Targeting MCL-1 protein to treat cancer: opportunities and challenges.

Evading apoptosis has been linked to tumor development and chemoresistance. One mechanism for this evasion is the overexpression of prosurvival B-cell lymphoma-2 (BCL-2) family proteins, which gives cancer cells a survival advantage. Mcl-1, a member of the BCL-2 family, is among the most frequently amplified genes in cancer. Targeting myeloid cell leukemia-1 (MCL-1) protein is a successful strategy to induce apoptosis and overcome tumor resistance to chemotherapy and targeted therapy. Various strategies to inhibit the antiapoptotic activity of MCL-1 protein, including transcription, translation, and the degradation of MCL-1 protein, have been tested. Neutralizing MCL-1's function by targeting its interactions with other proteins via BCL-2 interacting mediator (BIM)S2A has been shown to be an equally effective approach. Encouraged by the design of venetoclax and its efficacy in chronic lymphocytic leukemia, scientists have developed other BCL-2 homology (BH3) mimetics-particularly MCL-1 inhibitors (MCL-1i)-that are currently in clinical trials for various cancers. While extensive reviews of MCL-1i are available, critical analyses focusing on the challenges of MCL-1i and their optimization are lacking. In this review, we discuss the current knowledge regarding clinically relevant MCL-1i and focus on predictive biomarkers of response, mechanisms of resistance, major issues associated with use of MCL-1i, and the future use of and maximization of the benefits from these agents.

Mechanisms of MCL-1 Protein Stability Induced by MCL-1 Antagonists in B-Cell Malignancies.

PURPOSE: Several MCL-1 inhibitors (MCL-1i), including AMG-176 and AZD5991, have shown promise in preclinical studies and are being tested for the treatment of hematologic malignancies. A unique feature of these agents is induction and stability of Mcl-1 protein; however, the precise mechanism is unknown. We aim to study the mechanism of MCL-1i-induced Mcl-1 protein stability. EXPERIMENTAL DESIGN: Using several B-cell leukemia and lymphoma cell lines and primary chronic lymphocytic leukemia (CLL) lymphocytes, we evaluated molecular events associated with Mcl-1 protein stability including protein half-life, reverse-phase protein array, protein-protein interaction, phosphorylation, ubiquitination, and de-ubiquitination, followed by molecular simulation and modeling. RESULTS: Using both in vivo and in vitro analysis, we demonstrate that MCL-1i-induced Mcl-1 protein stability is predominantly associated with defective Mcl-1 ubiquitination and concurrent apoptosis induction in both cell lines and primary CLL subjects. These MCL1i also induced ERK-mediated Mcl-1Thr163 phosphorylation, which partially contributed to Mcl-1 stability. Disruption of Mcl-1:Noxa interaction followed by Noxa degradation, enhanced Mcl-1 de-ubiquitination by USP9x, and Mule destabilization are the major effects of these inhibitors. However, unlike other BH3 proteins, Mule:Mcl-1 interaction was unaffected by MCL-1i. WP1130, a global deubiquitinase (DUB) inhibitor, abrogated Mcl-1 induction reaffirming a critical role of DUBs in the observed Mcl-1 protein stability. Further, in vitro ubiquitination studies of Mcl-1 showed distinct difference among these inhibitors. CONCLUSIONS: We conclude that MCL-1i blocked Mcl-1 ubiquitination via enhanced de-ubiquitination and dissociation of Mcl-1 from Noxa, Bak and Bax, and Mule de-stabilization. These are critical events associated with increased Mcl-1 protein stability with AMG-176 and AZD5991.

Ataxia-telangiectasia mutated interacts with Parkin and induces mitophagy independent of kinase activity. Evidence from mantle cell lymphoma.

Ataxia telangiectasia mutated (ATM), a critical DNA damage sensor with protein kinase activity,is frequently altered in human cancers including mantle cell lymphoma (MCL). Loss of ATM protein is linked to accumulation of nonfunctional mitochondria and defective mitophagy, in both murine thymocytes and in A-T cells. However, the mechanistic role of ATM kinase in cancer cell mitophagy is unknown. Here, we provide evidence that FCCP-induced mitophagy in MCL and other cancer cell lines is dependent on ATM but independent of its kinase function. While Granta-519 MCL cells possess single copy and kinase dead ATM and are resistant to FCCP-induced mitophagy, both Jeko-1 and Mino cells are ATM proficient and induce mitophagy. Stable knockdown of ATM in Jeko-1 and Mino cells conferred resistance to mitophagy and was associated with reduced ATP production, oxygen consumption, and increased mROS. ATM interacts with the E3 ubiquitin ligase Parkin in a kinase-independent manner. Knockdown of ATM in HeLa cells resulted in proteasomal degradation of GFP-Parkin which was rescued by the proteasome inhibitor, MG132 suggesting that ATM-Parkin interaction is important for Parkin stability. Neither loss of ATM kinase activity in primary B cell lymphomas nor inhibition of ATM kinase in MCL, A-T and HeLa cell lines mitigated FCCP or CCCP-induced mitophagy suggesting that ATM kinase activity is dispensable for mitophagy. Malignant B-cell lymphomas without detectable ATM, Parkin, Pink1, and Parkin-Ub ser65 phosphorylation were resistant to mitophagy, providing the first molecular evidence of ATM's role in mitophagy in MCL and other B-cell lymphomas.

Activation of ATM kinase by ROS generated during ionophore-induced mitophagy in human T and B cell malignancies.

Ataxia telangiectasia mutated (ATM), a critical DNA damage sensor, also possesses non-nuclear functions owing to its presence in extra-nuclear compartments, including peroxisomes, lysosomes, and mitochondria. ATM is frequently altered in several human cancers. Recently, we and others have shown that loss of ATM is associated with defective mitochondrial autophagy (mitophagy) in ataxia-telangiectasia (A-T) fibroblasts and B-cell lymphomas. Further, we reported that ATM protein but not ATM kinase activity is required for mitophagy. However, the mechanism of ATM kinase activation during ionophore-induced mitophagy is unknown. In the work reported here, using several ionophores in A-T and multiple T-cell and B-cell lymphoma cell lines, we show that ionophore-induced mitophagy triggers oxidative stress-induced ATMSer1981 phosphorylation through ROS activation, which is different from neocarzinostatin-induced activation of ATMSer1981, Smc1Ser966, and Kap1Ser824. We used A-T cells overexpressed with WT or S1981A (auto-phosphorylation dead) ATM plasmids and show that ATM is activated by ROS-induced oxidative stress emanating from ionophore-induced mitochondrial damage and mitophagy. The antioxidants N-acetylcysteine and glutathione significantly inhibited ROS production and ATMSer1981 phosphorylation but failed to inhibit mitophagy as determined by retroviral infection with mt-mKeima construct followed by lysosomal dual-excitation ratiometric pH measurements. Our data suggest that while ATM kinase does not participate in mitophagy, it is activated via elevated ROS.

AMG-176, an Mcl-1 Antagonist, Shows Preclinical Efficacy in Chronic Lymphocytic Leukemia.

PURPOSE: Survival of CLL cells due to the presence of Bcl-2 and Mcl-1 has been established. Direct inhibition of Bcl-2 by venetoclax and indirect targeting of Mcl-1 with transcription inhibitors have been successful approaches for CLL. AMG-176 is a selective and direct antagonist of Mcl-1, which has shown efficacy in several hematologic malignancies; however, its effect on CLL is elusive. We evaluated biological and molecular effects of AMG-176 in primary CLL cells. EXPERIMENTAL DESIGN: Using samples from patients (n = 74) with CLL, we tested effects of AMG-176 on CLL and normal hematopoietic cell death and compared importance of CLL prognostic factors on this biological activity. We evaluated CLL cell apoptosis in the presence of stromal cells and identified cell death pathway including stabilization of Mcl-1 protein. Finally, we tested a couplet of AMG-176 and venetoclax in CLL lymphocytes. RESULTS: AMG-176 incubations resulted in time- and dose-dependent CLL cell death. At 100 and 300 nmol/L, there was 30% and 45% cell death at 24 hours. These concentrations did not result in significant cell death in normal hematopoietic cells. Presence of stroma did not affect AMG-176-induced CLL cell death. IGHV unmutated status, high ß2M and Mcl-1 protein levels resulted in slightly lower cell death. Mcl-1, but not Bcl-2 protein levels, in CLL cells increased with AMG-176. Low concentrations of venetoclax (1-30 nmol/L) were additive or synergistic with AMG-176. CONCLUSIONS: AMG-176 is active in inducing CLL cell death while sparing normal blood cells. Combination with low-dose venetoclax was additive or synergistic.

Molecular evidence of Zn chelation of the procaspase activating compound B-PAC-1 in B cell lymphoma.

The resistance of apoptosis in cancer cells is pivotal for their survival and is typically ruled by mutations or dysregulation of core apoptotic cascade. Mantle cell lymphoma (MCL) is a non-Hodgkin's B-cell malignancy expressing higher anti-apoptotic proteins providing survival advantage. B-PAC-1, a procaspase activating compound, induces apoptosis by sequestering Zn bound to procaspase-3, but the amino acids holding Zn in Caspase-3 is not known. Here we show that reintroduction of WT caspase-3 or 7 in Caspase3-7 double knock-out (DKO) mouse embryonic fibroblasts (MEF) promoted B-PAC-1 to induce apoptosis (27-43%), but not in DKO MEFs or MEFs expressing respective Casp3-7 catalytic mutants (12-13%). Using caspase-6 and -9 exosite analysis, we identified and mutated predicted Zn-ligands in caspase-3 (H108A, C148S and E272A) and overexpressed into DKO MEFs. Mutants carrying E272A abrogated Zn-reversal of apoptosis induced by B-PAC-1 via higher XIAP and smac expressions but not in H108A or C148S mutants. Co-immunoprecipitation analysis revealed stronger XIAP-caspase-3 interaction suggesting a novel mechanism of impulsive apoptosis resistance by disrupting predicted Zn-ligands in caspase-3. B-PAC-1 sponsored apoptosis in MCL cell lines (30-73%) via caspase-3 and PARP cleavages accompanied by loss of Mcl-1 and IAPs including XIAP while Zn substantially abrogated B-PAC-1-driven apoptosis (18-36%). In contrary, Zn is dispensable to inhibit staurosporin, bendamustine, ABT199 or MK206-induced apoptosis. Consistent to cell lines, B-PAC-1 stimulated cell death in primary B-lymphoma cells via caspase-3 cleavage with decline in both Mcl-1 and XIAP. This study underscores the first genetic evidence that B-PAC-1 driven apoptosis is mediated via Zn chelation.

Serum response factor expression is enriched in pancreatic ß cells and regulates insulin gene expression.

Serum response factor (SRF) is an essential regulator of myogenic and neurogenic genes and the ubiquitously expressed immediate-early genes. The purpose of this study is to determine SRF expression pattern in murine pancreas and examine the role of SRF in pancreatic gene expression. Immunohistochemical analysis of wild-type pancreas and LacZ staining of pancreas from SRF LacZ knock-in animals showed that SRF expression is restricted to ß cells. SRF bound to the rat insulin promoter II (RIP II) serum response element, an element conserved in both rat I and murine I and II insulin promoters. SRF activated RIP II, and SRF binding to RIP II and the exon 5-encoded 64-aa subdomain of SRF was required for this activation. Transient or stable knockdown of SRF leads to down-regulation of insulin gene expression, suggesting that SRF is required for insulin gene expression. Further, SRF physically interacted with the pancreas and duodenum homeobox-1 (Pdx-1) and synergistically activated RIP II. Elevated glucose concentration down-regulated SRF binding to RIP II SRE, and this down-regulation was associated with decreased RIP II activity and increased SRF phosphorylation on serine 103. Together, our results demonstrate that SRF is a glucose concentration-sensitive regulator of insulin gene expression.

Retroviral insertional activation of the Fli-3 locus in erythroleukemias encoding a cluster of microRNAs that convert Epo-induced differentiation to proliferation.

MicroRNAs (miRNAs) are a newly discovered class of posttranscriptional regulatory noncoding small RNAs. Recent evidence has shown that miRNA misexpression correlates with progression of various human cancers. Friend erythroleukemia has been used as an excellent system for the identification and characterization of oncogenes and tumor suppressor genes involved in neoplastic transformation. Using this model, we have isolated a novel integration site designated Fli-3, from a Friend murine leukemia virus (F-MuLV)-induced erythroleukemia. The Fli-3 transcription unit is a murine homologue of the human gene C13orf25 that includes a region encoding the mir-17-92 miRNA cluster. C13orf25 is the target gene of 13q31 chromosomal amplification in human B-cell lymphomas and other malignancies. The erythroleukemias that have acquired either insertional activation or amplification of Fli-3 express higher levels of the primary or mature miRNAs derived from mir-17-92. The ectopic expression of Fli-3 in an erythroblastic cell line switches erythropoietin (Epo)-induced differentiation to Epo-induced proliferation through activation of the Ras and PI3K pathways. Such a response is associated with alteration in the expression of several regulatory factors, such as Spi-1 and p27 (Kip1). These findings highlight the potential of the Fli-3 encoding mir-17-92 in the development of erythroleukemia and its important role in hematopoiesis.

Friend virus-induced erythroleukemias: a unique and well-defined mouse model for the development of leukemia.

Retroviruses lacking oncogenes have been known to induce various types of cancer when inoculated into animals. Among these, Friend virus, discovered by Charlotte Friend in 1957, is capable of inducing erythroleukemias when injected into susceptible strains of mice. Since its discovery, this murine model of leukemogenesis has been extensively used to study the multistage nature of cancer. In the past two decades, several oncogenes and tumour suppressor genes, which play critical roles in the induction and progression of Friend erythroleukemia, have been identified. Retroviral insertional activation of Fli-1 and Spi-1/PU.1, as well as loss of tumour suppressor genes such as p53 or p45 NFE2 have been shown to be critical for the induction and progression of Friend virus-induced erythroleukemias. The majority of these genetic changes have also been implicated in various types of human neoplastic transformations. In this review we will discuss the genetic changes associated with Friend Disease, the temporal order during induction and progression of disease, and the function of these genes in both normal erythroid development as well as malignant transformation.