Phosphorylation of the Proapoptotic BH3-Only Protein Bid Primes Mitochondria for Apoptosis during Mitotic Arrest

摘要

class="head no_bottom_margin" id="sec1title">IntroductionDuring mitosis, the spindle assembly checkpoint (SAC) normally prevents cells progressing to anaphase until all chromosomes are correctly attached to spindle microtubules (). However, if normal cells persist in mitosis for too long, they die by apoptosis. Antimitotic drugs such as paclitaxel keep the SAC active in order to selectively induce apoptosis in rapidly dividing cancer cells (). However, cancer cells can develop resistance to paclitaxel by either exiting mitosis before apoptosis is initiated (termed mitotic slippage) or by blocking the apoptotic response to delayed mitotic exit (). Mitotic slippage occurs due to the degradation of cyclin B1 before apoptosis can be activated (). On the other hand, how delayed mitotic exit activates apoptosis is poorly understood, despite the possibility that activating this mechanism could sensitize cancer cells to antimitotic drugs.The Bcl-2 family of proteins regulates apoptosis. Activation of the Bcl-2 proteins, Bax and Bak, leads to mitochondrial outer membrane permeabilization (MOMP) (). The BH3-only members of the Bcl-2 family either activate Bax and Bak or inhibit antiapoptotic proteins such as Bcl-XL and Mcl-1. Different BH3-only proteins respond to distinct apoptotic signals and are regulated both transcriptionally and by posttranslational modification. For example, PUMA is transcriptionally upregulated by p53 (), whereas Bad is phosphorylated via growth factor signaling (). Another BH3-only protein, Bid, is regulated by proteolytic cleavage by caspase-8 downstream of death receptor signaling (, ). Cleaved Bid then translocates to mitochondria where it activates MOMP. However, several studies have shown that Bid can be proapoptotic without being proteolytically cleaved (, ).Here, we show that Bid is phosphorylated during mitosis within its regulatory loop. This phosphorylation sensitizes mitochondria for MOMP if mitotic exit is delayed. Our data suggest that BH3 mimetics may represent a viable strategy for targeting paclitaxel-resistant cancer cells.