Bcl-X_L Is the primary mediator of p21 protection against hyperoxia-induced cell death

ABSTRACT

A tight balance between anti- and proapoptotic members of the Bcl-2 family controls cell survival and death. Exposure to hyperoxia shifts this balance towards a prodeath state that ultimately activates Bak- and Bax-dependent cell death. Mechanisms underlying this shift are undefined; however, the cell cycle inhibitor p21 delays the loss of antiapoptotic Mcl-1 and Bcl-X_L, and protects against hyperoxia. Here, H1299 human lung adenocarcinoma cells are used to investigate how these and other members of the Bcl-2 family cooperate with p21 to protect against hyperoxia. Expression of antiapoptotic Mcl-1 and Bcl-X_L, but not Bcl-2 or A1, declined during hyperoxia, whereas proapoptotic Bak, but not Bax, increased. Conditional overexpression of p21 selectively delayed the loss of Mcl-1 and Bcl-X_L, without affecting expression of the other members. siRNA knockdown of Mcl-1 and Bcl-X_L sensitized cells to hyperoxia, but only the loss of Bcl-X_L ablated the protective effects of p21. Conversely, overexpression of Mcl-1 and Bcl-X_L protected against hyperoxia, but only Bcl-X_L bound Bak and Bax. Altogether, these data suggest that Bcl-X_L is the primary mediator by which p21 protects against hyperoxia-induced Bak/Bax-dependent cell death.

KEYWORDS :

• apoptosis
• Bcl-2 proteins
• cell cycle inhibitor p21
• hyperoxia
• necrosis