Induction of calcification in rabbit aortas by high cholesterol diets: roles of calcifiable vesicles in dystrophic calcification

Atherosclerotic calcification may weaken the aorta wall and thereby lead to rupture of the vessel. The mechanism whereby aortas undergo calcification remains unclear. Previous reports in this laboratory showed that, after 2 months of cholesterol-supplemental feeding, an increase in calcifiability of membrane vesicles isolated from rabbit aortas precedes substantial arterial calcification. Further, the mineral was deposited by isolated calcifiable vesicles as an amorphous phase similar to minerals in human aortas at an early stage of atherosclerosis. In the current study, atherosclerotic calcification was induced by exposing rabbits to a 1% cholesterol-rich diet for 3 or 6 months. After 3 months of dietary interventions, atherosclerotic lesions were fully developed. Fatty streaks were evident in areas proximal to the heart and became less frequent in the distal areas. However, calcification was not yet identifiable histologically or by using Fourier transform spectroscopy (FT-IR). After 6 months of high cholesterol treatment, aortas were partially calcified. Histochemical staining for mineral revealed that calcification appeared to occur predominantly in the intimal areas immediately adjacent to the media. Fourier Transform Imaging analysis demonstrated that the mineral deposited in atherosclerotic rabbit aortas was a hydroxyapatite-like phase. To determine whether aorta vesicles play a role in mineral formation in aortas, vesicles were isolated from calcified aortas and then their calcifiability was compared to that in normal vesicles. Interestingly, during the course of vesicle isolation, we found that calcifiable vesicles with much higher calcifiability than normal vesicles could be readily isolated from atherosclerotic aortas simply by suspending minced tissues in PBS. The characteristics of the calcification process and the enzymatic contents of isolated vesicles were similar to those obtained using collagenase digestion. Correlatively, mineral deposited by calcifiable vesicles isolated from the calcified aortas was also of hydroxyapatite-like phases. Altogether, these observations indicate that (1) aortic calcification is a later event during atherogenesis, (2) calcifiable vesicles are loosely bound to the matrices of the lesions as the result of the disease process and (3) similarities in the mineral phases between those in aortas and by vesicles during atherogenesis further support the role of calcifiable vesicles in dystrophic calcification.     

Isolation of calcifiable vesicles from human atherosclerotic aortas

Advanced mineralization can cause brittleness of aortic walls with decreased elasticity thereby causing the wall to rupture. Although the precise mechanisms of dystrophic calcification remain unknown, morphological evidence reveals the presence of mineral-associated vesicles in the lesions and defective bioprosthetic valves. In an attempt to demonstrate the calcifiability of the vesicles, small segments of human atherosclerotic aortas with calcified lesions were removed at autopsy and then digested in a crude collagenase solution to release vesicles. A differential centrifugation was then used to isolate calcifiable vesicles, which was precipitated at 300,000 x g for 20 min. An exposure of the vesicles to a calcifying medium containing physiologic levels of Ca2+, Pi, and 1 mM ATP caused Ca deposition in a vesicle protein-concentration dependent manner. The calcifiability of the vesicles was further demonstrated by electron microscopy. Fourier transform spectroscopic analysis of the deposited mineral revealed the presence of a hydroxyapatite phase, closely resembling the native form of mineral in atherosclerotic plaques. In addition, calcifiable vesicles were enriched in ATP-hydrolyzing enzymes including Mg2+ or Ca2+-ATPase and NTP pyrophosphohydrolase that may be involved in normal and pathological calcification. Triton X-100 at 0.01% abolished 80% of both ATPase activity and ATP-initiated calcification. A comparison of vesicles isolated from non-atherosclerotic and atherosclerotic aortas indicated that atherosclerotic vesicles tended to have higher calcifiability. These observations suggest that the calcifiable vesicles play a part in dystrophic calcification of aortas in atherosclerosis.     

Effect of calcification on in vivo mechanical response of rabbit arteries to balloon dilation

BACKGROUND. Atherosclerosis has been associated with loss of artery wall distensibility in human cadavers and in experimental animal models, giving it the lay term "hardening of the arteries." METHODS AND RESULTS. To assess the effect of calcification on arterial distensibility, balloon pressure and volume were recorded during dilation of calcified aortas in Watanabe heritable hyperlipidemic (WHHL) rabbits in vivo. Calcification was induced by dietary supplements of cholesterol, vitamin D2, and calcium. Balloon pressure, volume, and time signals were acquired at high frequency with controls for temperature and balloon inflation rate. Resistance to balloon dilation was minimal in control rabbit aortas (delta Vmax = 5.0 +/- 3.5 microliters) and in excised nonatherosclerotic human coronary arteries, and it was small in aortas from cholesterol-fed rabbits (12.3 +/- 8 microliters), even when lipid levels were markedly elevated by a high cholesterol diet (611 +/- 347 mg/dl). With dietary cholesterol, vitamin D2, and calcium supplements, WHHL rabbits developed mild hypercalcemia (15 +/- 1.9 mg/dl), hypercholesterolemia (1,100 +/- 633 mg/dl), moderate-to-marked aortic calcification, and high resistance to balloon dilation (38 +/- 27) comparable to that seen in angioplasty patients. CONCLUSIONS. It is concluded that experimentally induced calcification decreases the distensibility of the rabbit aorta in vivo and that it yields to balloon dilation by plastic deformation closely resembling that seen in balloon angioplasty of human coronary arteries. These findings suggest that calcification contributes to arterial "hardening" associated with atherosclerosis.     

Isolation of calcifiable vesicles from aortas of rabbits fed with high cholesterol diets

Advanced arterial wall calcification in atherosclerosis imposes a serious rupturing effect on the aorta. However, the mechanism of dystrophic calcification linked to hyperlipidemia, that causes atherosclerosis remains unknown. Emerging morphological and biochemical evidence reveals that calcifiable vesicles may have a role in plaque calcification. To determine whether a high cholesterol diet can induce arterial calcification and produce or activate calcifiable vesicles in aortas, a rabbit model was used. After 2 months of daily high lipid feeding (supplemented with 2% cholesterol and 6% peanut oil), typical atherosclerotic lesions developed. However, the mineral, if present in aortas, was insufficient to be detected by Fourier transform-infrared spectroscopy (FT-IR) or alizarin red staining, indicative of a non-calcifying stage of atherosclerosis. Small segments of thoracic aortas were digested in a crude collagenase solution to release calcifiable vesicles. Vesicles were also isolated from normal aortas as control to consider the possibility that membrane vesicles may be produced by crude collagenase digestion, which could cause the degradation of some cells. Calcifiable vesicles were precipitated at 300,000 x g after subcellular particles were removed by centrifugation at 30,000 x g. Calcifiability of isolated vesicles was then tested using calcifying media containing physiological levels of Ca2+ and Pi and 1 mM ATP. Electron microscopic observations showed that the isolated vesicles were heterogeneous in size and shape and capable of depositing electron dense particles. Fourier transform infrared spectroscopic analysis of the deposited particles revealed the presence of an amorphous mineral phase. The spectroscopic mineral to matrix ratios, related to the amount of mineralization, indicated that vesicles from cholesterol-fed rabbits produced more minerals than control vesicles obtained from the normal aortas. Alizarin red staining for mineral further demonstrated substantially higher calcifiability of the experimental vesicles. A 3-5 h exposure of the vesicles to calcifying media caused significant deposition of 45Ca and 32Pi in a vesicle protein-concentration dependent manner. Similar to previously reported observations with human atherosclerotic aorta vesicles, rabbit vesicles were enriched in ATP-hydrolyzing enzymes including Mg2+- or Ca2+-ATPase and NTP pyrophosphohydrolase that are implicated in normal and pathological calcification. Altogether, these observations suggest that accumulation of the released calcifiable vesicles, as a result of high cholesterol diets, may have a role in dystrophic calcification in hyperlipidemia-related atherosclerosis.     

Effect of crystallization inhibitors on vascular calcifications induced by vitamin D: a pilot study in Sprague-Dawley rats.

BACKGROUND: Pathological calcification in soft tissues (ie, ectopic calcification) can have severe consequences. Hydroxyapatite is the common mineral phase present in all tissue calcifications. In general, the development of tissue calcifications requires a pre-existing injury as an inducer (heterogeneous nucleant), whereas further progression requires the presence of other promoter factors (such as hypercalcemia and/or hyperphosphatemia) and/or a deficiency in calcification repressor factors (crystallization inhibitors and cellular defense mechanisms). The present study investigated the capacity of etidronate (a bisphosphonate used in osteoporosis treatment) and phytate (a natural product) to inhibit vascular calcification in rats. METHODS AND RESULTS: Six male Sprague-Dawley rats in each of the 3 treatment groups were subcutaneously injected with either a placebo (physiological serum solution), etidronate (0.825 micromol x kg(-1) x day (-1)) or phytate (0.825 micromol x kg (-1) x day(-1)) for 8 days. Four days into this regimen, calcinosis was induced by subcutaneous injections of 500,000 IU/kg vitamin D at 0 h, 24 h and 48 h. Ninety-six hours after the final vitamin D injection, the rats were killed and aortas and their hearts were removed for histological and calcium analyses. The data showed that phytate-treated rats had lower levels of aortic calcium than placebo-treated rats. All groups had similar heart calcium levels. CONCLUSIONS: The present study found that phytate acted as a vascular calcification inhibitor. Thus, the action of polyphosphates could be important in protecting against vascular calcification.     

Osteoprotegerin inhibits artery calcification induced by warfarin and by vitamin D

The present experiments were carried out to test the hypothesis that arterial calcification is linked to bone resorption by determining whether the selective inhibition of bone resorption with osteoprotegerin will inhibit arterial calcification. In the first test, arterial calcification was induced by treating 22-day-old male rats with warfarin, a procedure that inhibits the gamma-carboxylation of matrix Gla protein and causes extensive calcification of the arterial media. Compared with rats treated for 1 week with warfarin alone, rats treated with warfarin plus osteoprotegerin at a dose of 1 mg/kg per day had dramatically reduced alizarin red staining for calcification in the aorta and in the carotid, hepatic, mesenteric, renal, and femoral arteries, and they had 90% lower levels of calcium and phosphate in the abdominal aorta (P<0.001) and in tracheal ring cartilage (P<0.01). More rapid arterial calcification was induced by treating 49-day-old male rats with toxic doses of vitamin D. Treatment for 96 hours with vitamin D caused widespread alizarin red staining for calcification in the aorta and the femoral, mesenteric, hepatic, renal, and carotid arteries, and osteoprotegerin completely prevented calcification in each of these arteries and reduced the levels of calcium and phosphate in the abdominal aorta to control levels (P<0.001). Treatment with vitamin D also caused extensive calcification in the lungs, trachea, kidneys, stomach, and small intestine, and treatment with osteoprotegerin reduced or prevented calcification in each of these sites. Measurement of serum levels of cross-linked N-teleopeptides showed that osteoprotegerin dramatically reduced bone resorption activity in each of these experiments (P<0.001). Therefore, we conclude that doses of osteoprotegerin that inhibit bone resorption are able to potently inhibit the calcification of arteries that is induced by warfarin treatment and by vitamin D treatment. These results support the hypothesis that arterial calcification is linked to bone resorption.     

阿托伐他汀对大鼠血管平滑肌细胞迁移和组织蛋白酶S表达的影响

目的 观察阿托伐他汀对白细胞介素1β诱导的大鼠血管平滑肌细胞迁移及组织蛋白酶S和核因子κB表达的影响.方法 取SD大鼠胸主动脉进行血管平滑肌细胞培养,采用Boyden小室实验评价不同浓度阿托伐他汀对白细胞介素1β诱导大鼠血管平滑肌细胞迁移的影响,用细胞免疫化学和逆转录聚合酶链反应法检测各组组织蛋白酶S和核因子κB表达的变化.结果 与正常对照组相比,白细胞介素1β组血管平滑肌细胞迁移增多,核因子κB和组织蛋白酶s表达明显增加(P<0.01).1 μmol/L和10 μmol/L阿托伐他汀组可呈剂量依赖性地抑制白细胞介素1β所致的细胞迁移以及组织蛋白酶S和核因子κB表达(P<0.01).结论 阿托伐他汀可能通过抑制核因子κB使组织蛋白酶S表达减少,从而抑制血管平滑肌细胞迁移.     

SB 242784, a selective inhibitor of the osteoclastic V-H+ATPase,inhibits arterial calcification in the rat

The present experiments were carried out to further test the hypothesis that arterial calcification is linked to bone resorption by determining whether the selective inhibition of bone resorption with SB 242784, a specific inhibitor of the osteoclastic V-H+-ATPase, will inhibit arterial calcification. Treatment for 96 hours with toxic doses of vitamin D caused widespread calcification in the aorta and in the femoral, mesenteric, hepatic, renal, and carotid arteries, and treatment with SB 242784 completely prevented the vitamin D-induced calcification of each of these arteries at a dose of 40 mg/kg per day and significantly reduced calcification at a dose of 10 mg/kg per day. Treatment with vitamin D also caused extensive calcification in the lungs, tracheal cartilage, and kidneys, and treatment with SB 242784 prevented or reduced calcification at each of these sites. Measurement of serum levels of cross-linked N-telopeptides, a specific measure of bone resorption activity, showed that treatment with vitamin D alone produced the expected 2.4-fold increase in bone resorption activity and that concurrent treatment with the 40-mg dose of SB 242784 reduced bone resorption activity to below control levels. With the inclusion of the present results, there are now three types of bone resorption inhibitors (each with an entirely different mode of action on the osteoclast) that share the ability to potently inhibit arterial calcification in the rat, the V-H+-ATPase inhibitor SB 242784, the cytokine osteoprotegerin, and the amino bisphosphonates alendronate and ibandronate.     

Phosphocitrate and its analogue N-sulpho-2-amino tricarballylate inhibit aortic calcification

This study reports the ability of phosphocitrate and its enzyme-resistant analogue N-sulpho-2-amino tricarballylate to inhibit aortic calcification. Dystrophic calcification of aorta was induced by transplanting fresh aortic segments in Millipore chambers to the peritoneal walls of recipient rats. Daily intraperitoneal injection of the new inhibitors remarkably reduced calcium accumulation by the aortae and prevented the appearance of hydroxyapatite-like crystalline structures. Phosphocitrate was the most effective of the anti-calcifying agents tested, preventing aortic calcification at 1 mumole/day/rat. N-sulpho-2-amino tricarballylate was less effective, reducing aortic calcification by 60% at 10 mumoles/day/rat. The new inhibitors might prove therapeutically useful in man to arrest soft tissue calcification.     

Vascular calcification in chronic renal failure: what have we learned from animal studies?

Accelerated atherosclerotic plaque calcification and extensive medial calcifications are common and highly detrimental complications of chronic kidney disease. Valid murine models have been developed to investigate both pathologically distinguishable complications, which allow for better insight into the cellular mechanisms underlying these vascular pathologies and evaluation of compounds that might prevent or retard the onset or progression of vascular calcification. This review describes various experimental models that have been used for the study of arterial intimal and/or medial calcification and discusses the extent to which this experimental research has contributed to our current understanding of vascular calcification, particularly in the setting of chronic renal failure.     

Inhibitors of HIV-1 replication that inhibit HIV integrase.

HIV-1 replication depends on the viral enzyme integrase that mediates integration of a DNA copy of the virus into the host cell genome. This enzyme represents a novel target to which antiviral agents might be directed. Three compounds, 3,5-dicaffeoylquinic acid, 1-methoxyoxalyl-3,5-dicaffeoylquinic acid, and L-chicoric acid, inhibit HIV-1 integrase in biochemical assays at concentrations ranging from 0.06-0.66 microgram/ml; furthermore, these compounds inhibit HIV-1 replication in tissue culture at 1-4 microgram/ml. The toxic concentrations of these compounds are fully 100-fold greater than their antiviral concentrations. These compounds represent a potentially important new class of antiviral agents that may contribute to our understanding of the molecular mechanisms of viral integration. Thus, the dicaffeoylquinic acids are promising leads to new anti-HIV therapeutics and offer a significant advance in the search for new HIV enzyme targets as they are both specific for HIV-1 integrase and active against HIV-1 in tissue culture.     

Turnover of Ethyl-Linoleate in Rat Plasma and Its Distribution in Various Organs

The fate of [¹⁴C]ethyl-linoleate (EthLin) after its intravenous administration was investigated in pentobarbital-anesthetized rats. The disappearance of [¹⁴C]EthLin from the plasma was very rapid and followed quite closely a biexponential function of time. Fitting of the experimental data to a two-compartmental mammillary model revealed that the labeled compounds are eliminated from the plasma with a half-life of <1 min during the early time following the intravenous injection and that a large portion of the EthLin is hydrolyzed instantly to linoleic acid and ethanol. About 9–11 % of the plasma [¹⁴C]EthLin or its breakdown products are irreversibly cleared from the plasma compartment each minute. Most of the ¹⁴C-labeled compounds that originated in the plasma were recovered in the rat liver and lungs and to a lesser extent in the heart, spleen, and kidneys. Two hr after the [¹⁴C]EthLin administration, -2.5–5.5% of the total radioactivity in the various organs was still associated with EthLin. Such accumulations, although relatively small, indicate that fatty acid ethyl esters (FAEEs) may be taken up from the plasma. Thus, some of the FAEEs that are formed in certain organs may spillover to the circulating blood where much of it would be hydrolyzed to free fatty acids, but reuptake from the plasma may still account, to some extent, to FAEE-induced damage in chronic alcohol abusers.     

Atherosclerosis and bisphosphonate

Bisphosphonates are widely used for the treatment of osteoporosis, hypercalcemia, Paget's disease, and osteolytic disease due to malignant tumor or its metastasis. Certain compounds of bisphosphonates have been shown to inhibit the development of experimental atherosclerosis without altering serum lipid profile. Recently, etidronate has been demonstrated to prevent the thickening of carotid arterial wall in diabetic patients associated with osteopenia. These findings may illuminate clinical application of bisphosphonates for the treatment of atherosclerosis and vascular calcification.     

The effect of type 1 diabetes mellitus on the gender difference in coronary artery calcification

OBJECTIVES

To examine whether the gender difference in coronary artery calcification, a measure of atherosclerotic plaque burden, is lost in type 1 diabetic patients, and whether abnormalities in established coronary heart disease risk factors explain this.

BACKGROUND

Type 1 diabetes abolishes the gender difference in coronary heart disease mortality because it is associated with a greater elevation of coronary disease risk in women than men. The pathophysiological basis of this is not understood.

METHODS

Coronary artery calcification and coronary risk factors were compared in 199 type 1 diabetic patients and 201 nondiabetic participants of similar age (30 to 55 years) and gender (50% female) distribution. Only one subject had a history of coronary disease. Calcification was measured with electron beam computed tomography.

RESULTS

In nondiabetic participants there was a large gender difference in calcification prevalence (men 54%, women 21%, odds ratio 4.5, p < 0.001), half of which was explained by established risk factors (odds ratio after ADJUSTMENT = 2.2). Diabetes was associated with a greatly increased prevalence of calcification in women (47%), but not men (52%), so that the gender difference in calcification was lost (p = 0.002 for the greater effect of diabetes on calcification in women than men). On adjustment for risk factors, diabetes remained associated with a threefold higher odds ratio of calcification in women than men (p = 0.02).

CONCLUSIONS

In type 1 diabetes coronary artery calcification is greatly increased in women and the gender difference in calcification is lost. Little of this is explained by known coronary risk factors.     

Diazepam inhibits cholesterol esterification by arterial ACAT and plasma LCAT, in vitro

Benzodiazepine drugs have been reported to have antiatherosclerotic effects in rabbits and roosters and to alter the pattern of circulating lipoproteins in man. The mechanism(s) of these effects has not been elucidated. The studies presented here indicate that diazepam, the most widely used benzodiazepine, is an inhibitor of cholesterol esterification by ACAT in vitro in atheromatous rabbit aortas, in microsomes isolated from atheromatous rabbit aortas, and in normal rat aortas. Diazepam also inhibited LCAT in plasma from man, monkey, rabbit, and rat, in vitro. The ability of diazepam to inhibit these enzyme systems may offer insight into possible in vivo mechanisms of action against atherosclerosis and of lipoprotein modification.     

Calcification in end-stage kidneys

This study was carried out to determine the frequency and to quantitate the severity of calcium-phosphate deposits in end-stage kidneys. In 57 of 59 end-stage kidneys obtained from patients with a variety of different renal diseases, calcium levels were greater than 2 standard deviations (SD) above control values. The mean calcium concentration was 157 ± 24 mmol/kg dry defatted tissue in the end-stage kidneys as compared to 17 ± 1 mmol/kg in the control kidneys. Histologically, calcium was deposited in the cortical tubular cells, basement membranes and interstitium. It would appear that calcification occurred during the course of renal failure rather than terminally in that the kidney calcium concentration bore no relationship to the calcium × phosphate product, and the calcium concentration in the kidneys of uremic patients undergoing dialysis (144 ± 23 mmol/kg) was no greater than that found in uremic patients not undergoing dialysis (188 ± 62 mmol/kg). It is suggested that calcification may damage the diseased kidney accelerating the rate of renal functional deterioration.     

Calcium channel antagonists in the treatment of asthma

The pathophysiologic processes that contribute to airway obstruction in asthma involve Ca²⁺-dependent excitation-contraction and stimulus-secretion coupling mechanisms. The emergence of new compounds that specifically inhibit Ca²⁺ flux across membrane ionic channels has stimulated widespread interest in the therapeutic potential of these agents in asthma. Studies with these agents in relevant in vitro test systems and animal models, however, have yielded conflicting results and have thus far failed to furnish strong support for a therapeutic role. In human studies, these agents have been found to inhibit exercise-induced bronchospasm, but their ability to inhibit the effects of other stimuli and to dilate airways is equivocal. In general, clinical trials with currently approved drugs—diltiazem, nifedipine, and verapamil— are limited by potency, formulation, and side effects of these agents. What future role, if any, Ca²⁺ channel antagonists will have in the treatment of asthma is likely to depend on the development of newer agents with greater tissue selectivity at the level of airway smooth muscle and mast cells.     

IL-37抑制血管钙化相关机制的研究现状

IL-37作为一种新型抗炎因子近年来被认为对血管钙化及动脉粥样硬化具有保护作用，在其他钙化反应如瓣膜钙化也发挥有利作用。IL-37可抑制炎症反应和自身免疫反应，并且显示出对成骨细胞分化也有一定的效果。IL-37或可作为一个新型靶点，减缓甚至治疗血管钙化相关心血管疾病。现对IL-37抑制钙化的具体机制作一综述。     

Current concepts of vascular calcification

Vascular calcification, such as coronary and aortic calcification, is a significant feature of vascular pathology. Two distinct forms of vascular calcification are well recognized. One is medial calcification, which occurs between the cell layers of smooth muscle cells, and is related to aging, diabetes and chronic renal failure. The other is atherosclerotic calcification, which occurs in the intima during the development of atheromatous disease. It has been shown that statins inhibit the progression of calcification in the aortic valve and the coronary artery. We have found that statins inhibit calcification of human aortic smooth muscle cells, which is induced by incubating the cells in high-phosphate medium. We also found that this is mediated by inhibiting cellular apoptosis, an essential mechanism for calcification, not by inhibiting inorganic phosphate (Pi) uptake by sodium-dependent phosphate cotransporter (NPC). Besides apoptosis and Pi uptake, such proteins as osteoprotegerin (OPG), matrix Gla protein (MGP), Klotho, fetuin-A, and apoE have been shown to negatively affect vascular calcification. Many previous reports suggest that vascular calcification appears to be regulated by promoting factors, such as Pi, apoptosis, modified LDL, advanced glycation end products, oxidative stress, vitaminD3, glucocorticoid, cbfa-1, osteopontin, and inhibitory factors, such as OPG, MGP, Klotho, fetuin-A, PTH/PTHrP, pyrophosphate, statins, and bisphosphonates. The precise mechanism of vascular calcification is of interest.     

Smooth muscle cells deficient in osteopontin have enhanced susceptibility to calcification in vitro

OBJECTIVE: Vascular calcification is an actively regulated process, correlating with cardiovascular morbidity and mortality especially in patients with diabetes and chronic renal diseases. Osteopontin (OPN) is abundantly expressed in human calcified arteries and inhibits vascular calcification in vitro and in vivo. How OPN functions in vascular calcification, however, is less clear. METHODS: Smooth muscle cells (SMCs) were isolated from aortas of OPN knock-out (OPN-/-) and wild type (OPN+/+) mice. RESULTS: OPN-/- SMCs were identical to OPN+/+ SMCs in morphology and stained positively for SM lineage proteins, desmin, smooth muscle alpha-actin and SM22alpha. No spontaneous calcification was observed in OPN-/- SMCs under normal culture conditions or in medium containing 1%, 3%, or 5% fetal bovine serum. However, when cultured in medium containing elevated concentrations of inorganic phosphate, an inducer of vascular calcification, a significantly higher calcification was observed in OPN-/- SMCs compared to OPN+/+ SMCs that, in response to elevated phosphate, synthesized and secreted OPN into the culture. Finally, retroviral transduction of mouse OPN cDNA into OPN-/- SMCs rescued the calcification phenotype of the cells. CONCLUSION: These results are the first to demonstrate an inhibitory role of endogenously produced OPN on SMC calcification, suggesting a novel feedback mechanism where OPN produced locally by the SMCs may serve as an important inducible inhibitor of vascular calcification.