Bleomycin Induces Caveolin-1 and -2 Expression in Epithelial Lung Cancer A549 Cells

Background: Bleomycin induces apoptosis in alveolar epithelial cells. The expression of caveolin-1 and -2 in lung epithelial-derived A549 cells was analysed in terms of apoptosis after exposure to bleomycin. Materials and Methods: Apoptosis was investigated using flow cytometry, ELISA, immunohistochemistry and Western blot analysis. Caveolin-1 and -2 were determined at the protein level (Western blot). Intracellular caveolin-1 distribution was studied with immunofluorescence, as well as sucrose density gradient centrifugation. Results: Caveolin-1 and -2 were up-regulated 1 h after exposure to bleomycin and preceding the occurrence of caspase-8, and of caspase-3 and caspase-9 cleavage products. Sucrose density gradient centrifugation revealed that bleomycin exposure led to a partial translocation of caveolin-1 from caveolin-rich membrane fractions to non-raft fractions. Successful inhibition of bleomycin-induced apoptosis by the broad-spectrum caspase inhibitor zVAD-fmk did not influence the amount of caveolin-1 and -2. Conclusion: ∆he early up- regulation of caveolin-1 and -2 following bleomycin exposure is a rather apoptosis-independent event related to other unknown mechanisms of bleomycin-mediated cell injury. Caveolae are flask-shaped invaginations of the plasma membrane and are present in most tissue cells. Caveolins are a family of integral membrane proteins (caveolin-1, -2, and -3) that are important structural components of caveolae (1). Although individual caveolin members are differentially expressed in various cell types (2), lung tissue has abundant caveolin-1, and -2 in both endothelial and alveolar type I cells (3, 4). Caveolae and caveolin-1 play a key role in a number of cellular functions, including tumor suppressor function and apoptosis (1), cholesterol trafficking (5), modulation of signalling events (6) and involvement in potocytosis and transcytosis (7). Recent data also describe caveolae as a portal for the entry of a range of bacteria and viruses (8).