Differential modulation of LTB₄ synthesis and degradation in mononuclear phagocytes.

B74

Leukotrienes have been implicated to play a prominent inductive role in carcinogenesis. We previously reported that tobacco smoke enhanced LTB4 production by stimulated alveolar macrophages (AM). This study aims to elucidate the underlying molecular mechanisms. AM obtained from current, former and never smokers were harvested and conditioned with LPS and/or tobacco smoke for up to 7 days. Conditioned culture supernatant, total RNA, and proteins were harvested to determine the effects of LPS and/or tobacco smoke on LTB₄ synthesis and metabolism. We found that LPS induced the release of LTB₄ from AM and down-regulated 5-lipoxygenase (5-LOX) and 5-LOX activating protein (FLAP) mRNA expression in a dose-dependent manner, followed by a decrease in 5-LOX protein production and normalization of LTB₄ levels, suggesting the presence of a negative feedback mechanism. LPS concomitantly induced expression and activity of the LTB₄ metabolizing enzyme, LTB₄ omega hydroxylase (LTB₄OH). The up-regulation of LTB₄OH mRNA expression was not observed in the monocytic leukemia-derived cell line, THP-1, or in AM obtained from current smokers. In vitro smoke exposure of ex-smoker's AM also abrogated the LPS-induced up-regulation of LTB₄OH mRNA expression. The differential modulation of LTB₄ synthesis and degradation in the setting of tobacco smoke exposure and malignancy suggest that mechanisms associated with sustained elevation of LTB₄ levels in the lung microenvironment may contribute to lung carcinogenesis.