Throughout the daily rhythm, the lung shows physiological and functional changes.
Disruption of the circadian rhythm is associated with alterations in the immune system
and its dysfunction as seen in chronic lung diseases such as asthma and COPD. The
circadian rhythm, being a 24-hour period, is regulated by the central clock in the
brain. However, circadian timers (Zeitgeber) are also found in peripheral tissues
such as the lung. Physiological changes of peripheral organs due to a malfunction
in its circadian rhythm may be an important factor of pre-disposition causing significant
differences in the response to xenobiotics.
To investigate a time-dependent sensitivity to inhalable particles, model nanoparticles
for combustion-derived air pollution (Carbon Black, Degussa, 14nm) were applied to
C57BL/6 mice at four different time points throughout the day. Twelve hours after
instillation, inflammatory parameters were analyzed in the bronchoalveolar lavage
(BAL). Significant differences in neutrophilic lung inflammation were observed depending
on the time point of particle application during the 24h period. Additional dose response
experiments demonstrate dramatic differences in sensitivity to various quantities
of carbon nanoparticles (CNP) during the daily time course. The relevance of the lung
internal rhythm was investigated by an inverted feeding experiment. In these approaches,
the circadian rhythm of peripheral organs is uncoupled from the central light stimulus
by feeding the mice during the light phase or at night. The previously observed sensitivity
to instillation was abrogated when interfering with the feeding rhythm. Reciprocally,
the effect of CNP on the pulmonary circadian rhythm was investigated at the mRNA level
of the zeitgeber gene per2 and was shown to be disrupted after CNP exposure.
We see strong differences in lung inflammation depending on the time point of exposure.
The peripheral clock in the lung and the associated inflammatory response to inhaled
particles was affected by a change in feeding times. The data indicate that CNP exposure
interferes with the organ-specific circadian rhythm of the lung. Still, the underlying
circadian mechanisms regulating the neutrophil-dominated acute inflammation remain
elusive. Further studies will concentrate on the involvement of different inflammatory
and lung epithelial cells mediating the circadian changes in sensitivity to particle
exposure.
