Disorders in the system of cyclic nucleotides in atherosclerosis: cyclic AMP and cyclic GMP content and activity of related enzymes in human aorta

Cellular models of atherosclerosis and their implication for testing natural substances with anti-atherosclerotic potential

BACKGROUND

Atherosclerosis remains a major problem in the modern society being a cause of life-threatening cardiovascular diseases. Subclinical atherosclerosis can be present for years before the symptoms become obvious, and first manifestations of the disease in a form of acute ischemia of organs are often fatal. The development of atherosclerosis is characterized by lipid accumulation in the aortic wall and formation of foam cells overloaded with large amounts of lipid inclusions in the cytoplasm. Current therapy of atherosclerosis is aimed mostly at the normalization of the blood lipid profile, and has no direct activity on the atherosclerotic plaque development. It is therefore necessary to continue the search for substances that possess a direct anti-atherosclerotic effect, preventing the cholesterol deposition in the arterial wall cells and reducing the existing plaques.

PURPOSE

Medicinal plants with potential anti-atherosclerotic activity are especially interesting in that regard, as plant-based medications are often characterized by good tolerability and are suitable for long-term therapy. The evaluation of novel active substances requires the establishment of reliable models of atherogenesis. In this review we discuss cellular models based on cultured human aortic cells. We also discuss several examples of successful application of these models for evaluation of anti-atherosclerotic activity of natural products of botanical origin based on measurable parameters, such as intracellular cholesterol accumulation.

CHAPTERS

We describe several examples of successful screening and clinical studies evaluating natural products that can be beneficial for prevention and treatment of atherosclerosis, including the subclinical (asymptomatic) forms.

CONCLUSION

Several substances of botanical origin have been demonstrated to be active for treatment and prevention of atherosclerosis. The obtained results encourage future studies of naturally occurring anti-atherosclerotic agents.

Long-term endothelial dysfunction is more pronounced after stenting than after balloon angioplasty in porcine coronary arteries

OBJECTIVE

To compare percutaneous transluminal coronary angioplasty (PTCA) and stent implantation with respect to the long-term changes they induce in the newly formed endothelium in porcine coronary arteries by studying both morphological and functional parameters of the endothelium at 2 weeks and 3 months after intervention.

BACKGROUND

Problems affecting PTCA or stent implantation have been overcome to a large extent by means of better techniques and the availability of new drugs. Late problems, however, still exist in that restenosis affects a large number of patients. With an increasing number of patients being treated with stents, the problem of in-stent restenosis is of even greater concern, as this seems difficult to treat. A functional endothelial lining is thought to be important in controlling the growth of the underlying vascular tissue. We hypothesized that the enhanced neointimal hyperplasia observed after stenting is associated with a more pronounced and prolonged endothelial dysfunction.

METHODS

Arteries were analyzed using a dye-exclusion test and planimetry of permeable areas. Thereafter, the arteries were processed for light and scanning electron microscopy for assessment of morphology and proliferative response.

RESULTS

Leakage of the endothelium for molecules such as Evans blue-albumin as well as prolonged endothelial proliferation is observed as late as 3 months after the intervention, and is more pronounced after stenting. Permeability is associated with distinct morphologic characteristics: endothelial retraction, the expression of surface folds, and the adhesion of leukocytes.

CONCLUSIONS

Stenting especially decreases long-term vascular integrity with respect to permeability and endothelial proliferation, and is associated with distinct morphologic characteristics.

Glucocorticoid inhibits cAMP production induced by vasoactive agents in aortic smooth muscle cells

It is well-known that atherosclerotic change and hypertension are common manifestations in patients with glucocorticoid excess. We previously reported that pituitary adenylate cyclase activating polypeptide (PACAP), prostaglandin E2 (PGE2) and carbacyclin, a stable analog of prostacyclin, have suppressive effects on vasopressin-induced DNA synthesis of rat aortic smooth muscle cells through cAMP production (Murase et al., J. Hypertens., 10 (1992) 1505; Oiso et al., Biochem. Cell. Biol., 71 (1993) 156). In the present study, we investigated the effect of glucocorticoid on cAMP production induced by PACAP, PGE2 and carbacyclin in aortic smooth muscle cells. The pretreatment with dexamethasone significantly inhibited cAMP accumulation induced by these vasoactive agents in a dose dependent manner in the range between 10 pM and 10 nM. These inhibitory effects of dexamethasone were dependent on the time of pretreatment up to 8 h. Dexamethasone inhibited cAMP accumulation induced by NaF, a GTP-binding protein activator, and forskolin which directly activates adenylate cyclase. Moreover, forskolin-induced adenylate cyclase activity was significantly reduced in membranes prepared from the cells treated with dexamethasone. These results strongly suggest that glucocorticoid inhibits cAMP production induced by vasoactive agents in primary cultured rat aortic smooth muscle cells and the inhibitory effect is exerted at the level of adenylate cyclase.

Hypercholesterolemia is associated with a reduced response of smooth muscle guanylyl cyclase to nitrovasodilators

A diminished relaxant response of atherosclerotic arteries to nitrovasodilators has been frequently observed in advanced stages of hypercholesterolemia. In the present study, we investigated whether this effect might be a result of reduced activity of smooth muscle guanylyl cyclase. Experimental atherosclerosis was induced by feeding rabbits a cholesterol-rich diet (1%) over a period of 4 months. Aortas were removed and homogenized, and guanylyl cyclase activity was measured in the 100,000 g supernatants. Sodium nitroprusside, which stimulated cyclic GMP (cGMP) formation in control tissues almost 200-fold (from 3 to 585 pmol cGMP.mg-1 x min-1), increased enzyme activities in atherosclerotic aortas only approximately 90-fold (from 3 to 257 pmol cGMP.mg-1 x min-1). Similarly, the maximal stimulatory effects of S-nitroso-glutathione were reduced from 200-fold (controls) to 114-fold in atherosclerotic tissues. Basal guanylyl cyclase activities were identical in both atherosclerotic and control vessels. Hypercholesterolemia also reduced the activity of smooth muscle adenylyl cyclase. In control aortas, basal and NaF-stimulated enzyme activities were 24 and 349 pmol cAMP.mg-1 x min-1, respectively, whereas cAMP formation was reduced in atherosclerotic aortas to 7 (basal) and 96 (NaF) pmol cAMP.mg-1.min-1. The stimulatory effect of NaF (approximately 14-fold) remained unchanged. Since adenylyl and guanylyl cyclase have important functions in regulating vascular tone, reduced activities of both enzymes may contribute to the diminished relaxant and/or enhanced vasoconstricting effects of vasoactive compounds in atherosclerotic blood vessels.

Heparan sulfate-degrading enzymes induce modulation of smooth muscle phenotype

Macrophages cocultured with rabbit aortic smooth muscle cells at a ratio of 1:3 degraded all the 35S-labeled heparan sulfate proteoglycan from the smooth muscle surface into free sulfate (Kav of 0.84 on Sepharose 6B). Concomitantly, the same macrophages induced a decrease in the volume fraction of myofilaments (Vvmyo) of the smooth muscle cells and a decrease in alpha-actin mRNA as a percentage of total actin mRNA. Both macrophage lysosomal lysate at neutral pH and heparinase degraded cell-free 35S-labeled matrix deposited by smooth muscle cells into fragments which eluted at a Kav of 0.63 and which were identified as heparan sulfate chains by their complete degradation in the presence of low pH nitrous acid. At acid pH the macrophage lysosomal lysate completely degraded the heparan sulfate to free sulfate (Kav 0.84). Both macrophage lysosomal lysate and commercial heparinase at neutral pH induced smooth muscle phenotypic change while other enzymes such as trypsin and chondroitin ABC lyase had no effect. It was therefore suggested that the active factor present in the macrophages is a lysosomal heparan sulfate-degrading endoglycosidase (heparinase). Only a small amount of heparan sulfate-degrading activity was released into the incubation medium by living macrophages, and there was no heparinase activity on their isolated plasma membranes, although proteolytic enzymes were evident in both instances. In pulse-chase studies, high Vvmyo smooth muscle cells were seen to constantly internalize and degrade 35S-labeled heparan sulfate proteoglycan from their own pericellular compartment, suggesting that this may be the mechanism by which smooth muscle phenotype is maintained under normal circumstances and that removal of heparan sulfate from the surface of smooth muscle cells and its degradation by macrophages temporarily interrupts this process, inducing smooth muscle phenotypic change.

Cyclic nucleotides and atherosclerosis: studies in primary culture of human aortic cells

A primary culture of cells derived from uninvolved and atherosclerotic intima of human aorta was used to elucidate the role of cyclic nucleotides in atherogenesis. The cells cultured from fatty streaks and atherosclerotic plaques had a 2- to 8-fold lower cyclic AMP level and a 1.5- to 2-fold higher level of cyclic GMP compared with those of a grossly normal intima. Medial cells cultured from nonlesioned and atherosclerotic aortic segments showed no differences in the cyclic nucleotide concentrations. Reduction of the intracellular cyclic AMP with 2'-deoxyadenosine or a cyclic GMP elevation with its dibutyryl derivative, or liposomes containing cyclic GMP stimulated the uptake of [3H]thymidine and protein synthesis in the cells cultured from unaffected intima. On the contrary, a rise of the intracellular cyclic AMP caused by adenylate cyclase activators, a phosphodiesterase inhibitor, dibutyryl cyclic AMP, and liposomes containing cyclic AMP inhibited cell proliferation and protein synthesis. Elevation of the intracellular cyclic AMP stimulated the hydrolysis of lipids which led to reduction of lipid levels in the cells cultured from atherosclerotic lesions. The results of this study corroborate the existence of a relationship between the alterations of intracellular cyclic nucleotide levels and the metabolic disorders occurring in atherosclerosis.