RUNX1 positively regulates the ErbB2/HER2 signaling pathway through modulating SOS1 expression in gastric cancer cells.

The dual function of runt-related transcriptional factor 1 (RUNX1) as an oncogene or oncosuppressor has been extensively studied in various malignancies, yet its role in gastric cancer remains elusive. Up-regulation of the ErbB2/HER2 signaling pathway is frequently-encountered in gastric cancer and contributes to the maintenance of these cancer cells. This signaling cascade is partly mediated by son of sevenless homolog (SOS) family, which function as adaptor proteins in the RTK cascades. Herein we report that RUNX1 regulates the ErbB2/HER2 signaling pathway in gastric cancer cells through transactivating SOS1 expression, rendering itself an ideal target in anti-tumor strategy toward this cancer. Mechanistically, RUNX1 interacts with the RUNX1 binding DNA sequence located in SOS1 promoter and positively regulates it. Knockdown of RUNX1 led to the decreased expression of SOS1 as well as dephosphorylation of ErbB2/HER2, subsequently suppressed the proliferation of gastric cancer cells. We also found that our novel RUNX inhibitor (Chb-M') consistently led to the deactivation of the ErbB2/HER2 signaling pathway and was effective against several gastric cancer cell lines. Taken together, our work identified a novel interaction of RUNX1 and the ErbB2/HER2 signaling pathway in gastric cancer, which can potentially be exploited in the management of this malignancy.

Autonomous feedback loop of RUNX1-p53-CBFB in acute myeloid leukemia cells.

Although runt-related transcription factor 1 (RUNX1) and its associating core binding factor-ß (CBFB) play pivotal roles in leukemogenesis, and inhibition of RUNX1 has now been widely recognized as a novel strategy for anti-leukemic therapies, it has been elusive how leukemic cells could acquire the serious resistance against RUNX1-inhibition therapies and also whether CBFB could participate in this process. Here, we show evidence that p53 (TP53) and CBFB are sequentially up-regulated in response to RUNX1 depletion, and their mutual interaction causes the physiological resistance against chemotherapy for acute myeloid leukemia (AML) cells. Mechanistically, p53 induced by RUNX1 gene silencing directly binds to CBFB promoter and stimulates its transcription as well as its translation, which in turn acts as a platform for the stabilization of RUNX1, thereby creating a compensative RUNX1-p53-CBFB feedback loop. Indeed, AML cells derived from relapsed cases exhibited higher CBFB expression levels compared to those from primary AML cells at diagnosis, and these CBFB expressions were positively correlated to those of p53. Our present results underscore the importance of RUNX1-p53-CBFB regulatory loop in the development and/or maintenance of AML cells, which could be targeted at any sides of this triangle in strategizing anti-leukemia therapies.

Paradoxical enhancement of leukemogenesis in acute myeloid leukemia with moderately attenuated RUNX1 expressions.

Besides being a classical tumor suppressor, runt-related transcription factor 1 (RUNX1) is now widely recognized for its oncogenic role in the development of acute myeloid leukemia (AML). Here we report that this bidirectional function of RUNX1 possibly arises from the total level of RUNX family expressions. Indeed, analysis of clinical data revealed that intermediate-level gene expression of RUNX1 marked the poorest-prognostic cohort in relation to AML patients with high- or low-level RUNX1 expressions. Through a series of RUNX1 knockdown experiments with various RUNX1 attenuation potentials, we found that moderate attenuation of RUNX1 contributed to the enhanced propagation of AML cells through accelerated cell-cycle progression, whereas profound RUNX1 depletion led to cell-cycle arrest and apoptosis. In these RUNX1-silenced tumors, amounts of compensative upregulation of RUNX2 and RUNX3 expressions were roughly equivalent and created an absolute elevation of total RUNX (RUNX1 + RUNX2 + RUNX3) expression levels in RUNX1 moderately attenuated AML cells. This elevation resulted in enhanced transactivation of glutathione S-transferase α 2 (GSTA2) expression, a vital enzyme handling the catabolization of intracellular reactive oxygen species (ROS) as well as advancing the cell-cycle progressions, and thus ultimately led to the acquisition of proliferative advantage in RUNX1 moderately attenuated AML cells. Besides, treatment with ethacrynic acid, which is known for its GSTA inhibiting property, actually prolonged the survival of AML mice in vivo. Collectively, our findings indicate that moderately attenuated RUNX1 expressions paradoxically enhance leukemogenesis in AML cells through intracellular environmental change via GSTA2, which could be a novel therapeutic target in antileukemia strategy.

Human SHPRH suppresses genomic instability through proliferating cell nuclear antigen polyubiquitination.

Differential modifications of proliferating cell nuclear antigen (PCNA) determine DNA repair pathways at stalled replication forks. In yeast, PCNA monoubiquitination by the ubiquitin ligase (E3) yRad18 promotes translesion synthesis (TLS), whereas the lysine-63-linked polyubiquitination of PCNA by yRad5 (E3) promotes the error-free mode of bypass. The yRad5-dependent pathway is important to prevent genomic instability during replication, although its exact molecular mechanism is poorly understood. This mechanism has remained totally elusive in mammals because of the lack of apparent RAD5 homologues. We report that a putative tumor suppressor gene, SHPRH, is a human orthologue of yeast RAD5. SHPRH associates with PCNA, RAD18, and the ubiquitin-conjugating enzyme UBC13 (E2) and promotes methyl methanesulfonate (MMS)-induced PCNA polyubiquitination. The reduction of SHPRH by stable short hairpin RNA increases sensitivity to MMS and enhances genomic instability. Therefore, the yRad5/SHPRH-dependent pathway is a conserved and fundamental DNA repair mechanism that protects the genome from genotoxic stress.

Role of Cbfb in hematopoiesis and perturbations resulting from expression of the leukemogenic fusion gene Cbfb-MYH11.

Core-binding factor beta (CBFbeta) and CBFalpha2 form a heterodimeric transcription factor that plays an important role in hematopoiesis. The genes encoding either CBFbeta or CBFalpha2 are involved in chromosomal rearrangements in more than 30% of cases of acute myeloid leukemia (AML), suggesting that CBFbeta and CBFalpha2 play important roles in leukemogenesis. Inv(16)(p13;q22) is found in almost all cases of AML M4Eo and results in the fusion of CBFB with MYH11, the gene encoding smooth muscle myosin heavy chain. Mouse embryos heterozygous for a Cbfb-MYH11 knock-in gene lack definitive hematopoiesis, a phenotype shared by Cbfb(-/-) embryos. In this study we generated a Cbfb-GFP knock-in mouse model to characterize the normal expression pattern of Cbfbeta in hematopoietic cells. In midgestation embryos, Cbfbeta was expressed in populations enriched for hematopoietic stem cells and progenitors. This population of stem cells and progenitors was not present in mouse embryos heterozygous for the Cbfb-MYH11 knock-in gene. Together, these data suggest that Cbfb-MYH11 blocks embryonic hematopoiesis at the stem-progenitor cell level and that Cbfb is essential for the generation of hematopoietic stem and progenitor cells. In adult mice, Cbfbeta was expressed in stem and progenitor cells, as well as mature myeloid and lymphoid cells. Although it was expressed in erythroid progenitors, Cbfbeta was not expressed during the terminal stages of erythropoiesis. Our data indicate that Cbfb is required for myeloid and lymphoid differentiation; but does not play a critical role in erythroid differentiation.