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Distinctive Roles of PHAP Proteins and Prothymosin- in a Death Regulatory Pathway Xuejun Jiang,1,2 Hyun-Eui Kim,1,2 Hongjun Shu,2 Yingming Zhao,2 Haichao Zhang,3 James Kofron,3 Jennifer Donnelly,3 Dave Burns,3 Shi-chung Ng,3 Saul Rosenberg,3 Xiaodong Wang1,2* A small molecule, -(trichloromethyl)-4-pyridineethanol (PETCM), was iden- tified by high-throughput screening as an activator of caspase-3 in extracts of a panel of cancer cells. PETCM was used in combination with biochemical fractionation to identify a pathway that regulates mitochondria-initiated caspase activation. This pathway consists of tumor suppressor putative HLA- DR–associated proteins (PHAP) and oncoprotein prothymosin- (ProT). PHAP proteins promoted caspase-9 activation after apoptosome formation, whereas ProT negatively regulated caspase-9 activation by inhibiting apoptosome for- mation. PETCM relieved ProT inhibition and allowed apoptosome formation at a physiological concentration of deoxyadenosine triphosphate. Elimination of ProT expression by RNA interference sensitized cells to ultraviolet irradiation– induced apoptosis and negated the requirement of PETCM for caspase activa- tion. Thus, this chemical-biological combinatory approach has revealed the regulatory roles of oncoprotein ProT and tumor suppressor PHAP in apoptosis. Cytochrome c release from mitochondria to the cytosol marks a defined moment in a mamma- lian cell’s response to a variety of apoptotic stimuli, in which the normal electron transfer chain is disrupted and caspases become active (1, 2). The released cytochrome c readily binds to apoptotic protease activating factor 1 (Apaf- 1) and induces a conformational change that allows stable binding of deoxyadenosine triphosphate/adenosine triphosphate (dATP/ ATP) to Apaf-1, an event that drives the forma- tion of a heptamer Apaf-1–cytochrome c com- plex called the apoptosome (3, 4). The apopto- some recruits and activates procaspase-9, which in turn activates the downstream caspases such as caspase-3, -6, and -7 (5, 6). These caspases cleave many intracellular substrates, ultimately leading to cell death (7). The mitochondrial caspase activation path- way is tightly regulated. One major regulatory step is at the release of cytochrome c from mitochondria, a process controlled by the Bcl-2 family of proteins (8, 9). The inhibitors of apo- ptosis (IAP) also regulate this pathway by di- rectly inhibiting caspase activity (2, 10). IAP proteins are antagonized by mitochondrial pro- teins such as Smac/Diablo and Omi/HtrA2 after they are released to cytoplasm (11–15). We have identified a death regulatory path- way by using a combined high-throughput chemical screen and biochemical fractionation approach. The pathway consists of tumor sup- pressor PHAP proteins and the oncoprotein ProT, each playing a distinctive role in regulat- ing apoptosome formation and activity. PETCM stimulates apoptosome forma- tion and caspase-3 activation. Caspase-3 in HeLa cell extracts can be activated by the ad- dition of 1 mM dATP through the mitochondria caspase activation pathway (16). To screen for small molecules that activate caspases, we screened 184,000 compounds for caspase-3 ac- tivators with HeLa cell extracts (S-100 frac- tion). The most potent positive hit from this large-scale screen was PETCM (Fig. 1A). This molecule has a simple chemical structure with no resemblance to dATP. Addition of PETCM to the S-100 fraction in the absence of exogenous dATP activated caspase-3 in a dose-dependent manner mea- sured by the liberation of colorimetric artifi- cial caspase-3 substrate (Fig. 1B). The effec- tive concentration for caspase-3 activation is between 0.1 and 0.2 mM. At 0.2 mM, PETCM was more efficient in activating caspase-3 than 1 mM dATP (Fig. 1C). In addition, cell extracts from many human can- cer lines, including colon cancer, prostate cancer, promyelocytic leukemia, T cell leu- kemia, bone marrow leukemia, malignant melanoma, lymphoma, and glioblastoma cells, were responsive to PETCM (17). To determine how this small molecule pro- motes activation of caspase-3, we analyzed ap- optosome formation by gel-filtration chromatog- raphy (Fig. 1D). Apaf-1 in a normal HeLa cell S-100 fraction was mostly in an inactive mono- meric form. After incubating with 1 mM dATP, most of the Apaf-1 shifted to a size of 1 million daltons, indicating apoptosome forma- tion. After the S-100 fraction was incubated with 0.2 mM PETCM, Apaf-1 exhibited a similar shift. The efficiency of apoptosome formation was better with 0.2 mM PETCM, which is con- sistent with the caspase-3 assay result (Fig. 1C). Stimulatory activity of PHAP proteins in the PETCM-initiated caspase activation pathway. To determine how PETCM pro- 1Howard Hughes Medical Institute, 2Department of Biochemistry, University of Texas Southwestern Med- ical Center, Dallas, TX 75390, USA. 3Abbott Labora- tories, D-460, AP10-LL, 100 Abbott Park Road, Abbott Park, IL 60064, USA. *To whom correspondence should be addressed. E- mail: xwang@biochem.swmed.edu Fig. 1. PETCM stimu- lates caspase-3 activa- tion and drives apo- ptosome formation in HeLa cell cytosol. (A) Structure of PETCM. (B) PETCM stimulates cas- pase-3 activity (DEVD activity) of HeLa S-100 in a dose-dependent manner. The colorimet- ric assay for capase-3 activity was performed as described (18). (C) Time course comparison of the stimulatory ef- fects of PETCM and dATP. PETCM anddATP were added as indicated. (D) PETCM drives apoptosome formation. HeLa S-100 was incubated with 1 mM dATP or 0.2 mM PETCM as indicated at 30°C for 1 hour. Mixtures were then resolved with a Superose 6 gel-filtration column. The column fractions were subjected to SDS–polyacrylamide gel electrophoresis and Apaf-1 in each fraction was detected with an immunoblot against Apaf-1. RESEARCH ARTICLE www.sciencemag.org SCIENCE VOL 299 10 JANUARY 2003 223 motes apoptosome formation and caspase-3 ac- tivation, we further fractionated HeLa cell ex- tracts with an anion-exchange column to search for proteins that mediate the PETCM effect (18). We obtained three fractions. The first fraction, Q-ft, flew through the column and contained cytochrome c (16); the second frac- tion, Q30, eluted with 0.3 M NaCl and con- tained Apaf-1 (19) and procaspase-9 (5); the third fraction, Q100, eluted with 1 M NaCl. When we incubated all three fractions together in the presence of 10 M dATP, the physiolog- ical concentration in cells, we observed little caspase-3 activation (Fig. 2A). In contrast, when we added 1 mM dATP, we observed robust caspase-3 activation. In the presence of 0.2 mM PETCM, we observed caspase-3 acti- vation at 10 M dATP, indicating that the combination of these three fractions mimicked what happened in the S-100 fraction. We ob- served no caspase-3 activation if we omitted dATP, indicating that PETCM function still requires dATP. As for the cell extracts (Fig. 1), the endogenous nucleotide was sufficient to support caspase-3 activation by PETCM. Omit- ting the Q-ft (cytochrome c) or Q30 (Apaf-1/ procaspase-9) fraction diminished the caspase-3 activating activity of PETCM. Surprisingly, omitting the Q100 fraction also reduced caspase-3 activating activity (Fig. 2A), suggest- ing that this fraction contained an unknown protein factor(s) that mediated the stimulating effect of PETCM. The stimulatory activity in the Q100 frac- tion was purified by chromatography (Fig. 2, B and C). For the final Mono Q column, a single activity peak at fractions 22 to 24 correlated with three proteins of 32, 29, and 35 kD. We identified these three proteins by mass spectrum analysis as putative HLA- DR–associated protein-1 (PHAPI, also called PP32 and LANP) (20–22), PHAPI2a (also called SSP29 and April) (23, 24), and a the- oretical protein in the National Center for Biotechnology Information database, which we named PHAPIII. The amino acid se- quences of the three proteins are more than 80% identical (fig. S1). They have a long acidic COOH-terminus and a leucine-rich re- gion in the middle (fig. S1). In mammalian cells, PHAP proteins are putative tumor sup- pressors (21, 25, 26), a function consistent with the proapoptotic activity identified here. ProT inhibits caspase-3 activation and PETCM antagonizes the inhibitory activity. Surprisingly, the stimulatory effect of PHAP proteins on caspase-3 activation was indepen- dent of PETCM (Fig. 3A). However, when the Q100 fraction, from which the PHAP proteins were purified, was also added, the stimulatory activity of the PHAP proteins was suppressed. PETCM reversed the suppression. This sug- gested that there was an inhibitory factor in the Q100 fraction as well. The PHAP proteins functioned only when the inhibitory factor was antagonized by PETCM. We purified a single inhibitory activity (Fig. 3, B and C) and iden- tified it by mass spectrum analysis as the onco- protein ProT (27, 28). ProT and PHAP distinctively regulate ap- optosome formation and activity. Recom- binant PHAPI stimulated caspase-3 activa- tion when added to the Q30 fraction plus cytochrome c and 10 M dATP. The activity was inhibited when we included recombinant ProT in the reaction mixture, and the inhibi- tory effect of ProT was reversed in the pres- ence of PETCM (Fig. 4A). In the presence of ProT, formation of apoptosome was efficient- ly blocked, and PETCM relieved this effect (Fig. 4B). In contrast, the presence of PHAPI did not affect the efficiency of apoptosome formation. Instead, we observed more acti- vated caspase-9, and increased caspase-9 was associated with apoptosome (Fig. 4C). Pull- Fig. 2. Identification of a stimulatory activity to mediate PETCM effect. (A) Fractions Q-ft, Q30, and Q100 were prepared (18), and the PETCM effect was examined with the frac- tions as follows. Fractions were mixed and different amounts of dATP and/or 0.2 mM PETCM were added as indicated. The reactions were carried out at 30°C for 1 hour. Caspase-3 activation of each mixture was measured by cleavage of [35S]methionine-labeled caspase-3 substrate, as described in (16). Procaspase-3 (PC3) and the cleaved products are marked by arrows. (B) Purification of stimulatory activity. Purification was performed as described (18). Activity of fractions from the final Mono Q column was assayed as follows. In a 20-l system, 3 l of Q30, 100 nM cytochrome c, 10 M dATP, and 0.2 mM PETCM were mixed in buffer A, and 2 l of each fraction was added as indicated. Caspase-3 activation of each mixture was measured as cleavage of PC3. (C) The final Mono Q fractions (30 l each) were resolved by SDS–polyacrylamide gel electrophoresis and the gel was stained with silver. The three purified proteins, PHAPI, PHAPI2a, and PHAPIII, are indicated. Fig. 3. Purification of inhibitory activity in the Q100 fraction. (A) Inhibitory activity in the Q100 fraction. The Q30 fraction, 100 nM cytochrome c, and 10 M dATP were mixed, and PHAP, the Q100 frac- tion, and/or PETCM were added as indi- cated. Caspase-3 activation of each mix- ture was measured as cleavage of pro- caspase-3 (PC3). (B) Purification of inhibitory activity. Purification was performed as described in (18). Activity of fractions from the final Mono Q chromatography was assayed as follows. The Q30 fraction, 100 nM cytochrome c, 10 M dATP, and 2 l of purified PHAP were mixed, and 4 l of each fraction was added as indicated. Caspase-3 activation of each mixture was measured as cleavage of PC3. (C) The final Mono Q fractions (10 l each) were resolved by SDS–polyacrylamide gel electrophoresis and the gel was stained with silver. R E S E A R C H A R T I C L E 10 JANUARY 2003 VOL 299 SCIENCE www.sciencemag.org 224 down experiments also showed more associ- ation of active caspase-9 with Apaf-1 in the presence of PHAPI (17). These results indi- cate that ProT and PHAP regulate caspase-3 activation at different steps. ProT inhibits caspase-3 activation by blocking apoptosome formation and therefore acts more upstream in this regulatory pathway; PHAPI does not affect apoptosome formation but accelerates its activity to promote more caspase-9 acti- vation. PETCM promotes caspase-3 activa- tion by removing the inhibition of ProT on apoptosome formation, allowing PHAPs to stimulate apoptosome activity. Interestingly, the PETCM effect cannot be reproduced in a reconstituted system containing purified Apaf-1, procaspase-9, cytochrome c, PHAP, and ProT. Therefore, an additional factor(s) present in the Q30 fraction is required (17). Elimination of ProT in vivo sensitizes HeLa cells to an apoptotic stimulus and bypasses PETCM action. To verify the apo- ptotic roles of PHAP and ProT in vivo, we used RNA interference (RNAi) to eliminate their expression in cells. RNAi against PHAP pro- teins was not successful, possibly because there are multiple forms of PHAP and they are stable proteins. RNAi against ProT did efficiently eliminate the ProT messenger RNA (mRNA) (Fig. 5A). Under this condition, we observed no apoptosis. However, when irradiated with ultra- violet (UV) light, the cells treated with ProT RNAi showed a higher rate of apoptosis (Fig. 5B). Twelve hours after UV irradiation, more than 70% of the ProT RNAi-treated cells showed apoptotic morphology, whereas control RNAi-treated cells showed only 25% cell death. Cell death correlated with the caspase-3 activation as higher caspase-3 activity was also observed in the ProT RNAi-treated cells (Fig. 5C). The RNAi experiment also confirmed that PETCM functions to antag- onize the inhibitory activity of ProT (Fig. 5D). Cell extracts from control RNAi-treat- ed cells were responsive to PETCM. In contrast, cell extracts from ProT RNAi- treated cells activated caspase-3 indepen- dently of PETCM. ProT and PHAP as apoptotic regulators. ProT is an oncoprotein required for cell prolif- eration (27, 28, 29–35). However, the biochem- ical mechanism for the oncogenic property of ProT was not clear (29). Our data indicate that one of the biochemical functions of ProT is to prevent apoptosome formation. Such a bio- chemical activity is consistent with its oncogen- ic function, because other previously known Fig. 4. Regulation of apoptosome by ProT and PHAP. (A) PHAP accelerates caspase-3 activation after PETCM an- tagonizes the inhibitory activity of ProT. The Q30 fraction, 100 nM cyto- chrome c, and 10 M dATP were mixed; 0.2 mM PETCM, 1 M recom- binant PHAPI (rPHAPI), and/or 2 M recombinant ProT (rProT) were added as indicated. Caspase-3 activation of each mixture was measured as cleavage of procaspase-3 (PC3). (B) ProT inhibits apoptosome formation and PETCM antagonizes the inhibitory activity. The Q30 fraction, 100 nM cytochrome c, and 10 M dATP were mixed and incubated at 30°C for 1 hour, with addition of 1 M ProT and 0.2 mM PETCM as indicated. Apoptosome formation of each reaction mixture was measured as described in Fig. 1. (C) PHAP enhances caspase-9 activation. The Q30 fraction, 100 nM cytochrome c, and 10 M dATP, in the absence or presence of 1 M rPHAPI as indicated, were mixed and incubated at 30°C for 1 hour. Apoptosome formation was measured as described in Fig. 1. Immunoblot analysis of both Apaf-1 and caspase-9 was performed. The 35-kD caspase-9 is the cleaved product of procaspase-9. Fig. 5. Elimination of ProT by RNAi-sensitized UV-induced apoptosis in HeLa cells. (A) Reverse transcriptase polymerase chain reaction (RT- PCR), showing elimination of ProT messenger by RNAi. Two days after transfection with ProT small interfering RNA (siRNA) or green fluorescent pro- tein (GFP) siRNA (18), RT-PCR of ProT was per- formed. RT-PCR of glyceraldehyde phosphate de- hydrogenase (GAPDH) was used as the control. (B) ProT RNAi sensitizes UV-induced cell death. Cells were treated with ProT or GFP RNAi. Top panel: Micrographs without UV treatment or 12 hours after UV irradiation. Bottom panel: Cell death counting with Hoechst staining at the in- dicated times after UV irradiation. (C) ProT RNAi increases UV-induced caspase-3 activation. Cells were treated with ProT or GFP RNAi and harvest- ed 8 hours after UV irradiation. Caspase-3 activity in the S-100 of RNAi-treated cells was measured by the fluorogenic caspase-3 assay (18). (D) Elim- ination of ProT by RNAi negates the PETCM re- quirement for caspase-3 activation. HeLa cells were transfected with ProT siRNA or GFP siRNA as indicated. After 2 days, cells were harvested and caspase-3 activity of the cell lysate was mea- sured after 2 hours of incubation at 30°C in the presence or absence of 0.2 mM PETCM as indi- cated. All the experiments were done at least three times with similar results. R E S E A R C H A R T I C L E www.sciencemag.org SCIENCE VOL 299 10 JANUARY 2003 225 negative regulators of apoptosis such as Bcl-2 (8, 9) and IAPs (10) have been shown to have oncogenic activities as well. The inhibition of apoptosome formation by ProT also offered an explanation for a long-standing puzzling obser- vation that up to a millimolar concentration of dATP is required to trigger efficient caspase-3 activation in vitro. The intracellular dATP un- der normal conditions is in the 10-M range and does not arise during apoptosis (36). The requirement for millimolar dATP also contra- dicts the direct binding studies with purified Apaf-1 and dATP. In this study, the dissocia- tion constant of dATP binding to Apaf-1 is at the micromolar level in the presence of cyto- chrome c, and micromolar amounts of dATP also efficiently stimulate caspase-3 activation in a reconstituted system containing purified Apaf-1, procaspase-9, and cytochrome c (3). It is clear now that most dATP is probably used for repressing ProT in HeLa cell S-100. When ProT is suppressed by PETCM, 10 M dATP is enough to trigger apoptosome formation and PHAP can subsequently accelerate the activity of the machinery. Unlike ProT, PHAP pro- teins function as tumor suppressors in mamma- lian cells to inhibit cell growth (21, 25, 26). They have been shown to inhibit protein phos- phatase 2A (37) and block histone acetylase (38). How these biochemical functions are linked to its cellular antigrowth function is not clear. But, in light of our finding that PHAP proteins promote apoptosis by accelerating caspase-9 activation, we suggest that it may inhibit cell growth by promoting apoptosis. In- terestingly, PHAP can interact with ataxin-1, a protein that is mutated in the neural degenera- tive disease spinocerebellar ataxia type 1 (22). This suggests a role of PHAP in the disease. Further, certain PHAP proteins are preferential- ly expressed in mouse cerebellum during its most active developmental period characterized by massive apoptosis (39–41). Because apo- ptosis and Apaf-1 are essential in this early brain developmental stage (42, 43), we suggest that PHAP, a stimulator of apoptosome activity, might also play a crucial role during brain development, a readily testable model. References and Notes 1. J. C. Goldstein et al., Nature Cell Biol. 2, 15 (2000). 2. X. Wang, Genes Dev. 15, 2922 (2001). 3. X. Jiang, X. Wang, J. Biol. Chem. 275, 31199 (2000). 4. D. Acehan et al., Mol. Cell 9, 423 (2002). 5. P. 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Cecconi et al., Cell 94, 727 (1998). 43. H. Yoshida et al., Cell 94, 739 (1998). 44. We thank F. Du and R. Harold for excellent technical support and Q. Liu for critical reading of the manu- script. Supported by the Howard Hughes Medical Institute, NIH grant GMRO1-57158, and Welch Foundation grant I-1412. SupportingOnline Material www.sciencemag.org/cgi/content/full/299/5604/223/ DC1 Materials and Methods Fig. S1 31 July 2002; accepted 29 October 2002 REPORTS Nanoparticle Assembly and Transport at Liquid-Liquid Interfaces Y. Lin,1 H. Skaff,1 T. Emrick,1* A. D. Dinsmore,2* T. P. Russell1* The self-assembly of particles at fluid interfaces, driven by the reduction in inter- facial energy, is well established. However, for nanoscopic particles, thermal fluc- tuations competewith interfacial energy and give rise to a particle-size–dependent self-assembly. Ligand-stabilized nanoparticles assembled into three-dimensional constructs at fluid-fluid interfaces, where the properties unique to the nanopar- ticleswere preserved. The small size of the nanoparticles led to aweak confinement of the nanoparticles at the fluid interface that opens avenues to size-selective particle assembly, two-dimensional phase behavior, and functionalization. Fluid interfaces afford a rapid approach to equilibrium and easy access to nanoparticles for subsequent modification. A photoinduced transformation is described in which nanoparticles, initially soluble only in toluene, were transported across an interface into water and were dispersed in the water phase. The characteristic fluorescence emission of the nanoparticles provided a direct probe of their spatial distribution. Directed self-assembly of nanoparticles opens new avenues of technology through the con- trolled fabrication of nanoscopic materials with unique optical, magnetic, and electronic properties (1–5). Ligand-stabilized colloidal nanoparticles are ideally suited to hierarchi- cal self-assembly, because the nanoparticle core dictates optical, electronic, or magnetic properties, whereas the surface-bound li- gands define the particle’s interactions with its surroundings. A fluid-fluid interface offers potential for such assembly (6, 7) and for the chemical manipulation of nanoparticles. At a fluid interface, the particles are highly mobile and rapidly achieve an equilibrium assembly. The rapid diffusion of nanoparticles and re- agents in either fluid also leads to very effi- cient interfacial chemistry. Surfaces of dis- persed droplets offer a substantially greater interfacial area than a planar interface. More- over, the size and shape of droplets can be controlled from microscopic to macroscopic 1Department of Polymer Science and Engineering, 2Department of Physics, University of Massachusetts, Amherst, MA 01003, USA. *To whom correspondence should be addressed. E- mail: tsemrick@mail.pse.umass.edu (T.E.); dinsmore@ physics.umass.edu (A.D.D.); russell@mail.pse.umass. edu (T.P.R.) R E S E A R C H A R T I C L E 10 JANUARY 2003 VOL 299 SCIENCE www.sciencemag.org 226
