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Apoptin, a small protein from the chicken anemia virus, has attracted attention because of its specificity in killing tumor cells. Localization of apoptin in the nucleus of tumor cells has been shown to be vital for proapoptotic activity, however, targeted expression of apoptin in the nucleus of normal cells does not harm the cells, indicating that nuclear localization of apoptin is insufficient for its cytotoxicity. Here, we demonstrate for the first time that apoptin interacts with the SH3 domain of p85, the regulatory subunit of phosphoinositide 3-kinase (PI3-K), through its proline-rich region. Apoptin derivatives devoid of this proline-rich region do not interact with p85, are unable to activate PI3-K, and show impaired apoptosis induction. Moreover, apoptin mutants containing the proline-rich domain are sufficient to elevate PI3-K activity and to induce apoptosis in cancer cells. Downregulation of p85 leads to nuclear exclusion of apoptin and impairs cell death induction, indicating that interaction with the p85 PI3-K subunit essentially contributes to the cytotoxic activity of apoptin.
Apoptin, a 14 kDa protein encoded by VP3 gene of chicken anemia virus, represents a novel potential anticancer therapeutic, because it induces apoptotic death specifically in tumor but not normal cells (Fischer and Schulze-Osthoff, 2005; Alvisi et al., 2006). The mechanism by which apoptin is able to distinguish between tumor and normal cells remains to be elucidated. The subcellular localization of apoptin appears to be important for the selective toxicity towards transformed and cancer cells. In cells resistant to apoptin-mediated cell death, the protein has a cytoplasmic localization, whereas in sensitive cells, apoptin is found in the nucleus (Danen-Van Oorschot et al., 2003). Apoptin was described to be specifically phosphorylated in transformed cells by an unknown kinase, which was proposed to be important for the nuclear localization and apoptotic activity of apoptin (Rohn et al., 2002). However, it was also shown that a C-terminally truncated apoptin mutant, in which the phosphorylation site was deleted, was still able to translocate to the nucleus and induce apoptosis (Guelen et al., 2004). Thus, the interaction with other molecules or additional modifications of apoptin that are not present in nontransformed cells may be required for the selective cytotoxicity of apoptin.
Employing a mass spectrometric approach, we previously found that apoptin interacts with the p85 regulatory subunit of phosphoinositide 3-kinase (PI3-K) (Maddika et al., 2007). Class I PI3-Ks play a central role in various regulatory processes, such as cell growth, survival and differentiation, by converting phosphatidylinositol-4,5-phosphate to phosphatidylinositol-3,4,5-phosphate, leading to the activation of AKT kinase and other downstream effectors (Fruman et al., 1998; Vanhaesebroeck and Waterfield, 1999). To investigate the relevance of an interaction of apoptin with the p85 subunit of PI3-K, we performed glutathione S-transferase (GST)–apoptin pull-down assays using total cell extracts of different tumor cell lines, includingPC-3 prostate cancer cells, L929 mouse fibrosarcoma cells, 293 transformed human embryonic kidney cells and MCF-7 human breast cancer cells. We detected that apoptin interacts with the p85 subunit in all the cell lines tested (Figure 1a), indicating that interaction of apoptin with p85 is neither cell type- nor species-specific.
To identify and map the sites on apoptin responsible for interaction with PI3-K, PC-3 cells were transfected to express either full-length green fluorescent protein (GFP)-apoptin or various deletion mutants tagged with an N-terminal GFP (Figure 1b). Apoptin was immunoprecipitated with anti-GFP antibodies at 24 h post-transfection, and the immune complexes were analysed for interaction with PI3-K by immunoblotting usinganti-p85 antibodies. PI3-K was found in the immunoprecipitates of full-length apoptin and apoptin derivatives that harbored amino acids 74–100 (a proline-rich sequence), implying that this region of apoptin is important for interaction with PI3-K (Figure 1b). We then tested whether interaction of PI3-K with apoptin is essential for apoptin-induced cell death by assessing the percentage of apoptosis induced by the different mutants. In agreement with the interaction data, only those mutants that had the intact interaction site for PI3-K were able to induce apoptosis both in PC-3 and MCF-7 cells (Figure 1c). Interestingly, the mutant Ala-108, which was reported to be noncytotoxic due the substitution of a Thr-phosphorylation site (Rohn et al., 2002), was still able to induce significant apoptosis, as long as its interaction site with the PI3-K remained intact.
Next, we investigated the apoptin interaction site on p85, which is attached to the p110 catalytic subunit and, depending on its conformation, can act as a repressor or inducer of kinase activity. The p85 subunit contains an N-terminal SH3 domain, two proline-rich domains and a breakpoint cluster region homology domain (Figure 2a). p85 also has two SH2 domains (nSH2, cSH2) that are separated by a coiled-coil region, called the inter (i) SH2 domain, which mediates the stable dimerization with the p110 catalytic subunit. Under resting conditions, p85 inhibits PI3-K activity. The binding of tyrosine-phosphorylated proteins to the nSH2 domain of p85 inhibits PI3-K and is most frequently responsible for kinase activation, however, alternate mechanisms have been reported. For example, the conformational switch between p85 and p110 holoenzyme can also occur upon the interaction of signaling mediators with the SH3 or breakpoint cluster region homology domain (Liu et al., 1993; Prasad et al., 1993; Pleiman et al., 1994; Zheng et al., 1994). To investigate the apoptin interaction site of the p85 subunit, we cotransfected PC-3 cells with a vector encoding full-length apoptin, together with full-length p85 or various deletion mutants. Both full-length p85 and the mutant lacking the iSH2 domain were immunoprecipitated by the anti-p85 antibody, while other mutants were immunoprecipitated using anti-hemagglutinin antibodies. Immunodetection of apoptin in the immune complexes revealed that apoptin interaction required an intact SH3 domain of p85 (Figure 2a).
To determine the functional significance of the interaction of apoptin with the p85-regulatory subunit, we measured the PI3-K activity by using a nonradioactive ELISA-based method. We tested whether the activation of PI3-K is a direct consequence of apoptin’s interaction by observing the PI3-K activity in cells transfected with different apoptin deletion mutants (Figure 2b). Full-length apoptin induced six-fold activation, while the 1–89 mutant, which may contain only a partially preserved interaction site, weakly activated PI3-K. The mutants 1–111, 74–121, Ala-108 and Glu-108 were able to activate PI3-K similar to the levels of activation by the full-length apoptin. In contrast, the 1–73 and 104–121 mutants were unable to activate beyond basal levels, indicating that the intact p85 interaction site on apoptin is crucial for PI3-K activation (Figure 2b).
On the other hand, the observed PI3-K activity in cells cotransfected with full-length apoptin and p85 deletion mutants confirmed that the interaction of apoptin at the SH3 domain of p85 is essential for PI3-K activity (Figure 2c). The p85 mutants with both the SH3 and iSH2 domains had constitutive PI3-K activity in the presence of apoptin, whereas others lacking these domains were severely impaired in their activity (Figure 2c). Interestingly, the mutant N+I+C +iSH2, containing only the SH2 domains but lacking the SH3 domain, conferred relatively higher PI3-K activity compared to other deletion mutants and the control, indicating that the intact SH3 domain in the wild-type p85 protein might have an inhibitory action on its activity (Figure 2c). In control experiments, different PI3-K inhibitors, including wortmannin and LY294002, and expression of a dominant-negative PI3-K mutant reduced the apoptin-induced PI3-K activation to the basal levels (Figure 2d). Thus, the interaction of apoptin with the SH3 domain might lead to a conformational change of p85, converting it from an inhibitory to an active state.
To investigate the functional significance and the requirement of PI3-K activity towards apoptin-induced cell death, we used RNA interference and tested whether knocking down endogenous p85 would affect resistance to apoptin-induced cell death. In PC-3 and MCF-7 cells, the expression of p85 was completely abrogated by p85-specific small interfering(si) RNA but not by a control siRNA (Figure 3a), as assayed 30 h post-transfection. The assay revealed that cells lackingp85 expression were strongly resistant towards apoptin-induced cell death compared to control siRNA-expressing cells (Figure 3b). Similar observations were made in 293 cells (data not shown).
We also tested whether PI3-K inhibition affects subcellular localization of apoptin. Apoptin was mainly localized in the nucleus of PC-3 and MCF-7 cells, but in the presence of wortmannin, apoptin was found mainly distributed in the cytoplasm, as shown either by immunostaining and confocal laser microscopic imaging (Figure 4a) or by subcellular fractionation and western blotting(Figure 4c). The effect of PI3-K inhibition on the localization of apoptin was further studied usingp85 siRNA. Upon inhibition of p85 expression, apoptin was almost completely excluded from the nucleus (Figure 4b), indicating that PI3-K activity is necessary for not only apoptin-induced cell death, but also for the nuclear localization of the protein.
In summary, we demonstrate that apoptin interacts with the p85 subunit of PI3-K in various cell types. Apoptin specifically associates with the SH3 domain of p85, leading to the activation of PI3-K presumably by a conformational change. Importantly, downregulation of the p85 subunit by RNA interference or PI3-K inhibition severely impairs apoptin-induced cell death.
p85 specifically interacts with the proline-rich domain of apoptin localized between amino acids 80 and 90. The PKPPSK sequence (amino acids 81–86) of apoptin is a perfect consensus motif (PxxPxR/K) recognized by SH3 domains. Mutants lacking this particular proline-rich region were unable to interact with the p85, were not able to activate PI3-K pathway and were nontoxic. This clearly implies that apoptin interacts with SH3 domain of p85 and activates PI3-K. Previously, it has been shown that phosphorylation of Thr-108 is specific for tumor cells and is required for apoptin-induced cell death (Rohn et al., 2002). However, phosphorylation of apoptin was severely impaired in a mutant lacking amino acids 80–90, even though the residues within the phosphorylation site remained intact (Rohn et al., 2002). Our results explain this interesting observation by showing that amino acids 80–90 are vital for the interaction with PI3-K, and this interaction may be required for further activation of downstream molecules. Mutants of Thr-108, that is Ala-108 and Glu-108, were still cytotoxic, indicating that phosphorylation of apoptin is not of crucial importance.
Our data further suggest that the PI3-K pathway is involved in localization of apoptin. Inhibition of the PI3-K pathway using wortmannin or RNA interference retained apoptin in the cytoplasm and led to its nuclear exclusion, hence impairing cell death. PI3-K has been mainly implicated in lipid signaling at the cell membrane, but increasing evidence indicates that all components of the PI3-K pathway also reside in the nucleus (Neri et al., 2002). The functions regulated by nuclear PI3-K are still rather unclear. Although PI3-K is important for apoptin localization, it may not be a direct regulator, as we did not observe any direct phosphorylation of apoptin by PI3-K in in vitro kinase assays (data not shown). Apoptin phosphorylation may, however, be regulated by downstream effectors of the PI3-K pathway, a possibility that we are currently investigating. In conclusion, the data presented here not only advance our knowledge on the mechanisms that are activated by apoptin to kill cancer cells, but they also demonstrate that the prosurvival PI3-K/Akt pathway, frequently aberrantly active in cancer, may have additional functions in cell death propagation.
We thank J Downward and T Mustelin for valuable reagents. SM was funded by a studentship from the Manitoba Health Research Council (MHRC) and the CancerCare Manitoba Foundation (CCMF). EW was supported through ITMHRT. ML thankfully acknowledges the support by the CFI-Canada Research Chair program, MICH-, CIHR, CIHR-IPM-, CCMF- and MHRC-financed programs.