Characterization of the activation of monoamine oxidases in conditions of cardiac damage and inflammation

Mitochondria represent the main site of both ROS formation and degradation. One of the main source of mitochondrial ROS is represented by monoamine oxidases (MAOs). MAOs are flavoenzymes located in the outer mitochondrial membrane, which catalyse the oxidative deamination of biogenic amines yielding aldehydes, ammonia, and hydrogen peroxide (H2O2). It has been demonstrated that these products contribute to the oxidative stress occurring in hearts subjected to pathological conditions, such as cardiac reperfusion injury and decompensated hypertrophy. As MAO inhibitors significantly protect the heart in these models of cardiac injury, we focused our attention on the molecular mechanisms underlying MAO activation. To address this issue we investigated (i) the availability of MAO substrates under conditions of oxidative stress or injury and (ii) their main cellular sources in the whole heart. By mass spectrometry (MS) analysis we identified several amines that became available upon different ex vivo protocols of cardiac injury, and we focused our attention on N-tele-methylhistamine (NMH) and its precursor histamine, which were found the most abundant. Thus, we evaluated the contribution of myocyte and non-myocyte cells as possible sources of MAO substrates. For this purpose we firstly excluded synaptic terminals that innervate heart by mice injection of a neurotoxin, 6-hydroxydopamine (6-OH-DOPA). Our findings demonstrate the relevant contribution of synaptic terminals to increase the availability of MAO substrates in the ex vivo model of oxidative stress. Next we considered isolated cardiomyocytes, as these cells undergo MAO-dependent oxidative stress. We showed that cardiomyocytes can synthesize MAO substrates and to induce MAO activity when exposed to oxidative stress. Taken together, we establish for the first time a relevant interaction between histamine metabolism and MAO activity in cardiac injury, that does not involve a receptor-dependent pathway. These findings can explain how MAO activity is turned on when cells are stressed. Moreover, it also helps understanding how the increase in MAO activity amplifies an initial oxidative stress. It has been largely shown that inflammation is usually involved in cardiac diseases and monocytes (Mn), macrophages (MF) as well as mast cells are involved in the inflammatory response. We focused our attention on immune phagocytic cells since the role of MAO has not been conclusively defined. Firstly, we demonstrated that both the MAO isoforms were expressed in both M1 and M2 MF. We focused our attention on MAO A, as it resulted the most expressed and significant isoform, especially in M2 MF. Then, we characterized its pathway of induction considering two different stimuli: LPS, that is a pro-inflammatory signal and IL-4 plus IL-13 (IL-4+IL-13), that is a combination of anti-inflammatory cytokines. It is well known that during differentiation and polarization MF generate ROS, which leads to the activation of different signalling pathways. However, the mechanisms that induce ROS formation and activate this signalling remain unclear. Here we demonstrated that MAO contributes to macrophage differentiation and polarization through its H2O2 production. Taken together, these data demonstrate for the first time that MAO-A plays a role in M2 MF differentiation and activation, besides its role in amine oxidative degradation. These novel insights about MAO activity in phagocytic cells suggest that this enzyme could represent a new target to modulate MF differentiation and activation under pathological conditions to avoid the side effects related to inflammation such as fibrosis, cardiac remodelling, and oxidative stress amplification in post-ischemic reperfusion injury.