Protein Z and ZPI levels and cardiovascular events

Protein Z (PZ) is a vitamin K-dependent protein that serves as a cofactor for the inhibition of factor (F) Xa by the serpin, protein Z-dependent proteinase inhibitor (ZPI) [1]. ZPI also inactivates FIXa and FXIa in a PZ-independent fashion [2,3]. PZ and ZPI circulate as a complex in plasma and the levels of PZ and ZPI are interrelated [4,5]. In murine models, PZ and ZPI deficiencies are prothrombotic, dramatically increasing the mortality of the heterozygotic and homozygotic FV Leiden (FVL) genotypes [6, and unpublished observations of J. Zhang, X. Tu and G. L. Broze]. The physiological role of the PZ/ZPI anticoagulant pathway in humans, however, is less clear. In the Leiden Thrombophilia Study (LETS), a potential association of low PZ with venous thrombosis was only demonstrated in men and older individuals (> 55) following subgroup analysis and no effect of a concomitant FVL genotype was detected [5]. In contrast, Martinelli et al. [7] reported no independent relationship of PZ with venous thrombosis, but that PZ deficiency synergistically enhanced the risks associated with FVL and hyperhomocysteinemia. In the LETS, no relationship between ZPI antigenic levels and venous thrombosis was found, but Van de Water et al. [8] has reported that heterozygosity for either of two nonsense mutations in ZPI was associated with venous thrombosis in a New Zealand population. Low levels of PZ have reportedly been associated with both an increased [9,10], and a decreased [11–14] risk of stroke. Two studies found an association between low PZ levels and the acute coronary syndrome, a relationship that was substantially increased in current smokers and that persisted following the acute event [15,16]. On the other hand, the Prospective Epidemiological Study of Myocardial Infarction (PRIME) trial did not detect a relationship between PZ and the subsequent risk of coronary events [17]. In an attempt to further define the role of the PZ/ZPI anticoagulant pathway, we analyzed PZ and ZPI levels in the prospective Atherosclerosis Risk in Communities (ARIC) study [18]. In ARIC, c. 16 000 middle-aged individuals have been followed for atherosclerotic-related cardiac and stroke events with an average follow-up of 9.7 ± 2.2 years. For our study, the initial plasma levels of PZ and ZPI were determined in individuals that developed coronary heart disease (CHD; 382 cases) and stroke (110 cases) and compared with the initial levels in 375 and 399 age, gender, and race matched non-cases, respectively. PZ and ZPI immunoassays were performed as previously described [5]. By Cox proportional hazards regression analyzes and after adjustment for other risk factors including age, gender, race, white blood cell count, hypertension, diabetes, total cholesterol, HDL cholesterol, and smoking, neither PZ nor ZPI levels were significantly associated with the time to development of CHD or stroke in the overall groups (Table 1). On subgroup analysis, ZPI levels were not associated with CHD or stroke, but there was a relationship between lower PZ levels and the development of CHD in females (P 1⁄4 0.022) and smokers (P 1⁄4 0.024) and a trend for the development of stroke in Blacks (P 1⁄4 0.060) (Table 1). Subgroup analyzesmust be viewed cautiously, but the result in smokers is perhaps consistent with the previous report of Fedi et al. [15], who found a 9.5-fold increased risk of the acute coronary syndrome in smokers with low PZ levels (<5th percentile). Interestingly, significantly higher levels of PZ were found in African-Americans (n 1⁄4 199; PZ 1⁄4 112.2 ± 41.3) than white Americans (n 1⁄4 557; PZ 1⁄4 93.5 ± 36.3) (P 1⁄4 0.0005). Two PZ genetic polymorphisms have been related to reduced PZ levels, A-13G and G79A, and it is conceivable that these polymorphisms are considerably less prevalent in AfricanAmericans than in, for example, western Australians (21% and 41%, respectively) [19].