Physiological and pathological cell death have been classifi ed according to morphological criteria into at least three categories: type I cell death or apoptosis; type II cell death or autophagic cell death; and type III cell death

This article is available online at http://www.jlr.org Physiological and pathological cell death have been classifi ed according to morphological criteria into at least three categories: type I cell death or apoptosis; type II cell death or autophagic cell death; and type III cell death or necrosis ( 1, 2 ). Lysosomes were known to be involved in autophagy ( 3, 4 ) and, after the massive rupture of lysosomes, in necrosis ( 5 ). In response to lethal stimuli, partial and selective lysosomal membrane permeabilization (LMP) occurs in certain lysosomes ( 6 ), and the redistributed lysosomal proteases induce apoptosis ( 7 ). Several lysosomal cathepsins (e.g., cathepsin B, D, L, etc.) have been implicated in apoptosis ( 8, 9 ). Recently, we found that, besides being a digestive enzyme, chymotrypsin is a novel member of lysosomal proteases ( 10 ). Chymotrypsin release as a consequence of LMP in a small proportion of lysosomes appears to play an important role in cell apoptosis, irrespective of whether the apoptosis is triggered by extrinsic or intrinsic mediators ( 11 ). Further investigation indicated that, during the initiation of apoptosis, both the activation of caspase 8 and the existence of Bid are necessary for the induction of LMP, but the precise mechanism of LMP remains to be elucidated. Abstract Upon apoptotic stimuli, lysosomal proteases, including cathepsins and chymotrypsin, are released into cytosol due to lysosomal membrane permeabilization (LMP), where they trigger apoptosis via the lysosomal-mitochondrial pathway of apoptosis. Herein, the mechanism of LMP was investigated. We found that caspase 8-cleaved Bid (tBid) could result in LMP directly. Although Bax or Bak might modestly enhance tBid-triggered LMP, they are not necessary for LMP. To study this further, large unilamellar vesicles (LUVs), model membranes mimicking the lipid constitution of lysosomes, were used to reconstitute the membrane permeabilization process in vitro. We found that phosphatidic acid (PA), one of the major acidic phospholipids found in lysosome membrane, is essential for tBid-induced LMP. PA facilitates the insertion of tBid deeply into lipid bilayers, where it undergoes homo-oligomerization and triggers the formation of highly curved nonbilayer lipid phases. These events induce LMP via pore formation mechanisms because encapsulated fl uorescein-conjugated dextran (FD)-20 was released more significantly than FD-70 or FD-250 from LUVs due to its smaller molecular size. On the basis of these data, we proposed tBid-PA interactions in the lysosomal membranes form lipidic pores and result in LMP. We further noted that chymotrypsin-cleaved Bid is more potent than tBid at binding to PA, inserting into the lipid bilayer, and promoting LMP. This amplifi cation mechanism likely contributes to the culmination of apoptotic signaling. — Zhao, K., H. Zhou, X. Zhao, D. W. Wolff, Y. Tu, H. Liu, T. Wei, and F. Yang. Phosphatidic acid mediates the targeting of tBid to induce lysosomal membrane permeabilization and apoptosis. J. Lipid Res . 2012. 53: 2102–2114.