Proteasome and immunoproteasome function in cigarette smoke-mediated chronic lung disease

Chronic obstructive pulmonary disease (COPD) is projected to be the
third leading cause of death by 2020 with cigarette smoke exposure
being the main risk factor. Cigarette smoke leads to oxidative
stress in the lung, resulting in protein damage and adaptive immune
responses. Also, smokers and COPD patients are more susceptible to
viral infections often followed by acute exacerbations of COPD
pathogenesis. Lungs of COPD patients exhibit increased numbers of
innate and adaptive immune cells, among these CD8+ T cells, whose
abundance correlates with disease severity. The proteasome degrades
more than 90 % of intracellular proteins - including damaged ones -
into small peptides and is important to protect the cell from
proteotoxic stress. Furthermore, the immunoproteasome, a
specialized proteasome subtype which is expressed by default in
antigen presenting cells and induced during infection, is involved
in shaping adaptive immune responses by enhancing antigen
presentation via major histocompatibility complex (MHC) I to
cytotoxic CD8+ T cells. The effects of cigarette smoke on
(immuno-)proteasome function have not been investigated so far. The
first publication included in this thesis (van Rijt et al. 2012)
explored the effects of acute cigarette smoke exposure on
proteasome expression and activity. We observed that short-term
exposure of cells to extracts of cigarette smoke directly impaired
proteasome activity, while proteasomal protein expression was not
altered. Oxidatively modified and polyubiquitinated proteins
accumulated, suggesting augmentation of oxidative stress in
cigarette smoke-treated cells. In lungs of mice acutely exposed to
cigarette smoke, a similar effect could be observed: one of the
three proteasome activities was significantly reduced, and
ubiquitinated substrates for the proteasome were found to be
accumulated, while proteasome expression levels were not changed.
The second publication in this thesis (Keller et al. 2015) shows
for the first time the cell-specific expression of
immunoproteasomes in the lung and their induction by interferon-γ
in vitro and by murid herpesvirus 68 (MHV-68) infection in vivo.
Within these experiments, activity-based probes were used to
clearly define the kinetics of standard and immunoproteasome
subunit incorporation. In human lungs from controls or early-stage
COPD patients, immunoproteasome expression was not changed.
Immunoproteasomes localized mainly to alveolar macrophages, but not
to parenchymal cells in both donors and end-stage COPD. Results
from recent experiments were accepted for publication in the
meantime (Kammerl et al. 2016): we investigated MHC I antigen
presentation in cigarette smoke extract-treated primary immune
cells and bronchoalveolar lavage (BAL) cells from mice exposed to
cigarette smoke for ten days. In vitro treatment of primary immune
cells with cigarette smoke extract led to a decrease in the
presentation of an immunoproteasome-dependent “self”-epitope. With
the help of activity-based probes, we observed a shift from immuno-
to standard proteasome activity in isolated alveolar macrophages
from smoke exposed mice. This shift, however, was not sufficient to
impact antigen presentation of an immunoproteasome-dependent
epitope. The altered ratio of standard and immunoproteasome might
be explained by transcriptional downregulation of immuno-, but not
standard proteasomes by cigarette smoke in isolated alveolar
macrophages of smoke-exposed mice, which was also observed in total
BAL cells of early-stage COPD patients. In the lungs of end-stage
COPD patients, activities of both standard and immunoproteasome
subunits were significantly decreased, while total proteasome
protein levels were not changed. Taken together, we show that
cigarette smoke directly impairs proteasome function in vitro and
in vivo, which may exacerbate oxidative stress resolution in
response to cigarette smoke, since the degradation of oxidatively
modified and misfolded proteins is impaired. In addition, we
observed alterations in immunoproteasome-dependent MHC I antigen
presentation, which may contribute to increased susceptibility to
virus-induced exacerbations, prolonged infection and possibly
result in autoimmune responses.