Prolongation of the lag period preceding the enhancement of thymidine and thymidylate kinase activity in regenerating rat liver by 5-azacytidine

Active RNA interference in mitochondria

RNA interference (RNAi) has been thought to be a gene-silencing pathway present in most eukaryotic cells to safeguard the genome against retrotransposition. Small interfering RNAs (siRNAs) have also become a powerful tool for studying gene functions. Given the endosymbiotic hypothesis that mitochondria originated from prokaryotes, mitochondria have been generally assumed to lack active RNAi; however, certain bacteria have Argonaute homologs and various reports suggest the presence of specific microRNAs and nuclear genome (nDNA)-encoded Ago2 in the mitochondria. Here we report that transfected siRNAs are not only able to enter the matrix of mitochondria, but also function there to specifically silence targeted mitochondrial transcripts. The mitoRNAi effect is readily detectable at the mRNA level, but only recordable on relatively unstable proteins, such as the mtDNA-encoded complex IV subunits. We also apply mitoRNAi to directly determine the postulated crosstalk between individual respiratory chain complexes, and our result suggests that the controversial observations previously made in patient-derived cells might result from differential adaptation in different cell lines. Our findings bring a new tool to study mitochondrial biology.

Identification of hypermethylation in hepatocyte cell adhesion molecule gene promoter region in bladder carcinoma

BACKGROUND

Epigenetic regulation such as aberrant hypermethylation of CpG islands in promoter plays a key role in tumorigenesis. 5-Aza-2'-deoxycytidine (5-aza-CdR) which is a potent inhibitor of DNA methylation can reverse the abnormal hypermethylation of the silenced tumor suppressor genes (TSGs). It has been reported that hepatocyte cell adhesion molecule (hepaCAM) acts as a tumor suppressor gene and expression of its mRNA and protein were down-regulated in bladder cancer. Over-expression of hepaCAM can inhibit cancer growth and arrest renal cancer cells at G0/G1 phase. In this study, we investigated the methylation status of hepaCAM gene, as well as the influence of 5-aza-CdR on expression of hepaCAM gene in bladder cancer cells.

METHODS

CpG islands in hepaCAM promoter and methprimers were predicted and designed using bioinformatics program. Methylation status of hepaCAM promoter was evaluated in bladder cancer tissues and two cell lines (T24 and BIU-87) by Methylation-specific PCR; Western blot and Immunofluorescence were used to detect expression of hepaCAM protein after 5-aza-CdR treatment; Flow cytometry assay was performed to determine effectiveness of 5-aza-CdR on cell cycle profile.

RESULTS

CpG island in promoter of hepaCAM gene was hyper-methylated both in bladder carcinoma tissues and cell lines (T24 and BIU-87). Otherwise, aberrant methylation of its promoter was associated with its decreased expression. Hypermethylation of hepaCAM gene was reversed and expression of its mRNA and protein were re-activated in two cell lines by DNA methyltransferases inhibitor 5-aza-CdR. Flow cytometry assay demonstrated that 5-aza-CdR can inhibit growth of cancer cells by arresting cancer cells at G0/G1 phase.

CONCLUSION

Abnormal hypermethylation in CpG island of hepaCAM promoter is involved in absence of hepaCAM gene expression when bladder cancer occurs. Re-activation of hepaCAM gene by 5-aza-CdR can inhibit growth of cancer cells and arrest cells at G0/G1 phase.

Epigenetic mechanisms in AML - a target for therapy

Epigenetics refers to a stable, mitotically perpetuated regulatory mechanism of gene expression without an alteration of the coding sequence. Epigenetic mechanism include DNA methylation and histone tail modifications. Epigenetic regulation is part of physiologic development and becomes abnormal in neoplasia, where silencing of critical genes by DNA methylation or histone deacetylation can contribute to leukemogenesis as an alternative to deletion or loss-of-function mutation. In acute myelogenous leukemia (AML), aberrant DNA methylation can be observed in multiple functionally relevant genes such as p15, p 73, E-cadherin, ID 4, RARbeta2. Abnormal activities of histone tail-modifying enzymes have also been seen in AML, frequently as a direct result of chromosomal translocations. It is now clear that these epigenetic changes play a significant role in development and progression of AML, and thus constitute important targets of therapy. The aim of targeting epigenetic effector protein or "epigenetic therapy" is to reverse epigenetic silencing and reactive various genes to induce a therapeutic effect such as differentiation, growth arrest, or apoptosis. Recent clinical studies have shown the relative safety and efficacy of such epigenetic therapies.

Decitabine--bedside to bench

PURPOSE OF THE REVIEW

Epigenetic changes marked by DNA methylation are known to contribute to the malignant transformation of cells by silencing critical genes. Decitabine inhibits DNA methyltransferase and has shown therapeutic effects in patients with hematologic malignancies. However, the connection between the clinical activity of decitabine and its demethylating activity is not clear. Herein, we summarize the results of recent clinical trials of decitabine in hematologic malignancies, and review the translational research into decitabine's mechanism of clinical activity.

RECENT FINDINGS

Low-dose decitabine has been studied recently in multiple clinical trials and has been shown to be effective for treatment of myelodysplastic syndromes. Correlative laboratory studies of clinical trials have shown that decitabine induces global hypomethylation as well as hypomethylation of gene-specific promoters and activation of gene expression. Past a given threshold, induction of higher degrees of hypomethylation is not directly associated with a better clinical outcome. Moreover, studies have suggested that patients with promoter hypermethylation of p15(INK4B) at baseline have paradoxically a lower chance of achieving response than those without hypermethylation. Furthermore, several other genes activated by decitabine were independent of hypomethylation in the promoter regions.

CONCLUSION

While at least part of decitabine's activity is through induction of hypomethylation and reactivation of critical genes, mechanisms independent from hypomethylation are also important for decitabine's antitumor activity.

Targeting the Epigenome for the Treatment and Prevention of Lung Cancer

Alterations in chromatin structure resulting from aberrant DNA methylation and perturbations of the histone code profoundly influence gene expression during pulmonary carcinogenesis. Recent studies indicate that DNA demethylating agents and histone deacetylase (HDAC) inhibitors synergistically induce gene expression and apoptosis in cultured lung cancer cells, and prevent lung cancer development in animals following exposure to tobacco carcinogens. Preliminary clinical trials have established proof of principle regarding the use of DNA demethylating agents and HDAC inhibitors for enhancing immunogenicity and apoptosis of lung cancer cells, and have revealed the complexities concerning the mechanisms by which chromatin remodeling agents mediate antitumor effects in vivo. These data support additional investigations pertaining to the epigenetics of lung cancer, and the evaluation of chromatin remodeling agents for the treatment and prevention of this disease.

The effects of 5-azacytidine and 5-azadeoxycytidine on chromosome structure and function: implications for methylation-associated cellular processes

5-Azacytidine (5-aza-C) analogs demonstrate a remarkable ability to induce heritable changes in gene and phenotypic expression. These cellular processes are associated with the demethylation of specific DNA sequences. On the other hand, 5-aza-C analogs have dramatic effects on chromosomes, leading to decondensation of chromatin structure, chromosomal instability and an advance in replication timing. Condensation inhibition of genetically inactive chromatin occurs when the DNA is still hemimethylated or fully methylated. In cell cultures prolonged for several replication cycles, chromosomal rearrangements and instability affect the 5-aza-C-sensitive regions. Moreover, the normally late-replicating inactive chromatin undergoes a transient temporal shift to an earlier DNA replication, characteristic of activatable chromatin. zThe induced alterations of chromosome structure and behavior may trigger the 5-aza-C-dependent process of cellular reprogramming. Apart from their differentiating and gene-modifying effects, 5-aza-C analogs can tumorigenically transform cells and modulate their metastatic potential. High doses of 5-aza-C analogs have cytotoxic and antineoplastic activities.

Suppression of nuclear ADP-ribosyltransferase activity in regenerating rat liver by 5-azacytidine and its relevance to the nuclear methylating activities

A single administration of 5-azacytidine (5-ACR) to partially hepatectomized rats 24 h following operation resulted in a dose-dependent reduction of nuclear ADP-ribosyltransferase (ADPRT) activity in the liver, when assayed after the nuclei were isolated 22 h after injection. No such a suppression by 5-ACR was observed in the liver of intact rats. Cytidine, a known agent which prevents the incorporation of 5-ACR into DNA, abolished the suppression of ADPRT, when it was given in combination with 5-ACR. The 5-ACR suppressed nuclei from regenerating liver showed no decreased DNA methylating activity, as estimated from the rate of radiolabel transfer from [methyl-3H]SAM to the bulk DNA. The methylation of nuclear RNA and protein was markedly reduced. These results suggest that the incorporation of 5-ACR into nucleic acids inactivates chromatin-bound ADPRT without inhibition of DNA methylation.

3-Deazauridine (NSC 126849): an interesting modulator of biochemical response

3-Deazauridine (NSC 126849) is a structural analog of uridine that inhibits the biosynthesis of Cytidine-5'-Triphosphate by competitive inhibition of Cytidine Triphosphate synthetase which is considered to be the primary mode of action of this nucleoside analog. Despite a paucity of clinical attention given to this drug as a single agent, it has generated much enthusiasm as a biological response modulator because of its synergistic effect with a number of antitumor agents including Cytosine Arabinoside, 5-aza-2'-deoxycytidine, 5-azacytidine, thymidine and D-galactosamine, although only the cytosine arabinoside/3-Deazauridine combination has been explored clinically. In this paper, the current status of the drug and future perspectives will be discussed.

Differential sensitivity of RSVts (temperature-sensitive Rous-sarcoma virus)-infected rat kidney cells to nucleoside antibiotics at permissive and non-permissive temperatures

Among a variety of anti-tumour agents tested, oxanosine and 5-azacytidine were found to be significantly more effective in inhibiting growth of rat kidney cells infected with a temperature-sensitive mutant of Rous sarcoma virus at a permissive temperature (33 degrees C) than at a non-permissive temperature (39 degrees C). These two nucleoside antibiotics were antagonistic to each other in cytotoxicity. They seem to share the same carrier-mediated membrane-transport system, because dipyridamole, a potent inhibitor of nucleoside transport, protected cells from the cytotoxicity of both drugs. Thymidine transport, which is twice as fast in cells at 33 degrees C as at 39 degrees C, was competitively inhibited by both drugs. Thus the differential toxicity of oxanosine and 5-azacytidine at the two temperatures is thought to be due to their increased transport via the thymidine-transport system, which is somehow under the influence of the active src-gene product.

Azapyrimidine nucleosides: metabolism and inhibitory mechanisms

Triazine nucleosides represent highly active compounds affecting different cellular processes. While 6-azauridine displays a rather selective inhibitory effect, biological action of 5-azacytidine reflects the polyvalent inhibitory mechanism of the drug (interaction with pyrimidine synthesis de novo, incorporation into RNA and DNA, depressed maturation of ribosomal RNA, inhibition of RNA and DNA methylation, etc.) and the analog displays pronounced cytostatic and immunosuppressive activity. 5-Aza-2'-deoxycytidine action is directed against DNA synthesis similar to that of 5-azacytosine arabinoside. N4-Substituted derivatives of 5-azacytidine affect gastric secretion and together with 5-azacytosine and 5-azacytidine represent a new type of drugs with antiulcer activity. 6-Amino-5-azacytosine nucleosides interfere with the metabolism of purines rather than pyrimidines as evidenced by the character of their inhibitory mechanism and measurement of conformation. 6-Azauridine (as 2',3',5'-triacetate) and 5-azacytidine were used with certain success in human chemotherapy, the first one as a drug affecting recalcitrant psoriasis, the second one for the treatment of different forms of leukemia. The inhibitory mechanisms of individual azapyrimidine nucleosides are discussed in relation to their known biological effects.

Decreased synthesis of DNA in regenerating rat liver after the administration of reserpine

1. Reserpine given to rats before the enhanced synthesis of DNA begins 14h after partial hepatectomy markedly depresses thymidine uptake into DNA at 24 hours.2. At this time decreased activity of liver thymidine kinase but unchanged thymidine 5'-nucleotidase were observed.3. Reserpine has no effect on DNA synthesis when administered simultaneously with the labelled thymidine 2 h before killing.4. With depressed DNA synthesis after reserpine administration there is no significant decrease of liver RNA synthesis.