Skp2-mediated degradation of p27 regulates progression into mitosis.

Although Skp2 has been thought to mediate the degradation of p27 at the G(1)-S transition, Skp2(-/-) cells exhibit accumulation of p27 in S-G(2) phase with overreplication. We demonstrate that Skp2(-/-)p27(-/-) mice do not exhibit the overreplication phenotype, suggesting that p27 accumulation is required for its development. Hepatocytes of Skp2(-/-) mice entered the endoduplication cycle after mitogenic stimulation, whereas this phenotype was not apparent in Skp2(-/-)p27(-/-) mice. Cdc2-associated kinase activity was lower in Skp2(-/-) cells than in wild-type cells, and a reduction in Cdc2 activity was sufficient to induce overreplication. The lack of p27 degradation in G(2) phase in Skp2(-/-) cells may thus result in suppression of Cdc2 activity and consequent inhibition of entry into M phase. These data suggest that p27 proteolysis is necessary for the activation of not only Cdk2 but also Cdc2, and that Skp2 contributes to regulation of G(2)-M progression by mediating the degradation of p27.

BRCA1 phosphorylation by Aurora-A in the regulation of G2 to M transition.

Aurora-A/BTAK/STK15 localizes to the centrosome in the G(2)-M phase, and its kinase activity regulates the G(2) to M transition of the cell cycle. Previous studies have shown that the BRCA1 breast cancer tumor suppressor also localizes to the centrosome and that BRCA1 inactivation results in loss of the G(2)-M checkpoint. We demonstrate here that Aurora-A physically binds to and phosphorylates BRCA1. Biochemical analysis showed that BRCA1 amino acids 1314-1863 binds to Aurora-A. Site-directed mutagenesis indicated that Ser(308) of BRCA1 is phosphorylated by Aurora-A in vitro. Anti-phospho-specific antibodies against Ser(308) of BRCA1 demonstrated that Ser(308) is phosphorylated in vivo. Phosphorylation of Ser(308) increased in the early M phase when Aurora-A activity also increases; these effects could be abolished by ionizing radiation. Consistent with these observations, acute loss of Aurora-A by small interfering RNA resulted in reduced phosphorylation of BRCA1 Ser(308), and transient infection of adenovirus Aurora-A increased Ser(308) phosphorylation. Mutation of a single phosphorylation site of BRCA1 (S308N), when expressed in BRCA1-deficient mouse embryo fibroblasts, decreased the number of cells in the M phase to a degree similar to that with wild type BRCA1-mediated G(2) arrest induced by DNA damage. We propose that BRCA1 phosphorylation by Aurora-A plays a role in G(2) to M transition of cell cycle.

ASC is a Bax adaptor and regulates the p53-Bax mitochondrial apoptosis pathway.

The apoptosis-associated speck-like protein (ASC) is an unusual adaptor protein that contains the Pyrin/PAAD death domain in addition to the CARD protein-protein interaction domain. Here, we present evidence that ASC can function as an adaptor molecule for Bax and regulate a p53-Bax mitochondrial pathway of apoptosis. When ectopically expressed, ASC interacted directly with Bax, colocalized with Bax to the mitochondria, induced cytochrome c release with a significant reduction of mitochondrial membrane potential and resulted in the activation of caspase-9, -2 and -3. The rapid induction of apoptosis by ASC was not observed in Bax-deficient cells. We also show that induction of ASC after exposure to genotoxic stress is dependent on p53. Blocking of endogenous ASC expression by small-interfering RNA (siRNA) reduced the apoptotic response and inhibited translocation of Bax to mitochondria in response to p53 or genotoxic insult, suggesting that ASC is required to translocate Bax to the mitochondria. Our findings demonstrate that ASC has an essential role in the intrinsic mitochondrial pathway of apoptosis through a p53-Bax network.

Modulation of p53 and p73 levels by cyclin G: implication of a negative feedback regulation.

Cyclin G is a transcriptional target gene of tumor suppressor p53. Recent studies present evidence that cyclin G may play a central role in the p53-Mdm2 autoregulated module, but the precise function of cyclin G remains elusive. Here, we show a negative effect of cyclin G on the stability of p53 and p73. Cyclin G expression resulted in a dramatic decrease of p53 protein levels in response to DNA damage and abrogated irradiation-mediated G1 arrest along with an increase of S phase in MCF7 cells containing wild-type p53. In p53-null Saos2 cells, cyclin G inhibited p73 induction in response to genotoxic stress and delayed the camptothecin-mediated cell cycle arrest. Cyclin G interacts with p53 as well as p73, and its binding to p53 or p73 presumably mediates downregulation of p53 and p73. We also found that cyclin G-mediated reduction of p53 but not of p73 is Mdm2-dependent. Moreover, inhibition of cyclin G by small interfering RNA (siRNA) caused the accumulation of p53 and p73 protein levels in response to DNA damage. Therefore, our results imply that cyclin G is transcriptionally activated by p53 or p73, and, in turn, cyclin G negatively regulates the stabilization of p53 family proteins through an unknown mechanism different from ubiquitination or transcriptional control.

Tumor suppression through angiogenesis inhibition by SUIT-2 pancreatic cancer cells genetically engineered to secrete NK4.

NK4, composed of the N-terminal hairpin and subsequent four-kringle domains of hepatocyte growth factor (HGF), acts not only as a competitive antagonist of HGF but also as an inhibitor of angiogenesis. By studying the antitumor effect of NK4, we evaluated the potential of gene therapy with NK4 as a treatment for pancreatic cancer. Expression vector pcDNA3-NK4 containing NK4 cDNA was used to transfect human pancreatic cancer cell line SUIT-2. Although the established NK4 transfectant continuously expressed NK4 protein, the expression was shown by migration assay to be insufficient to antagonize HGF in vitro. Proliferation of the NK4 transfectant did not differ significantly from that of a mock transfectant. In vivo, we used models of orthotopic implantation and liver metastasis to transplant NK4-transfected clone or mock-transfected clone into nude mice. Cell proliferation in vivo, evaluated by immunohistochemical staining of proliferating cell nuclear antigen, did not differ between NK4 and mock transfectants, and this was also the finding in the in vitro assay. However, the NK4-transfected clone showed significant inhibition of tumor progression in both the orthotopic implantation and liver metastasis models. The number of vessels within tumors was significantly decreased, and the apoptotic tumor cells were increased in number. The results of these experiments show that genetic modification of tumor cells with NK4 cDNA yields a significant antitumor effect and that this effect is mainly obtained by NK4's function as an angiogenesis inhibitor rather than as an HGF antagonist. We conclude that the potent angiogenesis inhibitor NK4 may be a promising molecule for gene therapy of pancreatic cancer.

Recovery of liver mass without proliferation of hepatocytes after partial hepatectomy in Skp2-deficient mice.

The abundance of p27(Kip1), an inhibitor of cell proliferation, is determined by Skp2-dependent proteolysis, the deregulation of which is associated with cancer progression. Lack of Skp2 results in p27(Kip1) accumulation as well as enlargement and polyploidy of hepatocytes. The role of Skp2 in cell growth and proliferation was investigated in Skp2-deficient mice subjected to partial hepatectomy. Skp2(-/-) mice exhibited restoration of liver mass without cell proliferation; rather, hepatocytes increased in size, an effect that was accompanied by increased polyploidy and p27(Kip1) accumulation. Lack of Skp2 thus impairs hepatocyte proliferation, which is compensated for by cellular enlargement, during liver regeneration.