Excessive accumulation of lipid droplets in hepatocytes is a
common feature of several diseases, including NAFLD [25]. Although
liver steatosis is currently considered benign, fatty livers
are particularly more susceptible to variety of insults. Indeed, excessive
lipid accumulation was found to predispose hepatocytes to
hepatocellular injury, which may lead to the progression of simple
steatosis to NASH and fibrosis [30]. Hepatocellular injury can be
caused by external as well as internal factors, including cytotoxic
effects of free FA excess, oxidative stress and lipid peroxidation
[31]. By using an in-vitro model of hepatic steatosis, this study
illustrates that steatosis per-se, without the addition of external
stimuli, induces nuclear alterations while not affecting cell viability.
Data presented here suggests FA-induced nuclear oxidative/
electrophilic stress as an important contributor to NAFLD
progression.
The exact mechanism by which lipid accumulation induces
nuclear stress is not entirely comprehended. Nevertheless, it
seems that lipid accumulation within hepatocytes might promote
mitochondrial FA oxidation as a compensatory mechanism which
subsequently enhances ROS generation. Furthermore, it was recently
demonstrated that palmitic acid overload can induce mitophagy
deficiency resulting in increased ROS production by damaged
mitochondria [32]. Taking into account unchanged cell
viability, enhanced mitochondrial activity accompanied by increase
in ROS production, we can speculate that FA treatment had
a similar effect on hepatocytes. It is important to note, that mild
oxidative stressor CoCl2 itself did not induce mitochondrial activity,
although significantly increased ROS production.
Increased ROS production resulted in the vast lipid peroxidation
and the production of biogenic reactive oxoLPP, as was
monitored by CHH labeling and fluorescence microscopy. OxoLPP,
formed in close proximity to nuclear membrane, can further affect
the nuclear compartment and induce electrophilic stress in this
organelle. Additionally, formation of a large number of low molecular
weight oxoLPP was confirmed by MS using derivatization
technique specific for carbonylated lipids [17]. Interestingly, high
similarity of generated oxoLPP was shown for both FA-treated cells
(FA and FA/CoCl2 groups), whereas CoCl2 treated group showed
specific set of new oxoLPP formation. Lipid peroxidation is one of
the main events induced by oxidative stress and it can be particularly
damaging to the liver nuclear membrane rich in PUFA [33].
Despite the fact that liver can metabolize reactive oxoLPP faster
than most of the other tissues [34], numerous publications indicated
high level of modifications of liver and hepatocyte proteins
by reactive oxoLPP, and HNE in particular [35–37].
Indeed, electrophilic stress resulted in a high number of proteins
modified by reactive oxoLPP in FA-treated cells. Results of
this work are in agreement with earlier studies demonstrating the
reactivity of oxoLPP towards nuclear biomolecules in hepatocytes
[33,38,39] and thus further suggest the importance of these alterations
in NAFLD progression. LC–MS allowed to identify over
300 proteins modified by oxoLPP in FA-treated cells and 751
protein modified in four experimental groups. By comparing the
list of modified proteins with published data, several dozen of
proteins were already shown as carbonylation targets. Thus, 30
proteins identified here were identical to the LPP-modified proteins
extracted from the liver of mice with early alcoholic liver
disease [23]. 59 and 23 identical proteins were present in the
study on RKO cells treated with LPP [26] and HeLa cells treated
with hydrogen peroxide [40]. Interestingly, 34 carbonylated proteins
were already identified in the human plasma samples from
obese patients with and without type II diabetes [41]. These similarities
might illustrate the specificity of certain protein as carbonylation
targets. However, in the current study enriched nuclear
protein fractions were used which allowed to get deeper coverage
of oxoLPP modified nuclear proteome in hepatocytes for the first
time, to the best of our knowledge.
Our study demonstrated specificity of protein targets in FAtreated
cells. Proteins involved in nuclear pore organization,
chromosome segregation, cell cycle control, centrosome organization,
regulation of transcription and splicing, ribosomal biogenesis
and DNA repair were specifically modified by oxoLPP. Clear
enrichment of modified proteins involved in nuclear riboprotein
complexes, chromatin remodeling, including histones and histone
methyl- and actyltransferases, as well as transcription and splicing
can provide the link between lipid accumulation, oxidative stress
and changes in protein expression levels as well as protein functional
activities. Our study indicated significant increase in phosphorylation
level of histone H2AX in FA-treated cells indicating
possible malfunction of replication/transcription and/or activation
of DNA repair mechanisms. It was recently demonstrated that
treatment of RKO cells with HNE and ONE resulted in multiple
adducts of oxoLPP with histones H2B, H2, H3 and H4. Furthermore,
modifications of H3 and H4 resulted in disruption of nucleosome
166 S. Anavi et al. / Redox Biology 4 (2015) 158–168
formation, which may challenge chromatin dynamics and histone
turnover [42]. Furthermore, modifications of lysines residues by
LPP might hamper epigenetic regulation via acetylation and methylation
on this residues. Interestingly, several key players of
epigenetic regulation such as N-lysine methyltransferase SETD8
and histone acetyltransferases KAT6B and KAT8 were shown
among modified proteins.
Additionally we demonstrated that nuclear shape seems to be
modified due to a mechanical pressure of the lipid droplets in FAtreated
cells. However, it is also possible that oxidative changes in
structural proteins resulted in this outcome. Changes in nuclear
shape were suggested to influence chromatin organization and
gene expression and are associated with the development and
progression of several pathological conditions [7–9]. We demonstrated
that several main proteins involved in a nuclear membrane
architecture were modified by reaction with oxoLPP, including
prelamin A/B and nesprin-1 and 2. These proteins directly determine
the shape, mechanical stiffness and extensibility of nuclear
membrane [43]. Additionally, several components of nuclear
pore complex, such as nucleoporins, were found to be modified.
The effect of moderate external stressor in control or FA-treated
cells was evaluated by using CoCl2 at relatively low concentrations.
Surprisingly, the addition of CoCl2 did not significantly influence
level of nuclear protein modifications despite significant increase
in ROS generation. Thus, the current study demonstrated that FAtreatment
alone without additional inducers of oxidative stress,
can result in ROS generation, most probably via defective mitophagy.
Enhanced generation of ROS resulted in the vast lipid
peroxidation and production of reactive oxoLPP capable to modify
nuclear proteins. Such electrophilic stress resulted in the modifications
of numerous proteins involved in nuclear pore organization,
chromosome segregation, cell cycle control, centrosome
organization, regulation of transcription and splicing, ribosomal
biogenesis and DNA repair. The loss of functional activity and
structural integrity of nuclear proteome may provide the link between
lipid accumulation and NAFLD progression.