Medial artery calcification. A neglected harbinger of cardiovascular complications in non-insulin-dependent diabetes mellitus

Impact of diabetes on coronary stenosis and coronary artery calcification detected by electron-beam computed tomography in symptomatic patients

OBJECTIVE

Ischemic heart disease is a pivotal complication for diabetic patients. Electron-beam computed tomography (EBCT) represents the only noninvasive method that allows for accurate quantification of coronary artery calcification that reflects underlying atherosclerotic disease. Although coronary calcium score (CCS) cut points that predict the presence of angiographic stenosis have been established in nondiabetic individuals, it is not known whether coronary calcifications in diabetic patients are associated with the presence of significant coronary stenoses. In this study, we evaluated the relationship between coronary calcifications and angiographic stenosis in symptomatic patients with or without type 2 diabetes.

RESEARCH DESIGN AND METHODS

In this study, 282 patients (204 men and 78 women) with chest pain, including 101 diabetic patients and 181 nondiabetic patients (mean age 63 +/- 9.6 years), underwent coronary angiography and EBCT with determination of CCS using Agatston's method. Luminal stenosis >or= 50% was defined as significant coronary stenosis.

RESULTS

Angiography identified 205 patients with significant stenoses (89 of 101 diabetic patients, 114 of 181 nondiabetic patients). The sensitivity and specificity of EBCT to detect significant coronary stenosis were not significantly different between diabetic and nondiabetic patients. In diabetic patients, a CCS >or=90 was associated with 75% sensitivity and 75% specificity, whereas a CCS >or=200 was associated with 64% sensitivity and 83% specificity.

CONCLUSIONS

We demonstrated that calcification of the coronary arteries in symptomatic diabetic patients is well associated with severity of coronary stenosis, as in nondiabetic patients.

Control of serum phosphorus: implications for coronary artery calcification and calcific uremic arteriolopathy (calciphylaxis)

There is mounting evidence that elevated serum phosphorus is an important cardiovascular risk factor in patients with end stage renal disease. Recent work has shown that vascular smooth muscle cells have the ability to undergo osteoblastic differentiation and produce an environment conducive to mineralization. Serum phosphorus is an important stimulator of this process and the adverse cardiovascular effects of hyperphosphatemia are most likely mediated via its ability to enhance the development of vascular calcification. Arterial calcification, whether it is intimal or medial in location, is a strong independent risk factor for cardiovascular morbidity and mortality. Both coronary artery calcification and calciphylaxis are prototypical examples of arterial calcification that have been associated with poor phosphate control. Furthermore, several investigators have recently suggested that the prescription of large doses of calcium to achieve phosphate control may augment, rather than diminish, the risk of vascular calcification. This is more likely to be true in the presence of low turnover bone disease, a diagnosis difficult to make with routine laboratory testing. A brief review of the molecular biology of vascular calcification supports the concept that warfarin administration may exacerbate the calcific process, particularly in the setting of hyperphosphatemia, as has been reported in patients with calciphylaxis. Recognizing the consequences of poor phosphate control, it is time to adopt strict target levels that aim to normalize serum phosphorus levels. The available evidence supports that this control should not be achieved through the use of supraphysiologic doses of supplemental calcium.

Vascular calcification and inorganic phosphate

Vascular calcification is highly correlated with elevated serum phosphate levels in uremic patients. To shed light on this process, we examined the ability of extracellular inorganic phosphate (Pi) levels to regulate human aortic smooth muscle cell (HSMC) culture mineralization in vitro. When cultured in media containing normal physiological levels of Pi (1.4 mmol/L Pi), HSMC grew in monolayers and did not mineralize. In contrast, HSMC cultured in media containing Pi levels comparable to those seen in hyperphosphatemic individuals (>1.4 mmol/L), showed dose-dependent increases in cell culture calcium deposition. Mechanistic studies showed that elevated Pi treatment of HSMC also enhanced the expression of the osteogenic markers, osteocalcin and Cbfa-1. The effects of elevated Pi on HSMC were mediated by a sodium-dependent phosphate cotransporter (NPC), as indicated by the ability of the specific NPC inhibitor, phosphonoformic acid (PFA), to dose-dependently inhibit Pi-induced calcium deposition as well as osteocalcin and Cbfa-1 gene expression. Using polymerase chain reaction and Northern blot analyses, the NPC in HSMC was identified as Pit-1 (Glvr-1), a member of the type III NPCs. Interestingly, platelet-derived growth factor-BB (PDGF-BB), a potent atherogenic stimulus, increased the maximum velocity (Vmax) but not the affinity (Km) of phosphate uptake, enhanced the expression of Pit-1 mRNA, and induced HSMC culture calcification in a time- and dose-dependent manner. Importantly, in the presence of PDGF, HSMC culture calcification occurred under normophosphatemic conditions. These data suggest that elevated Pi may directly stimulate HSMC to undergo phenotypic changes that predispose to calcification and may help explain both the phenomena of human metastatic calcification under hyperphosphatemic conditions as well as increased calcification in PDGF-rich atherosclerotic lesions.

Peripheral vascular disease and type 2 diabetes mellitus

Patients with Type 2 diabetes mellitus frequently have peripheral vascular disease, with a predilection for the lower legs. In this review potential mechanisms for this high prevalence and altered distribution are explored. It is hypothesised that the metabolic abnormalities in the prediabetic phase predispose to a more distal and aggressive atherosclerosis. Once diabetes has developed this process is accelerated due to chronic hyperglycaemia. Furthermore, endothelial damage, non-enzymatic glycosylation and polyneuropathy could lead to impaired vascular remodelling and collateral formation.

The role of osteoprogenitors in vascular calcification

Calcification is a component of vascular disease that usually occurs in concert with atheroma formation but through distinct pathophysiological processes. Vessel wall osteoprogenitor cells known as calcifying vascular cells can form bone matrix proteins and calcified nodules, analogous to osteoblastic differentiation in bone. These cells have been isolated from the tunica media of bovine and human arteries, and both in-vitro tissue culture models and mouse models of vascular calcification have been established. Studies of the effects of diabetes mellitus, hyperlipidemia, estrogens and glucocorticoids on calcifying vascular cell function provide insight into the relationship between common human disease states and vascular calcification.

Incidence and determinants of mortality and cardiovascular events in diabetes mellitus: a meta-analysis

Patients with diabetes mellitus are at increased risk of developing atherosclerotic disease. The extent of this additional risk and its determinants are not well known, but this information is needed for sample-size estimations in intervention studies. Therefore, a meta-analysis of epidemiologic studies on this subject was performed. Medline was searched from 1966 onwards, including the reference lists of all relevant publications. A total of 27 prospective follow-up studies in the English language that allowed calculation of the unadjusted incidence of one of the predefined outcome events were included. The influence of age, sex, type of diabetes, duration of diabetes, year of study, HbA1c, cholesterol level, blood pressure and smoking on these incidences was studied by means of univariate Poisson regression analysis. Overall total mortality was 2.9% per year (95% CI 2.8-3.0; 27 studies), and for death from all vascular causes was 1.4% per year (95% CI 1.3-1.4; 16 studies). Only two studies were found that reported on the incidence of the composite outcome 'event death from all vascular causes, non-fatal myocardial infarction, or non-fatal stroke'. In univariate analysis, age, year of study, total cholesterol and systolic blood pressure were positively related to total mortality and death from all vascular causes. After adjustment for age, or limiting the analyses to studies in patients with type 2 diabetes only (n = 11), these relationships remained statistically significant. In conclusion, the overall yearly total mortality in diabetes mellitus is 2.9% and for death from all vascular causes is 1.4%. There are few data on the incidence of composite cardiovascular outcome events.

Aging and cardiovascular disease. Use of subclinical measurements

This article examines the use of subclinical measurements in cardiovascular disease in the elderly. In the past 10 years, the development of new technology to measure subclinical disease has provided the opportunity to evaluate the prevalence of subclinical disease, the risk factors for subclinical disease, and the risk for clinical disease given the presence or absence of subclinical disease.

Diet-induced diabetes activates an osteogenic gene regulatory program in the aortas of low density lipoprotein receptor-deficient mice

Vascular calcification is common in people with diabetes and its presence predicts premature mortality. To clarify the underlying mechanisms, we used low density lipoprotein receptor-deficient (LDLR -/-) mice to study vascular calcification in the ascending aorta. LDLR -/- mice on a chow diet did not develop obesity, diabetes, atheroma, or vascular calcification. In contrast, LDLR -/- mice on high fat diets containing cholesterol developed obesity, severe hyperlipidemia, hyperinsulinemic diabetes, and aortic atheroma. A high fat diet without cholesterol also induced obesity and diabetes, but caused only moderate hyperlipidemia and did not result in significant aortic atheroma formation. Regardless of cholesterol content, high fat diets induced mineralization of the proximal aorta (assessed by von Kossa staining) and promoted aortic expression of Msx2 and Msx1, genes encoding homeodomain transcription factors that regulate mineralization and osseous differentiation programs in the developing skull. Osteopontin (Opn), an osteoblast matrix protein gene also expressed by activated macrophages, was up-regulated in the aorta by these high fat diets. In situ hybridization showed that peri-aortic adventitial cells in high fat-fed mice express Msx2. Opn was also detected in this adventitial cell population, but in addition was expressed by aortic vascular smooth muscle cells and macrophages of the intimal atheroma. High fat diets associated with hyperinsulinemic diabetes activate an aortic osteoblast transcriptional regulatory program that is independent of intimal atheroma formation. The spatial pattern of Msx2 and Opn gene expression strongly suggests that vascular calcification, thought to be limited to the media, is an active process that can originate from an osteoprogenitor cell population in the adventitia.

Epidemiology of risk factors for cardiovascular disease in diabetes and impaired glucose tolerance

Diabetes mellitus is a strong risk factor for all manifestations of atherosclerotic vascular disease, coronary heart disease (CHD), cerebrovascular disease and peripheral vascular disease. Diabetes can be classified into two main subtypes, insulin-dependent diabetes mellitus and non-insulin-dependent diabetes mellitus (NIDDM). This review focuses only on NIDDM. Also, in impaired glucose tolerance (IGT), a precursor stage of diabetes, the risk of macrovascular disease is substantially increased. NIDDM and IGT are known to be associated with several adverse cardiovascular risk factors, including hypertension, obesity, central obesity, hyperinsulinemia and serum lipid and lipoprotein abnormalities, characterized mainly by elevated serum total triglycerides and low high-density lipoprotein cholesterol. Practically no information is available on the role of different cardiovascular risk factors to predict macrovascular complications in subjects with IGT. The role of different cardiovascular risk factors with respect to the risk of CHD, stroke and peripheral vascular disease will be discussed.

Role of molecular regulation in vascular calcification

Calcium deposits account for most of the dry weight of atherosclerotic lesions. Previously considered uncommon, vascular calcification is now known to be present in 80% of significant lesions and in at least 90% of patients with coronary artery disease. Previously considered a passive process, it is increasingly recognized as an active, regulated process. Previously considered benign, it is now becoming recognized as a major risk factor for cardiovascular events, and a major contributor to systolic hypertension, heart failure, plaque rupture and stenosis. To confirm the similarity of vascular calcification with embryonic osteogenesis, we demonstrated the expression of bone morphogenetic protein in calcified human lesions, and we developed an in vitro model of vascular calcification that provides a useful experimental system for elucidating the molecular regulation of this process, which we have shown to include alkaline phosphatase induction and expression of bone matrix proteins and differentiation factors. Understanding the regulatory mechanisms of vascular calcification will allow future therapeutic approaches to prevent and possibly reverse this disease and its clinical consequences.