Takayuki Torigoe

Cellular pH regulators: potentially promising molecular targets for cancer chemotherapy

One of the major obstacles to the successful treatment of cancer is the complex biology of solid tumour development. Although regulation of intracellular pH has been shown to be critically important for many cellular functions, pH regulation has not been fully investigated in the field of cancer. It has, however, been shown that cellular pH is crucial for biological functions such as cell proliferation, invasion and metastasis, drug resistance and apoptosis. Hypoxic conditions are often observed during the development of solid tumours and lead to intracellular and extracellular acidosis. Cellular acidosis has been shown to be a trigger in the early phase of apoptosis and leads to activation of endonucleases inducing DNA fragmentation. To avoid intracellular acidification under such conditions, pH regulators are thought to be up-regulated in tumour cells. Four major types of pH regulator have been identified: the proton pump, the sodium-proton exchanger family (NHE), the bicarbonate transporter family (BCT) and the monocarboxylate transporter family (MCT). Here, we describe the structure and function of pH regulators expressed in tumour tissue. Understanding pH regulation in tumour cells may provide new ways of inducing tumour-specific apoptosis, thus aiding cancer chemotherapy.

Cisplatin Resistance and Transcription Factors

Cisplatin is one of the most potent and widely used anti-cancer agents in the treatment of various solid tumors. However, the development of resistance to cisplatin is a major obstacle in clinical treatment. Several mechanisms are thought to be involved in cisplatin resistance, including decreased intracellular drug accumulation, increased levels of cellular thiols, increased nucleotide excision-repair activity and decreased mismatch-repair activity. In general, the molecules responsible for each mechanism are upregulated in cisplatin-resistant cells; this indicates that the transcription factors activated in response to cisplatin might play crucial roles in drug resistance. It is known that the tumor-suppressor proteins p53 and p73, and the oncoprotein c-Myc, which function as transcription factors, influence cellular sensitivity to cisplatin. So far, we have identified several transcription factors involved in cisplatin resistance, including Y-box binding protein-1 (YB-1), CCAAT-binding transcription factor 2 (CTF2), activating transcription factor 4 (ATF4), zinc-finger factor 143 (ZNF143) and mitochondrial transcription factor A (mtTFA). Two of these-YB-1 and ZNF143-lack the high-mobility group (HMG) domain and can bind preferentially to cisplatin-modified DNA in addition to HMG domain proteins or DNA repair proteins, indicating that these transcription factors may also participate in DNA repair. In this review, we summarize the mechanisms of cisplatin resistance and focus on transcription factors involved in the genomic response to cisplatin.

Human mitochondrial transcription factor A binds preferentially to oxidatively damaged DNA.

Mitochondrial transcription factor A (mtTFA) is necessary for both transcription and maintenance of mtDNA, and is also one of the high mobility group (HMG) proteins that preferentially binds to cisplatin-damaged DNA. In this study we confirmed the preferential binding of mtTFA to cisplatin-damaged DNA, and also discovered that mtTFA binds to oxidatively damaged DNA. The affinity for oxidatively damaged DNA of mtTFA is higher for A/8-oxo-dG and C/8-oxo-dG than for G/8-oxo-dG and T/8-oxo-dG. Our findings suggest that mtTFA plays an important role in the recognition of oxidative DNA damage.

Vacuolar H(+)-ATPase: functional mechanisms and potential as a target for cancer chemotherapy.

Tumor cells in vivo often exist in a hypoxic microenvironment with a lower extracellular pH than that surrounding normal cells. Ability to upregulate proton extrusion may be important for tumor cell survival. Such microenvironmental factors may be involved in the development of resistant subpopulations of tumor cells. In solid tumors, both intracellular and extracellular pH differ between drug-sensitive and -resistant cells, and pH appears critical to the therapeutic effectiveness of anticancer agents. Four major types of pH regulators have been identified in tumor cells: the sodium–proton antiporter, the bicarbonate transporter, the proton–lactate symporter and proton pumps. Understanding mechanisms regulating tumor acidity opens up novel opportunities for cancer chemotherapy. In this minireview, we describe the structure and function of certain proton pumps overexpressed in many tumors—vacuolar H+-ATPases—and consider their potential as targets for cancer chemotherapy.

ZNF143 activates gene expression in response to DNA damage and binds to cisplatin-modified DNA

We have identified a cisplatin‐inducible gene, the mitochondrial ribosomal protein S11 (MRP S11) gene, by means of mRNA differential display. Functional analysis of the MRP S11 promoter showed that a Staf binding site in the promoter is required for both basal promoter activity and cisplatin‐inducible activity. We also found that Staf binding activity is significantly increased in nuclear extracts from cells treated with cisplatin. ZNF 143 and ZNF 76 are human homologues of the Xenopus transcriptional activator, Staf. ZNF 143 expression is induced by cisplatin but ZNF 76 expression is not. However, ZNF 143 expression is not induced by transplatin, which is clinically ineffective. ZNF143 is an inducible gene by other DNA damaging agents such as γ‐irradiation, etoposide and adriamycin. ZNF 143 also binds preferentially to cisplatin‐modified DNA. These results suggest that ZNF 143 participates in cellular responses to DNA damage.

DNA Topoisomerase II Poison TAS-103 Transactivates GC-Box-dependent Transcription via Acetylation of Sp1

Drug-induced modifications of transcription factors play important roles in both apoptosis and survival signaling. The data presented here show that the DNA topoisomerase II poison TAS-103 transactivated the SV40 promoter in a GC-box-dependent manner and induced Sp1 acetylation in cells expressing p300. This activity was not observed in cells lacking p300. TAS-103 treatment also enhanced the p300 content of the nucleus and the interaction of p300 with Sp1. Cellular susceptibility to TAS-103 was correlated with p300 expression but not with topoisomerase II expression. Furthermore, the presence of p300 significantly sensitized cancer cells to TAS-103 but not to cisplatin. Taken together, these findings demonstrate novel genomic responses to anticancer agents that modulate Sp1 acetylation and Sp1-dependent transcription in an apoptotic pathway.

Binding of RNA to p53 regulates its oligomerization and DNA-binding activity

The C-terminus of p53 is responsible for maintaining the latent, non-DNA-binding form of p53. However, the mechanism by which the C-terminus regulates DNA binding is not yet fully understood. We show here that p53 interacts with RNA via its C-terminal domain and that disruption of this interaction, by RNase A treatment, truncation or phosphorylation of the C-terminus, restores DNA-binding activity. Furthermore, the oligomerization of p53 is significantly enhanced by disrupting the interaction between p53 and RNA. These findings suggest that binding of RNA to p53 is involved in the mechanism of p53 latency.

DNA topoisomerase inhibitor, etoposide, enhances GC-box-dependent promoter activity via Sp1 phosphorylation.

Modification of transcription factors by anticancer agents plays an important role in both apoptotic and survival signaling. Here we report that both DNA topoisomerase I and II inhibitors such as SN‐38 and etoposide, but not cisplatin, 5‐fluorouracil or actinomycin D, can induce phosphorylation of the transcription factor Sp1. Furthermore, DNA topoisomerase inhibitors were shown to transactivate GC‐box‐dependent promoters such as the SV40 and vascular endothelial growth factor promoters. The phosphorylated form of Sp1 was detectable within 30 min of etoposide treatment and was greatly diminished by the presence of the PI3K inhibitor wortmannin and by DNA‐dependent protein kinase (DNA‐PK) knockdown. We also confirmed that the phosphorylated form of DNA‐PK was increased by treatment with both etoposide and SN‐38. Taken together, these findings demonstrate a novel genomic response to anticancer agents that induce Sp1 phosphorylation, and might contribute to tumor progression and drug resistance. (Cancer Sci 2007; 98: 858–863)

Malignant hemangiopericytoma in the pelvic cavity successfully treated by combined-modality therapy: report of a case.

A 55-year-old Japanese woman underwent extirpation of a malignant hemangiopericytoma in the pelvic cavity, followed by postoperative irradiation. An abdominal computed tomography scan 3 years later revealed a local recurrent tumor, 12 cm in diameter, in the pelvic cavity, for which transarterial embolization was done, followed by excision of the tumor employing Hartmanns procedure. Although an unresectable part of the recurrent tumor remained, postoperative irradiation reduced its size remarkably. The patient is still alive 7 years 2 months after her first operation, but with more recurrent tumors in the abdominal wall and around the bilateral iliac arteries. Because hemangiopericytoma often recurs or metastasizes after a prolonged disease-free interval, close long-term follow-up is necessary after the operation. Combined-modality therapy against the recurrent or unresectable disease may result in a good prognosis.

Intestinal bleeding from arteriovenous malformations of the small bowel in a patient with Cowden syndrome: report of a case.

This report presents the case of a patient with Cowden syndrome who had arteriovenous malformations (AVMs) at the jejunum and the ileum and experienced intestinal bleeding. A 54-year-old Japanese male presented with general fatigue and melena. Endoscopic examinations showed gastrointestinal polyposis from the esophagus to the rectum. However, the site of bleeding was not identified. There were some papules on his face and neck. He also had macrocephaly and had multiple papillomas along the gum-line. These findings indicated a clinical diagnosis of Cowden syndrome. Enhanced computed tomography (CT) and angiography analyses indicated the presence of AVMs at the jejunum and the ileum. He was treated with partial resection of the jejunum and ileum including these two AVMs. This was a rare case of two AVMs involving the small bowel in a patient with Cowden syndrome. Enhanced CT was very useful and convenient for the detection of gastrointestinal AVMs in this case.