Alveolar macrophage apoptosis and TNF-alpha, but not p53, expression correlate with murine response to bleomycin

Apoptosis is considered to be a protective mechanism that limits lung injur y. However, apoptosis might contribute to the inflammatory burden present i n the injured lung. The exposure of mice to bleomycin (BLM) is a well-estab lished model for the study of lung injury. BLM exposure induces DNA damage and enhances tumor necrosis factor (TNF)-alpha expression in the lung. To e valuate the importance of alveolar macrophage (AM) apoptosis in the pathoge nesis of lung injury, we exposed BLM-sensitive (C57BL/6) and BLM-resistant (BALB/c) mice to BLM (120 mg/kg) and studied the induction of apoptosis [by light-microscopy changes (2, 8, 12, 24, 48, and 72 h) and annexin V uptake by flow cytometry (24 h)], the secretion of TNF-alpha (measured by ELISA), and the expression of p53 (by immunoblotting) in AM retrieved from these m ice. BLM, but not vehicle, induced apoptosis in AM from both murine strains . The numbers of apoptotic AM were significantly greater (P < 0.001) in C57 BL/6 mice (52.9%) compared with BALB/c mice (40.8%) as demonstrated by anne xin V uptake. BLM induction of apoptosis in AM was preceded by an increased secretion of TNF-alpha in C57BL/6 but not in BALB/c mice. Furthermore, dou ble TNF-alpha receptor-deficient mice, developed on a C57BL/6 background, d emonstrated significantly (P < 0.001) lower numbers of apoptotic AM compare d with C57BL/6 and BALB/c mice. BLM also enhanced p53 expression in AM from both murine strains. However, p53-deficient mice developed BLM-induced lun g injury, exhibited similar lung cell proliferation (measured as proliferat ing cell nuclear antigen immunostaining), and accumulated similar amounts o f lung hydroxyproline (65 +/- 6.9 mu g/lung) as did C57BL/6 (62 +/- 6.5 mu g/lung) mice. Therefore, AM apoptosis is occurring during BLM-induced lung injury in a manner that correlates with murine strain sensitivity to BLM. F urthermore, TNF-alpha secretion rather than p53 expression contributes to t he difference in murine strain response to BLM.