ALTERED EXPRESSION AND LOCALIZATION OF 5-LIPOXYGENASE ACCOMPANY MACROPHAGE DIFFERENTIATION IN THE LUNG

The alveolar macrophage (AM) exhibits a greater capacity to synthesize bioactive leukotrienes from arachidonic acid than does its circulatin g precursor the peripheral blood monocyte. Macrophage differentiation in the lung entails cellular residence within both the pulmonary inter stitial and alveolar compartment. In the present study; we sought to d etermine 1) whether this enhanced metabolic activity was acquired duri ng maturation within the alveolar space and 2) the underlying mechanis ms responsible for this upregulation. Rat AMs were separated by Percol l gradient centrifugation into four density-defined subpopulations tho ught to reflect their degree of maturation. On stimulation with a calc ium ionophore, synthesis of leukotriene B-4 increased with the degree of maturation, although it was diminished in the oldest subpopulation This maturation-dependent upregulation was not explained by increases in arachidonic acid release but was associated with increased expressi on of 5-lipoxygenase (5-LO) protein as determined by immunoblot analys is. Whereas S-LO is primarily cytosolic in monocytes, it is known to b e primarily intranuclear in unfractionated AMs. Here, the localization of 5-LO was investigated by immunofluorescence microscopy and was fou nd to be predominantly nuclear in all AM subpopulations; by contrast, the protein was cytosolic in interstitial macrophages isolated by mech anical and enzymatic lung digestion. These divergent localization patt erns in AMs and interstitial macrophages were verified in situ by immu nohistochemical staining of sections of normal rat lung. When unfracti onated AMs were isolated and maintained in culture for 3 days, a shift in 5-LO distribution from nucleus to cytosol was observed. We conclud e that 1) nuclear import of 5-LO occurs within the alveolar space and is reversible on removal from the alveolar milieu and 2) leukotriene s ynthetic capacity increases further during AM residence within the alv eolar space as a result of a progressive increase in the amount of 5-L O protein.