Mode of action and effects of 5-azacytidine and of its derivatives in eukaryotic cells

Resistance to venetoclax and hypomethylating agents in acute myeloid leukemia

Despite the success of the combination of venetoclax with the hypomethylating agents (HMA) decitabine or azacitidine in inducing remission in older, previously untreated patients with acute myeloid leukemia (AML), resistance - primary or secondary - still constitutes a significant roadblock in the quest to prolong the duration of response. Here we review the proposed and proven mechanisms of resistance to venetoclax monotherapy, HMA monotherapy, and the doublet of venetoclax and HMA for the treatment of AML. We approach the mechanisms of resistance to HMAs and venetoclax in the light of the agents' mechanisms of action. We briefly describe potential therapeutic strategies to circumvent resistance to this promising combination, including alternative scheduling or the addition of other agents to the HMA and venetoclax backbone. Understanding the mechanisms of action and evolving resistance in AML remains a priority in order to maximize the benefit from novel drugs and combinations, identify new therapeutic targets, define potential prognostic markers, and avoid treatment failure.

The radiosensitizing effect of fluorocyclopentenyl-cytosine (RX-3117) in ovarian and lung cancer cell lines

RX-3117 (fluorocyclopentenyl-cytosine) is a novel cytidine analog currently being evaluated in a Phase Ib clinical trial in cancer patients with solid tumors. The radiosensitizing effect of RX-3117 was studied in A2780 ovarian cancer cells and non-small cell lung cancer cell lines and related to cell survival and the effect on cell cycle and cell cycle proteins. RX-3117 has a schedule-dependent radiosensitizing effect, but only at pre-incubation (dose modifying factors: 1.4-1.8), observed at pulse and fractionated irradiation. Radiosensitizion was also seen in a three-dimensional spheroid model. At the low radiosensitizing concentration, RX-3117 in combination with radiation led to an accumulation of cells in S-phase, which was accompanied with an increase of cell cycle proteins such as p-Chk2 and p-cdc25C. In addition, RX-3117 caused DNA damage and increased apoptosis. In conclusion, our in vitro experiments showed a radiosensitizing effect of RX-3117.

Quantitative trait variation is revealed in a novel hypomethylated population of woodland strawberry (Fragaria vesca)

BACKGROUND

Phenotypic variation is determined by a combination of genotype, environment and their interactions. The realization that allelic diversity can be both genetic and epigenetic allows the environmental component to be further separated. Partitioning phenotypic variation observed among inbred lines with an altered epigenome can allow the epigenetic component controlling quantitative traits to be estimated. To assess the contribution of epialleles on phenotypic variation and determine the fidelity with which epialleles are inherited, we have developed a novel hypomethylated population of strawberry (2n = 2x = 14) using 5-azacytidine from which individuals with altered phenotypes can be identified, selected and characterized.

RESULTS

The hypomethylated population was generated using an inbred strawberry population in the F. vesca ssp. vesca accession Hawaii 4. Analysis of whole genome sequence data from control and hypomethylated lines indicate that 5-azacytidine exposure does not increase SNP above background levels. The populations contained only Hawaii 4 alleles, removing introgression of alternate F. vesca alleles as a potential source of variation. Although genome sequencing and genetic marker data are unable to rule out 5-azacytidine induced chromosomal rearrangements as a potential source of the trait variation observed, none were detected in our survey. Quantitative trait variation focusing on flowering time and rosette diameter was scored in control and treated populations where expanded levels of variation were observed among the hypomethylated lines. Methylation sensitive molecular markers indicated that 5-azacytidine induced alterations in DNA methylation patterns and inheritance of methylation patterns were confirmed by bisulfite sequencing of targeted regions. It is possible that methylation polymorphisms might underlie or have induced genetic changes underlying the observable differences in quantitative phenotypes.

CONCLUSIONS

This population developed in a uniform genetic background provides a resource for the discovery of new variation controlling quantitative traits. Genome sequence analysis indicates that 5-azacytidine did not induce point mutations and the induced variation is largely restricted to DNA methylation. Using this resource, we have identified new variation and demonstrated the inheritance of both variant trait and methylation patterns. Although direct associations remain to be determined, these data suggest epigenetic variation might be subject to selection.

A clinical-molecular update on azanucleoside-based therapy for the treatment of hematologic cancers

The azanucleosides azacitidine and decitabine are currently used for the treatment of acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS) in patients not only eligible for intensive chemotherapy but are also being explored in other hematologic and solid cancers. Based on their capacity to interfere with the DNA methylation machinery, these drugs are also referred to as hypomethylating agents (HMAs). As DNA methylation contributes to epigenetic regulation, azanucleosides are further considered to be among the first true “epigenetic drugs” that have reached clinical application. However, intriguing new evidence suggests that DNA hypomethylation is not the only mechanism of action for these drugs. This review summarizes the experience from more than 10 years of clinical practice with azanucleosides and discusses their molecular actions, including several not related to DNA methylation. A particular focus is placed on possible causes of primary and acquired resistances to azanucleoside treatment. We highlight current limitations for the success and durability of azanucleoside-based therapy and illustrate that a better understanding of the molecular determinants of drug response holds great potential to overcome resistance.

Oxytocin improves the expression of cardiac specific markers in porcine bone marrow stem cells differentiation

Bone marrow stem cells (BMSCs) treated with 5-azacytidine possess myogenic differentiation potential. Oxytocin (OT) induces cardiomyogenesis in murine embryonic and cardiac stem cells. We attempted to isolate, characterize, and induce OT-mediated cardiomyogenic differentiation of porcine pBMSCs. Cells were treated as: control, OT, and 5-azacytidine groups. During early passages, transcripts of Oct4, GATA4, OT receptor, and phospholamban were expressed. RT-PCR showed upregulation of GATA4 in OT and 5-azacytidine-induced groups. Immunocytochemistry revealed higher expressions of cardiac troponin T and myosin heavy chain in OT than in 5-azacytidine-induced groups (p < 0.01). Western blot analysis showed upregulation of cardiac troponin I in OT-induced pBMSCs (p < 0.01). We infer pBMSCs should be induced during early passages, when expressing transcription factors related to pluripotency and cardiomyogenesis, as well as OT receptor. The more abundant expression of cardiac specific proteins in OT-treated pBMSCs suggests OT could be a more potent cardiomyogenic inducer of pBMSC.

Stochastic promoter activation affects Nanog expression variability in mouse embryonic stem cells

Mouse embryonic stem cells (mESCs) are self-renewing and capable of differentiating into any of the three germ layers. An interesting feature of mESCs is the presence of cell-to-cell heterogeneity in gene expression that may be responsible for cell fate decisions. Nanog, a key transcription factor for pluripotency, displays heterogeneous expression in mESCs, via mechanisms that are not fully understood. To understand this variability, we quantitatively analyzed Nanog transcription and found that Nanog was both infrequently transcribed, and transcribed in a pulsatile and stochastic manner. It is possible that such stochastic transcriptional activation could contribute to the heterogeneity observed in Nanog expression as "intrinsic noise." To discriminate the effects of both intrinsic noise from other (extrinsic) noise on the expression variability of Nanog mRNA, we performed allele-specific single-molecule RNA fluorescent in situ hybridization in a reporter cell line and found that intrinsic noise contributed to approximately 45% of the total variability in Nanog expression. Furthermore, we found that Nanog mRNA and protein levels were well correlated in individual cells. These results suggest that stochastic promoter activation significantly affects the Nanog expression variability in mESCs.

Metabolism, mechanism of action and sensitivity profile of fluorocyclopentenylcytosine (RX-3117; TV-1360)

A novel cytidine analog fluorocyclopentenylcytosine (RX-3117; TV-1360) was characterized for its cytotoxicity in a 59-cell line panel and further characterized for cytotoxicity, metabolism and mechanism of action in 15 additional cancer cell lines, including gemcitabine-resistant variants. In both panels sensitivity varied 75-fold (IC50: 0.4- > 30 μM RX-3117). RX-3117 showed a different sensitivity profile compared to cyclopentenyl-cytosine (CPEC) and azacytidine, substrates for uridine-cytidine-kinase (UCK). Dipyridamole, an inhibitor of the equilibrative-nucleoside-transporter protected against RX-3117. Uridine and cytidine protected against RX-3117, but deoxycytidine (substrate for deoxycytidine-kinase [dCK]) not, although it protected against gemcitabine, demonstrating that RX-3117 is a substrate for UCK and not for dCK. UCK activity was abundant in all cell lines, including the gemcitabine-resistant variants. RX-3117 was a very poor substrate for cytidine deaminase (66,000-fold less than gemcitabine). RX-3117 was rapidly metabolised to its nucleotides predominantly the triphosphate, which was highest in the most sensitive cells (U937, A2780) and lowest in the least sensitive (CCRF-CEM). RX-3117 did not significantly affect cytidine and uridine nucleotide pools. Incorporation of RX-3117 into RNA and DNA was higher in sensitive A2780 and low in insensitive SW1573 cells. In sensitive U937 cells 1 μM RX-3117 resulted in 90% inhibition of RNA synthesis but 100 μM RX-3117 was required in A2780 and CCRF-CEM cells. RX-3117 at IC50 values did not affect the integrity of RNA. DNA synthesis was completely inhibited in sensitive U937 cells at 1 μM, but in other cells even higher concentrations only resulted in a partial inhibition. At IC50 values RX-3117 downregulated the expression of DNA methyltransferase. In conclusion, RX-3117 showed a completely different sensitivity profile compared to gemcitabine and CPEC, its uptake is transporter dependent and is activated by UCK. RX-3117 is incorporated into RNA and DNA, did not affect RNA integrity, depleted DNA methyltransferase and inhibited RNA and DNA synthesis. Nucleotide formation is related with sensitivity.

Quantifying the contribution of chromatin dynamics to stochastic gene expression reveals long, locus-dependent periods between transcriptional bursts

BACKGROUND

A number of studies have established that stochasticity in gene expression may play an important role in many biological phenomena. This therefore calls for further investigations to identify the molecular mechanisms at stake, in order to understand and manipulate cell-to-cell variability. In this work, we explored the role played by chromatin dynamics in the regulation of stochastic gene expression in higher eukaryotic cells.

RESULTS

For this purpose, we generated isogenic chicken-cell populations expressing a fluorescent reporter integrated in one copy per clone. Although the clones differed only in the genetic locus at which the reporter was inserted, they showed markedly different fluorescence distributions, revealing different levels of stochastic gene expression. Use of chromatin-modifying agents showed that direct manipulation of chromatin dynamics had a marked effect on the extent of stochastic gene expression. To better understand the molecular mechanism involved in these phenomena, we fitted these data to a two-state model describing the opening/closing process of the chromatin. We found that the differences between clones seemed to be due mainly to the duration of the closed state, and that the agents we used mainly seem to act on the opening probability.

CONCLUSIONS

In this study, we report biological experiments combined with computational modeling, highlighting the importance of chromatin dynamics in stochastic gene expression. This work sheds a new light on the mechanisms of gene expression in higher eukaryotic cells, and argues in favor of relatively slow dynamics with long (hours to days) periods of quiet state.

Azacytidine impairs NK cell reactivity while decitabine augments NK cell responsiveness toward stimulation

Azacytidine and decitabine are approved for treatment of acute myeloid leukemias and myelodysplastic syndromes. While clinical responses are attributed to epigenetic effects and induction of apoptosis in malignant cells, these azanucleosides also affect antitumor immune responses. NK cells as components of innate immunity may confine development and progression of cancer. Numerous therapeutic strategies presently aim to reinforce NK reactivity against hematopoietic malignancies. We here comparatively analyzed the effect of the two clinically available azanucleosides and report that NK cytotoxicity and IFN-γ production are significantly impaired by pharmacological concentrations of azacytidine but enhanced by decitabine. This was not due to alterations in the target cells but caused by direct effects on NK cells depending on the chemical modifications by which azanucleosides differ from their physiological analogues. Although azacytidine impaired mRNA synthesis and induced apoptosis in NK cells, decitabine did not per se alter NK cell viability or reactivity but enhanced responsiveness to activating stimuli by inducing transcription of genes involved in NK reactivity. Tantalizingly, these effects were independent of incorporation of the azanucleosides into DNA during cell division. While azacytidine impairs NK antitumor immunity, decitabine augments NK reactivity by yet unidentified mechanisms and may thus serve well in therapeutic strategies combining its effects on malignant cells with its ability to enhance NK functions.

Identification and analysis of the promoter region of the human DHCR24 gene: involvement of DNA methylation and histone acetylation

Mutations in the DHCR24 gene, which encodes the cholesterol biosynthesis enzyme 3ß-hydroxysterol-∆24 reductase, result in an autosomal recessive disease called desmosterolosis. Further, reduced expression of DHCR24 is found in the temporal cortex of Alzheimer's disease patients. This suggests that variability in the regulatory regions of DHCR24 may contribute to the development of this neurodegenerative disease. In this work, we functionally characterised the proximal fragment of the human DHCR24 gene, for the first time. We show that the transcription of DHCR24 is initiated from a single CpG-rich promoter that is regulated by DNA methylation in some cell types. An activator sequence was also uncovered in the -1203/-665 bp region by reporter gene assays. Furthermore, sodium butyrate (a well-known HDAC inhibitor) increased DHCR24 expression in SH-SY5Y cells by recruiting acetylated core histones H3 and H4 to the enhancer region, as demonstrated by transient transfection and chromatin immunoprecipitation assays. Understanding the regulation of the DHCR24 gene may lead to alternative therapeutic strategies in at least some Alzheimer's patients.

DNA (Cytosine-C5) methyltransferase inhibition by oligodeoxyribonucleotides containing 2-(1H)-pyrimidinone (zebularine aglycon) at the enzymatic target site

Aberrant cytosine methylation in promoter regions leads to gene silencing associated with cancer progression. A number of DNA methyltransferase inhibitors are known to reactivate silenced genes; including 5-azacytidine and 2-(1H)-pyrimidinone riboside (zebularine). Zebularine is a more stable, less cytotoxic inhibitor compared to 5-azacytidine. To determine the mechanistic basis for this difference, we carried out a detailed comparisons of the interaction between purified DNA methyltransferases and oligodeoxyribonucleotides (ODNs) containing either 5-azacytosine or 2-(1H)-pyrimidinone in place of the cytosine targeted for methylation. When incorporated into small ODNs, the rate of C5 DNA methyltransferase inhibition by both nucleosides is essentially identical. However, the stability and reversibility of the enzyme complex in the absence and presence of cofactor differs. 5-Azacytosine ODNs form complexes with C5 DNA methyltransferases that are irreversible when the 5-azacytosine ring is intact. ODNs containing 2-(1H)-pyrimidinone at the enzymatic target site are competitive inhibitors of both prokaryotic and mammalian DNA C5 methyltransferases. We determined that the ternary complexes between the enzymes, 2-(1H)-pyrimidinone inhibitor, and the cofactor S-adenosyl methionine are maintained through the formation of a reversible covalent interaction. The differing stability and reversibility of the covalent bonds may partially account for the observed differences in cytotoxicity between zebularine and 5-azacytidine inhibitors.

Angiostatic activity of DNA methyltransferase inhibitors

Inhibitors of DNA methyltransferases (DNMT) and histone deacetylases can reactivate epigenetically silenced tumor suppressor genes and thereby decrease tumor cell growth. Little, however, is known on the effects of these compounds in endothelial cell biology and tumor angiogenesis. Here, we show that the DNMT inhibitors 5-aza-2'-deoxycytidine and zebularine markedly decrease vessel formation in different tumor models. We show that DNMT inhibitors are antiproliferative for tumor-conditioned endothelial cells, without affecting endothelial cell apoptosis and migration. Furthermore, these compounds inhibit angiogenesis in vitro and in vivo as shown by inhibition of endothelial cells sprouting in a three-dimensional gel and inhibition of microvessel formation in the chorioallantoic membrane, respectively. 5-Aza-2'-deoxycytidine, as well as the histone deacetylase inhibitor trichostatin A, reactivates the growth-inhibiting genes TSP1, JUNB, and IGFBP3, which are suppressed in tumor-conditioned endothelial cells. Despite enhanced DNMT activity and increased overall genomic methylation levels in tumor-conditioned endothelial cells, silencing of these genes seemed not to be regulated by direct promoter hypermethylation. For IGFBP3, gene expression in endothelial cells correlated with histone H3 acetylation patterns. In conclusion, our data show that DNMT inhibitors have angiostatic activity in addition to their inhibitory effects on tumor cells. This dual action of these compounds makes them promising anticancer therapeutics.

FDA drug approval summary: azacitidine (5-azacytidine, Vidaza) for injectable suspension

On May 19, 2004, azacitidine (5-azacytidine; Vidaza(trade mark); Pharmion Corporation, Boulder, CO, http://www.pharmion.com) for injectable suspension received regular approval by the U.S. Food and Drug Administration (FDA) for the treatment of all subtypes of myelodysplastic syndrome (MDS). This report summarizes the basis for this approval. Effectiveness was demonstrated in one randomized, controlled trial comparing azacitidine administered s.c. with best supportive care (observation group) and in two single-arm studies, one in which azacitidine was administered s.c. and in the other in which it was administered i.v. The dose of azacitidine, 75 mg/m2/day for 7 days every 28 days, was the same in all three studies. In the randomized trial, study participants were well matched with respect to age, sex, race, performance status, MDS subtype, and use of transfusion during the 3 months before study entry. Patients in the observation arm were permitted by protocol to cross over to azacitidine treatment if their disease progressed according to prespecified criteria. During the course of the study, more than half of the patients in the observation arm did cross over to the azacitidine treatment arm. The primary efficacy end point was the overall response rate. Response consisted of complete or partial normalization of blood cell counts and of bone marrow morphology. The response rate in the azacitidine arm was about 16%; there were no responses in the observation arm. The response rates in the two single-arm studies were similar (13% and 19%). The responses were sustained, with median durations of 11 months and 17 months respectively. Responding patients who were transfusion dependent at study entry lost the need for transfusions. In addition, about 19% of patients had less than partial responses (termed improvement), and two-thirds of them became transfusion independent. Common adverse events associated with azacitidine treatment were gastrointestinal (nausea, vomiting, diarrhea, constipation, and anorexia), hematologic (neutropenia, thrombocytopenia), fevers, rigors, ecchymoses, petechiae, injection site events, arthralgia, headache, and dizziness. Liver function abnormalities occurred in 16% of patients with intercurrent hepatobiliary disorders and in two patients with previously diagnosed liver cirrhosis. Renal failure occurred in patients during sepsis and hypotension. There were no deaths attributed to azacitidine. Azacitidine, the first drug approved by the U.S. FDA for MDS, has a favorable safety profile and provides a clinical benefit of eliminating transfusion dependence and complete or partial normalization of blood counts and bone marrow blast percentages in responding patients.

Epigenetic and chromatin modifiers as targeted therapy of hematologic malignancies

Epigenetic regulation of gene expression is mediated through alterations in the DNA methylation status, covalent modifications of core nucleosomal histones, rearrangement of histones, and by RNA interference. It is now abundantly clear that deregulation of epigenetic mechanisms cooperates with genetic alterations in the development and progression of cancer and leukemia. Epigenetic deregulation affects several aspects of tumor cell biology, including cell growth, cell cycle control, differentiation, DNA repair, and cell death. This raises the strong possibility that reversing deregulated epigenetic mechanisms may be an effective treatment strategy for leukemia and cancer. This treatment strategy may either be designed to separately or collectively target the specific perturbations in the epigenetic mechanisms found in human hematologic malignancies. The following review describes our current understanding of the important deregulated epigenetic mechanisms and the preclinical and clinical development of epigenetic and chromatin modifiers in the therapy of these disorders.

Phosphorylation of uridine and cytidine nucleoside analogs by two human uridine-cytidine kinases

Uridine-cytidine kinases (UCK) have important roles for the phosphorylation of nucleoside analogs that are being investigated for possible use in chemotherapy of cancer. We have cloned the cDNA of two human UCKs. The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides. UCK1 mRNA was detected as two isoforms of approximately 1.8 and approximately 2.7 kb. The 2.7-kb band was ubiquitously expressed in the investigated tissues. The band of approximately 1.8 kb was present in skeletal muscle, heart, liver, and kidney. The two isoforms of UCK2 mRNA of 1.2 and 2.0 kb were only detected in placenta among the investigated tissues. The genes encoding UCK1 and UCK2 were mapped to chromosome 9q34.2-9q34.3 and 1q22-1q23.2, respectively. We tested 28 cytidine and uridine nucleoside analogs as possible substrates of the enzymes. The enzymes phosphorylated several of the analogs, such as 6-azauridine, 5-fluorouridine, 4-thiouridine, 5-bromouridine, N(4)-acetylcytidine, N(4)-benzoylcytidine, 5-fluorocytidine, 2-thiocytidine, 5-methylcytidine, and N(4)-anisoylcytidine. The cloning and recombinant expression of the two human UCKs will be important for development of novel pyrimidine ribonucleoside analogs and the characterization of their pharmacological activation.

Expression of alpha1-6 fucosyltransferase in rat tissues and human cancer cell lines

GDP-L-Fuc:N-acetyl-beta-D-glucosaminide alpha1-6 fucosyltransferase (alpha1-6FucT) catalyzes the transfer of a fucosyl residue from GDP-fucose to the asparagine-linked GlcNAc residue of complex N-glycans via alpha1-6 linkage. These oligosaccharide structures are essential for the attachment of polysialic acid to the neural-cell-adhesion molecule, and its levels are useful for the differential diagnosis of hepatocellular carcinomas with respect to the microheterogeneity of alpha-fetoprotein. We have been successful in the purification and cDNA cloning of alpha1-6FucT from porcine brain and from a human gastric-cancer cell line. In the present study, mRNA expression of alpha1-6FucT in various rat tissues and human cancer cell lines was examined, along with the expression of alpha1-6FucT mRNA and the induction by treatment with several cytokines. Northern-blot analysis indicated high expression levels of alpha1-6FucT in brain and gastrointestinal-tract tissues of normal rats, as well as for a number of lung-cancer, gastric-cancer and colon-cancer cell lines. Although various cytokines did not induce alpha1-6FucT mRNA, differentiation of a tumor cell enhanced the mRNA by 2- to 3-fold. These results may provide new insight into studies on alpha1-6FucT in terms of carcinogenesis or differentiation.

5-azacytidine and 5-azadeoxycytidine inhibit human immunodeficiency virus type 1 replication in vitro

Chemotherapeutic agents which affect the integration, stability, or inducibility of the human immunodeficiency virus (HIV) provirus would have considerable value in treating acquired immunodeficiency syndrome. Two nucleoside analogs of cytosine, 5-azacytidine and 5-azadeoxycytidine, which seem to have such value because of their capabilities to affect both the stability and the methylation patterns of the nucleic acids into which they are incorporated, were tested for their ability to inhibit the replication of HIV type 1 (HIV-1) in human CEM T cells in vitro. 5-Azadeoxycytidine (1 microM) completely inhibited HIV replication in CEM cells, by the criteria of reduced viral antigen expression and decreased supernatant reverse transcriptase activity, with little toxicity for the treated cells. 5-azacytidine (1 microM) also inhibited HIV replication, but less effectively. When added 2 or more h after CEM cells were infected with HIV-1, both 5-azacytosine derivatives were less effective than they were when added at the time of infection. Even 2 h of exposure to 5-azadeoxycytidine was sufficient for inhibition of HIV replication. Although long exposure to either analog at concentrations of 1 microM would result in pronounced cellular cytotoxicity, the the fact that short exposures to the same dose of drug inhibit HIV replication but are not toxic for the cells implies that cellular toxicity itself is not an important mechanism of the antiviral action of the analogs.

Mechanism of action of 5-AZA-dC: induced DNA hypomethylation does not lead to aberrant gene expression in human leukemic CEM cells

To understand the mechanism of action of 5-AZA-2'-deoxycytidine, a potent antineoplastic agent, we studied its effect in CEM cells on DNA methylation while monitoring coinciding changes in DNA, RNA and protein synthesis. At concentrations near the inhibitory concentration, the drug induced a profound reduction of DNA methylation. Effects on DNA and RNA synthesis were also noted at 24 and 48 h of treatment. However, at all concentrations assayed, no change in polypeptide composition as monitored by 2-D gel electrophoresis, were observed. That the cytotoxic effect of 5-AZA-deoxycytidine is due to changes in gene expression induced by DNA methylation reduction is not supported.

Azapyrimidine analogues: inhibition of viral DNA synthesis and protein synthesis in SV40 infected BSC-1 cells

The replication of simian virus 40 DNA and protein synthesis in BSC-1 cells was analyzed in vitro after treatment with 5,6-dihydro-5-azacytidine (DH-5-AzaCR) or 5-azacytidine (5-AzaCR). Results demonstrated that after a 3-h treatment period with 100 micrograms/ml, DH-5-AzaCR exhibited a 77% inhibition of viral DNA synthesis, whereas 5-AzaCR resulted in a 50% inhibition. Stimulation of DNA synthesis occurred when infected cultures were treated with low doses (0.1 to 0.5 microgram/ml) of 5-AzaCR for 3 h and after 1 and 2 h of treatment with 100 micrograms/ml of 5-AzaCR; however, stimulation did not occur with DH-5-AzaCR. DNA synthesized in the presence of either drug demonstrated altered conformations when analyzed on agarose gels; however this alteration was negligible after DH-5-AzaCR treatment, but more pronounced in the presence of 5-AzaCR. Restriction enzyme analysis suggests that DH-5-AzaCR may not be a hypomethylating agent as is 5-AzaCR. Inhibition of total protein synthesis (cellular and viral) was essentially complete 6 h after treatment with DH-5-AzaCR, whereas 5-AzaCR completely inhibited protein synthesis after 3 h. These data indicate that 5-AzaCR does not exhibit a direct dose relationship to the inhibition of DNA synthesis whereas DH-5-AzaCR may show some dose relationship, and that DH-5-AzaCR is a more potent inhibitor of DNA synthesis as compared to 5-AzaCR.

Sudden outbreak of a leukemia-like lesion in female CBA mice after repeated injections of 5-azacytidine

A total of 11 i.p. injections of 1 mg/kg 5-azacytidine given within 21 weeks induced a sudden outbreak of a leukemia-like disease in female CBA mice about 2 weeks after the last injection. The outbreak of disease was highly synchronous; 68% of animals who had survived the treatment period succumbed within a period of 5 days, 2/3 of them on a single day. The lesion had a characteristic form commonly presenting as a result of hemorrhagic effusion in the thoracic and/or abdominal cavities, associated with diffuse infiltration of lymphatic and fatty tissue by lymphoblasts. It appears that a critical combination of factors was responsible for the unexpected appearance of the lesion. Female BALB/c mice treated identically were not affected. The factors indicating that the lesion was induced by an epigenetic mechanism are discussed.

Negative and positive regulation of human leukocyte antigen class I gene transcription in K562 leukemia cells

The mechanism of transcriptional activation of human leukocyte antigen class I genes by gamma interferon and 5-azacytidine was studied in K562 human leukemia cells. Nuclear run-on transcription assays with various protein and RNA synthesis inhibitors yield evidence for both stimulation of a positive regulatory factor and inhibition of an mRNA that codes for a labile repressor. A novel mechanism is proposed to explain how 5-azacytidine can activate repressed genes without affecting DNA methylation.

Negative and positive regulation of human leukocyte antigen class I gene transcription in K562 leukemia cells

The mechanism of transcriptional activation of human leukocyte antigen class I genes by gamma interferon and 5-azacytidine was studied in K562 human leukemia cells. Nuclear run-on transcription assays with various protein and RNA synthesis inhibitors yield evidence for both stimulation of a positive regulatory factor and inhibition of an mRNA that codes for a labile repressor. A novel mechanism is proposed to explain how 5-azacytidine can activate repressed genes without affecting DNA methylation.

Genetic analysis of the 5-azacytidine sensitivity of Escherichia coli K-12

DNA containing 5-azacytidine (5-azaC) has been shown to form stable detergent-resistant complexes with cytosine methylases. We reasoned that if 5-azaC treatment causes protein-DNA cross-links in vivo, then mutations in DNA repair and recombination genes may increase the sensitivity of a cell to 5-azaC. We found that although recA (defective) and lexA (induction-negative) mutants of Escherichia coli were very sensitive to the drug, mutations in uvrA and ung genes had little effect on cell lethality. The sensitivity of recA strains to 5-azaC was dose dependent and was enhanced by the overproduction of a DNA cytosine methylase in the cell. Unexpectedly, a strain of E. coli carrying a recA mutation and a deletion of the DNA cytosine methylase gene (dcm) was found to be significantly sensitive to 5-azaC. Study of mutations in the pyrimidine salvage pathway of E. coli suggests that direct phosphorylation of 5-azaC, rather than phosphorylation of its degradation products, is largely responsible for the lethal effects of the drug. The addition of uracil to the growth medium has little effect on cell lethality of recA mutants, but it partially reversed the inhibition of cell growth caused by 5-azaC. This reversal of the bacteriostatic effects of the drug could not be achieved by adding cytosine or orotic acid to the growth medium and required the presence of functional UMP-pyrophosphorylase (gene upp) in the cell.