M-CSF induces PI 3-kinase activation, resulting in reactive oxygen species (ROS) production. Previously, we reported that ROS mediate macrophage colony-stimulating factor (M-CSF)-induced extracellular regulated kinase (Erk) activation and monocyte survival. In this work, we hypothesized that M-CSF-stimulated ROS products modulated Akt1 and p38 activation. Furthermore, we sought to clarify the source of these ROS and the role of ROS and Akt in monocyte/macrophage survival. Macrophages from p47(phox-/-) mice, lacking a key component of the NADPH oxidase complex required for ROS generation, had reduced cell survival and Akt1 and p38 mitogen-activated protein kinase (MAPK) phosphorylation compared with wild-type macrophages in response to M-CSF stimulation, but had no difference in M-CSF-stimulated Erk. To understand how ROS affected monocyte survival and signaling, we observed that NAC and DPI decreased cell survival and Akt1 and p38 MAPK phosphorylation. Using bone marrow-derived macrophages from mice expressing constitutively activated Akt1 (Myr-Akt1) or transfecting Myr-Akt1 constructs into human peripheral monocytes, we concluded that Akt is a positive regulator of monocyte survival. Moreover, the p38 MAPK inhibitor, SB203580, inhibited p38 activity and M-CSF-induced monocyte survival. These findings demonstrate that ROS generated from the NADPH oxidase complex contribute to monocyte/macrophage survival induced by M-CSF via regulation of Akt and p38 MAPK.

The ability to target and accumulate monocytes and macrophages in areas of tissue inflammation plays an important role in innate and humoral immunity. However, when this process becomes uncontrolled, tissue injury and dysfunction may ensue. This paper will focus on understanding the role and action of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in regulating the molecular and biochemical pathways responsible for the regulation of the survival of human monocytes. We and others have found that ROS and RNS serve as important intracellular signaling molecules that influence cellular survival. Human monocytes are influenced by intracellular production of ROS and RNS, which affects both monocyte survival and death, depending on the form of nitric oxide presented to the cell. This review will address potential mechanisms by which ROS and RNS promote the survival of human monocytes and macrophages.

In the absence of survival factors, blood monocytes undergo spontaneous apoptosis, which involves the activation of caspase-3. Although nitric oxide can block caspase-3 activation and promote cell survival, it can also induce apoptosis. We hypothesized that nitrosothiols that promote protein S-nitrosylation would reduce caspase-3 activation and cell survival, whereas nitric oxide donors (such as 1-propamine 3-(2-hydroxy-2-nitroso-1-propylhydrazine (PAPA) NONOate and diethylamine (DEA) NONOate) that do not target thiol residues would not. Using human monocytes as a model, we observed that nitrosothiol donors S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine suppressed caspase-9 and caspase-3 activity and DNA fragmentation. In contrast, PAPA or DEA NONOate did not promote monocyte survival events and appeared to inhibit monocyte survival induced by macrophage colony-stimulating factor. The caspase-3-selective inhibitor DEVD-fluoromethyl ketone reversed DNA fragmentation events, and the caspase-9 inhibitor LEHD-fluoromethyl ketone reversed caspase-3 activity in monocytes treated with PAPA or DEA NONOate in the presence of macrophage colony-stimulating factor. These results were not caused by differences in glutathione levels or the kinetics of nitric oxide release. Moreover, S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine directly blocked the activity of recombinant caspase-3, which was reversed by the reducing agent dithiothreitol, whereas PAPA or DEA NONOate did not block the enzymatic activity of caspase-3. These data support the hypothesis that nitrosylation of protein thiol residues by nitric oxide is critical for promoting the survival of human monocytes.

Previous reports from our laboratory showed M-CSF promotes PI 3-kinase activation resulting in the production of reactive oxygen species (ROS) and PI 3-kinase inhibitors, and the antioxidants diphenyleneiodonium (DPI) and n-acetyl cysteine (NAC) suppressed M-CSF-stimulated Erk activation. In this study, we hypothesized that M-CSF-induced generation of ROS affected Akt1 activation and sought to define the role of Akt1 in monocyte/macrophage survival and differentiation. We found that the production of ROS following M-CSF-treatment was inhibited by the antioxidant DPI. The addition of either DPI or NAC to the monocytes in the presence of M-CSF resulted in decrease cell survival as measured by Annexin V/PI and DNA fragmentation. In the cells treated with the antioxidants, there was a reduction in pAKT protein levels compared to M-CSF alone treatment suggesting that ROS contributed to Akt activity and cell survival. Macrophages from p47phox −/− mice, lacking a key component of the NADPH oxidase complex required for ROS generation were examined for M-CSF-induced survival and Akt1 activation. Bone marrow macrophages from p47phox −/ − mice and wild type (WT) littermates were isolated and differentiated in RPMI-1640 medium in the presence of 20 ng/ml of M-CSF for 5 days. We observed a reduction in Akt1 phosphorylation, cellular survival and increase in apoptosis measured by Annexin V/PI staining in p47phox −/ − macrophages compared to WT controls. Since macrophages from the p47phox−/− mice had reduced Akt1 activity and cell survival to M-CSF stimulation, we next wanted to independently evaluate the role of Akt in macrophage survival and differentiation. We therefore examined macrophages from mice that had targeted expression of activated Akt1 (Myr-Akt1) in mononuclear phagocytes. Bone marrow from Myr-Akt1 mice and (WT) littermates was isolated and cultured in the presence of 20 ng/ml of M-CSF for 5 days. We found that Myr-Akt1 cells had enhanced survival and reduced apoptosis versus WT cells. Interestingly, Myr-Akt1 mice had normal circulating numbers of monocytes, but had splenomegaly and increased numbers of mature macrophages in their spleens by CD68 staining. Since the expression of Myr-Akt1 in BMM enhanced cell survival, we were interested in the effect in human monocytes. Peripheral blood monocytes (PBM) were transiently transfected with Myr-Akt1 constructs. We observed a decrease in Annexin V/PI staining indicating promotion of cell survival in PBM expressing Myr-Akt compared to untransfected PBM. Since Akt1 appeared to be important for macrophage survival/differentiation, we examined its contribution in human alveolar macrophage homeostasis and found that freshly isolated macrophages had constitutive Akt1 activity and had increased levels of Akt1 protein. This data support a critical role for Akt1 in macrophage differentiation in mice. Together, our findings may provide insight in the pathogenesis of monocyte/macrophage homeostasis.