Id proteins are overexpressed in human oral squamous cell carcinomas

Background: The helix-loop-helix (HLH) proteins Id-1, Id-2 and Id-3 have been demonstrated to inhibit the activity of transcription factors and play an important role in regulating cell growth and tissue-specific differentiation. Methods: To elucidate the involvement of Id in human oral squamous cell carcinoma (OSCC), we examined 83 surgical specimens and eight normal gingival mucosae for the expression of Id proteins by immunohistochemistry; in addition, some specimens of the OSCC and the normal gingivae were also examined for the expression of Id-1 mRNA by in situ hybridization (ISH), while Western blots were performed on six of the tumours and on cell lysates of five OSCC cell lines. We also explored the correlation between Id expressions and cellular proliferation indicating Ki-67 or clinical parameters. Results: We discovered a higher and more frequent expression of Id-1 protein (27.7%) in human OSCC compared to that of Id-2 and Id-3 proteins (6.0 and 7.2%, respectively). Western blot analysis detected Id-1 protein in four of six tumour samples and in all cell lines. ISH demonstrated strong cytoplasmic localization of Id-1 mRNA in tumour samples at significantly higher levels compared to those of normal gingival mucosae. Conclusion: Id-1 protein expression significantly correlates with lymph node status, indicating that increased Id expression may relate to the metastatic behaviour in human OSCC.