Adult respiratory distress syndrome (ARDS), a disease process often caused by gram negative sepsis, is responsible for much morbidity and mortality in critically ill patients. ARDS is characterized by an influx of inflammatory cells into the lung and consequent destruction of interstitium and alveoli. The mechanism underlying cellular damage likely involves cytokine mediators and reactive oxygen species which are products of neutrophils and macrophages. The lung's response to injury includes up-regulation of antioxidant enzymes and other stress response genes including heme oxygenase-1 (HO-1). HO-1 is a 32 kD protein that catalyzed the rate-limiting step in the oxidative degradation of heme to bilirubin. There is evidence that HO-1 plays a role in cellular protection by increasing levels of bilirubin which has antioxidant properties while decreasing pro-oxidant, heme-containing compounds. Additionally, increases in HO-1 are coupled to increases in ferritin which sequesters intracellular free iron, making the iron unavailable to participate in the Fenton reaction, hydroxyl radical formation, and subsequent cellular damage. We have observed marked induction the HO-1 gene expression in a rat model of LPS-induced lung injury and in a macrophage cell line treated with LPS. We hypothesize that enhanced activation of the HO-1 gene plays an important role in mediating the host's defense and/or adaptation to acuted lung injury and sepsis. To test this hypothesis we will examine the regulation of HO-1 gene expression and the functional significance of HO-1 in in vivo and in vitro models of ARDS by addressing the following specific aims: 1) to determine the transcriptional regulation of HO-1 gene induction in macrophages after LPS; 2) to determine the transcriptional regulation of HO-1 gene activation by interleukin-6 (IL-6), a secretory product of LPS- activated macrophages, and; 3) to determine the functional significance of HO-1 gene induction in vivo and in vitro after cellular stress. Our initial studies will focus on identifying the critical cis- and trans- acting regulatory elements responsible for mediating induction of the HO- 1 gene after LPS and IL-6. We will address the functional significance of HO-1 expression after various stresses by manipulating HO-1 levels and studying the effects of stress on a) morbidity and mortality in experimental animals and b) cellular functions including growth, viability, and the tendency to undergo apoptosis.
