Faculty Bibliography

Chronic environmental exposure to toxic metal(loid)s significantly contributes to human cancer development and progression. It is estimated that approximately 90% of cancer deaths are a result of metastasis of malignant cells, which is initiated by epithelial-mesenchymal transition (EMT) during early carcinogenesis. EMT is regulated by many families of genes and microRNAs (miRNAs) that control signaling pathways for cell survival, death, and/or differentiation. Recent mechanistic studies have shown that toxic metal(loid)s alter the expression of miRNAs responsible for regulating the expression of genes involved in EMT. Altered miRNA expressions have the potential to be biomarkers for predicting survival and responses to treatment in cancers. Significantly, miRNAs can be developed as therapeutic targets for cancer patients in the clinic. In this mini review, we summarize key findings from recent studies that highlight chemical-miRNA-gene interactions leading to the perturbation of EMT after exposure to toxic metal(loid)s including arsenic, cadmium, nickel, and chromium.

Reactive oxygen species (ROS) are by-products of metabolism of oxygen and they play an important role in normal homeostasis and cell signaling, as well as in the initiation of diseases including cancer when their production is upregulated. Thus, it is imperative to understand the cellular and molecular basis by which ROS impact on various biological and pathological processes. In this report, we show that human keratinocyte cell line (HaCaT) treated with hydrogen peroxide displayed an increased activity of AhR, leading to enhanced expression of its downstream targets including cytochrome P450 genes. Intriguingly, preincubation of the complete culture medium with hydrogen peroxide accelerated AhR activation and its downstream signaling. Subsequent mass spectrometric analysis reveals that the oxidant elicits the production of oxindole, a tryptophan catabolic product. We further demonstrate that 2-oxindole (a major form of oxindole) is capable of activating AhR, strongly suggesting that ROS may exert a significant impact on AhR signaling. Consistent with this, we also observe that hexavalent chromium [Cr(VI)], a heavy metal known to generate ROS in vivo, enhances AhR protein levels, as well as stimulates expression of CYP1A2 in an AhR-dependent manner. Significantly, we show that hydrogen peroxide and 2-oxindole induce expression of IDO1 and PD-L1, two immune checkpoint proteins. Given the role of IDO1 and PD-L1 in mediating T cell activity and/or differentiation, we postulate that ROS in the tumor microenvironment may play a crucial role in immune suppression via perturbing AhR signaling.

Ras proteins are small GTPases that participate in multiple signal cascades, regulating crucial cellular processes including cell survival, proliferation, and differentiation. Mutations or deregulated activities of Ras are frequently the driving force for oncogenic transformation and tumorigenesis. Posttranslational modifications play a crucial role in mediating the stability, activity, or subcellular localization/trafficking of numerous cellular regulators including Ras proteins. A series of recent studies reveal that Ras proteins are also regulated by sumoylation. All three Ras protein isoforms (HRas, KRas, and NRas) are modified by SUMO3. The conserved lysine42 appears to be the primary site for mediating sumoylation. Expression of KRas^V12/R42 mutants compromised the activation of the Raf/MEK/ERK signaling axis, leading to a reduced rate of cell migration and invasion in vitro in multiple cell lines. Moreover, treatment of transformed pancreatic cells with a SUMO E2 inhibitor blocks cell migration in a concentration-dependent manner, which is associated with a reduced level of both KRas sumoylation and expression of mesenchymal cell markers. Furthermore, mouse xenograft experiments reveal that expression of a SUMO-resistant mutant appears to suppress tumor development in vivo. Combined, these studies indicate that sumoylation functions as an important mechanism in mediating the roles of Ras in cell proliferation, differentiation, and malignant transformation and that the SUMO-modification system of Ras oncoproteins can be explored as a new druggable target for various human malignancies.

OBJECTIVES/OBJECTIVE:Besides its role in regulating phosphatidylinositol-3 kinase (PI3K) signalling in the cytosol, PTEN also has a nuclear function. In this study, we attempted to understand the mechanism of chromatin PTEN in suppressing chromosomal instability during cell division. MATERIALS AND METHODS/METHODS:Immunocoprecipitation, ectopic expression, and deletional analyses were used to identify the physical interaction between Chromobox Homolog protein 8 (CBX8) and PTEN, as well as the functional domain(s) of PTEN mediating the interaction. Cell synchronization followed by immunoblotting was employed to study cell cycle regulation of CBX8 and the functional interaction between chromatin PTEN and CBX8. Small interfering RNAs (siRNAs) were used to study the role of PTEN and CBX8 in modulating histone epigenetic markers during the cell cycle. RESULTS:Polycomb group (PcG) proteins including CBXs function to repress gene expression in a wide range of organisms including mammals. We recently showed that PTEN interacted with CBX8, a component of Polycomb Repressing Complex 1 (PRC1), and that CBX8 co-localized with PTEN in the nucleus. CBX8 levels were high, coinciding with its phosphorylation in mitosis. Phosphorylation of CBX8 was associated with monoubiquitinated PTEN and phosphorylated-BubR1 on chromatin. Moreover, CBX8 played an important role in cell proliferation and mitotic progression. Significantly, downregulation of either PTEN or CBX8 induced H3K27Me3 epigenetic marker in mitotic cells. CONCLUSION/CONCLUSIONS:CBX8 is a new component that physically interacts with chromatin PTEN, playing an important role in regulating mitotic progression.

Ras genes are among the most frequently mutated oncogenes in human malignancies. To date, there are no successful anti-cancer drugs in the clinic that target Ras proteins or their pathways. Therefore, it is imperative to identify and characterize new components that regulate Ras activity or mediates its downstream signaling. To this end, we used a combination of affinity-pulldown and mass spectrometry to search for proteins that are physically associated with KRas. One of the top hits was Radil, a gene product with a Ras-association (RA) domain. Radil is known to be a downstream effector of Rap1, inhibiting RhoA signaling to regulate cell adhesion and migration. We demonstrate that Radil interacted with all three isoforms of Ras including HRas, NRas, and KRas, although it exhibited the strongest interaction with KRas. Moreover, Radil interacts with GTP-bound Ras more efficiently, suggesting a possibility that Radil may be involved in Ras activation. Supporting this, ectopic expression of Radil led to transient activation of MEK and ERK; Radil knockdown resulted in weakened activation of Ras downstream signaling components, which was coupled with decreased cell proliferation and invasion, and reduced expression of mesenchymal cell markers. Moreover, Radil knockdown greatly reduced the number of adhesion foci and depolymerized actin filaments, molecular processes that facilitate cancer cell migration. Taken together, our current studies strongly suggest that Radil is an important player for regulating Ras signaling, cell adhesion, and the epithelial-mesenchymal transition, and may provide new directions for Ras-related anti-cancer drug development.

Ras proteins participate in multiple signal cascades, regulating crucial cellular processes including cell survival, proliferation, and differentiation. We have previously reported that Ras proteins are modified by sumoylation and that lysine-42 (K42) plays an important role in mediating the modification.  In the current study, we further investigated the role of K42 in regulating cellular activities of K-Ras. Inducible expression of K-RasV12 led to the activation of downstream components including c-RAF, MEK1, and ERKs whereas expression of K-RasV12/R42 mutant compromised the activation of the RAF/MEK/ERK signaling axis. Expression of K-RasV12/R42 also led to reduced phosphorylation of several other protein kinases including JNK, Chk2, and FAK. Significantly, K-RasV12/R42 expression inhibited cellular migration and invasion in vitro in multiple cell lines including transformed pancreatic cells. Given K-Ras plays a crucial role in mediating oncogenesis in pancreas, we treated transformed pancreatic cells of both BxPC-3 and MiaPaCa-2 with 2-D08, an SUMO E2 inhibitor. Treatment with the compound inhibited cell migration in a concentration-dependent manner, which was correlated with a reduced level of K-Ras sumoylation. Moreover, 2-D08 suppressed expression of ZEB1 (a mesenchymal cell marker) with concomitant induction of ZO-1 (an epithelial cell marker). Combined, our studies strongly suggest that post-translational modification(s) including sumoylation mediated by K42 plays a crucial role in K-Ras activities in vivo.

RAS proteins are GTPases that participate in multiple signal cascades, regulating crucial cellular processes including cell survival, proliferation, differentiation, and autophagy. Mutations or deregulated activities of RAS are frequently the driving force for oncogenic transformation and tumorigenesis. Given the important roles of the small ubiquitin-related modifier (SUMO) pathway in controlling the stability, activity, or subcellular localization of key cellular regulators, we investigated here whether RAS proteins are posttranslationally modified (i.e. SUMOylated) by the SUMO pathway. We observed that all three RAS protein isoforms (HRAS, KRAS, and NRAS) were modified by the SUMO3 protein. SUMOylation of KRAS protein, either endogenous or ectopically expressed, was observed in multiple cell lines. The SUMO3 modification of KRAS proteins could be removed by SUMO1/sentrin-specific peptidase 1 (SENP1) and SENP2, but not by SENP6, indicating that RAS SUMOylation is a reversible process. A conserved residue in RAS, Lys-42, was a site that mediates SUMOylation. Results from biochemical and molecular studies indicated that the SUMO-E3 ligase PIASγ specifically interacts with RAS and promotes its SUMOylation. Moreover, SUMOylation of RAS appeared to be associated with its activation. In summary, our study reveals a new posttranslational modification for RAS proteins. Since we found that HRAS, KRAS, and NRAS can all be SUMOylated, we propose that SUMOylation might represent a mechanism by which RAS activities are controlled.

Quantum dots (QDs) have shown great potential for biomedical use in a broad range including diagnostic agents. However, the regulatory mechanism of dermal toxicity is poorly understood. In this study, we investigated how QDs-induced apoptosis is regulated in human keratinocytes. We also examined the effect of carboxylic acid-coated QDs (QD 565 and QD 655) on reactive oxygen species (ROS) production and apoptosis-related cellular signalling. The viability of keratinocyte was inhibited by two types of QDs in a concentration-dependent manner. QDs induce ROS production and blockade of AKT phosphorylation. Moreover, the cleavage of AKT-dependent pro-apoptotic proteins such as poly (ADP-ribose) polymerase, caspases-3 and caspases-9 was significantly increased. We also found that a decrease in cellular ROS level by ROS scavenger, N-acetylcysteine (NAC), resulting in the abolishment of QDs-induced AKT de-phosphorylation and cellular apoptosis. Interestingly, QD 655 had a more cytotoxic effect including oxidative stress and AKT-dependent apoptosis than QD 565. In addition, QD 655 had the cytotoxic potential in the human skin equivalent model (HSEM). These data show that QD-induced intracellular ROS levels may be an important parameter in QD-induced apoptosis. These findings from this study indicate that intracellular ROS levels might determine the apoptotic potential of keratinocyte by QD via blockade of AKT phosphorylation.

Nuclear PTEN plays an important role during mitosis. To understand the molecular basis by which PTEN mediates mitotic progression, we examined whether PTEN regulated the formation of mitotic checkpoint complex (MCC). We observed that arsenic trioxide, a mitotic inducer, stimulated nuclear translocation of PTEN in a time-dependent manner. PTEN physically interacted with Cdc20 and Mad2, two important components of MCC. Arsenic treatment diminished the physical association of PTEN with BubR1 and Bub3 but not with Cdc20 and Mad2. Our further studies revealed that downregulation of PTEN via RNAi enhanced formation of MCC during the cell cycle. Moreover, PTEN silencing induced chromosomal instability. Given the crucial role of PTEN in suppressing tumor development, our study strongly suggests that PTEN also functions to maintain chromosomal stability, partly through suppressing unscheduled formation of MCC.

WWP2 is a ubiquitin E3 ligase belonging to the Nedd4-like family. Given that WWP2 target proteins including PTEN that are crucial for regulating cell proliferation or suppressing tumorigenesis, we have asked whether WWP2 plays a role in controlling cell cycle progression. Here we report that WWP2 is necessary for normal cell cycle progression as its silencing significantly reduces the cell proliferation rate. We have identified that an isoform of WWP2 (WWP2-V4) is highly expressed in the M phase of the cell cycle. Silencing of WWP2 accelerates the turnover of cyclin E, which is accompanied by increased levels of phospho-histone H3 (p-H3) and cyclin B. Moreover, silencing of WWP2 results in compromised phosphorylation of AktS473, a residue whose phosphorylation is tightly associated with the activation of the kinase. Combined, these results strongly suggest that WWP2 is an important component in regulating the Akt signaling cascade, as well as cell cycle progression.