Identification and characterization of BH3 domain protein Bim and its isoforms in human hepatocellular carcinomas

Reovirus changes the expression of anti-apoptotic and proapoptotic proteins with the c-kit downregulation in canine mast cell tumor cell lines

Although reovirus has reached phase II and III clinical trials in human cancers, the exact mechanism of reovirus oncolysis is still not completely understood. Previously, we have shown that canine mast cell tumor (MCT) cell lines were highly susceptible to reovirus, as compared with other kinds of canine cancer cell lines. In this study, we showed that reovirus infection not only led to the dephosphorylation but also downregulation of c-kit in four canine MCT cell lines, where c-kit activation is required for proliferation. Consistent with c-kit dysregulation, downstream signaling of c-kit, the level of Ras-GTP and phosphorylation of all the downstream effectors of Ras (Raf, MEK, and ERK) and Akt decreased in all the cell lines after reovirus infection, except for Akt in one of cell lines. Pro-apoptotic and anti-apoptotic proteins such as Bim, Bad and Mcl-1 were also altered by reovirus infection in these cell lines. In short, reovirus infection degraded c-kit in all the canine MCT cell lines, leading to the downregulation of downstream signaling of c-kit, which may relate to the cell death induced by reovirus.

Influence of BCL2L11 polymorphism on osteonecrosis during treatment of childhood acute lymphoblastic leukemia

Osteonecrosis (ON) is corticosteroid-related complication, reported in children with acute lymphoblastic leukemia (ALL). We have previously found that polymorphisms in BCL2L11 gene coding for pro-apoptotic Bim protein influence reduction of overall survival (OS) in a corticosteroid (CS) dose-dependent manner in childhood ALL patients. The same set of SNPs was here investigated for an association with CS-related ON assessed retrospectively in 304 children with ALL from Quebec (QcALL cohort) who received Dana-Farber Cancer Institute (DFCI) ALL treatment protocols. Two-year cumulative incidence of symptomatic ON was 10.6%. Two BCL2L11 polymorphisms, the 891T>G (rs2241843) in all QcALL patients and 29201C>T (rs724710) in high-risk group were significantly associated with ON, P = 0.009 and P = 0.003, respectively. The association remained significant in multivariate model (HR_891TT = 2.4, 95% CI 1.2-4.8, P = 0.01 and HR_29201CC = 5.7, 95% CI 1.6-20.9, P = 0.008). Both polymorphisms influenced viability of dexamethasone treated lymphoblastoid cell lines (P ≤ 0.03). The 891T>G influenced Bim gamma isoform levels (0.03) and its association with ON was also confirmed in replication DFCI cohort (N = 168, P = 0.03). QcALL children had a high incidence of ON during therapy, which was highly associated with BCL2L11 polymorphisms.

Differential Impacts of Alternative Splicing Networks on Apoptosis

Apoptosis functions as a common mechanism to eliminate unnecessary or damaged cells during cell renewal and tissue development in multicellular organisms. More than 200 proteins constitute complex networks involved in apoptotic regulation. Imbalanced expressions of apoptosis-related factors frequently lead to malignant diseases. The biological functions of several apoptotic factors are manipulated through alternative splicing mechanisms which expand gene diversity by generating discrete variants from one messenger RNA precursor. It is widely observed that alternatively-spliced variants encoded from apoptosis-related genes exhibit differential effects on apoptotic regulation. Alternative splicing events are meticulously regulated by the interplay between trans-splicing factors and cis-responsive elements surrounding the regulated exons. The major focus of this review is to highlight recent studies that illustrate the influences of alternative splicing networks on apoptotic regulation which participates in diverse cellular processes and diseases.

The Role of Alternative Splicing in the Control of Immune Homeostasis and Cellular Differentiation

Alternative splicing of pre-mRNA helps to enhance the genetic diversity within mammalian cells by increasing the number of protein isoforms that can be generated from one gene product. This provides a great deal of flexibility to the host cell to alter protein function, but when dysregulation in splicing occurs this can have important impact on health and disease. Alternative splicing is widely used in the mammalian immune system to control the development and function of antigen specific lymphocytes. In this review we will examine the splicing of pre-mRNAs yielding key proteins in the immune system that regulate apoptosis, lymphocyte differentiation, activation and homeostasis, and discuss how defects in splicing can contribute to diseases. We will describe how disruption to trans-acting factors, such as heterogeneous nuclear ribonucleoproteins (hnRNPs), can impact on cell survival and differentiation in the immune system.

miR-106b-25/miR-17-92 clusters: Polycistrons with oncogenic roles in hepatocellular carcinoma

MicroRNAs are small endogenously expressed RNA molecules which are involved in the process of silencing gene expression through translational regulation. The polycistronic miR-17-92 cluster is the first microRNA cluster shown to play a role in tumorigenesis. It has two other paralogs in the human genome, the miR-106b-25 cluster and the miR-106a-363 cluster. Collectively, the microRNAs encoded by these clusters can be further grouped based on the seed sequences into four families, namely the miR-17, the miR-92, the miR-18 and the miR-19 families. Over-expression of the miR-106b-25 and miR-17-92 clusters has been reported not only during the development of cirrhosis but also subsequently during the development of hepatocellular carcinoma. Members of these clusters have also been shown to affect the replication of hepatitis B and hepatitis C viruses. Various targets of these microRNAs have been identified, and these targets are involved in tumor growth, cell survival and metastasis. In this review, we first describe the regulation of these clusters by c-Myc and E2F1, and how the members of these clusters in turn regulate E2F1 expression forming an auto-regulatory loop. In addition, the roles of the various members of the clusters in affecting relevant target gene expression in the pathogenesis of hepatocellular carcinoma will also be discussed.

A single nucleotide polymorphism in cBIM is associated with a slower achievement of major molecular response in chronic myeloid leukaemia treated with imatinib

Purpose

BIM is essential for the response to tyrosine-kinase inhibitors (TKI) in chronic myeloid leukaemia (CML) patients. Recently, a deletion polymorphism in intron 2 of the BIM gene was demonstrated to confer an intrinsic TKI resistance in Asian patients. The present study aimed at identifying mutations in the BIM sequence that could lead to imatinib resistance independently of BCR-ABL mutations.

Experimental Design

BIM coding sequence analysis was performed in 72 imatinib-treated CML patients from a French population of our centre and in 29 healthy controls (reference population) as a case-control study. Real-time quantitative PCR (RT qPCR) was performed to assess Bim expression in our reference population.

Results

No mutation with amino-acid change was found in the BIM coding sequence. However, we observed a silent single nucleotide polymorphism (SNP) c465C>T (rs724710). A strong statistical link was found between the presence of the T allele and the high Sokal risk group (p = 0.0065). T allele frequency was higher in non responsive patients than in the reference population (p = 0.0049). Similarly, this T allele was associated with the mutation frequency on the tyrosine kinase domain of BCR-ABL (p<0.001) and the presence of the T allele significantly lengthened the time to achieve a major molecular response (MMR). Finally, the presence of the T allele was related to a decreased basal expression of the Bim mRNA in the circulating mononuclear cells of healthy controls.

Conclusion

These results suggest that the analysis of the c465C>T SNP of BIM could be useful for predicting the outcome of imatinib-treated CML patients.

Bim polymorphisms: influence on function and response to treatment in children with acute lymphoblastic leukemia

PURPOSE

Corticosteroids induce apoptosis in the malignant lymphoid cells and are critical component of combination therapy for acute lymphoblastic leukemia (ALL). Several genome-wide microarray studies showed major implication of proapoptotic Bim in mediating corticosteroid-related resistance in leukemia cells.

EXPERIMENTAL DESIGN

We investigated Bim gene polymorphisms and their association with childhood ALL outcome, and the mechanism underlying the observed finding.

RESULTS

Lower overall survival (OS) was associated with Bim C29201T located in Bcl-2 homology 3 (BH3) domain (P = 0.01). An association remained significant in multivariate model (P = 0.007), was more apparent in high-risk patients (P = 0.004) and patients treated with dexamethasone (P = 0.009), and was subsequently confirmed in the replication patient cohort (P = 0.03). RNA analysis revealed that C29201T affects generation of γ isoforms (γ1) that lack proapoptotic BH3 domain. The phenotypic effect was minor suggesting the influence of additional factors that may act in conjunction with Bim genotype. Combined analysis with Mcl gene polymorphism (G-486T) revealed profound reduction in OS in individuals with both risk genotypes (P < 0.0005 in discovery and P = 0.002 in replication cohort) and particularly in high-risk patients (P ≤ 0.008).

CONCLUSIONS

Increased expression of prosurvival Mcl1 and presence of Bim isoforms lacking proapoptotic function might explain marked reduction of OS in a disease and dose-dependent manner in ALL patients carrying Bim- and Mcl1-risk genotypes.

Zinc-induced modulation of SRSF6 activity alters Bim splicing to promote generation of the most potent apoptotic isoform BimS

Bim is a member of the pro-apoptotic BH3-only Bcl-2 family of proteins. Bim gene undergoes alternative splicing to produce three predominant splicing variants (BimEL, BimL and BimS). The smallest variant BimS is the most potent inducer of apoptosis. Zinc (Zn(2+)) has been reported to stimulate apoptosis in various cell types. In this study, we examined whether Zn(2+) affects the expression of Bim in human neuroblastoma SH-SY5Y cells. Zn(2+) triggered alterations in Bim splicing and induced preferential generation of BimS, but not BimEL and BimL, in a dose- and time-dependent manner. Other metals (cadmium, cobalt and copper) and stresses (oxidative, endoplasmic reticulum and genotoxic stresses) had little or no effect on the expression of BimS. To address the mechanism of Zn(2+)-induced preferential generation of BimS, which lacks exon 4, we developed a Bim mini-gene construct. Deletion analysis using the Bim mini-gene revealed that predicted binding sites of the SR protein SRSF6, also known as SRp55, are located in the intronic region adjacent to exon 4. We also found that mutations in the predicted SRSF6-binding sites abolished generation of BimS mRNA from the mutated Bim mini-gene. In addition, a UV cross-linking assay followed by Western blotting showed that SRSF6 directly bound to the predicted binding site and Zn(2+) suppressed this binding. Moreover, Zn(2+) stimulated SRSF6 hyper-phosphorylation. TG003, a cdc2-like kinase inhibitor, partially prevented Zn(2+)-induced generation of BimS and SRSF6 hyper-phosphorylation. Taken together, our findings suggest that Zn(2+) inhibits the activity of SRSF6 and promotes elimination of exon 4, leading to preferential generation of BimS.

Coexistent mutations of KRAS and PIK3CA affect the efficacy of NVP-BEZ235, a dual PI3K/MTOR inhibitor, in regulating the PI3K/MTOR pathway in colorectal cancer

Colorectal cancer (CRC) with mutational activation of KRAS is observed frequently. In addition, PIK3CA mutations commonly coexist with KRAS mutations and lead to additive activation of the PI3K/MTOR signaling pathway. Here, we investigated how CRC cells that harbor KRAS and PIK3CA mutations affect sensitivity to inhibition of PI3K/MTOR with NVP-BEZ235 (BEZ235). We selected CRC patient samples and assessed their mutational status. CRC patients with KRAS or PIK3CA mutations show activation of AKT and MTOR, particularly when KRAS and PIK3CA mutations coexist. Suppression of PI3K/MTOR by BEZ235 results in a growth inhibitory effect and enhanced apoptosis via BIM activation in KRAS mutant cells. Mutational activation of KRAS when accompanied by a PIK3CA mutation converges at PI3K/MTOR pathway activation, resulting in resistance to BEZ235. BIM knockdown blocked the apoptotic response to BEZ235 in KRAS mutant cells, suggesting that PI3K inhibition leads to BIM accumulation. Moreover, BEZ235 treatment resulted in induction of FOXO3A activity and its induced transcription of BIM activation, which sensitized cells to cytotoxic agents leading to apoptosis in double mutant cells in vitro and in vivo. Taken together, our data suggest that targeting PI3K/MTOR sensitizes cells to apoptosis, implying that activation of PI3K/MTOR signaling via KRAS or PIK3CA mutation is an important pathway in CRC cell growth. Based on these results, coexistent KRAS and PIK3CA mutations confer resistance to BEZ235 via suppression of BIM-induced apoptosis, suggesting that combined treatment with conventional chemoagents is a potential strategy in the clinic.

MicroRNA-10b is a prognostic indicator in colorectal cancer and confers resistance to the chemotherapeutic agent 5-fluorouracil in colorectal cancer cells

PURPOSE

Recent evidence has shown that altered patterns of microRNA (miRNA) expression correlate with various human cancers. We investigated the clinical significance of miR-10b and its involvement in chemotherapeutic resistance to 5-fluorouracil (5-FU), which is a key component of common chemotherapy regimens in colorectal cancer.

METHODS

Quantitative RT-PCR was used to evaluate the clinicopathologic significance of miR-10b expression in 88 colorectal cancer cases. We also investigated the chemotherapeutic sensitivity to 5-FU in miR-10b-overexpressing colorectal cancer cells. To explore the mechanism of chemoresistance in miR-10b transfected cells, we examined whether miR-10b inhibits the pro-apoptotic BH3-only Bcl-2 family member BIM(BCL2L11), a key mediator of chemotherapy-induced cell death.

RESULTS

High level miR-10b expression was found to be significantly associated with high incidence of lymphatic invasion (P = 0.0257) and poor prognosis (P = 0.0057). Multivariate analysis indicated that high miR-10b expression is an independent prognostic factor for survival. In vitro studies revealed that miR-10b directly inhibits pro-apoptotic BIM, and the overexpression of miR-10b confers chemoresistance in colorectal cancer cells to 5-FU.

CONCLUSIONS

MiR-10b is a novel prognostic marker in colorectal cancer. Moreover, the expression of miR-10b is a potential indicator of chemosensitivity to the common 5-FU-based chemotherapy regimen.

The splicing factor SRSF1 regulates apoptosis and proliferation to promote mammary epithelial cell transformation

The splicing-factor oncoprotein SRSF1 (also known as SF2/ASF) is upregulated in breast cancers. We investigated SRSF1’s ability to transform human and mouse mammary epithelial cells in vivo and in vitro. SRSF1-overexpressing COMMA-1D cells formed tumors, following orthotopic transplantation to reconstitute the mammary gland. In 3-D culture, SRSF1-overexpressing MCF-10A cells formed larger acini than control cells, reflecting increased proliferation and delayed apoptosis during acinar morphogenesis. These effects required the first RNA-recognition motif and nuclear functions of SRSF1. SRSF1 overexpression promoted alternative splicing of BIM and BIN1 isoforms that lack pro-apoptotic functions and contribute to the phenotype. Finally, SRSF1 cooperated specifically with MYC to transform mammary epithelial cells, in part by potentiating eIF4E activation, and these cooperating oncogenes are significantly co-expressed in human breast tumors. Thus, SRSF1 can promote breast cancer, and SRSF1 itself or its downstream effectors may be valuable targets for therapeutics development.

Follicle-stimulating hormone regulates pro-apoptotic protein Bcl-2-interacting mediator of cell death-extra long (BimEL)-induced porcine granulosa cell apoptosis

The pro-apoptotic protein Bim (B-cell lymphoma-2 (Bcl-2)-interacting modulator of cell death) has recently been identified and shown to promote cell death in response to several stimuli. In this report, we investigated the role of Bim in porcine follicular atresia. Initially, Bim cDNA was cloned and characterized from porcine ovarian tissue. Porcine Bim had three alternative splicing variants (Bim-extra long, Bim-long, and Bim-short), all containing the consensus Bcl-2 homology 3 domain. We then found the Bim-extra long (Bim(EL)) protein, the most abundant isoform of Bim, was strongly expressed and co-localized with apoptotic (TUNEL-positive) granulosa cells from porcine atretic follicles. Furthermore, overexpression of Bim(EL) triggered apoptosis in granulosa cells. In primary granulosa cell cultures under basal conditions, we observed that Bim(EL) expression was dampened by treatment with follicle-stimulating hormone (FSH). The role of the PI3K/Akt pathway in the regulation of repression was clarified by the use of the PI3K inhibitor, LY294002, and by transfection with Akt siRNA. Forkhead Box Protein O3a (FoxO3a), a well defined transcriptional activator of Bim, was phosphorylated at Ser-253 and inactivated after FSH stimulation. Also, FSH abolished FoxO3a nuclear accumulation in response to LY294002. Finally, chromatin immunoprecipitation assays demonstrated that FoxO3a directly bound and activated the bim promoter. Taken together, we conclude that Bim(EL) induces porcine granulosa cell apoptosis during follicular atresia, and its expression is regulated by FSH via the PI3K/Akt/FoxO3a pathway.

Increased expression of the pro-apoptotic protein BIM, a mechanism for cAMP/protein kinase A (PKA)-induced apoptosis of immature T cells

The second messenger cAMP is proapoptotic for numerous cell types, but the mechanism for this proapoptotic action is not defined. Here, we use murine CD4(+)/CD8(+) S49 lymphoma cells and isolated thymocytes to assess this mechanism. In WT S49 cells, cAMP acts via protein kinase A (PKA) to induce G(1) phase cell cycle arrest and apoptosis. Treatment of WT and cAMP-Deathless (D-) S49 cells, which lack cAMP-promoted apoptosis, with the PKA agonist 8-(4-chlorophenylthio)-cAMP (CPT-cAMP) differentially regulates transcripts for numerous proapoptotic and antiapoptotic proteins. In contrast, kin-S49 cells (which lack PKA) show no cAMP-promoted changes in transcript expression. In this study, we use knockdown and overexpression approaches to define the role in cAMP/PKA-promoted apoptosis of the proapoptotic factor BIM (Bcl-2 interacting mediator of cell death), whose expression prominently increases in response to CPT-cAMP treatment of WT but not D- or kin- S49 cells. Conditional expression of BimL, one of the three major forms of Bim, increases apoptosis of WT, D-, and kin-S49 cells, whereas inhibition of cAMP-mediated induction of Bim isoforms by shRNAi attenuates CPT-cAMP-mediated apoptosis of WT S49 cells. Bim protein levels increase in subpopulations of CPT-cAMP-treated cells that undergo apoptosis. Thymic CD4(+)/CD8(+) cells isolated from Bim(-/-) mice corroborated the requirement of Bim expression for cAMP-promoted apoptosis. Thus, up-regulation of Bim appears to be an important determinant of cAMP/PKA-mediated apoptosis in immature T cells and may be a mechanism for such apoptosis in other cell types as well.

Global gene profiling of spontaneous hepatocellular carcinoma in B6C3F1 mice: similarities in the molecular landscape with human liver cancer

Hepatocellular carcinoma (HCC) is an important cause of morbidity and mortality worldwide. Although the risk factors of human HCC are well known, the molecular pathogenesis of this disease is complex, and in general, treatment options remain poor. The use of rodent models to study human cancer has been extensively pursued, both through genetically engineered rodents and rodent models used in carcinogenicity and toxicology studies. In particular, the B6C3F1 mouse used in the National Toxicology Program (NTP) two-year bioassay has been used to evaluate the carcinogenic effects of environmental and occupational chemicals, and other compounds. The high incidence of spontaneous HCC in the B6C3F1 mouse has challenged its use as a model for chemically induced HCC in terms of relevance to the human disease. Using global gene expression profiling, we identify the dysregulation of several mediators similarly altered in human HCC, including re-expression of fetal oncogenes, upregulation of protooncogenes, downregulation of tumor suppressor genes, and abnormal expression of cell cycle mediators, growth factors, apoptosis regulators, and angiogenesis and extracellular matrix remodeling factors. Although major differences in etiology and pathogenesis remain between human and mouse HCC, there are important similarities in global gene expression and molecular pathways dysregulated in mouse and human HCC. These data provide further support for the use of this model in hazard identification of compounds with potential human carcinogenicity risk, and may help in better understanding the mechanisms of tumorigenesis resulting from chemical exposure in the NTP two-year carcinogenicity bioassay.

Role of the miR-106b-25 microRNA cluster in hepatocellular carcinoma

MicroRNAs are tiny RNA molecules which serve as important post-transcriptional regulators of gene expression. Dysregulated expression of microRNAs has been observed in human cancers, indicating that microRNAs may function as oncogenes or as tumor suppressors. To date, the microRNAs encoded by the oncogenic miR-17-92 cluster, and its paralog the miR-106b-25 cluster, are among those which are differentially expressed in human cancers. In this study, we examined and confirmed the over-expression of these clusters in hepatocellular carcinoma and in hepatoma-derived cells. At least 50% of the tumor samples showed a greater than two-fold increase in the expression for miR-18 and for the miR-106b-25 cluster when compared with the corresponding paired non-tumor samples. Knock-down studies for the miR-106b-25 cluster, which includes miR-106b, miR-93 and miR-25, showed that the expression of the cluster is necessary for cell proliferation and for anchorage-independent growth. In tumors with high expression of this cluster, reduced expression of the BH3-only protein Bim, a miR-25 target, was observed. We further identified the transcription factor E2F1 as a target gene for miR-106b and miR-93 and it is likely that one of the roles of the miR-106b-25 cluster is to prevent excessively high E2F1 expression, which may then cause apoptosis. We conclude that there is aberrant expression of microRNAs encoded by the oncogenic miR-17-92 cluster and the miR-106b-25 cluster in hepatocellular carcinoma. The consistent overexpression of the miR-106b-25 cluster and its role in cell proliferation and anchorage-independent growth points to the oncogenic potential of this cluster.

Wilms' tumor 1 gene expression in hepatocellular carcinoma promotes cell dedifferentiation and resistance to chemotherapy

The Wilms' tumor 1 gene (WT1) encodes a transcription factor involved in cell growth and development. As we previously reported, WT1 expression is hardly detectable in normal hepatic tissue but is induced in liver cirrhosis. Although WT1 has been found to be overexpressed in a number of malignancies, the role of WT1 in hepatocarcinogenesis has not been clarified. We found that WT1 is expressed in several human hepatocellular carcinoma (HCC) cell lines, including PLC/PRF/5 and HepG2, and in HCC tumor tissue in 42% of patients. WT1 small interfering RNAs did not affect proliferation rate of HCC cells but abrogated their resistance to anoikis. Transcriptome analysis of PLC/PRF/5 cells after WT1 knockdown showed up-regulation of 251 genes and down-regulation of 321. Ninety percent of the former corresponded to metabolic genes, mostly those characterizing the mature hepatocyte phenotype. On the contrary, genes that decreased upon WT1 inhibition were mainly related to defense against apoptosis, cell cycle, and tumor progression. In agreement with these findings, WT1 expression increased the resistance of liver tumor cells to doxorubicin, a compound used to treat HCC. Interestingly, doxorubicin strongly enhanced WT1 expression in both HCC cells and normal human hepatocytes. Among different chemotherapeutics, induction of WT1 transcription was restricted to topoisomerase 2 inhibitors. When WT1 expression was prohibited, doxorubicin caused a marked increase in caspase-3 activation. In conclusion, WT1 is expressed in a substantial proportion of HCC contributing to tumor progression and resistance to chemotherapy, suggesting that WT1 may be an important target for HCC treatment.