For real-time RT-PCR analysis, the total RNA of cells was extracted by using the RNA easy kit (Qiagen, Valencia, CA). Briefly, the total RNA (1 mg) of each sample was reversely transcribed in 20 mL using 0.5 mg of oligo dT and 200 U Superscript II RT (Invitrogen, Carlsbad, CA). The amplification was carried out in a total volume of 20 ml containing 0.5 mM of each primer, 4 mM 