Role of Monoamine Oxidase A and Diet-Induced Monocyte Dysfunction, Macrophage Reprogramming, and Atherosclerosis

Monocytes and macrophages are essential for tissue homeostasis, but in the context of metabolic disorders they
become dysfunctional and promote chronic inflammatory diseases, including atherosclerosis. However, the
underlying mechanisms are not well-understood. We showed that chronic exposure of blood monocytes to
nutrient stress induced by a “Western”-style high-calorie diet (HCD) stimulates the formation of reactive oxygen
species (ROS) and promotes protein thiol oxidation, resulting in monocyte dysfunction and the reprogramming
of blood monocytes into a pro-inflammatory, pro-atherogenic phenotype, hyper-sensitive to chemoattractants.
These metabolically “primed” blood monocytes give rise to reprogrammed and dysfunctional macrophages,
sensitive to oxysterol-induced cell death, with defective autophagy and dysregulated activation profiles.
Monocyte priming by nutrient stress is mediated by the H2O2-dependent S-glutathionylation, inactivation and
degradation of mitogen-activated protein kinase phosphatase 1 (MKP-1), a master regulator of both monocyte
adhesion and migration and macrophage function and plasticity. However, the source of HCD-induced H2O2 and
“oxidative stress” in “primed” blood monocytes is not known. We have now identified monoamine oxidase A (Mao
A) and NADPH oxidase 4 (Nox4) as novel sources of H2O2 induced by nutrient stress in monocytes and
macrophages and as mediators of nutrient stress-induced monocyte priming and dysfunction. We hypothesize
that the induction of Mao A in monocytes in response to a HCD accelerates atherogenesis by promoting
H2O2 production and the inactivation of MKP-1, resulting in monocyte priming and reprogramming, and
giving rise to dysfunctional, hyper-inflammatory and pro-atherogenic monocyte-derived macrophages
with impaired inflammation resolving capabilities. Furthermore, we propose that by inactivating the thiol
transferase glutaredoxin 1 (Grx1) and disrupting thiol redox homeostasis, Mao A-derived H2O2 promotes the
induction of Nox4, amplifying the oxidative stress response triggered by HCD. To test these hypotheses and to
elucidate the underlying mechanisms, we propose the following Specific Aims:
Specific Aim 1: Determine the mechanisms by which high-calorie diet-triggered induction of Mao A
promotes monocyte dysfunction, dysregulates macrophage plasticity, and accelerates atherogenesis.
Specific Aim 2: Determine the contribution of Nox4 to high-calorie diet-induced monocyte priming,
macrophage dysfunction, and atherogenesis.
Specific Aim 3: Determine the molecular mechanisms by which high-calorie diets trigger monocyte
priming and reprogramming in metabolically healthy human subjects and whether and to what extent
these mechanisms differ from mice.

Grant Number: 5R01HL153120-05
NIH Institute/Center: NIH
Principal Investigator: Reto Asmis