Enhanced antitumor activity of vitamin C via p53 in cancer cells.

Ascorbate is an important natural antioxidant that can selectively kill cancer cells at pharmacological concentrations. Despite its benefit, it is quite difficult to predict the antitumor effects of ascorbate, because the relative cytotoxicity of ascorbate differs between cancer cell lines. Therefore, it is essential to examine the basis for this fundamental disagreement. Because p53 is activated by DNA-damaging stress and then regulates various cellular conditions, we hypothesized that p53 can sensitize cancer cells to ascorbate. Using isogenic cancer cells, we observed that the presence of p53 can affect ascorbate cytotoxicity, and also reactivation of p53 can make cancer cells sensitive to ascorbate. p53-dependent enhancement of ascorbate cytotoxicity is caused by increased reactive oxygen species generation via a differentially regulated p53 transcriptional network. We also found that transcriptionally activated p53 was derived from MDM2 ubiquitination by ascorbate and subsequently its signaling network renders cancer cells more susceptible to oxidative stress. Similar to the p53 effect on in vitro ascorbate cytotoxicity, inhibition of tumor growth is also stronger in p53-expressing tumors than in p53-deficient ones in vivo. This is the first observation that ascorbate cytotoxicity is positively related to p53 expression, activating its transcriptional network to worsen intracellular oxidative stress and consequently enhancing its cytotoxicity. Based on our study, reactivation of p53 may help to achieve more consistent cytotoxic effects of ascorbate in cancer therapies.

The role of hLHX6-HMR as a methylation biomarker for early diagnosis of cervical cancer.

The homo sapiens LIM homeobox domain LHX6 gene, hLHX6, is a putative transcription regulator with homeo-domain. Multiple cytosine guanine dinucleotides (CpG island) are found in the genomic sequences between exon 4a and exon 5 of the gene encoding hLHX6s (alternative short iso-form of hLHX6 gene). This specific CpG island, hLHX6-HMR, is found frequently hypermethylated in 7 cervical cancer cell lines as shown in MSP, BSP, and COBRA assays. Methylation densities were also investigated with human tissue samples with a distinctive degree of malignant transformation. Our data showed that the hLHX6-HMR was rarely or partly methylated in the normal and CIN I cells, respectively. In contrast, it was frequently hypermethylated in CIN II, CIN III, and invasive carcinoma cells. In summary, this methylation study led to two conclusions. First, hLHX6-HMR hypermethylation is exclusively associated with cervical carcinogenesis. Second, the epigenetic change in hLHX6-HMR seems to start at CIN I, relatively early stage of cervical cancer development. Therefore, hLHX6-HMR can be used as an effective and sensitive methylation biomarker for early diagnosis of cervical cancer.

p34SEI-1 inhibits apoptosis through the stabilization of the X-linked inhibitor of apoptosis protein: p34SEI-1 as a novel target for anti-breast cancer strategies.

The p34(SEI-1) protein exerts oncogenic effects via regulation of the cell cycle, which occurs through a direct interaction with cyclin-dependent kinase 4. Such regulation can increase the survival of various types of tumor cells. Here, we show that the antiapoptotic function of p34(SEI-1) increases tumor cell survival by protecting the X-linked inhibitor of apoptosis protein (XIAP) from degradation. Our findings show that p34(SEI-1) inhibits apoptosis. This antiapoptotic effect was eliminated by the suppression of p34(SEI-1) expression. We also determined that direct binding of p34(SEI-1) to the BIR2 domain prevents ubiquitination of XIAP. Interestingly, p34(SEI-1) expression is absent or weak in normal tissues but is strongly expressed in tissues obtained from patients with breast cancer. Furthermore, the expression levels of p34(SEI-1) and XIAP seem to be coordinated in human breast cancer cell lines and tumor tissues. Thus, our findings reveal that p34(SEI-1) uses a novel apoptosis-inhibiting mechanism to stabilize XIAP.

Vitamin C inhibits p53-induced replicative senescence through suppression of ROS production and p38 MAPK activity.

We previously reported that tumor cells expressing p53 increase intracellular levels of reactive oxygen species (ROS). In this study, we described an inhibitory effect of vitamin C on replicative senescence. Vitamin C was found to inhibit p53-induced senescence in human bladder cancer EJ cells. The senescence-like phenotype (SLP) induced by p53, which showed a morphological change and an irreversible cell cycle arrest, was not observed in vitamin C-treated EJ cells. In addition, vitamin C did not significantly affect normal cell proliferation. We investigated the molecular mechanisms of the inhibitory effect of vitamin C on the development of replicative senescence in EJ cells. We found that vitamin C inhibited this p53-induced ROS generation. Moreover, p38 kinase which was activated during p53-induced senescence was not observed in vitamin C-treated EJ cells. SB203580, a chemical inhibitor of p38 kinase, was found to consistently inhibit p53-induced senescence. Therefore, it is suggested that vitamin C inhibits p53-induced senescence by preventing ROS generation, which in turn leads to the activation of p38 MAPKinase. These results reveal the inhibitory mechanism of vitamin C on cellular senescence.