Pathophysiological Consequences of Protein N-Homocysteinylation - Homocysteine in Protein Structure/Function and Human Disease - page 113

How excessive accumulation of Hcy triggers an innate immune response is
unknown, but accumulating evidence suggests that protein N-homocysteinylation
and the resulting protein damage are involved (Fig. 6.1 ). Hyperhomocysteinemia is
known to cause elevation of Hcy-thiolactone [93] and N-Hcy-protein levels
[79, 113, 115] in humans (Table 3.12 ) and mice (Table 5.1 ). Moreover, N-Hcy-
proteins are susceptible to protein damage (Fig. 5.11 ) [68, 96] via a thiyl radical
mechanism (Reaction 5.2 ), which leads to the generation of protein carbonyls
[311]. In fact, the extent of protein damage assessed by measurements of protein
carbonyls is significantly greater in liver proteins from Cbs / mice compared with
wild-type littermates [397]. Protein carbonyls are also elevated in plasma of human
CBS-deficient patients, and Hcy-lowering therapy partially prevents protein car-
bonyl accumulation [398].
N-Hcy-proteins, particularly N-Hcy-LDL, are highly immunogenic [356].
N-Hcy-LDL is present in the human blood [79] and is taken up by macrophages
faster than unmodified LDL [336]. Furthermore, levels of anti-N-Hcy-protein
autoantibodies are weakly, but significantly, correlated with plasma CRP levels
(r ¼

0.002) [135], suggesting that N-Hcy-protein is proinflammatory.
This suggestion is further supported by recent findings [169] showing that the
treatment with N-Hcy-albumin increases binding of monocytes to ex vivo cultured
endothelial cells and induces expression of proinflammatory chemokines and
cytokines such as VCAM1, ICAM-1, and MCP1 in both monocytes and endothelial
cells. In addition, the treatment with N-Hcy-albumin upregulates numerous genes,
including five that are implicated in endothelial cell activation: CCL2, HSPD1,
ADAM17, TFP1, and NRP1 [169]. It should be noted that these proinflammatory
effects on cell adhesion and gene expression are observed at 1-

0.24, p ¼

M N-Hcy-albumin,
an elevated concentration (twofold above normal, see Table 2.1 ) that is observed in
uremic hyperhomocysteinemic patients [301]. Although proinflammatory
cytokines can also be induced in human cultured cells by Hcy, much higher
concentrations (usually submillimolar or higher) are required [45, 399, 400].
Taken together, these results strongly suggest that N-Hcy-protein is involved in
the induction of inflammatory response associated with hyperhomocysteinemia
(Fig. 6.1 ).

μ

6.4 N-Hcy-Fibrinogen and Thrombosis

Patients with severe hyperhomocysteinemia due to CBS deficiency suffer from life-
threatening thromboembolic events by the age of 30 [46]. Mild to moderate
hyperhomocysteinemia, more prevalent than severe hyperhomocysteinemia, is
also associated with increased risk for myocardial infarction, stroke, and thrombo-
sis [401]. The identification of N-homocysteinylation site at Lys562, located in an
unstructured region involved in plasmin and tPA binding (Fig. 5.3 ) , in the
prothrombotic N-Hcy-fibrinogen that accumulates in the circulation of CBS-defi-
cient patients [79, 115] suggests that the modified fibrinogen can impair fibrinolysis