Measurements of proteoglycan-lipoprotein interaction by gel mobility shift assay

Imatinib inhibits vascular smooth muscle proteoglycan synthesis and reduces LDL binding in vitro and aortic lipid deposition in vivo

The ‘response to retention’ hypothesis of atherogenesis proposes that proteoglycans bind and retain low-density lipoproteins (LDL) in the vessel wall. Platelet-derived growth factor (PDGF) is strongly implicated in atherosclerosis and stimulates proteoglycan synthesis. Here we investigated the action of the PDGF receptor inhibitor imatinib on PDGF-mediated proteoglycan biosynthesis in vitro, lipid deposition in the aortic wall in vivo and the carotid artery ex vivo. In human vSMCs, imatinib inhibited PDGF mediated ³⁵S-SO₄ incorporation into proteoglycans by 31% (P < 0.01) and inhibited PDGF-mediated size increases in both chemically cleaved and xyloside associated glycosaminoglycan (GAG) chains by 19%, P < 0.05 and 27%, P < 0.05, respectively. Imatinib decreased PDGF stimulation of the 6:4 position sulphation ratio of disaccharides. The half maximal saturation value for LDL binding for proteoglycans from PDGF stimulated cells in the presence of imatinib was approximately 2.5-fold higher than for PDGF treatment alone. In high fat fed ApoE^−/– mice, imatinib reduced total lipid staining area by ∼31% (P < 0.05). Carotid artery lipid accumulation in imatinib treated mice was also reduced. Furthermore, we demonstrate that imatinib inhibits phosphorylation of tyrosine 857, the autophosphorylation site of the PDGF receptor, in vSMCs. Thus imatinib inhibits GAG synthesis on vascular proteoglycans and reduces LDL binding in vitro and in vivo and this effect is mediated via the PDGF receptor. These findings validate a novel mechanism to prevent cardiac disease.

Serum amyloid A, but not C-reactive protein, stimulates vascular proteoglycan synthesis in a pro-atherogenic manner

Inflammatory markers serum amyloid A (SAA) and C-reactive protein (CRP) are predictive of cardiac disease and are proposed to play causal roles in the development of atherosclerosis, in which the retention of lipoproteins by vascular wall proteoglycans is critical. The purpose of this study was to determine whether SAA and/or CRP alters vascular proteoglycan synthesis and lipoprotein retention in a pro-atherogenic manner. Vascular smooth muscle cells were stimulated with either SAA or CRP (1 to 100 mg/L) and proteoglycans were then isolated and characterized. SAA, but not CRP, increased proteoglycan sulfate incorporation by 50 to 100% in a dose-dependent manner (P < 0.0001), increased glycosaminoglycan chain length, and increased low-density lipoprotein (LDL) binding affinity (K(d), 29 microg/ml LDL versus 90 microg/ml LDL for SAA versus control proteoglycans; P < 0.005). Furthermore, SAA up-regulated biglycan via the induction of endogenous transforming growth factor (TGF)-beta. To determine whether SAA stimulated proteoglycan synthesis in vivo, ApoE(-/-) mice were injected with an adenovirus expressing human SAA-1, a null virus, or saline. Mice that received adenovirus expressing SAA had increased TGF-beta concentrations in plasma and increased aortic biglycan content compared with mice that received either null virus or saline. Thus, SAA alters vascular proteoglycans in a pro-atherogenic manner via the stimulation of TGF-beta and may play a causal role in the development of atherosclerosis.

Tesaglitazar, a dual peroxisome proliferator-activated receptor alpha/gamma agonist, reduces atherosclerosis in female low density lipoprotein receptor deficient mice

OBJECTIVE

The transcription factors, peroxisome proliferator-activated receptors (PPAR) alpha (alpha) and gamma (gamma), which are involved in lipid and glucose homeostasis, also exert modulatory actions on vascular cells where they exhibit anti-inflammatory and anti-proliferative properties. Hence, PPAR agonists potentially can affect atherogenesis both via metabolic effects and direct effects on the vessel wall. We tested whether the dual PPAR-alpha/gamma agonist, tesaglitazar (TZ), would reduce atherosclerosis in a non-diabetic, atherosclerosis-prone mouse model, independent of effects on plasma lipids.

METHODS AND RESULTS

Low-density lipoprotein receptor deficient (LDLr-/-) mice were fed a Western type diet consisting of 21% butterfat and 0.15% cholesterol, with or without TZ 0.5 micromol/kg of diet, for 12 weeks. TZ reduced atherosclerosis in the female, but not male, LDLr-/- mice without affecting cholesterol and triglyceride levels, HDL binding to biglycan, or the inflammatory markers serum amyloid A (SAA) and serum amyloid P (SAP). TZ also decreased adiposity in both genders.

CONCLUSIONS

TZ reduced atherosclerosis in the female LDLr-/- mice via lipid-independent mechanisms, probably at least in part by direct actions on the vessels. The body weight changes in these mice are different from the effects of dual PPAR agonists seen in humans.

Glycosaminoglycan-lipoprotein interaction

Glycosaminoglycans (GAGs) bound to various proteoglycans (PGs) present in the cardiovascular system have been proposed to perform a wide range of functions. These include conferring viscoelastic properties; interacting with and modulating growth factors and enzymes; and as receptors and co-receptors in lipoprotein metabolism. Binding of apoB-100 lipoproteins, particularly low density lipoproteins (LDL), to GAGs of extracellular matrix PGs in arteries has been proposed to be an initiating event in development of atherosclerosis. This study was initiated with the aim of getting an overview of the binding patterns of different lipoprotein subclasses with individual GAG categories. We thus evaluated the interaction of lipoproteins with GAGs commonly found in the cardiovascular system using a gel mobility-shift assay developed for this purpose. The same procedure was used to measure lipoproteins binding to metabolically [(35)S]-labeled whole PGs prepared from three cell types, arterial smooth muscle cells, THP-1 macrophages and from HepG2 cells. The effect of GAG composition on PGs on lipoprotein binding was evaluated by enzymatic degradation of the carbohydrate chains. Heparan sulfate was found to bind beta very low density lipoproteins (beta-VLDL) and a chylomicron remnant model (beta-VLDL+apoE), but not LDL. Dermatan sulfate was found to bind LDL, but not beta-VLDL or the chylomicron remnant model. Chondroitin sulfate and heparin were found to bind all lipoproteins tested (LDL, beta-VLDL and beta-VLDL+apoE) although with different affinities. We can conclude that each lipoprotein subclass tested binds a specific assortment of the GAGs tested. The observations made contribute to the understanding of new and complex mechanisms by which carbohydrate and lipid metabolism may be linked.

Changes in matrix proteoglycans induced by insulin and fatty acids in hepatic cells may contribute to dyslipidemia of insulin resistance

Insulin resistance and type 2 diabetes are associated with elevated circulating levels of insulin, nonesterified fatty acids (NEFAs), and lipoprotein remnants. Extracellular matrix proteoglycan (PG) alterations are also common in macro- and microvascular complications of type 2 diabetes. In liver, extracellular heparan sulfate (HS) PGs contribute to the uptake of triglyceride-rich lipoprotein remnants. We found that HepG2 cells cultured with 10 or 50 nmol/l insulin or 300 micromol/l albumin-bound linoleic acid changed their PG secretion. The glycosaminoglycans (GAGs) of the secreted PGs from insulin-treated HepG2 cells were enriched in chondroitin sulfate (CS) PGs. In contrast, cells exposed to linoleic acid secreted PGs with decreased content of CS. Insulin caused a moderate increase in mRNA for versican (secreted CS PG), whereas linoleic acid markedly decreased mRNA for versican in HepG2 cells, as did the peroxisomal proliferator-activated receptor-alpha agonist bezafibrate. The effects of insulin or linoleic acid on syndecan 1, a cell surface HS PG, were similar to those on versican, but less pronounced. The livers of obese Zucker fa/fa rats, which are insulin-resistant and have high levels of insulin, NEFAs, and triglyceride-rich remnants, showed increased expression of CS PGs when compared with lean littermates. These changes in PG composition decreased the affinity of remnant beta-VLDL particles to PGs isolated from insulin-treated HepG2 cells and obese rat livers. The results indicated that insulin and NEFAs modulate the expression of PGs in hepatic cells. We speculate that in vivo this exchange of CS for HS may reduce the clearance of remnant beta-VLDLs and contribute to the dyslipidemia of insulin resistance.