The lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is the major receptor for oxidized low-density lipoprotein (ox-LDL) in endothelial cells (1). It is also expressed in macrophages and vascular smooth muscle cells and plays an important role in atherosclerosis (2). LOX-1 may also have important effects in adipocytes, as LOX-1 expression is increased twofold 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 increased in 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.
We investigated the association between soluble lectin-like oxidized low-density lipoprotein receptor-1 (sLOX-1) levels and obesity in older women. Fifty-one postmenopausal women (10 lean, 22 overweight, and 19 obese) were included in this small retrospective analysis. Plasma sLOX-1 levels were measured using a chemiluminescent enzyme-linked immunoassay. 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, hormone replacement therapy, and body fat. In conclusion, obese women have higher sLOX-1 levels, which may reflect increased LOX-1 expression in adipose tissue.
Research Methods and Procedures
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.
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), or obese (30.0–39.9 kg/m2). Body fat and plasma lipid levels were measured as described previously (8,9). Plasma sLOX-1 levels were measured by a sandwich chemiluminescent enzyme-linked immunoassay 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 enzyme-linked immunoassay (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 coefficients of variation were 1.8–6.4% and 4.4–10.7%, respectively.
All statistical analyses were performed using SAS version 9.1 (SAS Institute, Cary, NC). Plasma sLOX-1 levels were normalized with a log transformation. χ2-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.
Body weight and body fat were significantly different among lean, overweight, and obese women (Table 1). Plasma sLOX-1 levels were 79% and 44% higher in obese women compared to lean and overweight women, respectively (Figure 1). 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 hormone replacement therapy; however, further adjustment for body fat attenuated these relationships (P > 0.05).
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 tissue) 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 of 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 obese 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.
This research was supported by National Institutes of Health grants AG-17474 and AG-15389.
The authors declared no conflict of interest.