Pathophysiological Consequences of Protein N-Homocysteinylation - Homocysteine in Protein Structure/Function and Human Disease - page 104

create misfolded protein aggregates. The appearance of misfolded/aggregated
proteins in the lumen of the ER activates the UPR that, when overwhelmed, leads
to cell death via apoptosis; protein aggregates are known to be inherently toxic to
cells [371].
The molecular basis for the impairment in ER function by hyperhomocys-
teinemia is not known but may include protein N-homocysteinylation by Hcy-
thiolactone (Reaction 3.4 ) that can cause protein misfolding and induce ER stress.
As discussed in the following sections, protein modification by Hcy-thiolactone can
also create altered proteins with newly acquired interactions or impaired function
and can lead to induction of autoimmune responses.
The findings that N-Hcy-proteins have the propensity to aggregate [78, 139, 153,
336] and induce cell death in cultured cells, first demonstrated for N-Hcy-LDL and
human endothelial cells [170], are consistent with this concept. In addition to its
propensity to aggregate, N-Hcy-albumin [78], which accumulates in hyperhomo-
cysteinemia [213], forms amyloid-like protofibrils that are toxic to cells [171].
Recent findings show that N-homocysteinylation induces amyloidal transformation
in several other proteins, further discussed in Sect. 5.4 of this topic. It should be
noted that ER stress and UPR in cultured human endothelial cells and in mice are
induced by hyperhomocysteinemia [18, 58, 59, 369], which also elevates plasma
Hcy-thiolactone [64, 74, 93] and N-Hcy-protein levels [78, 113].
Furthermore, treatments with Hcy-thiolactone are more effective than treatments
with Hcy in inducing ER stress and UPR in retinal epithelial cells [60] and apoptotic
death in cultured human vascular endothelial cells [61, 164]. The greater sensitivity
of cells to Hcy-thiolactone suggests that protein modification by Hcy-thiolactone
leads to the UPR and induction of the apoptotic pathway. Consistent with this
scenario, cellular levels of N-Hcy-protein are elevated under conditions of
hyperhomocysteinemia-induced ER stress both in cultured cells [74] and in the
mouse liver [113]. N-Hcy-proteins are known to undergo major structural changes
[78, 96] which cause misfolding and formation of toxic amyloid aggregates [171].
Recent studies show that folic acid limitation increases cellular Hcy levels, induces
N-homocysteinylation of the motor protein dynein, and induces the formation of N-
Hcy-protein aggregates in rat neuronal cells [299]. Proteolytic degradation of N-
Hcy-protein can generate antigenic peptides, which can be displayed on cell surface
and induce an autoimmune response.
6.1 The Hcy-Thiolactone Hypothesis
Studies of Hcy-thiolactone metabolism shed light on a mechanism underlying
pathological consequences of elevated Hcy levels. The Hcy-thiolactone hypothesis
(Fig. 6.1 ), originally formulated in 1997 [62], states that the metabolic conversion
of Hcy to Hcy-thiolactone followed by the nonenzymatic protein modification by
Hcy-thiolactone—protein N-homocysteinylation—is an underlying biochemical
mechanism that contributes to the pathology of hyperhomocysteinemia, such as