PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Obesity (Silver Spring). Author manuscript; available in PMC 2009 June 1.
Published in final edited form as:
PMCID: PMC2677801
NIHMSID: NIHMS93980

Elevated Soluble Lectin-like Oxidized LDL Receptor 1 (LOX-1) Levels in Obese Postmenopausal Women

Abstract

We investigated the association between soluble lectin-like oxidized LDL receptor 1 (sLOX-1) levels and obesity in older women. Fifty-one (10 lean, 22 overweight, and 19 obese) postmenopausal women were included in this small retrospective analysis. Plasma sLOX-1 levels were measured using a chemiluminescent ELISA. Plasma levels of sLOX-1 were significantly higher in obese women (55.33±4.49 pg/mL) compared to lean (30.91±6.19 pg/mL, p=0.002) and overweight women (38.31±4.18 pg/mL, p=0.017). Plasma sLOX-1 levels were positively associated with body weight, BMI, total body fat, and trunk fat. The relationship between sLOX-1 and BMI was attenuated after adjustment for age, HRT, and body fat. In conclusion, obese women have higher sLOX-1 levels, which may reflect increased LOX-1 expression in adipose tissue.

Keywords: obesity, postmenopausal women, receptors

Introduction

The lectin-like oxidized LDL receptor 1 (LOX-1) is the major receptor for oxidized LDL (ox-LDL) in endothelial cells (1). It is also expressed in macrophages and vascular smooth muscle cells and plays a key role in atherosclerosis (2). LOX-1 may also have important effects in adipocytes, as LOX-1 expression is increased 2-fold in the adipose tissue of obese vs. lean mice (3). LOX-1 can be cleaved by serine proteases and released into the circulation as soluble LOX-1 (sLOX-1) (4, 5). Serine proteases increase with obesity (6), potentially promoting sLOX-1 formation. Elevated levels of soluble receptors may reflect increased membrane expression and disease activities, as seen in patients with acute coronary syndromes (7). However, no studies have examined whether sLOX-1 is associated with obesity. Thus, the purpose of this study was to investigate the relationship between sLOX-1 levels and obesity in postmenopausal women. We hypothesize that obese women will have higher LOX-1 expression, as measured by circulating sLOX-1 levels.

Research Methods and Procedures

Participants

Fifty-one postmenopausal women (10 lean, 22 overweight, and 19 obese) who were recruited to participate in a larger exercise training study were included in this analysis. The eligibility requirements, screening process, and dietary controls have been described previously (8). None of the women were on medications affecting lipid or glucose metabolism. This study was approved by the University of Maryland, College Park Institutional Review Board. All participants provided their written informed consent.

Clinical Assessments

Body mass index (BMI) was calculated as weight (kg) divided by height (m) squared and women were categorized as lean (18.5–24.9 kg/m2), overweight (25.0–29.9 kg/m2), and obese (30.0–39.9 kg/m2). Body fat and plasma lipid levels were measured as previously described (8, 9). Plasma sLOX-1 levels were measured by a sandwich chemiluminescent ELISA using two different human LOX-1-specific monoclonal antibodies and a recombinant human LOX-1 extracellular domain as an assay standard. This procedure was modified from the previously described sandwich ELISA (7). Monoclonal antibodies directed to human LOX-1 were established by standard hybridoma techniques after immunizing mice with a recombinant protein corresponding to the extracellular domain of human LOX-1. Intra-assay and inter-assay CVs were 1.8–6.4% and 4.4–10.7%, respectively.

Statistical Analysis

All statistical analyses were performed using SAS version 9.1 (Cary, NC). Plasma sLOX-1 levels were normalized with a log transformation. Chi-square tests and analysis of variance were used to compare differences in categorical and continuous variables, respectively. Regression was used to examine relationships between sLOX-1 and other variables. Statistical significance was set at p≤0.05.

Results

Body weight and body fat were significantly different among lean, overweight, and obese women (Table). Plasma sLOX-1 levels were 79% and 44% higher in obese women compared to lean and overweight women, respectively (Figure). There was no difference in sLOX-1 levels between lean and overweight women. Plasma sLOX-1 levels were associated with body weight (β=0.008±0.002, p=0.001), BMI (β=0.02±0.007, p=0.002), trunk fat (β=0.009±0.004, p=0.04), and total body fat (β=0.01±0.005, p=0.05), but not abdominal fat, lean mass, plasma lipid levels, or blood pressure. Body weight and BMI remained significantly associated with sLOX-1 levels after adjusting for age and HRT; however, further adjustment for body fat attenuated these relationships (p>0.05).

Figure
Plasma sLOX-1 levels in lean, overweight, and obese women. *p=0.002 vs. lean women, p=0.017 vs. overweight women
Table
Subject Characteristics

Discussion

LOX-1 is a scavenger receptor expressed in endothelial cells, smooth muscle cells, macrophages, and adipocytes (3, 10). Soluble LOX-1 is produced mainly by proteolytic cleavage of LOX-1 at the cell surface, and the expression of membrane-bound LOX-1 precedes the release of sLOX-1 (5). Thus, plasma sLOX-1 may be a surrogate marker of cell-surface LOX-1 expression. We found that plasma sLOX-1 levels were elevated in obese women compared to lean and overweight women. As expected, the association between sLOX-1 and BMI appears to be mediated by the amount of fat mass. To our knowledge, this is the first study to report an association between sLOX-1 levels and obesity. While these findings are consistent with increased LOX-1 expression in obesity, the source of sLOX-1 in our women (vascular or adipose) cannot be determined.

Increased LOX-1 expression in obesity may be a physiological response to changes in adipocyte cholesterol content (3). Cholesterol uptake in adipocytes occurs largely via the LDL receptor, but scavenger receptors are also important (3, 11). As adipocyte triglyceride storage increases, cholesterol is redistributed to the plasma membrane. This redistribution signals that the adipocyte is cholesterol-deficient and upregulates genes required for cholesterol synthesis and uptake, including possibly LOX-1 (3, 11). LOX-1 expression enhances ox-LDL and fatty acid uptake and increases adipocyte cholesterol content; however, if this occurs in excess, it could contribute to obesity (3).

Two limitations in this study were the small sample size and the lack of direct measurements (e.g. adipocyte LOX-1 expression, cholesterol content, and cell size/number). Nevertheless, we had sufficient power to detect differences between lean and overweight women. Our findings that sLOX-1 levels are increased in obese women, along with previous findings that LOX-1 expression is increased in obese mice and induces ox-LDL and fatty acid uptake, support a role for LOX-1 in adipocyte metabolism. We believe that our study provides preliminary evidence for a relationship between sLOX-1 (i.e. LOX-1) and obesity in postmenopausal women. Future studies will need to determine whether LOX-1 is a consequence of or plays a role in the development of obesity.

Acknowledgments

This research was supported by NIH grants AG-17474 and AG-15389.

References

1. Sawamura T, Kume N, Aoyama T, et al. An endothelial receptor for oxidized low-density lipoprotein. Nature. 1997;386(6620):73–7. [PubMed]
2. Chen M, Masaki T, Sawamura T. LOX-1, the receptor for oxidized low-density lipoprotein identified from endothelial cells: implications in endothelial dysfunction and atherosclerosis. Pharmacol Ther. 2002;95(1):89–100. [PubMed]
3. Chui PC, Guan HP, Lehrke M, Lazar MA. PPARgamma regulates adipocyte cholesterol metabolism via oxidized LDL receptor 1. J Clin Invest. 2005;115(8):2244–56. [PMC free article] [PubMed]
4. Kume N, Kita T. Roles of lectin-like oxidized LDL receptor-1 and its soluble forms in atherogenesis. Curr Opin Lipidol. 2001;12(4):419–23. [PubMed]
5. Murase T, Kume N, Kataoka H, et al. Identification of soluble forms of lectin-like oxidized LDL receptor-1. Arterioscler Thromb Vasc Biol. 2000;20(3):715–20. [PubMed]
6. Maslowska M, Vu H, Phelis S, et al. Plasma acylation stimulating protein, adipsin and lipids in non-obese and obese populations. Eur J Clin Invest. 1999;29(8):679–86. [PubMed]
7. Hayashida K, Kume N, Murase T, et al. Serum soluble lectin-like oxidized low-density lipoprotein receptor-1 levels are elevated in acute coronary syndrome: a novel marker for early diagnosis. Circulation. 2005;112(6):812–8. [PubMed]
8. Halverstadt A, Phares DA, Wilund KR, Goldberg AP, Hagberg JM. Endurance exercise training raises high-density lipoprotein cholesterol and lowers small low-density lipoprotein and very low-density lipoprotein independent of body fat phenotypes in older men and women. Metabolism. 2007;56(4):444–50. [PubMed]
9. Nicklas BJ, Rogus EM, Colman EG, Goldberg AP. Visceral adiposity, increased adipocyte lipolysis, and metabolic dysfunction in obese postmenopausal women. Am J Physiol. 1996;270(1 Pt 1):E72–E78. [PubMed]
10. Mehta JL, Chen J, Hermonat PL, Romeo F, Novelli G. Lectin-like, oxidized low-density lipoprotein receptor-1 (LOX-1): a critical player in the development of atherosclerosis and related disorders. Cardiovasc Res. 2006;69(1):36–45. [PubMed]
11. Dugail I, Le LS, Varret M, Le LX, Dagher G, Ferre P. New insights into how adipocytes sense their triglyceride stores. Is cholesterol a signal? Horm Metab Res. 2003;35(4):204–10. [PubMed]