Alveolar
hypoxia, a common feature of many respiratory disorders, has been previously reported to induce functional changes, particularly a decrease of transepithelial Na and fluid transport. In polarized epithelia, cytoskeleton plays a regulatory role in transcellular and paracellular transport of
ions and fluid. We hypothesized that exposure to
hypoxia could damage cytoskeleton organization, which in turn, may adversely affect ion and fluid transport. Primary rat alveolar epithelial cells (AEC) were exposed to either mild (3% O(2)) or severe (0.5% O(2))
hypoxia for 18 h or to normoxia (21% O(2)). First, mild and severe
hypoxia induced a disorganization of actin, a major
protein of the cytoskeleton, reflected by disruption of
F-actin filaments. Second,
alpha-spectrin, an apical cytoskeleton
protein, which binds to actin cytoskeleton and Na
transport proteins, was cleaved by
hypoxia. Pretreatment of AEC by a
caspase inhibitor (
z-VAD-fmk; 90 microM) blunted
hypoxia-induced
spectrin cleavage as well as
hypoxia-induced decrease in surface membrane alpha-ENaC and concomitantly induced a partial recovery of
hypoxia-induced decrease of
amiloride-sensitive Na transport at 3% O(2). Finally, tight junctions (TJs)
proteins, which are linked to actin and are a determinant of paracellular permeability, were altered by mild and severe
hypoxia:
hypoxia induced a mislocalization of
occludin from the TJ to cytoplasm and a decrease in
zonula occludens-1 protein level. These modifications were associated with modest changes in paracellular permeability at 0.5% O(2,) as assessed by small 4-kD
dextran flux and transepithelial resistance measurements. Together, these findings indicate that
hypoxia disrupted cytoskeleton and TJ organization in AEC and may participate, at least in part, to
hypoxia-induced decrease in Na transport.