An increased turnover of ECM fragments in blood may reflect remodeling and early fibrotic disease.29 In a smaller sub-sample of the iPOWER cohort we have previously demonstrated imbalanced turnover of certain collagens when compared with healthy controls30 but have failed to find a relation between fibrosis on cardiac magnetic resonance imaging and CFVR assessed non-invasively, perhaps due to lack of statistical power.31 In this larger study we aimed to verify the increased ECM activity and determine whether DM patients, who are particularly prone to developing myocardial fibrosis, had elevated ECM turnover as a marker of myocardial fibrosis and whether this was related to impaired coronary microvascular function.
We found that women with angina pectoris and DM had significantly higher levels of ECM biomarkers although CMD did not seem to be associated with these biomarkers. Furthermore, high levels of ECM biomarkers were associated with metabolic disturbances as reflected in higher BMI, HbA1c, triglycerides and lower HDL.
Cardiovascular risk factors other than diabetes were highly prevalent in symptomatic women with DM. Ageing, hypertension and metabolic disturbances such as obesity and DM have previously been associated with myocardial fibrosis. In DM, glycation end-product deposition25 and metabolic dysregulation have been described as triggers of fibroblast activation, cardiac ECM-remodeling and fibrosis. Adipositas and hypertension may also activate fibroblasts and thereby induce collagen accumulation and deposition.8
Biomarkers of collagen type IV and VI turnover, TIM and MIM have previously been associated with fibrotic disease or fibrotic related conditions. Collagen type IV is primarily found in the basement membrane30 and has a stabilizing function of microvessels during angiogenesis.32 Increased levels have been correlated to endocardial hypertrophy and liver fibrosis.33–35 Collagen type V and VI are important components of the interstitial connective tissue and contribute to the quality of the ECM by regulating the fibril size of collagen type I and III.30 PRO-C6 have been associated with diabetes16,19 and together with PRO-C4 associated with poor prognosis in HFpEF-patients.21 C4M and C6M have been linked to severe liver fibrosis.22 Also, C4M has recently been found to predict major cardiovascular events and to be associated with carotid atherosclerosis.23 Decreased titin in the sarcomere is thought to cause fibrosis,24,26 and circulating levels of MMP-cleaved mimecan (MIM) has previously been identified as a marker of extracellular matrix remodeling in mice.10 Although elevated in symptomatic women with DM compared with asymptomatic women, TIM was no longer correlated to DM and HbA1c after Bonferroni adjustment, whereas a strong correlation remained with BMI and blood cholesterol levels. Mimecan is a small proteoglycan with important functions in myofibril formation and angiogenesis. It is upregulated and released in heart disease such as after myocardial infarction, in conditions with pressure overload such as in hypertension, but is also released in inflammatory disease such as vasculitis.10,14
In our previous study of collagen turnover in the iPOWER cohort including 71 symptomatic patients,30 PRO-C6, C4M and C6M were increased when compared to asymptomatic controls, whereas no significant difference between groups was observed for PRO-C4, possibly due to lack of statistical power. C5M and C6M were found to be lower in iPOWER women than in controls. However, the previous study did not include patients with DM and are thus not directly comparable with the current results where the high values are related to the presence of DM.
To our knowledge, this is the first study to demonstrate a consistent and significant overexpression of multiple biomarkers of fibrosis in women with angina pectoris, DM and risk factors for myocardial fibrosis. Although many correlations were weak, most were highly significant even after conservative Bonferroni adjustment. Further, all biomarkers were consistently associated with DM and metabolic risk factors: BMI, HbA1c, HDL-cholesterol and triglycerides. Also, we performed Bonferroni corrected pairwise correlations and consequently, the association with DM and HbA1c disappeared for TIM and C5M.
We found no relation between CMD and ECM biomarkers. This would indicate that non-endothelial dependent CMD, as assessed in this study by dipyridamole stress, is not causally related to the development of myocardial fibrosis. Other explanations, as discussed below, is that the ECM biomarker level does not only reflect cardiac remodeling but general fibrotic activity, making direct comparisons difficult. Also, a relation between increased ECM turnover and CMD caused by endothelial inflammation may have been missed in this study as we have only assessed non-endothelial dependent CMD.8,12,36 Obesity, arterial hypertension and DM may induce chronic, systemic inflammation and consequently endothelial dysfunction, ECM remodeling, cardiac fibrosis and finally HFpEF.37
Another explanation for the lack of relation between CMD and biomarkers is that the measured biomarker activity may reflect early stages of fibrotic disease that later may develop into manifest myocardial fibrosis, CMD and/or HFpEF.38 Risk factors for HFpEF such as female sex, ageing, hypertension, obesity, and DM38 are all well presented in our population but our population of women at risk did not have HFpEF. However, they did show signs of ventricular hyper-contractibility on echocardiography (higher LVEF), and of poorer diastolic function and higher left ventricular filling pressure compared with controls.
Strengths and limitations
In the iPOWER study, participants were consecutively included and systematically examined. All participants except the asymptomatic controls had a clinical invasive coronary angiography performed ruling out obstructive coronary artery disease (defined by > 50% stenosis of coronary arteries). The prevalence of cardiovascular risk factors was high. However, if we had been able to include participants with more impaired ventricular function or more pronounced CMD, the population might have had more myocardial fibrosis which we might have been able to measure by increased levels of circulating biomarkers. We did not measure the endothelial dependent component of coronary microvascular function and have therefore not examined the relationship between endothelial-dependent microvascular function and biomarker turnover
Further research is needed with the aim of detecting cardio-specific biomarkers of fibrosis. Until then, it is possible to misinterpret fibrosis as myocardial when biomarkers may be increased due to fibrosis in other organs than the heart.