LDL associates with pro-inflammatory monocyte subset differentiation and increases in chemokine receptor profile expression in African Americans☆
Graphical abstract
LDL associated changes on monocyte subsets and CCR2/5 expression.
Introduction
Obesity remains a major contributor to cardiovascular disease (CVD) risk and mortality worldwide [1] and is described as a chronic, low-grade inflammatory disease that is often associated with hyperlipidemia and hyperglycemia [2]. Atherosclerosis, the main underlying cause of CVD, is driven by the infiltration of immune cells into the vessel wall, with monocytes being a significant contributor [3]. Monocytes, among other cell types, are recruited to the vascular wall by chemokine receptors, notably the CC Motif chemokine receptors 2 and 5 (CCR2 [4] and CCR5 [5], respectively). Peripheral blood monocytes are heterogenous and possess either a classical (CM; CD14++CD16−), intermediate (IM; CD14++CD16+), or nonclassical (NCM; CD14+CD16++) phenotype [6]. Specifically, elevated IM population frequencies have been traced to adverse cardiac events in patients with coronary artery disease [7], end-stage renal disease [8], psoriasis [9], and rheumatoid arthritis [10]. Increased IM and NCM populations have also been observed in individuals with obesity [[11], [12], [13]]. Additionally, notable monocyte subset sex differences have been reported with women having a less favorable monocyte subset profile [11]. In addition, our group has recently shown in a smaller exploratory study that African Americans (AAs) have higher IM and NCM populations when compared to Caucasians, even in the absence of known CVD and seemingly independent of age and sex [14]. Remarkably, hypercholesteremia has been shown to directly impact hematopoiesis by inducing monocytosis [15]. Clinical correlations have also assessed a direct link between lipid profile and monocyte subsets in humans [16]. However, these studies have not been performed in diverse study cohorts nor have they assessed the causal effects of these lipids in vitro. Therefore, studies are needed to investigate the association between cholesterol content and its direct contribution to pro-inflammatory monocyte lineages in a population at high risk for obesity, hyperlipidemia, and CVD, like AAs.
In this study we aim to: 1) identify whether lipid profile is associated with a shift in monocyte lineage towards NCM; among a community-based cohort of African Americans; 2) determine if LDL cholesterol is directly associated with monocyte subset differentiation and chemokine receptor expression using ex vivo and in vitro approaches; and 3) elucidate whether surface marker expression of CD14, CD16, CCR2, and CCR5 is a result of translation-dependent protein synthesis. We hypothesize that individual LDL content will be associated with increased populations of NCM in ex vivo experiments and changes in chemokine receptor expression in in vitro experiments. Also, we hypothesize that chemokine receptor expression differences are related to internalization mechanisms rather than de novo synthesis.
Section snippets
Methods
A cohort of African Americans residing in Washington, D.C. were included in this study (Table 1). Study approval was obtained from the Institutional Review Board at the NIH in accordance with the Principles of Declaration of Helsinki. All guidelines for good clinical practice and those set forth in the Belmont Report were followed. Data for all study participants were obtained under the clinical protocols NCT01143454 and NCT00001846. All study participants provided written informed consent.
LDL and TG levels in serum are associated with monocyte subset shifts
Freshly isolated PBMCs from a healthy individual were treated with bio-banked serum from 40 individuals in an ex vivo experimental setup (Patient characteristics summarized in Table 1). LDL and TG were found to be significantly associated with the various monocyte subsets (Table 2). A negative association was observed between serum LDL levels and CM (β = -0.32; p = 0.046) which became non-significant after adjusting for FRS score, BMI, and clinical use of statin treatment. A positive
Discussion
Our ex vivo experimental setup showed that LDL and TG were associated with monocyte subset differentiation highlighting their important role as CVD risk factors. Interestingly, BMI did not correlate with monocyte subset phenotypes in our clinical cohort; this may relate to limitations of BMI as a marker of CVD risk, especially in racially/ethnically diverse populations [18]. Other clinical markers of CVD risk such as HOMA-IR, serum insulin levels, CRP, and HDL cholesterol did not associate with
Conclusion
Our data suggest that LDL contributes to monocyte subset differentiation and to chemotactic receptor expression, supporting its role as a crucial CVD risk factor. In our experiments, high concentration LDL decreases the CM and increases the IM/NCM populations. Due to a small, homogeneous cohort, the findings in this report should be replicated utilizing larger, racially/ethnically diverse cohorts in order to assess the intersectionality of hyperlipidemia, and race/ethnicity as a marker of
CRediT authorship contribution statement
Cristhian A. Gutierrez-Huerta: Investigation, Data curation, Formal analysis, Software, Writing – original draft. Briana S. Turner: Investigation, Validation, Writing – review & editing. Sophie E. Claudel: Formal analysis, Software. Nicole Farmer: Data curation. Rafique Islam: Data curation. Valerie M. Mitchell: Data curation. Billy S. Collins: Data curation. Yvonne Baumer: Methodology, Supervision, Project administration, Writing – review & editing. Alan T. Remaley: Data curation. Tiffany M.
Declaration of Competing Interest
The authors of this manuscript report no conflict of interests.
Acknowledgements
The authors want to thank all the support received by the NHLBI, the NIMHD, and the NIH. The research reported in this publication is solely the responsibility of the authors and does not necessarily represent the official views of the National Heart Lung and Blood Institute, the National Institute on Minority Health and Health Disparities, or the National Institutes of Health. We are grateful for the participants in our clinical trials and the donors and staff at the NIH Blood Bank and the
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This author takes responsibility for all aspects of the reliability and freedom from bias of the data presented and their discussed interpretation.