OVEREXPRESSION OF HEME OXYGENASE-1 IN HUMAN PULMONARY EPITHELIAL-CELLS RESULTS IN CELL-GROWTH ARREST AND INCREASED RESISTANCE TO HYPEROXIA

Heme oxygenase (HO) catalyzes the rate-limiting step in the degradatio n of heme to biliverdin, which is reduced by biliverdin reductase to b ilirubin, Heme oxygenase-1 (HO-1) is inducible not only by its heme su bstrate, but also by a variety of agents causing oxidative stress, Alt hough much is known about the regulation of HO-1 expression, the funct ional significance of HO-1 induction after oxidant insult is still poo rly understood, We hypothesize and provide evidence that HO-1 inductio n serves to protect cells against oxidant stress. Human pulmonary epit helial cells (A549 cells) stably transfected with the rat HO-1 cDNA ex hibit marked increases of HO-1 mRNA levels which were correlated with increased HO enzyme activity, Cells that overexpress HO-1 (A549-A4) ex hibited a marked decrease in cell growth compared with wild-type A549 (A549-WT) cells or A549 cells transfected with control DNA (A549-neo), This slowing of cell growth was associated with an increased number o f cells in G(0)/G(1) phase during the exponential growth phase and dec reased entry into the S phase, as determined by flow cytometric analys is of propidium iodide-stained cells and pulse experiments with bromod eoxyuridine, Furthermore, the A549-A4 cells accumulated at the G(2)/M phase and failed to progress through the cell cycle when stimulated wi th serum, whereas the A549-neo control cells exhibited normal cell cyc le progression, interestingly, the A549-A4 cells also exhibited marked resistance to hyperoxic oxidant insult, Tin protoporphyrin, a selecti ve inhibitor of HO, reversed the growth arrest and ablated the increas ed survival against hyperoxia observed in the A549-A4 cells overexpres sing HO-1, Taken together, our data suggest that overexpression of HO- 1 results in cell growth arrest, which may facilitate cellular protect ion against non-heme-mediated oxidant insult such as hyperoxia.