Background: Salivary gland cancer (SGC) is one of the common malignancies of the head and neck area. It develops in the minor and major salivary glands and sometimes metastasizes to other organs, particularly to the lungs. Inhibitors of differentiation (Id) proteins are negative regulators of basic helix-loop-helix transcription factors that control malignant cell behavior and tumor aggressiveness in many tissues. In this study, our goal was to determine the potential role of Id proteins, particularly Id1, during human SGC cell progression.
Methods: We first determined the expression levels of Id1 and Id2 in four SGC cell lines: two adenocarcinoma of the salivary gland (HSG and HSY) and two adenoid cystic carcinoma (ACC2 and ACCM) cell lines. We then used constructs that expressed antisense cDNAs to Id1 or Id2 to knockdown the expression of these proteins in cell lines where they were highly expressed, and determined the effects of the knockdown on cell proliferation, migration and invasion.
Results: Id1 mRNA and protein were detectable in all cell lines, and expression of Id2 was variable, from absent to high. The ACC2 and ACCM cell lines expressed both Id1 and Id2, but Id1 was expressed at a higher level in the more aggressive ACCM cell line in comparison to ACC2 cells as confirmed by Id1 promoter-reporter assays. We therefore focused on the ACCM cells for the remainder of the study. We found that proliferation and invasiveness of ACCM cells were strongly reduced after Id1 knockdown whereas Id2 suppression had only a slight effect. Results of scratch and colony formation assays also confirmed that ACCM cell aggressiveness was significantly reduced upon Id1 knockdown. Finally, this knockdown resulted in reduced c-myc and enhanced cyclin-dependent kinase inhibitor p21 expression.
Conclusions: These results demonstrate that Id1 plays an important role in the control of human SGC cell aggressiveness and suggest a potential role as a marker of diagnosis, prognosis and progression of SGCs. Id1 suppression could represent a novel and effective approach for the treatment of salivary gland cancer.