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Introduction

Polo-like kinase 1 (PLK1) is a critical regulator of the many stages of mitosis. Accumulating evidence indicates that over-expression of Plk1 correlates with clinical outcome. In this work, we evaluate for the first time, the expression of Plk1 in primary human nasopharyngeal carcinoma (NPC) biopsies, and demonstrate its potential role as a therapeutic target.

Experimental Procedures

Plk1 expression was assessed using immunohistochemistry in 40 NPC archival samples linked to outcome data. Plk1 was targeted using siRNA to evaluate both functional significance, as well as its therapeutic potential in NPC. RNA interference was achieved using a pooled small interfering RNA (siPlk1) to knock down the expression of PLK1 in the Epstein-Barr Virus (EBV)-positive NPC cell line C666-1.

Results

Plk1 immunoexpression was observed in 28 of 40 specimens (70%), which in turn, was associated with a higher likelihood of recurrence (p=0.018). SiPlk1 (30nM) reduced C666-1 viability in a time and dose-dependent manner. The cytotoxic effect was enhanced with the addition of radiation (6 Gy), down to 15% viability at day 7. This cytotoxicity appeared to be mediated by apoptosis (33.5% at 48 hours), along with activation of caspases 3/7 (2.1-fold increase over control). Cell cycle analysis demonstrated an increase in the G2/M cell population to 30.6%. Immunofluorescence demonstrated the G2/M arrest being associated with aberrant spindle formation, resulting in mitotic arrest.Western blotting corroborated that down-regulation of Plk1 was associated with an increase in Cyclin B1 expression, consistent with the known downstream targets of Plk1. Finally, the therapeutic potential of Plk1 was assessed using an in vivo tumor-formation assay. Transfection of C666-1 cells with SiPlk1 delayed tumor formation by 44 days, compared to control mice. When combined with radiation (6 Gy), this resulted in a further delay in tumor development of up to 75 days, compared to the 60 days in the siCNTR-treated group.

Conclusion

Our data demonstrate over-expression of Plk1 in human NPC samples, which was associated with a higher risk of recurrent disease. In addition, targeting Plk1 was effective in reducing NPC survival both in vitro and in vivo, particularly when combined with ionizing radiation, indicating that Plk1 likely represents an important target for NPC therapy.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA