Circulating soluble form of LR11, a regulator of smooth muscle cell migration, is a novel marker for intima-media thickness of carotid arteries in type 2 diabetes

Highlights

• We analyzed the correlation of sLR11 levels with IMT in middle-aged, non-obese patients with T2D.

• sLR11 correlated positively with IMT, together with age and FPG.

• Circulating sLR11 may be a novel marker representing the pathophysiology of intimal SMCs.

Abstract

Background

Smooth muscle cell (SMC) migration from the media to the intima, a process affecting plaque stability in advanced-stage atherosclerosis, is under the control of LR11. To delineate the clinical significance of the circulating soluble form of LR11 (sLR11) in patients with type 2 diabetes (T2D), we analyzed the correlation of sLR11 levels with intima-media thickness (IMT) of carotid arteries.

Methods

Plasma sLR11 levels were measured in 165 patients with T2D (mean age 56.2 ± 10.4 y, 58.2% males, and BMI 24.6 ± 3.6) by ELISA. Averaged IMT levels of common carotid arteries were determined by ultrasonography.

Results

Circulating sLR11 levels were 9.8 ± 3.5 ng/ml, and correlated positively with the classical atherosclerosis risk factors age, sex, systolic blood pressure, low-density lipoprotein-cholesterol (LDL-C), fasting plasma-glucose (FPG), and glycosylated hemoglobin. Multivariate linear regression analysis indicated that only FPG was associated with sLR11; sLR11 correlated positively with IMT, together with age and FPG, but less with LDL-C. Among the serum risk factors for IMT, multivariate linear regression analysis uncovered that sLR11 was independently associated with IMT. Subsequent logistic analysis revealed that FPG correlated best with IMT values at a cut-off of 0.80 mm and sLR11 at a cut-off of 0.90 mm, respectively, while LDL-C showed lower discriminatory power at any IMT cut-off values.

Conclusion

Increased sLR11 concentrations are highly associated with increased IMT as well as with FPG in middle-aged, non-obese patients with T2D. Circulating sLR11 may be a novel marker representing the pathophysiology of intimal SMCs in patients with T2D.

Introduction

The migration of vascular smooth muscle cells (SMCs) from the medial layer to the intimal layer is a key step in the development of advanced atherosclerosis. The initial steps are a functional disturbance of endothelial cells and the infiltration of activated monocytes/macrophages from the circulation [1], [2]. Following migration, intimal SMCs, upon conversion in the media from a ‘contractile’ phenotype to a de-differentiated ‘synthetic’ phenotype, regulate various potential functions including matrix production, protease release and cytokine secretion [1], [2], [3]. Thus, migrated intimal SMCs are believed to be a player in forming atherosclerotic plaques and determining their fragility upon interaction with other vascular and inflammatory cells. Using animal models and cultured SMCs, we showed that disturbed interactions of SMCs with other vascular cells or in phenotype conversion of SMCs in dyslipidemia can lead to atherosclerosis progression and raised plaque fragility [4]. However, the clinical significance in diabetes mellitus of the intimal SMCs migrated under pathophysiological conditions has not yet been elucidated for the process of atherosclerosis.

LR11 (also called sorLA), an unusually complex and highly conserved member of the family of LDL receptor relatives (LRs), has been discovered and molecularly characterized by us and others [5], [6], [7]. The receptor mediates the plasma membrane localization of urokinase-type plasminogen activator receptor (uPAR), as the shed soluble form of the receptor (sLR11) binds to and colocalizes with uPAR on the cell surface [8]. LR11 is highly expressed in intimal SMCs at the intima-media border in the plaque area of experimental models of atherosclerosis [9], [10]. Furthermore, overexpression of LR11 in SMCs enhances their migration via elevated concentrations of uPAR [10], [11]. We have developed a sandwich enzyme-linked immunosorbent assay (ELISA) for the exact quantitation of circulating sLR11 using specific monoclonal antibodies against human LR11 [12], [13]. With this method, it could be shown that the concentrations of sLR11 were increased in patients with familial hypercholesterolemia [14], coronary artery diseases [15], [16], [17], type 2 diabetes (T2D) [15], [18], [19], and in patients with hematological malignancies [13], [20], [21], [22], [23]. Notably, several independent studies have shown that the concentrations of sLR11 were increased in relation to markers of glycemic disturbances among the classical risk factors for atherosclerosis [11], [14], [15], [16], [18], [19]. Thus, sLR11 has been suggested to reflect the conditions or degrees of migrated intimal SMCs, particularly in the vascular complications observed in diabetes.

Based on the knowledge generated by the above described broad spectrum of studies, we have now investigated in patients with T2D the relationship of sLR11 with the degrees of carotid IMT. Elevated IMT has been shown to predict vascular events, myocardial infarction, and stroke in asymptomatic adults [24].