To investigate the upstream kinases associated with MRB- mediated cyclin D1 degradation, HCT116 and SW480 cells were pretreated with PD98059 (ERK1/2 inhibitor), SB203580 (p38 in- hibitor) or SP600125 (JNK inhibitor), and then co-treated with MRB. As shown in Fig. 5 A–F, inhibition of ERK1/2, p38 or JNK did not affect MRB-mediated cyclin D1 degradation. In addition, we investigated whether other kinases such as I κK- αand GSK βaf- fects cyclin D1 degradation, and found that MRB induced cyclin D1 degradation irrespective of the inhibition of I κK- αby BAY11-7082 in HCT116 and SW480 cells ( Fig. 6 A and B). However, GSK βinhi- bition by LiCl attenuated MRB-mediated cyclin D1 degradation in HCT116 and SW480 cells ( Fig. 6 C and D). We also found that GSK βinhibition by LiCl suppressed MRB-mediated threonine-286 phos- phorylation of cyclin D1 ( Fig. 6 E).