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A novel isoform of the 8p22 tumor suppressor gene DLC1 suppresses tumor growth and is frequently silenced in multiple common tumors

Oncogene volume 30, pages 1923–1935 (2011)Cite this article

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Abstract

The critical 8p22 tumor suppressor deleted in liver cancer 1 (DLC1) is frequently inactivated by aberrant CpG methylation and/or genetic deletion and implicated in tumorigeneses of multiple tumor types. Here, we report the identification and characterization of its new isoform, DLC1 isoform 4 (DLC1-i4). This novel isoform encodes an 1125-aa (amino acid) protein with distinct N-terminus as compared with other known DLC1 isoforms. Similar to other isoforms, DLC1-i4 is expressed ubiquitously in normal tissues and immortalized normal epithelial cells, suggesting a role as a major DLC1 transcript. However, differential expression of the four DLC1 isoforms is found in tumor cell lines: Isoform 1 (longest) and 3 (short thus probably nonfunctional) share a promoter and are silenced in almost all cancer and immortalized cell lines, whereas isoform 2 and 4 utilize different promoters and are frequently downregulated. DLC1-i4 is significantly downregulated in multiple carcinoma cell lines, including 2/4 nasopharyngeal, 8/16 (50%) esophageal, 4/16 (25%) gastric, 6/9 (67%) breast, 3/4 colorectal, 4/4 cervical and 2/8(25%) lung carcinoma cell lines. The functional DLC1-i4 promoter is within a CpG island and is activated by wild-type p53. CpG methylation of the DLC1-i4 promoter is associated with its silencing in tumor cells and was detected in 38–100% of multiple primary tumors. Treatment with 5-aza-2′-deoxycytidine or genetic double knockout of DNMT1 and DNMT3B led to demethylation of the promoter and reactivation of its expression, indicating a predominantly epigenetic mechanism of silencing. Ectopic expression of DLC1-i4 in silenced tumor cells strongly inhibited their growth and colony formation. Thus, we identified a new isoform of DLC1 with tumor suppressive function. The differential expression of various DLC1 isoforms suggests interplay in modulating the complex activities of DLC1 during carcinogenesis.

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Acknowledgements

This project was supported by an A*STAR grant (Johns Hopkins Singapore, QT/WS/RA), a Chinese University of Hong Kong grant (QT) and a grant from Singapore National Research Foundation and the Ministry of Education under the Research Center of Excellence Program (WS/RA). We thank Drs Bert Vogelstein, George Tsao, (Dolly Huang), Michael Obster and Guiyuan Li for some cell lines; and DSMZ (German Collection of Microorganisms & Cell Cultures) for the KYSE cell lines (Shimada et al., Cancer 69: 277–284, 1992). We also thank Drs Wenling Han, Thomas Putti, Luke Tan for some tumor samples.

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Authors and Affiliations

  1. Division of Biomedical Sciences, Johns Hopkins Singapore, Singapore

    J S W Low, Q Tao, H K Goh, W L Woo, R F Ambinder, M Shamay & W-S Hsieh

  2. Cancer Science Institute of Singapore, National University of Singapore, Singapore

    J S W Low, H K Goh, R F Ambinder & W-S Hsieh

  3. Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins School of Medicine, Baltimore, MD, USA

    Q Tao, R F Ambinder, M Shamay & W-S Hsieh

  4. Department of Clinical Oncology, Cancer Epigenetics Laboratory, State Key Laboratory of Oncology in South China, Sir YK Pao Center for Cancer, Hong Kong Cancer Institute and Li Ka Shing Institute of Health Sciences, The Chinese University of Hong Kong, Hong Kong

    Q Tao, K M Ng, X-S Shu & A T C Chan

  5. Department of Pathology, University of Hong Kong, Hong Kong

    G Srivastava

  6. Laboratory of Experimental Carcinogenesis, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD, USA

    N C Popescu

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Low, J., Tao, Q., Ng, K. et al. A novel isoform of the 8p22 tumor suppressor gene DLC1 suppresses tumor growth and is frequently silenced in multiple common tumors. Oncogene 30, 1923–1935 (2011). https://doi.org/10.1038/onc.2010.576

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