4A). An opposite effect was obtained when silencing the PLK2, PLK3, or PLK4 gene by siRNA in SNU-423 and HLE cell lines (expressing high levels of the PLK2, PLK3, and PLK4 genes). Indeed, suppression of PLK2, PLK3, or PLK4 was accompanied by a significant growth acceleration in the two cell lines (Fig. 3B-D) and resistance to apoptosis (Fig. 4B-D), suggesting that down-regulation of PLK2, PLK3, and PLK4 play

a protumorigenic role in human hepatocarcinogenesis. Next, we assessed the possible interplay between PLKs by determining the levels of PLK1-4 genes following siRNA-mediated silencing of the other members of the PLK family. Interestingly, suppression of both PLK2 and PLK3 led to up-regulation of PLK1 (Supporting Figs. 2 and 3), implying a modulatory role of PLK2 and PLK3 over PLK1 expression. No additional modifications in gene expression were detected following silencing of MAPK inhibitor PLK1 and PLK4 by siRNA (Supporting Figs. 2 and Epigenetics inhibitor 3). Thus, the present data suggest that PLK1 promotes the growth of human HCC cells, whereas the down-regulation of PLK2, PLK3, and PLK4 antagonizes the antiproliferative and

proapoptotic functions exerted by these proteins in nontumor cells. Because the most pronounced antitumorigenic effects on HCC cell growth were obtained by targeting PLK1, our following studies focused on the role of PLK1 in the regulation of cell cycle and apoptosis in HCC cells. Silencing of PLK1 expression by siRNA in Hep3B and HepG2 cells resulted in a block in G2/M phase (Fig. 5A) as well as in a strong increase of the sub-G1 fraction indicating apoptosis (data not shown), as confirmed by the detection of cleaved PARP protein

these (Fig. 5B). In addition, inhibition of PLK1 expression was followed by down-regulation of the antiapoptotic protein survivin (Fig. 5B), supporting the recent finding that PLK1 promotes cell survival through inhibition of survivin degradation in esophageal cancer cells.25 Previous evidence indicated that PLK1 can bind to p53 and abrogate its tumor suppressor functions,26 and recent reports have demonstrated that PLK1 is able to phosphorylate the tumor suppressor p73, with consequent inhibition of its transcriptional activity, thereby suppressing apoptosis.27, 28 Thus, we determined whether the activation of p53 and p73 proteins could be involved in the apoptotic response following PLK1 inhibition. In accordance with our hypothesis, up-regulation of p53 and p73 protein levels as well as activation of their target genes p21CIP1 and BAX was detected in HepG2 cells (p53 wild-type) following PLK1 inhibition (Fig. 6A). In Hep3B cells (p53 deletion), apoptosis induction was paralleled by a rise in p73 expression and the induction of p21CIP1 and BAX (Fig. 6A). Furthermore, siRNA-mediated silencing resulted in BAX activation in HepG2 and Hep3B cells, as demonstrated by its translocation to the mitochondria and subsequent release of cytochrome C into the cytoplasm (Fig. 6B).