Donia M. Moujalled

The MCL1 inhibitor S63845 is tolerable and effective in diverse cancer models.

Avoidance of apoptosis is critical for the development and sustained growth of tumours. The pro-survival protein myeloid cell leukemia 1 (MCL1) is overexpressed in many cancers, but the development of small molecules targeting this protein that are amenable for clinical testing has been challenging. Here we describe S63845, a small molecule that specifically binds with high affinity to the BH3-binding groove of MCL1. Our mechanistic studies demonstrate that S63845 potently kills MCL1-dependent cancer cells, including multiple myeloma, leukaemia and lymphoma cells, by activating the BAX/BAK-dependent mitochondrial apoptotic pathway. In vivo, S63845 shows potent anti-tumour activity with an acceptable safety margin as a single agent in several cancers. Moreover, MCL1 inhibition, either alone or in combination with other anti-cancer drugs, proved effective against several solid cancer-derived cell lines. These results point towards MCL1 as a target for the treatment of a wide range of tumours.

In TNF-stimulated cells, RIPK1 promotes cell survival by stabilizing TRAF2 and cIAP1, which limits induction of non-canonical NF-κB and activation of caspase-8.

RIPK1 is involved in signaling from TNF and TLR family receptors. After receptor ligation, RIPK1 not only modulates activation of both canonical and NIK-dependent NF-κB, but also regulates caspase-8 activation and cell death. Although overexpression of RIPK1 can cause caspase-8-dependent cell death, when RIPK1−/− cells are exposed to TNF and low doses of cycloheximide, they die more readily than wild-type cells, indicating RIPK1 has pro-survival as well as pro-apoptotic activities (1, 2). To determine how RIPK1 promotes cell survival, we compared wild-type and RIPK1−/− cells treated with TNF. Although TRAF2 levels remained constant in TNF-treated wild-type cells, TNF stimulation of RIPK1−/− cells caused TRAF2 and cIAP1 to be rapidly degraded by the proteasome, which led to an increase in NIK levels. This resulted in processing of p100 NF-κB2 to p52, a decrease in levels of cFLIPL, and activation of caspase-8, culminating in cell death. Therefore, the pro-survival effect of RIPK1 is mediated by stabilization of TRAF2 and cIAP1.

Targeting p38 or MK2 Enhances the Anti-Leukemic Activity of Smac-Mimetics.

Birinapant is a smac-mimetic (SM) in clinical trials for treating cancer. SM antagonize inhibitor of apoptosis (IAP) proteins and simultaneously induce tumor necrosis factor (TNF) secretion to render cancers sensitive to TNF-induced killing. To enhance SM efficacy, we screened kinase inhibitors for their ability to increase TNF production of SM-treated cells. We showed that p38 inhibitors increased TNF induced by SM. Unexpectedly, even though p38 is required for Toll-like receptors to induce TNF, loss of p38 or its downstream kinase MK2 increased induction of TNF by SM. Hence, we show that the p38/MK2 axis can inhibit or promote TNF production, depending on the stimulus. Importantly, clinical p38 inhibitors overcame resistance of primary acute myeloid leukemia to birinapant.

TAK1 is required for survival of mouse fibroblasts treated with TRAIL, and does so by NF-κB dependent induction of cFLIPL

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is known as a “death ligand”—a member of the TNF superfamily that binds to receptors bearing death domains. As well as causing apoptosis of certain types of tumor cells, TRAIL can activate both NF-κB and JNK signalling pathways. To determine the role of TGF-β-Activated Kinase-1 (TAK1) in TRAIL signalling, we analyzed the effects of adding TRAIL to mouse embryonic fibroblasts (MEFs) derived from TAK1 conditional knockout mice. TAK1−/− MEFs were significantly more sensitive to killing by TRAIL than wild-type MEFs, and failed to activate NF-κB or JNK. Overexpression of IKK2-EE, a constitutive activator of NF-κB, protected TAK1−/− MEFs against TRAIL killing, suggesting that TAK1 activation of NF-κB is critical for the viability of cells treated with TRAIL. Consistent with this model, TRAIL failed to induce the survival genes cIAP2 and cFlipL in the absence of TAK1, whereas activation of NF-κB by IKK2-EE restored the levels of both proteins. Moreover, ectopic expression of cFlipL, but not cIAP2, in TAK1−/− MEFs strongly inhibited TRAIL-induced cell death. These results indicate that cells that survive TRAIL treatment may do so by activation of a TAK1–NF-κB pathway that drives expression of cFlipL, and suggest that TAK1 may be a good target for overcoming TRAIL resistance.

In mouse embryonic fibroblasts, neither caspase-8 nor cellular FLICE-inhibitory protein (FLIP) is necessary for TNF to activate NF-κB, but caspase-8 is required for TNF to cause cell death, and induction of FLIP by NF-κB is required to prevent it

Binding of TNF to TNF receptor-1 can give a pro-survival signal through activation of p65/RelA NF-κB, but also signals cell death. To determine the roles of FLICE-inhibitory protein (FLIP) and caspase-8 in TNF-induced activation of NF-κB and apoptosis, we used mouse embryonic fibroblasts derived from FLIP and caspase-8 gene-deleted mice, and treated them with TNF and a smac-mimetic compound that causes degradation of cellular inhibitor of apoptosis proteins (cIAPs). In cells treated with smac mimetic, TNF and Fas Ligand caused wild-type and FLIP−/− MEFs to die, whereas caspase-8−/− MEFs survived, indicating that caspase-8 is necessary for death of MEFs triggered by these ligands when IAPs are degraded. By contrast, neither caspase-8 nor FLIP was required for TNF to activate p65/RelA NF-κB, because IκB was degraded, p65 translocated to the nucleus, and an NF-κB reporter gene activated normally in caspase-8−/− or FLIP−/− MEFs. Reconstitution of FLIP−/− MEFs with the FLIP isoforms FLIP-L, FLIP-R, or FLIP-p43 protected these cells from dying when treated with TNF or FasL, whether or not cIAPs were depleted. These results show that in MEFs, caspase-8 is necessary for TNF- and FasL-induced death, and FLIP is needed to prevent it, but neither caspase-8 nor FLIP is required for TNF to activate NF-κB.

Enhancing venetoclax activity in acute myeloid leukemia by co-targeting MCL1

Targeted therapies are frequently combined with standard cytotoxic drugs to enhance clinical response. Targeting the B-cell lymphoma 2 (BCL-2) family of proteins is an attractive option to combat chemoresistance in leukemia. Preclinical and clinical studies indicate modest single-agent activity with selective BCL-2 inhibitors (for example, venetoclax). We show that venetoclax synergizes with cytarabine and idarubicin to increase antileukemic efficacy in a TP53-dependent manner. Although TP53 deficiency impaired sensitivity to combined venetoclax and chemotherapy, higher-dose idarubicin was able to suppress MCL1 and induce cell death independently of TP53. Consistent with an MCL1-specific effect, cell death from high-dose idarubicin was dependent on pro-apoptotic Bak. Combining higher-dose idarubicin with venetoclax was able to partially overcome resistance in Bak-deficient cells. Using inducible vectors and venetoclax to differentially target anti-apoptotic BCL-2 family members, BCL-2 and MCL1 emerged as critical and complementary proteins regulating cell survival in acute myeloid leukemia. Dual targeting of BCL-2 and MCL1, but not either alone, prolonged survival of leukemia-bearing mice. In conclusion, our findings support the further investigation of venetoclax in combination with standard chemotherapy, including intensified doses of idarubicin. Venetoclax should also be investigated in combination with direct inhibitors of MCL1 as a chemotherapy-free approach in the future.

Correction: Targeting p38 or MK2 Enhances the Anti-Leukemic Activity of Smac-Mimetics (Cancer Cell (2016) 30(3) (499–500) (S1535610816000350) (10.1016/j.ccell.2016.01.006))

Corrections Targeting p38 or MK2 Enhances the Anti-Leukemic Activity of Smac-Mimetics Najoua Lalaoui, Kay Hänggi, Gabriela Brumatti, Diep Chau, Nhu-Y.N. Nguyen, Lazaros Vasilikos, Lisanne M. Spilgies, Denise A. Heckmann, Chunyan Ma, Margherita Ghisi, Jessica M. Salmon, Geoffrey M. Matthews, Elisha de Valle, Donia M. Moujalled, Manoj B. Menon, Sukhdeep Kaur Spall, Stefan P. Glaser, Jennifer Richmond, Richard B. Lock, StephenM. Condon, Raffi Gugasyan, Matthias Gaestel, Mark Guthridge, RickyW. Johnstone, LenkaMunoz, AndrewWei, Paul G. Ekert, David L. Vaux, W. Wei-Lynn Wong, and John Silke* *Correspondence: silke@wehi.edu.au http://dx.doi.org/10.1016/j.ccell.2016.08.009 (Cancer Cell 29, 145–158; February 8, 2016) After the publication of this paper, the authors found four small errors in Figure 3. In Figure 3A, the label p38 on the right side of the second blot from the top should be p38, and the label MK2 on the right side of the second blot from the bottom should be MK2. In addition, the pTAK1 and pMK2 blots in Figure 3C were inadvertently stretched out during figure preparation. These errors have now been corrected here and in the article online. The authors apologize for these errors and any inconvenience that may have resulted.

TDP-43 mutations causing amyotrophic lateral sclerosis are associated with altered expression of RNA-binding protein hnRNP K and affect the Nrf2 antioxidant pathway

&NA; TAR DNA binding protein 43 (TDP‐43) is a major disease‐associated protein involved in the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitin‐positive inclusions (FTLD‐U). Our previous studies found a direct association between TDP‐43 and heterogeneous nuclear ribonucleoprotein K (hnRNP K). In this study, utilizing ALS patient fibroblasts harboring a TDP‐43M337V mutation and NSC‐34 motor neuronal cell line expressing TDP‐43Q331K mutation, we show that hnRNP K expression is impaired in urea soluble extracts from mutant TDP‐43 cell models. This was confirmed in vivo using TDP‐43Q331K and inducible TDP‐43A315T murine ALS models. We further investigated the potential pathological effects of mutant TDP‐43‐mediated changes to hnRNP K metabolism by RNA binding immunoprecipitation analysis. hnRNP K protein was bound to antioxidant NFE2L2 transcripts encoding Nrf2 antioxidant transcription factor, with greater enrichment in TDP‐43M337V patient fibroblasts compared to healthy controls. Subsequent gene expression profiling revealed an increase in downstream antioxidant transcript expression of Nrf2 signaling in the spinal cord of TDP‐43Q331K mice compared to control counterparts, yet the corresponding protein expression was not up‐regulated in transgenic mice. Despite the elevated expression of antioxidant transcripts, we observed impaired levels of glutathione (downstream Nrf2 antioxidant) in TDP‐43M337V patient fibroblasts and astrocyte cultures from TDP‐43Q331K mice, indicative of elevated oxidative stress and failure of some upregulated antioxidant genes to be translated into protein. Our findings indicate that further exploration of the interplay between hnRNP K (or other hnRNPs) and Nrf2‐mediated antioxidant signaling is warranted and may be an important driver for motor neuron degeneration in ALS.

Erratum: TNF-stimulated cells, RIPK1 promotes cell survival by stabilizing TRAF2 and cIAP1, which limits induction of non-canonical NF-κB and activation of caspase-8 (Journal of Biological Chemistry (2016) 291 (2547))

The figure legend of Fig. 1B incorrectly stated that 25 mg of protein was loaded. The correct amount is 25 g. The description of Fig. 1C incorrectly stated that the concentration of chloroquine used was 200 mM. The correct concentration is 200 M. The description of Fig. 1D applies to Fig. 1E, and the description for Fig. 1D should instead read “wild-type and three different RIPK1 / MEF lines were treated with 100 ng/ml Fc-TNF for 0, 5, and 15 min. Cells were then lysed in DISC lysis buffer and split into soluble and insoluble fractions. 25 g of protein from both fractions were run on SDS-PAGE and analyzed for levels of indicated proteins by Western blotting analysis.” An asterisk denotes a nonspecific band.

Combining BH3-mimetics to target both BCL-2 and MCL1 has potent activity in pre-clinical models of acute myeloid leukemia

Improving outcomes in acute myeloid leukemia (AML) remains a major clinical challenge. Overexpression of pro-survival BCL-2 family members rendering transformed cells resistant to cytotoxic drugs is a common theme in cancer. Targeting BCL-2 with the BH3-mimetic venetoclax is active in AML when combined with low-dose chemotherapy or hypomethylating agents. We now report the pre-clinical anti-leukemic efficacy of a novel BCL-2 inhibitor S55746, which demonstrates synergistic pro-apoptotic activity in combination with the MCL1 inhibitor S63845. Activity of the combination was caspase and BAX/BAK dependent, superior to combination with standard cytotoxic AML drugs and active against a broad spectrum of poor risk genotypes, including primary samples from patients with chemoresistant AML. Co-targeting BCL-2 and MCL1 was more effective against leukemic, compared to normal hematopoietic progenitors, suggesting a therapeutic window of activity. Finally, S55746 combined with S63845 prolonged survival in xenograft models of AML and suppressed patient-derived leukemia but not normal hematopoietic cells in bone marrow of engrafted mice. In conclusion, a dual BH3-mimetic approach is feasible, highly synergistic, and active in diverse models of human AML. This approach has strong clinical potential to rapidly suppress leukemia, with reduced toxicity to normal hematopoietic precursors compared to chemotherapy.