A novel fluorometric oligonucleotide assay to measure O( 6)-methylguanine DNA methyltransferase, methylpurine DNA glycosylase, 8-oxoguanine DNA glycosylase and abasic endonuclease activities: DNA repair status in human breast carcinoma cells overexpressing methylpurine DNA glycosylase.

DNA repair status plays a major role in mutagenesis, carcinogenesis and resistance to genotoxic agents. Because DNA repair processes involve multiple enzymatic steps, understanding cellular DNA repair status has required several assay procedures. We have developed a novel in vitro assay that allows quantitative measurement of alkylation repair via O(6)-methylguanine DNA methyltransferase (MGMT) and base excision repair (BER) involving methylpurine DNA glycosylase (MPG), human 8-oxoguanine DNA glycosylase (hOGG1) and yeast and human abasic endonuclease (APN1 and APE/ref-1, respectively) from a single cell extract. This approach involves preparation of cell extracts in a common buffer in which all of the DNA repair proteins are active and the use of fluorometrically labeled oligonucleotide substrates containing DNA lesions specific to each repair protein. This method enables methylation and BER capacities to be determined rapidly from a small amount of starting sample. In addition, the stability of the fluorometric oligonucleotides precludes the substrate variability caused by continual radiolabeling. In this report this technique was applied to human breast carcinoma MDA-MB231 cells overexpressing human MPG in order to assess whether up-regulation of the initial step in BER alters the activity of selected other BER (hOGG1 and APE/ref-1) or direct reversal (MGMT) repair activities.

Nitric oxide inhibits human aldosteronogenesis without guanylyl cyclase stimulation.

Deta nonoate (deta-NO), a zwitterion nitric oxide (NO) donor, potently inhibited forskolin- and angiotensin II-stimulated aldosterone production in human adrenocortical H295R cells in a concentration-dependent manner (0.1-1000 microM). The half-maximal and maximal inhibition of forskolin-evoked aldosteronogenesis occurred at 0.6 and 100 microM deta-NO, respectively. The respective half-maximal and maximal deta-NO-mediated inhibition of angiotensin II-stimulated aldosterone generation occurred at 150 microM and 1 mM. In H295R cells, deta-NO and sodium nitroprusside did not stimulate cGMP production, and the soluble guanylyl cyclase inhibitor oxadiazoloquinoxalinone (10 microM) did not block deta-NO-mediated attenuation of aldosteronogenesis. 25-Hydroxycholesterol (10 microM)-facilitated aldosterone synthesis was also diminished with half-maximal and maximal inhibition occurring at 120 microM and 1 mM deta-NO, respectively. Taken together, these results demonstrate that NO inhibits human aldosteronogenesis without stimulating guanylyl cyclase in H295R cells.

Discovery and characterization of LY2784544, a small-molecule tyrosine kinase inhibitor of JAK2V617F.

Owing to the prevalence of the JAK2V617F mutation in myeloproliferative neoplasms (MPNs), its constitutive activity, and ability to recapitulate the MPN phenotype in mouse models, JAK2V617F kinase is an attractive therapeutic target. We report the discovery and initial characterization of the orally bioavailable imidazopyridazine, LY2784544, a potent, selective and ATP-competitive inhibitor of janus kinase 2 (JAK2) tyrosine kinase. LY2784544 was discovered and characterized using a JAK2-inhibition screening assay in tandem with biochemical and cell-based assays. LY2784544 in vitro selectivity for JAK2 was found to be equal or superior to known JAK2 inhibitors. Further studies showed that LY2784544 effectively inhibited JAK2V617F-driven signaling and cell proliferation in Ba/F3 cells (IC50=20 and 55 nM, respectively). In comparison, LY2784544 was much less potent at inhibiting interleukin-3-stimulated wild-type JAK2-mediated signaling and cell proliferation (IC50=1183 and 1309 nM, respectively). In vivo, LY2784544 effectively inhibited STAT5 phosphorylation in Ba/F3-JAK2V617F-GFP (green fluorescent protein) ascitic tumor cells (TED50=12.7 mg/kg) and significantly reduced (P<0.05) Ba/F3-JAK2V617F-GFP tumor burden in the JAK2V617F-induced MPN model (TED50=13.7 mg/kg, twice daily). In contrast, LY2784544 showed no effect on erythroid progenitors, reticulocytes or platelets. These data suggest that LY2784544 has potential for development as a targeted agent against JAK2V617F and may have properties that allow suppression of JAK2V617F-induced MPN pathogenesis while minimizing effects on hematopoietic progenitor cells.

Prolonged inhibition of O(6)-methylguanine DNA methyltransferase in human tumor cells by O(6)-benzylguanine in vitro and in vivo.

We previously demonstrated that sustained depletion of methylguanine DNA methyltransferase (MGMT) activity is required for optimal reversal of chloroethylnitrosourea resistance in tumor cells. The purpose of this study was to design O(6)-benzylguanine (BG) treatments that deplete MGMT activity in tumor cells and xenograft tumors in a prolonged manner. When SF767 cells were treated with a bolus dose of BG (25 microM for 1 h), >95% of MGMT activity was depleted but 33% of the activity recovered within 24 h. In contrast, MGMT activity was completely depleted for 24 h when cells were pretreated with a low dose of BG (2.5 microM) for 24 h, followed by the bolus dose and same low-dose treatment for 24 h. This combination regimen of pre- and post-treatments with a bolus dose sensitized cells N,N'-bis(2-chloroethyl)-N-nitrosourea in vitro by approximately 2-fold more than the bolus dose alone. Similar BG treatment with Alzet micro-osmotic pumps produced sustained inhibition of MGMT activity in vivo. In xenograft SF767 tumors, low-dose pre- and post-treatments (8 mg/kg over 24 h) combined with an i.p. bolus dose (80 mg/kg) of BG inhibited >95% of MGMT activity for 24 h after the bolus. The bolus dose alone did not deplete MGMT for 24 h. These results demonstrate that combination low-dose and bolus BG treatment is superior to the bolus dose alone in depleting MGMT activity in a sustained manner in vitro and in vivo. When combined with N,N'-bis(2-chloroethyl)-N-nitrosourea treatment, this BG regimen also should also produce greater antitumor activity than the single bolus dose evaluated clinically.

Comparison of single- versus double-bolus treatments of O(6)-benzylguanine for depletion of O(6)-methylguanine DNA methyltransferase (MGMT) activity in vivo: development of a novel fluorometric oligonucleotide assay for measurement of MGMT activity.

Previous studies have demonstrated that optimal reversal of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) resistance requires complete inactivation of the DNA repair protein O(6)-methylguanine DNA methyltransferase (MGMT) for at least 24 h following BCNU administration. In preparation for clinical trials at this institution, this study was undertaken to compare the efficacy of a conventional single-bolus dose versus double-bolus dose treatments with O(6)-benzylguanine (BG) in depleting MGMT activity in vivo. In xenograft human glioma SF767 tumors, a single 30-mg/kg bolus dose of BG completely inhibited MGMT activity for at least 8 h, but approximately 50% of the basal MGMT activity recovered within 24 h. To sustain the MGMT depletion for 24 h, a second bolus injection of BG at escalating doses was administered 8 h after the first dose. Second bolus doses of 5, 10, and 15 mg/kg BG attenuated the MGMT recovery in a dose-dependent manner compared with the single 30-mg/kg BG dose alone. When the 15-mg/kg BG dose was administered 8 h after the 30-mg/kg initial dose, MGMT activity was completely inactivated in the tumor xenografts for 24 h. This double-bolus BG treatment also depleted MGMT activity in normal murine tissues, including the liver, kidney, lung, brain, spleen, and bone marrow; and the kinetics of MGMT recovery varied among these tissues. When combined with BCNU treatment, the double-bolus BG treatment would be expected to produce greater antitumor activity in future trials than the conventional single-bolus BG treatment.

Modulation of 1,3-bis-(2-chloroethyl)-1-nitrosourea resistance in human tumor cells using hammerhead ribozymes designed to degrade O6-methylguanine DNA methyltransferase mRNA.

O6-Methylguanine DNA Methyltransferase (MGMT) protects tumor cells from the cytotoxic effects of the DNA alkylating agent 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU). To improve the therapeutic index of BCNU, biochemical strategies to deplete MGMT activity have been developed. In the present study, a molecular strategy for modulating BCNU resistance was explored using hammerhead ribozymes (Rz) designed to degrade the long-lived MGMT mRNA. The ribozymes were designed against eight GUC sites within the MGMT mRNA. cDNAs of these ribozymes were cloned into an expression vector and then all eight vectors were pooled and stably transfected into HeLa cells. Several HeLa/Rz clones sensitive to a sublethal dose of BCNU were identified using a short-term cell proliferation assay. The ribozyme inserts were amplified from genomic DNA by polymerase chain reaction and sequenced in the BCNU-sensitive clones. The ribozyme inserts Rz161, 178, and 212, targeted against nucleotide 161, 178, and 212, respectively, in the MGMT mRNA, were found to be present in these clones. MGMT activity, Western, and Northern blot analyses revealed that two of the HeLa/Rz clones contained very low levels of MGMT activity, protein, and mRNA. Investigation of CpG methylation within the MGMT promoter indicated that the lack of MGMT expression in these HeLa/Rz clones was not likely due to methylation silencing of the MGMT gene. By colony formation, the cell killing induced by 100 microM BCNU was increased by 2 to 3 logs in the HeLa/Rz clones compared with wild-type HeLa cells.

O6-methylguanine-DNA-methyltransferase expression and gene polymorphisms in relation to chemotherapeutic response in metastatic melanoma.

In a retrospective study, O(6)-methylguanine-DNA-methyltransferase (MGMT) expression was analysed by immunohistochemistry using monoclonal human anti-MGMT antibody in melanoma metastases in patients receiving dacarbazine (DTIC) as single-drug therapy or as part of combination chemotherapy with DTIC-vindesine or DTIC-vindesine-cisplatin. The correlation of MGMT expression levels with clinical response to chemotherapy was investigated in 79 patients with metastatic melanoma. There was an inverse relationship between MGMT expression and clinical response to DTIC-based chemotherapy (P=0.05). Polymorphisms in the coding region of the MGMT gene were also investigated in tumours from 52 melanoma patients by PCR/SSCP and nucleotide sequence analyses. Single-nucleotide polymorphisms (SNPs) in exon 3 (L53L and L84F) and in exon 5 (I143V/K178R) were identified. There were no differences in the frequencies of these polymorphisms between these melanoma patients and patients with familial melanoma or healthy Swedish individuals. Functional analysis of variants MGMT-I143V and -I143V/K178R was performed by in vitro mutagenesis in Escherichia coli. There was no evidence that these variants decreased the MGMT DNA repair activity compared to the wild-type protein. All melanoma patients with the MGMT 53/84 polymorphism except one had tumours with high MGMT expression. There was no significant correlation between any of the MGMT polymorphisms and clinical response to chemotherapy, although an indication of a lower response rate in patients with SNPs in exon 5 was obtained. Thus, MGMT expression appears to be more related to response to chemotherapy than MGMT polymorphisms in patients with metastatic melanoma.