Early human atherosclerosis: accumulation of lipid and proteoglycans in intimal thickenings followed by macrophage infiltration

Rosiglitazone but not losartan prevents Nrf-2 dependent CD36 gene expression up-regulation in an in vivo atherosclerosis model

Background

Thiazolidinediones exert anti-inflammatory and anti-oxidative roles and attenuate atherosclerosis by mechanisms partially independent of their metabolizing actions. High doses of angiotensin type 1 receptor (AT₁R) blocker losartan (LST) seem to promote fat cell formation by preserving PPARγ activity.

Methods

C57BL/6J diet-induced atherosclerotic susceptible mice randomly received a normal or a high-fat high-cholesterol (HFHC) diet and were treated with rosiglitazone (RG), LST or a vehicle for 12 weeks.

Results

HFHC was associated with increased PPARγ gene expression without an over regulation of PPARγ responsive genes, whereas RG and LST treatments were found to maintain PPARγ activity without resulting in increased PPARγ gene expression. A better anti-inflammatory and antioxidant profile in mice treated with RG regarding LST was observed in spite of a similar PPARγ preserved activity. Chromatin immunoprecipitation (ChIP) assays revealed that animals under HFHC diet treated with RG showed a significant nuclear factor erythroid 2-like 2 (Nrf2)-dependent down-regulation of the expression of the CD36 gene.

Conclusion

The PPARγ agonist RG exerts antioxidant properties that significantly reduced Nrf-2-dependent CD-36 up-regulation in mice under HFHC diet. Because LST treatment was also associated with a preserved PPARγ activity, our data suggests that these RG antioxidant effects are partially independent of its PPARγ metabolizing properties.

Prediction of the Localization of High-Risk Coronary Atherosclerotic Plaques on the Basis of Low Endothelial Shear Stress

Background— Low endothelial shear stress (ESS) promotes the development of atherosclerosis; however, its role in the progression of atherosclerotic plaques and evolution to inflamed high-risk plaques has not been studied. Our hypothesis was that the lowest values of ESS are responsible for the development of high-risk coronary atherosclerotic plaques associated with excessive expansive remodeling.

Methods and Results— Twenty-four swine, treated with streptozotocin to induce diabetes and fed a high-fat diet, were allocated into early (n=12) and late (n=12) atherosclerosis groups. Intima-media thickness was assessed by intravascular ultrasound in the coronary arteries at weeks 4 and 8 in the early group and weeks 23 and 30 in the late group. Plaques started to develop after week 8, leading to marked heterogeneity in plaque severity at week 30. ESS was calculated in plaque-free subsegments of interest (n=142) in the late group at week 23. Coronary arteries (n=31) of this group were harvested at week 30, and the subsegments of interest were identified and analyzed histopathologically. Low ESS was an independent predictor of the development of high-risk plaques, characterized by intense lipid accumulation, inflammation, thin fibrous cap, severe internal elastic lamina degradation, and excessive expansive remodeling. The severity of high-risk plaque characteristics at week 30 was significantly correlated with the magnitude of low ESS at week 23.

Conclusions— The magnitude of low ESS determines the complexity and heterogeneity of atherosclerotic lesions and predicts the development of high-risk plaque.

Technology insight: in vivo coronary plaque classification by intravascular ultrasonography radiofrequency analysis

Acute coronary syndromes or sudden coronary death are often the first manifestations of coronary artery disease. In the majority of patients, acute coronary syndrome events are caused by plaque rupture in flow-limiting and non-flow-limiting angiographically intermediate stenoses. Histopathologic analyses have shown that plaque composition is related to the occurrence of acute clinical events and, therefore, to the vulnerability of the plaque. The emerging importance of adaptive coronary remodeling processes, such as the compensatory enlargement of the coronary artery in response to initial lesion development, has focused our interest on the nonstenotic lesions of the coronary tree. In vivo intravascular ultrasonography can demonstrate the discrepancies between the actual extent of coronary atherosclerosis and that seen by angiographic imaging. The spectral analysis of intravascular ultrasonography derived radiofrequency data enables more precise analysis of plaque composition and type than grayscale intravascular ultrasonography.

Increased endothelial expression of Toll-like receptor 2 at sites of disturbed blood flow exacerbates early atherogenic events

Toll-like receptors (TLRs) are pattern recognition receptors of innate immunity. TLRs initiate inflammatory pathways that may exacerbate chronic inflammatory diseases like atherosclerosis. En face laser scanning confocal microscopy (LSCM) of isolated aortic segments revealed the distribution of intimal TLR2 expression and the atheroprotective outcomes resulting from a TLR2 deficiency. TLR2 expression was restricted to endothelial cells in regions of disturbed blood flow, such as the lesser curvature region, in atherosclerosis-prone, low-density lipoprotein receptor-deficient (LDLr(-/-)) mice. Diet-induced hyperlipidemia in LDLr(-/-) mice increased this regional endothelial TLR2 expression. Bone marrow (BM) reconstitution of LDLr(-/-) and LDLr(-/-)TLR2(-/-) mice created chimeric mice with green fluorescent protein (GFP) expression in BM-derived cells (BMGFP(+)). Lesser curvature BMGFP(+) leukocyte accumulation, lipid accumulation, foam cell generation and endothelial cell injury were all increased by hyperlipidemia, whereas hyperlipidemic double mutant BMGFP(+)LDLr(-/-)TLR2(-/-) mice had reduced BMGFP(+) leukocyte accumulation, lipid accumulation, foam cells, and endothelial cell injury. This is the first report of in vivo site-specific expression of endothelial cell TLR2. Expression of this receptor on endothelial cells contributed to early atherosclerotic processes in lesion-prone areas of the mouse aorta.

Smad and p38 MAP kinase-mediated signaling of proteoglycan synthesis in vascular smooth muscle

Atherosclerosis is the underlying pathological process of most cardiovascular disease. A critical component of the "response to retention" hypothesis of atherogenesis is proteoglycan/low density lipoprotein (LDL) binding. Transforming growth factor beta (TGF-beta) is present in atherosclerotic lesions, regulates vascular smooth muscle cell (VSMC) proteoglycan synthesis via an unknown signaling pathway, and increases proteoglycan/LDL binding. This pathway was investigated using the activin receptor-like kinase 5 (ALK5) inhibitor SB431542 and inhibitors of p38 MAP kinase as a possible downstream or alternative mediator. TGF-beta stimulated and SB431542 inhibited the phosphorylation of Smad2/3. In human VSMC, TGF-beta increased [(35)S]sulfate incorporation into proteoglycans associated with a 19% increase in glycosaminoglycan (GAG) chain size by size exclusion chromatography. SB431542 caused a concentration-dependent decrease in TGF-beta-mediated [(35)S]sulfate incorporation with 92% inhibition at 3 mum. Two different p38 MAP kinase inhibitors, SB203580 and SB202190, but not the inactive analogue SB202474, concentration-dependently blocked TGF-beta-mediated [(35)S]sulfate incorporation. TGF-beta increased [(3)H]glucosamine incorporation into glycosaminoglycans by 180% and [(35)S]Met/Cys incorporation into proteoglycan core proteins by 35% with both effects completely inhibited by SB431542. Blocking both Smad2/3 and p38 MAP kinase pathways prevented the effect of TGF-beta to increase proteoglycan to LDL binding. TGF-beta mediates its effects on proteoglycan synthesis in VSMCs via the ALK5/Smad2/3 phosphorylation pathway as well as via the p38 MAP kinase signaling cascade. Further studies of downstream pathways controlling proteoglycan synthesis may identify potential therapeutic targets for the prevention of atherosclerosis and cardiovascular disease.

Anti-proliferative activity of oral anti-hyperglycemic agents on human vascular smooth muscle cells: thiazolidinediones (glitazones) have enhanced activity under high glucose conditions

Background

Inhibition of vascular smooth muscle cell (vSMC) proliferation by oral anti-hyperglycemic agents may have a role to play in the amelioration of vascular disease in diabetes. Thiazolidinediones (TZDs) inhibit vSMC proliferation but it has been reported that they anomalously stimulate [³H]-thymidine incorporation. We investigated three TZDs, two biguanides and two sulfonylureas for their ability of inhibit vSMC proliferation. People with diabetes obviously have fluctuating blood glucose levels thus we determined the effect of media glucose concentration on the inhibitory activity of TZDs in a vSMC preparation that grew considerably more rapidly under high glucose conditions. We further explored the mechanisms by which TZDs increase [³H]-thymidine incorporation.

Methods

VSMC proliferation was investigated by [³H]-thymidine incorporation into DNA and cell counting. Activation and inhibition of thymidine kinase utilized short term [³H]-thymidine uptake. Cell cycle events were analyzed by FACS.

Results

VSMC cells grown for 3 days in DMEM with 5% fetal calf serum under low (5 mM glucose) and high (25 mM glucose) increased in number by 2.5 and 4.7 fold, respectively. Rosiglitazone and pioglitazone showed modest but statistically significantly greater inhibitory activity under high versus low glucose conditions (P < 0.05 and P < 0.001, respectively). We confirmed an earlier report that troglitazone (at low concentrations) causes enhanced incorporation of [³H]-thymidine into DNA but did not increase cell numbers. Troglitazone inhibited serum mediated thymidine kinase induction in a concentration dependent manner. FACS analysis showed that troglitazone and rosiglitazone but not pioglitazone placed a slightly higher percentage of cells in the S phase of a growing culture. Of the biguanides, metformin had no effect on proliferation assessed as [³H]-thymidine incorporation or cell numbers whereas phenformin was inhibitory in both assays albeit at high concentrations. The sulfonylureas chlorpropamide and gliclazide had no inhibitory effect on vSMC proliferation assessed by either [³H]-thymidine incorporation or cell numbers.

Conclusion

TZDs but not sulfonylureas nor biguanides (except phenformin at high concentrations) show favorable vascular actions assessed as inhibition of vSMC proliferation. The activity of rosiglitazone and pioglitazone is enhanced under high glucose conditions. These data provide further in vitro evidence for the potential efficacy of TZDs in preventing multiple cardiovascular diseases. However, the plethora of potentially beneficial actions of TZDs in cell and animal models have not been reflected in the results of major clinical trials and a greater understanding of these complex drugs is required to delineate their ultimate clinical utility in preventing macrovascular disease in diabetes.

Retention of Low-Density Lipoprotein in Atherosclerotic Lesions of the Mouse

Direct binding of apolipoprotein (apo)B-containing lipoproteins to proteoglycans is the initiating event in atherosclerosis, but the processes involved at later stages of development are unclear. Here, we investigated the importance of the apoB–proteoglycan interaction in the development of atherosclerosis over time and investigated the role of lipoprotein lipase (LPL) to facilitate low-density lipoprotein (LDL) retention at later stages of development. Atherosclerosis was analyzed in apoB transgenic mice expressing LDL with normal (control LDL) or reduced proteoglycan-binding (RK3359-3369SA LDL) activity after an atherogenic diet for 0 to 40 weeks. The initiation of atherosclerosis was delayed in mice expressing RK3359-3369SA LDL, but they eventually developed the same level of atherosclerosis as mice expressing control LDL. Retention studies in vivo showed that although higher levels of 131 I-tyramine cellobiose–labeled control LDL ( 131 I-TC-LDL) were retained in nonatherosclerotic aortae compared with RK3359-3369SA 131 I-TC-LDL, the retention was significantly higher and there was no difference between the groups in atherosclerotic aortae. Lower levels of control 125 I-TC-LDL and RK3359-3369SA 125 I-TC-LDL were retained in atherosclerotic aortae from ldlr −/− mice transplanted with lpl −/− compared with lpl +/+ bone marrow. Uptake of control LDL or RK3359-3369SA LDL into macrophages with specific expression of human catalytically active or inactive LPL was increased compared with control macrophages. Furthermore, transgenic mice expressing catalytically active or inactive LPL developed the same extent of atherosclerosis. Thus, retention of LDL in the artery wall is initiated by direct LDL–proteoglycan binding but shifts to indirect binding with bridging molecules such as LPL.

Role of the extracellular matrix and its receptors in smooth muscle cell function: implications in vascular development and disease

PURPOSE OF REVIEW

Cardiovascular disease affects millions of people worldwide, while the sarcoglycan deficient cardiomyopathies are rare disorders. One important common feature, however, is the vascular smooth muscle. Here we focus on the roles of extracellular matrix components and their receptors in the functions of vascular smooth muscle cells.

RECENT FINDINGS

Recent observations highlight the importance of integrins and the dystrophin-glycoprotein complex in development and cardiomyopathy. For example, integrin alpha4 and alpha7 subunits are important for distributing vascular smooth muscle cells during blood vessel development. Studies on delta-sarcoglycan deficient animals have revealed abnormal vascular smooth muscle proliferation and apoptosis. Furthermore, data suggest that perlecan, by affecting smooth muscle cell proliferation, participates in the atherosclerotic process. Overexpression of decorin leads to reduced progression of atherosclerosis and thrombospondin-1 has been implicated in regulation of smooth muscle cell contractility via inhibition of nitric oxide. Novel findings on versican suggest that the binding of versican to fibulin is of great importance for regulating smooth muscle cell function.

SUMMARY

By regulating migration, proliferation and apoptosis as well as extracellular matrix synthesis and assembly, proteoglycans, integrins and the dystrophin-glycoprotein complex may be of great importance both during development and in vascular disease.

Fatty acid-induced atherogenic changes in extracellular matrix proteoglycans

PURPOSE OF REVIEW

Nonesterified fatty acids change the expression and properties of the extracellular matrix proteoglycans of arterial and hepatic cells. We review how this may contribute to arterial disease in insulin resistance and type 2 diabetes.

RECENT FINDINGS

Elevated nonesterified fatty acids characterize the dyslipidemia of insulin resistance and type 2 diabetes. In hepatocytes high levels of fatty acids cause changes in proteoglycans leading to a matrix with decreased affinity for VLDL remnants. Furthermore, liver proteoglycans from insulin resistant hyperlipidemic Zucker rats showed alterations also associated with decreased remnant affinity. In arterial smooth muscle cells overexposure to fatty acids augmented expression of matrix proteoglycans for which LDL showed increased affinity. Fatty acids appeared to compromise insulin signaling by protein kinase C activation. The observed fatty acid-induced changes in matrix proteoglycans in liver and arteries can be an important component of the atherogenicity of the dyslipidemia of insulin resistance and type 2 diabetes.

SUMMARY

Overexposure to fatty acids can contribute to generate a remnant-rich dyslipidemia and to precondition the arterial intima for lipoprotein deposition via changes in expression of matrix proteoglycans. Normalizing fatty acid should be a key target in treatment of the atherogenic dyslipidemia of insulin resistance.

Free cholesterol in atherosclerotic plaques: where does it come from?

PURPOSE OF REVIEW

Free cholesterol in plaques is an emerging contributing factor to lesion instability and, until recently, apoptosis of lipid-laden macrophages was considered the major source of free cholesterol. The validity of this concept is beginning to be challenged since there is recent evidence of erythrocyte membrane-derived cholesterol in plaques. Therefore, intraplaque hemorrhage may not be a passive event, as once considered as studies continue to support the relationship of intraplaque hemorrhage and necrotic core expansion.

RECENT FINDINGS

The association of intraplaque hemorrhage, accumulated free cholesterol, and necrotic core expansion is beginning to unfold and recent MRI studies suggest the value of intraplaque hemorrhage as a predictor of recurrent cerebrovascular events. The amount of erythrocyte membrane-derived cholesterol is also suggested to be a measure of lesion vulnerability in acute coronary syndromes. Recent inhibitors studies of vascular permeability factors further emphasize the importance of intraplaque hemorrhage in plaque progression. Finally, DNA microarray analysis is starting to reveal key molecules involved in the accumulation of free cholesterol that are selectively induced in high-risk plaques.

SUMMARY

These recent findings emphasize the importance of intraplaque hemorrhage as a contributor of free cholesterol in plaques and point to its provocative role in lesion destabilization.