RPPA-based proteomics recognizes distinct epigenetic signatures in chronic lymphocytic leukemia with clinical consequences.

The chronic lymphocytic leukemia (CLL) armamentarium has evolved significantly, with novel therapies that inhibit Bruton Tyrosine Kinase, PI3K delta and/or the BCL2 protein improving outcomes. Still, the clinical course of CLL patients is highly variable and most previously recognized prognostic features lack the capacity to predict response to modern treatments indicating the need for new prognostic markers. In this study, we identified four epigenetically distinct proteomic signatures of a large cohort of CLL and related diseases derived samples (n = 871) using reverse phase protein array technology. These signatures are associated with clinical features including age, cytogenetic abnormalities [trisomy 12, del(13q) and del(17p)], immunoglobulin heavy-chain locus (IGHV) mutational load, ZAP-70 status, Binet and Rai staging as well as with the outcome measures of time to treatment and overall survival. Protein signature membership was identified as predictive marker for overall survival regardless of other clinical features. Among the analyzed epigenetic proteins, EZH2, HDAC6, and loss of H3K27me3 levels were the most independently associated with poor survival. These findings demonstrate that proteomic based epigenetic biomarkers can be used to better classify CLL patients and provide therapeutic guidance.

AXL/MERTK inhibitor ONO-7475 potently synergizes with venetoclax and overcomes venetoclax resistance to kill FLT3-ITD acute myeloid leukemia.

FMS-like Tyrosine Kinase 3 (FLT3) mutation is associated with poor survival in acute myeloid leukemia (AML). The specific Anexelekto/MER Tyrosine Kinase (AXL) inhibitor, ONO-7475, kills FLT3-mutant AML cells with targets including Extracellular- signal Regulated Kinase (ERK) and Myeloid Cell Leukemia 1 (MCL1). ERK and MCL1 are known resistance factors for Venetoclax (ABT-199), a popular drug for AML therapy, prompting the investigation of the efficacy of ONO-7475 in combination with ABT-199 in vitro and in vivo. ONO-7475 synergizes with ABT-199 to potently kill FLT3-mutant acute myeloid leukemia cell lines and primary cells. ONO-7475 is effective against ABT-199-resistant cells including cells that overexpress MCL1. Proteomic analyses revealed that ABT-199-resistant cells expressed elevated levels of pro-growth and anti-apoptotic proteins compared to parental cells, and that ONO-7475 reduced the expression of these proteins in both the parental and ABT-199-resistant cells. ONO-7475 treatment significantly extended survival as a single in vivo agent using acute myeloid leukemia cell lines and PDX models. Compared to ONO-7474 monotherapy, the combination of ONO-7475/ABT-199 was even more potent in reducing leukemic burden and prolonging the survival of mice in both model systems. These results suggest that the ONO-7475/ABT-199 combination may be effective for AML therapy.

TAM kinases as regulators of cell death.

Receptor Tyrosine Kinases are critical regulators of signal transduction that support cell survival, proliferation, and differentiation. Dysregulation of normal Receptor Tyrosine Kinase function by mutation or other activity-altering event can be oncogenic or can impact the transformed malignant cell so it becomes particularly resistant to stress challenge, have increased proliferation, become evasive to immune surveillance, and may be more prone to metastasis of the tumor to other organ sites. The TAM family of Receptor Tyrosine Kinases (TYRO3, AXL, MERTK) is emerging as important components of malignant cell survival in many cancers. The TAM kinases are important regulators of cellular homeostasis and proper cell differentiation in normal cells as receptors for their ligands GAS6 and Protein S. They also are critical to immune and inflammatory processes. In malignant cells, the TAM kinases can act as ligand independent co-receptors to mutant Receptor Tyrosine Kinases and in some cases (e.g. FLT3-ITD mutant) are required for their function. They also have a role in immune checkpoint surveillance. At the time of this review, the Covid-19 pandemic poses a global threat to world health. TAM kinases play an important role in host response to many viruses and it is suggested the TAM kinases may be important in aspects of Covid-19 biology. This review will cover the TAM kinases and their role in these processes.

Heat shock factor 1 (HSF1-pSer326) predicts response to bortezomib-containing chemotherapy in pediatric AML: a COG report.

Bortezomib (BTZ) was recently evaluated in a randomized phase 3 clinical trial by the Children's Oncology Group (COG) that compared standard chemotherapy (cytarabine, daunorubicin, and etoposide [ADE]) vs standard therapy with BTZ (ADEB) for de novo pediatric acute myeloid leukemia (AML). Although the study concluded that BTZ did not improve outcome overall, we examined patient subgroups benefiting from BTZ-containing chemotherapy using proteomic analyses. The proteasome inhibitor BTZ disrupts protein homeostasis and activates cytoprotective heat shock responses. Total heat shock factor 1 (HSF1) and phosphorylated HSF1 (HSF1-pSer326) were measured in leukemic cells from 483 pediatric patients using reverse phase protein arrays. HSF1-pSer326 phosphorylation was significantly lower in pediatric AML compared with CD34+ nonmalignant cells. We identified a strong correlation between HSF1-pSer326 expression and BTZ sensitivity. BTZ significantly improved outcome of patients with low-HSF1-pSer326 with a 5-year event-free survival of 44% (ADE) vs 67% for low-HSF1-pSer326 treated with ADEB (P = .019). To determine the effect of HSF1 expression on BTZ potency in vitro, cell viability with HSF1 gene variants that mimicked phosphorylated (S326A) and nonphosphorylated (S326E) HSF1-pSer326 were examined. Those with increased HSF1 phosphorylation showed clear resistance to BTZ vs those with wild-type or reduced HSF1-phosphorylation. We hypothesize that HSF1-pSer326 expression could identify patients who benefit from BTZ-containing chemotherapy.

LGALS1 acts as a pro-survival molecule in AML.

The galectin LGALS1 is a glycan binding protein that regulates intracellular (e.g. signal transduction) and extracellular processes (e.g. immunity, leukocyte mobilization) that support cell survival. The protein is best known for its role in RAS signaling. LGALS1 is important in acute lymphoblastic leukemia but its role in acute myeloid leukemia is not well defined. We previously found suppression of LGALS1 in AML cell lines OCI-AML3 and THP-1 sensitized both cell lines to BCL2 inhibitor ABT-737. In this study, we used an in vivo murine OCI-AML3 xenograft model to test whether reduction expression of LGALS1 affects survival. Mice bearing the OCI-AML3 cells with LGALS1 shRNA survived significantly longer than mice with control OCI-AML3 cells. Gene expression profiling using RNASeq was performed using the control and LGALS1 shRNA of p53 WT OCI-AML3 and p53 mutant THP-1 cells. The data reveal distinct differences between the two cell lines in number of genes affected, in pathways associated with these genes, in expression of oncogenes, and in the transcription factors involved. The p53 pathway is prominent in OCI-AML3 cells. An examination of LGALS1 mRNA in an AML patient population reveals elevated LGALS1 mRNA is associated with shorter disease free survival and increased blasts in the BM. This data with the xenograft model data presented suggest LGALS1 may be important in the AML microenvironment. In summary, the data presented here suggest that a strategy targeting LGALS1 may benefit AML patients.

TP53/miR-34a-associated signaling targets SERPINE1 expression in human pancreatic cancer.

Pancreatic ductal adenocarcinoma (PDAC) is a disease of aging. The TP53 gene product regulates cell growth, aging, and cancer. To determine the important targets of TP53 in PDAC, we examined the expression of 440 proteins on a reverse phase protein array (RPPA) in PDAC-derived MIA-PaCa-2 cells which either had WT-TP53 or lacked WT-TP53. MIA-PaCa-2 cells have a TP53 mutation as well as mutant KRAS and represent a good in vitro model to study PDAC. RPPA analysis demonstrated expression of tumor promoting proteins in cells that lacked WT-TP53; and this feature could be reversed significantly when the cells were transfected with vector encoding WT-TP53 or treated with berberine or a modified berberine (BBR). Expression of miR-34a-associated signaling was elevated in cells expressing WT-TP53 compared to cells expressing mTP53. Results from in vivo studies using human PDAC specimens confirmed the in vitro results as the expression of miR-34a and associated signaling was significantly decreased in PDAC specimens compared to non-cancerous tissues. This study determined SERPINE1 as a miR-34a target with relevance to the biology of PDAC. Thus, we have identified a key target (SERPINE1) of the TP53/miR-34a axis that may serve as a potential biomarker for early detection of pancreatic cancer.

Comment on "PP2A inhibition sensitizes cancer stem cells to ABL tyrosine kinase inhibitors in BCR-ABL human leukemia".

LB100 does not sensitize CML stem cells to tyrosine kinase inhibitor-induced apoptosis.

LGALS3 is connected to CD74 in a previously unknown protein network that is associated with poor survival in patients with AML.

BACKGROUND: Galectin 3 (LGALS3) gene expression is associated with poor survival in acute myeloid leukemia (AML) but the prognostic impact of LGALS3 protein expression in AML is unknown. LGALS3 supports diverse survival pathways including RAS mediated cascades, protein expression and stability of anti-apoptotic BCL2 family members, and activation of proliferative pathways including those mediated by beta Catenin. CD74 is a positive regulator of CD44 and CXCR4 signaling and this molecule may be critical for AML stem cell function. At present, the role of LGALS3 and CD74 in AML is unclear. In this study, we examine protein expression of LGALS3 and CD74 by reverse phase protein analysis (RPPA) and identify new protein networks associated with these molecules. In addition, we determine prognostic potential of LGALS3, CD74, and their protein networks for clinical correlates in AML patients. METHODS: RPPA was used to determine relative expression of LGALS3, CD74, and 229 other proteins in 231 fresh AML patient samples and 205 samples were from patients who were treated and evaluable for outcome. Pearson correlation analysis was performed to identify proteins associated with LGALS3 and CD74. Progeny clustering was performed to generate protein networks. String analysis was performed to determine protein:protein interactions in networks and to perform gene ontology analysis. Kaplan-Meir method was used to generate survival curves. FINDINGS: LGALS3 is highest in monocytic AML patients and those with elevated LGALS3 had significantly shorter remission duration compared to patients with lower LGALS3 levels (median 21.9 vs 51.3 weeks, p = 0.016). Pearson correlation of LGALS3 with 230 other proteins identifies a distinct set of 37 proteins positively correlated with LGALS3 expression levels with a high representation of proteins involved in AKT and ERK signaling pathways. Thirty-one proteins were negatively correlated with LGALS3 including an AKT phosphatase. Pearson correlation of proteins associated with CD74 identified 12 proteins negatively correlated with CD74 and 16 proteins that are positively correlated with CD74. CD74 network revealed strong association with CD44 signaling and a high representation of apoptosis regulators. Progeny clustering was used to build protein networks based on LGALS3 and CD74 associated proteins. A strong relationship of the LGALS3 network with the CD74 network was identified. For AML patients with both the LGALS3 and CD74 protein cluster active, median overall survival was only 24.3 weeks, median remission duration was 17.8 weeks, and no patient survived beyond one year. INTERPRETATION: The findings from this study identify for the first time protein networks associated with LGALS3 and CD74 in AML. Each network features unique pathway characteristics. The data also suggest that the LGALS3 network and the CD74 network each support AML cell survival and the two networks may cooperate in a novel high risk AML population. FUND: Leukemia Lymphoma Society provided funds to SMK for RPPA study of AML patient population. Texas Leukemia provided funds to PPR and SMK to study CD74 and LGALS3 expression in AML patients using RPPA. No payment was involved in the production of this manuscript.

Role of protein phosphatases in the cancer microenvironment.

Cancer cells depend on a supportive niche (the tumor microenvironment) that promotes tumor cell survival while protecting the malignant cells from therapeutic challenges and the host's defense systems. Cancer cells and the support cells in the tumor microenvironment communicate via cytokines/chemokines, cell:cell contact, or alterations in the metabolic state of the niche (e.g. hypoxia) that promote growth and survival of the tumor cell, influence metastasis, and defeat immune surveillance. These signaling pathways involve dysregulation of not only protein kinases but also protein phosphatases as normal signal transduction processes require both activation and deactivation. For instance, aberrant receptor signaling can result from constitutive activation of a tyrosine kinase such as FLT3 or inactivation of a tyrosine protein phosphatase such as SHP-2 (PTPN11). Activation of serine/threonine kinases such as AKT and ERK are often observed during the development of drug resistance while genomic and non-genomic suppression of serine/threonine protein phosphatases such as PP2A achieve similar results. It is fairly clear that the various protein phosphatases will impact processes that support drug resistance. Of growing interest is the emerging model whereby the support cells in the tumor microenvironment actually serve as drivers of tumorigenesis. This phenomenon has been most prominently observed in osteoblast cells in leukemic niches. At least one protein phosphatase, PTPN11, has emerged as a critical driver of this process in juvenile myelomonocytic leukemia. This review will cover the role of various serine/threonine and tyrosine protein phosphatases in processes that are central to tumor microenvironment function.

Galectins as regulators of cell survival in the leukemia niche.

The microenvironment within the bone marrow (BM) contains support cells that promote leukemia cell survival and suppress host anti-tumor defenses. Galectins are a family of beta-galactoside binding proteins that are critical components in the tumor microenvironment. Galectin 1 (LGALS1) and Galectin 3 (LGALS3) as regulators of RAS signaling intracellularly and as inhibitors of immune cells extracellularly are perhaps the best studied members for their role in leukemia biology. Interest in Galectin 9 (LGALS9) is growing as this galectin has been identified as an immune checkpoint molecule. LGALS9 also supports leukemia stem cells (LSCs) though a mechanism of action is not clear. LGALS1 and LGALS3 each participate in a diverse number of survival pathways that promote drug resistance by supporting pro-tumor molecules such BCL2, MCL-1, and MYC and blocking tumor suppressors like p53. Acute myeloid leukemia (AML) BM mesenchymal stromal cells (MSC) have protein signatures that differ from healthy donor MSC. Elevated LGALS3 protein in AML MSC is associated with refractory disease/relapse demonstrating that MSC derived galectin impacts patient survival. LGALS3 is a critical determining factor whether MSC differentiate into adipocytes or osteoblasts so the galectin influences the cellular composition of the leukemia niche. Both LGALS3 and LGALS1 when secreted can suppress immune function. Both galectins can induce apoptosis of T cells. LGALS3 also modulates T cell receptor endocytosis and impairs interferon mediated chemokine production by binding glycosylated interferon. LGALS3 as a TIM3 binding partner acts to suppress T cell function. Galectins also impact leukemia cell mobilization and may participate in homing mechanisms. LGALS3 participates in transport mechanism of integrins, receptors, and other molecules that control cell adhesion and cell:cell interactions. The diversity of these various functions demonstrate the importance of these galectins in the leukemia niche. This review will cover the role of LGALS1, LGALS3, and LGALS9 in the various processes that are critical for maintaining leukemia cells in the tumor microenvironment.

Role of MSC-derived galectin 3 in the AML microenvironment.

In acute myeloid leukemia (AML), high Galectin 3 (LGALS3) expression is associated with poor prognosis. The role of LGALS3 derived from mesenchymal stromal cells (MSC) in the AML microenvironment is unclear; however, we have recently found high LGALS3 expression in MSC derived from AML patients is associated with relapse. In this study, we used reverse phase protein analysis (RPPA) to correlate LGALS3 expression in AML MSC with 119 other proteins including variants of these proteins such as phosphorylated forms or cleaved forms to identify biologically relevant pathways. RPPA revealed that LGALS3 protein was positively correlated with expression of thirteen proteins including MYC, phosphorylated beta-Catenin (p-CTNNB1), and AKT2 and negatively correlated with expression of six proteins including integrin beta 3 (ITGB3). String analysis revealed that proteins positively correlated with LGALS3 showed strong interconnectivity. Consistent with the RPPA results, LGALS3 suppression by shRNA in MSC resulted in decreased MYC and AKT expression while ITGB3 was induced. In co-culture, the ability of AML cell to adhere to MSC LGALS3 shRNA transductants was reduced compared to AML cell adhesion to MSC control shRNA transductants. Finally, use of novel specific LGALS3 inhibitor CBP.001 in co-culture of AML cells with MSC reduced viable leukemia cell populations with induced apoptosis and augmented the chemotherapeutic effect of AraC. In summary, the current study demonstrates that MSC-derived LGALS3 may be critical for important biological pathways for MSC homeostasis and for regulating AML cell localization and survival in the leukemia microenvironmental niche.

Distinct protein signatures of acute myeloid leukemia bone marrow-derived stromal cells are prognostic for patient survival.

Mesenchymal stromal cells (MSC) support acute myeloid leukemia (AML) cell survival in the bone marrow (BM) microenvironment. Protein expression profiles of AML-derived MSC are unknown. Reverse phase protein array analysis was performed to compare expression of 151 proteins from AML-MSC (n=106) with MSC from healthy donors (n=71). Protein expression differed significantly between the two groups with 19 proteins over-expressed in leukemia stromal cells and 9 over-expressed in normal stromal cells. Unbiased hierarchical clustering analysis of the samples using these 28 proteins revealed three protein constellations whose variation in expression defined four MSC protein expression signatures: Class 1, Class 2, Class 3, and Class 4. These cell populations appear to have clinical relevance. Specifically, patients with Class 3 cells have longer survival and remission duration compared to other groups. Comparison of leukemia MSC at first diagnosis with those obtained at salvage (i.e. relapse/refractory) showed differential expression of 9 proteins reflecting a shift toward osteogenic differentiation. Leukemia MSC are more senescent compared to their normal counterparts, possibly due to the overexpressed p53/p21 axis as confirmed by high ß-galactosidase staining. In addition, overexpression of BCL-XL in leukemia MSC might give survival advantage under conditions of senescence or stress and overexpressed galectin-3 exerts profound immunosuppression. Together, our findings suggest that the identification of specific populations of MSC in AML patients may be an important determinant of therapeutic response.

Targeting signaling and apoptotic pathways involved in chemotherapeutic drug-resistance of hematopoietic cells.

A critical problem in leukemia as well as other cancer therapies is the development of chemotherapeutic drug-resistance. We have developed models of hematopoietic drug resistance that are based on expression of dominant-negative TP53 [TP53 (DN)] or constitutively-active MEK1 [MEK1(CA)] oncogenes in the presence of chemotherapeutic drugs. In human cancer, functional TP53 activity is often lost in human cancers. Also, activation of the Raf/MEK/ERK pathway frequently occurs due to mutations/amplification of upstream components of this and other interacting pathways. FL5.12 is an interleukin-3 (IL-3) dependent hematopoietic cell line that is sensitive to doxorubicin (a.k.a Adriamycin). FL/Doxo is a derivative cell line that was isolated by culturing the parental FL5.12 cells in doxorubicin for prolonged periods of time. FL/Doxo + TP53 (DN) and FL/Doxo + MEK1 (CA) are FL/Doxo derivate cell lines that were infected with retrovirus encoding TP53 (DN) or MEK1 (CA) and are more resistant to doxorubicin than FL/Doxo cells. This panel of cell lines displayed differences in the sensitivity to inhibitors that suppress mTORC1, BCL2/BCLXL, MEK1 or MDM2 activities, as well as, the proteasomal inhibitor MG132. The expression of key genes involved in cell growth and drug-resistance (e.g., MDM2, MDR1, BAX) also varied in these cells. Thus, we can begin to understand some of the key genes that are involved in the resistance of hematopoietic cells to chemotherapeutic drugs and targeted therapeutics.

Anexelekto/MER tyrosine kinase inhibitor ONO-7475 arrests growth and kills FMS-like tyrosine kinase 3-internal tandem duplication mutant acute myeloid leukemia cells by diverse mechanisms.

Nearly one-third of patients with acute myeloid leukemia have FMS-like tyrosine kinase 3 mutations and thus have poor survival prospects. Receptor tyrosine kinase anexelekto is critical for FMS-like tyrosine kinase 3 signaling and participates in FMS-like tyrosine kinase 3 inhibitor resistance mechanisms. Thus, strategies targeting anexelekto could prove useful for acute myeloid leukemia therapy. ONO-7475 is an inhibitor with high specificity for anexelekto and MER tyrosine kinase. Herein, we report that ONO-7475 potently arrested growth and induced apoptosis in acute myeloid leukemia with internal tandem duplication mutation of FMS-like tyrosine kinase 3. MER tyrosine kinase-lacking MOLM13 cells were sensitive to ONO-7475, while MER tyrosine kinase expressing OCI-AML3 cells were resistant, suggesting that the drug acts via anexelekto in acute myeloid leukemia cells. Reverse phase protein analysis of ONO-7475 treated cells revealed that cell cycle regulators like cyclin dependent kinase 1, cyclin B1, polo-like kinase 1, and retinoblastoma were suppressed. ONO-7475 suppressed cyclin dependent kinase 1, cyclin B1, polo-like kinase 1 gene expression suggesting that anexelekto may regulate the cell cycle, at least in part, via transcriptional mechanisms. Importantly, ONO-7475 was effective in a human FMS-like tyrosine kinase 3 with internal tandem duplication mutant murine xenograft model. Mice fed a diet containing ONO-7475 exhibited significantly longer survival and, interestingly, blocked leukemia cell infiltration in the liver. In summary, ONO-7475 effectively kills acute myeloid leukemia cells in vitro and in vivo by mechanisms that involve disruption of diverse survival and proliferation pathways.

GSK-3 as a novel prognostic indicator in leukemia.

While leukemias represent a diverse set of diseases with malignant cells derived from myeloid or lymphoid origin, a common feature is the dysregulation of signal transduction pathways that influence leukemogeneisis, promote drug resistance, and favor leukemia stem cells. Mutations in PI3K, PTEN, RAS, or other upstream regulators can activate the AKT kinase which has central roles in supporting cell proliferation and survival. A major target of AKT is Glycogen Synthase Kinase 3 (GSK3). GSK3 has two isoforms (alpha and beta) that were studied as regulators of metabolism but emerged as central players in cancer in the early 1990s. GSK3 is unique in that the isoforms are constitutively active. Active GSK3 promotes destruction of oncogenic proteins such as beta Catenin, c-MYC, and MCL-1 and thus has tumor suppressor properties. In AML, inactivation of GSK3 is associated with poor overall survival. Interestingly in some leukemias GSK3 targets a tumor suppressor and thus the kinases can act as tumor promoters in those instances. An example is GSK3 targeting p27Kip1 in AML with MLL translocation. This review will cover the role of GSK3 in various leukemias both as tumor suppressor and tumor promoter. We will also briefly cover current state of GSK3 inhibitors for leukemia therapy.

Combining Anti-Mir-155 with Chemotherapy for the Treatment of Lung Cancers.

Purpose: The oncogenic miR-155 is upregulated in many human cancers, and its expression is increased in more aggressive and therapy-resistant tumors, but the molecular mechanisms underlying miR-155-induced therapy resistance are not fully understood. The main objectives of this study were to determine the role of miR-155 in resistance to chemotherapy and to evaluate anti-miR-155 treatment to chemosensitize tumors.Experimental Design: We performed in vitro studies on cell lines to investigate the role of miR-155 in therapy resistance. To assess the effects of miR-155 inhibition on chemoresistance, we used an in vivo orthotopic lung cancer model of athymic nude mice, which we treated with anti-miR-155 alone or in combination with chemotherapy. To analyze the association of miR-155 expression and the combination of miR-155 and TP53 expression with cancer survival, we studied 956 patients with lung cancer, chronic lymphocytic leukemia, and acute lymphoblastic leukemia.Results: We demonstrate that miR-155 induces resistance to multiple chemotherapeutic agents in vitro, and that downregulation of miR-155 successfully resensitizes tumors to chemotherapy in vivo We show that anti-miR-155-DOPC can be considered non-toxic in vivo We further demonstrate that miR-155 and TP53 are linked in a negative feedback mechanism and that a combination of high expression of miR-155 and low expression of TP53 is significantly associated with shorter survival in lung cancer.Conclusions: Our findings support the existence of an miR-155/TP53 feedback loop, which is involved in resistance to chemotherapy and which can be specifically targeted to overcome drug resistance, an important cause of cancer-related death. Clin Cancer Res; 23(11); 2891-904. ©2016 AACR.

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.

Atg7 suppression enhances chemotherapeutic agent sensitivity and overcomes stroma-mediated chemoresistance in acute myeloid leukemia.

Autophagy is a cellular adaptive mechanism to stress, including that induced by chemotherapeutic agents. Reverse phase protein array suggested that high expression of the essential autophagy-related protein, Atg7, was associated with shorter remission in newly diagnosed acute myeloid leukemia (AML) patient samples, indicating a role in chemoresistance. Knockdown of Atg7 in AML cells using short hairpin RNA markedly increased apoptosis and DNA damage following treatment with cytarabine and idarubicin. Interestingly, coculture of AML cells with stromal cells increased autophagy and chemoresistance in the AML cells exposed to chemotherapeutic agents, and this was reversed following Atg7 knockdown. This effect was further enhanced by concomitant knockdown of Atg7 in both AML and stromal cells. These findings strongly suggest that Atg7, and likely microenvironment autophagy in general, plays an important role in AML chemoresistance. Mechanistic studies revealed that Atg7 knockdown induced a proapoptotic phenotype in AML cells, which was manifested by an increased NOXA expression at the transcriptional level. Finally, in a mouse model of human leukemia, Atg7 knockdown extended overall survival after chemotherapy. Thus, the inhibition of Atg7 appears to be a valid strategy to enhance chemosensitivity, and it could indeed improve outcomes in AML therapy.