E2F-1 gene transfer enhances invasiveness of human head and neck carcinoma cell lines.

The transcription factor E2F-1, a downstream regulator of the p16-cyclinD-Rb pathway, is required for cell cycle progression. Evidence shows that overexpression of E2F-1 can either promote or inhibit development of tumors, depending on tissue or experimental conditions. To study whether the E2F-1 gene plays a role in tumor progression, the expression of E2F-1 protein was evaluated in 10 human head and neck squamous cell carcinoma cell lines using Western blot analysis. In addition, the invasive ability of these cell lines was determined by evaluating the penetration of cell lines into the tracheal wall in an in vivo invasion assay using deepithelialized tracheas transplanted into the s.c. tissue of Scid mice. This study showed that the aggressive cell lines had higher expression of E2F-1 than the less invasive cell lines. To evaluate the hypothesis that E2F-1 enhances invasiveness, we selected two cell lines, SCC9 and SCC12, for a gene transfer experiment. These cell lines exhibited low invasive ability with low expression of E2F-1. Two stable clones with overexpression of transfected E2F-1 gene and two clones with their respective vector-alone control were selected from each cell line for in vivo invasion evaluation. The clones containing the transfected E2F-1 gene had significantly higher invasive ability than their respective vector-alone clones. Flow cytometry showed that parental, transfected E2F-1, and vector-alone cells had a similar proliferation pattern under normal culture conditions. Nevertheless, transfected E2F-1 cells exhibited a higher portion of cells in S phase than the control cells after serum-starvation and refeeding. The results indicated that overexpression of E2F-1 plays a positive role in cell cycle reentry from quiescence and is associated with increased in vivo invasiveness.