When starved for amino acids, Saccharomyces cerevisiae accumulates uncharged tRNAs to activate its sole eukaryotic initiation factor (eIF) 2␣ kinase GCN2. Subsequent phosphorylation of eIF2␣ impedes general translation, but translationally derepresses the transcription factor GCN4, which induces expression of various biosynthetic genes to elicit general amino acid control response. By contrast, when supplied with enough nutrients, the yeast activates the target of rapamycin signaling pathway to stimulate translation initiation by facilitating the assembly of eIF4F. A cross-talk was suggested between the two pathways by rapamycin-induced translation of GCN4 mRNA. Here we show that rapamycin causes an increase in phosphorylated eIF2␣ to translationally derepress GCN4. This increment is not observed in the cells expressing mammalian non-GCN2 eIF2␣ kinases in place of GCN2. It is thus suggested that rapamycin does not inhibit dephosphorylation of eIF2␣ but rather activates the kinase GCN2. This activation seems to require an interaction between the kinase and uncharged tRNAs, because rapamycin, similar to amino acid starvation, fails to induce eIF2␣ phosphorylation in the cells with GCN2 defective in tRNA binding. However, in contrast with amino acid starvation, rapamycin activates GCN2 without increasing the amount of uncharged tRNAs, but presumably by modifying the tRNA binding affinity of GCN2.

Dysfunction of p53 is observed in the many malignant tumors. In cervical cancer, p53 is inactivated by degradation through the complex with human papilloma virus (HPV) oncoprotein E6 and E6-associated protein (E6AP), an E3 ubiquitin protein ligase. In endometrial cancer, overexpression of p53 in immunohistochemistry is a significant prognostic factor. A discrepancy between p53 overexpression and TP53 mutations is observed in endometrioid endometrial cancer, indicating that the accumulation of p53 protein can be explained by not only gene mutations but also dysregulation of the factors such as ERβ and MDM2. Furthermore, the double-positive expression of immunoreactive estrogen receptor (ER) β and p53 proteins is closely associated with the incidence of metastasis and/or recurrence. High-grade serous ovarian carcinoma (HGSC) arises from secretary cells in the fallopian tube. The secretary cell outgrowth (SCOUT) with TP53 mutations progresses to HGSC via the p53 signature, serous intraepithelial lesion (STIL), and serous intraepithelial carcinoma (STIC), indicating that TP53 mutation is associated with carcinogenesis of HGSC. Clinical application targeting p53 has been approved for some malignant tumors. Gene therapy by the adenovirus-mediated p53 gene transfer system is performed for head and neck cancer. A clinical phase III trial using MDM2/X inhibitors, idasanutlin (RG7388) combined with cytarabine, is being performed involving relapse/refractory acute myeloid leukemia patients. The use of adenoviruses as live vectors which encode wild-type p53 has given promising results in cervical cancer patients.

ABSTRACT:The antitumor 3-ethynyl nucleosides, 1-(3-C-ethynyl-␤-D-ribopentofuranosyl)cytosine (ECyd) and 1-(3-C-ethynyl-␤-D-ribopentofuranosyl)uridine (EUrd), are potent inhibitors of RNA polymerases and show excellent antitumor activity against various human solid tumors in xenograft models. ECyd is being investigated in phase I clinical trials as a novel anticancer drug possessing a unique antitumor action. ECyd and EUrd require the activity of uridine/cytidine kinase (UCK) to produce the corresponding active metabolite. The UCK family consists of two members, UCK1 and UCK2, and both UCKs are expressed in many tumor cells. It was unclear, however, whether UCK1 or UCK2 is responsible for the phosphorylation of the 3-ethynyl nucleosides. We therefore established cell lines that are highly resistant to the 3-ethynyl nucleosides from human fibrosarcoma HT-1080 and gastric carcinoma NUGC-3. All the resistant cell lines showed a high cross-resistance to ECyd and EUrd. As a result of cDNA sequence analysis, we found that UCK2 mRNA expressed in EUrd-resistant HT-1080 cells has a 98-base pair deletion of exon 5, whereas EUrd-resistant NUGC-3 cells were harboring the point mutation at nucleotide position 484 (C to T) within exon 4 of UCK2 mRNA. This mutation was confirmed by genome sequence analysis of the UCK2 gene. Moreover, the expression of UCK2 protein was decreased in these resistant cells. In contrast, no mutation in the mRNA or differences in protein expression levels of UCK1 were shown in the EUrdresistant HT-1080 and NUGC-3 cells. These results suggest that UCK2 is responsible for the phosphorylation and activation of the antitumor 3-ethynyl nucleosides.The 3Ј-ethynyl nucleosides, 1-(3-C-ethynyl-␤-D-ribo-pentofuranosyl)cytosine (ECyd) and 1-(3-C-ethynyl-␤-D-ribo-pentofuranosyl)uracil (EUrd), are antitumor ribo-nucleoside analogs of cytidine and uridine, respectively. We previously reported that both ECyd and EUrd exhibit significant cytotoxicity and antitumor activity on various types of animal and human solid tumors in in vivo models (Hattori et al., 1996;Tanaka et al., 1997;Takatori et al., 1999). ECyd, which is currently undergoing a phase I study, is a unique anticancer drug possessing a potent inhibitory effect on RNA polymerases. Both ECyd and EUrd incorporated into cells are rapidly phosphorylated to a monophosphate form by uridine/cytidine kinase (UCK; EC 2.7.1.48), and these monophosphates are subsequently phosphorylated to the corresponding di-(ethynyl cytidine diphosphate and ethynyl uridine diphosphate) and triphosphates (ethynyl cytidine triphosphate and ethynyl uridine triphosphate) (Shimamoto et al., 2002a). The triphosphates are active metabolites and inhibit RNA synthesis by blocking RNA polymerases I, II, and III Matsuda et al., 1999;Takatori et al., 1999). Intracellular accumulation of the triphosphates is therefore considered to be critical for the 3Ј-ethynyl nucleosides to exert a cytotoxic activity. We have reported that the sensitivity of tumor cells to EUrd is well correlated with the in...

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