Tight junction protein 1 (TJP1) has recently been proposed as a biomarker to identify multiple myeloma (MM) patients most likely to respond to bortezomib- and carfilzomib-based proteasome inhibitor regimens. Herein we report increased expression of TJP1 during the adaptive response mediating carfilzomib resistance in the LP-1/Cfz MM cell line. Moreover, increased TJP1 expression delineated a subset of relapsed/refractory MM patients on bortezomib-based therapy sharing an LP-1/Cfz-like phenotype characterized by activation of interacting transcriptional effectors of the Hippo signaling cascade (TAZ and TEAD1) and an adult tissue stem cell signature. siRNA-mediated knockdown of TJP1 or TAZ/TEAD1 partially sensitized LP-1/Cfz cells to carfilzomib. Connectivity Map analysis identified translation inhibitors as candidate therapeutic agents targeting this molecular phenotype. We confirmed this prediction by showing that homoharringtonine (omacetaxine mepesuccinate) — the first translation inhibitor to be approved by the U.S. Food and Drug Administration — displayed potent cytotoxic activity on LP-1/Cfz cells. Homoharringtonine treatment reduced the levels of TAZ and TEAD1 as well as the MM-protective proteins Nrf2 and MCL1. Thus, our data suggest the importance of further studies evaluating translation inhibitors in relapsed/refractory MM. On the other hand, use of TJP1 as a MM biomarker for proteasome inhibitor sensitivity requires careful consideration.

Multiple Myeloma (MM) is a B-cell malignancy characterized by the accumulation of clonal plasma cells in the bone marrow, with drug resistance being a major cause of therapeutic failure. We established a carfilzomib-resistant derivative of the LP-1 MM cell line (LP-1/Cfz) and found that the transcription factor NF-E2 p45-related factor 2 (Nrf2; gene symbol NFE2L2) contributes to carfilzomib resistance. The mechanism of Nrf2 activation involved enhanced translation of Nrf2 as well as its positive regulator, the autophagy receptor sequestosome 1 (SQSTM1)/p62. The eukaryotic translation initiation factor gene EIF4E3 was among the Nrf2 target genes upregulated in LP-1/Cfz cells, suggesting existence of a positive feedback loop. In line with this, we found that siRNA knockdown of eIF4E3 decreased Nrf2 protein levels. On the other hand, elevated SQSTM1/p62 levels were due at least in part to activation of the PERK-eIF2α pathway. LP-1/Cfz cells had decreased levels of reactive oxygen species as well as elevated levels of fatty acid oxidation and prosurvival autophagy. Genetic and pharmacologic inhibition of the Nrf2-EIF4E3 axis or the PERK-eIF2α pathway, disruption of redox homeostasis or inhibition of fatty acid oxidation or autophagy conferred sensitivity to carfilzomib. Our findings were supported by clinical data where increased EIF4E3 expression was predictive of Nrf2 target gene upregulation in a subgroup of patients with chemoresistant minimal residual disease and relapsed/refractory MM. Thus, our data offer a preclinical rationale for including inhibitors of the SQSTM1/p62-Nrf2 pathway to the treatment regimens for certain advanced stage MM patients.

Multiple myeloma (MM) is a clonal plasma cell malignancy which is frequently diagnosed in patients over 65 years of age. Because of an aging population, the incidence of MM has increased nearly 1 percent annually since 1975 and it is anticipated that the number of cases will nearly double by 2034 [1]. The introduction of novel agents such as the proteasome inhibitor bortezomib (BTZ) has significantly improved overall survival of patients with MM during the past decade. Therapeutic efficacy is related to the exquisite dependence of MM cells on proteasomal degradation of unfolded proteins to maintain proteostasis. However, progression towards BTZ-refractory disease occurs in the majority of MM patients. Unfortunately, once MM patients become refractory to BTZ, their median overall survival has been reported to be less than 1 year [2]. Moreover, in a pivotal phase 2 study with the second-generation proteasome inhibitor carfilzomib (CFZ), less than a 25% response rate was achieved in BTZ-treated patients who had relapsed [3]. These results indicate that the majority of MM cells that became resistant to BTZ were also resistant to CFZ. Clearly, to extend the life expectancy of patients with MM, it is essential to elucidate the underlying mechanisms of acquired proteasome inhibitor resistance.

Multiple myeloma (MM) is an incurable clonal plasma cell malignancy. Because of a high rate of immunoglobulin synthesis, the endoplasmic reticulum of MM cells is subjected to elevated basal levels of stress. Consequently, proteasome inhibitors, which exacerbate this stress by inhibiting ubiquitin-proteasome-mediated protein degradation, are an important new class of chemotherapeutic agents being used to combat this disease. However, MM cells still develop resistance to proteasome inhibitors such as carfilzomib. Toward this end, we have established carfilzomib-resistant derivatives of MM cell lines. We found that resistance to carfilzomib was associated with elevated levels of prosurvival autophagy, and Kruppel-like factor 4 (KLF4) was identified as a contributing factor. Expression levels as well as nuclear localization of KLF4 protein were elevated in MM cells with acquired carfilzomib resistance. Chromatin immunoprecipitations indicated that endogenous KLF4 bound to the promoter regions of the SQSTM1 gene encoding the ubiquitin-binding adaptor protein sequestosome/p62 that links the proteasomal and autophagic protein degradation pathways. Ectopic expression of KLF4 induced upregulation of SQSTM1. On the other hand, inhibitors of autophagy sensitized MM cells to carfilzomib, even in carfilzomib-resistant derivatives having increased expression of the multidrug resistance protein P-glycoprotein. Thus, we report here a novel function for KLF4, one of the Yamanaka reprogramming factors, as being a contributor to autophagy gene expression which moderates preclinical proteasome inhibitor efficacy in MM.

INTRODUCTION:Inappropriate activation of the TLX1 (T-cell leukemia homeobox 1) gene by chromosomal translocation is a recurrent event in human T-cell Acute Lymphoblastic Leukemia (T-ALL). Ectopic expression of TLX1 in murine bone marrow progenitor cells using a conventional retroviral vector efficiently yields immortalized cell lines and induces T-ALL-like tumors in mice after long latency.METHODS:To eliminate a potential contribution of retroviral insertional mutagenesis to TLX1 immortalizing and transforming function, we incorporated the TLX1 gene into an insulated self-inactivating retroviral vector.RESULTS:Retrovirally transduced TLX1-expressing murine bone marrow progenitor cells had a growth/survival advantage and readily gave rise to immortalized cell lines. Extensive characterization of 15 newly established cell lines failed to reveal a common retroviral integration site. This comprehensive analysis greatly extends our previous study involving a limited number of cell lines, providing additional support for the view that constitutive TLX1 expression is sufficient to initiate the series of events culminating in hematopoietic progenitor cell immortalization. When TLX1-immortalized cells were co-cultured on OP9-DL1 monolayers under conditions permissive for T-cell differentiation, a latent T-lineage potential was revealed. However, the cells were unable to transit the DN2 myeloid-T (DN2mt)-DN2 T-lineage determined (DN2t) commitment step. The differentiation block coincided with failure to upregulate the zinc finger transcription factor gene Bcl11b, the human ortholog of which was shown to be a direct transcriptional target of TLX1 downregulated in the TLX1+ T-ALL cell line ALL-SIL. Other studies have described the ability of TLX1 to promote bypass of mitotic checkpoint arrest, leading to aneuploidy. We likewise found that diploid TLX1-expressing DN2mt cells treated with the mitotic inhibitor paclitaxel bypassed the mitotic checkpoint and displayed chromosomal instability. This was associated with elevated expression of TLX1 transcriptional targets involved in DNA replication and mitosis, including Ccna2 (cyclin A2), Ccnb1 (cyclin B1), Ccnb2 (cyclin B2) and Top2a (topoisomerase IIα). Notably, enforced expression of BCL11B in ALL-SIL T-ALL cells conferred resistance to the topoisomerase IIα poison etoposide.CONCLUSION:Taken together with previous findings, the data reinforce a mechanism of TLX1 oncogenic activity linked to chromosomal instability resulting from dysregulated expression of target genes involved in mitotic processes. We speculate that repression of BCL11B expression may provide part of the explanation for the observation that aneuploid DNA content in TLX1+ leukemic T cells does not necessarily portend an unfavorable prognosis. This TLX1 hematopoietic progenitor cell immortalization/T-cell differentiation assay should help further our understanding of the mechanisms of TLX1-mediated evolution to malignancy and has the potential to be a useful predictor of disease response to novel therapeutic agents in TLX1+ T-ALL.

Multiple myeloma (MM) is characterized by the malignant expansion of differentiated plasma cells. Although many chemotherapeutic agents display cytotoxic activity toward MM cells, patients inevitably succumb to their disease because the tumor cells become resistant to the anticancer drugs. The cancer stem cell hypothesis postulates that a small subpopulation of chemotherapy‐resistant cancer cells is responsible for propagation of the tumor. Herein we report that efflux of the pluripotent stem cell dye CDy1 identifies a subpopulation in MM cell lines characterized by increased expression of P‐glycoprotein, a member of the ABC (ATP‐binding cassette) superfamily of transporters encoded by ABCB1 . We also demonstrate that ABCB1 ‐overexpressing MM cells are resistant to the second‐generation proteasome inhibitor carfilzomib that recently received accelerated approval for the treatment of therapy‐refractive MM by the U.S. Food and Drug Administration. Moreover, increased resistance to carfilzomib in sensitive MM cells following drug selection was associated with upregulation of ABCB1 cell‐surface expression which correlated with increased transporter activity as measured by CDy1 efflux. We further show that chemosensitization of MM cells to carfilzomib could be achieved in vitro by cotreatment with vismodegib, a hedgehog pathway antagonist which is currently in MM clinical trials. CDy1 efflux may therefore be a useful assay to determine whether high expression of ABCB1 is predictive of poor clinical responses in MM patients treated with carfilzomib. Our data also suggest that inclusion of vismodegib might be a potential strategy to reverse ABCB1 ‐mediated drug resistance should it occur. Am. J. Hematol. 88:265–272, 2013. © 2013 Wiley Periodicals, Inc.

In this study, we utilized an integrated bioinformatics and computational biology approach in search of new BH3-only proteins belonging to the BCL2 family of apoptotic regulators. The BH3 (BCL2 homology 3) domain mediates specific binding interactions among various BCL2 family members. It is composed of an amphipathic α-helical region of approximately 13 residues that has only a few amino acids that are highly conserved across all members. Using a generalized motif, we performed a genome-wide search for novel BH3-containing proteins in the NCBI Consensus Coding Sequence (CCDS) database. In addition to known pro-apoptotic BH3-only proteins, 197 proteins were recovered that satisfied the search criteria. These were categorized according to α-helical content and predictive binding to BCL-xL (encoded by BCL2L1) and MCL-1, two representative anti-apoptotic BCL2 family members, using position-specific scoring matrix models. Notably, the list is enriched for proteins associated with autophagy as well as a broad spectrum of cellular stress responses such as endoplasmic reticulum stress, oxidative stress, antiviral defense, and the DNA damage response. Several potential novel BH3-containing proteins are highlighted. In particular, the analysis strongly suggests that the apoptosis inhibitor and DNA damage response regulator, AVEN, which was originally isolated as a BCL-xLinteracting protein, is a functional BH3-only protein representing a distinct subclass of BCL2 family members.

TP53 is a central hub integrating stress signals from oncogenic genetic lesions and cytotoxic anti-cancer agents. The function of p53 as a regulator of transcription is well documented. More recently it was shown to directly interact with BCL2 family members and induce mitochondrial cell death. Stress-induced activation of p53 leads to cell cycle arrest that allows metabolic adjustments and repair mechanisms to take place prior to the next cycle; p53 may also induce senescence or apoptosis depending on the strength and nature of stress stimuli and/or cell type. In 50% of human cancers, the TP53 gene is deleted or mutated with a high proportion of gain of oncogenic function mutations. It is noteworthy therefore that TP53 is rarely mutated in T-ALL. However, its tumor suppressor activity is circumvented by genetic lesions. We will discuss here the most frequent T-ALL genetic abnormalities of INK4A/ARF, NOTCH1 and PTEN genes and how they affect TP53 expression and function. Current understanding of the signaling pathways governed by these oncogenes is advanced enough to find points of intersection with p53 downstream targets and attempt to translate the accumulated knowledge to the clinic. In addition, we will discuss the results of our analysis of publicly available expression profiling data indicating the existence of a TP53-anchored transcriptional program targeted by T-ALL oncogenes such as NOTCH, MYC and TLX1 in primary leukemic cells and how it can be exploited for cancer intervention. Overall, the goal of this chapter is to describe how T-ALL pathobiology affects the p53interacting network, highlighting some new potential therapeutic targets as well as some still unresolved questions.

Despite considerable progress in the treatment of T cell acute lymphoblastic leukemia (T-ALL), it is still the highest risk malignancy among ALL. The outcome of relapsed patients remains dismal. The pro-survival role of NOTCH1 and NFkB in T-ALL is well documented; also, both factors were reported to be predictive of relapse. The NOTCH1 signaling pathway, commonly activated in T-ALL, was shown to enhance the transcriptional function of NFkB via several mechanisms. Thus, pharmacological inhibition of NOTCH1-NFkB signaling was suggested to be incorporated into existing T-ALL treatment protocols. However, conventional chemotherapy is based on activation of various types of stress, such as DNA damage, mitotic perturbations or endoplasmic reticulum overload. NFkB is frequently activated in response to stress and, depending on yet unknown mechanisms, it either protects cells from the drug action or mediates apoptosis. Here, we report that T-ALL cells respond to NFkB inhibition in opposite ways depending on whether they were treated with a stress-inducing chemotherapeutic agent or not. Moreover, we found that NOTCH1 enhances NFkB apoptotic function in the stressed cells. The data argue for further studies of NFkB status in T-ALL patients on different treatment protocols and the impact of activating NOTCH1 mutations on treatment response. Mol Cancer Res; 9(8); 979–84.

Despite considerable progress in the treatment of T cell acute lymphoblastic leukemia (T-ALL), it is still the highest risk malignancy among ALL. The outcome of relapsed patients remains dismal. The pro-survival role of NOTCH1 and NFκB in T-ALL is well documented; also, both factors were reported to be predictive of relapse. The NOTCH1 signaling pathway, commonly activated in T-ALL, was shown to enhance the transcriptional function of NFκB via several mechanisms. Thus, pharmacological inhibition of NOTCH1-NFκB signaling was suggested to be incorporated into existing T-ALL treatment protocols. However, conventional chemotherapy is based on activation of various types of stress, such as DNA damage, mitotic perturbations or endoplasmic reticulum overload. NFκB is frequently activated in response to stress and, depending on yet unknown mechanisms, it either protects cells from the drug action or mediates apoptosis. Here, we report that T-ALL cells respond to NFκB inhibition in opposite ways depending on whether they were treated with a stress-inducing chemotherapeutic agent or not. Moreover, we found that NOTCH1 enhances NFκB apoptotic function in the stressed cells. The data argue for further studies of NFκB status in T-ALL patients on different treatment protocols and the impact of activating NOTCH1 mutations on treatment response.

In vivo biotinylation tagging, based on a method in which a protein of interest is tagged with a peptide that is biotinylated in vivo by coexpression of Escherichia coli BirA biotin ligase, has been successfully used for the isolation of protein protein and protein-DNA complexes in mammalian cells. We describe a modification of this methodology in which cells stably expressing the tagged gene of interest and the BirA gene can be selected by fluorescence-activated cell sorting (FACS). We recently implemented this approach to isolate and characterize proteins associated with TLX1, a homeodomain transcription factor with leukemogenic function. The modified technique utilizes two components: a lentiviral vector coexpressing the gene of interest containing a biotinylation tag on a bicistronic transcript together with a downstream yellow fluorescent protein (YFP) gene; and a second lentiviral vector encoding a fusion protein composed of bacterial BirA linked to the green fluorescent protein (GFP). This FACS-based binary in vivo biotinylation tagging system allows precise control over the levels of BirA-mediated biotinylation as well as the expression of the gene of interest, which is especially important if high-level expression negatively impacts cell growth or viability.

IntroductionTP53 is a central hub integrating stress signals from oncogenic genetic lesions and cytotoxic anti-cancer agents.The function of p53 as a regulator of transcription is well documented.More recently it was shown to directly interact with BCL2 family members and induce mitochondrial cell death.Stress-induced activation of p53 leads to cell cycle arrest that allows metabolic adjustments and repair mechanisms to take place prior to the next cycle; p53 may also induce senescence or apoptosis depending on the strength and nature of stress stimuli and/or cell type.In 50% of human cancers, the TP53 gene is deleted or mutated with a high proportion of gain of oncogenic function mutations.It is noteworthy therefore that TP53 is rarely mutated in T-ALL.However, its tumor suppressor activity is circumvented by genetic lesions.We will discuss here the most frequent T-ALL genetic abnormalities of INK4A/ARF, NOTCH1 and PTEN genes and how they affect TP53 expression and function.Current understanding of the signaling pathways governed by these oncogenes is advanced enough to find points of intersection with p53 downstream targets and attempt to translate the accumulated knowledge to the clinic.In addition, we will discuss the results of our analysis of publicly available expression profiling data indicating the existence of a TP53-anchored transcriptional program targeted by T-ALL oncogenes such as NOTCH, MYC and TLX1 in primary leukemic cells and how it can be exploited for cancer intervention.Overall, the goal of this chapter is to describe how T-ALL pathobiology affects the p53interacting network, highlighting some new potential therapeutic targets as well as some still unresolved questions. INK4A-ARF inactivation circumvents oncogene-induced p53

Translocations resulting in ectopic expression of the TLX1 homeobox gene (previously known as HOX11 ) are recurrent events in human T‐cell acute lymphoblastic leukemia (T‐ALL). Transduction of primary murine hematopoietic stem/progenitor cells with retroviral vectors expressing TLX1 readily yields immortalized hematopoietic progenitor cell lines. Understanding the processes involved in TLX1 ‐mediated cellular immortalization should yield insights into the growth and differentiation pathways altered by TLX1 during the development of T‐ALL. In recent clinical gene therapy trials, hematopoietic clonal dominance or T‐ALL‐like diseases have occurred as a direct consequence of insertional activation of the EVI1 , PRDM16 or LMO2 proto‐oncogenes by the retroviral vectors used to deliver the therapeutic genes. Additionally, the generation of murine hematopoietic progenitor cell lines due to retroviral integrations into Evi1 or Prdm16 has also been recently reported. Here, we determined by linker‐mediated nested polymerase chain reaction the integration sites in eight TLX1 ‐immortalized hematopoietic cell lines. Notably, no common integration site was observed among the cell lines. Moreover, no insertions into the Evi1 or Prdm16 genes were identified although insertion near Lmo2 was observed in one instance. However, neither Lmo2 nor any of the other genes examined surrounding the integration sites showed differential vector‐influenced expression compared to the cell lines lacking such insertions. While we cannot exclude the possibility that insertional side effects transiently provided a selective growth/survival advantage to the hematopoietic progenitor populations, our results unequivocally rule out insertions into Evi1 and Prdm16 as being integral to the TLX1 ‐initiated immortalization process. © 2009 Wiley‐Liss, Inc.

Abstract Inappropriate activation of the TLX1 homeobox gene (previously known as HOX11) by chromosomal translocation is a recurrent event in human T-cell acute lymphoblastic leukemia (T-ALL). Enforced expression of TLX1 in murine hematopoietic stem/progenitor cells using a conventional retroviral vector yields immortalized hematopoietic progenitor cell lines. Recent clinical gene therapy trials have shown that retroviral vectors can cause insertional activation of proto-oncogenes which has resulted in the development of hematopoietic clonal dominance or T-ALL-like diseases. Our lab has developed an insulated self-inactivating retroviral vector, RMSinOFB, which has been safety-modified to virtually eliminate insertional genotoxicity during transgene delivery. To observe the effect of reducing retroviral genotoxicity during TLX1 gene transfer, we subcloned the TLX1 gene into the RMSinOFB vector for ectopic expression in murine hematopoietic stem/progenitor cells. Post transduction, TLX1-expressing bone marrow cells had a growth/survival advantage, and readily gave rise to immortalized cell lines. To determine whether insertional mutagenesis played a role in TLX1-mediated immortalization, we identified the integration sites in 15 TLX1-immortalized hematopoietic cell lines by LM-PCR. No common immortalizing genes, such as Evi1 or Prdm16, were identified among the cell lines which underlines the importance of TLX1 transcriptional properties for cell immortalization. Additionally, when tested by RT-PCR no differential vector-influenced expression was observed for candidate genes nearest the integration sites compared to the cell lines lacking such insertions. Furthermore, gene ontology analysis of genes within 100-kb windows of the integration sites did not reveal overrepresentation of a particular functional group. Although several laboratories recently reported the generation of hematopoietic progenitor cell lines due to insertional mutagenesis by retroviral vector backbones, these results confirm and extend our previous findings suggesting that constitutive TLX1 expression alone is sufficient to initiate the series of events culminating in hematopoietic progenitor cell immortalization. We are currently investigating whether non insertion site-related mutations, such as inactivation of the Cdkn2a tumor suppressor locus which collaborates with TLX1 during human T-cell leukemogenesis, may contribute to the process. Investigating the molecular mechanism of TLX1-mediated cellular immortalization in our in vitro model should provide a better understanding of the early stages of TLX1-associated T-ALL development. 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 2965.