Factor VII-activating protease

Factor VII Activating Protease (FSAP) and Its Importance in Hemostasis—Part I: FSAP Structure, Synthesis and Activity Regulation: A Narrative Review

Factor VII activating protease (FSAP) was first isolated from human plasma less than 30 years ago. Since then, many research groups have described the biological properties of this protease and its role in hemostasis and other processes in humans and other animals. With the progress of knowledge about the structure of FSAP, several of its relationships with other proteins or chemical compounds that may modulate its activity have been explained. These mutual axes are described in the present narrative review. The first part of our series of manuscripts on FSAP describes the structure of this protein and the processes leading to the enhancement and inhibition of its activities. The following parts, II and III, concern the role of FSAP in hemostasis and in the pathophysiology of human diseases, with particular emphasis on cardiovascular diseases.

Factor VII Activating Protease Expression in Human Platelets and Accumulation in Symptomatic Carotid Plaque

Background

Factor VII activating protease (FSAP) is of interest as a marker for vascular inflammation and plaque destabilization. The aim of this study was to analyze the expression profile of FSAP in endarterectomy specimens that were taken from patients with asymptomatic and symptomatic carotid atherosclerotic plaques and to compare them with circulating FSAP levels.

Methods and Results

Plasma FSAP concentration, activity, and mRNA expression were measured in endarterectomy specimens and in monocytes and platelets. Plaque and plasma FSAP levels were higher in symptomatic patients (n=10) than in asymptomatic patients (n=14). Stronger FSAP immunostaining was observed in advanced symptomatic lesions, in intraplaque hemorrhage‐related structures, and in lipid‐rich areas within the necrotic core. FSAP was also colocalized with monocytes and macrophages (CD11b/CD68‐positive cells) and platelets (CD41‐positive cells) of the plaques. Moreover, human platelets expressed FSAP in vitro, at both the mRNA and protein levels. Expression is stimulated by thrombin receptor‐activating peptide and ADP and reduced by acetylsalicylic acid.

Conclusions

Plasma FSAP levels were significantly increased in patients with symptomatic carotid stenosis and thus may be involved in plaque development This plaque‐associated FSAP may be produced by platelets or macrophages or may be taken up from the circulation. To establish FSAP’s utility as a circulating or plaque biomarker in patients with symptomatic carotid atherosclerotic plaques, further studies are needed.

Carotid plaque composition by CT angiography in asymptomatic subjects: a head-to-head comparison to ultrasound

OBJECTIVES

To describe carotid plaque composition by computed tomography angiography (CTA) in asymptomatic subjects and to compare this to carotid plaque assessment by ultrasound, coronary plaques by coronary CTA, and inflammatory biomarkers in plasma.

METHODS

Middle-aged asymptomatic men, n = 43, without known cardiovascular disease and diabetes were included. Plaques in coronary and carotid arteries were evaluated using CTA. Total plaque volumes and plaque composition were assessed by a validated plaque analysis software. The 60% centile cut point was used to divide the population into low or high carotid total plaque volumes. The occurrence of carotid plaques and intima-media thickness (IMT) was estimated by ultrasound.

RESULTS

Carotid plaque by ultrasound was undiagnosed in 13 of 28 participants (46%) compared to CTA. Participants having carotid plaques by ultrasound had significantly higher absolute volumes of all CTA-defined carotid plaque subtypes and a higher fraction of calcified plaque. A high carotid total plaque volume was independently associated with age (adjusted odds ratio (OR) 1.41 [95% confidence interval (CI) 1.14-1.74], p = 0.001), IMT (adjusted OR 2.26 [95% CI 1.10-4.65], p = 0.03), and D-dimer (adjusted OR 8.86 [95% CI 1.26-62.37], p = 0.03). All coronary plaque features were significantly higher in participants with a high carotid total plaque volume.

CONCLUSION

The occurrence of carotid plaques in asymptomatic individuals is underestimated by ultrasound compared to plaque assessment by CTA. Carotid plaque composition by CTA is different in individuals with and without carotid plaques by ultrasound.

KEY POINTS

• The occurrence of carotid plaques by ultrasound was underestimated in 46% of participants who had plaques by carotid CTA. • Participants with carotid plaques by ultrasound had higher volumes of all plaque subtypes and a higher calcified plaque component as determined by carotid CTA compared to participants without carotid plaques by ultrasound. • A high carotid total plaque volume was independently associated with age, intima-media thickness, and D-dimer.

Sex difference in fibrin clot lysability: Association with coronary plaque composition

INTRODUCTION

Fibrin clot lysability is associated with development of cardiovascular disease (CVD). We evaluated sex-differences in fibrin clot lysability and the association with coronary plaque composition determined by computed tomography angiography (CTA).

METHODS

Middle-aged citizens without known CVD were randomly selected from a national registry. A coronary CTA assessed volumes of calcified-, non-calcified-, low-density non-calcified-, and total- plaque using a validated plaque quantification software. A non-enhanced cardiac CT scan assessed the Agatston score. Fibrin structure properties were determined using turbidimetric methods. Plasma concentrations of C-reactive protein and fibrinogen were assessed.

RESULTS

138 individuals (71 women) participated. Men more frequently had coronary plaques compared to women, P < 0.05. Coronary plaque features were comparable between men and women, P > 0.05. Women with total plaque volume > 0 mm³ had lower fibrin clot lysability compared to women with total plaque volume = 0 mm³, adjusted difference [95% confidence interval] 10.28 [1.42-19.15], P = 0.02, and a fibrinogen-dependent lower fibrin clot lysability compared to men with and without coronary plaques, 6.82 [-2.67-16.31], P = 0.16, and 8.73 [-0.43-17.89], P = 0.06, respectively. Fibrinogen correlated with all the coronary plaque features (correlation coefficient r = 0.42-0.57) only in women with total plaque volume > 0 mm³, all P < 0.01.

CONCLUSION

Asymptomatic women with coronary plaques assessed by coronary CTA have reduced fibrin clot lysability compared to both women without coronary plaques and men, suggesting a sex-dependent link between coronary atherosclerosis and fibrin clot lysability.

Sex differences in coronary plaque composition evaluated by coronary computed tomography angiography in newly diagnosed Type 2 diabetes: association with low-grade inflammation

AIM

To determine differences in coronary plaque composition and inflammatory biomarkers between men and women with newly diagnosed Type 2 diabetes without known cardiovascular disease.

METHODS

A total of 88 people with newly diagnosed (<1 year) Type 2 diabetes underwent contrast-enhanced coronary computed tomography angiography. Advanced coronary plaque analysis was performed using semi-automated software. Plasma concentrations of inflammatory biomarkers were determined.

RESULTS

There were no significant differences between men (n=60) and women (n=28) regarding age or cardiovascular risk factors (all P>0.05). The median (quartiles) serum levels of fibrinogen [10.9 (9.8-12.6) μmol/l vs 9.7 (8.8-10.9) μmol/l], fibrin d-dimer [0.3 (0.2-0.4) mg/l vs 0.27 (0.2-0.4) mg/l] and C-reactive protein [3.1 (1.1-5.2) mg/l vs (0.8-2.6) 1.6 mg/l] were significantly higher in women (all P<0.05). Overall, men more often had multi-vessel involvement [28 men (47%) vs 4 women (14%)], and higher total plaque burden [median (quartiles) 11.6 (2.3-36.0)% vs 2.0 (0.4-5.4)%; both P<0.05]. The median (quartiles) total plaque volume [269.9 (62.6-641.9) mm³ vs 61.1 (7.6-239.9) mm³ ] and absolute calcified plaque volume [33.5 (8.3-148.3) mm³ vs 4.7 (0.9-17.3) mm³ ] were higher in men (both P<0.05). Women had a lower relative proportion of the calcified plaque component [median (quartiles) 7.8 (4.7-15.4)% vs 23.7 (8.4-31.1)%] and a higher relative proportion (median [quartiles]) of the non-low-density non-calfied plaque component [77.6 (66.0-86.0)% vs 63.6 (54.0-72.9)%; both P<0.05].

CONCLUSIONS

In people with newly diagnosed Type 2 diabetes, women had lower absolute coronary plaque volumes but a more unfavourable plaque composition and enhanced systemic inflammation compared with men.

Genome-wide analysis of genetic determinants of circulating factor VII-activating protease (FSAP) activity

Essentials Knowledge of genetic regulators of plasma factor VII activating protease (FSAP) levels is limited. We performed a genome-wide analysis of variants influencing FSAP activity in Scandinavian cohorts. We replicated an association for Marburg-1 and identified an association for a HABP2 stop variant. We identified a novel locus near ADCY2 as a potential additional regulator of FSAP activity.

SUMMARY

Background Factor VII-activating protease (FSAP) has roles in both coagulation and fibrinolysis. Recent data indicate its involvement in several other processes, such as vascular remodeling and inflammation. Plasma FSAP activity is highly variable among healthy individuals and, apart from the low-frequency missense variant Marburg-I (rs7080536) in the FSAP-encoding gene HABP2, determinants of this variation are unclear. Objectives To identify novel genetic variants within and outside of the HABP2 locus that influence circulating FSAP activity. Patients/Methods We performed an exploratory genome-wide association study (GWAS) on plasma FSAP activity amongst 3230 Swedish subjects. Directly genotyped rare variants were also analyzed with gene-based tests. Using GWAS, we confirmed the strong association between the Marburg-I variant and FSAP activity. HABP2 was also significant in the gene-based analysis, and remained significant after exclusion of Marburg-I carriers. This was attributable to a rare HABP2 stop variant (rs41292628). Carriers of this stop variant showed a similar reduction in FSAP activity as Marburg-I carriers, and this finding was replicated. A secondary genome-wide significant locus was identified at a 5p15 locus (rs35510613), and this finding requires future replication. This common variant is located upstream of ADCY2, which encodes a protein catalyzing the formation of cAMP. Results and Conclusions This study verified the Marburg-I variant to be a strong regulator of FSAP activity, and identified an HABP2 stop variant with a similar impact on FSAP activity. A novel locus near ADCY2 was identified as a potential additional regulator of FSAP activity.