Although the enzyme activity of HDAC6 can be inhibited by LBH589 in both LNCaP and PC-3 PCa cells, LBH589 selectively depletes either HDAC6 or Aurora kinases in LNCaP and PC-3 PCa cells with distinct biological outcomes, respectively. This study raises the important question of why LBH589 selectively depletes either HDAC6 or Aurora kinases through a proteasome degradation pathway in different PCa cells. Understanding the molecular mechanisms behind this discrepancy in the therapeutic response of LBH589 on different PCa cells can provide more insights for the clinical application of LBH589. The results here prove that LBH589 SU5416 induces ERK activation by inhibiting HDAC6 activity in certain cells. ERK activation is controlled by the upstream Ras/Raf/MEK pathway. Dephosphorylation of S259 of c-Raf by two phosphatases, PP1 or PP2A, results in c-Raf release from 14-3-3 and allows for the reactivation of c-Raf, which in turn triggers ERK activity. HDAC1, 6, and 10 have been reported to form a complex with PP1, respectively. HDACIs selectively disrupt the HDAC-PP1 complex and increase the association of PP1 and Akt, which contributes to the anti-neoplastic activities of HDACI. The present study shows that LBH589 disrupts the HDAC6/PP1�� complex and promotes the interaction between PP1�� and acetylated 14-3-3��. When PP1�� is associated with 14-3-3��, PP1�� still maintains its phosphatase activity. With LBH589 switching its interacting partner, PP1�� may alter its affinity or specificity to substrates. Again, an important question is raised as to whether HDACs are involved in cell cycle regulation by altering the substrates�� affinity or specificity of PP1��. In addition to ERK activation, inhibition of HDAC6 by LBH589 also induces Cdc25C hyper-phosphorylation by removal of inhibitory phosphorylation of serine 216 of Cdc25C. LBH589- induced dephosphorylation of S216 of Cdc25C is also regulated by PP1�� and 14-3-3�� with the same mechanisms responsible for S259 dephosphorylation of c-Raf. Thus, HDAC6 not only participates in the regulation of c- Raf/PP1/ERK signaling pathway but also coordinates the ERK signaling cascade to M phase cell cycle transition. This study proposes a model to explain how LBH589 induces prometaphase arrest. When HDAC6 binds with an HDACI, such as LBH589 in this study, it may cause a conformational change in HDAC6, Reversine leading to the dissociation of PP1�� and the enhancement of 14-3-3�� acetylation. Acetylated 14-3-3�� has high affinity for binding with PP1�� and modulating the affinity of PP1�� binding to its substrates.