Augmentation of apoptosis responses in p53-deficient L1210 cells by compounds directed at blocking NFkappaB activation

A mouse leukemia L1210 cell line (Y8) selected for resistance to deoxyadenosine was found to be deficient in the expression of p53 mRNA and protein while maintaining the expression of WAF1/p21 mRNA and protein even under basal conditions. The Y8 cells were shown to be more sensitive to apoptosis induced by a variety of agents when compared to the parental wild-type (WT) L1210 cells. Roscovitine, an inhibitor of cdk 2 and cdk5, was one of the agents that caused increased apoptosis in the Y8 cells through a pathway that ultimately involved the activation of caspase-3 activity. In these studies, the effects of leflunomide and parthenolide (drugs reported to alter the activation of NFkappaB in a variety of cell types) were studied for their cell cycle and apoptotic effects in WT and Y8 cells as single agents and in combination with roscovitine. Leflunomide at IC50 concentrations had little effect on the cell cycle distribution of either the WT or Y8 cells while at higher concentrations caused a G0/G1 block in Y8 cells. Parthenolide, at IC50 concentrations, caused a G0/G1 cell cycle block in the WT and Y8 cells but at higher concentrations caused a G2/M block in the Y8 cells. The combinations of leflunomide and roscovitine or parthenolide and roscovitine did not alter, in a significant way the cell cycle distribution of the Y8 cells. However, in the presence of the combinations of leflunomide and roscovitine or parthenolide and roscovitine there were large increases in the fraction of Y8 cells undergoing early apoptosis without a corresponding increase in the necrotic fraction of cells. These data show that combinations of agents directed at different pathways or different steps of pathways involved in apoptosis can cause the cells to reach an apoptotic threshold that results in synergistic apoptosis.

Increased sensitivity to sodium salicylate-induced apoptosis in drug-resistant leukemia L1210 cells

Mouse leukemia L1210 cells selected for resistance to deoxyadenosine contain ribonucleotide reductase that is not feedback inhibited by dATP. These deoxyadenosine-resistant cells (Y8) also do not express p53 protein but do have WAF1 and Gadd45 mRNA and protein. The Y8 cells show increased sensitivity to DNA damaging agents and kinase inhibitors. In these studies we show that in the presence of sodium salicylate (NaSal), the parental wild-type (WT) cells block in G2/M phase of the cell cycle while the Y8 cells show a marked increased in the G0/G1 population of cells. The Y8 cells are more sensitive to apoptosis induced by NaSal than the WT cells. NaSal treatment causes the induction of caspase-3-like activity in Y8 cells but no induction of caspase-3 activity in the WT cells. The caspase inhibitor, Ac-DEVD-CHO, decreased the percentage of Y8 cells in the early apoptotic fraction, but this decrease was reflected by an increase in the percent of cells in the late apoptotic/necrotic fraction. SB20358, a p38-MAP kinase inhibitor did not protect the Y8 cells from NaSal-induced apoptosis indicating that the p38-MAP kinase pathway was not involved in the NaSal-induced apoptotic pathway in the p53-independent Y8 cells.

Apoptosis induced by inhibitors of nucleotide synthesis in deoxyadenosine-resistant leukemia L1210 cells that lack p53 expression

An L1210 cell line (Y8) selected for resistance to deoxyadenosine contains ribonucleotide reductase that is not subject to inhibition by dATP. In addition, the Y8 cells have other phenotypic expressions that include increased sensitivity to apoptosis induced by various agents such as radiation, doxorubicin, anisomycin and roscovitine. The Y8 cells were found to be more sensitive to apoptosis induced by methotrexate (MTX), tiazofurin (TZ), deoxyguanosine (dGuo) and N-(phosphonoacetyl)-L-aspartate (PALA). Deoxyguanosine, at concentrations that did not cause apoptosis in the Y8 cells, prevented the apoptotic response of the Y8 cells to MTX and TZ. Deoxycytidine had no effect. Since caspase-3 activation is involved in apoptotic pathways, the effects of the caspase-3 inhibitor, Ac-DEVD-CHO, were studied on the dGuo-, MTX- or TZ-induced apoptosis in the Y8 cells. Ac-DEVD-CHO caused a marked decrease in the fraction of cells in the early phase of apoptosis. However, there was a corresponding increase in the fraction of cells in the late apoptotic/necrotic stages of cell death. This is in marked contrast to the dGuo-induced decrease in apoptosis seen in the MTX- and TZ-treated Y8 cells in which there were no increases in the late apoptotic/necrotic fraction of cells. These data show that alterations of nucleotide pools in the Y8 cells cause marked increases in the apoptotic response which may indicate that the Y8 cells are much more susceptible to the effects of misincorporation of nucleotides into DNA than are the parental WT L1210 cells.

Altered sensitivity of deoxyadenosine-resistant mouse leukemia L1210 cells to apoptosis induced by 7-hydroxystaurosporine

The deoxyadenosine-resistant mouse leukemia L1210 cell line (Y8) has previously been shown to be more sensitive to apoptosis induced by DNA damaging agents and by protein synthesis inhibitors than the parental wild-type L1210 (WT) cells. These responses occur independently of p53 as both cell lines lack wild-type p53 function. Recent evidence suggests that a serine/threonine kinase is involved in the divergent cellular responses of the WT and Y8 cells. In the present study, the effects of 7-hydroxystaurosporine (UCN-01), a relatively specific serine/threonine kinase inhibitor, were examined in the WT and Y8 cells. Both cell lines were equally sensitive to the growth inhibitory effects of UCN-01. However, the Y8 cells accumulated in G0/G1 and became apoptotic. Apoptosis induced by UCN-01 in the Y8 cells was mediated by a caspase-3-like activity which could be partially blocked by Ac-DEVD-CHO, a caspase-3 inhibitor. UCN-01 did not alter the phosphorylation status of cdc2 nor cyclin B1 and cdc2 protein levels in either cell line.

Enhanced roscovitine-induced apoptosis is mediated by a caspase-3-like activity in deoxyadenosine-resistant mouse leukemia L1210 cells

The deoxyadenosine-resistant mouse leukemia L1210 cell line (Y8) has previously been shown to have phenotypic differences that appear to be unrelated to the altered properties observed at the level of ribonucleotide reductase (RR). One of these changes is that the Y8 cells do not express p53. In response to DNA damaging agents, x-irradiation and doxorubicin, both the parental wild-type L1210 (WT) and Y8 cells undergo G2/M arrest, which is consistent with cells lacking wild-type p53 function. However, Y8 cells are much more sensitive to apoptosis induced by these agents than WT cells. Previous studies have also shown that expression of certain genes involved in cell cycle regulation is different between WT and Y8 cells. Recent evidence suggests that a serine/threonine kinase is involved in the divergent cellular responses of these cells. In the present study, the effects of roscovitine, a cyclin-dependent kinase inhibitor, were examined on the WT and Y8 cells. The WT cells blocked in G2/M, whereas Y8 cells became apoptotic. Apoptosis induced by roscovitine in the Y8 cells was mediated by a caspase-3-like activity. NF kappa B was activated to a much greater extent by roscovitine in the WT cells than in Y8 cells. The data also indicate that cyclin B1/cdc2 plays a role in the divergent p53-independent G2/M block and apoptotic responses of the WT and Y8 cells, respectively. Several key factors such as cathepsin B, caspase-1, release of cytochrome c into the cytosol, TNF-alpha signaling, FasL/Fas signaling, c-myc overexpression, and E2F-1 overexpression and induction were shown not to be involved in the apoptotic pathway(s) in the Y8 cells.

Lack of competition of substrates for P-glycoprotein in MCF-7 breast cancer cells overexpressing MDR1

The MCF-7/Adr cells overexpress MDR-1 which contributes to the drug-resistant phenotype. Our studies show: 1. The retention of daunomycin in the MCF-7/Adr cells relates to a temperature-dependent and energy-dependent process. 2. The MCF-7/Adr cells retain less rhodamine-123 than the parental MCF-7 cells. 3. The MCF-7/Adr cells retain less daunomycin than the parental MCF-7 cells as measured by mean fluorescence or radioactive daunomycin. 4. Cyclosporin A and verapamil effectively block the effluxes of rhodamine-123 and daunomycin from the MCF-7/Adr cells. 5. On short-term incubation, 2-deoxyglucose lowers the NTP levels to a greater extent than sodium azide, showing the importance of glycolysis in the MCF-7 cell lines. 6. Although the MCF-7/Adr cells show cross-resistance to VP-16, actinomycin D and vinblastine, these drugs do not compete with daunomycin for the efflux pump. 7. These data suggest that either there must be multiple MDR-1 pumps that differ in substrate specificity or that there are distinct substrate sites on MDR-1.

Triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone; 3-AP): an inhibitor of ribonucleotide reductase with antineoplastic activity

The enzyme RR catalyzes the conversion of ribonucleoside diphosphates to their deoxyribonucleotide counterparts. RR is critical for the generation of the cytosine, adenine, and guanine deoxyribonucleotide 5'-triphosphate building blocks of DNA, which are present in cells as exceedingly small intracellular pools. Therefore, interference with the function of RR might well result in an agent with significant antineoplastic activity, particularly against rapidly proliferating tumor cells. HUr is the only inhibitor of RR in clinical usage; this agent, however, is a relatively poor inhibitor of the enzyme and has a short serum half-life. Consequently, HUr is a relatively weak anticancer agent. In an effort to develop a more potent inhibitor of RR with utility as an anticancer agent, we have synthesized 3-AP and demonstrated (a) potent inhibition of L1210 leukemia cells in vitro, (b) curative capacity for mice bearing the L1210 leukemia, (c) marked inhibition of RR, and (d) sensitivity of HUr-resistant cells to 3-AP. These findings collectively demonstrate the clinical potential of 3-AP as an antineoplastic agent.

Mouse leukemia L1210 cells selected for resistance to the ribonucleotide reductase inhibitor, 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone show altered response to DNA damaging agents

An L1210 cell line (MQ-580) selected for resistance to the ribonucleotide reductase inhibitor, 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone (MAIQ), had been previously shown to have altered properties related to the non-heme iron subunit of ribonucleotide reductase (RR). In addition, the MQ-580 cells had other metabolic alterations that were consistent with multidrug resistance. Neither the wild-type (WT) nor the MQ-580 cells showed a G0/G1 block in response to X-irradiation. Since the MQ-580 cells also showed resistance to adriamycin (ADR), the WT cells were more sensitive to the effects of ADR than the MQ-580 cells. However, when the MQ-580 cells were treated with the combination of ADR plus verapamil, the MQ-580 cells showed cellular responses that included cell cycle block in G2/M and increased apoptosis. The WT cells, in response to the combination of ADR plus verapamil, blocked in the S-phase of the cell cycle with an increased necrotic cell population in comparison to treatment with ADR alone. While MAIQ could be shown to cause an apoptotic response in the WT and MQ-580 cells, the concentrations of MAIQ required to induce apoptosis were 20- to 40-times the IC50 value. However, even though the MQ-580 cells were 8-fold more resistant to MAIQ than the WT cells, the MQ-580 cells were more sensitive to MAIQ-induced apoptosis. These data show that alterations at the RR site lead to phenotypic expressions not overtly related to RR. However, because of the critical role that RR plays in cell division and DNA repair, the changes observed with respect to cell cycle and apoptosis, may in fact, be a direct consequence of the alteration at RR.

Effect of doxorubicin on wild-type and deoxyadenosine-resistant mouse leukemia L1210 cells

Wild-type (WT) mouse leukemia L1210 cells express steady-state levels of the mRNA and protein for p53. However, the p53 expressed by the wild-type cells is a mutant form of p53. A deoxyadenosine-resistant L1210 cell line (Y8) derived from the parental WT L1210 cells does not maintain constitutive levels of p53 mRNA or protein. Upon DNA damage, induced by doxorubicin, neither the WT nor the Y8 cells block in G0/G1; the cells block in G2/M. However, treatment of the Y8 cells with doxorubicin results in a much greater fraction of cells becoming apoptotic compared to the WT cells. Doxorubicin treatment resulted in the induction of p53 mRNA in the WT cells, but not the Y8 cells. WAF1, c-myc and Bax mRNAs were also induced by doxorubicin in the WT cells but not in the Y8 cells. The constitutive levels of WAF1 and Gadd45, unexpectedly seen in the p53-deficient Y8 cells, decreased following doxorubicin treatment. The comparison of the effects of DNA damage, as measured by mRNA levels, induced by X-irradiation or doxorubicin were found to vary between the WT and Y8 cells and for the particular mRNA studied. The effect of doxorubicin or X-irradiation on the cell cycle could be overridden in the WT cells by caffeine. Comparisons of DNA damage induced by doxorubicin or X-irradiation show that although the Y8 cells are more sensitive to these damaging agents than the WT cells, the effects on gene expressions are not identical.

Effect of ionizing radiation on wild-type and mutant mouse leukemia L1210 cells

Wild-type (WT) mouse leukemia L1210 cells express steady-state levels of p53 mRNA and protein. However, the p53 expressed by the wild-type L1210 cells was found to be a mutant form of p53 (relative to normal mouse fibroblast p53 sequence) having a point mutation in the DNA binding domain of p53. A deoxyadenosine-resistant L1210 cell line (Y8) derived from the parental WT cells had previously been shown to lack the expression of p53 but to respond to cycloheximide (CHX) treatment by superinduction of p53 mRNA. The mRNA for p53 induced by CHX had the same sequence as the p53 from normal mouse fibroblasts. Although the Y8 cells had no constitutive levels of p53 mRNA or protein, the Y8 cells expressed constitutive levels of WAF1 mRNA and protein. Gadd45 mRNA was also present in the Y8 cells. Subjecting the WT or Y8 cells to ionizing radiation did not result in a G0/G1 cell cycle block; the cells blocked in G2/M. The Y8 cells were much more sensitive to the irradiation treatment than the WT cells, resulting in marked increases in apoptosis in the Y8 cells. Although radiation treatment induced p53 mRNA, but no p53 protein, in the Y8 cells, WAF1 mRNA was induced in the Y8 cells. These data indicate that there are p53-independent pathway(s) that may still involve WAF1 and Gadd45 with respect to cell cycle control and apoptosis.

Altered sensitivity of deoxyadenosine-resistant mouse leukemia L1210 cells to various kinase inhibitors

The deoxyadenosine-resistant mouse leukemia L1210 cell line (Y8) has previously been shown to have phenotypic differences that appear unrelated to the altered properties observed at the level of ribonucleotide reductase (RR). In response to various stress factors, the parental wild-type (WT) L1210 cell line undergoes cell cycle arrest; Y8 cells become apoptotic. These responses are p53-independent. Cell cycle regulation also appears different between the two cell lines, suggesting that Y8 cells are more apoptotic because of alterations in their cell cycle compared to WT cells. In order to study the relationships between cell cycle regulation and apoptosis, the effects of 2-aminopurine (2-AP), wortmannin, and PD98059, were studied on WT and Y8 cells. 2-AP induced G2/M block in both WT and Y8 cells with differences in G0/G1 and S phase contents between the two cell lines. Wortmannin induced G0/G1 block in Y8 cells, while exhibiting no effect on WT cells. PD98059 had no effect on the cell cycle of either WT or Y8 cells. In response to each inhibitor, Y8 cells underwent apoptosis to a much greater extent than the parental WT cell line. These data suggest that the specific pathways that converge on the cell cycle are altered and may be involved in the differences between a tumor cell to block in cell cycle or to undergo apoptosis.

p53-independent anisomycin induced G1 arrest and apoptosis in L1210 cell lines

An L1210 cell line selected for resistance to deoxyadenosine (Y8) has been shown to have barely detectable levels of p53 mRNA and no measurable p53 protein in comparison to the parental mouse wild-type (WT) L1210 cells. We now show that the protein synthesis inhibitors, anisomycin and cycloheximide, arrest WT cells in the G1 phase of the cell cycle and induce apoptosis in Y8 cells via a p53-independent mechanism. There was a decrease in Rb phosphorylation but without the induction of WAF1 protein. Anisomycin treatment activated NF kappa B in the WT cells as early as 0.5 hr after treatment but did not activate NF kappa B in the Y8 cells. Cycloheximide was neither as potent as anisomycin in arresting WT cells at G1 and inducing apoptosis in Y8 cells, nor as potent in decreasing Rb phosphorylation. The finding that caffeine could override the G1 arrest induced by anisomycin and cycloheximide in the WT cells further supports the idea that the effects of anisomycin or cycloheximide on the cell cycle are at the level of cell cycle regulation, and are likely not mediated exclusively through the inhibition of protein synthesis.

Cellular responses in mouse leukemia L1210 cells made resistant to deoxyadenosine

Recent studies have implicated nucleotides in diverse and unexpected functions related to p53 levels, p53-dependent G0/G1 cell cycle arrest, and the role of dATP in the activation of the caspase-induced apoptosis. Using deoxyadenosine-resistant L1210 cells (ED2 and Y8) that had ribonucleotide reductase that was not sensitive to inhibition by dATP and also exhibited other metabolic alterations, the properties of these cells with respect to the role(s) of nucleotides in these functions were explored. In the ED2 and Y8 cells that did not express p53 protein, the pools of UTP, CTP, ATP, and GTP were markedly decreased. The decreased cellular levels of UTP and CTP did not result in these cells being more sensitive to either PALA or acivicin. The ED2 and Y8 cells did not block in G0/G1 in response to PALA treatment even though the basal cellular concentrations of UTP and CTP were reduced 50 to 80%. While it has been shown that dATP in combination with cytochrome c is involved in the apoptotic pathway, the concentration of exogenous deoxyadenosine required to induce apoptosis in the parental L1210 cells was far in excess of the concentration required to inhibit cell growth. Deoxyadenosine did not cause an increase in apoptosis in the deoxyadenosine-resistant Y8 cells. These data suggest that the new roles ascribed to nucleotides may be specific for the particular cell type under very specific conditions.

Alterations in the properties of mouse leukemia L1210 cell lines selected by different methods for resistance to deoxyguanosine

Mouse leukemia L1210 cells were generated for resistance to deoxyguanosine by two different methods. In one case the L1210 cells were subjected to gradual increases in deoxyguanosine (dGuo-R); in the second approach, the cells were subjected to deoxyguanosine at a concentration ten times the IC50 value and plated out on soft agar (D-92). The dGuo-R and D-92 cell lines had different phenotypic expressions. The dGuo-R cells showed a higher degree of resistance to dGuo than the D-92 cells. The levels of resistance to other cytotoxic drugs such as araC or 2-chloro-2'-deoxyadenosine did not necessarily correlate with the degree of resistance to dGuo. Deoxycytidine kinase activity was decreased in both of the cell lines, although there was a larger decrease in the dGuo-R cell line. The levels of kinase activities toward the other substrates were not all coordinately decreased in these cell lines. The degree of resistance of these cell lines to dGuo cannot be ascribed solely to an alteration at the deoxycytidine kinase site.

Overexpression of the multidrug resistance genes mdr1, mdr3, and mrp in L1210 leukemia cells resistant to inhibitors of ribonucleotide reductase

L1210 MQ-580 is a murine leukemia cell line resistant to the cytotoxic activity of the alpha-(N)-heterocyclic carboxaldehyde thiosemicarbazone class of inhibitors of ribonucleotide reductase. The line is cross-resistant to etoposide, daunomycin, and vinblastine. L1210 MQ-580 cells expressed 8-fold resistance to 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP), a relatively newly developed inhibitor of ribonucleotide reductase. The accumulation of [14C]3-AP by L1210 MQ-580 cells was 5- to 6-fold less than by parental L1210 cells. An increased rate of efflux of 3-AP was responsible for the lower steady-state concentration of 3-AP in resistant cells. In reverse transcription-polymerase chain reaction assays, L1210 MQ-580 cells were found to overexpress the multidrug resistance genes mdr1, mdr3, and mrp, but not the mdr2 gene, compared with parental L1210 cells. Measurement of the steady-state concentration of doxorubicin, a potential substrate for both the mdr and mrp gene products, demonstrated that L1210 MQ-580 cells accumulated 4-fold less anthracycline than parental cells. These findings indicate that drug efflux is a major determinant of the pattern of cross-resistance of L1210 MQ-580 cells. To extrapolate these observations to the human homologues of the mdr1, mdr3, and mrp murine genes, the effects of 3-AP were measured in L1210/VMDRC0.06 and NIH3T3 36-8-32 cells transfected with human MDR1 and MRP cDNAs, respectively. The transfectants were 2- to 3-fold resistant to the cytotoxic effects of 3-AP and accumulated less [14C]3-AP than their parental mock-transfected counterparts. Moreover, the cytotoxic activity of 3-AP was significantly greater in two double mrp gene knockout cell lines than in parental W 9.5 embryonic stem cells. Thus, the results suggest that 3-AP is a substrate for both the P-glycoprotein and MRP and that baseline MRP expression has the capacity to exert a protective role against the toxicity of this agent.

Altered efflux properties of mouse leukemia L1210 cells resistant to 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone

A mouse leukemia L1210 cell line, denoted MQ-580, that was selected for resistance to the ribonucleotide reductase inhibitor, 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone (MAIQ), in addition to having altered properties at the ribonucleotide reductase site had other alterations that contributed to its resistant phenotype; these included the expression of p-glycoprotein and the multi-drug resistance associated protein (MRP). The efflux of rhodamine 123 (Rh-123) or daunomycin (Dau) was greatly increased in MQ-580 cells compared to parental wild-type (WT) cells. The effluxes of Rh-123 and Dau were ATP- and temperature-dependent. The p-glycoprotein inhibitors, verapamil, cyclosporin A and reserpine blocked the efflux of both Rh-123 and Dau. In contrast, the inhibitors of MRP, MK571, BSO-treatment, arsenite and genistein did not block the efflux of either Rh-123 or Dau from MQ-580 cells. These findings suggest that the p-glycoprotein is the major transporter involved in effluxing Rh-123 and Dau from MQ-580 cells.

Cellular delivery of nucleoside diphosphates: a prodrug approach

PURPOSE

This study is concerned with cellular delivery/generation of 2'-azido-2'-deoxyuridine and -deoxycytidine diphosphate (N3UDP or N3CDP), potent inhibitors of ribonucleotide reductase. It characterizes the phosphorylation steps involved in the conversion of 2'-azido-2'-deoxyuridine (N3Urd) and 2'-azido-2'-deoxycytidine (N3Cyd) to the corresponding diphosphates and explores a prodrug approach in cellular delivery of the inhibitor which circumvents the requirement of deoxynucleoside kinases.

METHODS

Cell growth of CHO and 3T6 cells of known deoxycytidine kinase level was determined in the presence of N3Urd and N3Cyd. Activity of ribonucleotide reductase was determined in the presence of the azidonucleosides as well as their mono- or di-phosphates in a Tween 80-containing permeabilizing buffer. A prodrug of 5'-monophosphate of N3Urd was prepared and its biological activity was evaluated with CHO cells as well as with cells transfected with deoxycytidine kinase.

RESULTS

N3Urd failed to inhibit the growth of both cell lines, while N3Cyd was active against 3T6 cells and moderately active against CHO cells. These results correlate with the deoxycytidine kinase levels found in the cells. Importance of the kinase was further established with the finding that the nucleoside analogs were inactive as reductase inhibitors in a permeabilized cell assay system while their mono- and di-phosphates were equally active. The prodrug was active in cell growth inhibition regardless of the deoxycytidine kinase level.

CONCLUSIONS

The azidonucleosides become potent inhibitors of the reductase by two sequential phosphorylation steps. The present study indicates that the first step to monophosphate is rate-limiting, justifying a prodrug approach with the monophosphate.

Deoxyadenosine-resistant mouse leukemia L1210 cell lines with alterations in early response genes and p53

L1210 cell lines selected for resistance to deoxyadenosine exhibit altered steady-state levels of the mRNA for the early response genes and p53. In the deoxyadenosine-resistant cell lines (Y8 and ED2), the levels of the mRNAs for p53 and c-jun were markedly decreased while the steady-state levels for mRNAs for c-myc, c-fos and jun B were elevated in the Y-8 and ED2 cell lines. The levels of the mRNAs for PCNA and c-myb were the same in the wild type and mutant cell lines. The levels of the mRNAs for krox-24 were extremely low in the wild type and mutant cell lines. Cycloheximide (CHX) treatment of the cells resulted in the increase in the mRNA levels for c-jun, jun B, krox 24 and p53 in the Y-8 and ED2 cell lines. The time courses and the extents of the increases in the mRNA levels following CHX treatment were not the same for all of these mRNAs. The level of p53 RNA increased with no lag following CHX treatment while the levels of the mRNAs for c-myc, c-jun and krox-24 increased after a one-hour lag period. The level of the mRNA for p53 and c-myc increased 20- and 7-fold, respectively while the mRNA level for knox-24 increased 80-fold following CHX treatment. The Y8 and ED2 cell lines that lack steady-state levels of p53 show decreased sensitivity to cisplatin and increased frequency of gene amplification as measured by PALA resistance in a manner similar to other cell lines lacking p53. On the other hand, the ED2 and Y8 cell lines do not show a G1-block in response to PALA treatment. The cell lines appear to offer an experimental system in which to study the interactions between/among these early response genes and the p53-dependent and independent pathways.

Synthesis and biological activity of 3- and 5-amino derivatives of pyridine-2-carboxaldehyde thiosemicarbazone

A series of 3- and 5-alkylamino derivatives, as well as other structurally modified analogues of pyridine-2-carboxaldehyde thiosemicarbazone, have been synthesized and evaluated as inhibitors of CDP reductase activity and for their cytotoxicity in vitro and antineoplastic activity in vivo against the L1210 leukemia. Alkylation of 3- and 5-amino-2-(1,3-dioxolan-2-yl)pyridines (1, 2) resulted in corresponding 3-methylamino, 5-methylamino, 3-allylamino, 5-ethylamino, 5-allylamino, 5-propylamino, and 5-butylamino derivatives (5, 6, and 11-15), which were then condensed with thiosemicarbazide to yield the respective thiosemicarbazones (7, 8, and 16-20). Oxidation of 3,5-dinitro-2-methylpyridine (21) with selenium dioxide, followed by treatment with ethylene glycol and p-toluenesulfonic acid, produced the cyclic ethylene acetal, 23. Oxidation of 2-(1,3-dioxolan-2-yl)-4-methyl-5-nitropyridine (26) with selenium dioxide, followed by sequential treatment with sodium borohydride, methanesulfonyl chloride, and morpholine afforded the morpholinomethyl derivative 30. Catalytic hydrogenation of 23 and 30 with Pd/C yielded the corresponding amino derivatives 24 and 31. Catalytic hydrogenation of 5-cyano-2-methylpyridine (33) with Raney nickel, followed by treatment with acetic anhydride, gave the amide derivative 35. N-Oxidation of 35, followed by rearrangement with acetic anhydride, produced the acetate derivative, 5-[(acetylamino)methyl]-2-(acetoxymethyl)pyridine (37). Repetition of the N-oxidation and rearrangement procedures with compound 37 yielded the diacetate derivative 39. Condensation of compounds 24, 31, and 39 with thiosemicarbazide afforded the respective 3,5-diaminopyridine-, 4-(4-morpholinylmethyl)-5-aminopyridine-, and 5-(aminomethyl)pyridine-2-carboxaldehyde thiosemicarbazones (25, 32, and 40). The most biologically active compounds synthesized were the 5-(methylamino)-, 5-(ethylamino)-, and 5-(allylamino)pyridine-2-carboxaldehyde thiosemicarbazones (8, 17, and 18), which were potent inhibitors of ribonucleotide reductase activity with corresponding IC50 values of 1.3, 1.0, and 1.4 microM and which produced significant prolongation of the survival time of L1210 leukemia-bearing mice, with corresponding optimum % T/C values of 223, 204, and 215 being obtained when administered twice daily for six consecutive days at dosages of 60, 80, and 80 mg/kg, respectively.

Characterization of mouse leukemia L1210 cells resistant to the ribonucleotide reductase inhibitor 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone

Mouse leukemia L1210 cells were generated for resistance to 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone (MAIQ), a potent inhibitor of ribonucleotide reductase that is directed in the nonheme iron subunit (NHI) of the enzyme. The resistant cells, MQ-580, showed an 8-fold increase in IC50 toward MAIQ, a 4-fold increase in IC50 toward hydroxyurea, and also showed resistance to other ribonucleotide reductase inhibitors. In addition, the MQ-580 cell line was resistant to nonribonucleotide reductase inhibitors such as etoposide, daunomycin and vinblastine, but not to cisplatin. The mRNA for the NHI subunit was increased 7-fold in the MQ-580 cells with essentially no change in the mRNA level for the effector-binding subunit. The ribonucleotide reductase activity in the cell-free extracts prepared from the MQ-580 cells was only slightly elevated (30%). However, passage of the cell-free extract from the MQ-580 cells over Sephadex G-25 resulted in a 4.8-fold increase in specific activity over that of the wild-type cells. While the reductase activity in the cell-free extract from the MQ-580 cells did not show altered sensitivity to MAIQ, the reductase activity in the cell-free extract from the MQ-580 cells was much more sensitive to the effects of the iron-chelating agents Desferal and EDTA. The cell pellets from the MQ-580 cells were much darker in color than the pellets from the wild-type cells or hydroxyurea-resistant cells. The supernatant fraction from the MQ-580 cells after-SDS-PAGE showed the appearance of a strong Coomassie blue-staining band at 50 kDA that was not apparent in either the wild-type or hydroxyurea-resistant cells. This new resistant cell line offers an opportunity to explore differences in resistance mechanisms of drugs (e.g. MAIQ and hydroxyurea) that are directed at the same subunit of ribonucleotide reductase.

Differences in the properties of mammalian ribonucleotide reductase toward its substrates

These studies, using three different reagents, show that the substrate properties of ribonucleotide reductase are specific but can be variable depending upon the nature of the interaction of the reagent with the holoenzyme or the individual subunit. Etheno-CDP, which acts as a competitive inhibitor with respect to CDP, interacts with the active site of the holoenzyme. This interaction was the result of rather tight structural requirements as epsilon-ADP did not result in a similar level of inhibition of either CDP or ADP reductase activities. The YL 1/2 antibody which binds very tightly to the NHI subunit has a much greater effect on CDP reductase activity than ADP reductase activity. The nonapeptide that corresponds to the C-terminus amino acid sequence of the NHI subunit and which binds to the EB subunit and aborts the formation of the NHI-EB active complex has a greater effect on ADP reductase activity than on CDP reductase activity. The use of reagents such as these can be helpful in dissecting the subtle but important differences in the substrate properties of mammalian ribonucleotide reductase.

Structure-function relationships for a new series of pyridine-2-carboxaldehyde thiosemicarbazones on ribonucleotide reductase activity and tumor cell growth in culture and in vivo

The synthesis of a new series of pyridine-2-carboxaldehyde thiosemicarbazones (HCTs) that have amino groups in the 3- and 5-positions has allowed the comparison of the structure/function relationships with regard to inhibition of ribonucleotide reductase activity, L1210 cell growth in culture and L1210 leukemia in vivo. 3-Aminopyridine-2-carboxaldehyde thiosemicarbazones are more active than the corresponding 3-hydroxy-derivatives. The 3-amino-2-pyridine carboxaldehyde thiosemicarbazones were also more active then the 5-amino-2-carboxaldehyde thiosemicarbazones in inhibiting ribonucleotide reductase activity and L1210 cell growth in culture and in vivo. N-Acetylation of the 3-amino derivative resulted in a compound that was much less active both in vitro and in vivo; N-acetylation of the 5-amino derivative did not alter the in vitro inhibitory properties, but did eliminate the antitumor properties in vivo. When the most active HCTs were studied in more detail, it was found that the incorporation of [3H]thymidine into DNA was inhibited completely without the inhibition of [3H]uridine incorporation into RNA. Further, the conversion of [14C]cytidine to deoxycytidine nucleotides and incorporation into DNA was inhibited by the HCTs without an effect on the incorporation of cytidine into RNA. These data support the conclusion that ribonucleotide reductase is the major site of action of these HCTs. The 3-aminopyridine-2-carboxaldehyde thiosemicarbazones emerge as strong candidates for development for clinical trials in cancer patients.

Inhibitors of ribonucleotide reductase. Comparative effects of amino- and hydroxy-substituted pyridine-2-carboxaldehyde thiosemicarbazones

A new series of alpha-(N)-heterocyclic carboxaldehyde thiosemicarbazones (HCTs) was studied for their effects on L1210 cell growth in culture, cell cycle transit, nucleic acid biosynthesis and ribonucleotide reductase activity. 3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) and 3-amino-4-methylpyridine-2-carboxaldehyde thiosemicarbazone (3-AMP) were the most active compounds tested with respect to inhibition of cell growth and ribonucleotide reductase activity. 5-Aminopyridine-2-carboxaldehyde thiosemicarbazone (5-AP) and 4-methyl-5-aminopyridine-2-carboxaldehyde thiosemicarbazone (5-AMP) were slightly less active. 3-AP, 3-AMP, 5-AP and 5-AMP inhibited the incorporation of [3H]thymidine into DNA without affecting the rate of incorporation of [3H]uridine into RNA. The uptake and incorporation of [14C]cytidine into cellular ribonucleotides and RNA, respectively, were not decreased by 3-AP or 3-AMP; however, the incorporation of cytidine into DNA via ribonucleotide reductase was inhibited markedly. Thus, a pronounced decrease in the formation of [14C]deoxyribonucleotides from radioactive cytidine occurred in the acid-soluble fraction of 3-AP- and 3-AMP-treated L1210 cells. Consistent with an inhibition of DNA replication that occurred at relatively low concentrations of 3-AP and 3-AMP, cells gradually accumulated in the S-phase of the cell cycle; at higher concentrations of 3-AP and 3-AMP, a more rapid accumulation of cells in the G0/G1 phase of the cell cycle occurred, with the loss of the S-phase population, implying that a second less sensitive metabolic lesion was created by the HCTs. N-Acetylation of 3-AMP resulted in a compound that was 10-fold less active as an inhibitor of ribonucleotide reductase activity and 8-fold less active as an inhibitor of L1210 cell growth. N-Acetylation of either 5-AP or 5-AMP did not alter the inhibitory properties of these compounds. The results obtained provide an experimental rationale for the further development of the HCTs, particularly 3-AP and 3-AMP, as potential drugs for clinical use in the treatment of cancer.

Substituted 2-acylpyridine-alpha-(N)-hetarylhydrazones as inhibitors of ribonucleotide reductase activity and L1210 cell growth

A series of substituted 2-acylpyridine-alpha-(N)-hetarylhydrazones was prepared and studied for their effects on mammalian ribonucleotide reductase activity using a highly purified enzyme preparation from Ehrlich tumor cells and on mouse leukemia L1210 cell growth in culture. Pyridine-2-aldehyde-2-pyridylhydrazone (PH 22), ethyl-2-pyridylketone-I-phthalazinylhydrazone (PH 22-25) and pyridine-2-aldehyde-2'-quinolylhydrazone (PQ 22) inhibited purified ribonucleotide reductase activity and inhibited L1210 cell growth in culture. PH 22-25 inhibited [3H]thymidine incorporation into DNA and inhibited ribonucleotide reductase activity in situ (as measured bvy [14C]cytidine metabolism and as a result inhibited DNA synthesis. There was no effect on RNA synthesis. These data indicate that these substituted hydrazones are potent inhibitors of tumor cell growth through the inhibition of ribonucleotide reductase.

2'-Deoxy-2'-methylene derivatives of adenosine, guanosine, tubercidin, cytidine and uridine as inhibitors of L1210 cell growth in culture

The 2'-deoxy-2'-methylene derivatives of adenosine (MdAdo), guanosine (MdGuo), tubercidin (MdTu), cytidine (MdCyd) and uridine (MdUrd) were synthesized as mechanism-based inhibitors directed at ribonucleotide reductase. It was shown that MdCyd 5'-diphosphate irreversibly inactivated ribonucleotide reductase from Escherichia coli (Baker et al., J Med Chem 34: 1879-1884, 1991). In studies reported here, MdAdo/EHNA, MdGuo and MdCyd inhibited L1210 cell growth with IC50 values of 3.4, 10.6 and 1.4 microM, respectively. Since MdAdo is a substrate for adenosine deaminase, the presence of EHNA was required to give maximal growth inhibition. 8-Aminoguanosine was not required to maximize the cytotoxic effects of MdGuo. The 2'-deoxy-2'-methylene derivatives of tubercidin and uridine did not inhibit L1210 cell growth at concentrations as high as 50 microM (MdTu) or 100 microM (MdUrd). L1210 cell lines resistant to hydroxyurea (directed at the non-heme iron subunit of ribonucleotide reductase) or deoxyadenosine (directed at the effector binding subunit of ribonucleotide reductase) were not resistant to MdCyd. An L1210 cell line that was highly resistant to dGuo due to the loss of a relatively specific deoxyribonucleoside kinase (Cory et al., J Biol Chem 268: 405-409, 1993) had a 6.6-fold increase in the IC50 value toward MdCyd, but showed only a 2-fold increase in resistance to MdGuo. Another L1210 cell line that was markedly deficient in adenosine kinase activity was highly resistant to MdAdo. Analysis by flow cytometry showed that MdCyd showed the transit of the cells through the G2/M phase of the cell cycle resulting in the buildup of the G2/M population. MdAdo, MdGuo and MdCyd inhibited the incorporation of [14C]cytidine into DNA without an effect on RNA synthesis or total cellular uptake of [14C]cytidine. The conversion of [14C]cytidine to deoxycytidine nucleotides was partially inhibited by MdGuo, but not by MdAdo or MdCyd. These data show that the 2'-deoxy-2'-methylene derivatives of adenosine, guanosine and cytidine are activated via specific nucleoside kinases and that the modes of action of these compounds are not identical.

Use of nucleoside kinase-deficient mouse leukemia L1210 cell lines to determine metabolic routes of activation of antitumor nucleoside analogs

Mouse leukemia L1210 cell lines that were selected for resistance to deoxyguanosine (dGuo-R) or lacked adenosine kinase activity (ED2) were used to evaluate the nature of the nucleoside kinase that was required to phosphorylate nucleoside analogs to their respective active nucleotide form. The dGuo-R cells had reduced levels of kinase activity toward araC, dGuo and 2-CldAdo as substrates with essentially no loss of activity toward dCyd. This cell line showed resistance to dGuo, araC, araG, FdAdo, and Fara A but not to dAdo or araA. The ED2 cell line was resistant to pyrazofurin and 6-methylmercaptopurine riboside and to araA/EHNA but not to 2-CldAdo or 2-Cl-2'-FaraA. The study of the effects of newer nucleoside analogs such as dFdCyd, MdAdo, MdCyd and MdGuo in these cell lines showed that some of these agents are primarily phosphorylated by deoxyribonucleoside kinase (dFdCyd) or by adenosine kinase (MdAdo) or in some instances by multiple kinases (FaraA). These cell lines will be useful in defining the nature of the kinase(s) responsible for activating new nucleoside analogs and defining cross-resistance patterns.

Lack of immunosuppressive effect of low-dose oral methotrexate on lymphocytes in rheumatoid arthritis

Whether methotrexate (MTX) is effective in rheumatoid arthritis (RA) because of immunosuppressive and/or anti-inflammatory mechanisms of action is controversial. Many lines of investigation point to the latter. We evaluated DNA synthesis in peripheral blood lymphocytes (PBL) from 33 RA patients on oral MTX (7.5-15 mg/wk) and in 30 healthy controls by flow cytometric cell cycle analysis (CCA). DNA synthesis was also evaluated with a thymidilate synthetase activity assay (TSA) (3H-deoxyuridine incorporation) in 12 patients and 21 controls (12 on MTX and NSAID, and 9 healthy subjects). The patients had taken MTX for at least 3 months and were in different stages of clinical activity. There were no significant differences in TSA or in the cell cycle phase distributions (especially the S phase) between treated RA patients and controls. These data suggest that low-dose oral MTX does not inhibit DNA synthesis and therefore does not have an immunosuppressive effect on lymphocytes from patients with RA.

Deoxyguanosine-resistant leukemia L1210 cells. Loss of specific deoxyribonucleoside kinase activity

A mouse leukemia L1210 cell line was selected for resistance to deoxyguanosine. The deoxyguanosine-resistant cells (dGuo-R) were 126-fold less sensitive to deoxyguanosine than the wild-type cells. The IC50 values for araC and araG were increased, but only 10-12-fold in the dGuo-R cells when compared with the wild-type cells. The dGuo-R cell line showed an increased level of resistance to 2-fluoro-2'-deoxyadenosine and 2-fluoroadenine arabinoside (11-14-fold), but essentially no increase in resistance to deoxyadenosine or adenine arabinoside. Deoxyribonucleoside kinase activity was decreased only slightly (19%) when deoxycytidine was utilized as substrate; when cytosine arabinoside or deoxyguanosine was used as the substrate, the kinase activity in the extracts from the dGuo-R cells was only 10% of the enzyme activity in the extracts from the wild-type cells. The determination of the kinetic parameters, Km and Vmax, indicated that there were marked decreases in the Vmax values for deoxyguanosine and cytosine arabinoside as substrates, but not for deoxycytidine as substrate; the Km values for deoxycytidine and cytosine arabinoside were increased in the extracts from the dGuo-R cells. By use of high-performance liquid chromatography, the kinase activities in the extracts from the wild-type and resistant cells could be resolved. There was the specific loss of kinase activity toward cytosine arabinoside and deoxyguanosine as substrates. These data indicate that the dGuo-R cells have decreased levels of a specific deoxyribonucleoside kinase activity.

Structural aspects of N-hydroxy-N'-aminoguanidine derivatives as inhibitors of L1210 cell growth and ribonucleotide reductase activity

Previous studies have shown that N-hydroxy-N'-aminoguanidine (HAG) derivatives [RCH = NNHC(= NH)NHOH-tosylate] inhibit ribonucleotide reductase activity and block the growth of leukemia L1210 cells and human colon carcinoma, HT-29, cells in culture. In the current studies, the role of the side chains and the location of the bond of the side chain moiety to HAG were investigated using a new series of HAG derivatives which contained as the R-group--cyclohexyl, phenyl-, pyridyl- or napthyl moieties. The effects of these compounds as inhibitors of L1210 cell growth and ribonucleotide reductase activity were compared with the parent compound. N-hydroxy-N'-aminoguanidine was less inhibitory to ribonucleotide reductase activity and L1210 cell growth than hydroxyurea. The phenyl-HAG compounds which included 1-benzyloxybenzylidene- and 4-cyclohexylmethoxybenzylidene-HAG inhibited CDP reductase with IC50s which ranged from 50-110 microM. 1-Naphthylmethylene-HAG was more inhibitory than 2-naphthylmethylene-HAG and more inhibitory than the phenyl-HAG compounds. 2-Pyridylmethylene-HAG was more inhibitory than 3-pyridylmethylene- or 4-pyridylmethylene-HAG. While HAG inhibited CDP and ADP reductase activities essentially to the same extent, the HAG-derivatives inhibited ADP reductase activity to a greater extent than CDP reductase activity. Cyclohexylmethylene-HAG did not inhibit either L1210 cell growth or ribonucleotide reductase activity. There was good correlation between the inhibition of ribonucleotide reductase activity and L1210 cell growth by these HAG-derivatives. These data indicate that not only is the nature of the side chain substitution important, but also the location of the HAG-moiety on the ring position.

Altered steady-state levels of the messenger RNAs for c-myc and p53 in L1210 cell lines resistant to deoxyadenosine

L1210 cell lines, selected for resistance to deoxyadenosine due to the loss of allosteric inhibition of ribonucleotide reductase by dATP, had altered steady-state levels of the mRNAs for c-myc, fos, and p53. Wild-type L1210 cells had constitutive steady-state levels of c-myc and p53 with little or no fos mRNA. Two different deoxyadenosine-resistant cell lines (Y8 and ED2) had elevated steady-state levels of c-myc and fos but essentially no p53 mRNA. Hydroxyurea-resistant L1210 cells had the same levels of c-myc, fos, and p53 as the wild-type cells. There was no amplification of the gene for c-myc in the Y8 or ED2 cell lines. The half-life for c-myc mRNA was essentially the same in the wild-type and the Y8 and ED2 cells. Nuclear runoff experiments showed that the rates of transcription for c-myc in the Y8 and ED2 cells were elevated and could account for the increased steady-state levels of c-myc in these two cell lines. The transcription rate for p53 mRNA was not decreased in the Y8 and ED2 cells and therefore did not account for the loss of the steady-state levels of p53 in the cells. Cycloheximide treatment of the Y8 and ED2 cells resulted in a marked increase in the steady-state p53 mRNA level, indicating that a protein which was rapidly turned over was responsible for the extremely short half-life of p53 mRNA in these two cell lines.

Factors affecting the mRNA levels for the non-heme iron and effector-binding subunits of ribonucleotide reductase

Ribonucleotide reductase which catalyzes the rate-limiting step in the de novo synthesis of dNTPs is composed of two non-identical protein subunits which are not under coordinate control in terms of synthesis and degradation. The mRNAs for the effector-binding (EB) and non-heme iron (NHI) subunits are likewise not under coordinate control during cell cycle traverse. Inhibitors directed at the specific subunits of ribonucleotide reductase block DNA synthesis. These current studies show that drugs such as IMPY or hydroxyurea which specifically inhibit the NHI subunit cause a marked increase in the steady-state level of the mRNA for the NHI subunit while resulting in a decrease in the level of mRNA for the EB subunit. In cells treated with deoxyadenosine, the patterns of the mRNAs for the NHI and EB subunits were different from those seen in the IMPY- or hydroxyurea-treated cells. Control experiments utilizing inhibitors (aphidicolin or araC) directed at DNA polymerase showed that the pattern of changes in the mRNA levels for the NHI and EB subunits were specific for the reductase inhibitors. These changes in the mRNAs for the NHI and EB subunits may be due to drug-induced alterations in transcription rates and/or degradation rates for the specific mRNAs.

Use of an aqueous soluble tetrazolium/formazan assay for cell growth assays in culture

A new tetrazolium analog of 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) was evaluated as a substitute for MTT in the microculture screening assay for in vitro cell growth. This new tetrazolium compound, 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2- (4-sulfophenyl)-2H-tetrazolium, inner salt (MTS), in the presence of phenazine methosulfate (PMS), gave a water-soluble formazan product that had an absorbance maximum at 490-500 nm in phosphate-buffered saline. The amount of colored product formed was proportional to the number of cells and the time of incubation of the cells with MTS/PMS. MTS/PMS was reactive in all the cell lines tested which included mouse leukemia L1210 cells, mouse Ehrlich tumor cells, mouse 3T3 fibroblasts, and human colon tumor cells (HT-29). HT-29 and 3T3 fibroblasts reduced MTS/PMS more efficiently than they reduced MTT. Comparable to the amount of product formed from MTT, MTS/PMS gave excellent product formation. The IC50 value for pyrazoloimidazole obtained using MTS/PMS was 200 microM; for 5-fluoro-2'-deoxyuridine, the IC50 value was 0.9 nM. These values compared very favorably with the IC50 values obtained by direct cell counts. Further, the same IC50 values were obtained when the absorbances of the formazan product in the 96-well plates were determined after different times of incubation.

Cell-cycle associated transcriptional regulation of ribonucleotide reductase in L1210 leukemia cells and drug-resistant variants

Previous studies from this laboratory have shown that the steady-state levels of the mRNA for the non-heme iron (NHI) subunit of ribonucleotide reductase were markedly elevated in hydroxyurea-resistant L1210 cell lines with minimal changes in the mRNA levels for the effector-binding (EB) subunit. In the present study, wild-type L1210 cells and their drug-resistant variants [hydroxyurea-resistant (HU-7); deoxyadenosine-resistant (Y-8); and deoxyadenosine/pyrazoloimidazole-resistant (ED2)] were synchronized by EGTA treatment in the G0/G1-phase of the cell cycle. Upon the addition of CaCl2, the cells reentered the cell cycle. The steady-state levels and the transcriptional rates of the mRNAs for the EB subunit and glyceraldehyde-3-phosphate dehydrogenase were measured and found to be similar in the drug-resistant variants compared to the wild-type cells. While the steady-state level of the mRNA for the NHI subunit was increased 35-fold in the HU-7 cell line, the transcription rate was increased only 7-fold. The increase in the transcription rate did not account for the large increase in the steady-state level. These data indicate that the increased steady-state level of the mRNA for the NHI subunit in the HU-7 L1210 cell line was not due to cell-cycle differences and that post-transcriptional processing and/or stability may play a role as well.

Studies on the mechanisms of inhibition of L1210 cell growth by 3,4-dihydroxybenzohydroxamic acid and 3,4-dihydroxybenzamidoxime

Didox and Amidox inhibit L1210 cell growth in culture. At least one of the mechanisms in the mode(s) of action of the compounds is directed at the ribonucleotide reductase site. Partially purified preparations of ribonucleotide reductase activity are inhibited by Amidox and Didox. The formation of deoxycytidine nucleotides from [14C]cytidine in intact L1210 cells is also blocked. Didox and Amidox cause the decrease in the intracellular pools of the four dNTPs. Hydroxyurea-resistant L1210 cells are not cross-resistant to either Didox or Amidox. These data suggest that Didox and Amidox are not inhibiting ribonucleotide reductase through a mechanism similar to hydroxyurea.

Inhibition of ribonucleotide reductase and growth of human colon carcinoma HT-29 cells and mouse leukemia L1210 cells by N-hydroxy-N'-aminoguanidine derivatives

A series of N-hydroxy-N'-aminoguanidine (HAG) derivatives were studied and compared for their effects on ribonucleotide reductase activity in cell-free extracts; on nucleic acid synthesis and the growth of human colon carcinoma HT-29 cells; and on mouse leukemia L1210 cells in culture. The HAG derivatives [RCH=NNHC(=NH)NHOH-tosylate] studied could be grouped as: (1) hydroxybenzylidines; (2) methoxybenzylidines; and (3) nitrobenzylidines substituted at the R position. 2'-Hydroxybenzylidine-HAG, the lead compound, was relatively active in both HT-29 cells and L1210 cells (20 +/- 5 and 13 +/- 4 microM for 50% inhibition of HT-29 and L1210 cell growth respectively). The monohydroxybenzylidene compounds were generally more active than the dihydroxy- and trihydroxybenzylidene-HAG derivatives. The methoxybenzylidene-HAGs were as active as the monohydroxybenzylidene-HAGs. 2'-Hydroxy-4'-methoxybenzylidene-HAG was much more active than 2',4'-dihydroxybenzylidene-HAG. The mononitrobenzylidene-HAGs were more active than the dinitrobenzylidene-HAG compound. In general, L1210 cells were more sensitive to the effects of the HAG compounds than were HT-29 cells. There was good agreement between the concentration of drug required to inhibit the growth of HT-29 cells and that required to inhibit the growth of L1210 cells. There was also good correlation between the ability of HAG derivatives to inhibit ribonucleotide reductase activity and to inhibit tumor cell growth. Some derivatives, such as 2',3',4'- and 3',4',5'-trihydroxybenzylidene-HAG inhibited L1210 cell growth by 50% at lower concentrations (7.8 and 11.9 microM respectively) than the concentrations needed for 50% inhibition of HT-29 cell growth (196 and 234 microM respectively) and ribonucleotide reductase activity (122 and 188 microM respectively). The studies of nucleic acid synthesis in L1210 cells using [3H]cytidine as a precursor showed that 2',3',4'-trihydroxybenzylidine-HAG inhibited DNA synthesis at a lower concentration (29 microM for 50% inhibition) than was needed for the inhibition of RNA synthesis and formation of [3H]deoxycytidine nucleotides in the acid-soluble fraction (320 and 820 microM for 50% inhibition respectively). These results indicate that 2',3',4'-trihydroxybenzylidine-HAG inhibits DNA synthesis in L1210 cells through other mechanisms rather than exclusively through the inhibition of ribonucleotide reductase activity.

Effect of ribonucleotide reductase inhibitors on the growth of human colon carcinoma HT-29 cells in culture

The effects of ribonucleotide reductase inhibitors on the growth of the human colon carcinoma cell line HT-29 were examined. Inhibitors were chosen for these studies that were specifically directed at each of the subunits of ribonucleotide reductase. The concentrations of drugs required to inhibit the growth of HT-29 cells by 50% (IC50) for hydroxyurea, 2,3-dihydro-lH-pyrazole-[2,3a]imidazole (IMPY), and 4-methyl-5-amino-l-formyl-isoquinoline thiosemicarbazone (MAIQ) were 206, 996, and 3.2 microM, respectively. Although the IC50 for deoxyadenosine alone was greater than 2,000 microM, in the presence of 5 microM erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), which protects deoxyadenosine from deamination by adenosine deaminase, it was reduced to 112 microM. The IC50 for deoxyguanosine was 1,060 microM. The addition of 8-aminoguanosine to protect deoxyguanosine from phosphorolysis by purine nucleoside phosphorylase did not increase the toxicity of deoxyguanosine in HT-29 cells. The combination of MAIQ or IMPY and deoxyadenosine/EHNA gave strong synergistic inhibition of HT-29 cell growth. The results of these studies indicate that ribonucleotide reductase inhibitors effectively block the growth of human colon carcinoma HT-29 cells and that combinations of inhibitors directed at the individual subunits of reductase result in synergistic inhibition of HT-29 cell growth in culture.

5-Hexyl-2'-deoxyuridine blocks the cytotoxic effects of 5-fluorodeoxyuridine or deoxyadenosine in leukemia L1210 cells in culture

Antitumor agents which block the de novo synthesis of nucleotides can be circumvented by the presence of salvage pathways for the reutilization of nucleobases and nucleosides. Studies have been carried out which show that 5-hexyl-2'-deoxyuridine (HdUrd) effectively blocks the cytotoxic effects of deoxyadenosine and fluorodeoxyuridine in L1210 cells. Although HdUrd (500 microM) had essentially no effect on the growth of L1210 cells in culture, the total uptake of [14C]cytidine into these cells was inhibited 99% by this concentration of HdUrd. The inhibitory effects of fluorodeoxyuridine (FdUrd) and deoxyadenosine could be completely prevented by the presence of HdUrd (200 microM). The growth inhibitory effects of fluorouracil were not prevented by HdUrd. Dipyridamole prevented the inhibition of L1210 cell growth by FdUrd but not by deoxyadenosine or fluorouracil. 5-Isopropyl-, 5-pentyl-, and 5-octyldeoxyuridine were not effective in preventing the cytotoxic effects of deoxyadenosine. The data suggest that HdUrd might be useful in blocking the salvage of nucleosides, thereby potentiating the effects of inhibitors of de novo nucleotide synthesis.

Effects of cytosine arabinoside and hydroxyurea on the synthesis of deoxyribonucleotides and DNA replication in L1210 cells

Experiments were carried out in L1210 cells to examine the importance of 'substrate cycles' in regulating the intracellular levels of deoxyribonucleoside 5'-triphosphate. L1210 cells were incubated with [14C]cytidine or [14C]adenosine in the presence and absence of hydroxyurea or cytosine arabinoside (araC). These incubations were carried out for either 30 or 120 min. Inhibition of ribonucleotide reductase by hydroxyurea resulted in the blockage of the flux of ribonucleotides to deoxyribonucleotides (greater than 90%) as expected. When DNA synthesis was inhibited with araC, there was a marked decrease in the incorporation of [14C]cytidine or [14C]adenosine into DNA as deoxyribonucleotides. However, there was not a corresponding increase in the deoxyribonucleotide levels in the acid-soluble fraction or deoxyribonucleosides in the culture medium. AraC treatment decreased the total formation of deoxyribonucleotides. These data indicate that L1210 cells do not regulate the intracellular pools of dNTPs via 'substrate cycles' which involve activation of phosphatases when DNA synthesis is blocked or activation of kinases when ribonucleotide reductase is inhibited.

Antineoplastic effect of the combination of 2,3-dihydro-1H-pyrazole[2,3a]imidazole plus deoxyadenosine/erythro-9-(2-hydroxyl-3-nonyl)adenine in mice with L1210 leukemia cells

Administration of 2,3-dihydro-1H-pyrazole[2,3a]imidazole (IMPY, 150 mg/kg) followed 8 hr later by injection of deoxyadenosine/erythro-9-(2-hydroxyl-3-nonyl)adenine (dAdo/EHNA, 175 mg/17.5 mg/kg) on days 2, 3, 6, and 7 increased the mean survival time of L1210 tumor bearing mice (210%). The sequential treatment was more efficacious than the simultaneous administration of these drugs. Administration of IMPY or dAdo/EHNA, alone, at the same doses as in the combination, did not prolong the life-span of tumor bearing mice. To determine the basis for the increased survival due to the sequential treatment with IMPY and dAdo/EHNA, cell cycle analysis and deoxyribonucleoside triphosphate concentrations were measured. Cytotoxicity of IMPY and dAdo/EHNA is known to be achieved through the inhibition of ribonucleotide reductase. IMPY is a specific inhibitor of the nonheme-iron subunit of ribonucleotide reductase, whereas deoxyadenosine in the presence of the adenosine deaminase inhibition, EHNA, is converted to deoxyadenosine 5'-triphosphate (dATP), which is a specific inhibitor of the effector-binding-subunit of ribonucleotide reductase. Our studies showed that L1210 cells accumulated in early S-phase, whereas intracellular dATP and deoxyguanosine triphosphate (dGTP) pools were depleted 8 hr after IMPY administration. dAdo/EHNA administration 8 hr after IMPY injection caused an increase in the intracellular concentration of dATP while maintaining the depletion of the dGTP pool and prolonged the S-phase as compared to the administration of IMPY alone.

Selective resistance of L1210 cell lines to inhibitors directed at the subunits of ribonucleotide reductase

L1210 cell lines were generated which were resistant to specific ribonucleotide reductase inhibitors. Hydroxyurea-resistant L1210 cells (HU-7) were cross-resistant to IMPY but sensitive to deoxyadenosine and deoxyguanosine. Deoxyadenosine-resistant L1210 cells (Y-8) were cross-resistant to 2-fluorodeoxyadenosine and showed only a small increase in resistance to hydroxyurea or IMPY. L1210 cells which were generated in the presence of deoxyadenosine/EHNA/IMPY/Desferal were markedly resistant to deoxyadenosine, deoxyguanosine and 2-fluorodeoxyadenosine with moderate increases in resistance to IMPY. The HU-7, Y-8 and ED2 cell lines were sensitive to the inhibitory effects of MAIQ and HAG-IQ. The HU-7 L1210 cell line had elevated levels of ribonucleotide reductase activity and this activity showed normal inhibition by hydroxyurea, IMPY, dATP, dGTP and dTTP. The Y-8 L1210 cell line did not have elevated levels of ribonucleotide reductase activity, but had altered allosteric properties relative to dATP. The ED2 L1210 cell line had elevated levels of ribonucleotide reductase activity and had altered allosteric properties relative to dATP. These data show that resistance to ribonucleotide reductase inhibitors is specifically generated in response to the particular drug. The biochemical basis can be related to either increased levels of ribonucleotide reductase activity or loss of feedback control by dATP or both.

Mechanisms of drug resistance to inhibitors directed at the individual subunits of ribonucleotide reductase

Ribonucleotide reductase consists of two non-identical subunits, non-heme iron subunit (NHI) and effector-binding subunit (EB), that are encoded by different genes and that can be specifically and independently inhibited. L1210 cell lines were selected for resistance to hydroxyurea (HU-7), deoxyadenosine (Y-8), or the combination of pyrazoloimidazole and deoxyadenosine (ED2). The gene for the NHI was amplified in the HU-7, Y-8, and ED2 cell lines; there was no amplification of the gene for the EB. The mRNA for the NHI was increased in the HU-7, Y-8, and ED2 cells, but there was no change in the mRNA levels for the EB. Reductase activity was increased in the HU-7, ED2 cells, but not in the Y-8 cells. The reductase activities in the Y-8 and the ED2 cells were not subject to feedback inhibition by dATP. These data show that the mechanisms of resistance to inhibitors directed at this reductase are varied and do not require increased enzyme activity. Further, gene amplification or increased mRNA levels did not necessarily result in increased levels of cellular enzyme.

Metabolic activation of 2,6-diaminopurine and 2,6-diaminopurine-2'-deoxyriboside to antitumor agents

2,6-Diaminopurine (DAP) and 2,6-diaminopurine 2'-deoxyriboside (DAPdR) are analogs of adenine and deoxyadenosine, respectively. It was the purpose of this study to compare these analogs under identical conditions in order to define their inhibitory properties and the underlying mechanism in L1210 mouse leukemia cells. In a 5-day cell growth experiment, DAP exerted a significantly stronger antiproliferative effect than DAPdR. Correspondingly, colony formation of L1210 cells in soft agarose was inhibited by DAP to a greater extent than by DAPdR. A differential distribution of L1210 cells in the cell cycle resulted from an exposure to DAP and DAPdR. While DAPdR arrested cells in the G1/G0 phase of the cell cycle, DAP appeared to lead to an accumulation of G2/M cells. The diaminopurines were combined with modulatory agents to test the antiproliferative action of the combinations. Deoxycytidine partially rescued the cells from the growth inhibitory action of DAPdR without affecting the growth of DAP-treated cells. When adenine was used, the antiproliferative effect of DAPdR was slightly enhanced while the effect of DAP was completely abolished. 8-Aminoguanosine, a specific inhibitor of purine nucleoside phosphorylase, synergistically potentiated the cytostatic effect of DAPdR. However, this inhibitor did not alter DAP effects. At the biochemical level, the target of DAPdR was ribonucleotide reductase which was in line with a drastic expansion of the dGTP pool in DAPdR-treated cells. In cells exposed to DAP, high levels of DAP riboside triphosphate were measured; concomitantly, the ATP level dropped markedly. Enzymological studies revealed that DAPdR is an excellent substrate of adenosine deaminase giving rise to the formation of deoxyguanosine. DAP was found to be activated in the purine nucleoside phosphorylase reaction and in a phosphoribosyl-pyrophosphate-dependent reaction. The data from this comparative study suggest that DAPdR and DAP possess different toxicity mechanisms. DAPdR and DAP possess different toxicity mechanisms. DAPdR acts as a precursor of deoxyguanosine, and DAP is metabolically activated to DAP-containing ribonucleotide analogs. These different metabolic routes seem to account for the different effects of DAP and DAPdR at the cellular level.

Liver microsomal inactivation of 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone as an inhibitor of ribonucleotide reductase

Studies were carried out to determine the effects of preincubation of 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone (MAIQ) with hepatic microsomes on the ability of MAIQ to inhibit CDP reductase activity in vitro. An aliquot from the 100,000 x g supernatant fraction from this incubation was used in the CDP reductase assay. MAIQ incubated in the absence of microsomes inhibited CDP reductase activity in a dose-dependent manner. At high MAIQ concentration (5 microM) CDP reductase activity was inhibited 95%. When MAIQ (5 microM) was first incubated in the presence of hepatic microsomes and NADPH, CDP reductase activity was inhibited only 30%. This attenuation of MAIQ inhibition was dependent on time of incubation and microsomal protein concentration and showed an obligatory requirement for NADPH or NADH. Significant attenuation was observed at pyridine nucleotide concentrations as low as 0.1 mM. Heat denaturation of microsomal proteins inactivated their ability to attenuate the MAIQ inhibition. Microsomes prepared from Ehrlich tumor cells were ineffective as inactivators of MAIQ. Results of our studies show that hepatic microsomes contain an enzyme(s) which can inactive MAIQ as an inhibitor of CDP reductase.

Cross-resistance patterns in hydroxyurea-resistant leukemia L1210 cells

Hydroxyurea is an inhibitor of ribonucleotide reductase and is specifically directed at the non-heme iron subunit (which contains the free radical) of this enzyme. Leukemia L1210 cells, grown in the presence of increasing concentrations of hydroxyurea, developed resistance to hydroxyurea. For hydroxyurea, the wild-type L1210 cells required a drug concentration of 85 microM to inhibit cell growth by 50%, and the hydroxyurea-resistant (HU-7-S7) cells required a concentration of approximately 2000 microM. The resistant L1210 cells were cross-resistant to 2,3-dihydro-1H-pyrazolo[2,3-a]imidazole/Desferal. However, these HU-7-S7 cells remained sensitive to 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone and 1-isoquinolylmethylene-N-hydroxy-N'-amino-guanidine tosylate (inhibitors directed at the same subunit as hydroxyurea). The HU-7-S7 cells retained their sensitivity to deoxyadenosine/erythro-9-(2-hydroxy-3-nonyl)adenine and deoxyguanosine/8-amino-guanosine (inhibitors directed at the effector-binding subunit of ribonucleotide reductase). The L1210 cells developed for resistance to hydroxyurea were sensitive to the non-ribonucleotide reductase inhibitors, methotrexate and 1-beta-D-arabinofuranosylcytosine. Ribonucleotide reductase activity was elevated in the HU-7-S7 cells (CDP reductase, 5.5-fold increase; ADP reductase, 13.2-fold increase). The addition of exogenous effector-binding subunit caused much greater stimulation of reductase activities in the extracts from the resistant cells than from the wild-type cells. The reductase activity in cell-free extracts from the resistant cells was inhibited by hydroxyurea, 2,3-dihydro-1H-pyrazolo[2,3-a]imidazole and dATP to the same extent as the activity from the wild-type L1210 cells. These data indicate that resistance to hydroxyurea in these L1210 cells is to some extent related to increased reductase activity. However, the specificity of resistance of these L1210 cells to inhibitors of ribonucleotide reductase depends on the nature of the inhibitor and the subunit at which the inhibitor is directed.

Ribonucleotide reductase activity and growth of glutathione-depleted mouse leukemia L1210 cells in vitro

L1210 cells treated with L-buthionine-(S/R)-sulfoximine (BSO) had glutathione (GSH) and non-protein thiol levels only 15% that of control. These GSH-depleted cells grew as well as the control L1210 cells and there was no decrease in ribonucleotide reductase activity in situ as measured by the conversion of [14C]cytidine to deoxytidine nucleotides and incorporation into DNA. Further, when these BSO-stressed cells were treated with hydroxyurea or IMPY, there was no potentiation of the inhibition caused by hydroxyurea or IMPY alone. These data indicate that the glutathione/glutaredoxin system of ribonucleotide reductase is not the sole carrier of reducing equivalents from NADPH for the reduction of the 2'-position of the corresponding ribonucleoside 5'-diphosphate; and that glutathione is not critical in regenerating the tyrosyl free-radical on the M2 subunit which is destroyed by the hydroxyurea or 2,3-dihydro-1H-pyrazolo-[2,3-alpha]imidazole (IMPY) treatment.