MCL-1 Promoter Insertions Dial-Up Aggressiveness of Chronic Leukemia

LTβR overexpression promotes plasma cell accumulation

Multiple myeloma (MM), a malignancy of plasma cells (PCs), has diverse genetic underpinnings and in rare cases these include amplification of the lymphotoxin b receptor (Ltbr) locus. LTβR has well defined roles in supporting lymphoid tissue development and function through actions in stromal and myeloid cells, but whether it is functional in PCs is unknown. Here we showed that Ltbr mRNA was upregulated in mouse PCs compared to follicular B cells, but deficiency in the receptor did not cause a reduction in PC responses to a T-dependent or T-independent immunogen. However, LTβR overexpression (OE) enhanced PC formation in vitro after LPS or anti-CD40 stimulation. In vivo, LTβR OE led to increased antigen-specific splenic and bone marrow (BM) plasma cells responses. LTβR OE PCs had increased expression of Nfkb2 and of the NF-kB target genes Bcl2 and Mcl1, factors involved in the formation of long-lived BM PCs. Our findings suggest a pathway by which Ltbr gene amplifications may contribute to MM development through increased NF-kB activity and induction of an anti-apoptotic transcriptional program.

Assessment of the Antitumor Potential of Umbelliprenin, a Naturally Occurring Sesquiterpene Coumarin

Cancer is one of the greatest causes of mortality worldwide. The prevalence rates of different types of cancer is increasing around the world as well. Limitations in chemotherapy and radiotherapy, owing to multiple side effects including cytotoxic effects of antitumor compounds on normal cells as well as the development of resistance to these treatment options in patients, create a serious threat to successful treatment of cancer. The use of natural compounds to prevent and treat cancers has been found to be quite effective, with fewer adverse effects found in patients. Umbelliprenin (UMB) is a naturally occurring sesquiterpene compound found in Ferula species and recently in Artemisia absinthium. Many studies have highlighted the antitumor potential of UMB in different cancer cell lines as well as in animal models. UMB exerts its anticancer actions by regulating extrinsic and intrinsic apoptotic pathways; causing inhibition of the cell cycle at the G0/G1 phase; and attenuating migration and invasion by modulating the Wnt signaling, NF-ĸB, TGFβ, and Fox3 signaling pathways. UMB also affects the key hallmarks of tumor cells by attenuating tumor growth, angiogenesis, and metastasis. This review provides an insight into the role of UMB as a potential antitumor drug for different malignancies and highlights the signaling cascades affected by UMB treatment in diverse tumor cell lines and preclinical models.

hBfl-1/hNOXA Interaction Studies Provide New Insights on the Role of Bfl-1 in Cancer Cell Resistance and for the Design of Novel Anticancer Agents

Upregulation of antiapoptotic Bcl-2 proteins in certain tumors confers cancer cell resistance to chemotherapy or radiations. Members of the antiapoptotic Bcl-2 proteins, including Bcl-2, Mcl-1, Bcl-xL, Bcl-w, and Bfl-1, inhibit apoptosis by selectively binding to conserved α-helical regions, named BH3 domains, of pro-apoptotic proteins such as Bim, tBid, Bad, or NOXA. Five antiapoptotic proteins have been identified that interact with various selectivity with BH3 containing pro-apoptotic counterparts. Cancer cells present various and variable levels of these proteins, making the design of effective apoptosis based therapeutics challenging. Recently, BH3 profiling was introduced as a method to classify cancer cells based on their ability to resist apoptosis following exposure to selected BH3 peptides. However, these studies were based on binding affinities measured with model BH3 peptides and Bcl-2-proteins taken from mouse sequences. While the majority of these interactions are conserved between mice and humans, we found surprisingly that human NOXA binds to human Bfl-1 potently and covalently via conserved Cys residues, with over 2 orders of magnitude increased affinity over hMcl-1. Our data suggest that some assumptions of the original BH3 profiling need to be revisited and that perhaps further targeting efforts should be redirected toward Bfl-1, for which no suitable specific inhibitors or pharmacological tools have been reported. In this regard, we also describe the initial design and characterizations of novel covalent BH3-based agents that potently target Bfl-1. These molecules could provide a novel platform on which to design effective Bfl-1 targeting therapeutics.

Novel Biomarker Proteins in Chronic Lymphocytic Leukemia: Impact on Diagnosis, Prognosis and Treatment

In many cancers, cells undergo re-programming of metabolism, cell survival and anti-apoptotic defense strategies, with the proteins mediating this reprogramming representing potential biomarkers. Here, we searched for novel biomarker proteins in chronic lymphocytic leukemia (CLL) that can impact diagnosis, treatment and prognosis by comparing the protein expression profiles of peripheral blood mononuclear cells from CLL patients and healthy donors using specific antibodies, mass spectrometry and binary logistic regression analyses and other bioinformatics tools. Mass spectrometry (LC-HR-MS/MS) analysis identified 1,360 proteins whose expression levels were modified in CLL-derived lymphocytes. Some of these proteins were previously connected to different cancer types, including CLL, while four other highly expressed proteins were not previously reported to be associated with cancer, and here, for the first time, DDX46 and AK3 are linked to CLL. Down-regulation expression of two of these proteins resulted in cell growth inhibition. High DDX46 expression levels were associated with shorter survival of CLL patients and thus can serve as a prognosis marker. The proteins with modified expression include proteins involved in RNA splicing and translation and particularly mitochondrial proteins involved in apoptosis and metabolism. Thus, we focused on several metabolism- and apoptosis-modulating proteins, particularly on the voltage-dependent anion channel 1 (VDAC1), regulating both metabolism and apoptosis. Expression levels of Bcl-2, VDAC1, MAVS, AIF and SMAC/Diablo were markedly increased in CLL-derived lymphocytes. VDAC1 levels were highly correlated with the amount of CLL-cancerous CD19+/CD5+ cells and with the levels of all other apoptosis-modulating proteins tested. Binary logistic regression analysis demonstrated the ability to predict probability of disease with over 90% accuracy. Finally, based on the changes in the levels of several proteins in CLL patients, as revealed from LC-HR-MS/MS, we could distinguish between patients in a stable disease state and those who would be later transferred to anti-cancer treatments. The over-expressed proteins can thus serve as potential biomarkers for early diagnosis, prognosis, new targets for CLL therapy, and treatment guidance of CLL, forming the basis for personalized therapy.

Expression of executioner procaspases and their activation by a procaspase-activating compound in chronic lymphocytic leukemia cells

Intrinsic and extrinsic apoptotic pathways converge to activate common downstream executioner caspases (caspase-3, -6, and -7), resulting in cell death. In chronic lymphocytic leukemia (CLL), neoplastic B cells evade apoptosis owing to the overexpression of survival proteins. We hypothesized that direct activation of procaspases could bypass the apoptosis resistance induced by the upstream prosurvival proteins. The procaspase-activating compounds (PAC-1), including B-PAC-1 (L14R8), convert inactive executioner procaspases to their active cleaved forms by chelation of labile zinc ions. Both at transcript and protein levels, primary CLL cells express high levels of latent procaspases (3, -7, and -9). B-PAC-1 treatment induced CLL lymphocyte death which was higher than that in normal peripheral blood mononuclear cells or B cells, and was independent of prognostic markers and microenvironmental factors. Mechanistically, B-PAC-1 treatment activated executioner procaspases and not other Zn-dependent enzymes. Exogenous zinc completely, and pancaspase inhibitors partially, reversed B-PAC-1-induced apoptosis, elucidating the zinc-mediated mechanism of action. The cell demise relied on the presence of caspase-3/7 but not caspase-8 or Bax/Bak proteins. B-PAC-1 in combination with an inhibitor of apoptosis protein antagonist (Smac066) synergistically induced apoptosis in CLL samples. Our investigations demonstrated that direct activation of executioner procaspases via B-PAC-1 treatment bypasses apoptosis resistance and is a novel approach for CLL therapeutics.

Impact of bone marrow stromal cells on Bcl-2 family members in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most common form of adult leukemia in the Western world. High levels of Bcl-2 family anti-apoptotic proteins are responsible for apoptosis resistance. Besides anti-apoptotic proteins, the microenvironment provides substantial survival signals to CLL leukemic cells. However, in-depth knowledge on the role of individual Bcl-2 family members in the context of the microenvironment is still limited. We performed a comprehensive analysis of transcripts and proteins of 18 Bcl-2 family members using an "apoptosis array microfluidic card" in primary cells before and after stromal co-cultures. Our data showed that five of six anti-apoptotic members (excluding Bcl-b), two of three pro-apoptotic members (excluding Bok) and six of nine BH3-only members were present at detectable mRNA levels in CLL cells. Importantly, stromal-mediated extended survival of CLL cells was strongly associated with elevated global transcription. Upon co-culturing with stromal cells, there was an early response of an increase in anti- (2/5) and pro-apoptotic protein (3/8) transcripts on day 1, while an increase in anti-apoptotic proteins was observed on day 3, with no significant change in pro-apoptotic proteins. Our study revealed a differential pattern of expression of both transcripts and proteins following stromal co-cultures, proposing a significance of Bcl-2 family members in the stromal microenvironment.

ABT-737 resistance in B-cells isolated from chronic lymphocytic leukemia patients and leukemia cell lines is overcome by the pleiotropic kinase inhibitor quercetin through Mcl-1 down-regulation

Chronic lymphocytic leukemia (CLL) is the most frequent form of leukemia in adult population and despite numerous studies, it is considered an incurable disease. Since CLL is characterized by overexpression of pro-survival Bcl-2 family members, treatments with their antagonists, such as ABT-737, represent a promising new therapeutic strategy. ABT-737 is a BH3 mimetic agent which binds Bcl-2, Bcl-XL and Bcl-w with high affinity, while weakly interacts with Mcl-1 and Bfl-1. Previous studies demonstrated that quercetin, a flavonoid naturally present in food and beverages, was able to sensitize B-cells isolated from CLL patients to apoptosis when associated with death ligands or fludarabine, through a mechanism involving Mcl-1 down-regulation. Here, we report that the association between ABT-737 and quercetin synergistically induces apoptosis in B-cells and in five leukemic cell lines (Combination Index <1). Peripheral blood mononuclear cell from healthy donors were not affected by quercetin treatment. The molecular pathways triggered by quercetin have been investigated in HPB-ALL cells, characterized by the highest resistance to both ABT-737 and quercetin when applied as single molecules, but highly sensitivity to the co-treatment. In this cell line, quercetin down-regulated Mcl-1 through the inhibition of PI3K/Akt signaling pathway, leading to Mcl-1 instability. The same mechanism was confirmed in B-cells. These results may open new clinical perspectives based on a translational approach in CLL therapy.

Deregulated expression of A1, Bcl-2, Bcl-xL, and Mcl-1 antiapoptotic proteins and Bid, Bad, and Bax proapoptotic genes in polycythemia vera patients

Apoptosis deregulation might have a role in the pathophysiology of polycythemia vera (PV). This study evaluated Bcl-2 molecule expression in CD34+ cells and leukocytes in 12 PV patients. Gene expression was investigated by real time PCR using SybrGreen Quantitect kit and protein expression was evaluated by western-blotting. JAK2 V617F mutation was detected according to Baxter et al (2005). CD34+ cells from PV patients presented higher levels of A1 and Mcl-1 expression (median: 22.6 and 5.2, respectively) in comparison with controls (0.9 and 0.5, p=0.004 and p=0.020); while Bcl-2 and Bcl-xL expression decreased in PV patients (0.18 and 1.19) compared with controls (1.39 and 2.01, p=0.006 and p=0.020). CD34+ cells in PV patients showed an elevated Bid expression (14.4) in comparison with healthy subjects (1.0; p=0.002). Patients' leukocytes showed an A1 augmentation (7.41, p=0.001) and a reduced expression of Bax (0.19; p=0.040) and Bad (0.2; p=0.030). There was no correlation between JAK2 V617F allele burden and molecular expression. PV patients showed alterations in Bcl-2 members' expression, which may interfere with control of apoptotic machinery and contribute to disease pathogenesis.

MCL-1 localizes to sites of DNA damage and regulates DNA damage response

MCL-1, a pro-survival member of the BCL-2 family, was previously shown to have functions in ATR-dependent Chk1 phosphorylation following DNA damage. To further delineate these functions, we explored possible differences in DNA damage response caused by lack of MCL-1 in mouse embryo fibroblasts (MEFs). As expected, Mcl-1(-/-) MEFs had delayed Chk1 phosphorylation following etoposide treatment, compared to wild type MEFs. However, their response to hydroxyurea, which causes a G(1)/S checkpoint response, was not significantly different. In addition, appearance of gamma-H2AX was delayed in the Mcl-1(-/-) MEFs treated with etoposide. We next investigated whether MCL-1 is present, together with other DNA damage response proteins, at the sites of DNA damage. Immunoprecipitation of etoposide-treated extracts with anti-MCL-1 antibody showed association of MCL-1 with gamma-H2AX as well as NBS1. Immunofluorescent staining for MCL-1 further showed increased co-staining of MCL-1 and NBS1 following DNA damage. By using a system that creates DNA double strand breaks at specific sites in the genome, we demonstrated that MCL-1 is recruited directly adjacent to the sites of damage. Finally, in a direct demonstration of the importance of MCL-1 in allowing proper repair of DNA damage, we found that treatment for two brief exposures to etoposide , followed by periods of recovery, which mimics the clinical situation of etoposide use, resulted in greater accumulation of chromosomal abnormalities in the MEFs that lacked MCL-1. Together, these data indicate an important role for MCL-1 in coordinating DNA damage mediated checkpoint response, and have broad implications for the importance of MCL-1 in maintenance of genome integrity.

A survey of the anti-apoptotic Bcl-2 subfamily expression in cancer types provides a platform to predict the efficacy of Bcl-2 antagonists in cancer therapy

We investigated the mRNA expression levels of all six antiapoptotic Bcl-2 subfamily members in 68 human cancer cell lines using qPCR techniques and measured the ability of known Bcl-2 inhibitors to induce cell death in 36 of the studied tumor cell lines. Our study reveals that Mcl-1 represents the anti-apoptotic Bcl-2 subfamily member with the highest mRNA levels in the lung, prostate, breast, ovarian, renal, and glioma cancer cell lines. In leukemia/lymphoma and melanoma cancer cell lines, Bcl-2 and Bfl-1 had the highest levels of mRNA, respectively. The observed correlation between the cell killing properties of known Bcl-2 inhibitors and the relative mRNA expression levels of anti-apoptotic Bcl-2 proteins provide critical insights into apoptosis-based anticancer strategies that target Bcl-2 proteins. Our data may explain current challenges of selective Bcl-2 inhibitors in the clinic, given that severe expression of Bcl-2 seems to be limited to leukemia cell lines. Furthermore, our data suggest that in most cancer types a strategy targeted to Mcl-1 inhibition, or combination of Bfl-1 and Mcl-1 inhibition for melanoma, may prove to be more successful than therapies targeting only Bcl-2.

FLT3-ITD up-regulates MCL-1 to promote survival of stem cells in acute myeloid leukemia via FLT3-ITD-specific STAT5 activation

Myeloid cell leukemia-1 (MCL-1) is an essential survival factor for hematopoiesis. In humans, hematopoietic stem cells (HSCs) express MCL-1 at the highest level in response to FMS-like tyrosine kinase-3 (FLT3) signaling. We here show that this FLT3-dependent stem cell maintenance system also plays a critical role in survival of leukemic stem cells (LSCs) in acute myeloid leukemia (AML). The CD34(+)CD38(-) LSC fraction expresses high levels of FLT3 as well as MCL-1, even compared with normal HSCs. Treatment with FLT3 ligand induced further MCL-1 up-regulation in LSCs in all AML cases tested. Interestingly, the group of samples expressing the highest levels of MCL-1 constituted AML with FLT3-internal tandem duplications (ITD). In FLT3-ITD AML cell lines, cells expressed a high level of MCL-1, and an inhibition of MCL-1 induced their apoptotic cell death. A tyrosine kinase inhibitor suppressed MCL-1 expression, and induced apoptosis that was reversed by the enforced MCL-1 expression. Finally, transduction of FLT3-ITD into HSCs strongly activated MCL-1 expression through its signal transducer and activator of transcription 5 (STAT5)-docking domains. This effect was completely abrogated when STAT5 activation was blocked. Thus, the acquisition of FLT3-ITD ensures LSC survival by up-regulating MCL-1 via constitutive STAT5 activation that is independent of wild-type FLT3 signaling.

Deguelin inhibits expression of IκBα protein and induces apoptosis of B-CLL cells in vitro

We investigated if deguelin, a naturally occurring rotenoid, was able to inhibit nuclear factor kappa B (NF-kappaB)-binding protein (IkappaBalpha) expression and to induce apoptosis in B-cell chronic lymphocytic leukemia (B-CLL) cells in vitro. Deguelin-induced cell death in the majority of B-CLL cells and was found to be more toxic toward B-CLL cells than to the normal mononuclear or B-cells, suggesting selectivity towards the malignant cells. Deguelin was found to reduce IkappaBalpha protein expression, and thus interacts with the NFkappaB pathway. The induced apoptosis was characterized by processing of caspase-9 and -3 and poly-(ADP)-ribose-polymerase cleavage. Exposure of B-CLL cells to deguelin resulted in Bcl2-associated protein (Bax) conformational changes and downregulation of the key survival protein myeloid cell leukemia sequence 1 (Mcl-1), which is associated with response to treatment in B-CLL patients. Deguelin retained its ability to induce apoptosis in B-CLL cells in the presence of interleukin-4, a pro-survival cytokine in B-CLL, and when cultured with 50% human serum. These data indicate that deguelin is able to induce apoptosis in B-CLL cells in the presence of pro-survival signals and thus merits further investigation for clinical application either as a single agent or in combination with other anticancer agents.

Sphingolipid players in the leukemia arena

Sphingolipids function as bioactive mediators of different cellular processes, mostly proliferation, survival, differentiation and apoptosis, besides being structural components of cellular membranes. Involvement of sphingolipid metabolism in cancerogenesis was demonstrated in solid tumors as well as in hematological malignancies. Herein, we describe the main biological and clinical aspects of leukemias and summarize data regarding sphingolipids as mediators of apoptosis triggered in response to anti-leukemic agents and synthetic analogs as inducers of cell death as well. We also report the contribution of molecules that modulate sphingolipid metabolism to development of encouraging strategies for leukemia treatment. Finally we address how deregulation of sphingolipid metabolism is associated to occurrence of therapy resistance both in vitro and in vivo. Sphingolipids can be considered promising therapeutic tools alone or in combination with other compounds, as well as valid targets in the attempt to eradicate leukemia and overcome drug resistance.

Inducer-and cell type-specific regulation of antiapoptotic MCL1 in myeloid leukemia and multiple myeloma cells exposed to differentiation-inducing or microtubule-disrupting agents

The antiapoptotic BCL2 family member MCL1 is rapidly upregulated upon exposure of ML-1 myeloid leukemia cells to either differentiation-inducing phorbol 12'-myristate 13'-acetate (PMA) or chemotherapeutic microtubule disrupting agents (MTDAs). This report examined how signaling for MCL1 upregulation is coupled to these two different phenotypic changes, and tested for upregulation in other hematopoietic cancers. With PMA, ERK stimulated MCL1 mRNA expression and ML-1 cell differentiation, and ERK additionally stabilized expression of the MCL1 protein. However, with MTDAs, transient ERK and ensuing JNK activation contributed to initial MCL1 upregulation and viability-retention, but sustained JNK activation eventually resulted in cell death. MCL1 was upregulated by PMA in THP-1 and U937 myeloid leukemia cells, but by MTDAs only in THP-1 cells. MCL1 expression was constitutively elevated in multiple myeloma cell lines, and was not affected by PMA/ERK or MTDAs. Thus, MCL1 expression level and sensitivity to regulation are important considerations in selecting approaches for targeting this antiapoptotic gene product to kill cancer cells.

The natural product honokiol induces caspase-dependent apoptosis in B-cell chronic lymphocytic leukemia (B-CLL) cells

B-cell chronic lymphocytic leukemia (B-CLL) remains an incurable disease that requires innovative new approaches to improve therapeutic outcome. Honokiol is a natural product known to possess potent antineoplastic and antiangiogenic properties. We examined whether honokiol can overcome apoptotic resistance in primary tumor cells derived from B-CLL patients. Honokiol induced caspase-dependent cell death in all of the B-CLL cells examined and was more toxic toward B-CLL cells than to normal mononuclear cells, suggesting greater susceptibility of the malignant cells. Honokiol-induced apoptosis was characterized by the activation of caspase-3, -8, and -9 and cleavage of poly(adenosine diphosphate-ribose) polymerase (PARP). Exposure of B-CLL cells to honokiol resulted in up-regulation of Bcl2-associated protein (Bax) and down-regulation of the expression of the key survival protein myeloid-cell leukemia sequence 1 (Mcl-1), which is associated with response to treatment in B-CLL patients. In addition, B-CLL cells pretreated with interleukin-4 (IL-4), a cytokine known to support B-CLL survival, underwent apoptosis when subsequently incubated with honokiol, indicating that honokiol could also overcome the prosurvival effects of IL-4. Furthermore, honokiol enhanced cytotoxicity induced by fludarabine, cladribine, or chlorambucil. These data indicate that honokiol is a potent inducer of apoptosis in B-CLL cells and should be examined for further clinical application either as a single agent or in combination with other anticancer agents. (Blood. 2005;106:690-697)

Mcl-1 is overexpressed in multiple myeloma and associated with relapse and shorter survival

We and others have shown that Mcl-1 was essential for the survival of human myeloma cells in vitro. Furthermore, this antiapoptotic protein is upregulated by interleukin-6, which plays a critical role in multiple myeloma (MM). For these reasons, we have evaluated the expression of Mcl-1 in vivo in normal, reactive and malignant plasma cells (PC), that is, myeloma cells from 51 patients with MM and 21 human myeloma cell lines (HMCL) using flow cytometry. We show that Mcl-1 is overexpressed in MM in comparison with normal bone marrow PC. In total, 52% of patients with MM at diagnosis (P=0.017) and 81% at relapse (P=0.014 for comparison with diagnosis) overexpress Mcl-1. Of note, only HMCL but not reactive plasmacytoses have abnormal Mcl-1 expression, although both PC expansions share similar high proliferation rates. Of interest, Bcl-2 as opposed to Mcl-1, does not discriminate malignant from normal PC. Finally, the level of Mcl-1 expression is related to disease severity, the highest values at diagnosis being associated with the shortest event-free survival (P=0.002). In conclusion, Mcl-1, which has been shown to be essential for the survival of human myeloma cells in vitro, is overexpressed in vivo in MM in relation with relapse and shorter survival. Mcl-1 represents a potential therapeutical target in MM.

Recent progress in the discovery and development of cyclin-dependent kinase inhibitors

Cyclin-dependent kinases (CDKs) have long been known to be the main facilitators of the cell proliferation cycle. However, they also play important roles in the regulation of the RNA polymerase II transcription cycle. Cancer cells display aberrant cell cycle regulation to gain proliferative advantages and they also appear to have an exaggerated dependence on RNA polymerase II transcriptional activity to sustain pro-survival and antiapoptotic signalling. A picture is now starting to emerge that both the cell-cycle and transcriptional functions of CDKs can be exploited pharmacologically with CDK inhibitors that possess appropriate selectivity profiles. In this article, recent advances into these mechanistic insights and how they can guide clinical development in terms of choice of indication are reviewed, as well as combinations with existing chemotherapies. An overview is also given of recent clinical trial results with the lead CDK inhibitor drug candidates seliciclib (CYC202, (R)-roscovitine; Cyclacel) and alvocidib (flavopiridol; Aventis-NCI), as well as the development of other clinical entries and advanced preclinical compounds. The discussion focuses on oncology, but we point out recent results with CDK inhibitors in virology and nephrology.

Apoptosis-based therapies for hematologic malignancies

Apoptosis is an intrinsic cell death program that plays critical roles in tissue homeostasis, especially in organs where high rates of daily cell production are offset by rapid cell turnover. The hematopoietic system provides numerous examples attesting to the importance of cell death mechanisms for achieving homeostatic control. Much has been learned about the mechanisms of apoptosis of lymphoid and hematopoietic cells since the seminal observation in 1980 that glucocorticoids induce DNA fragmentation and apoptosis of thymocytes and the demonstration in 1990 that depriving colony-stimulating factors from factor-dependent hematopoietic cells causes programmed cell death. From an understanding of the core components of the apoptosis machinery at the molecular and structural levels, many potential new therapies for leukemia and lymphoma are emerging. In this review, we introduce some of the drug discovery targets thus far identified within the core apoptotic machinery and describe some of the progress to date toward translating our growing knowledge about these targets into new therapies for cancer and leukemia.