Proteoglycans Synthesized by Arterial Smooth Muscle Cells in the Presence of Transforming Growth Factor-β1 Exhibit Increased Binding to LDLs

Abstract

Abstract-The "response-to-retention" hypothesis of atherogenesis states that atherogenic lipoproteins, such as low density lipoprotein (LDL), are retained in vessels by proteoglycans and undergo proatherosclerotic modifications. Transforming growth factor (TGF)-␤1 has been identified in atherosclerotic vessels and has been shown to stimulate the synthesis of chondroitin sulfate-and dermatan sulfate-containing proteoglycans by arterial smooth muscle cells (ASMCs), but whether it promotes lipid retention has not been addressed. We investigated whether TGF-␤1 modulates the biosynthesis of proteoglycans by ASMCs in a manner that promotes binding to LDL. Proteoglycans isolated from TGF-␤1-treated ASMCs exhibited enhanced binding to native LDL compared with the binding of proteoglycans isolated from control cultures (K d 18 g/mL LDL versus 81 g/mL LDL, respectively). The increase in proteoglycan-LDL binding caused by TGF-␤1 could be attributed primarily to the glycosaminoglycan portion of the proteoglycans, since the glycosaminoglycan chains liberated from the core proteins of these proteoglycans synthesized in the presence of TGF-␤1 exhibited increased LDL binding as well. Furthermore, glycosaminoglycan chains initiated on xyloside (an initiator of glycosaminoglycan synthesis) in the presence of TGF-␤1 were longer and displayed enhanced binding to LDL compared with the LDL binding of xyloside-initiated glycosaminoglycan chains from control cultures. These results indicate that TGF-␤1 promotes LDL-proteoglycan interaction primarily by its effects on the glycosaminoglycan synthetic machinery of the ASMCs. Therefore, this study supports a proatherogenic role for TGF-␤1. Key Words: proteoglycans Ⅲ glycosaminoglycans Ⅲ smooth muscle cells Ⅲ transforming growth factor-␤1 Ⅲ lipoproteins C ardiovascular disease resulting from coronary artery atherosclerosis is the major cause of morbidity and mortality in industrialized countries. [1][2][3] The biochemical and metabolic mechanisms responsible for the initiation and progression of atherosclerosis are not fully understood. Retention of lipoproteins by extracellular matrix molecules in the vascular wall is believed to play an important role in atherogenesis. 4 -7 The "response-to-retention" hypothesis states that apoB-and apoE-containing lipoproteins bind to and are retained by vascular matrix molecules, particularly proteoglycans. 5 The interaction of proteoglycans and lipoproteins predominantly occurs as an ionic interaction between negatively charged residues on the proteoglycans and positively charged residues on the apoproteins or via bridging molecules such as apoE and lipoprotein lipase. The retention of lipoproteins is increased by measures that increase the number of negatively charged residues on the glycosaminoglycan chain, either by an increase in chain length or increases in the degree of sulfation (see reviews 4 -8 ).Transforming growth factor (TGF)-␤1 is a cytokine that is increased in atherogenesis and has been shown to promote atherosclerotic lesion formation...

Thiazolidinediones reduce the LDL binding affinity of non-human primate vascular cell proteoglycans

“…We have previously demonstrated that LDL binding affinity of intact proteoglycans and glycosaminoglycan chains relates to the relative size and sulphation of the molecules [17,30]. In the present experiments we confirm these previous observations, demonstrating that proteoglycans synthesised in the presence of troglitazone or rosiglitazone are smaller and have decreased LDL binding affinity.…”

“…Our inability to demonstrate decreased LDL binding affinity for peak 2 glycosaminoglycans synthesised in the presence of troglitazone could be due to a limitation of the assay, or to troglitazone having less of an effect on the LDL binding affinity of dermatan sulphate chains than on that of chondroitin sulphate chains. The LDL binding affinity of free glycosaminoglycan chains is lower than the binding affinity of intact proteoglycans, as previously described [17,30].…”

Smad2-dependent glycosaminoglycan elongation in aortic valve interstitial cells enhances binding of LDL to proteoglycans

“…To synthesize xyloside-initiated GAG chains (an independent measure of GAG synthesis) treatment media was supplemented with methyl β-D-xylopyranoside (0.5 mmol/l, Sigma). Secreted proteoglycans were isolated and concentrated from the conditioned medium as described previously [34]. Radiolabel incorporation into proteoglycans was quantified using the CPC precipitation assay [38].…”

“…In atherosclerosis, TGF-β1 leads to the activation of proteoglycan synthesis in vascular smooth muscle cells and induces elongation of the glycosaminoglycan (GAG) chains [33,34]. Elongated GAG chains are capable of increased binding of lipid [34,35] and this interaction underlies the "response to retention" hypothesis of Williams and Tabas regarding the origin of atherosclerosis [36].…”

“…In atherosclerosis, TGF-β1 leads to the activation of proteoglycan synthesis in vascular smooth muscle cells and induces elongation of the glycosaminoglycan (GAG) chains [33,34]. Elongated GAG chains are capable of increased binding of lipid [34,35] and this interaction underlies the "response to retention" hypothesis of Williams and Tabas regarding the origin of atherosclerosis [36]. In this study we investigated TβR signaling in diseased human aortic valves and the effects of TGF-β1 on por cine aortic valve interstitial cell proteoglycan synthesis and lipid binding to determine whether the proteoglycans produced by aortic valve interstitial cells could have a role in the initiation of the early sclerotic stage of CAVD.…”