Apoptosis and release of CD44s in bleomycin-treated L132 cells

The anti-cancer drug bleomycin (BLM) induces lung injury and triggers apoptosis of alveolar epithelial cells. In epithelia, among other functions, the adhesion protein CD44 promotes the contact to components of the extracellular matrix like hyaluronate. A functional link between apoptosis and the loss of CD44 has been observed in colon carcinoma cells and involvement of CD44 in apoptosis of lung cells has been reported in several studies. The present in vitro study examined the expression of CD44s (CD44 standard) in two human epithelial lung cell lines, L132 and A549, during BLM-induced apoptosis. A loss of CD44s by lung epithelial cells and an increase of the soluble form of this adhesion protein in culture supernatants upon exposure to BLM were observed. Apoptosis was characterized by an activation of caspase-3 as well as by release of cytochrome C into the cytosol as shown for L132 cells. Inhibition of apoptosis by the broad-range caspase inhibitor Z-VAD-fmk reduced CD44 release by both cell lines demonstrating that CD44 release is a result of apoptotic processes. Kinetic experiments failed to discriminate between the initiation of apoptosis and CD44 release. Blocking experiments using antagonistic anti-CD95 receptor antibodies revealed that BLM may cause apoptosis and CD44 release in a CD95-independent manner.