Jung A. Choi

Opposite effects of Ha-Ras and Ki-Ras on radiation-induced apoptosis via differential activation of PI3K/Akt and Rac/p38 mitogen-activated protein kinase signaling pathways

It has been well known that Ras signaling is involved in various cellular processes, including proliferation, differentiation, and apoptosis. However, distinct cellular functions of Ras isozymes are not fully understood. Here we show the opposing roles of Ha-Ras and Ki-Ras genes in the modulation of cell sensitivity to ionizing radiation. Overexpression of active isoform of Ha-Ras (12V-Ha-Ras) in Rat2 cells increases resistance to the ionizing radiation. Constitutive activation of phosphoinositide-3-kinase (PI3K) and Akt is detected specifically in 12V-Ha-Ras-overexpressing cells. The specific PI3K inhibitor LY294002 inhibits PI3K/Akt signaling and potentiates the radiation-induced apoptosis, suggesting that activation of the PI3K/Akt signaling pathway is involved in the increased radio-resistance in cells overexpressing 12V-Ha-Ras. Overexpression of activated Ki-Ras (12V-Ki-Ras), on the other hand, markedly increases radiation sensitivity. The p38 mitogen-activated protein kinase (MAPK) activity is selectively enhanced by ionizing radiation in cells overexpressing 12V-Ki-Ras. The specific p38 MAPK inhibitor, PD169316, or dominant-negative p38 MAPK decreases radiation-induced cell death. We further show that the mechanism that underlies potentiation of cell death in cells overexpressing 12V-Ki-Ras involves Bax translocation to the mitochondrial membrane. Elevated Bax translocation following ionizing irradiation in 12V-Ki-Ras-overexpressing cells is completely inhibited by PD169316 or dominant-negative p38 MAPK. In addition, introduction of cells with RacN17, a dominant-negative mutant of Rac, resulted in a marked inhibition of radiation-induced Bax translocation and apoptotic cell death as well as p38 MAPK activation. Taken together, these findings explain the opposite effects of Ha-Ras and Ki-Ras on modulation of radiosensitivity, and suggest that differential activation of PI3K/Akt and Rac/p38 MAPK signaling by Ha-Ras and Ki-Ras may account for the opposing response to the ionizing radiation. These data provide an explanation for the diverse biological functions of Ras isozymes, and partly accounts for the differential response of transformed cells to anticancer treatments.

BLT2 Up-Regulates Interleukin-8 Production and Promotes the Invasiveness of Breast Cancer Cells

Background The elevated production of interleukin (IL)–8 is critically associated with invasiveness and metastatic potential in breast cancer cells. However, the intracellular signaling pathway responsible for up-regulation of IL-8 production in breast cancer cells has remained unclear. Methodology/Principal Findings In this study, we report that the expression of BLT2 is markedly up-regulated in the highly aggressive human breast cancer cell lines MDA-MB-231 and MDA-MB-435 compared with MCF-10A immortalized human mammary epithelial cells, as determined by RT-PCR, real-time PCR and FACS analysis. Blockade of BLT2 with BLT2 siRNA knockdown or BLT2 inhibitor treatment downregulated IL-8 production and thereby diminished the invasiveness of aggressive breast cancer cells, analyzed by Matrigel invasion chamber assays. We further characterized the downstream signaling mechanism by which BLT2 stimulates IL-8 production and identified critical mediatory roles for the generation of reactive oxygen species (ROS) and the consequent activation of the transcription factor NF-κB. Moreover, blockade of BLT2 suppressed the formation of metastatic lung nodules by MDA-MB-231 cells in both experimental and orthotopic metastasis models. Conclusions/Significance Taken together, our study demonstrates that a BLT2–ROS–NF-κB pathway up-regulates IL-8 production in MDA-MB-231 and MDA-MB-435 cells, thereby contributing to the invasiveness of these aggressive breast cancer cells. Our findings provide insight into the molecular mechanism of invasiveness in breast cancer.

Ras Promotes Transforming Growth Factor-β (TGF-β)-induced Epithelial-Mesenchymal Transition via a Leukotriene B4 Receptor-2-linked Cascade in Mammary Epithelial Cells

Background: The mechanism by which oncogenic Ras contributes to epithelial-mesenchymal transition (EMT) is not clearly defined. Results: The “BLT2-ROS-NF-κB”-linked cascade lies downstream of Ras and promotes EMT. Conclusion: Activation of leukotriene B4 receptor-2 (BLT2)-linked pathway contributes to EMT in mammary epithelial cells. Significance: These findings suggest that BLT2 is a novel regulator of EMT in mammary epithelial cells. Inflammation and inflammatory mediators are inextricably linked with epithelial-mesenchymal transition (EMT) through complex pathways in the tumor microenvironment. However, the mechanism by which inflammatory mediators, such as the lipid inflammatory mediators, eicosanoids, contribute to EMT is largely unknown. In the present study we observed that BLT2, leukotriene B4 receptor-2, is markedly up-regulated by oncogenic Ras and promotes EMT in response to transforming growth factor-β (TGF-β) in mammary epithelial cells. Blockade of BLT2 by the BLT2 inhibitor LY255283 or by siRNA reduced EMT induced by Ras in the presence of TGF-β. In addition, stimulation of BLT2 by the addition of a BLT2 ligand, such as leukotriene B4, restored EMT in the presence of TGF-β in human immortalized mammary epithelial MCF-10A cells. We further searched BLT2 downstream components and identified reactive oxygen species and nuclear factor κB as critical components that contribute to EMT. Taken together, these results demonstrate for the first time that a BLT2-linked inflammatory pathway contributes to EMT. This provides valuable insight into the mechanism of EMT in mammary epithelial cells. In addition, considering the implications of EMT with the stemness of cancer cells, our finding may contribute to a better understanding of tumor progression.

Cell-penetrating H4 tail peptides potentiate p53-mediated transactivation via inhibition of G9a and HDAC1.

Histone acetylation has a central role in establishing an active chromatin environment. The functional contribution of histone acetylation to chromatin transcription is accomplished by a dominant action of histone acetyltransferases over repressive histone-modifying activities at gene promoters; misregulation of these dynamic events can lead to various diseases. Here, we describe the synthesis and characterization of transducible peptides derived from histone H4 N-terminal tail as a molecular tool to establish and maintain the active state of p53 target genes. Cellular experiments demonstrate a distinct increase in p53 transactivation by acetylated H4 tail peptides, but only a modest change by unmodified H4 tail peptides. The molecular basis underlying the observed effects involves the selective interaction of the tail peptides with G9a histone methyltransferase and histone deacetylase 1 (HDAC1) and the disruption of their occupancy at p53 target promoters. Furthermore, treatment of xenograft models and cancer cell lines with the tail peptides sharply decline tumor cell growth and enhances apoptosis in response to DNA damage. These results indicate that H4 tail peptide mimics upregulate p53 transcription pathway and may be used as a novel strategy for anticancer therapy.

Inhibition of breast cancer invasion by TIS21 /BTG2/Pc3 -Akt1-Sp1-Nox4 pathway targeting actin nucleators, mDia genes

The mammalian homolog of Drosophila diaphanous (mDia), actin nucleator, has been known to participate in the process of invasion and metastasis of cancer cells via regulating a number of actin-related biological processes. We have previously reported that tumor suppressor TIS21/BTG2/Pc3 (TIS21) inhibits invadopodia formation by downregulating reactive oxygen species (ROS) in MDA-MB-231 cells. We herein report that TIS21/BTG2/Pc3 downregulates diaphanous-related formin (DRF) expression via reducing NADPH oxidase 4 (Nox4)-derived ROS generation by Akt1 activation and subsequently impairs invasion activity of the highly invasive breast cancer cells. Knockdown of Akt1 by RNA interference recovered the TIS21/BTG2/Pc3-inhibited F-actin remodeling and ROS generation by recovering Nox4 expression. Furthermore, Sp1-mediated Nox4 transcription was downregulated by TIS21/BTG2/Pc3-Akt1 signals, leading to the inhibition of cancer cell invasion via F-actin remodeling by mDia genes. To our best knowledge, this is the first study to show that TIS21/BTG2/Pc3-Akt1 inhibited Sp1-Nox4-ROS cascade, subsequently reducing invasion activity via inhibition of mDia family genes.