Cigarette smoke (CS) harbors a strong oxidative stress potential, which broadly impacts exposed cells. A special feature of the cellular response to CS corresponds to the strong transcriptional upregulation of antioxidant genes, among which, as evaluated by cDNA microarray analysis, the gene encoding heme oxygenase-1 (HO-1) was found to be the most strongly expressed gene, both in vitro and in vivo. Exposure of

murine 3T3 fibroblast cells with strongly impaired HO-1 expression to otherwise subcytotoxic concentrations of aqueous extracts of CS resulted in a significant increase in the extent of cell death, suggesting that CS-induced HO-1 expression is potentially cytoprotective in this system. Cell death induced by CS in Swiss 3T3 cells with attenuated HO-1 expression showed specific apoptotic traits, as indicated by the induction of fasL, followed by a significant increase in the number of Annexin V-positive=propidium iodide-negative cells, which was partly sensitive to caspase-8 inhibition. The apoptotic signal was traced to Jun-N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) signal transduction pathways, as inhibition of either pathway resulted in a significant decrease in the degree of apoptosis in CS-exposed cells with attenuated HO-1 expression. In contrast to wild-type 3T3 fibroblasts, a substantial fraction of human myeloid U937 cells undergo apoptosis when exposed to similarly low concentrations of CS. Similar to CS-exposed HO-1-deprived 3T3 cells, apoptosis in CS-treated U937 cells appears to be executed in an autocrine manner as indicated by fasL expression and activation of caspase-8. Although CSexposed U937 cells also strongly express HO-1, the underlying kinetics significantly differs from that seen in 3T3 cells, particularly by a later onset of HO-1 expression. Notably, after 6 hr of CS exposure, U937 cells undergoing apoptosis are completely devoid of HO-1 protein, whereas the fraction of viable cells efficiently expresses HO-1. Altogether, these results strongly support the general concept that under stressing conditions, HO-1 plays a central antiapoptotic, cytoprotective role.