Eicosanoids and exosomes : a link between macrophages and lung cancer

Abstract

Chronic inflammation increases the risk of lung cancer. Macrophages (MO) are important players in inflammation, with regulatory and executive functions. Eicosanoids and exosomes can be both triggers and mediators of these functions. Cysteinyl leukotrienes (CysLTs) are the most potent mediators of broncho-constriction in the lungs, a function exerted via CysLT1 receptor. Their function in asthma is well described, but little is known about CysLTs and lung cancer.

In the first study we investigated how the interaction between pulmonary epithelium and leukocytes affects CysLTs formation. Monocytic cells and eosinophils formed LTC4, which was exported and promptly converted to LTD4 by pulmonary epithelial cells in a transcellular manner. The lung cancer cell line A549 expressing γ-glutamyl transpeptidase 1 (GGT-1) showed a high activity. Exosomes released by A549 cells also contained GGT-1 and efficiently converted LTC4 to LTD4. On the other hand, healthy bronchial epithelial cells (PBEC) expressing GGT-5 formed LTD4 12 times more slowly. The results highlight an active role for epithelial cells and their exosomes in biosynthesis of LTD4, which may be of particular relevance in the lung, given that LTD4 is the most potent agonist of CysLT1. MOs can be differentiated from blood monocytes with GM-CSF and M-CSF, resulting in cells primed toward the inflammatory M1- and resolving M2-states. A comprehensive analysis of eicosanoid formation in these two in vitro models is missing and our second study focused on this gap. By LC-MS analysis, we observed that both MO phenotypes released pro-resolving lipid mediators (PGE metabolite, LXA4) in resting conditions. When the same cells were incubated (30 min) with bacterial stimuli, there was a shift to pro-inflammatory eicosanoids: M-CSF MOs produced high amounts of LTC4, relevant for M2 functions in asthma. GM-CSF cells expressed the highest levels of cPLA2, 5-LO and FLAP; and in ionophore incubations these cells also produced the highest levels of 5-HETE. However, MCSF MO formed more products apparently due to a better response to bacterial stimuli, demonstrated by enhanced mobilization and activation of cPLA2 and 5-LO. In conclusion, GM-CSF and M-CSF can regulate specific pathways in MOs, and it appears that eicosanoid biosynthesis primarily reflect the cellular response and activation mechanisms, rather than the protein expression profile.

In colon cancer a pro-tumorigenic effect of LTD4 but not LTC4 has been demonstrated. A pro-tumorigenic effect has been shown also for exosomes. To extend the findings of our first study, we used pleura exudates from lung cancer patients to isolate primary cancer cells and exosomes. Both cells and exosomes metabolized LTC4 to LTD4, and we also found that exosomes stimulated CysLTs formation in the cancer cells. Cancer cells from all patients expressed CysLT1, and exosomes promoted their migration and survival in a CysLT1 dependent manner, as demonstrated by the inhibition by montelukast (MK) treatment, a CysLT1 antagonist used to treat asthma.

In cancer, interactions between the transformed cancer cells and other recruited cell types in the tumor are important. Tumor associated macrophages (TAMs) provide cancer cells with a suitable low-grade inflammation milieu including growth promoting factors. Taken together, the results in this thesis suggest a novel pro-tumorigenic mechanism based on this theme, driven by the exosomes/CysLT1 cascade: TAMs provide LTC4 that lung cancer cells and their exosomes convert to LTD4. Via CysLT1 receptor this promotes survival and migration of the cancer cells. A protective effect in lung cancer has been previously described for MK and our results suggest a possible mechanism for this, driven by the exosomes/ CysLT1 cascade, further encouraging the use of this drug in lung cancer treatment.