Nitric oxide and resolution of inflammation

Activated macrophages play a central role in both the development and resolution of inflammation. These immune cells need to be functional in harmful conditions with high levels of reactive oxygen and nitrogen species that can damage their basic cell components, which may alter their metabolism. An excessive accumulation of these cell alterations drives macrophages inexorably to cell death, which has been associated to the development of several inflammatory diseases and even with aging in a process termed as “immunosenescence”. Macrophages, however, exhibit a prolonged survival in this hostile environment because they equip themselves with a complex network of protective mechanisms. Here we provide an overview of these self-defense mechanisms with special attention being paid to bioactive lipid mediators, NRF2 signaling and metabolic reprogramming.

Nowadays, it is well recognized that amino acids are powerful molecules responsible for regulatory control over fundamental cellular processes. However, our understanding of the signaling cascades involved in amino acid sensing in organisms, as well as signal initiation, is largely limited. This is also the case of semi-essential amino acid l-arginine, which has multiple metabolic fates, and it is considered as one of the most versatile amino acids. Recently, some new and important facts have been published considering the role of l-arginine in the regulation of inflammatory processes in several human and mouse models, mediated also via the regulation of macrophage activation. Therefore, this mini review focuses on the actual summarization of information about (i) l-arginine bioavailability in organism, (ii) l-arginine-dependent regulation of nitric oxide synthase expression and nitric oxide production, and importantly (iii) its role in the activation of intracellular signaling pathways and G-protein-coupled receptors in macrophages.

Lilium lancifolium is commonly used to treat bronchitis, pneumonia, etc. In this study, we investigated the anti-inflammatory effects of methanol extracts of the root of Lilium lancifolium (LL extracts) in LPS-stimulated Raw264.7 cells.

Levels of NO, PGE₂ and pro-inflammatory cytokines (IL-6 and TNF-α) in the supernatant fraction were determined using sandwich ELISA. Expression of COX-2 and iNOS, phosphorylation of MAPK subgroups (ERK and JNK), and NF-κB activation in extracts were detected via Western blot and immunocytochemistry assays.

The LL extract significantly inhibited NO, PGE₂, IL-6 and TNF-α production in LPS-stimulated cells, and suppressed iNOS and COX-2 expression. A mechanism-based study showed that phosphorylation of ERK1/2 and JNK and translocation of the NF-κB p65 subunit into nuclei were inhibited by the LL extract. Furthermore, interleukin-4 and interleukin-13 production in Con A-induced splenocytes was suppressed.

These results indicate that anti-inflammatory effects of methanol extracts from Lilium lancifolium are due to downregulation of iNOS and COX-2 via suppression of NF-κB activation and nuclear translocation as well as blocking of ERK and JNK signaling in LPS-stimulated Raw264.7 cells.

Apoptosis plays an important role in a great number of pathological processes, including atherosclerotic disease. Although apoptosis occurs in the major cell types found in atherosclerotic lesions (e.g. macrophages, endothelial cells, and smooth muscle cells), the mechanism involved in this process differs depending on the stage, the localization and the cell composition of the plaque. In this study, we have compared the effects of different apoptotic inducers on the cells that form the atherosclerotic plaque. We have demonstrated that monocytes and macrophages are more susceptible to apoptosis than smooth muscle cells and endothelial cells. These findings provide insights about the potential role of apoptosis in the atherosclerotic disease and suggest strategies to treat vascular diseases by exploiting the differential sensitivity of cells to cell death.

Macrophages participate actively in the inflammatory response by releasing cytokines, chemokines and factors that recruit additional cells to sites of infection or tissue injury or alteration. In addition to this, activated macrophages rapidly activate the expression of genes responsible for the high-output synthesis of reactive oxygen and nitrogen species (NO, O₂⁻, H₂O₂ and peroxynitrite, among others) and bioactive lipids derived from arachidonic acid. All of these agents contribute to the regulation of the inflammatory response. Most of these molecules, when synthesized at these high concentrations, exert pro-apoptotic effects in many cell types. Macrophages themselves are a notable and important exception, being resistant to apoptotic death upon activation. This resistance is necessary to enable these cells to perform their functional role during the early phases of an inflammatory response. However, after cumulative damage, or when the synthesis of inflammatory mediators decreases, macrophages undergo the characteristic mitochondrial-dependent cell death program, contributing in this way to the resolution of the inflammatory reaction. In the case of infectious diseases, this also helps to prevent the development of parasitic strategies by phagocytosed pathogens.

We have investigated the role of the nitric oxide (NO) and prostaglandins (PGs), respectively, synthesized by nitric oxide synthase 2 (NOS-2) and cyclooxygenase-2 (COX-2), in the outcome of liver regeneration after partial hepatectomy (PH).

Liver mass recovery and molecular parameters related to cell proliferation and apoptotic death have been determined.

NOS-2 and COX-2 are normally both expressed in the remnant liver after PH, and inhibition of either one delays regeneration. We found, however, that simultaneous suppression of the activities of NOS-2 (by gene knockout) and COX-2 (by pharmacological inhibition) resulted in animal death between 24 and 72 h after PH. Analysis of liver mass recovery and molecular parameters related to cell proliferation and apoptotic death revealed increased liver-cell apoptosis and an insufficient proliferative response. Broad-specificity caspase inhibitors, such as z-Val-Ala-Asp.fmk (z-VAD), or administration of NO-donors, rescued animals from death, revealing a critical apoptotic bias at this stage of proliferation.

These findings demonstrate that simultaneous signaling by NO and prostaglandins plays an important role in the mechanism of liver regeneration after PH by protecting the remnant tissue from apoptotic death.