Bcl2 phosphorylation and active PKC α are associated with poor survival in AML

Protein Kinase C (PKC) Isozymes as Diagnostic and Prognostic Biomarkers and Therapeutic Targets for Cancer

Protein kinase C (PKC) is a large family of calcium- and phospholipid-dependent serine/threonine kinases that consists of at least 11 isozymes. Based on their structural characteristics and mode of activation, the PKC family is classified into three subfamilies: conventional or classic (cPKCs; α, βI, βII, and γ), novel or non-classic (nPKCs; δ, ε, η, and θ), and atypical (aPKCs; ζ, ι, and λ) (PKCλ is the mouse homolog of PKCι) PKC isozymes. PKC isozymes play important roles in proliferation, differentiation, survival, migration, invasion, apoptosis, and anticancer drug resistance in cancer cells. Several studies have shown a positive relationship between PKC isozymes and poor disease-free survival, poor survival following anticancer drug treatment, and increased recurrence. Furthermore, a higher level of PKC activation has been reported in cancer tissues compared to that in normal tissues. These data suggest that PKC isozymes represent potential diagnostic and prognostic biomarkers and therapeutic targets for cancer. This review summarizes the current knowledge and discusses the potential of PKC isozymes as biomarkers in the diagnosis, prognosis, and treatment of cancers.

Deregulated calcium signaling in blood cancer: Underlying mechanisms and therapeutic potential

Intracellular calcium signaling regulates diverse physiological and pathological processes. In solid tumors, changes to calcium channels and effectors via mutations or changes in expression affect all cancer hallmarks. Such changes often disrupt transport of calcium ions (Ca²⁺) in the endoplasmic reticulum (ER) or mitochondria, impacting apoptosis. Evidence rapidly accumulates that this is similar in blood cancer. Principles of intracellular Ca²⁺ signaling are outlined in the introduction. We describe different Ca²⁺-toolkit components and summarize the unique relationship between extracellular Ca²⁺ in the endosteal niche and hematopoietic stem cells. The foundational data on Ca²⁺ homeostasis in red blood cells is discussed, with the demonstration of changes in red blood cell disorders. This leads to the role of Ca²⁺ in neoplastic erythropoiesis. Then we expand onto the neoplastic impact of deregulated plasma membrane Ca²⁺ channels, ER Ca²⁺ channels, Ca²⁺ pumps and exchangers, as well as Ca²⁺ sensor and effector proteins across all types of hematologic neoplasms. This includes an overview of genetic variants in the Ca²⁺-toolkit encoding genes in lymphoid and myeloid cancers as recorded in publically available cancer databases. The data we compiled demonstrate that multiple Ca²⁺ homeostatic mechanisms and Ca²⁺ responsive pathways are altered in hematologic cancers. Some of these alterations may have genetic basis but this requires further investigation. Most changes in the Ca²⁺-toolkit do not appear to define/associate with specific disease entities but may influence disease grade, prognosis, treatment response, and certain complications. Further elucidation of the underlying mechanisms may lead to novel treatments, with the aim to tailor drugs to different patterns of deregulation. To our knowledge this is the first review of its type in the published literature. We hope that the evidence we compiled increases awareness of the calcium signaling deregulation in hematologic neoplasms and triggers more clinical studies to help advance this field.

The complexities of PKCα signaling in cancer

Protein kinase C α (PKCα) is a ubiquitously expressed member of the PKC family of serine/threonine kinases with diverse functions in normal and neoplastic cells. Early studies identified anti-proliferative and differentiation-inducing functions for PKCα in some normal tissues (e.g., regenerating epithelia) and pro-proliferative effects in others (e.g., cells of the hematopoietic system, smooth muscle cells). Additional well documented roles of PKCα signaling in normal cells include regulation of the cytoskeleton, cell adhesion, and cell migration, and PKCα can function as a survival factor in many contexts. While a majority of tumors lose expression of PKCα, others display aberrant overexpression of the enzyme. Cancer-related mutations in PKCα are uncommon, but rare examples of driver mutations have been detected in certain cancer types (e. g., choroid gliomas). Here we review the role of PKCα in various cancers, describe mechanisms by which PKCα affects cancer-related cell functions, and discuss how the diverse functions of PKCα contribute to tumor suppressive and tumor promoting activities of the enzyme. We end the discussion by addressing mutations and expression of PKCα in tumors and the clinical relevance of these findings.

Patient-tailored design for selective co-inhibition of leukemic cell subpopulations

The extensive drug resistance requires rational approaches to design personalized combinatorial treatments that exploit patient-specific therapeutic vulnerabilities to selectively target disease-driving cell subpopulations. To solve the combinatorial explosion challenge, we implemented an effective machine learning approach that prioritizes patient-customized drug combinations with a desired synergy-efficacy-toxicity balance by combining single-cell RNA sequencing with ex vivo single-agent testing in scarce patient-derived primary cells. When applied to two diagnostic and two refractory acute myeloid leukemia (AML) patient cases, each with a different genetic background, we accurately predicted patient-specific combinations that not only resulted in synergistic cancer cell co-inhibition but also were capable of targeting specific AML cell subpopulations that emerge in differing stages of disease pathogenesis or treatment regimens. Our functional precision oncology approach provides an unbiased means for systematic identification of personalized combinatorial regimens that selectively co-inhibit leukemic cells while avoiding inhibition of nonmalignant cells, thereby increasing their likelihood for clinical translation.

Targeting the Allosteric Pathway That Interconnects the Core-Functional Scaffold and the Distal Phosphorylation Sites for Specific Dephosphorylation of Bcl-2

Protein phosphorylation is the most significant post-translational modification for regulating cellular activities, but site-specific modulation of phosphorylation is still challenging. Using three-dimensional NMR spectra, molecular dynamics simulations, and alanine mutations, we identified that the interaction network between pT69/pS70 and R106/R109 residues prevents the phosphorylation sites from exposure to phosphatase and subsequent dephosphorylation. A Bcl-2-dephosphorylation probe, S1-6e, was designed by installing a carboxylic acid group to a Bcl-2 inhibitor. The carboxyl group competitively disrupts the interaction network between R106/R109 and pT69/pS70 and subsequently facilitates Bcl-2 dephosphorylation in living cells. As a result, S1-6e manifests a more effective apoptosis induction in pBcl-2-dependent cancer cells than other inhibitors exhibiting a similar binding affinity for Bcl-2. We believe that targeting the allosteric pathways interconnecting the core-functional domain and the phosphorylation site can be a general strategy for a rational design of site-specific dephosphorylating probes, since the allosteric pathway has been discovered in a variety of proteins.

GSK-J4 induces cell cycle arrest and apoptosis via ER stress and the synergism between GSK-J4 and decitabine in acute myeloid leukemia KG-1a cells

Background

GSK-J4 is the inhibitor of H3K27me3 demethylase. Recent studies demonstrated that GSK-J4 could affect the proliferation and apoptosis of a variety of cancer cells. However, the effects and underlying mechanisms of GSK-J4 on the proliferation and apoptosis of human acute myeloid leukemia (AML) KG-1a cells have not been explored thoroughly.

Methods

The effect of GSK-J4 on cell proliferation was assessed with CCK8, while cell cycle distribution and apoptosis were analyzed using flow cytometry. The proteins related to cell cycle, cell apoptosis, endoplastic reticulum (ER) stress and PKC-α/p-Bcl2 pathway were detected by Western blotting. The expression level of PKC-α mRNA was measured by quantitative real-time PCR.ER stress inhibitor 4-phenyl butyric acid (4-PBA) was used to explore the role of ER stress in GSK-J4 induced cell-cycle arrest and cell apoptosis. The combination effects of Decitabine and GSK-J4 on KG-1a cells proliferation and apoptosis were also evaluated by CCK8, flow cytometry and immunoblot analysis.

Results

GSK-J4 reduced cell viability and arrested cell cycle progression at the S phase by decreasing the expression of CyclinD1 and CyclinA2 and increasing that of P21. Moreover, GSK-J4 enhanced the expression of apoptosis-related proteins (cle-caspase-9 and bax) and inhibited PKC-a/p-Bcl2 pathway to promote cell apoptosis. In addition, ER stress-related proteins (caspase-12, GRP78 and ATF4) were increased markedly after exposure to GSK-J4. The effects of GSK-J4 on cell cycle, apoptosis and PKC-a/p-Bcl2 pathway were attenuated after treatment with ER stress inhibitor. Furthermore, decitabine could significantly inhibit the proliferation and induce the apoptosis of KG-1a cells after combined treatment with GSK-J4.

Conclusion

Taken together, this study provided evidence that ER stress could regulate the process of GSK-J4-induced cell cycle arrest, cell apoptosis and PKC-α/p-bcl2 pathway inhibition and demonstrated a potential combinatory effect of decitabine and GSK-J4 on leukemic cell proliferation and apoptosis.

The Novel Phospholipid Mimetic KPC34 Is Highly Active Against Acute Myeloid Leukemia with Activated Protein Kinase C

Acute myeloid leukemia (AML) is an aggressive malignancy with poor outcomes. Nucleoside analogs are subject to resistance mechanisms including downregulation of equilibrative nucleoside transporter (ENT1) and deoxycytidine kinase (dCK). KPC34 is a novel phospholipid mimetic that when cleaved by phospholipase C (PLC) liberates gemcitabine monophosphate and a diacylglycerol mimetic that inhibits the classical isoforms of protein kinase C (PKC). KPC34 acts independently of ENT1 and dCK. KPC34 was active against all AML cell lines tested with IC₅₀s in the nanomolar range. Enforced expression of PLC increased response to KPC34 in vivo. In an orthotopic, xenograft model, KPC34 treatment resulted in a significant increase in survival compared to control animals and those treated with high-dose cytarabine. In a PDX model with activated PKC, there was a significant survival benefit with KPC34, and at progression, there was attenuation of PKC activation in the resistant cells. In contrast, KPC34 was ineffective against a syngeneic, orthotopic AML model without activated PKC. However, when cells from that model were forced to express PKC, there were significantly increased sensitivity in vitro and survival benefit in vivo. These data suggest that KPC34 is active against AML and that the presence of activated PKC can be a predictive biomarker.

Tetraspanin CD82 drives acute myeloid leukemia chemoresistance by modulating protein kinase C alpha and β1 integrin activation

A principal challenge in treating acute myeloid leukemia (AML) is chemotherapy refractory disease. As such, there remains a critical need to identify key regulators of chemotherapy resistance in AML. In this study, we demonstrate that the membrane scaffold, CD82, contributes to the chemoresistant phenotype of AML. Using an RNA-seq approach, we identified the increased expression of the tetraspanin family member, CD82, in response to the chemotherapeutic, daunorubicin. Analysis of the TARGET and BEAT AML databases identifies a correlation between CD82 expression and overall survival of AML patients. Moreover, using a combination of cell lines and patient samples, we find that CD82 overexpression results in significantly reduced cell death in response to chemotherapy. Investigation of the mechanism by which CD82 promotes AML survival in response to chemotherapy identified a crucial role for enhanced protein kinase c alpha (PKCα) signaling and downstream activation of the β1 integrin. In addition, analysis of β1 integrin clustering by super-resolution imaging demonstrates that CD82 expression promotes the formation of dense β1 integrin membrane clusters. Lastly, evaluation of survival signaling following daunorubicin treatment identified robust activation of p38 mitogen-activated protein kinase (MAPK) downstream of PKCα and β1 integrin signaling when CD82 is overexpressed. Together, these data propose a mechanism where CD82 promotes chemoresistance by increasing PKCα activation and downstream activation/clustering of β1 integrin, leading to AML cell survival via activation of p38 MAPK. These observations suggest that the CD82-PKCα signaling axis may be a potential therapeutic target for attenuating chemoresistance signaling in AML.

Implications of Bcl-2 and its interplay with other molecules and signaling pathways in prostate cancer progression

INTRODUCTION

Among several genetic alterations involved in the progression of prostate cancer, B cell lymphoma gene number 2 (BCL-2) is an important target molecule in the progression of androgen-independent prostate cancer (AIPC) after androgen ablation or castration. Nevertheless, the molecular mechanism of BCL-2 in prostate cancer progression remains elusive and controversial. In the current review, we discuss the critical role of BCL-2 in the carcinogenesis of prostate cancer with experimental evidences on the BCL-2 molecular networks in AIPC and androgen-dependent prostate cancer (ADPC) and subsequently suggest perspective research targeting BCL-2. Areas covered: This review focused on the molecular implications of BCL-2 in association with other molecules and signaling pathways involved in the progression and carcinogenesis of prostate cancer. Expert opinion: BCL-2 plays a pivotal role in the progression of AIPC than in ADPC since androgen represses BCL-2. BCL-2 acts as a pro-survival molecule in association with androgen-related signaling in the progression of ADPC, while BCL-2 upregulation, PTEN loss, PI3K/AKT phosphorylation and receptor tyrosine kinase (RTK) activation are primarily involved in AIPC. To identify more effective prostate cancer therapy, further mechanistic studies are required with BCL-2 inhibitors in AIPC and ADPC, considering a multi-target therapy against BCL-2 and its related signaling.

The Tim-3-galectin-9 Secretory Pathway is Involved in the Immune Escape of Human Acute Myeloid Leukemia Cells

Acute myeloid leukemia (AML) is a severe and often fatal systemic malignancy. Malignant cells are capable of escaping host immune surveillance by inactivating cytotoxic lymphoid cells. In this work we discovered a fundamental molecular pathway, which includes ligand-dependent activation of ectopically expressed latrophilin 1 and possibly other G-protein coupled receptors leading to increased translation and exocytosis of the immune receptor Tim-3 and its ligand galectin-9. This occurs in a protein kinase C and mTOR (mammalian target of rapamycin)-dependent manner. Tim-3 participates in galectin-9 secretion and is also released in a free soluble form. Galectin-9 impairs the anti-cancer activity of cytotoxic lymphoid cells including natural killer (NK) cells. Soluble Tim-3 prevents secretion of interleukin-2 (IL-2) required for the activation of cytotoxic lymphoid cells. These results were validated in ex vivo experiments using primary samples from AML patients. This pathway provides reliable targets for both highly specific diagnosis and immune therapy of AML.

Primary cilia are present on human blood and bone marrow cells and mediate Hedgehog signaling

Primary cilia are nonmotile, microtubule-based organelles that are present on the cellular membrane of all eukaryotic cells. Functional cilia are required for the response to developmental signaling pathways such as Hedgehog (Hh) and Wnt/β-catenin. Although the Hh pathway has been shown to be active in leukemia and other blood cancers, there have been no reports describing the presence of primary cilia in human blood or leukemia cells. In the present study, we show that nearly all human blood and bone marrow cells have primary cilia (97-99%). In contrast, primary cilia on AML cell lines (KG1, KG1a, and K562) were less frequent (10-36% of cells) and were often shorter and dysmorphic, with less well-defined basal bodies. Finally, we show that treatment of blood cells with the Hh pathway ligand Sonic Hedgehog (SHh) causes translocation of Smoothened (SMO) to the primary cilia and activation of Hh target genes, demonstrating that primary cilia in blood cells are functional and participate in Hh signaling. Loss of primary cilia on leukemia cells may have important implications for aberrant pathway activation and response to SMO inhibitors currently in clinical development.

The broken "Off" switch in cancer signaling: PP2A as a regulator of tumorigenesis, drug resistance, and immune surveillance

Aberrant activation of signal transduction pathways can transform a normal cell to a malignant one and can impart survival properties that render cancer cells resistant to therapy. A diverse set of cascades have been implicated in various cancers including those mediated by serine/threonine kinases such RAS, PI3K/AKT, and PKC. Signal transduction is a dynamic process involving both "On" and "Off" switches. Activating mutations of RAS or PI3K can be viewed as the switch being stuck in the "On" position resulting in continued signaling by a survival and/or proliferation pathway. On the other hand, inactivation of protein phosphatases such as the PP2A family can be seen as the defective "Off" switch that similarly can activate these pathways. A problem for therapeutic targeting of PP2A is that the enzyme is a hetero-trimer and thus drug targeting involves complex structures. More importantly, since PP2A isoforms generally act as tumor suppressors one would want to activate these enzymes rather than suppress them. The elucidation of the role of cellular inhibitors like SET and CIP2A in cancer suggests that targeting these proteins can have therapeutic efficacy by mechanisms involving PP2A activation. Furthermore, drugs such as FTY-720 can activate PP2A isoforms directly. This review will cover the current state of knowledge of PP2A role as a tumor suppressor in cancer cells and as a mediator of processes that can impact drug resistance and immune surveillance.

Tetraspanin CD82 Regulates the Spatiotemporal Dynamics of PKCα in Acute Myeloid Leukemia

Patients with acute myeloid leukemia (AML) have increased myeloid cells within their bone marrow that exhibit aberrant signaling. Therefore, therapeutic targets that modulate disrupted signaling cascades are of significant interest. In this study, we demonstrate that the tetraspanin membrane scaffold, CD82, regulates protein kinase c alpha (PKCα)-mediated signaling critical for AML progression. Utilizing a palmitoylation mutant form of CD82 with disrupted membrane organization, we find that the CD82 scaffold controls PKCα expression and activation. Combining single molecule and ensemble imaging measurements, we determine that CD82 stabilizes PKCα activation at the membrane and regulates the size of PKCα membrane clusters. Further evaluation of downstream effector signaling identified robust and sustained activation of ERK1/2 upon CD82 overexpression that results in enhanced AML colony formation. Together, these data propose a mechanism where CD82 membrane organization regulates sustained PKCα signaling that results in an aggressive leukemia phenotype. These observations suggest that the CD82 scaffold may be a potential therapeutic target for attenuating aberrant signal transduction in AML.

Anti- and Pro-apoptotic Bcl2 Proteins Distribution and Metabolic Profile in Human Coronary Aorta Endothelial Cells Before and After HypPDT

Understanding apoptosis regulatory mechanisms in endothelial cells (ECs) has great importance for the development of novel therapy strategies for cancer and cardiovascular pathologies. An oxidative stress with the generation of reactive oxygen species (ROS) is a common mechanism causing ECs' dysfunction and apoptosis. The generation of ROS can be triggered by various stimuli including photodynamic therapy (PDT). In most PDT treatments, photosensitizer (PS) is administered systemically, and thus, possibility of high exposure to PS in the ECs remains high. PS accumulation in ECs may be clinically relevant even without PDT, if PS molecules affect the pro-apoptotic cascade without illumination. In the present work, we focused on Hypericin (Hyp) and HypPDT effects on the cell viability, oxidative stress, and the distribution of Bcl2 family members in human coronary artery endothelial (HCAEC) cells. Our findings show that the presence of Hyp itself has an effect on cell viability, oxidative stress, and the distribution of Bcl2 family members, without affecting the mitochondria function. In contrast, HypPDT resulted in mitochondria dysfunction, further increase of oxidative stress and effect on the distribution of Bcl2 family members, and in primarily necrotic type of death in HCAEC cells.

Hedgehog signaling in cancer stem cells: a focus on hematological cancers

The stem cell paradigm was first demonstrated in hematopoietic stem cells. Whilst classically it was cytokines and chemokines which were believed to control stem cell fate, more recently it has become apparent that the stem cell niche and highly conserved embryonic pathways play a key role in governing stem cell behavior. One of these pathways, the hedgehog signaling pathway, found in all organisms, is vitally important in embryogenesis, performing the function of patterning through early stages of development, and in adulthood, through the control of somatic stem cell numbers. In addition to these roles in health however, it has been found to be deregulated in a number of solid and hematological malignancies, components of the hedgehog pathway being associated with a poor prognosis. Further, these components represent viable therapeutic targets, with inhibition from a drug development perspective being readily achieved, making the hedgehog pathway an attractive potential therapeutic target. However, although the concept of cancer stem cells is well established, how these cells arise and the factors which influence their behavior are not yet fully understood. The role of the hedgehog signaling pathway and its potential as a therapeutic target in hematological malignancies is the focus of this review.

Protein Kinase C (PKC) Isozymes and Cancer

Protein kinase C (PKC) is a family of phospholipid-dependent serine/threonine kinases, which can be further classified into three PKC isozymes subfamilies: conventional or classic, novel or nonclassic, and atypical. PKC isozymes are known to be involved in cell proliferation, survival, invasion, migration, apoptosis, angiogenesis, and drug resistance. Because of their key roles in cell signaling, PKC isozymes also have the potential to be promising therapeutic targets for several diseases, such as cardiovascular diseases, immune and inflammatory diseases, neurological diseases, metabolic disorders, and multiple types of cancer. This review primarily focuses on the activation, mechanism, and function of PKC isozymes during cancer development and progression.

Glucocorticoid sensitisation in Mixed Lineage Leukaemia-rearranged acute lymphoblastic leukaemia by the pan-BCL-2 family inhibitors gossypol and AT-101

AIM OF THE STUDY

Resistance to glucocorticoids (GCs) remains a major problem in the treatment of infants with acute lymphoblastic leukaemia (ALL) carrying Mixed Lineage Leukaemia (MLL) translocations. Despite intensive research, the mechanism(s) underlying GC resistance remain poorly understood. Recent studies suggested an important role for the pro-survival BCL-2 family member MCL1 in GC resistance in MLL-rearranged ALL.

METHODS

We exposed GC-resistant MLL-rearranged SEMK2 cells to potent MCL1-inhibiting agents, including gossypol, AT-101, rapamycin, SU9516 and obatoclax (GX15-070) and determined GC sensitisation using 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) assays. Using Western blotting we analysed the protein expression of most BCL-2 family members in MLL-rearranged SEMK2 cells after treatment with potent MCL-1 inhibiting agents.

RESULTS

Only gossypol and its synthetic analogue AT-101 induced GC sensitivity in MLL-rearranged ALL cells. Remarkably, the GC-sensitising effects of gossypol and AT-101 appeared not to be mediated by down-regulation MCL1 or other anti-apoptotic BCL-2 family members, but rather involved up-regulation of multiple pro-apoptotic BCL-2 family members, in particular that of BIM and BID.

CONCLUDING REMARKS

In conclusion, gossypol and AT-101 induce GC sensitivity in MLL-rearranged ALL cells, most likely mediated by the activation of BID and BIM without the necessity to down-regulate anti-apoptotic BCL-2 family members like MCL1. Hence, co-administration of either gossypol or AT-101 during GC treatment of GC-resistant MLL-rearranged ALL patients may overcome GC resistance and improve prognosis in this high-risk childhood leukaemia.

Sensitization of human pancreatic cancer cells harboring mutated K-ras to apoptosis

Pancreatic cancer is a devastating human malignancy and gain of functional mutations in K-ras oncogene is observed in 75%-90% of the patients. Studies have shown that oncogenic ras is not only able to promote cell growth or survival, but also apoptosis, depending upon circumstances. Using pancreatic cancer cell lines with or without expressing mutated K-ras, we demonstrated that the inhibition of endogenous PKC activity sensitized human pancreatic cancer cells (MIA and PANC-1) expressing mutated K-ras to apoptosis, which had no apoptotic effect on BxPC-3 pancreatic cancer cells that contain a normal Ras as well as human lung epithelial BAES-2B cells. In this apoptotic process, the level of ROS was increased and PUMA was upregulated in a p73-dependent fashion in MIA and PANC-1 cells. Subsequently, caspase-3 was cleaved. A full induction of apoptosis required the activation of both ROS- and p73-mediated pathways. The data suggest that PKC is a crucial factor that copes with aberrant K-ras to maintain the homeostasis of the pancreatic cancer cells harboring mutated K-ras. However, the suppression or loss of PKC disrupts the balance and initiates an apoptotic crisis, in which ROS and p73 appear the potential, key targets.

Sensitization of epithelial growth factor receptors by nicotine exposure to promote breast cancer cell growth

Introduction

Tobacco smoke is known to be the main cause of lung, head and neck tumors. Recently, evidence for an increasing breast cancer risk associated with tobacco smoke exposure has been emerging. We and other groups have shown that nicotine, as a non-conventional carcinogen, has the potential to facilitate cancer genesis and progression. However, the underlying mechanisms by which the smoke affects the breast, rather than the lung, remain unclear. Here, we examine possible downstream signaling pathways of the nicotinic acetylcholine receptor (nAChR) and their role in breast cancer promotion.

Methods

Using human benign MCF10A and malignant MDA-MB-231 breast cells and specific inhibitors of possible downstream kinases, we identified nAChR effectors that were activated by treatment with nicotine. We further tested the effects of these effector pathways on the regulation of E2F1 activation, cell cycle progression and on Bcl-2 expression and long-term cell survival.

Results

In this study, we demonstrated a novel signaling mechanism by which nicotine exposure activated Src to sensitize epidermal growth factor receptor (EGFR)-mediated pathways for breast cancer cell growth promotion. After the ligation of nAChR with nicotine, EGFR was shown to be activated and then internalized in both MCF10A and MDA-MB-231 breast cancer cells. Subsequently, Src, Akt and ERK1/2 were phosphorylated at different time points following nicotine treatment. We further demonstrated that through Src, the ligation of nicotine with nAChR stimulated the EGFR/ERK1/2 pathway for the activation of E2F1 and further cell progression. Our data also showed that Akt functioned directly downstream of Src and was responsible for the increase of Bcl-2 expression and long-term cell survival.

Conclusions

Our study reveals the existence of a potential, regulatory network governed by the interaction of nicotine and nAChR that integrates the conventional, mitogenic Src and EGFR signals for breast cancer development.

Targeting PKC-mediated signal transduction pathways using enzastaurin to promote apoptosis in acute myeloid leukemia-derived cell lines and blast cells

Recent studies in acute myeloid leukemia (AML) suggest activation of pro-proliferative signaling cascades including those mediated by protein kinase C (PKC) represent a poor prognostic factor for patients. The classical PKC isoforms α and β generally support survival signaling and have emerged as important targets for anti-cancer therapy. Enzastaurin is a PKC β inhibitor and is in clinical trials for lymphomas, gliomas, and lung cancer. Presently, it is not known if enzastaurin could be effective against AML. In the current study, we found that high dose enzastaurin was found to promote apoptosis in the AML-derived cell lines and in blast cells from AML patients. The mechanism of cell death, however, likely does not involve PKC β as another PKC β inhibitor was not toxic to AML cell lines and did not promote enzastaurin-induced cell killing. While enzastaurin is fairly specific for PKC β, the agent can inhibit other PKC isoforms at higher concentrations. Enzastaurin was effective at inhibiting PKC α phosphorylation and membrane localization in the AML cell lines and suppressed phosphorylation of BCL2. Furthermore, enzastaurin suppressed activation of ERK (which can be activated by PKC α). Analysis of the serine/threonine phosphorylation profile in HL60 cells after enzastaurin treatment revealed that the drug inhibits the phosphorylation of a distinct set of proteins while promoting phosphorylation of another set of proteins. This suggests the drug may regulate multiple signaling pathways. Taken together, these findings suggest that enzastaurin could be effective in the therapy of AML.

Inflammation and survival pathways: chronic lymphocytic leukemia as a model system

A primary response to inflammation is an increased survival of the target cell. Several pathways have been identified that promote maintenance of the cell. The principal mechanism for the extended survival is through induction of anti-apoptotic Bcl-2 family proteins. Bcl-2 was the founding member of this family with five additional members, Bcl-X(L), Bcl-W, Bcl-B, Bfl-1, and Mcl-1, discovered mostly in hematological malignancies. Another mechanism that could add to cell survival is the Pim kinase pathway. This family of enzymes is associated with Myc-driven transcription, cell cycle regulation, degradation of pro-apoptotic proteins, and protein translation. Chronic lymphocytic leukemia serves as an optimal model to understand the mechanism by which these two protein families provide survival advantage to cells. In addition, since this malignancy is known to be maintained by microenvironment milieu, this further adds advantage to investigate mechanisms by which these pro-survival proteins are induced in the presence of stromal support. Multiple mechanisms exists that result in increase in transcript and protein level of anti-apoptotic Bcl-2 family members. Following these inductions, post-translational modifications occur resulting in increased stability of pro-survival proteins, while Pim-mediated phosphorylation inhibits pro-apoptotic protein activity. Furthermore, there is a cross-talk between these two (Bcl-2 family proteins and Pim family proteins) pathways that co-operate with each other for CLL cell survival and maintenance. Vigorous efforts are being made to create small molecules that affect these proteins directly or indirectly. Several of these pharmacological inhibitors are in early clinical trials for patients with hematological malignancies.

Role for PKC δ in Fenretinide-Mediated Apoptosis in Lymphoid Leukemia Cells

The synthetic Vitamin A analog fenretinide is a promising chemotherapeutic agent. In the current paper, the role of PKC δ was examined in fenretinide-induced apoptosis in lymphoid leukemia cells. Levels of proapoptotic cleaved PKC δ positively correlated with drug sensitivity. Fenretinide promoted reactive oxygen species (ROS) generation. The antioxidant Vitamin C prevented fenretinide-induced PKC δ cleavage and protected cells from fenretinide. Suppression of PKC δ expression by shRNA sensitized cells to fenretinide-induced apoptosis possibly by a mechanism involving ROS production. A previous study demonstrated that fenretinide promotes degradation of antiapoptotic MCL-1 in ALL cells via JNK. Now we have found that fenretinide-induced MCL-1 degradation may involve PKC δ as cleavage of the kinase correlated with loss of MCL-1 even in cells when JNK was not activated. These results suggest that PKC δ may play a complex role in fenretinide-induced apoptosis and may be targeted in antileukemia strategies that utilize fenretinide.

Nicotine, through upregulating pro-survival signaling, cooperates with NNK to promote transformation

Cigarette smoking is a mixture of thousands of compounds, many of which are carcinogens, such as NNK [4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone]. Nicotine, as an addictive substance in cigarette, has been shown to promote growth of non-neuronal cells. It is unclear how nicotine cooperates with tobacco-related carcinogens during tumorigenesis. Here, by concurrent treatment of nicotine and NNK, we investigate the effect of the cooperation of these two compounds on cell growth and apoptosis in various different lung epithelial (RLE) or cancer (LKR) cells. We demonstrated that short-term nicotine exposure moderately activated mitogenic signaling pathways (such as PKC, ERK, and Akt) and a mediocre protection against cisplatin-mediated apoptosis. In contrast, NNK strongly stimulated mitogenic signaling and rendered the cells a high resistance to cisplatin. The pre-ligation of nAChR by nicotine interfered with NNK-mediated mitogenic signaling and resistance to cisplatin, the magnitude of which was similar as that exposed to nicotine alone. Interestingly, a week after the exposure to nicotine or nicotine plus NNK, Bcl-2 expression was augmented, accompanied with the increased resistance to cisplatin-induced apoptosis. In comparison, long-term NNK treatment provided very little protection of the cells from cisplatin. We also showed that the combination treatment promoted more cells to grow in an anchorage-independent fashion than NNK exposure alone. Thus, the data suggest that through occupying nAChR, nicotine appears to modulate NNK-mediated signaling and persistently sustain pro-survival activities to promote transformation.

Connexin-based signaling in acute myelogenous leukemia (AML)

Normal and malignant hematopoiesis are regulated by intercellular communication in the hematopoietic microenvironments, and both soluble mediators as well as direct cell-cell contact play important functional roles. Gap junctions are complex membrane structures that transfer molecules between neighboring cells and thereby alter intracellular signaling and metabolism. The gap junction building blocks, the connexins, are also involved in gap junction-independent intercellular communication by forming hemichannels that transfer substances between the intra- and extracellular spaces. Connexins are furthermore involved in cell regulation as single molecules by modulating intracellular pathways and possibly gene transcription. The role of connexins in leukemogenesis and leukemic cell functions are not well characterized. In this review, we describe the known effects of gap junctions and connexins in acute myelogenous leukemia and the diverse potential of connexins in acute myelogenous leukemia chemosensitivity, intracellular signaling and cell death regulation.

Protein phosphatase 2A inactivates Bcl2's antiapoptotic function by dephosphorylation and up-regulation of Bcl2-p53 binding

Bcl2 is associated with chemoresistance and poor prognosis in patients with various hematologic malignancies. DNA damage-induced p53/Bcl2 interaction at the outer mitochondrial membrane results in a Bcl2 conformational change with loss of its antiapoptotic activity in interleukin-3-dependent myeloid H7 cells. Here we find that specific disruption of protein phosphatase 2A (PP2A) activity by either expression of small t antigen or depletion of PP2A/C by RNA interference enhances Bcl2 phosphorylation and suppresses cisplatin-stimulated p53/Bcl2 binding in association with prolonged cell survival. By contrast, treatment of cells with C2-ceramide (a potent PP2A activator) or expression of the PP2A catalytic subunit (PP2A/C) inhibits Bcl2 phosphorylation, leading to increased p53/Bcl2 binding and apoptotic cell death. Mechanistically, PP2A-mediated dephosphorylation of Bcl2 in vitro promotes its direct interaction with p53 as well as a conformational change in Bcl2. PP2A directly interacts with the BH4 domain of Bcl2 as a docking site to potentially "bridge" PP2A to Bcl2's flexible loop domain containing the target serine 70 phosphorylation site. Thus, PP2A may provide a dual inhibitory effect on Bcl2's survival function by both dephosphorylating Bcl2 and enhancing p53-Bcl2 binding. Activating PP2A to dephosphorylate Bcl2 and/or increase Bcl2/p53 binding may represent an efficient and novel approach for treatment of hematologic malignancies.

Protein kinase C signalling in leukemia

The protein kinase C (PKC) family of proteins includes several kinases that share structural homology, but at the same time exhibit substantial functional diversity. There is a significant amount of evidence establishing distinct patterns of expression and function for different PKC isoforms and groups in different leukemias. Although most members of this family promote leukemic cell survival and growth, others exhibit opposing effects and participate in the generation of antileukemic responses. This review summarizes work in this field on the relevance of distinct members of the PKC family in the pathophysiology of myeloid and lymphoid leukemias. The clinical-therapeutic potential of such ongoing work for the treatment of future development of novel approaches for the treatment of different types of leukemias is discussed.

BCL-2 phosphorylation modulates sensitivity to the BH3 mimetic GX15-070 (Obatoclax) and reduces its synergistic interaction with bortezomib in chronic lymphocytic leukemia cells

Chronic lymphocytic leukemia (CLL) is a B-cell lymphoid neoplasm with deregulated apoptosis and overexpression of several antiapoptotic BCL-2 proteins. GX15-070/Obatoclax is a small-molecule BH3 mimetic compound that has shown activity against several hematologic malignancies and solid tumors. In the present work, we report that GX15-070 led to the disruption of BCL-2/BIM and MCL-1/BAK complexes in CLL cells, followed by the activation of the mitochondrial apoptotic pathway. CLL cells showed lower sensitivity to GX15-070 than primary mantle cell lymphoma (MCL) ones, in correlation with higher levels of phosphorylated BCL-2 at serine 70 residue (pBCL-2(Ser70)) in CLL cells. Decrease in BCL-2 phosphorylation by extracellular signal-regulated kinase (ERK)1/2 inhibition increased CLL sensitivity to GX15-070, while blocking BCL-2 dephosphorylation using a PP2A antagonist reduced the activity of this BH3 mimetic. GX15-070 activity was increased by cotreatment with the proteasome inhibitor bortezomib. However, as proteasome inhibition led to the accumulation of phosphorylated BCL-2, the degree of interaction between GX15-070 and bortezomib was regulated by basal pBCL-2(Ser70) levels. These results support the role of BCL-2 phosphorylation as a mechanism of resistance to BH3 mimetic compounds, and demonstrate that combination approaches including ERK inhibitors could enhance BH3 mimetics activity both alone or in combination with proteasome inhibitors.

Rapid and selective death of leukemia stem and progenitor cells induced by the compound 4-benzyl, 2-methyl, 1,2,4-thiadiazolidine, 3,5 dione (TDZD-8)

Leukemia is thought to arise from malignant stem cells, which have been described for acute and chronic myeloid leukemia (AML and CML) and for acute lymphoblastic leukemia (ALL). Leukemia stem cells (LSCs) are relatively resistant to current chemotherapy and likely contribute to disease relapse and progression. Consequently, the identification of drugs that can efficiently eradicate LSCs is an important priority. In the present study, we investigated the antileukemia activity of the compound TDZD-8. Analysis of primary AML, blast crisis CML (bcCML), ALL, and chronic lymphoblastic leukemia (CLL) specimens showed rapid induction of cell death upon treatment with TDZD-8. In addition, for myeloid leukemias, cytotoxicity was observed for phenotypically primitive cells, in vitro colony-forming progenitors, and LSCs as defined by xenotransplantation assays. In contrast, no significant toxicity was observed for normal hematopoietic stem and progenitor cells. Notably, cell death was frequently evident within 2 hours or less of TDZD-8 exposure. Cellular and molecular studies indicate that the mechanism by which TDZD-8 induces cell death involves rapid loss of membrane integrity, depletion of free thiols, and inhibition of both the PKC and FLT3 signaling pathways. We conclude that TDZD-8 uses a unique and previously unknown mechanism to rapidly target leukemia cells, including malignant stem and progenitor populations.

Mechanisms of apoptosis sensitivity and resistance to the BH3 mimetic ABT-737 in acute myeloid leukemia

BCL-2 proteins are critical for cell survival and are overexpressed in many tumors. ABT-737 is a small-molecule BH3 mimetic that exhibits single-agent activity against lymphoma and small-cell lung cancer in preclinical studies. We here report that ABT-737 effectively kills acute myeloid leukemia blast, progenitor, and stem cells without affecting normal hematopoietic cells. ABT-737 induced the disruption of the BCL-2/BAX complex and BAK-dependent but BIM-independent activation of the intrinsic apoptotic pathway. In cells with phosphorylated BCL-2 or increased MCL-1, ABT-737 was inactive. Inhibition of BCL-2 phosphorylation and reduction of MCL-1 expression restored sensitivity to ABT-737. These data suggest that ABT-737 could be a highly effective antileukemia agent when the mechanisms of resistance identified here are considered.