c-Myc-deficient B lymphocytes are resistant to spontaneous and induced cell death

CD40 signaling-mediated delay in terminal differentiation of B cells enables alternate fate choices during early divisions

Memory B cells and differentiated plasma cells combine to confer sustained humoral immunity. Nonetheless, we are yet to understand how B cells decide between these fates. Although pan-T cell help augments plasma cell differentiation, signaling via CD40 alone is considered to be inhibitory. Here, we examine the capacity of CD40 signaling to interfere with lipopolysaccharide-induced differentiation. Whereas lipopolysaccharide stimulation yielded only short-lived plasmablasts, co-stimulation of CD40 enhanced activation, proliferation, survival, and isotype-switching, leading to alternate fate choices such as germinal center and memory B cells during early divisions. Contrary to the notion that CD40 signaling simply arrests differentiation, the survivors, at later time points, developed into long-lived mature plasma cells, after progressively losing their ability to get restimulated. Counterintuitively, as constitutive lipopolysaccharide stimulation itself hampered differentiation, we identified that the proliferation potential of cells acted alongside CD40 signaling. Accordingly, we propose a bi-layered regulation of differentiation - CD40 signaling and proliferation potential of cells independently inhibit the commitment to and maturation of differentiation, respectively. Elucidating such cell fate decision mechanisms will aid in better vaccine design and disease management.

Is nuclear sirtuin SIRT6 a master regulator of immune function?

The ability to ward off pathogens with minimal damage to the host determines the immune system's robustness. Multiple factors, including pathogen processing, identification, secretion of mediator and effector molecules, and immune cell proliferation and differentiation into various subsets, constitute the success of mounting an effective immune response. Cellular metabolism controls all of these intricate processes. Cells utilize diverse fuel sources and switch back and forth between different metabolic pathways depending on their energy needs. The three most critical metabolic pathways on which immune cells depend to meet their energy needs are oxidative metabolism, glycolysis, and glutaminolysis. Dynamic switching between these metabolic pathways is needed for optimal function of the immune cells. Moreover, switching between these metabolic pathways needs to be tightly regulated to achieve the best results. Immune cells depend on the Warburg effect for their growth, proliferation, secretory, and effector functions. Here, we hypothesize that the sirtuin, SIRT6, could be a negative regulator of the Warburg effect. We also postulate that SIRT6 could act as a master regulator of immune cell metabolism and function by regulating critical signaling pathways.

IL-6 produced by prostate epithelial cells stimulated with Trichomonas vaginalis promotes proliferation of prostate cancer cells by inducing M2 polarization of THP-1-derived macrophages

Trichomonas vaginalis (Tv), a protozoan parasite causing sexually-transmitted disease, has been detected in tissue of prostatitis, benign prostatic hyperplasia (BPH) and prostate cancer (PCa). IL-6, a mediator of chronic inflammation, induces the progression of prostate cancer, and influences the polarization of M2 macrophages, which are the main tumor-associated macrophages. We investigated whether IL-6 produced by human prostate epithelial cells stimulated with Tv induces the M2 polarization of THP-1-derived macrophages, which in turn promotes the progression of PCa. Conditioned medium was prepared from Tv-infected (TCM) and uninfected (CM) prostate epithelial cells (RWPE-1). Thereafter conditioned medium was prepared from macrophages after incubation with CM (M-CM) or TCM (M-TCM). RWPE-1 cells infected with Tv produced IL-6 and chemokines such as CCL2 and CXCL8. When human macrophages were treated with conditioned medium of RWPE-1 cells co-cultured with Tv (TCM), they became polarized to M2-like macrophages as indicated by the production of IL-10 and TGF-β, and the expression of CD36 and arginase-1, which are M2 macrophage markers. Moreover, proliferation of the M2-like macrophages was also increased by TCM. Blockade of IL-6 signaling with IL-6 receptor antibody and JAK inhibitor (Ruxolitinib) inhibited M2 polarization of THP-1-derived macrophages and proliferation of the macrophages. To assess the effect of crosstalk between macrophages and prostate epithelial cells inflamed by Tv infection on the growth of prostate cancer (PCa) cells, PC3, DU145 and LNCaP cells were treated with conditioned medium from THP-1-derived macrophages stimulated with TCM (M-TCM). Proliferation and migration of the PCa cells were significantly increased by the M-TCM. Our findings suggest that IL-6 produced in response to Tv infection of the prostate has an important effect on the tumor microenvironment by promoting progression of PCa cells following induction of M2 macrophage polarization.

In male, T. vaginalis infection have been proposed to involve in several prostate diseases such as prostatitis, benign prostatic hyperplasia and prostate cancer. However, studies for these mechanisms have been rare. We have previously reported that T. vaginalis induce the production of inflammatory cytokines in prostate cells. Among these cytokines, IL-6 have been reported to play an important role in M2 macrophage polarization, which lead to formation of tumor microenvironment in various cancers. Here we show that IL-6 produced by T. vaginalis infection in prostate epithelial cells induces M2 polarization of macrophages and these macrophages promote proliferation of prostate cancer cells. These findings suggest that T. vaginalis indirectly induces progression of prostate cancer by creating a tumor microenvironment through an inflammatory response.

MYC in Germinal Center-derived lymphomas: Mechanisms and therapeutic opportunities

The rearrangement of immunoglobulin loci during the germinal center reaction is associated with an increased risk of chromosomal translocations that activate oncogenes such as MYC, BCL2 or BCL6, thus contributing to the development of B-cell lymphomas. MYC and BCL2 activation are initiating events in Burkitt's (BL) and Follicular Lymphoma (FL), respectively, but can occur at later stages in other subtypes such as Diffuse Large-B Cell Lymphoma (DLBCL). MYC can also be activated during the progression of FL to the transformed stage. Thus, either DLBCL or FL can give rise to aggressive double-hit lymphomas (DHL) with concurrent activation of MYC and BCL2. Research over the last three decades has improved our understanding of the functions of these oncogenes and the basis for their cooperative action in lymphomagenesis. MYC, in particular, is a transcription factor that contributes to cell activation, growth and proliferation, while concomitantly sensitizing cells to apoptosis, the latter being blocked by BCL2. Here, we review our current knowledge about the role of MYC in germinal center B-cells and lymphomas, discuss MYC-induced dependencies that can sensitize cancer cells to select pharmacological inhibitors, and illustrate their therapeutic potential in aggressive lymphomas-and in particular in DHL, in combination with BCL2 inhibitors.

Functional interplay between c-Myc and Max in B lymphocyte differentiation

The Myc family of oncogenic transcription factors regulates myriad cellular functions. Myc proteins contain a basic region/helix-loop-helix/leucine zipper domain that mediates DNA binding and heterodimerization with its partner Max. Among the Myc proteins, c-Myc is the most widely expressed and relevant in primary B lymphocytes. There is evidence suggesting that c-Myc can perform some of its functions in the absence of Max in different cellular contexts. However, the functional in vivo interplay between c-Myc and Max during B lymphocyte differentiation is not well understood. Using in vivo and ex vivo models, we show that while c-Myc requires Max in primary B lymphocytes, several key biological processes, such as cell differentiation and DNA replication, can initially progress without the formation of c-Myc/Max heterodimers. We also describe that B lymphocytes lacking Myc, Max, or both show upregulation of signaling pathways associated with the B-cell receptor. These data suggest that c-Myc/Max heterodimers are not essential for the initiation of a subset of important biological processes in B lymphocytes, but are required for fine-tuning the initial response after activation.

Crosstalk between hepatic tumor cells and macrophages via Wnt/β-catenin signaling promotes M2-like macrophage polarization and reinforces tumor malignant behaviors

Tumor-associated macrophages (TAMs) are a major component of tumor microenvironment (TME) and play pivotal roles in the progression of hepatocellular carcinoma (HCC). Wnt signaling is evolutionarily conserved and participates in liver tumorigenesis. Several studies have shown that macrophage-derived Wnt ligands can activate Wnt signaling in tumor cells. However, whether Wnt ligands secreted by tumor cells can trigger Wnt signaling in macrophages is still elusive. In this study, we first verified that canonical Wnt/β-catenin signaling was activated during monocyte-to-macrophage differentiation and in M2-polarized macrophages. Knockdown of β-catenin in M2 macrophages exhibited stronger antitumor characteristics when cocultured with Hepa1-6 HCC cells in a series of experiments. Activation of Wnt signaling promoted M2 macrophage polarization through c-Myc. Moreover, co-culturing naïve macrophages with Hepa1-6 HCC cells in which Wnt ligands secretion was blocked by knockdown of Wntless inhibited M2 polarization in vitro. Consistently, the growth of HCC tumor orthotopically inoculated with Wntless-silenced Hepa1-6 cells was impeded, and the phenotype of M2-like TAMs was abrogated due to attenuated Wnt/β-catenin signaling in TAMs, leading to subverted immunosuppressive TME. Finally, we confirmed the correlation between M2 macrophage polarization and nuclear β-catenin accumulation in CD68⁺ macrophages in human HCC biopsies. Taken together, our study indicates that tumor cells-derived Wnt ligands stimulate M2-like polarization of TAMs via canonical Wnt/β-catenin signaling, which results in tumor growth, migration, metastasis, and immunosuppression in HCC. To block Wnts secretion from tumor cells and/or Wnt/β-catenin signal activation in TAMs may be potential strategy for HCC therapy in future.

MYC in Regulating Immunity: Metabolism and Beyond

Myelocytomatosis oncogene (MYC) family members, including cellular MYC (c-Myc), neuroblastoma derived MYC (MYCN), and lung carcinoma derived MYC (MYCL), have all been implicated as key oncogenic drivers in a broad range of human cancers. Beyond cancer, MYC plays an important role in other physiological and pathological processes, namely immunity and immunological diseases. MYC largely functions as a transcription factor that promotes the expression of numerous target genes to coordinate death, proliferation, and metabolism at the cellular, tissue, and organismal levels. It has been shown that the expression of MYC family members is tightly regulated in immune cells during development or upon immune stimulations. Emerging evidence suggests that MYC family members play essential roles in regulating the development, differentiation and activation of immune cells. Through driving the expression of a broad range of metabolic genes in immune cells, MYC family members coordinate metabolic programs to support immune functions. Here, we discuss our understanding of MYC biology in immune system and how modulation of MYC impacts immune metabolism and responses.

The activities of MYC, MNT and the MAX-interactome in lymphocyte proliferation and oncogenesis

The MYC family of proteins plays essential roles in embryonic development and in oncogenesis. Efforts over the past 30 years to define the transcriptional activities of MYC and how MYC functions to promote proliferation have produced evolving models of MYC function. One picture that has emerged of MYC and its partner protein MAX is of a transcription factor complex with a seemingly unique ability to stimulate the transcription of genes that are epigenetically poised for transcription and to amplify the transcription of actively transcribed genes. During lymphocyte activation, MYC is upregulated and stimulates a pro-proliferative program in part through the upregulation of a wide variety of metabolic effector genes that facilitate cell growth and cell cycle progression. MYC upregulation simultaneously sensitizes cells to apoptosis and activated lymphocytes and lymphoma cells have pro-survival attributes that allow MYC-driven proliferation to prevail. For example, the MAX-interacting protein MNT is upregulated in activated lymphocytes and was found to protect lymphocytes from MYC-dependent apoptosis. Here we review the activities of MYC, MNT and other MAX interacting proteins in the setting of T and B cell activation and oncogenesis. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Toll-like receptor agonists induce apoptosis in mouse B-cell lymphoma cells by altering NF-κB activation

Toll-like receptor 9 (TLR9) recognizes microbial DNA containing unmethylated cytosyl guanosyl (CpG) sequences, induces innate immune responses, and facilitates antigen-specific adaptive immunity. Recent studies report that in addition to stimulating innate immunity, TLR9 ligands induce apoptosis of TLR9 expressing cancer cells. To understand the mechanism of TLR9-induced apoptosis, we compared the effects of CpG containing oligodeoxynucleotides (CpG ODN) on a mouse B-cell lymphoma line, CH27, with those on mouse splenic B cells. CpG ODN inhibited constitutive proliferation and induced apoptosis in the CH27 B-cell lymphoma line. In contrast, CpG ODN-treated primary B cells were stimulated to proliferate and were rescued from spontaneous apoptosis. The induction of apoptosis required the ODNs to contain the CpG motif and the expression of TLR9 in lymphoma B cells. A decrease in Bcl-xl expression and an increase in Fas and Fas ligand expression accompanied lymphoma B-cell apoptosis. Treatment with the Fas ligand-neutralizing antibody inhibited CpG ODN-induced apoptosis. CpG ODN triggered a transient NF-κB activation in the B-cell lymphoma cell line, which constitutively expresses a high level of c-Myc, while CpG ODN induced sustained increases in NF-κB activation and c-Myc expression in primary B cells. Furthermore, an NF-κB inhibitor inhibited the proliferation of the CH27 B-cell lymphoma line. Our data suggest that the differential responses of lymphoma and primary B cells to CpG ODN are the result of differences in NF-κB activation. The impaired NF-κB activation in the CpG ODN-treated B-cell lymphoma cell line alters the balance between NF-κB and c-Myc, which induces Fas/Fas ligand-dependent apoptosis.

The proto-oncogene c-myc regulates antibody secretion and Ig class switch recombination

The immune response involves the generation of Ab-secreting cells and memory B cells through a process called terminal B lymphocyte differentiation. This program requires the transcriptional repressor Blimp-1, which inhibits c-myc expression and terminates proliferation. Although the role of c-Myc in cell proliferation is well characterized, it is not known whether it has other functions in terminal differentiation. In this study, we show that c-Myc not only regulates cell proliferation, but it is also essential for Ab-secreting cell function and differentiation in vivo. c-Myc-deficient B lymphocytes hypersecrete IgM and do not undergo Ig class switch recombination (CSR). CSR has been previously linked to proliferation, and in this study we mechanistically link class switching and proliferation via c-Myc. We observed that c-Myc regulates CSR by transcriptionally activating the B cell-specific factor activation-induced cytidine deaminase. By linking cell proliferation and CSR, c-Myc is thus a critical component for a potent immune response.

In vivo inhibition of c-MYC in myeloid cells impairs tumor-associated macrophage maturation and pro-tumoral activities

Although tumor-associated macrophages (TAMs) are involved in tumor growth and metastasis, the mechanisms controlling their pro-tumoral activities remain largely unknown. The transcription factor c-MYC has been recently shown to regulate in vitro human macrophage polarization and be expressed in macrophages infiltrating human tumors. In this study, we exploited the predominant expression of LysM in myeloid cells to generate c-Myc^fl/fl LysM^cre/+ mice, which lack c-Myc in macrophages, to investigate the role of macrophage c-MYC expression in cancer. Under steady-state conditions, immune system parameters in c-Myc^fl/fl LysM^cre/+ mice appeared normal, including the abundance of different subsets of bone marrow hematopoietic stem cells, precursors and circulating cells, macrophage density, and immune organ structure. In a model of melanoma, however, TAMs lacking c-Myc displayed a delay in maturation and showed an attenuation of pro-tumoral functions (e.g., reduced expression of VEGF, MMP9, and HIF1α) that was associated with impaired tissue remodeling and angiogenesis and limited tumor growth in c-Myc^fl/fl LysM^cre/+ mice. Macrophage c-Myc deletion also diminished fibrosarcoma growth. These data identify c-Myc as a positive regulator of the pro-tumoral program of TAMs and suggest c-Myc inactivation as an attractive target for anti-cancer therapy.

Absence of sclerostin adversely affects B-cell survival

Increased osteoblast activity in sclerostin-knockout (Sost(-/-)) mice results in generalized hyperostosis and bones with small bone marrow cavities resulting from hyperactive mineralizing osteoblast populations. Hematopoietic cell fate decisions are dependent on their local microenvironment, which contains osteoblast and stromal cell populations that support both hematopoietic stem cell quiescence and facilitate B-cell development. In this study, we investigated whether high bone mass environments affect B-cell development via the utilization of Sost(-/-) mice, a model of sclerosteosis. We found the bone marrow of Sost(-/-) mice to be specifically depleted of B cells because of elevated apoptosis at all B-cell developmental stages. In contrast, B-cell function in the spleen was normal. Sost expression analysis confirmed that Sost is primarily expressed in osteocytes and is not expressed in any hematopoietic lineage, which indicated that the B-cell defects in Sost(-/-) mice are non-cell autonomous, and this was confirmed by transplantation of wild-type (WT) bone marrow into lethally irradiated Sost(-/-) recipients. WT→Sost(-/-) chimeras displayed a reduction in B cells, whereas reciprocal Sost(-/-) →WT chimeras did not, supporting the idea that the Sost(-/-) bone environment cannot fully support normal B-cell development. Expression of the pre-B-cell growth stimulating factor, Cxcl12, was significantly lower in bone marrow stromal cells of Sost(-/-) mice, whereas the Wnt target genes Lef-1 and Ccnd1 remained unchanged in B cells. Taken together, these results demonstrate a novel role for Sost in the regulation of bone marrow environments that support B cells.

Postnatal liver growth and regeneration are independent of c-myc in a mouse model of conditional hepatic c-myc deletion

BACKGROUND

The transcription factor c-myc regulates genes involved in hepatocyte growth, proliferation, metabolism, and differentiation. It has also been assigned roles in liver development and regeneration. In previous studies, we made the unexpected observation that c-Myc protein levels were similar in proliferating fetal liver and quiescent adult liver with c-Myc displaying nucleolar localization in the latter. In order to investigate the functional role of c-Myc in adult liver, we have developed a hepatocyte-specific c-myc knockout mouse, c-mycfl/fl;Alb-Cre.

RESULTS

Liver weight to body weight ratios were similar in control and c-myc deficient mice. Liver architecture was unaffected. Conditional c-myc deletion did not result in compensatory induction of other myc family members or in c-Myc's binding partner Max. Floxed c-myc did have a negative effect on Alb-Cre expression at 4 weeks of age. To explore this relationship further, we used the Rosa26 reporter line to assay Cre activity in the c-myc floxed mice. No significant difference in Alb-Cre activity was found between control and c-mycfl/fl mice. c-myc deficient mice were studied in a nonproliferative model of liver growth, fasting for 48 hr followed by a 24 hr refeeding period. Fasting resulted in a decrease in liver mass and liver protein, both of which recovered upon 24 h of refeeding in the c-mycfl/fl;Alb-Cre animals. There was also no effect of reducing c-myc on recovery of liver mass following 2/3 partial hepatectomy.

CONCLUSIONS

c-Myc appears to be dispensable for normal liver growth during the postnatal period, restoration of liver mass following partial hepatectomy and recovery from fasting.

Role of c-MYC in alternative activation of human macrophages and tumor-associated macrophage biology

In response to microenvironmental signals, macrophages undergo different activation, including the "classic" proinflammatory phenotype (also called M1), the "alternative" activation induced by the IL-4/IL-13 trigger, and the related but distinct heterogeneous M2 polarization associated with the anti-inflammatory profile. The latter is induced by several stimuli, including IL-10 and TGF-β. Macrophage-polarized activation has profound effects on immune and inflammatory responses and in tumor biology, but information on the underlying molecular pathways is scarce. In the present study, we report that alternative polarization of macrophages requires the transcription factor c-MYC. In macrophages, IL-4 and different stimuli sustaining M2-like polarization induce c-MYC expression and its translocation to the nucleus. c-MYC controls the induction of a subset (45%) of genes associated with alternative activation. ChIP assays indicate that c-MYC directly regulates some genes associated with alternative activation, including SCARB1, ALOX15, and MRC1, whereas others, including CD209, are indirectly regulated by c-MYC. c-MYC up-regulates the IL-4 signaling mediators signal transducer and activator of transcription-6 and peroxisome proliferator-activated receptorγ, is also expressed in tumor-associated macrophages, and its inhibition blocks the expression of protumoral genes including VEGF, MMP9, HIF-1α, and TGF-β. We conclude that c-MYC is a key player in alternative macrophage activation, and is therefore a potential therapeutic target in pathologies related to these cells, including tumors.

Complex regulation of cell-cycle inhibitors by Fbxw7 in mouse embryonic fibroblasts

The F-box protein Fbxw7 (also known as Fbw7, SEL-10, hCdc4 or hAgo) mediates the ubiquitylation and thereby contributes to the degradation of proteins that positively regulate cell cycle. Conditional ablation of Fbxw7 in mouse embryonic fibroblasts (MEFs) induces cell-cycle arrest accompanied by abnormal accumulation of the intracellular domain of Notch1 (NICD1) and c-Myc. However, the molecular mechanisms by which the accumulation of NICD1 and c-Myc induces cell-cycle arrest have remained unclear. We have now examined the expression of cell-cycle inhibitors in Fbxw7-deficient MEFs and found that the abundance of p27(Kip1) and p57(Kip2) is paradoxically decreased. This phenomenon appears to be attributable to the accumulation of NICD1, given that it was recapitulated by overexpression of NICD1 and blocked by ablation of RBP-J. Conversely, the expression of p16(Ink4a) and p19(ARF) was increased in an NICD1-independent manner in Fbxw7-null MEFs. The increased expression of p19(ARF) was recapitulated by overexpression of c-Myc and abolished by ablation of c-Myc, suggesting that the accumulation of c-Myc is primarily responsible for that of p19(ARF). In contrast, the upregulation of p16(Ink4a) appeared to be independent of c-Myc. These results indicate that cell-cycle inhibitors undergo complex regulation by the Fbxw7-mediated proteolytic system.

Myc's other life: stem cells and beyond

Over the last three decades genetic and biochemical studies have revealed the pleiotropic effects of the Myc oncoprotein. While cell line studies have defined the intracellular processes regulated by Myc such as proliferation, differentiation, and metabolic growth, in vivo studies have confirmed these functions, and revealed roles in acquisition and maintenance of stem cell properties. These roles may be partially mediated by Myc's capacity to modify the chromatin landscape on a global scale. Myc also regulates numerous protein-coding transcripts, and many noncoding RNAs (rRNAs, tRNAs, and miRNAs). As Myc activity directly correlates with protein expression, further complexity is provided by post-translational modifications that regulate Myc in normal stem cells or deregulate it in malignant stem cells.

A Myc-regulated transcriptional network controls B-cell fate in response to BCR triggering

Background

The B cell antigen receptor (BCR) is a signaling complex that mediates the differentiation of stage-specific cell fate decisions in B lymphocytes. While several studies have shown differences in signal transduction components as being key to contrasting phenotypic outcomes, little is known about the differential BCR-triggered gene transcription downstream of the signaling cascades.

Results

Here we define the transcriptional changes that underlie BCR-induced apoptosis and proliferation of immature and mature B cells, respectively. Comparative genome-wide expression profiling identified 24 genes that discriminated between the early responses of the two cell types to BCR stimulation. Using mice with a conditional Myc-deletion, we validated the microarray data by demonstrating that Myc is critical to promoting BCR-triggered B-cell proliferation. We further investigated the Myc-dependent molecular mechanisms and found that Myc promotes a BCR-dependent clonal expansion of mature B cells by inducing proliferation and inhibiting differentiation.

Conclusion

This work provides the first comprehensive analysis of the early transcriptional events that lead to either deletion or clonal expansion of B cells upon antigen recognition, and demonstrates that Myc functions as the hub of a transcriptional network that control B-cell fate in the periphery.

Apoptosis and cancer: the genesis of a research field

In multicellular organisms, the total number of cells is a balance between the cell-generating effects of mitosis and cell death that is induced through apoptosis. A disruption of this delicate balance can lead to the development of cancer. This Timeline article focuses on how the field of apoptosis biology has developed in the context of its contribution to our understanding of cell death, or lack of it, in the development of malignant disease. It traces the course of research from key discoveries in fundamental biology to potential therapeutic applications.

Reflecting on 25 years with MYC

Just over 25 years ago, MYC, the human homologue of a retroviral oncogene, was identified. Since that time, MYC research has been intense and the advances impressive. On reflection, it is astonishing how each incremental insight into MYC regulation and function has also had an impact on numerous biological disciplines, including our understanding of molecular oncogenesis in general. Here we chronicle the major advances in our understanding of MYC biology, and peer into the future of MYC research.

Gamma-glutamylcysteine synthetase mediates the c-Myc-dependent response to antineoplastic agents in melanoma cells

This study aims to investigate the role of gamma-glutamylcysteine synthetase (gamma-GCS), the rate-limiting enzyme for glutathione (GSH) synthesis, in the c-Myc-dependent response to antineoplastic agents. We found that specific c-Myc inhibition depleted cells of GSH by directly reducing the gene expression of both heavy and light subunits of the gamma-GCS enzyme and increased their susceptibility to antineoplastic drugs with different mechanisms of action, such as cisplatin (CDDP), staurosporine (STR), and 5-fluorouracil (5-FU). The effect caused by c-Myc inhibition on CDDP and STR response, but not to 5-FU treatment, is directly linked to the impairment of the gamma-GCS expression, because up-regulation of gamma-GCS reverted drug sensitivity, whereas the interference of GSH synthesis increased drug susceptibility as much as after c-Myc down-regulation. The role of gamma-GCS in the c-Myc-directed drug response depends on the capacity of drugs to trigger reactive oxygen species (ROS) production. Indeed, although 5-FU exposure did not induce any ROS, CDDP- and STR-induced oxidative stress enhanced the recruitment of c-Myc on both gamma-GCS promoters, thus stimulating GSH neosynthesis and allowing cells to recover from ROS-induced drug damage. In conclusion, our data demonstrate that the gamma-GCS gene is the downstream target of c-Myc oncoprotein, driving the response to ROS-inducing drugs. Thus, gamma-GCS impairment might specifically sensitize high c-Myc tumor cells to chemotherapy.

Excision of the Frt-flanked neo (R) cassette from the CD19cre knock-in transgene reduces Cre-mediated recombination

Intercross of mice transgenic for Flp-recombinase with the CD19cre mouse strain leads to excision of the Frt-flanked neo (R) cassette from the CD19cre knock-in transgene. This significantly reduces the expression level of Cre by the CD19cre transgene and consequently decreases the extent of Cre-mediated recombination of loxP-flanked alleles, most likely due to the fact that this neo (R) cassette contains polyoma enhancer sequences. We wish to draw attention to this finding, since the Flp-deleter mouse strain is commonly used to remove Frt-flanked selection cassettes in vivo from conditional alleles. Therefore conditional alleles have to be separated from the Flp-deleter transgene by breeding before crosses with CD19cre mice are initiated. In addition our findings suggest that gene expression from the CD19 locus can be increased by the insertion of exogenous enhancer sequences, without compromising B cell specificity.

The Oscar-worthy role of Myc in apoptosis

The discovery that the Myc oncoprotein could drive cells to undergo apoptosis in addition to its well-established role in cellular proliferation came in the early 1990s, at the beginning of a period of explosive research on cell death. Experimental evidence revealed that Myc sensitises cells to a wide range of death stimuli and abrogating this biological activity plays a profound role in tumorigenesis. Our understanding of the molecular mechanism and genetic programme of Myc-induced apoptosis remains shrouded in mystery and the focus of much attention. In this review, we will discuss established data, recent advances and future objectives regarding the regulatory processes and the functional cooperators that effect and abrogate apoptosis induced by Myc.

c-Myc mediates pre-TCR-induced proliferation but not developmental progression

Constitutive and cell-autonomous signals emanating from the pre-T-cell receptor (pre-TCR) promote proliferation, survival and differentiation of immature thymocytes. We show here that induction of pre-TCR signaling resulted in rapid elevation of c-Myc protein levels. Cre-mediated thymocyte-specific ablation of c-Myc in CD25(+)CD44(-) thymocytes reduced proliferation and cell growth at the pre-TCR checkpoint, resulting in thymic hypocellularity and a severe reduction in CD4(+)CD8(+) thymocytes. In contrast, c-Myc deficiency did not inhibit pre-TCR-mediated differentiation or survival. Myc(-/-) double-negative (DN) 3 cells progressed to the double-positive (DP) stage and up-regulated TCRalphabeta surface expression in the absence of cell proliferation, in vivo as well as in vitro. These observations indicate that distinct signals downstream of the pre-TCR are responsible for proliferation versus differentiation, and demonstrate that c-Myc is only required for pre-TCR-induced proliferation but is dispensable for developmental progression from the DN to the DP stage.

Myc stimulates nuclearly encoded mitochondrial genes and mitochondrial biogenesis

Although several genes involved in mitochondrial function are direct Myc targets, the role of Myc in mitochondrial biogenesis has not been directly established. We determined the effects of ectopic Myc expression or the loss of Myc on mitochondrial biogenesis. Induction of Myc in P493-6 cells resulted in increased oxygen consumption and mitochondrial mass and function. Conversely, compared to wild-type Myc fibroblasts, Myc null rat fibroblasts have diminished mitochondrial mass and decreased number of normal mitochondria. Reconstitution of Myc expression in Myc null fibroblasts partially restored mitochondrial mass and function and normal-appearing mitochondria. Concordantly, we also observed in primary hepatocytes that acute deletion of floxed murine Myc by Cre recombinase resulted in diminished mitochondrial mass in primary hepatocytes. Our microarray analysis of genes responsive to Myc in human P493-6 B lymphocytes supports a role for Myc in mitochondrial biogenesis, since genes involved in mitochondrial structure and function are overrepresented among the Myc-induced genes. In addition to the known direct binding of Myc to many genes involved in mitochondrial structure and function, we found that Myc binds the TFAM gene, which encodes a key transcriptional regulator and mitochondrial DNA replication factor, both in P493-6 lymphocytes with high ectopic MYC expression and in serum-stimulated primary human 2091 fibroblasts with induced endogenous MYC. These observations support a pivotal role for Myc in regulating mitochondrial biogenesis.