# Noncatalytic Functions Are Required for MPO and PON1 in Modulating the Involvement of Monocytes and Endothelial Cells in Atherosclerosis

## TL;DR

This study shows that MPO and PON1 proteins, not their enzyme activities, influence cholesterol efflux and adhesion molecule expression in cells related to atherosclerosis.

## Contribution

The study reveals that noncatalytic functions of MPO and PON1 modulate monocyte and endothelial cell behavior in atherosclerosis.

## Key findings

- MPO protein reduces cholesterol efflux in THP-1 cells, while PON1 increases it.
- MPO enhances adhesion molecule expression in HUVECs, whereas PON1 reduces it.
- MPO protein impairs microvascular structure by increasing IL-6 and TNFα via NF-κB p65.

## Abstract

High‐density lipoproteins (HDLs) are deeply implicated in atherosclerosis. HDL, myeloperoxidase (MPO), and paraoxonase‐1 (PON1) form a functional ternary complex where PON1 partially inhibits the MPO activity, and MPO in turn partially inactivates PON1. The activity of MPO is dependent on the concentration of hydrogen peroxide, but the extremely low concentrations of hydrogen peroxide in serums severely constrain MPO activity. PON1 has the activities of organophosphatase, arylesterase, and thiolactonase, but these hydrolase activities are extraneous to antioxidative stress. Thus, we proposed that MPO and PON1 may be involved in atherosclerosis by acting as proteins, rather than enzyme activities. Cholesterol efflux assay, ATP‐binding cassette transporter A1 (ABCA1)–dependent cholesterol efflux, and LCAT activity assay were performed. The effect of MPO, PON1, and serums from the individuals with ASCVD and healthy individuals on cholesterol efflux of human acute monocytic leukemia cell line (THP‐1 cells) was compared. Noncatalytic functions of MPO and PON1 were analyzed using recombinant proteins and neutralizing antibodies. Wound healing assay and tube formation assay were used to analyze noncatalytic functions of MPO and PON1 in modulating the involvement of human umbilical vein endothelial cells (HUVECs). We found that MPO protein decreased the cholesterol efflux; by contrast, PON1 protein increased the cholesterol efflux of THP‐1 cells. Importantly, MPO antibody partially restored cholesterol efflux, but PON1 antibody partially reduced cholesterol efflux of THP‐1 cells. Moreover, ABCA1 was necessary for controlling the involvement of MPO and PON1 in modulating cholesterol efflux of THP‐1 cells. There existed the confrontations between the noncatalytic functions of PON1 and MPO in migration of endothelial cells. Instead, MPO protein enhanced the expression of intercellular adhesion molecule‐1 (ICAM‐1) and E‐selectin of HUVECs; nonetheless, PON1 protein reduced the expression of these adhesion molecules. Of note, PON1 protein was unable to balance out the induction of MPO protein for these adhesion molecules in that the expression of these adhesion molecules generated by the combination of MPO protein and PON1 protein was similar to that of MPO. The activation of THP‐1 cells induced by MPO protein directly impaired in vitro microvascular structure via increasing the expression of IL‐6 and TNFα regulated by NF‐κB p65 of THP‐1 cells. Together, the noncatalytic functions entail MPO and PON in modulating the involvement of monocytes and endothelial cells in atherosclerosis.