Bcr-Abl Exerts Its Antiapoptotic Effect Against Diverse Apoptotic Stimuli Through Blockage of Mitochondrial Release of Cytochrome C and Activation of Caspase-3

Hotspots of single-strand DNA “breakome” are enriched at transcriptional start sites of genes

Single-strand breaks (SSBs) represent one of the most common types of DNA damage, yet not much is known about the genome landscapes of this type of DNA lesions in mammalian cells. Here, we found that SSBs are more likely to occur in certain positions of the human genome—SSB hotspots—in different cells of the same cell type and in different cell types. We hypothesize that the hotspots are likely to represent biologically relevant breaks. Furthermore, we found that the hotspots had a prominent tendency to be enriched in the immediate vicinity of transcriptional start sites (TSSs). We show that these hotspots are not likely to represent technical artifacts or be caused by common mechanisms previously found to cause DNA cleavage at promoters, such as apoptotic DNA fragmentation or topoisomerase type II (TOP2) activity. Therefore, such TSS-associated hotspots could potentially be generated using a novel mechanism that could involve preferential cleavage at cytosines, and their existence is consistent with recent studies suggesting a complex relationship between DNA damage and regulation of gene expression.

[MET/ERK and MET/JNK Pathway Activation Is Involved in BCR-ABL Inhibitor-resistance in Chronic Myeloid Leukemia]

Resistance to the breakpoint cluster region-abelson (BCR-ABL) tyrosine kinase inhibitor (TKI), imatinib, poses a major problem in the treatment of chronic myeloid leukemia (CML). Imatinib resistance often results from a secondary mutation in BCR-ABL1. However, the basis of this BCR-ABL1-independent resistance in the absence of such mutation remains to be elucidated. The aim of the present study is to identify the mechanism of imatinib resistance in CML. To gain insight into BCR-ABL1-independent imatinib resistance mechanisms, we performed an array-based comparative genomic hybridization. We identified various resistance-related genes, focusing on the receptor tyrosine kinase MET. Treatment with an MET inhibitor resensitized K562/IR cells to BCR-ABL TKIs. A treatment combining imatinib and a MET inhibitor in K562/IR cells inhibited extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK) activation, but did not affect AKT activation. Moreover, the combination of MET inhibitor and imatinib suppressed tumor growth in vivo. These results indicate that the activation of MET/ERK and MET/JNK are potential mechanisms of BCR-ABL TKI resistance. Our findings provide new and important information concerning the mechanisms of imatinib resistance in CML, and reveal new proteins potentially involved in BCR-ABL TKI resistance.

Synergistic interactions between DMAG and mitogen-activated protein kinase kinase 1/2 inhibitors in Bcr/abl+ leukemia cells sensitive and resistant to imatinib mesylate

PURPOSE

To characterize interactions between the heat shock protein 90 antagonist 17-dimethylaminoethylamino-17-demethoxygeldanamycin (DMAG) and the mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase 1/2 inhibitor PD184352 in Bcr/abl(+) leukemia cells sensitive and resistant to imatinib mesylate.

EXPERIMENTAL DESIGN

K562 and LAMA 84 cells were exposed to varying concentrations of DMAG and PD184352 for 48 hours; after which, mitochondrial integrity, caspase activation, and apoptosis were monitored. Parallel studies were done in imatinib mesylate-resistant cells, including BaF3 cells transfected with plasmids encoding clinically relevant Bcr/abl mutations conferring imatinib mesylate resistance (e.g., E255K, M351T, and T315I) and primary CD34(+) bone marrow cells from patients refractory to imatinib mesylate.

RESULTS

Cotreatment of Bcr/abl(+) cells with minimally toxic concentrations of DMAG and PD184352 resulted in synergistic induction of mitochondrial injury (cytochrome c release and Bax conformational change), events associated with the pronounced and sustained inactivation of ERK1/2 accompanied by down-regulation of Bcl-x(L). Conversely, cells ectopically expressing Bcl-x(L) displayed significant protection against PD184352/DMAG-mediated lethality. This regimen effectively induced apoptosis in K562 cells overexpressing Bcr/abl, in BaF3 cells expressing various clinically relevant Bcr/abl mutations, and in primary CD34(+) cells from patients resistant to imatinib mesylate, but was relatively sparing of normal CD34(+) bone marrow cells.

CONCLUSIONS

A regimen combining the heat shock protein 90 antagonist DMAG and the mitogen-activated protein kinase/ERK kinase 1/2 inhibitor potently induces apoptosis in Bcr/abl(+) cells, including those resistant to imatinib mesylate through various mechanisms including Bcr/abl kinase mutations, through a process that may involve sustained ERK1/2 inactivation and Bcl-x(L) down-regulation. This strategy warrants further attention in Bcr/abl(+) hematopoietic malignancies, particularly those resistant to Bcr/abl kinase inhibitors.

Fusion tyrosine kinases: a result and cause of genomic instability

Reciprocal chromosomal translocations may arise as a result of unfaithful repair of spontaneous DNA double-strand breaks, most probably induced by oxidative stress, radiation, genotoxic chemicals and/or replication stress. Genes encoding tyrosine kinases are targeted by these mechanisms resulting in the generation of chimera genes encoding fusion tyrosine kinases (FTKs). FTKs display transforming activity owing to their constitutive kinase activity causing deregulated proliferation, apoptosis, differentiation and adhesion. Moreover, FTKs are able to facilitate DNA repair, prolong activation of G(2)/M and S cell cycle checkpoints, and elevate expression of antiapoptotic protein Bcl-X(L), making malignant cells less responsive to antitumor treatment. FTKs may also stimulate the generation of reactive oxygen species and enhance spontaneous DNA damage in tumor cells. Unfortunately, FTKs compromise the fidelity of DNA repair mechanisms, which contribute to the accumulation of additional genetic abnormalities leading to the resistance to inhibitors such as imatinib mesylate and malignant progression of the disease.

Disruption of the inhibitor of apoptosis protein survivin sensitizes Bcr-abl-positive cells to STI571-induced apoptosis

The Bcr-abl oncogene induces hematopoietic cell transformation and protects cells from apoptosis; however, the mechanisms whereby Bcr-abl blocks apoptosis are poorly defined. We examined whether the inhibitor of apoptosis protein (IAP) family, in particular survivin, are regulated by Bcr-abl. Overexpression of Bcr-abl in Mo7e or BaF3 hematopoietic cells elevated survivin mRNA and protein concomitant with a 4-fold increase in survivin promoter activity. The region of the survivin promoter responding to Bcr-abl was narrowed down to a 116 bp fragment between nucleotides -1,194 and -1,078. The IAP family member IAP-like protein-2 was also up-regulated by Bcr-abl. Disruption of Bcr-abl in Bcr-abl-transduced BaF3 cells by small interfering RNA resulted in 3- to 4-fold reduction in survivin protein confirming the link between Bcr-abl and survivin. Survivin disruption in Bcr-abl-transduced Mo7e cells, or in K562 cells that endogenously express Bcr-abl, by transfection with dominant-negative or antisense survivin constructs promoted apoptosis induced by the Bcr-abl tyrosine kinase inhibitor STI571, which was accompanied by caspase-dependent cleavage of Bcr-abl, mitochondrial membrane potential disruption, and enhanced mitochondrial cytochrome c release. Although ectopic survivin protected K562 cells from apoptosis induced by STI571, it did not protect cells from apoptosis induced either by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) or the combination of TRAIL plus Hemin. Our results identify a new signal pathway downstream of Bcr-abl, in addition to the Bcl-2 family involved in the antiapoptotic effects of Bcr-abl, and suggest that anti-survivin therapy may have utility in patients with chronic myelogenous leukemia.

BLM helicase is activated in BCR/ABL leukemia cells to modulate responses to cisplatin

Bloom protein (BLM) is a 3'-5' helicase, mutated in Bloom syndrome, which plays an important role in response to DNA double-strand breaks and stalled replication forks. Here, we show that BCR/ABL tyrosine kinase, which also modulates DNA repair capacity, is associated with elevated expression of BLM. Downregulation of BLM by antisense cDNA or dominant-negative mutant inhibits homologous recombination repair (HRR) and increases sensitivity to cisplatin in BCR/ABL-positive cells. Bone marrow cells from mice heterozygous for BLM mutation, BLM(Cin/+), transfected with BCR/ABL display increased sensitivity to cisplatin compared to those obtained from the wild-type littermates. BCR/ABL promotes interactions of BLM with RAD51, while simultaneous overexpression of BLM and RAD51 in normal cells increases drug resistance. These data suggest that BLM collaborates with RAD51 to facilitate HRR and promotes the resistance of BCR/ABL-positive leukemia cells to DNA-damaging agents.

MEK kinase 1 mediates the antiapoptotic effect of the Bcr-Abl oncogene through NF-κB activation

Bcr-Abl tyrosine kinase, a chimeric oncoprotein responsible for chronic myelogenous leukemia, constitutively activates several signal transduction pathways that stimulate cell proliferation and prevent apoptosis in hematopoietic cells. The antiapoptotic function of Bcr-Abl is necessary for hematopoietic transformation, and also contributes to leukemogenesis. Herein, we show for the first time that cell transformation induced by Bcr-Abl leads to increased expression and kinase activity of MEK kinase 1 (MEKK1), which acts upstream of the c-Jun N-terminal kinase (JNK), extracellular signal regulated kinase (ERK) and NF-kappaB signaling pathways. Inhibition of MEKK1 activity using a dominant-negative MEKK1 mutant (MEKK1km) diminished the ability of Bcr-Abl to protect cells from genotoxin-induced apoptosis, but had no effect on the proliferation of Bcr-Abl-transformed cells. Expression of MEKK1km also reduced NF-kappaB activation, and inhibited antiapoptotic c-IAP1 and c-IAP2 mRNA expression in response to the genotoxin. By contrast, neither JNK nor ERK activation was affected. These results indicate that MEKK1 is a downstream target of Bcr-Abl, and that the antiapoptotic effect of Bcr-Abl in chronic myelogenous leukemia cells is mediated via the MEKK1-NF-kappaB pathway.

BCR/ABL regulates response to DNA damage: the role in resistance to genotoxic treatment and in genomic instability

BCR/ABL regulates cell proliferation, apoptosis, differentiation and adhesion. In addition, BCR/ABL can induce resistance to cytostatic drugs and irradiation by modulation of DNA repair mechanisms, cell cycle checkpoints and Bcl-2 protein family members. Upon DNA damage BCR/ABL not only enhances reparation of DNA lesions (e.g. homologous recombination repair), but also prolongs activation of cell cycle checkpoints (e.g. G2/M) providing more time for repair of otherwise lethal lesions. Moreover, by modification of anti-apoptotic members of the Bcl-2 family (e.g. upregulation of Bcl-x(L)) BCR/ABL provides a cytoplasmic 'umbrella' protecting mitochondria from the 'rain' of apoptotic signals coming from the damaged DNA in the nucleus, thus preventing release of cytochrome c and activation of caspases. The unrepaired and/or aberrantly repaired (but not lethal) DNA lesions resulting from spontaneous and/or drug-induced damage can accumulate in BCR/ABL-transformed cells leading to genomic instability and malignant progression of the disease. Inhibition of BCR/ABL kinase activity by STI571 (Gleevec, imatinib mesylate) reverses drug resistance and, in combination with standard chemotherapeutics can exert strong anti-leukemia effect.

Proteinase-3, a serine protease which mediates doxorubicin-induced apoptosis in the HL-60 leukemia cell line, is downregulated in its doxorubicin-resistant variant

We report here that expression of proteinase 3 (PR3), a serine protease, is down-regulated in the HL60/ADR multidrug resistant variant of the human myelogenous leukemia cell line HL-60, and that down-regulation of PR3 is associated with doxorubicin (DOX) resistance in these cells. To determine whether PR3 is involved in DOX-induced apoptosis in HL-60 cells, and whether its loss causes resistance to DOX, we inhibited PR3 expression by an anti-sense PR3 oligodeoxynucleotide and showed that inhibition of PR3 expression results in a significant reduction in DOX-induced DNA fragmentation and increased resistance to DOX-induced apoptosis. Our results revealed that PR3-mediated DOX-induced apoptosis in HL-60 cells is independent of the loss of mitochondrial membrane potential (deltapsi(m)) and activation of the caspase-8 and -9 pathways. Moreover, while PR3 is involved in the cleavage of caspase-3, PR3-mediated DOX-induced DNA fragmentation and apoptosis were not prevented by a specific inhibitor of caspase-3. These data suggest that activation of caspase-3 alone is not sufficient to trigger PR3-mediated DOX-induced apoptosis. Treatment with an anti-PR3 oligomer significantly decreased reactive oxygen species (ROS) generation in cells treated with low concentrations of DOX, revealing a role for PR3 in enhancing production of DOX-induced ROS. Moreover, DOX-induced apoptosis at 0.001-0.01 microM was only inhibited in HL-60 cells pre-treated with the antioxidant N-acetyl-cysteine in the absence of anti-PR3, revealing that DOX-induced apoptosis in these cells is PR3- and ROS-dependent. Our results show that PR3 is involved in DOX-induced ROS-dependent apoptosis and that its loss is associated with resistance to DOX in HL-60 cells.

Tyrosine kinase oncogenes in normal hematopoiesis and hematological disease

Tyrosine kinase oncogenes are formed as a result of mutations that induce constitutive kinase activity. Many of these tyrosine kinase oncogenes that are derived from genes, such as c-Abl, c-Fes, Flt3, c-Fms, c-Kit and PDGFRbeta, that are normally involved in the regulation of hematopoiesis or hematopoietic cell function. Despite differences in structure, normal function, and subcellular location, many of the tyrosine kinase oncogenes signal through the same pathways, and typically enhance proliferation and prolong viability. They represent excellent potential drug targets, and it is likely that additional mutations will be identified in other kinases, their immediate downstream targets, or in proteins regulating their function.

Bax translocation is crucial for the sensitivity of leukaemic cells to etoposide-induced apoptosis

Bax translocation from cytosol to mitochondria is believed to be a crucial step for triggering cytochrome c release from mitochondria. However, it is unclear whether Bax translocation is associated with Bax induction by DNA damaging agents. The induction of Bax in response to DNA damaging agents has been considered to be linked with p53. In this study, we used the p53 negative human chronic myeloid leukaemia K562 cell line. Bax up-regulation occurred at the whole cell level after DNA damage induced by etoposide. However, after incubation with etoposide, Bax failed to translocate to mitochondria and as a result, the apoptotic process was blocked. A Bax stable transfectant, the K/Bax cell line, expressed more Bax protein in the cytosol, mitochondria and nuclei. This Bax overexpression induced cytochrome c release, a reduction of cytochrome c oxidase activity and mitochondrial membrane potential (Delta(Psi)m). However, Bax-induced apoptosis was blocked downstream of mitochondria in K562 cells. The increased levels of mitochondrial Bax sensitized cells to etoposide-induced activation of caspases-2, -3 and -9 and apoptosis. However, after transient transfection with the Apaf-1 gene, K/Bax cells were sensitized to etoposide-induced caspase activation and apoptosis to a larger extent compared with Bax or Apaf-1 transfection alone. We therefore conclude that two mechanisms contribute to the resistance of K562 cells to etoposide-induced apoptosis; firstly failure of Bax targeting to mitochondria and, secondly, deficiency of Apaf-1. Uncoupling of Bax translocation from Bax induction can occur in response to etoposide-induced DNA damage.

NF-kappaB/RelA transactivation is required for atypical protein kinase C iota-mediated cell survival

In chronic myelogenous leukemia (CML), the oncogene bcr-abl encodes a dysregulated tyrosine kinase that inhibits apoptosis. We showed previously that human erythroleukemia K562 cells are resistant to antineoplastic drug (taxol)-induced apoptosis through the atypical protein kinase C iota isozyme (PKC iota), a kinase downstream of Bcr-Abl. The mechanism(s) by which PKC iota mediates cell survival to taxol is unknown. Here we demonstrate that PKC iota requires the transcription factor nuclear factor-kappaB (NF-kappaB) to confer cell survival. At apoptosis-inducing concentrations, taxol weakly induces IkappaB(alpha) proteolysis and NF-kappaB translocation in K562 cells, but potently induces its transcriptional activity. Inhibition of NF-kappaB activity (by blocking IkappaB(alpha) degradation) significantly sensitizes cells to taxol-induced apoptosis. Likewise, K562 cells expressing antisense PKC iota mRNA or kinase dead PKC iota (PKC iota-KD) are sensitized to taxol; these cells are rescued from apoptosis by NF-kappaB overexpression. Expression of constitutively active PKC iota (PKC iota-CA) upregulates NF-kappaB transactivation and rescues cells from apoptosis in the absence of Bcr-Abl tyrosine kinase activity. Using a chimeric GAL4-RelA transactivator, we find that taxol potently activates GAL4-RelA-dependent transcription. This activation was further upregulated by expression of PKC iota-CA and inhibited by expression of PKC iota-KD. Our results indicate that RelA transactivation is an important downstream target of the PKC iota-mediated Bcr-Abl signaling pathway and is required for resistance to taxol-induced apoptosis.

Regulation of cell death by the Abl tyrosine kinase

The c-abl proto-oncogene encodes a protein tyrosine kinase that is distributed in the nucleus and the cytoplasm of proliferating cells. In the nucleus, c-Abl activity is negatively regulated by the retinoblastoma protein (RB) and positively regulated by DNA damage signals. Activation of the c-Abl kinase by DNA damage requires the function of ATM, which regulates cell cycle checkpoint, DNA repair and apoptosis in response to DNA damage. Cells lacking c-Abl can activate cell cycle checkpoints and DNA repair, but show defects in apoptosis. The apoptosis defect of c-Abl deficient cells is correlated with a defect in the induction and activation of p73, which is a functional homologue of the p53 tumor suppressor protein and has pro-apoptotic activity. The inhibition of c-Abl by RB is consistent with RB's ability to block apoptosis; while the activation of c-Abl by ATM is consistent with ATM's ability to activate cell death. The oncogenic Bcr-Abl tyrosine kinase is a potent inhibitor of apoptosis, and it is retained exclusively in the cytoplasm of transformed cells. Interestingly, when Bcr-Abl is trapped inside of the nucleus through a combined disruption of its cytoplasmic retention and its nuclear export, this oncogenic Abl kinase induces apoptosis. Taken together, the current results support a role for the nuclear c-Abl tyrosine kinase in the regulation of apoptosis. Whether the cytoplasmic c-Abl kinase can actively inhibit apoptosis remains to be determined; however, a deliberate retention of c-Abl in the cytoplasm could potentially contribute to the attenuation of apoptosis response.

CGP57148B (STI-571) induces differentiation and apoptosis and sensitizes Bcr-Abl–positive human leukemia cells to apoptosis due to antileukemic drugs

The differentiation and apoptosis-sensitizing effects of the Bcr-Abl–specific tyrosine kinase inhibitor CGP57148B, also known as STI-571, were determined in human Bcr-Abl–positive HL-60/Bcr-Abl and K562 cells. First, the results demonstrate that the ectopic expression of the p185 Bcr-Abl fusion protein induced hemoglobin in the acute myeloid leukemia (AML) HL-60 cells. Exposure to low-dose cytosine arabinoside (Ara-C; 10 nmol/L) increased hemoglobin levels in HL-60/Bcr-Abl and in the chronic myeloid leukemia (CML) blast crisis K562 cells, which express the p210 Bcr-Abl protein. As compared with HL-60/neo, HL-60/Bcr-Abl and K562 cells were resistant to apoptosis induced by Ara-C, doxorubicin, or tumor necrosis factor-α (TNF-α), which was associated with reduced processing of caspase-8 and Bid protein and decreased cytosolic accumulation of cytochrome c (cyt c). Exposure to CGP57148B alone increased hemoglobin levels and CD11b expression and induced apoptosis of HL-60/Bcr-Abl and K562 cells. CGP57148B treatment down-regulated antiapoptotic XIAP, cIAP1, and Bcl-xL, without affecting Bcl-2, Bax, Apaf-1, Fas (CD95), Fas ligand, Abl, and Bcr-Abl levels. CGP57148B also inhibited constitutively active Akt kinase and NFκB in Bcr-Abl–positive cells. Attenuation of NFκB activity by ectopic expression of transdominant repressor of IκB sensitized HL-60/Bcr-Abl and K562 cells to TNF-α but not to apoptosis induced by Ara-C or doxorubicin. Importantly, cotreatment with CGP57148B significantly increased Ara-C– or doxorubicin-induced apoptosis of HL-60/Bcr-Abl and K562 cells. This was associated with greater cytosolic accumulation of cyt c and PARP cleavage activity of caspase-3. These in vitro data indicate that combinations of CGP57148B and antileukemic drugs such as Ara-C may have improved in vivo efficacy against Bcr-Abl–positive acute leukemia.

Early events in leukemogenesis in P190Bcr-abl transgenic mice

The activated tyrosine kinase, Bcr-abl, is implicated in a number of hematopoietic malignancies. The exact biological mechanism by which the kinases transforms cells is still not well delineated. Previous data has suggested that the inhibition of apoptosis and the deregulation of cell cycle progression as the result of P210Bcr-abl expression might contribute to leukemogenesis. In vitro systems in which Bcr-Abl is over-expressed have concluded that similar growth regulatory pathways are affected as a result of the expression of both P210 and P190Bcr-abl. Here, we utilized an in vitro P190Bcr-abl leukemia mouse model to dissect the early events that contribute to transformation by this isoform of Bcr-Abl. In this mouse model P190Bcr-abl is expressed as a low but physiologically relevant level in that all mice develop pre-B leukemia lymphomas. We show that cell cycle and apoptotic responses to DNA damage are intact in bone marrow and spleen cells of such animals. We also demonstrate a normal induction of p21WAF-1/CIP1 in both hematopoietic and non-hematopoietic tissue as a result of genotoxic stress. We suggest that P190Bcr-abl induced transformation is different than that of P210Bcr-abl.

Versatility of BCR/ABL-expressing leukemic cells in circumventing proapoptotic BAD effects

BAD, the proapoptotic member of the “BH3-only” subfamily of BCL-2 proteins, is inactivated by phosphorylation at serines 112 and 136 and by sequestration in the cytoplasm where it interacts with members of the 14-3-3 family. In BCR/ABL-expressing cells, BAD is constitutively phosphorylated and mainly cytoplasmic, whereas in cells expressing BCR/ABL mutants unable to protect from apoptosis, BAD is nonphosphorylated. We show here that both the wild-type (WT) and the S112A/ S136A double mutant (DM) BAD are more potent inducers of apoptosis in parental than in BCR/ABL-expressing 32D myeloid precursor cells. Stable lines of parental cells expressing DM BAD could not be established and most clones from WT BAD retrovirus-infected parental cells lost BAD expression. On IL-3 withdrawal from parental 32D cells, BAD was rapidly dephosphorylated by the serine-threonine phosphatase 1, and localized in the mitochondria, whereas it remained phosphorylated and did not localize to the mitochondria in the cohort of BCR/ABL-expressing cells escaping apoptosis induced by WT BAD. Moreover, these cells showed high levels of BCL-2 and BCL-XL expression. The cohort of BCR/ABL-expressing cells resistant to apoptosis induced by DM BAD showed only high levels of BCL-2 and BCL-XL. These findings suggest that BCR/ABL-expressing cells are more versatile than normal hematopoietic progenitors in counteracting the apoptotic potential of BAD, and raise the possibility that tumor cells activate multiple antiapoptotic pathways for survival in the face of death-inducing stimuli.

The role of STATs in myeloid differentiation and leukemia

Myeloid differentiation is a highly regulated process governed by various cytokines, such as EPO, TPO, G-CSF, IL-3, IL-5 and GM-CSF. These cytokines act in part through activation of the STAT transcription factor family. In particular, various isoforms of STAT3 and STAT5 are activated during myeloid differentiation in a cell-type and maturation-state dependent fashion. In vitro studies have shown that STAT proteins are essential for cytokine-regulated processes such as cellular proliferation, differentiation as well as survival. Similarly, various STAT knock-outs have highlighted the role of STATs in myeloid differentiation in vivo. STATs also appear to play an important role in various myeloid malignancies, which are characterized by arrested maturation and cytokine-independent proliferation of myeloid progenitors. Constitutive activation of STAT3 and/or STAT5 resulting in enhanced transcription of anti-apoptotic- cell-cycle progression genes is likely to contribute to the pathogenesis of various myeloid leukemia's. Oncogene (2000).

Susceptibility of HIV-1-TAT transfected cells to undergo apoptosis. Biochemical mechanisms

The effects of HIV-1 Tat protein on mitochondria membrane permeability and apoptosis were analysed in lymphoid cells. In this report we show that stable-transfected HIV-Tat cells are primed to undergo apoptosis upon serum withdrawal. This effect was observed in both the Jhan T cell line and the K562 cells, the latter expressing the bcr-abl chimeric gene, which confers resistance to apoptosis induced by different stimuli. Using a cytofluorimetric approach we have determined that serum withdrawal induces a disruption of the transmembrane mitochondrial potential (Deltapsim) followed by an increase of reactive oxygen species (ROS) and the subsequent DNA nuclear loss in K562-Tat cells but not in the K562-pcDNA cell line. These pre-apoptotic events were associated with the cleavage of the caspase-3, while the expression of Bcl-2, Bcl-XL and Bax proteins was not affected by the presence of Tat. Regardless of the steady state of the Bax protein, we found that in both K562 and K562-Tat cells, this protein is located in the nucleus, but after serum withdrawal its localization was mainly in the cytoplasm. The activity of caspase-3 detected in K562-Tat cells after serum withdrawal paralleled with the mitochondria permeability transition. Nevertheless, in Jhan-Tat cells the inhibition of this caspase with the specific inhibitor, z-DEVD-cmk, did not affect the disruption of the mitochondria potential induced by serum withdrawal. Interestingly, we found that HIV-Tat protein accumulates at the mitochondria in the K562-Tat cells cultured under low serum conditions, and this mitochondrial localization correlated with the Deltapsim disruption detected in these cells. In addition, HIV-1 Tat protein synergies with protoporphyrin IX (PPIX), a ligand of the mitochondrial benzodiazepine receptor, in the induction of apoptosis in both Jhan and K562 cells. Thus, HIV-1 Tat protein may induce apoptosis by a mechanism that involves mitochondrial PT and may contribute to the lymphocyte depletion seen in AIDS patients.