Abstract
Background:
Hypercoagulability in type 2 diabetes mellitus (T2DM) patients increases their risk of cardiovascular diseases.
Objective:
The aim of this work was to investigate the hypercoagulation mechanism in T2DM patients in terms of circulating tissue factor (TF).
Methods:
Whole blood coagulation tests by damped oscillation rheometry and dielectric blood coagulometry (DBCM) were performed.
Results:
The average coagulation time was significantly shorter for T2DM patients than for healthy controls. In vitro addition of either anti-TF or anti-activated factor VII (FVIIa) antibody to hypercoagulable blood samples prolonged coagulation times for one group of patients, while coagulation times remained short for another group. The levels of circulating TF were estimated in the former group by measuring the coagulation times for blood samples from healthy subjects with addition of various concentrations of TF and comparing them with the coagulation times for the group. The results indicated that the levels of circulating TF were on the order of subpicomolar at most.
Conclusions:
Circulating TF is at least partially responsible for a hypercoagulable group of T2DM patients, while an abnormality in the intrinsic coagulation pathway probably occurs in the other group.
Keywords
Introduction
Type 2 diabetes mellitus (T2DM) is associated with an increased risk of thrombosis and cardiovascular diseases [1,2]. In T2DM patients, there is a high rate of abnormal modulation of plasma proteins involved in coagulation [3]. Such abnormalities may induce a hypercoagulable state and increase the risk of fatal thrombotic events. Notably, previous studies reported that the level of tissue factor (TF) is increased in circulating blood in patients with T2DM [4–6]. Because TF is normally not expressed in endocapillary cells, it has been thought that TF is not exposed to circulating blood unless there is vascular damage. However, both flow cytometry studies [7,8] and TF activity measurements using activated factor X (FXa) assay [9,10] demonstrated the expression of TF on the surface of monocytes. It is known that circulating TF exists in three forms: monocyte TF, microparticle TF shed from monocytes or other leukocytes [6,11], and soluble TF [12]. The mechanism underlying the increased circulating TF observed in T2DM has been discussed in terms of the inhibitory effect of insulin on TF synthesis in monocytes, which is decreased by insulin resistance in T2DM [9,10]. On the other hand, Butenas et al. called attention to the significant overestimation of circulating TF that may result from either binding of antibodies to the inactive forms of TF in immunoassays or use of non-physiologically excess concentrations of activated factor VII (FVIIa) in FXa assays [13].
Even though there is still controversy about accurate quantification, the observation of increased circulating TF in T2DM patients compared to healthy subjects appears to be valid at least qualitatively. Furthermore, circulating TF pathway inhibitor (TFPI) is also increased in T2DM, probably owing to some homeostatic mechanism to prevent thrombosis, as mentioned by El-Hagracy et al. [14]. The net influence of circulating TF on blood coagulability might vary greatly among individual patients. Therefore, a blood coagulation test that is sufficiently sensitive to detect hypercoagulability would provide an easy method for individualized evaluation of thrombosis risk.
Standard blood coagulation screening tests such as the prothrombin time international normalized ratio (PT-INR) and activated partial thromboplastin time (aPTT) have limited sensitivity to hypercoagulation [15] because of the use of a large excess of coagulation initiators. Damped oscillation rheometry, in contrast, allows quantitative measurement of clotting time without artificial activation of blood coagulation, thereby providing a better estimate of hypercoagulability [16,17]. In addition, dielectric blood coagulometry (DBCM) was developed, and a good correlation with damped oscillation rheometry has been shown [18]. Recently, Chiba et al. [19] used DBCM to demonstrate hypercoagulation in respiratory diseases including interstitial pneumonitis, lung cancer, chronic obstructive pulmonary diseases, and sleep apnea syndrome. Hayashi et al. [20] summarized the principles of DBCM in a recent paper.
In the present work, blood coagulation measurements of T2DM patient samples were performed using both damped oscillation rheometry and DBCM to study the mechanism of hypercoagulation in T2DM, and the clinical utility of DBCM was also assessed. First, the rheological approach was employed with and without in vitro addition of anti-TF or anti-FVIIa antibodies to confirm that circulating TF plays an important role in the hypercoagulability in T2DM. Next, a feasibility study of DBCM was conducted to evaluate hypercoagulability in T2DM patients. Finally, DBCM was used for nonclinical model experiments carried out using blood from healthy volunteers. Extrinsic pathway inhibition by either anti-TF or anti-FVIIa antibody was confirmed for blood samples spiked with TF, and then the levels of circulating TF were estimated by measuring coagulation times as the TF concentration was varied, and they were compared with the coagulation times of T2DM patients.
Materials and methods
Blood sources and reagents
This study was approved by the Ethics Committee of Tokyo Medical and Dental University. Whole blood samples (1.8 mL) mixed with 0.2 mL of 3.13% trisodium citrate were obtained from healthy volunteers and T2DM patients who had been hospitalized in the Department of Endocrinology and Metabolism, Tokyo Medical and Dental University Hospital. Exclusion criteria included age younger than 20 years or hemoglobin A1c (HbA1c) lower than 6.5% at the examination. All patients involved agreed to participate in the study after they provided their informed consent and fulfilled the eligibility criteria for enrollment. The clinical study of T2MD was performed in Tokyo Medical and Dental University, and the nonclinical part of the study was carried out at the laboratory of Sony Corporation at the university.
Anti-TF monoclonal antibody and anti-FVIIa polyclonal antibody were obtained from Cosmo Bio, Ltd. (Tokyo, Japan) and Funakoshi Ltd. (Tokyo, Japan), respectively, and they were dissolved in distilled water to a concentration of 1.0 mg/mL. To restart the blood coagulation process, a 250 mM aqueous solution of calcium chloride (Wako Pure Chemical Industries, Ltd., Tokyo, Japan) was added to the blood samples at a final concentration of 85 μL/mL, which were mixed just before rheological or DBCM measurement was started. The reagent for the PT test, Dade Innovin, was obtained from Sysmex Corp. (Kobe, Japan) and dissolved in 4 mL of distilled water according to the manufacturer’s instructions. The TF concentration of this stock solution was approximately 6–7 nM, according to the literature [21–23]. In the present study, this solution was diluted 100 times in distilled water and further diluted in PBS for serial dilutions, which, together with the calcium chloride solution, were added to and mixed with blood samples just before DBCM measurement was started in the nonclinical part of the study.
Damped oscillation rheometry
Blood from each patient was dispensed into two polypropylene tubes. Anti-TF or anti-FVIIa antibody solutions (20.2 μL/mL) were added into one of the tubes (final antibody concentration of 20 μg/mL), and the same volume of HEPES buffer (pH 7.4) was added into the other tube (a control) to equalize the dilution rate of blood samples. The samples were gently mixed well and incubated at 37°C for 30 minutes.
Two damped oscillation rheometer apparatuses were used to simultaneously measure samples in the absence and presence of the antibodies. The details of the method were described in the previous papers [16,17]. Briefly, incubated blood samples were dispensed in a polypropylene sample tube just after recalcification. The sample tube was placed in the holder of the rheometer, which is connected via a torsion wire to a magnetic coil and maintained at 37°C. Direct current (DC) was introduced into the coil between magnetic poles, which generated an initial rotational displacement in the measuring system. When current was turned off, the system started a rotational damped oscillation, which produced an induced electromotive force in the coil. Thus, a damped oscillation curve was detected as an output voltage every 20 s, and a logarithmic damping factor (LDF) was obtained from the curve. In the present experiments, the frequency was 1 Hz, the initial rotational angle was 20 degrees, and the shear rate on the blood sample was estimated to be below 1 s−1 [24,25]. After the measurement, the coagulation time of the sample (
DBCM
Dielectric responses in blood samples during the coagulation process at 37°C were recorded for 60 minutes at intervals of one or two minutes and a frequency range covering at least 1 kHz to 10 MHz using DBCM prototype setups developed by Sony Corporation [18–20]. The effects of erythrocyte sedimentation were minimized by the prototypes’ design. Two kinds of methods were available. One was a rotating method, where the prototype system automatically rotated a sealed sample holder by 180 degrees every minute. The sample holder was a plate capacitor-type consisting of a polypropylene cylinder tube with two titanium electrodes (9 mm in diameter), squeezed into the top and bottom ends of the tube. The other one was a non-rotating method based on use of polypropylene disposable cartridge with titanium electrode inserts to be a plate capacitor-type. This cartridge was designed so that the sedimentation boundaries would not reach electrode height during measurements (see Supplemental Fig. S1 in the reference [20]). Note that the applied AC voltages for measurements were lower than 300 mV (effective voltage), where non-linear dielectric responses were negligible.
In the first phase of the DBCM study, the coagulation process of T2DM blood samples was monitored in the absence of anti-TF and anti-FVIIa antibodies. In the second phase, the inhibition effects of anti-TF and anti-FVIIa antibodies were tested, and the levels of circulating TF in T2MD samples were estimated by nonclinical studies on samples from healthy subjects. To test the effects of the antibodies, the diluted Dade Innovin solution was added to the sample blood at TF concentration of 0.6–0.7 pM, and the sample was incubated at 37°C for 30 min with and without anti-TF or anti-FVIIa antibody (20.2 μL/mL), where the rate of sample dilution was kept the same at 9% for all tested samples using physiological saline. The calcium chloride solution was added just before the start of DBCM measurement. For the estimation of the levels of circulating TF in T2MD samples, different concentrations of TF were added to the blood samples together with the calcium chloride solution just before a measurement was started. The dilution rate for all samples was kept equal at 12%.
Statistical analysis
Differences in the DBCM data sets were verified using Student’s t-test with the Bonferroni correction for multiple comparisons. Three data sets, obtained by the rotating and non-rotating methods for T2DM and by the rotating method for healthy controls were tested. Significance was defined as
Results and discussion
Patient characteristics
The characteristics, laboratory data, and meditations of the T2DM subjects in this study are presented in Table 1. The mean values of HbA1c indicate that the majority of the subjects had poorly controlled plasma glucose levels.
Characteristics of patients
Characteristics of patients
Data are shown as mean (SD)/interquartile range.
LDL-C: low-density lipoprotein cholesterol, sCr: serum creatinine concentration, OHA: oral hypoglycemic agent.

Typical LDF curves, where open and filled circles correspond to the data without and with added anti-TF or anti-FVIIa antibody, respectively, and positions of
Figure 1 shows typical examples of rheological results for samples from T2DM and healthy subjects. The start of fibrin polymerization induces drastic changes of viscosity and viscoelasticity of blood samples, and it appears as a rapid decrease of LDF. Such a time point is reflected in the definition of
Figure 1(a) and (b) represent two groups of T2DM samples that showed different responsiveness to added anti-TF antibody. For approximately half of the T2DM samples tested, inhibition of TF induced a prolonged coagulation process suggested by increased
An abnormality in the intrinsic coagulation pathway is another potential cause of the hypercoagulability in T2DM. Feener et al. [26] recently reviewed the role of plasma kallikrein in diabetes. Kallikrein is a serine protease that converts factor XII (FXII) to activated FXII (FXIIa) upstream of the intrinsic coagulation cascade. Kedzierska et al. [27] reported higher plasma concentrations of prekallikrein (the precursor of kallikrein) in human type 1 diabetics than in non-diabetic subjects, while Clermont et al. [28] reported increased plasma prekallikrein in diabetic rats. Furthermore, Iwata and Kaibara [29] reported the presence of erythroelastase-IX, which converts factor IX (FIX) to an activated form (FIXa), on human erythrocyte membranes, and erythroelastase-IX activity was found to be higher in T2DM patients than in healthy controls [17]. In the intrinsic coagulation cascade, FIX is activated by activated factor XI (FXIa), and FXIa is produced by the catalytic activity of FXIIa, which is activated by kallikrein. Thus, increased activity of either kallikrein or erythroelastase-IX may induce a hypercoagulable state of the intrinsic coagulation cascade. From the arguments above, it is expected that
The examples in the pair of Fig. 1(a) and (c) and the pair of Fig. 1(b) and (d) are considered to correspond to the hypercoagulability related to the extrinsic and intrinsic coagulation pathways, respectively. The damped oscillation rheometer can clearly differentiate the two groups. However, during the study, it was found to have difficulty in performing measurements for blood samples with elevated sedimentation rates. Figure 1(f) shows a typical case where
Hypercoagulability in T2DM studied by DBCM
The main dielectric response of blood is known to be interfacial polarization on erythrocyte membranes that is usually observed over a frequency range of hundreds of kHz to tens of MHz, while a large parasitic contribution from electrode polarization is also observed at lower frequencies [31]. Figure 2(a) shows typical dielectric dispersion curves as a function of time for a T2DM sample, where the stepwise decreases in permittivity (

Typical DBCM response for a T2DM sample. Panel (a) shows the change in the dielectric dispersion curve during the progression of blood coagulation, and panel (b) is the same data normalized to the dielectric dispersion curve at the first time point. The frequency range for this measurement was 100 Hz to 110 MHz.
From this observation, it was found that the time-dependent curve of normalized

Normalized permittivity change at 10.7 MHz extracted from Fig. 2(b).

Comparison of
The effectiveness of anti-TF and anti-FVIIa antibodies for inhibition of TF was checked under a well-controlled condition using a model system to establish the basis of our discussion on the circulating TF in T2DM in Section 3.2. Figure 5 shows normalized DBCM responses for a healthy subject with and without in vitro addition of TF and/or the antibodies. With addition of 0.6–0.7 pM TF without the antibody, the DBCM response is shifted to the left direction in shorter times (from the curve (1) to (2) in Fig. 5). The sample corresponding to curve (2) is considered a simplified model of the hypercoagulation state of T2DM due to circulating TF. Addition of both anti-TF and anti-FVIIa antibodies to this model system partially compensated for the effects of TF and shifted the DBCM responses (curves (3) and (4)) in the right direction toward the control (curve (1)). Addition of either anti-TF or anti-FVIIa antibody without TF did not affect the DBCM response, because curves (5) and (6) in Fig. 5 were very close to the control curve (1). Therefore, it was confirmed that DBCM can monitor the inhibition of TF by the antibodies, and that circulating TF is a probable contributor to the hypercoagulability in the subset of T2DM corresponding to Fig. 1(a) and (c).

DBCM responses from a healthy subject at 10 MHz normalized by the minimum and maximum values of permittivity. The black curve (1) shows the control sample without TF and antibodies, and the red curve (2) demonstrates the acceleration of the DBCM response by addition of TF to the sample blood. Simultaneous addition of TF and anti-TF antibody (3) or TF and anti-FVIIa antibody (4) prolongs the DBCM response in comparison with (2). On the other hand, addition of anti-TF antibody (5) or anti-FVIIa antibody (6) without TF shows responses similar to the control (1).
Assuming that circulating TF is the main factor responsible for shortened

TF concentration dependence of
For the hypercoagulable T2DM group,
Based on our estimate, however, the baseline TF concentration in healthy subjects is unknown. To be precise, therefore, the discussed TF level in T2DM is only a relative value. Nevertheless, Butenas et al. concluded that physiologically active TF in blood from healthy subjects is less than 20 femtomolars [13]. If that is the baseline level of TF in healthy subjects, the present rough estimation of the TF level in T2DM is still valid as the plasma concentration. Another limitation of the present work is TFPI, because its effects were not taken into account. Because TFPI may be increased in T2DM [14,33], the present analysis can lead to underestimation due to differences in TFPI concentrations between T2DM and healthy subjects.
The present study indicated that there are at least two hypercoagulability mechanisms involved in T2DM. One mechanism is dependent on circulating TF levels, and the other is TF-independent. Although the clinical importance of circulating TF, including the risk of fatal thrombotic events, still remains an open question, the possibility of discriminating the types of hypercoagulability shown in this study is promising for the development of a personalized medicine tool in the future. Because DBCM is a sensitive tool for testing hypercoagulability, further development of DBCM-based assays will enable the evaluation of individual coagulation states. At the same time, clarifications of how comorbidities, medications, and physical characteristics affect hypercoagulability in T2DM are very important, but multivariate analyses of data from a much larger scale of study than the present work are needed for this.
