Inhibition of cyclin D1 expression by cyclin D1 shRNAs in human oral squamous cell carcinoma cells is associated with increased cisplatin chemosensitivity

Cyclin D1 is a well-known cell cycle regulator. Recently, its pro-survival function has been revealed in several tumors. Because increasing expression of cyclin D1 is a common event in oral squamous cell carcinoma (OSCC) and has been correlated with cisplatin resistance, we investigated if cyclin D1 inhibition could increase cisplatin chemosensitivity of OSCC. Five cyclin D1 shRNAs were prepared and 3 were selected for subsequent experiments. IC50 values for cisplatin were determined by an MTT assay. Cisplatin-induced apoptosis and cell cycle block were investigated. A tumor transplantation model was generated to examine the cisplatin sensitivity of Tca/cisplatin after in vivo cyclin D1 silencing. The role of nuclear factor-kappaB (NF-kappaB) and cyclin-dependent kinase 4 (CDK4) in cyclin D1-mediated cisplatin resistance was also examined. The most effective shRNA resulted in 84.51% knockdown of the cyclin D1 protein level. After the transfection with the 2 most effective shRNAs, the cisplatin IC50 decreased from 5.88 microg/ml to 1.36 microg/ml and 2.47 microg/ml, although overexpression of cyclin D1 rendered OSCC cells more resistant to cisplatin treatment (IC50 increased from 6.43 microg/ml to 12.24 microg/ml). This decreasing IC50 was correlated with the down-regulation of cisplatin-induced NF-kappaB activity in cyclin D1 knockdown cells, and was independent of CDK4 function. In vivo tumor transplantation models also confirmed a cisplatin-sensitizing effect of cyclin D1 shRNA in OSCC. A TUNEL assay validated the increase in apoptosis as induced by cisplatin in cyclin D1 knockdown cells. Cyclin D1 may be an important target for future therapy in patients with OSCC.