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J. Biol. Chem., Vol. 278, Issue 22, 19917-19925, May 30, 2003

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Early Events in the Anoikis Program Occur in the Absence of Caspase Activation^*

Pengbo Wang, Anthony J. Valentijn, Andrew P. Gilmore and Charles H. Streuli 

From the School of Biological Sciences, University of Manchester, Stopford Building, Oxford Road, Manchester M13 9PT, United Kingdom

Adhesion of many cell types to the extracellular matrix is essential to maintain their survival. In the absence of integrin-mediated signals, normal epithelial cells undergo a form of apoptosis termed anoikis. It has been proposed that the activation of initiator caspases is an early event in anoikis, resulting in Bid cleavage and cytochrome c release from mitochondria. We have previously demonstrated that the loss of integrin signaling in mammary epithelial cells results in apoptosis and that this is dependent upon translocation of Bax from the cytosol to the mitochondria. In this paper, we ask whether caspases are required for Bax activation and the associated changes within mitochondria. We show that Bax activation occurs extremely rapidly, within 15 min after loss of integrin-mediated adhesion to extracellular matrix. The conformational changes associated with Bax activation are independent of caspases including the initiator caspase-8. We also examined downstream events in the apoptosis program and found that cytochrome c release occurs after a delay of at least 1 h, with subsequent activation of the effector caspase-3. This delay is not due to a requirement for new protein synthesis, since cycloheximide has no effect on the kinetics of Bax activation, cytochrome c release, caspase-3 cleavage, or apoptosis. Together, our data indicate that the cellular decision for anoikis in mammary epithelial cells occurs in the absence of caspase activation. Moreover, although the conformational changes in Bax are rapid and synchronous, the subsequent events occur stochastically and with considerable delays.

Received for publication, October 9, 2002 , and in revised form, February 14, 2003.

^* This work was supported by the Wellcome Trust and the Medical Research Council. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed. Tel.: 44-161-275-5626; Fax: 44-161-275-1505; E-mail: cstreuli{at}man.ac.uk.

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