DNA binding-independent transcriptional activation of the vascular endothelial growth factor gene (VEGF) by the Myb oncoprotein

A Feedback Loop Involving MicroRNA-150 and MYB Regulates VEGF Expression in Brain Microvascular Endothelial Cells After Oxygen Glucose Deprivation

Vascular endothelial growth factor (VEGF) plays a pivotal role in regulating cerebral angiogenesis after stroke. Meanwhile, excessive VEGF expression induces increased microvascular permeability in brain, probably leading to neurological deterioration. Therefore, the appropriate level of VEGF expression is significant to the recovery of brain exposed to stroke. In this work, we demonstrate that microRNA-150 (miR-150) and its predicted target MYB form a negative feedback loop to control the level of post-stroke VEGF expression. Repression of MYB leads to decreased expression of miR-150 in brain microvascular endothelial cells (BMVECs) exposed to oxygen glucose deprivation (OGD), thus miR-150 was predicted to be down-regulated by MYB. Moreover, MYB was confirmed to be a direct target of miR-150 by using dual luciferase reporter assay. In our previous work, we have validated VEGF as another direct target of miR-150. Therefore, MYB participates in regulation of VEGF via miR-150 under OGD, forming a feedback loop with miR-150. We also find that high levels of miR-150 inhibitors combined with MYB silence contribute to further enhancement of VEGF expression in BMVECs in response to OGD. These observations suggest that the feedback loop comprised of miR-150 and MYB, which is a pivotal endogenous epigenetic regulation to control the expression levels of VEGF in BMVECs subjected to OGD.

MYB oncoproteins: emerging players and potential therapeutic targets in human cancer

MYB transcription factors are highly conserved from plants to vertebrates, indicating that their functions embrace fundamental mechanisms in the biology of cells and organisms. In humans, the MYB gene family is composed of three members: MYB, MYBL1 and MYBL2, encoding the transcription factors MYB, MYBL1, and MYBL2 (also known as c-MYB, A-MYB, and B-MYB), respectively. A truncated version of MYB, the prototype member of the MYB family, was originally identified as the product of the retroviral oncogene v-myb, which causes leukaemia in birds. This led to the hypothesis that aberrant activation of vertebrate MYB could also cause cancer. Despite more than three decades have elapsed since the isolation of v-myb, only recently investigators were able to detect MYB genes rearrangements and mutations, smoking gun evidence of the involvement of MYB family members in human cancer. In this review, we will highlight studies linking the activity of MYB family members to human malignancies and experimental therapeutic interventions tailored for MYB-expressing cancers.

Extracellular vesicle-encapsulated microRNA-424 exerts inhibitory function in ovarian cancer by targeting MYB

Background

Recent studies have suggested a crucial role of mesenchymal stem cell (MSC)-derived extracellular vesicles (EVs) in ovarian cancer treatment. We, therefore, set out to explore the mechanism through which MSC-derived EVs delivered microRNA-424 (miR-424) to influence the development of ovarian cancer.

Methods

Bioinformatics analyses were first performed to screen ovarian cancer-related differentially expressed genes and to predict regulatory miRNAs. Then, dual-luciferase reporter gene assay was carried out to verify the relationship between miR-424 and MYB. Subsequently, the characterized MSCs and isolated EVs were co-cultured with ovarian cancer cells, followed by determination of the expression patterns of miR-424, MYB, vascular endothelial growth factor (VEGF), and VEGF receptor (VEGFR), respectively. In addition, the effects of EVs-delivered miR-424 on the proliferation, migration, invasion and tube formation of ovarian cancer cells were assessed using gain- and loss-of-function approaches. Lastly, tumor xenograft was induced in nude mice to illustrate the influence of EVs-loaded miR-424 on ovarian cancer in vivo.

Results

Our data exhibited that MYB was highly-expressed and miR-424 was poorly-expressed in ovarian cancer. More importantly, MYB was identified as a target gene of miR-424. Additionally, the transfer of miR-424 by MSC-derived EVs was found to repress the proliferation, migration, and invasion of ovarian cancer cells, with a reduction in the expressions of VEGF and VEGFR. Furthermore, MSC-derived EVs over-expressing miR-424 could inhibit the proliferation, migration, and tube formation of human umbilical vein endothelial cells, and also suppressed tumorigenesis and angiogenesis of ovarian tumors in vivo.

Conclusion

Collectively, our findings indicate that MSC-derived EVs transfer miR-424 to down-regulate MYB, which ultimately led to the inhibition of the tumorigenesis and angiogenesis of ovarian cancer. Hence, this study offers a potential prognostic marker and a therapeutic target for ovarian cancer.

Intestinal-specific activatable Myb initiates colon tumorigenesis in mice

Transcription factor Myb is overexpressed in most colorectal cancers (CRC). Patients with CRC expressing the highest Myb are more likely to relapse. We previously showed that mono-allelic loss of Myb in an Adenomatous polyposis coli (APC)-driven CRC mouse model (Apc^Min/+) significantly improves survival. Here we directly investigated the association of Myb with poor prognosis and how Myb co-operates with tumor suppressor genes (TSGs) (Apc) and cell cycle regulator, p27. Here we generated the first intestinal-specific, inducible transgenic model; a MybER transgene encoding a tamoxifen-inducible fusion protein between Myb and the estrogen receptor-α ligand-binding domain driven by the intestinal-specific promoter, Gpa33. This was to mimic human CRC with constitutive Myb activity in a highly tractable mouse model. We confirmed that the transgene was faithfully expressed and inducible in intestinal stem cells (ISCs) before embarking on carcinogenesis studies. Activation of the MybER did not change colon homeostasis unless one p27 allele was lost. We then established that MybER activation during CRC initiation using a pro-carcinogen treatment, azoxymethane (AOM), augmented most measured aspects of ISC gene expression and function and accelerated tumorigenesis in mice. CRC-associated symptoms of patients including intestinal bleeding and anaemia were faithfully mimicked in AOM-treated MybER transgenic mice and implicated hypoxia and vessel leakage identifying an additional pathogenic role for Myb. Collectively, the results suggest that Myb expands the ISC pool within which CRC is initiated while co-operating with TSG loss. Myb further exacerbates CRC pathology partly explaining why high MYB is a predictor of worse patient outcome.

Myeloid zinc-finger 1 (MZF-1) suppresses prostate tumor growth through enforcing ferroportin-conducted iron egress

Although previous studies suggest that myeloid zinc-finger 1 (MZF-1) is a multifaceted transcription factor that may function as either an oncogene or a tumor suppressor, the molecular bases determining its different traits remain elusive. Increasing evidence suggests that disorders in iron metabolism affect tumorigenesis and tumor behaviors, and that excess tumor iron stimulates tumor progression through various mechanisms such as enhancing DNA replication and energy metabolism. Ferroportin (FPN) is the only known iron exporter in mammalian cells, and it determines global iron egress out of cells. FPN reduction leads to decreased iron efflux and increased intracellular iron that consequentially aggravates the oncogenic effects of iron. MZF-1 was recently identified as a transcription factor that regulates FPN expression. Thus far, however, the molecular mechanisms underlying the MZF-1-FPN signaling in cancers are largely unknown. Here, we found a significant reduction of FPN levels in prostate tumors relative to adjacent tissues, and demonstrated a crucial role of FPN in tumor growth through controlling tumor iron concentration. Inhibition of MZF-1 expression led to reduced FPN concentration, coupled with resultant intracellular iron retention, increased iron-related cellular activities and enhanced tumor cell growth. In contrast, increase of MZF-1 expression restrained tumor cell growth by promoting FPN-driven iron egress. Importantly, we demonstrated that AP4 and c-Myb jointly modulated MZF-1 transcription, and that miR-492 was also directly involved in regulating MZF-1 concentration through binding to the 3' untranslated regions of its mRNA. These results correlate with reduced AP4 and c-Myb expression and elevated miR-492 expression found in prostate tumors as compared with adjacent tissues that resulted in diminished MZF-1 and FPN. Moreover, we demonstrated that alterations of AP4, c-Myb and miR-492 levels significantly affected tumor cell growth. Targeting molecules within the MZF-1-FPN signaling thus appears to be a promising approach to restrain prostate cancer.

Expression and prognostic role of c-Myb as a novel cell cycle protein in esophageal squamous cell carcinoma

PURPOSE

The c-Myb transcription factor controls differentiation and proliferation in hematopoietic and other cell types, and has latent in regulation during the cell cycle. Recent studies suggested that deregulation of c-Myb expression plays a key role in oncogenesis. To investigate the potential roles of c-Myb in esophageal carcinoma, expression of c-Myb was examined in human esophageal carcinoma samples.

METHODS

Immunohistochemistry and Western blot analysis were performed for c-Myb in 87 esophageal carcinoma samples. The data were correlated with clinicopathological features. The univariate and multivariate survival analyses were also performed to determine their prognostic significance.

RESULTS

c-Myb was overexpressed in esophageal carcinoma as compared with the adjacent normal tissue. High expression of c-Myb was associated with histological grade and was positively correlated with proliferation marker Ki-67 (P = 0.001). Univariate analysis showed that c-Myb expression was associated with poor prognosis (P < 0.001). Multivariate analysis indicated that c-Myb was an independent prognostic marker for esophageal carcinoma (P < 0.001). While in vitro, following release from serum starvation of TE-1 esophageal carcinoma cell, the expression of c-Myb was upregulated.

CONCLUSIONS

Our results suggested that c-Myb overexpression is involved in the pathogenesis of esophageal carcinoma; it may be a favorable independent poor prognostic parameter for esophageal carcinoma.

Time series gene expression profiling and temporal regulatory pathway analysis of BMP6 induced osteoblast differentiation and mineralization

Background

BMP6 mediated osteoblast differentiation plays a key role in skeletal development and bone disease. Unfortunately, the signaling pathways regulated by BMP6 are largely uncharacterized due to both a lack of data and the complexity of the response.

Results

To better characterize the signaling pathways responsive to BMP6, we conducted a time series microarray study to track BMP6 induced osteoblast differentiation and mineralization. These temporal data were analyzed using a customized gene set analysis approach to identify temporally coherent sets of genes that act downstream of BMP6. Our analysis identified BMP6 regulation of previously reported pathways, such as the TGF-beta pathway. We also identified previously unknown connections between BMP6 and pathways such as Notch signaling and the MYB and BAF57 regulatory modules. In addition, we identify a super-network of pathways that are sequentially activated following BMP6 induction.

Conclusion

In this work, we carried out a microarray-based temporal regulatory pathway analysis of BMP6 induced osteoblast differentiation and mineralization using GAGE method. This novel temporal analysis is more informative and powerful than the classical static pathway analysis in that: (1) it captures the interconnections between signaling pathways or functional modules and demonstrates the even higher level organization of molecular biological systems; (2) it describes the temporal perturbation patterns of each pathway or module and their dynamic roles in osteoblast differentiation. The same set of experimental and computational strategies employed in our work could be useful for studying other complex biological processes.

c-Myb is required for pro-B cell differentiation

The c-Myb transcription factor is required for normal adult hematopoiesis. However, the embryonic lethality of Myb-null mutations has been an impediment to identifying roles for c-Myb during lymphocyte development. We have used tissue-specific inactivation of the Myb locus in early progenitor cells to demonstrate that c-Myb is absolutely required for the differentiation of CD19(+) B-lineage cells and B cell differentiation is profoundly blocked beyond the pre-pro-B cell stage in Myb(f/f) Mb1-cre mice. We demonstrate that c-Myb is required for the intrinsic survival of CD19(+) pro-B cells as well as the proper expression of the alpha-chain of the IL-7 receptor (CD127) and Ebf1. However, survival of c-Myb-deficient CD19(+) pro-B cells cannot be rescued by transduction with CD127-producing retrovirus, suggesting that c-Myb controls a survival pathway independent of CD127. Furthermore, c-Myb-deficient progenitor cells inefficiently generate CD19(+) B-lineage cells during stromal cell culture but this process can be partially rescued with exogenous Ebf1. Thus, c-Myb does not appear to be required for commitment to B cell differentiation but is crucial for B cell differentiation to the CD19(+) pro-B cell stage as well as survival of CD19(+) pro-B cells. Surprisingly, forced c-Myb expression in lymphoid-primed multipotent progenitors favors differentiation toward the myeloid lineage, suggesting that proper c-Myb expression is crucial for B-lineage development.

Prolactin signaling through the short isoform of the mouse prolactin receptor regulates DNA binding of specific transcription factors, often with opposite effects in different reproductive issues

BACKGROUND

It has been well established that prolactin (PRL) signals through the long form of its receptor (PRL-RL) and activates the Jak/Stat pathway for transcription of PRL target genes. However, signaling pathways mediated through the short PRL-R isoform (PRL-RS) remains controversial. Our recent finding that PRL signaling through PRL-RS represses two transcription factors critical for follicular development lead us to examine other putative PRL/PRL-RS target transcription factors in the decidua and ovary, two well-known target tissues of PRL action in reproduction.

METHODS

In this investigation we used mice expressing PRL-RS on a PRL-R knockout background and a combo protein/DNA array to study the transcription factors regulated by PRL through PRL-RS only.

RESULTS

We show that PRL activation of the PRL-RS receptor either stimulates or inhibits the DNA binding activity of a substantial number of transcription factors in the decidua as well as ovary. We found few transcription factors to be similarly regulated in both tissues, while most transcription factors are oppositely regulated by PRL in the decidua and ovary. In addition, some transcription factors are regulated by PRL only in the ovary or only in the decidua. Several of these transcription factors are involved in physiological pathways known to be regulated by PRL while others are novel.

CONCLUSION

Our results clearly indicate that PRL does signal through PRL-RS in the decidua as well as the ovary, independently of PRL-RL, and activates/represses transcription factors in a tissue specific manner. This is the first report showing PRL/PRL-RS regulation of specific transcription factors. Many of these transcription factors were not previously known to be PRL targets, suggesting novel physiological roles for this hormone.

Role of c-Myb during prolactin-induced signal transducer and activator of transcription 5a signaling in breast cancer cells

Implicated in the pathogenesis of breast cancer, prolactin (PRL) mediates its function in part through the prolactin receptor (PRLr)-associated Janus kinase 2 (Jak2)/signal transducer and activator of transcription 5 (Stat5) signaling complex. To delineate the mechanisms of Stat5a regulation in breast cancer, transcription factor-transcription factor (TF-TF) array analysis was employed to identify associated transcriptional regulators. These analyses revealed a PRL-inducible association of Stat5a with the transcription factor and protooncogene c-Myb. Confirmatory co-immunoprecipitation studies using lysates from both T47D and MCF7 breast cancer cells revealed a PRL-inducible association between these transcription factors. Ectopic expression of c-Myb enhanced the PRL-induced expression from both composite and synthetic Stat5a-responsive luciferase reporters. Chromatin immunoprecipitation assays also revealed a PRL-inducible association between c-Myb and endogenous Stat5a-responsive CISH promoter, which was associated with an enhanced expression of CISH gene product at the RNA and protein levels. Small interfering RNA-mediated c-Myb knockdown impaired the PRL-induced mRNA expression of five Stat5-responsive genes. DNA binding-defective mutants of c-Myb, incapable of activating expression from a c-Myb-responsive reporter, maintained their ability to enhance a Stat5a-responsive reporter. At a cellular level, ectopic expression of c-Myb resulted in an increase in T47D proliferation. Taken together, these results indicate that c-Myb potentiates Stat5a-driven gene expression, possibly functioning as a Stat5a coactivator, in human breast cancer.

Adenovirus-mediated expression of dominant negative c-myb induces apoptosis in head and neck cancer cells and inhibits tumor growth in animal model

The recent demonstration of aberrant expression of the c-myb proto-oncogene in various cancers suggests that c-myb plays an important role in the development of cancer. On this basis, it has been proposed that ablation of c-myb function might be an effective approach for therapy of c-myb dependent malignancies. We previously used a dominant negative c-myb (DN-myb) construct to induce apoptosis in K562 cells. In this study, DN-myb was expressed in an adenovirus-mediated gene delivery system and introduced into head and neck squamous cell carcinoma cells (HNSCC) in vitro and in vivo to examine its tumor suppressive function and its potential in HNSCC gene therapy. Over expression of DN-myb in HNSCC cells inhibited in vitro cell proliferation, expression of growth factors such as IGF-I, -II, IGF-1R, and VEGF, inhibited Akt/PKB pathway activation, and enhanced induction of apoptosis. Similarly, in vivo administration of DN-myb retarded tumor-growth. Our results support a role for DN-myb in inducing apoptosis and tumor suppression, and, furthermore, suggest that DN-myb gene therapy might provide a powerful tool for treatment of c-myb dependent malignancies such as HNSCC.