (LPS 0.1-10 micrograms/ml) and NO synthesis was monitored by measuring increased concentrations of NO2 in the medium. In initial experiments employment of NG-monomethyl-L-arginine (L-NMMA) and dexamethasone, two specific inhibitors of nitric oxide synthase (NOS), confirmed the presence of inducible NOS activity in the cells. Exposure of cells to rat ANP99-126 in the range of 10(-8) to 10(-6) M significantly decreased LPS induced NO synthesis over 24 hours of incubation. Thus, ANP may alter macrophage function by affecting their nitric oxide synthesizing pathway.

Effects of atrial natriuretic peptide and cGMP on uptake of IgG complexes by macrophages

Macromolecular handling by macrophages and glomerular mesangial cells may be important in the development of renal injury. We undertook the present study to determine whether atrial natriuretic peptide (ANP), a particulate guanylate cyclase stimulator, plays a direct role in uptake of immunoglobulin G (IgG) complexes by macrophages. Macrophages incubated with ANP at 10(-5) and 10(-6) M showed significantly suppressed uptake of IgG complexes compared with control. Macrophage uptake of IgG complexes was also significantly suppressed by the soluble guanylate cyclase stimulator sodium nitroprusside. Dibutyryl guanosine 3',5'-cyclic monophosphate and dibutyryl adenosine 3',5'-cyclic monophosphate both significantly suppressed IgG complex uptake as well. ANP was found to significantly enhance macrophage guanosine 3',5'-cyclic monophosphate (cGMP) levels compared with control cells, and this effect was antagonized by angiotensin II. Angiotensin II significantly enhanced uptake of IgG complexes and suppressed macrophage adenosine 3',5'-cyclic monophosphate synthesis, and both effects were antagonized by coincubation with ANP. These results suggest that ANP modulates uptake of IgG complexes by macrophages and that this effect may be mediated via cGMP.