Acquired mutations in BAX confer resistance to BH3-mimetic therapy in acute myeloid leukemia

Randomized trials in acute myeloid leukemia (AML) have demonstrated improved survival by the BCL-2 inhibitor venetoclax combined with azacitidine in older patients, and clinical trials are actively exploring the role of venetoclax in combination with intensive chemotherapy in fitter patients with AML. As most patients still develop recurrent disease, improved understanding of relapse mechanisms is needed. We find that 17% of patients relapsing after venetoclax-based therapy for AML have acquired inactivating missense or frameshift/nonsense mutations in the apoptosis effector gene BAX. In contrast, such variants were rare after genotoxic chemotherapy. BAX variants arose within either leukemic or preleukemic compartments, with multiple mutations observed in some patients. In vitro, AML cells with mutated BAX were competitively selected during prolonged exposure to BCL-2 antagonists. In model systems, AML cells rendered deficient for BAX, but not its close relative BAK, displayed resistance to BCL-2 targeting, whereas sensitivity to conventional chemotherapy was variable. Acquired mutations in BAX during venetoclax-based therapy represent a novel mechanism of resistance to BH3-mimetics and a potential barrier to the long-term efficacy of drugs targeting BCL-2 in AML.

Non-coding RNAs and ferroptosis: potential implications for cancer therapy

Ferroptosis is a recently defined form of regulated cell death, which is biochemically and morphologically distinct from traditional forms of programmed cell death such as apoptosis or necrosis. It is driven by iron, reactive oxygen species, and phospholipids that are oxidatively damaged, ultimately resulting in mitochondrial damage and breakdown of membrane integrity. Numerous cellular signaling pathways and molecules are involved in the regulation of ferroptosis, including enzymes that control the cellular redox status. Alterations in the ferroptosis-regulating network can contribute to the development of various diseases, including cancer. Evidence suggests that ferroptosis is commonly suppressed in cancer cells, allowing them to survive and progress. However, cancer cells which are resistant to common chemotherapeutic drugs seem to be highly susceptible to ferroptosis inducers, highlighting the great potential of pharmacologic modulation of ferroptosis for cancer treatment. Non-coding RNAs (ncRNAs) are considered master regulators of various cellular processes, particularly in cancer where they have been implicated in all hallmarks of cancer. Recent work also demonstrated their involvement in the molecular control of ferroptosis. Hence, ncRNA-based therapeutics represent an exciting alternative to modulate ferroptosis for cancer therapy. This review summarizes the ncRNAs implicated in the regulation of ferroptosis in cancer and highlights their underlying molecular mechanisms in the light of potential therapeutic applications.

Clonal hematopoiesis, myeloid disorders and BAX-mutated myelopoiesis in patients receiving venetoclax for CLL

The BCL2 inhibitor venetoclax has established therapeutic roles in chronic lymphocytic leukemia (CLL) and acute myeloid leukemia (AML). As BCL2 is an important determinant of survival of both myeloid progenitor and B cells, we investigated whether clinical and molecular abnormalities arise in the myeloid compartment during long-term continuous venetoclax treatment of CLL in 89 patients (87 with relapsed/refractory CLL). Over a median follow-up of 75 (range 21-98) months, persistent cytopenias (≥1 of neutropenia, thrombocytopenia, anemia) lasting ≥4 months and unrelated to CLL occurred in 25 patients (28%). Of these patients, 20 (80%) displayed clonal hematopoiesis, including 10 with therapy-related myeloid neoplasms (t-MNs). t-MNs occurred exclusively in patients previously exposed to fludarabine-alkylator combination therapy with a cumulative 5-year incidence of 10.4% after venetoclax initiation, consistent with rates reported for patients exposed to fludarabine-alkylator combination therapy without venetoclax. To determine whether the altered myelopoiesis reflected the acquisition of mutations, we analyzed samples from patients with no or minimal bone marrow CLL burden (n = 41). Mutations in the apoptosis effector BAX were identified in 32% (13/41). In cellular assays, C-terminal BAX mutants abrogated outer mitochondrial membrane localization of BAX and engendered resistance to venetoclax killing. BAX-mutated clonal hematopoiesis occurred independently of prior fludarabine-alkylator combination therapy exposure and was not associated with t-MNs. Single-cell sequencing revealed clonal co-occurrence of mutations in BAX with DNMT3A or ASXL1. We also observed simultaneous BCL2 mutations within CLL cells and BAX mutations in the myeloid compartment of the same patients, indicating lineage-specific adaptation to venetoclax therapy.

Nanobody cocktails potently neutralize SARS-CoV-2 D614G N501Y variant and protect mice

Neutralizing antibodies are important for immunity against SARS-CoV-2 and as therapeutics for the prevention and treatment of COVID-19. Here, we identified high-affinity nanobodies from alpacas immunized with coronavirus spike and receptor-binding domains (RBD) that disrupted RBD engagement with the human receptor angiotensin-converting enzyme 2 (ACE2) and potently neutralized SARS-CoV-2. Epitope mapping, X-ray crystallography, and cryo-electron microscopy revealed two distinct antigenic sites and showed two neutralizing nanobodies from different epitope classes bound simultaneously to the spike trimer. Nanobody-Fc fusions of the four most potent nanobodies blocked ACE2 engagement with RBD variants present in human populations and potently neutralized both wild-type SARS-CoV-2 and the N501Y D614G variant at concentrations as low as 0.1 nM. Prophylactic administration of either single nanobody-Fc or as mixtures reduced viral loads by up to 104-fold in mice infected with the N501Y D614G SARS-CoV-2 virus. These results suggest a role for nanobody-Fc fusions as prophylactic agents against SARS-CoV-2.

Flexible Usage and Interconnectivity of Diverse Cell Death Pathways Protect against Intracellular Infection

Programmed cell death contributes to host defense against pathogens. To investigate the relative importance of pyroptosis, necroptosis, and apoptosis during Salmonella infection, we infected mice and macrophages deficient for diverse combinations of caspases-1, -11, -12, and -8 and receptor interacting serine/threonine kinase 3 (RIPK3). Loss of pyroptosis, caspase-8-driven apoptosis, or necroptosis had minor impact on Salmonella control. However, combined deficiency of these cell death pathways caused loss of bacterial control in mice and their macrophages, demonstrating that host defense can employ varying components of several cell death pathways to limit intracellular infections. This flexible use of distinct cell death pathways involved extensive cross-talk between initiators and effectors of pyroptosis and apoptosis, where initiator caspases-1 and -8 also functioned as executioners when all known effectors of cell death were absent. These findings uncover a highly coordinated and flexible cell death system with in-built fail-safe processes that protect the host from intracellular infections.

Subtle Changes in the Levels of BCL-2 Proteins Cause Severe Craniofacial Abnormalities

Apoptotic cell death removes unwanted cells and is regulated by interactions between pro-survival and pro-apoptotic members of the BCL-2 protein family. The regulation of apoptosis is thought to be crucial for normal embryonic development. Accordingly, complete loss of pro-survival MCL-1 or BCL-XL (BCL2L1) causes embryonic lethality. However, it is not known whether minor reductions in pro-survival proteins could cause developmental abnormalities. We explored the rate-limiting roles of MCL-1 and BCL-XL in development and show that combined loss of single alleles of Mcl-1 and Bcl-x causes neonatal lethality. Mcl-1^+/-;Bcl-x^+/- mice display craniofacial anomalies, but additional loss of a single allele of pro-apoptotic Bim (Bcl2l11) restores normal development. These findings demonstrate that the control of cell survival during embryogenesis is finely balanced and suggest that some human craniofacial defects, for which causes are currently unknown, may be due to subtle imbalances between pro-survival and pro-apoptotic BCL-2 family members.

An analysis of the role of follicular lymphoma-associated fibroblasts to promote tumor cell viability following drug-induced apoptosis

Treatment response of follicular lymphomas (FL) is highly variable. We, therefore, investigated the role of FL cancer-associated fibroblasts (CAFs) on tumor cell viability, in particular in response to treatment with cytotoxic drugs. Stromal cells outgrown from FL patients were characterized and pure CAF populations were co-cultivated with FL cells. To analyze fibroblast-mediated effects, cells in co-culture were treated with ABT-737 and Bortezomib. The adherent cell population was positive for all fibroblastic markers tested and showed increased mRNA-expression of the activation marker FAP. No effect on FL cell viability was noted when co-cultivating them with CAFs. However, stromal cells protected tumor cells from apoptosis in response to cytotoxic treatment. This might be explained by mRNA-induction of ABCC1 and ABCG2 and up-regulation of BCL2L1 in FL cells. Our finding of protective mechanisms mediated by CAFs is of pivotal impact for further studies of cytotoxic agents in FL.

Discrepant NOXA (PMAIP1) transcript and NOXA protein levels: a potential Achilles' heel in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an aggressive lymphoid neoplasm with transient response to conventional chemotherapy. We here investigated the role of the Bcl-2 homology domain 3-only protein NOXA for life–death decision in MCL. Surprisingly, NOXA (PMAIP1) mRNA and NOXA protein levels were extremely discrepant in MCL cells: NOXA mRNA was found to be highly expressed whereas NOXA protein levels were low. Chronic active B-cell receptor signaling and to a minor degree cyclin D1 overexpression contributed to high NOXA mRNA expression levels in MCL cells. The phoshatidyl-inositol-3 kinase/AKT/mammalian target of rapamycin pathway was identified as the major downstream signaling pathway involved in the maintenance of NOXA gene expression. Interestingly, MCL cells adapt to this constitutive pro-apoptotic signal by extensive ubiquitination and rapid proteasomal degradation of NOXA protein (T_½∼15–30 min). In addition to the proteasome inhibitor Bortezomib, we identified the neddylation inhibitor MLN4924 and the fatty acid synthase inhibitor Orlistat as potent inducers of NOXA protein expression leading to apoptosis in MCL. All inhibitors targeted NOXA protein turnover. In contrast to Bortezomib, MLN4924 and Orlistat interfered with the ubiquitination process of NOXA protein thereby offering new strategies to kill Bortezomib-resistant MCL cells. Our data, therefore, highlight a critical role of NOXA in the balance between life and death in MCL. The discrepancy between NOXA transcript and protein levels is essential for sensitivity of MCL to ubiquitin-proteasome system inhibitors and could therefore provide a druggable Achilles' heel of MCL cells.

Abstract 1717: High NOXA (PMAIP1) transcript levels combined with a short-lived NOXA protein primes mantle cell lymphoma (MCL) cells for death by inhibition of the ubiquitin proteasome system

MCL is an aggressive type of non-Hodgkin lymphoma with poor prognosis and short survival. The genetic hallmark of MCL is the t(11;14) translocation leading to an aberrant expression of the oncogene cyclin D1. Several other deregulated pathways also contribute to the pathogenesis of MCL, including the DNA damage response-, B-cell receptor- and PI3K/mTOR pathway. Deregulated oncogenic pathways not only induce proliferation but also frequently lead to a constitutive stress signal. A common response to cellular stress is the transcriptional up-regulation of pro-apoptotic genes. By analyzing mRNA expression of these genes in MCL cells we found high levels of the pro-apoptotic Bcl-2 family member NOXA (PMAIP1). NOXA transcript was significantly higher in MCL cell lines as well as in samples from MCL patients when compared to other cancer cell lines and PBMCs. To analyze if these high transcript levels depend on activation of oncogenic pathways, we treated the MCL cells with a panel of inhibitors of different signaling pathways and found that the inhibition of the PI3K/mTOR pathway led to a significant reduction of NOXA mRNA levels. These results indicate that this signaling axis is not only acting pro-survival but also mediates up-regulation of NOXA mRNA.

In contrast to the high transcript levels, NOXA protein is low in MCL cells. We found that NOXA protein is unstable with half-life times of 15-30 min. Inhibitors of the ubiquitin proteasome system (UPS) such as bortezomib, MG-132 and the cullin-ubiquitin ligase inhibitor MLN4924 stabilized NOXA and led to a strong accumulation of the protein. This was accompanied by a rapid induction of cell death. Interestingly, similar effects could be observed using the fatty acid synthase inhibitor orlistat indicating that fatty acid metabolism is also involved in the UPS. Reducing the high NOXA mRNA levels by treating the cells with the PI3K/mTOR dual-inhibitor Bez235 or using RNA interference prior to treatment with UPS inhibitors significantly reduced cell death and NOXA protein accumulation. These results indicate that the high NOXA mRNA levels are essential for the response of MCL cells to proteasomal inhibitors.

In summary, our results show that MCL have a constitutive signal mediated by the PI3K/mTOR pathway resulting in a high expression of NOXA mRNA. The cells survive by rapidly degrading the NOXA protein. Therefore, an effective strategy to kill MCL cells is to target the high NOXA protein turnover in these cells by inhibiting the proteasome ubiquitin system. This mechanism apparently underlies the in vitro activity of bortezomib which is already used in the clinic for treatment of patients with mantle cell lymphoma. Moreover, also other inhibitors such as MLN4924 or increasing the metabolic stress by orlistat might be promising to selectively kill MCL cells.

Citation Format: Michael A. Dengler, Andrea Weilbacher, Matthias Gutekunst, Annette M. Staiger, Heike Horn, German Ott, Heiko van der Kuip, Walter E. Aulitzky. High NOXA (PMAIP1) transcript levels combined with a short-lived NOXA protein primes mantle cell lymphoma (MCL) cells for death by inhibition of the ubiquitin proteasome system. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1717. doi:10.1158/1538-7445.AM2013-1717

Abstract 1721: Testicular germ cell tumors are hypersensitive to p53 activation based on their Oct-4/Noxa-mediated cellular context rather than on differential p53 activity

TGCTs are highly apoptosis-prone cancers which can be cured by Cisplatin-based chemotherapy. We have previously shown that p53 plays a major role in this phenotype. Furthermore, we demonstrated that sensitivity of these tumors is dictated by p53 accumulation in general and is not restricted to Cisplatin. Hypersensitivity seems to be an intrinsic feature of pluripotent TGCTs, since both differentiation and loss of the pluripotency factor Oct-4 induce resistance.

We sought to investigate the link between the Oct-4-mediated pluripotent context and the proapoptotic p53 response in these tumors. For this, we tested if loss of Oct-4 also reduces p53-mediated apoptosis upon non-genotoxic p53 activation. Indeed, RNAi-mediated silencing of Oct-4 significantly reduced sensitivity to the p53 activator Nutlin-3. Hence, we investigated a possible differential activation of p53 in the context of Oct-4-positive TGCT cells. For this, a qPCR system that allows simultaneous acquisition of 46 bona fide p53 target genes was used to detect p53’s transactivation capacity in NTERA-2D1 and 2102EP cells upon Cisplatin treatment in the presence and absence of Oct-4. In spite of p53’s central role in Cisplatin-induced apoptosis, no significant alterations in transactivation of p53 target genes were observed upon Cisplatin treatment in both cell lines. We also analyzed the cellular distribution of p53 upon Cisplatin treatment and found that p53 was not only accumulated in the nucleus but was increased to a similar extent in the cytoplasm. Importantly, RNAi-mediated silencing of Oct-4 did neither influence accumulation of p53 in the nucleus nor in the cytoplasm. These data indicate that Oct-4 does not directly modulate p53 activity but provides a cellular context that augments the proapoptotic activity of p53.

We have previously proposed that Oct-4-dependent high constitutive Noxa protein levels account for the low apoptotic threshold in TGCTs. In contrast to Cisplatin or Nutlin-3, treatment with MG132 resulted in apoptosis in Oct-4-depleted NTERA-2D1 and 2102EP cells. This was due to simultaneous accumulation of p53 and Noxa, as RNAi-mediated silencing of either protein rescued the cells from death. It is of importance that p53 knockdown did not compromise Noxa accumulation upon MG132 treatment, indicating that both p53 activation and the Noxa-mediated apoptosis-prone context are required for high sensitivity.

In conclusion, our data demonstrate that in spite of its impact on hypersensitivity, p53 activity is not altered in Oct-4-positive TGCT cells when compared to relatively resistant Oct-4-depleted cells. Although a robust p53 response is essential for hypersensitivity of TGCTs, its predominantly proapoptotic characteristic requires the apoptosis-prone cellular context of pluripotent Oct-4-positive TGCT cells, which includes high Noxa protein levels.

Citation Format: Matthias Gutekunst, Thomas Mueller, Andrea Weilbacher, Michael A. Dengler, Moshe Oren, Walter E. Aulitzky, Heiko van der Kuip. Testicular germ cell tumors are hypersensitive to p53 activation based on their Oct-4/Noxa-mediated cellular context rather than on differential p53 activity. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1721. doi:10.1158/1538-7445.AM2013-1721

Imaging of myocardial infarction using ultrasmall superparamagnetic iron oxide nanoparticles: a human study using a multi-parametric cardiovascular magnetic resonance imaging approach

AIMS

The purpose of this clinical trial was to investigate whether cardiovascular magnetic resonance imaging (CMR) using ferumoxytol (Feraheme™, FH), an ultrasmall superparamagnetic iron oxide nanoparticle (USPIO), allows more detailed characterization of infarct pathology compared with conventional gadolinium-based necrosis/fibrosis imaging in patients with acute myocardial infarction.

METHODS AND RESULTS

Fourteen patients who had experienced an acute ST-elevation myocardial infarction were included in this study. Following coronary angiography, a first baseline study (pre-FH) was performed followed by subsequent CMR studies (post-FH) 48 h after intravenous ferumoxytol administration. The CMR studies comprised cine-CMR, T(2)-weighted short tau inversion recovery spin echo imaging, T(2)-mapping, and T(1)-weighted late gadolinium enhancement (LGE) imaging. The median extent of short-axis in-plane LGE was 30% [inter-quartile range (IQR) 26-40%]. The median in-plane extent of T(2)-weighted 'hypoenhancement' in the region of myocardial infarction, which was not present prior to ferumoxytol administration in any patient, was 19% (IQR 14-22%; P < 0.001 compared with the extent of LGE). The median in-plane extent of areas showing signal void in T(2)-mapping images post-FH in the region of myocardial infarction was 16% (IQR 12-18%; P < 0.001 compared with the extent of LGE; P = 0.34 compared with the extent of T(2)-weighted hypoenhancement). A substantial drop in absolute T(2)-values was observed not only in the infarct core and peri-infarct zone, but also in the remote 'healthy' myocardium, although there was only a minor change in the skeletal muscle. Substantial ferumoxytol uptake was detected only in cultured macrophages, but not in peripheral blood monocytes from study patients.

CONCLUSION

We could demonstrate in humans that USPIO-based contrast agents enable a more detailed characterization of myocardial infarct pathology mainly by detecting infiltrating macrophages. Considering the multi-functionality of USPIO-based particles and their superior safety profile compared with gadolinium-based compounds, these observations open up new vistas for the clinical application of USPIO.

Abstract 4675: Fatty acid metabolism is a possible target for treatment of cyclin D1 over-expressing mantle cell lymphoma

Cyclin D1 over-expression has been linked to the development and progression of several types of cancer including mantle cell lymphoma (MCL), an aggressive type of B-cell lymphoma characterized by a t(11;14)(q13;q32) chromosomal translocation. Recent studies have shown that cyclin D1 is a multifunctional protein not only regulating the cell cycle but also affecting other cellular processes including DNA repair, apoptosis, as well as glucose, fatty acid, and lipid metabolism. In this study, we investigated the effects of the fatty acid synthase and lipase inhibitor Orlistat on cyclin D1 over-expressing MCL cell lines. In contrast to non-malignant peripheral blood lymphocytes and normal fibroblasts all MCL cell lines examined were sensitive to Orlistat. This enhanced sensitivity was dependent on cyclin D1 overexpression since siRNA mediated cyclin D1 knockdown almost completely rescued MCL cells from Orlistat induced apoptosis. Cell death upon Orlistat treatment was accompanied by loss of mitochondrial membrane potential and dependent on caspase activation since pre-incubation of the cells with the pan-caspase inhibitor zVAD-fmk completely blocked induction of apoptosis. We therefore investigated potential Orlistat-mediated changes in the expression levels of the pro- and anti-apoptotic Bcl-2 family proteins and found NOXA to be strongly induced whereas the expression of other Bcl-2 proteins did not change significantly. RNAi mediated knockdown of NOXA inhibited induction of cell death demonstrating the predominant role of this protein for the proapoptotic effect of Orlistat in MCL cells. Interestingly, silencing of cyclin D1 reduced the expression of NOXA upon Orlistat treatment further indicating a connection between cyclin D1, fatty acid metabolism, and the induction of NOXA. Since inhibition of fatty acid metabolism by Orlistat was found to disturb the balance between pro- and anti-apoptotic proteins in MCL cells we analyzed possible combinatory effects with Bcl-2 family modulators. Co-treatment of the cells with Orlistat and the BH3 mimetic ABT737 led to a rapid induction of cell death and an almost complete loss of cell viability in cyclin D1 over-expressing cells. A similar synergistic effect could be observed by combining Orlistat and 2-deoxy-D-glucose, a glycolysis inhibitor known to reduce MCL1 protein. These combinatory effects were selective for MCL cells as the same treatments had only minor or no effects on cell viability of primary PBMCs and fibroblasts from healthy donors. In summary, our results for the first time indicate that fatty acid metabolism may be an attractive target for therapy of cyclin D1 over-expressing MCL cells. Furthermore, these observations may contribute to the development of rational strategies combining fatty acid metabolism inhibitors with Bcl-2 family modulators for treatment of MCL.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4675. doi:1538-7445.AM2012-4675

Abstract 2002: OCT-3/4 expression is associated with high levels of the pro-apoptotic BH3 only protein NOXA in testicular germ cell tumors (TGCTs)

Embryonal carcinoma (EC) cells are the pluripotent precursors of differentiated structures of testicular germ cell tumors (TGCT) characterized by expression of the embryonal transcription factor OCT-3/4. Loss of OCT-3/4 during differentiation abrogates the extraordinary high sensitivity to cisplatin in these cells. We previously observed a constitutively high expression level of NOXA and a major role of this pro-apoptotic BH3-only protein for cisplatin hypersensitivity in the EC cell lines NTERA-2D1 and 2102EP. We now investigated if these high NOXA protein levels may be directly linked to the OCT-3/4 status in TGCT cells. Therefore, we analyzed NOXA protein expression in an extended TGCT cell line panel of 5 OCT-3/4 negative and 5 OCT-3/4 positive cell lines. Importantly, NOXA protein was highly correlated with OCT-3/4 levels and also with cisplatin sensitivity. These data indicate that constitutively high NOXA levels might be responsible for the low threshold for cisplatin-induced apoptosis in OCT-3/4 positive pluripotent cells. A direct link between OCT-3/4 and NOXA could also be demonstrated by RNAi mediated silencing experiments performed in NTERA-2D1 and 2102EP cells. Silencing of OCT-3/4 resulted in a significant downregulation of NOXA transcript and an almost complete loss of NOXA protein accompanied by a loss of cisplatin sensitivity. This was observed in both differentiation competent NTERA-2D1 cells as well as in differentiation incompetent 2102EP cells pointing to a direct OCT-3/4 dependent NOXA regulation rather than a bystander effect of differentiation upon loss of OCT-3/4. We further validated our in vitro data by comparing OCT-3/4 and NOXA expression levels in patient samples derived from ECs with other non-seminomatous GCTs, seminomas, and tumor entities from lung, breast, and ovary. In agreement with our in vitro observations, OCT-3/4 and NOXA were found to be highly expressed selectively in seminomas and ECs. This could also be confirmed on protein level in primary tissue samples derived from 5 ECs and 8 seminomas by western blot analysis. In conclusion, our data for the first time demonstrate a close correlation between OCT-3/4 and NOXA protein levels and strengthen the previously hypothesized role of constitutively high NOXA levels for the exquisite sensitivity of TGCTs to cisplatin.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2002. doi:1538-7445.AM2012-2002

p53 hypersensitivity is the predominant mechanism of the unique responsiveness of testicular germ cell tumor (TGCT) cells to cisplatin

Consistent with the excellent clinical results in testicular germ cell tumors (TGCT), most cell lines derived from this cancer show an exquisite sensitivity to Cisplatin. It is well accepted that the high susceptibility of TGCT cells to apoptosis plays a central role in this hypersensitive phenotype. The role of the tumor suppressor p53 in this response, however, remains controversial. Here we show that siRNA-mediated silencing of p53 is sufficient to completely abrogate hypersensitivity not only to Cisplatin but also to non-genotoxic inducers of p53 such as the Mdm2 antagonist Nutlin-3 and the proteasome inhibitor Bortezomib. The close relationship between p53 protein levels and induction of apoptosis is lost upon short-term differentiation, indicating that this predominant pro-apoptotic function of p53 is unique in pluripotent embryonal carcinoma (EC) cells. RNA interference experiments as well as microarray analysis demonstrated a central role of the pro-apoptotic p53 target gene NOXA in the p53-dependent apoptotic response of these cells. In conclusion, our data indicate that the hypersensitivity of TGCT cells is a result of their unique sensitivity to p53 activation. Furthermore, in the very specific cellular context of germ cell-derived pluripotent EC cells, p53 function appears to be limited to induction of apoptosis.

Abstract 4693: High expression levels of NOXA are important for p53-mediated hypersensitivity in testicular germ cell tumor (TGCT) cells

Consistent with the excellent clinical results in testicular germ cell tumors (TGCT), most cell lines derived from this cancer such as NTERA-2D1 show an exquisite sensitivity to Cisplatin. It is well accepted that the high susceptibility of TGCT cells to apoptosis is responsible for this hypersensitive phenotype. However, the role of the p53 pathway remains controversial.

Using RNA interference-mediated gene silencing, we systematically investigated the impact of DNA damage kinases upstream of p53, p53 itself, as well as apoptosis-related p53 targets on this hypersensitivity. Unexpectedly, neither knockdown of ATM, ATR, or DNA-PK alone nor ATM/ATR double knockdown or ATM/ATR/DNA-PK triple knockdown had any significant protective effect on survival of NTERA-2D1 cells upon Cisplatin. However, siRNA-mediated silencing of p53 was sufficient to completely abrogate hypersensitivity demonstrating that in these cells p53 acts preferentially as a pro-apoptotic factor. This is also supported by the finding that NTERA-2D1 cells were not only hypersensitive to Cisplatin but also to non-genotoxic inducers of p53 such as the Mdm2 antagonist Nutlin-3 and the proteasome inhibitor Bortezomib. Screening of different p53 targets revealed that the pro-apoptotic function of p53 was dependent on NOXA, since silencing of this p53 target almost completely mimicked the effect of the p53 knockdown. Importantly, both constitutive and Cisplatin-induced levels of NOXA were significantly reduced in resistant NTERA-2D1 cells obtained by either short-term differentiation or cultivation with increasing Cisplatin concentrations. Furthermore, comparison of a panel of p53 wildtype tumor cell lines revealed exceptionally high NOXA levels in the most sensitive cell line NTERA-2D1.

In conclusion, our data indicate that the hypersensitivity of TGCT cells is a result of their unique sensitivity to pro-apoptotic functions of p53 mediated by exceptionally high levels of NOXA.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4693. doi:10.1158/1538-7445.AM2011-4693

Abstract 1266: Oncogenic stress induced by Bcr-Abl over-activation leads to cell death mediated by a massively enhanced aerobic glycolysis

It is well established that normal cells respond to over-activation of oncogenes by induction of genetically encoded programs such as apoptosis or senescence. Therefore, too much activity of a growth promoting oncogene seems to disturb the cellular homeostasis, a phenomenon which is known as oncogenic stress. To investigate the molecular mechanisms behind this phenomenon we used cell clones overexpressing the oncogene Bcr-Abl and cultivated them continuously in the presence of the Bcr-Abl inhibitor Imatinib. Removal of Imatinib led to a robust induction of Bcr-Abl autophosphorylation and concomitant overstimulation of the Bcr-Abl downstream pathways PI3K/AKT-, RAS/MAPK-, and JAK/STAT. Importantly, this acute over-activation of Bcr-Abl resulted in a delayed non-apoptotic cell death starting not before 48 hours after Imatinib withdrawal. This cell death was preceded by a massively enhanced aerobic glycolysis and glutaminolysis (Warburg effect) and amino acid metabolism leading to an elevated cellular ATP and protein content. During the first 30 hours after Imatinib deprivation the enhanced metabolism had no effect on proliferation but resulted in cellular swelling, cytoplasmic vacuolization, and massive induction of ER stress markers indicating that Bcr-Abl over-activation induces ER stress response. However, cell death upon Bcr-Abl over-activation was not dependent on this severe ER stress since siRNA mediated knockdown of CHOP and BIM had no effect on cell viability. In contrast, both inhibition of aerobic glycolysis by 2-deoxy-glucose as well as reduction of glutamine concentration in the medium were sufficient to block Imatinib deprivation induced cell death. These data demonstrate that the Bcr-Abl over-activation mediated enhanced metabolism is responsible for oncogenic stress induced cell death. Interestingly, screening of 1200 marketed small molecule inhibitors (Prestwick chemical library) for their potential effects on oncogenic stress response uncovered selectively steroids to effectively antagonize both ER stress and cell death upon Bcr-Abl over-activation. These agents were also able to normalize cellular metabolism upon Imatinib withdrawal, supporting the view that “overshooting” metabolism upon oncogenic over-activation leads to cell death.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1266. doi:10.1158/1538-7445.AM2011-1266

Abstract 4522: Oncogenic stress-induced cell death following Imatinib deprivation in Bcr-Abl overexpressing Imatinib-resistant ALL cells

Imatinib resistance is a major problem in treatment of Bcr-Abl positive leukemias, particularly in patients with ALL and advanced CML. One of the major mechanisms of this resistance is overexpression of Bcr-Abl.

We investigated the effects of Imatinib deprivation in Bcr-Abl overexpressing Imatinib-resistant ALL cell lines. Removal of Imatinib from culture medium led to a huge induction of Bcr-Abl autophosphorylation and concomitant overstimulation of PI3K-, MAPK-, and JAK/STAT signalling leading to an elevated cell size and cellular protein content. Massive induction of cell death was then observed at later time points (48 hours after Imatinib deprivation) accompanied by loss of outer and inner mitochondrial membrane integrity. Using a protein kinase inhibitor library, we identified inhibitors of glycogen synthase kinase-3 (GSK3) and p38-MAPK as the most potent compounds which completely prevented induction of cell death upon removal of Imatinib whereas the same inhibitors had synergistic effects with Imatinib in Imatinib-sensitive ALL cells. In contrast to GSK3- and p38, inhibition of PI3K had no effect on oncogenic stress upon Imatinib withdrawal in the resistant clones.

Importantly, oncogenic stress-induced cell death could not be blocked by the pan-caspase inhibitor z-VAD-fmk whereas the RIP1 inhibitor Necrostatin partially rescued the cells upon Imatinib deprivation. We also detected a posttranslational modification of RIP1 upon Imatinib withdrawal that was completely absent in cells pre-treated with GSK3- or p38 inhibitors indicating a central role of RIP1 for oncogenic stress-induced cell death. In conclusion, we found a massive induction of a necroptosis-like cell death in Bcr-Abl overexpressing ALL cells as a consequence of oncogenic stress upon Imatinib deprivation. GSK3 and p38 turned out to play the most prominent role for oncogenic stress induced cell death. These data implicate that a discontinuous treatment with Imatinib (periods of Imatinib treatment followed by short periods without Imatinib) may prevent the appearance of cell clones with Bcr-Abl overexpression.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4522.