Poh Kuan Chong

Differential Expression of Novel Tyrosine Kinase Substrates during Breast Cancer Development

To identify novel tyrosine kinase substrates that have never been implicated in cancer, we studied the phosphoproteomic changes in the MCF10AT model of breast cancer progression using a combination of phosphotyrosyl affinity enrichment, iTRAQ™ technology, and LC-MS/MS. Using complementary MALDI- and ESI-based mass spectrometry, 57 unique proteins comprising tyrosine kinases, phosphatases, and other signaling proteins were detected to undergo differential phosphorylation during disease progression. Seven of these proteins (SPAG9, Toll-interacting protein (TOLLIP), WBP2, NSFL1C, SLC4A7, CYFIP1, and RPS2) were validated to be novel tyrosine kinase substrates. SPAG9, TOLLIP, WBP2, and NSFL1C were further proven to be authentic targets of epidermal growth factor signaling and Iressa (gefitinib). A closer examination revealed that the expression of SLC4A7, a bicarbonate transporter, was down-regulated in 64% of the 25 matched normal and tumor clinical samples. The expression of TOLLIP in clinical breast cancers was heterogeneous with 25% showing higher expression in tumor compared with normal tissues and 35% showing the reverse trend. Preliminary studies on SPAG9, on the other hand, did not show differential expression between normal and diseased states. This is the first time SLC4A7 and TOLLIP have been discovered as novel tyrosine kinase substrates that are also associated with human cancer development. Future molecular and functional studies will provide novel insights into the roles of TOLLIP and SLC4A7 in the molecular etiology of breast cancer.

Novel breast cancer metastasis-associated proteins.

With the use of the breast cancer metastatic model, which comprises four isogenic cell lines, iTRAQ-based ESI-LC/MS/MS proteomics was employed to catalog protein expression changes as cancer cells acquire increasing metastatic potential. From more than 1000 proteins detected, 197 proteins, including drug-targetable kinases, phosphatases, proteases and transcription factors, displayed differential expression when cancer cells becomes more metastatic. Overall, the number of protein expression changes was evenly distributed across mildly ( approximately 30%), moderately ( approximately 40%) and aggressively ( approximately 30%) metastatic cancer cells. Some changes were found to be specific to one while others were required for two or more phenotypes. KEGG Orthology suggests major reprogramming in cell metabolism and to smaller extents in genetic and environmental information processing. Ten novel metastasis-associated proteins were identified and the iTRAQ-based expression profiles of 7 proteins were verified to be congruent with antibody-based methods. With the use of tissue microarrays comprising 50 matched cases of invasive and metastatic lesions, the expression profiles of SH3GLB1 and SUB1, SND1, TRIM28 were validated to be down- and up-regulated, respectively, during clinical progression of carcinoma in situ to invasive and metastatic carcinomas. Our study has unraveled proteome-wide molecular aberrations and potentially new players in breast cancer metastasis.

Cathepsin S Mediates Gastric Cancer Cell Migration and Invasion via a Putative Network of Metastasis-Associated Proteins

Cancer progression is governed by multifaceted interactions of cancer cells with their microenvironment and one of these ways is through secreted compounds. Substances released by gastric cancer cells have not being profiled in a proteome-wide manner. ITRAQ-based tandem mass spectrometry was employed to quantify proteins secreted by HFE145 normal, MKN7 well-differentiated, and MKN45 poorly differentiated gastric cancer cell lines. The expression levels of 237 proteins were found to be significantly different between normal and cancer cells. Further examination of 16 gastric cell lines and 115 clinical samples validated the up-regulation of CTSS expression in gastric cancer. Silencing CTSS expression suppressed the migration and invasion of gastric cancer cells in vitro. Subsequent secretomics revealed that CTSS silencing resulted in changes in expression levels of 197 proteins, one-third of which are implicated in cellular movement. Proteome-wide comparative secretomes of normal and gastric cancer cells were produced that constitute a useful resource for gastric cancer research. CTSS was demonstrated to play novel roles in gastric cancer cell migration and invasion, putatively via a network of proteins associated with cell migration, invasion, or metastasis. Cathepsin S is member of a large group of extracellular proteases, which are attractive drug targets. The implicated role of CTSS in gastric cancer metastasis provides an opportunity to test existing compounds against CTSS for adjuvant therapy and/or treatment of metastatic gastric cancers.

Upregulation of plasma C9 protein in gastric cancer patients

Gastric cancer is one of the leading causes of cancer‐related deaths worldwide. Current biomarkers used in the clinic do not have sufficient sensitivity for gastric cancer detection. To discover new and better biomarkers, protein profiling on plasma samples from 25 normal, 15 early‐stage and 21 late‐stage cancer was performed using an iTRAQ‐LC‐MS/MS approach. The level of C9 protein was found to be significantly higher in gastric cancer compared with normal subjects. Immunoblotting data revealed a congruent trend with iTRAQ results. The discriminatory power of C9 between normal and cancer states was not due to inter‐patient variations and was independent from gastritis and Helicobacter pylori status of the patients. C9 overexpression could also be detected in a panel of gastric cancer cell lines and their conditioned media compared with normal cells, implying that higher C9 levels in plasma of cancer patients could be attributed to the presence of gastric tumor. A subsequent blind test study on a total of 119 plasma samples showed that the sensitivity of C9 could be as high as 90% at a specificity of 74%. Hence, C9 is a potentially useful biomarker for gastric cancer detection.

ITIH3 Is a Potential Biomarker for Early Detection of Gastric Cancer

Gastric cancer has one of the highest morbidities and mortalities worldwide. Early detection is key measure to improve the outcome of gastric cancer patients. In our efforts to identify potential markers for gastric cancer detection, we coupled xenotransplantation mouse model with a plasma proteomic approach. MKN45 gastric cancer cells were subcutaneously injected into nude mice and plasma samples from mice bearing different sizes of tumors were collected and subjected to iTRAQ and mass spectrometry analysis. ITIH3 protein was found to be more highly expressed in plasma of tumor bearing mice compared to control. Subsequent screening of ITIH3 expression in 167 clinical plasma samples, including 83 cancer-free subjects and 84 gastric cancer patients, revealed higher ITIH3 level in the plasma of gastric cancer patients. A receiver operating characteristics (ROC) curve estimated a maximal sensitivity of 96% at 66% specificity for ITIH3 in gastric cancer detection. In addition, plasma from early stage gastric cancer patient has significantly (p < 0.001) higher level of ITIH3 compared to that from noncancer subject. Our data suggest that ITIH3 may be a useful biomarker for early detection of gastric cancer.

Reduced plasma APOA1 level is associated with gastric tumor growth in MKN45 mouse xenograft model.

There is no suitable diagnostic and prognostic biomarker for gastric cancer. The biggest hurdles in biomarker discovery are (i) the low abundance of cancer cell-specific proteins that limits their detection and (ii) complex inter-patient variations that complicate the discovery process. To circumvent these issues, we conducted proteomics on the plasma of gastric cancer mouse xenograft and attempted to identify proteins released by cancer cells. MKN45 gastric cancer cells were subcutaneously implanted into immune-incompetent nude mice. Plasma samples collected from mice with different tumor sizes (low, mid and high tumor loads) were subjected to iTRAQ and mass spectrometric analyses. Detection of human APOA1 in mouse plasma was verified and its expression level was shown to be lower in mice with large tumors compared to those with small tumors. Studies on a panel of about 14 gastric cancer cell lines supported the notion that APOA1 in mouse plasma was of human gastric cancer cell origin. While the clinical utility of APOA1 remains to be ascertained with a larger scale study, the current work supported the feasibility of using mouse xenograft model for gastric cancer biomarker discovery.

Proteome-Wide Profiling of the MCF10AT Breast Cancer Progression Model

Background Mapping the expression changes during breast cancer development should facilitate basic and translational research that will eventually improve our understanding and clinical management of cancer. However, most studies in this area are challenged by genetic and environmental heterogeneities associated with cancer. Methodology/Principal Findings We conducted proteomics of the MCF10AT breast cancer model, which comprises of 4 isogenic xenograft-derived human cell lines that mimic different stages of breast cancer progression, using iTRAQ-based tandem mass spectrometry. Of more than 1200 proteins detected, 98 proteins representing at least 20 molecular function groups including kinases, proteases, adhesion, calcium binding and cytoskeletal proteins were found to display significant expression changes across the MCF10AT model. The number of proteins that showed different expression levels increased as disease progressed from AT1k pre-neoplastic cells to low grade CA1h cancer cells and high grade cancer cells. Bioinformatics revealed that MCF10AT model of breast cancer progression is associated with a major re-programming in metabolism, one of the first identified biochemical hallmarks of tumor cells (the “Warburg effect”). Aberrant expression of 3 novel breast cancer-associated proteins namely AK1, ATOX1 and HIST1H2BM were subsequently validated via immunoblotting of the MCF10AT model and immunohistochemistry of progressive clinical breast cancer lesions. Conclusion/Significance The information generated by this study should serve as a useful reference for future basic and translational cancer research. Dysregulation of ATOX1, AK1 and HIST1HB2M could be detected as early as the pre-neoplastic stage. The findings have implications on early detection and stratification of patients for adjuvant therapy.

Phosphoproteomics, oncogenic signaling and cancer research

The past 5 years have seen an explosion of phosphoproteomics methods development. In this review, using epidermal growth‐factor signaling as a model, we will discuss how phosphoproteomics, along with bioinformatics and computational modeling, have impacted key aspects of oncogenic signaling such as in the temporal fine mapping of phosphorylation events, and the identification of novel tyrosine kinase substrates and phosphorylation sites. We submit that the next decade will see considerable exploitation of phosphoproteomics in cancer research. Such a phenomenon is already happening as exemplified by its use in promoting the understanding of the molecular etiology of cancer and target‐directed therapeutics.

CXCL3 is a Potential Target for Breast Cancer Metastasis

Secreted proteins are an attractive minefield for cancer drug targets. An iTRAQ-based tandem mass spectrometry approach was employed to relatively quantify proteins in the secretomes of four isogenic breast cancer cell lines with increasing metastatic potential. CXCL3 was found to be upregulated in aggressive cancer cells. SiRNA and antibody neutralization studies supported a role of CXCL3 in metastatic processes. Meta-analysis of the mRNA level of CXCL3 in 1881 breast tumors supported a role of CXCL3 in clinical breast cancer. Our results support a functional role of CXCL3 in breast cancer metastasis and as a viable target for cancer therapy.

Identification and Characterization of Sulfolobus solfataricus P2 Proteome Using Multidimensional Liquid Phase Protein Separations

We have identified and characterized the proteome of Sulfolobus solfataricus P2 using multidimensional liquid phase protein separations. Multidimensional liquid phase chromatography was performed using ion exchange chromatography in the first dimension, followed by reverse-phase chromatography using 500 microm i.d. poly(styrene-divinylbenzene) monoliths in the second dimension to separate soluble protein lysates from S. solfataricus. The 2DLC protein separations from S. solfataricus protein lysates enabled the generation of a 2D liquid phase map analogous to the traditional 2DE map. Following separation of the proteins in the second dimension, fractions were collected, digested in solution using trypsin and analyzed using mass spectrometry. These approaches offer significant reductions in labor intensity and the overall time taken to analyze the proteome in comparison to 2DE, taking advantage of automation and fraction collection associated with this approach. Furthermore, following proteomic analysis using 2DLC, the data obtained was compared to previous 2DE and shotgun proteomic studies of a soluble protein lysate from S. solfataricus. In comparison to 2DE, the results show an overall increase in proteome coverage. Moreover, 2DLC showed increased coverage of a number of protein subsets including acidic, basic, low abundance and small molecular weight proteins in comparison to 2DE. In comparison to shotgun studies, an increase in proteome coverage was also observed. Furthermore, 187 unique proteins were identified using 2DLC, demonstrating this methodology as an alternative approach for proteomic studies or in combination with 2DE and shotgun workflows for global proteomics.