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We have demonstrated previously that the Myc oncoprotein blocks cancer cell differentiation by forming a novel transcriptional repressor complex with histone deacetylase and inhibiting gene transcription of tissue transglutaminase (TG2). Moreover, induction of TG2 gene transcription and transamidase activity is essential for the differentiating effects of retinoids in cancer cells. Here, we show that two structurally distinct TG2 protein isoforms, the full-length (TG2-L) and the short form (TG2-S), exert opposing effects on cell differentiation. Repres- sion of TG2-L with small interfering RNA, which did not affect TG2-S expression, induced dramatic neuritic differentiation in neurobtastoma cells. In contrast, overexpression of TG2-S or a GTP-binding-deficient mutant of TG2-L (RS80A), both of which lack the GTP-binding Arg-580 residue, induced neuroblastoma cell differentiation, which was blocked by an inhibitor of transamidase activity. Whereas N-Myc repressed and retinoid activated both TG2 isoforms, repression of TG2-L, but not simultaneous repression of TG2-L and TG2-S, enhanced neuroblastoma cell differentiation due to N-Myc small interfering RNA or retinoid. Moreover, suppression of vasoactive intestinal peptide (VIP) expression alone induced neuroblastoma cell differentiation, and VIP was up-regulated by TG2-L, but not TG2-S. Taken together, our data indicate that TG2-L and TG2-S exert opposite effects on cell differentiation due to differences in GTP binding and modulation of VIP gene transcription. Our findings highlight the potential importance of repressing the GTP binding activity of TG2-L or activating the transamidase activity of TG2-L or TG2-S for the treatment of neuroblastoma, and possibly also other Myc-induced malignancies, and for enhancing retinoid anticancer effects. [ABSTRACT FROM AUTHOR]

Histone deacetylase (HDAC) inhibitors reactivate tumor suppressor gene transcription; induce cancer cell differentiation, growth arrest, and programmed cell death; and are among the most promising new classes of anticancer drugs. Myc oncoproteins can block cell differentiation and promote cell proliferation and malignant transformation, in some cases by modulating target gene transcription. Here, we show that tissue transglutaminase (TG2) was commonly reactivated by HDAC inhibitors in neuroblastoma and breast cancer cells but not normal cells and contributed to HDAC inhibitor-induced growth arrest. TG2 was the gene most significantly repressed by N-Myc in neuroblastoma cells in a cDNA microarray analysis and was commonly repressed by N-Myc in neuroblastoma cells and c-Myc in breast cancer cells. Repression of TG2 expression by N-Myc in neuroblastoma cells was necessary for the inhibitory effect of N-Myc on neuroblastoma cell differentiation. Dual step cross-linking chromatin immunoprecipitation and protein coimmunoprecipitation assays showed that N-Myc acted as a transrepressor by recruiting the HDAC1 protein to an Spi-binding site in the TG2 core promoter in a manner distinct from it's action as a transactivator at E-Box binding sites. HDAC inhibitor treatment blocked the N-Myc-mediated HDAC1 recruitment and TG2 repression in vitro. In neuroblastoma-bearing N-Myc transgenic mice, HDAC inhibitor treatment induced TG2 expression and demonstrated marked antitumor activity in vivo. Taken together, our data indicate the critical roles of HDAC1 and TG2 in Myc-induced oncogenesis and have significant implications for the use of HDAC inhibitor therapy in Myc-driven oncogenesis. [ABSTRACT FROM AUTHOR]

Background: Histone deacetylase inhibitors (HDACIs) have many effects on cancer cells, such as growth inhibition, induction of cell death, differentiation, and anti-angiogenesis, all with a wide therapeutic index. However, clinical trials demonstrate that HDACIs are more likely to be effective when used in combination with other anticancer agents. Moreover, the molecular basis for the anticancer action of HDACIs is still unknown. In this study, we compared different combinations of HDACIs and anti-cancer agents with anti-angiogenic effects, and analysed their mechanism of action. Results: Trichostatin A (TSA) and α-interferon (IFNα) were the most effective combination across a range of different cancer cell lines, while normal non-malignant cells did not respond in the same manner to the combination therapy. There was a close correlation between absence of basal p21WAF1 expression and response to TSA and IFNα treatment. Moreover, inhibition of p21WAF1 expression in a p21WAF1-expressing breast cancer cell line by a specific siRNA increased the cytotoxic effects of TSA and IFNα. In vitro assays of endothelial cell function showed that TSA and IFNα decreased endothelial cell migration, invasion, and capillary tubule formation, without affecting endothelial cell viability. TSA and IFNα co-operatively inhibited gene expression of some pro-angiogenic factors: vascular endothelial growth factor, hypoxia-inducible factor 1α and matrix metalloproteinase 9, in neuroblastoma cells under hypoxic conditions. Combination TSA and IFNα therapy markedly reduced tumour angiogenesis in neuroblastoma-bearing transgenic mice. Conclusion: Our results indicate that combination TSA and IFNα therapy has potent cooperative cytotoxic and anti-angiogenic activity. High basal p21WAF1 expression appears to be acting as a resistance factor to the combination therapy. [ABSTRACT FROM AUTHOR]

Myc oncoproteins exert tumorigenic effects by regulating expression of target oncogenes. Histone H3 lysine 79 (H3K79) methylation at Myc-responsive elements of target gene promoters is a strict prerequisite for Myc-induced transcriptional activation, and DOT1L is the only known histone methyltransferase that catalyzes H3K79 methylation. Here, we show that N-Myc upregulates DOT1L mRNA and protein expression by binding to the DOT1L gene promoter. shRNA-mediated depletion of DOT1L reduced mRNA and protein expression of N-Myc target genes ODC1 and E2F2 DOT1L bound to the Myc Box II domain of N-Myc protein, and knockdown of DOT1L reduced histone H3K79 methylation and N-Myc protein binding at the ODC1 and E2F2 gene promoters and reduced neuroblastoma cell proliferation. Treatment with the small-molecule DOT1L inhibitor SGC0946 reduced H3K79 methylation and proliferation of MYCN gene-amplified neuroblastoma cells. In mice xenografts of neuroblastoma cells stably expressing doxycycline-inducible DOT1L shRNA, ablating DOT1L expression with doxycycline significantly reduced ODC1 and E2F2 expression, reduced tumor progression, and improved overall survival. In addition, high levels of DOT1L gene expression in human neuroblastoma tissues correlated with high levels of MYCN, ODC1 , and E2F2 gene expression and independently correlated with poor patient survival. Taken together, our results identify DOT1L as a novel cofactor in N-Myc-mediated transcriptional activation of target genes and neuroblastoma oncogenesis. Furthermore, they characterize DOT1L inhibitors as novel anticancer agents against MYCN-amplified neuroblastoma. Cancer Res; 77(9); 2522-33. ©2017 AACR ., (©2017 American Association for Cancer Research.)

Histone deacetylase inhibitors (HDACIs) modulate gene transcription and are among the most promising new classes of anticancer drugs. OGX-011, an anti-sense oligonucleotide targeting clusterin, sensitises cancer cells to chemo- and radiotherapies. By reviewing microarray gene profiling data reported in the literature, we identified clusterin as one of only two genes commonly up-regulated by most HDACIs in cancer cell lines of different organ origins. Suppression of clusterin gene expression synergistically enhanced high-dosage HDACI-induced cell death through cytochrome C-mediated mitochondrial apoptosis in HDACI-resistant cancer cells, and synergistically enhanced low-dosage HDACI-induced growth arrest in both HDACI-sensitive and HDACI-resistant tumour cells, but not in normal cells. In mice xenografted with neuroblastoma cells, combination of OGX-011 and the HDACI, valproate, synergistically repressed tumour growth. Our data indicate that HDACI-induced clusterin over-expression renders cancer cells resistant to HDACI-induced growth arrest and apoptosis, and suggests the addition of OGX-011 to HDACIs in future clinical trials in cancer patients.