MANGANESE SUPEROXIDE-DISMUTASE EXPRESSION IN ALVEOLAR TYPE-II EPITHELIAL-CELLS FROM NONVENTILATED AND HYPOPERFUSED LUNGS

Lungs that have been hypoxic and hypoperfused because of atelectasis a nd the resulting decrease in pulmonary arterial blood flow develop spe cific decreases in manganese superoxide dismutase (MnSOD) activity and are sensitive to oxidant injury during reoxygenation. Since the MnSOD protein is concentrated in mitochondria of alveolar epithelial type I I cells (ATII), we hypothesized that expression of MnSOD would be decr eased in these cells also as a result of hypoxia. To investigate wheth er regulation of MnSOD expression occurred before or after transcripti on, we determined whether MnSOD protein content or steady-state mRNA l evel changed after hypoxia as well. ATII cells were isolated by elasta se digestion from lungs of adult rabbits after right lungs had been hy poxic and hypoperfused for 7 days because of unilateral atelectasis. M nSOD activity was measured by inhibition of cytochrome c reduction in the presence of 1 mM KCN, MnSOD protein content was measured on immuno blots, and MnSOD mRNA was quantified on slot blot autoradiograms. MnSO D activity was 8.4 +/- 1.9 U/mg protein in ATII cells from control lun gs and 6.8 +/- 1.5 U/mg protein in ATII cells from hypoxic and hypoper fused lungs (n = 9, P = 0.037). MnSOD protein content was 5.1 +/- 1.4 mu g/mg protein in ATII cells from control and 4.1 +/- 1.2 mu g/mg pro tein in ATII cells from hypoxic and hypoperfused lungs (P = 0.021). AT II cell MnSOD mRNA/18S ribosomal RNA (ratio of arbitrary absorbance un its) determined by RNA slot blots was 2.18 +/- 1.26 in ATII cells from control lungs and 2.94 +/- 0.88 in ATII cells from hypoxic lungs (n = 7, P > 0.05). Neither recovery of ATII cells nor the apparent recover y of mitochondrial protein (assayed by immunoblotting for cytochrome c oxidase subunit IV) was affected by previous lung tissue hypoxia and hypoperfusion in vivo. Both MnSOD activity and protein expression in A TII cells from hypoxic lungs decreased concomitantly, indicating that no change in MnSOD enzyme specific activity occurred. No change occurr ed in steady-state mRNA level, so regulation of MnSOD in ATII cells ma y occur through changes in synthesis or degradation of the protein rat her than at the transcriptional level.