Expression and stabilization of the MCT-1 protein by DNA damaging agents

MCTS1 promotes invasion and metastasis of oral cancer by modifying the EMT process

Background

The oncogene, malignant T-cell-amplified sequence 1 (MCTS1), has been found to be highly expressed in a variety of cancer cell lines. It has been shown to be involved in cell cycle progression and to confer a growth advantage for lymphomas and breast cancer. Nevertheless, the role of MCTS1 in contributing to the development of oral cancer remains elusive.

Methods

We analyzed the gene expression profiles of MCTS1 in normal oral keratinocytes and cancerous cells. Cellular proliferation, invasion, and migration experiments were performed to detect the effect of MCTS1 on the biological evolution of oral cancer. The in vitro results were verified by the in vivo lymphatic metastasis test. The underlying mechanism of MCTS1 in promoting oral cancer invasion and metastasis correlated with the epithelial-mesenchymal transition (EMT) process as revealed by western blotting.

Results

The results showed that MCTS1 was aberrantly expressed in oral cancer cells. MCTS1 overexpression significantly promoted tumor cell growth, proliferation, migration, and invasion. MCTS1-mediated lymphatic metastasis was verified in vivo using an intraplantar tumor model. Biomarkers associated with EMT progression were positively or negatively regulated upon knockdown or overexpression of MCTS1, respectively.

Conclusions

Higher MCTS1 expression in oral cancer may be connected with an unfavorable prognosis due to involvement of MCTS1. MCTS1 potentiates the growth and proliferation of oral cancer cells and subsequent metastasis by regulating cell cycle and modifying the EMT process.

Keywords

Oral cancer; oncogene; malignant T-cell-amplified sequence 1 (MCTS1); metastasis; invasion.

Genome Profiling for Aflatoxin B1 Resistance in Saccharomyces cerevisiae Reveals a Role for the CSM2/SHU Complex in Tolerance of Aflatoxin B1-Associated DNA Damage

Exposure to the mycotoxin aflatoxin B1 (AFB₁) strongly correlates with hepatocellular carcinoma (HCC). P450 enzymes convert AFB₁ into a highly reactive epoxide that forms unstable 8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 (AFB₁-N ⁷-Gua) DNA adducts, which convert to stable mutagenic AFB₁ formamidopyrimidine (FAPY) DNA adducts. In CYP1A2-expressing budding yeast, AFB₁ is a weak mutagen but a potent recombinagen. However, few genes have been identified that confer AFB₁ resistance. Here, we profiled the yeast genome for AFB₁ resistance. We introduced the human CYP1A2 into ∼90% of the diploid deletion library, and pooled samples from CYP1A2-expressing libraries and the original library were exposed to 50 μM AFB₁ for 20 hs. By using next generation sequencing (NGS) to count molecular barcodes, we initially identified 86 genes from the CYP1A2-expressing libraries, of which 79 were confirmed to confer AFB₁ resistance. While functionally diverse genes, including those that function in proteolysis, actin reorganization, and tRNA modification, were identified, those that function in postreplication DNA repair and encode proteins that bind to DNA damage were over-represented, compared to the yeast genome, at large. DNA metabolism genes also included those functioning in checkpoint recovery and replication fork maintenance, emphasizing the potency of the mycotoxin to trigger replication stress. Among genes involved in postreplication repair, we observed that CSM2, a member of the CSM2 (SHU) complex, functioned in AFB₁-associated sister chromatid recombination while suppressing AFB₁-associated mutations. These studies thus broaden the number of AFB₁ resistance genes and have elucidated a mechanism of error-free bypass of AFB₁-associated DNA adducts.

PKC inhibition of sotrastaurin has antitumor activity in diffuse large B-cell lymphoma via regulating the expression of MCT-1

MCT-1 (multiple copies in T-cell lymphoma-1), a novel oncogene, was originally identified in T-cell lymphoma. A recent study has demonstrated that MCT-1 is highly expressed in 85% of diffuse large B-cell lymphomas (DLBCL). PKC (protein kinase C) plays an essential role in signal transduction for multiple biologically active substances for activating cellular functions and proliferation. In this study, we found that the mRNA and protein expression levels of MCT-1 were visibly decreased after knocking down PKC by siRNA in SUDHL-4 and OCI-LY8 DLBCL cell lines. A selective PKC inhibitor, sotrastaurin, effectively inhibited cell proliferation and induced cell apoptosis in a dose- and time-dependent manner. Meanwhile, we also observed that the cell cycle was arrested in the G1 phase in sotrastaurin-treated cells. In addition, MCT-1 was down-regulated in the sotrastaurin treatment group in vivo. Furthermore, we demonstrated that the PKC inhibitor sotrastaurin induced cell apoptosis and cell cycle arrest in DLBCL cells potentially through regulating the expression of MCT-1. Our data suggest that targeting PKC may be a potential therapeutic approach for lymphomas and related malignancies that exhibit high levels of MCT-1 protein.

MCT-1 protein interacts with the cap complex and modulates messenger RNA translational profiles

MCT-1 is an oncogene that was initially identified in a human T cell lymphoma and has been shown to induce cell proliferation as well as activate survival-related pathways. MCT-1 contains the PUA domain, a recently described RNA-binding domain that is found in several tRNA and rRNA modification enzymes. Here, we established that MCT-1 protein interacts with the cap complex through its PUA domain and recruits the density-regulated protein (DENR/DRP), containing the SUI1 translation initiation domain. Through the use of microarray analysis on polysome-associated mRNAs, we showed that up-regulation of MCT-1 was able to modulate the translation profiles of BCL2L2, TFDP1, MRE11A, cyclin D1, and E2F1 mRNAs, despite equivalent levels of mRNAs in the cytoplasm. Our data establish a role for MCT-1 in translational regulation, and support a linkage between translational control and oncogenesis.

Phosphorylation of MCT-1 by p44/42 MAPK is required for its stabilization in response to DNA damage

We discovered a novel oncogene in a T-cell lymphoma cell line, multiple copies in T-cell lymphoma-1 (MCT-1), that has been shown to decrease cell-doubling time, shorten the duration of G(1) transit time and/or G(1)-S transition, and transform NIH3T3 fibroblasts. We subsequently demonstrated that there were significantly increased levels of MCT-1 protein in a subset of primary diffuse large B-cell lymphomas. Levels of MCT-1 protein were shown to be increased after exposure to DNA damaging agents. This increase did not require new protein synthesis, suggesting that post-translational mechanisms were involved. Phosphorylation is one potential mechanism by which the activity of molecules involved in cell cycle/survival is rapidly modulated. The RAS/mitogen-activated/extracellular-regulated kinase (MEK)/extracellular signal-regulated kinases (ERK) pathway plays a prominent role in the regulation of cell growth and proliferation through phosphorylation-dependent regulation of several substrates. The MCT-1 protein is predicted to have numerous putative phosphorylation sites. Using a combination of genetic and pharmacological approaches, we established that phosphorylation of MCT-1 protein by p44/p42 mitogen-activated protein kinases is critical for stabilization of MCT-1 protein and for its ability to promote cell proliferation. Our data suggests that targeting the RAS/MEK/ERK signal transduction cascade may provide a potential therapeutic approach in lymphomas and related malignancies that exhibit high levels of MCT-1 protein.

The MCT-1 oncogene product impairs cell cycle checkpoint control and transforms human mammary epithelial cells

Multiple copies in T-cell maligancy (MCT-1) is a putative oncogene initially identified in a human T-cell lymphoma. Forced expression of MCT-1 has recently been shown to induce cell transformation and proliferation, as well as to activate survival-related PI-3K/AKT pathways protecting cells from apoptosis. MCT-1 protein is stabilized in response to DNA damage. The impact of MCT-1 overexpression on DNA damage response remains unknown. Here, we show that MCT-1 deregulates cell cycle checkpoints. The phosphorylation of genomic stabilizers H2AX and NBS1 are enhanced in MCT-1-overexpressing cells. Forced expression of MCT-1 significantly increases the number of DNA damage-induced foci involving gamma-H2AX and 53BP1. In MCT-1-overexpressing cells, the proportion of S-phase cell population is preferentially increased after exposure to gamma-irradiation compared to controls. Knockdown of endogenous MCT-1 using an siRNA approach attenuates the H2AX phosphorylation and the G1/S checkpoint defect. Furthermore, MCT-1 is capable of transforming immortalized human mammary epithelial cells and promoting genomic instability. These data shed light on the role of MCT-1 in the cellular response to DNA damage and its involvement in malignant transformation.

Identification and characterization of a novel enhancer for the human MCT-1 oncogene promoter

Cloning and characterization of the promoter region for the MCT-1 oncogene is described. We used luciferase assays to identify cis-acting elements responsible for human MCT-1 promoter function. The MCT-1 promoter is TATA-less with a consensus initiator element located at the transcription start site and facilitated by two Sp1 sites that directs basal transcription. Deletion of a region of the MCT-1 promoter (-133 to -122) resulted in significant decrease in luciferase activity, suggesting that this region contains a positive cis-acting element. Using mobility shift assays with a 26-mer oligonucleotide, which contains this fragment and its flanking regions, we demonstrated the presence of sequence-specific DNA-binding protein in both Jurkat and Hela nuclear extracts that we designated as LMBF (for lymphoid MCT-1 binding factor). This 26-mer oligonucleotide containing the LMBF binding site is required for maximum transcriptional activity of the MCT-1 promoter. Although the 26-mer oligonucleotide contains a sequence with strong homology to a heat-shock factor consensus, competitive electrophoretic mobility shift assay (EMSA) analysis demonstrated that the binding protein is not a known member of heat shock family. Furthermore, this sequence when placed in reverse orientation downstream of the luciferase gene was able to enhance luciferase activity driven by a minimal promoter. These data are consistent with this sequence behaving as an enhancer. Finally, Southwestern blot analysis revealed a 96-kDa protein capable of binding a probe containing the LMBF binding site.

Expression of the candidate MCT-1 oncogene in B- and T-cell lymphoid malignancies

Our laboratory has recently discovered a novel candidate oncogene, MCT-1, amplified in human T-cell lymphoma and mapped to chromosome Xq22-24. This region is amplified in a subset of primary B-cell non-Hodgkin lymphoma (NHL), suggesting that increased copy number of a gene(s) located in this region confers a growth advantage to some primary human lymphomas. We examined a diverse panel of lymphoid malignancies for the expression of MCT-1. We demonstrated that there are significantly increased levels of MCT-1 protein in a panel of T-cell lymphoid cell lines and in non-Hodgkin lymphoma cell lines. Furthermore, we identified a subset of primary diffuse large B-cell lymphomas that exhibited elevated levels of MCT-1 protein. Interestingly, all transformed follicular lymphomas in our study demonstrated elevated protein levels of MCT-1. There was no detectable MCT-1 protein in leukemic cells from patients with chronic lymphocytic leukemia or in any healthy lymphoid tissue examined. Lymphoid cell lines overexpressing MCT-1 exhibited increased growth rates and displayed increased protection against apoptosis induced by serum starvation when compared with matched controls. We found that MCT-1-overexpressing cells show constitutively higher levels of phosphorylated PKB/Akt protein, especially under serum starvation. Activation of survival pathways may be an additional function of the MCT-1 gene. Our data suggest that high levels of MCT-1 protein may be associated with a high-risk subset of lymphoid neoplasms and may further support the potential role of MCT-1 in promoting human lymphoid tumor development.