Oxazaphosphorine-specific resistance in human MCF-7 breast carcinoma cell lines expressing transfected rat class 3 aldehyde dehydrogenase

Emerging role of NRF2 in chemoresistance by regulating drug-metabolizing enzymes and efflux transporters

Chemoresistance is a disturbing barrier in cancer therapy, which always results in limited therapeutic options and unfavorable prognosis. Nuclear factor E2-related factor 2 (NRF2) controls the expression of genes encoding cytoprotective enzymes and transporters that protect against oxidative stress and electrophilic injury to maintain intrinsic redox homeostasis. However, recent studies have demonstrated that aberrant activation of NRF2 due to genetic and/or epigenetic mutations in tumor contributes to the high expression of phase I and phase II drug-metabolizing enzymes, phase III transporters, and other cytoprotective proteins, which leads to the decreased therapeutic efficacy of anticancer drugs through biotransformation or extrusion during chemotherapy. Therefore, a better understanding of the role of NRF2 in regulation of these enzymes and transporters in tumors is necessary to find new strategies that improve chemotherapeutic efficacy. In this review, we summarized the recent findings about the chemoresistance-promoting role of NRF2, NRF2-regulated phase I and phase II drug-metabolizing enzymes, phase III drug efflux transporters, and other cytoprotective genes. Most importantly, the potential of NRF2 was proposed to counteract drug resistance in cancer treatment.

Protection from oxidative and electrophilic stress in the Gsta4-null mouse heart

4-Hydroxynonenal (4-HNE) mediates many pathological effects of oxidative and electrophilic stress and signals to activate cytoprotective gene expression regulated by NF-E2-related factor 2 (Nrf2). By exhibiting very high levels of 4-HNE-conjugating activity, the murine glutathione transferase alpha 4 (GSTA4-4) helps regulate cellular 4-HNE levels. To examine the role of 4-HNE in vivo, we disrupted the murine Gsta4 gene. Gsta4-null mice exhibited no cardiac phenotype under normal conditions and no difference in cardiac 4-HNE level as compared to wild-type mice. We hypothesized that the Nrf2 pathway might contribute an important compensatory mechanism to remove excess cardiac 4-HNE in Gsta4-null mice. Cardiac nuclear extracts from Gsta4-null mice exhibited significantly higher Nrf2 binding to antioxidant response elements. We also observed responses in critical Nrf2 target gene products: elevated Sod2, Cat, and Akr1b7 mRNA levels and significant increases in both cardiac antioxidant and anti-electrophile enzyme activities. Gsta4-null mice were less sensitive and maintained normal cardiac function following chronic doxorubicin treatment, known to increase cardiac 4-HNE levels. Hence, in the absence of GSTA4-4 to modulate both physiological and pathological 4-HNE levels, the adaptive Nrf2 pathway may be primed to contribute to a preconditioned cardiac phenotype in the Gsta4-null mouse.

Aldehyde dehydrogenases in cellular responses to oxidative/electrophilic stress

Reactive oxygen species (ROS) are continuously generated within living systems and the inability to manage ROS load leads to elevated oxidative stress and cell damage. Oxidative stress is coupled to the oxidative degradation of lipid membranes, also known as lipid peroxidation. This process generates over 200 types of aldehydes, many of which are highly reactive and toxic. Aldehyde dehydrogenases (ALDHs) metabolize endogenous and exogenous aldehydes and thereby mitigate oxidative/electrophilic stress in prokaryotic and eukaryotic organisms. ALDHs are found throughout the evolutionary gamut, from single-celled organisms to complex multicellular species. Not surprisingly, many ALDHs in evolutionarily distant, and seemingly unrelated, species perform similar functions, including protection against a variety of environmental stressors such as dehydration and ultraviolet radiation. The ability to act as an "aldehyde scavenger" during lipid peroxidation is another ostensibly universal ALDH function found across species. Upregulation of ALDHs is a stress response in bacteria (environmental and chemical stress), plants (dehydration, salinity, and oxidative stress), yeast (ethanol exposure and oxidative stress), Caenorhabditis elegans (lipid peroxidation), and mammals (oxidative stress and lipid peroxidation). Recent studies have also identified ALDH activity as an important feature of cancer stem cells. In these cells, ALDH expression helps abrogate oxidative stress and imparts resistance against chemotherapeutic agents such as oxazaphosphorine, taxane, and platinum drugs. The ALDH superfamily represents a fundamentally important class of enzymes that contributes significantly to the management of electrophilic/oxidative stress within living systems. Mutations in various ALDHs are associated with a variety of pathological conditions in humans, highlighting the fundamental importance of these enzymes in physiological and pathological processes.

Overcoming challenges of ovarian cancer stem cells: novel therapeutic approaches

Understanding the genetic and molecular mechanisms of ovarian cancer has been the focus of research efforts working toward the greater goal of improving cancer therapy for patients with residual disease after initial treatment with conventional surgery and neoadjuvant chemotherapy. The focus of this review will be centered on new therapeutic strategies based on Cancer Stem Cells studies of chemoresistant subpopulations, the prevention of metastasis, and individualized therapy in order to find the most successful combination of treatments to effectively treat human ovarian cancer. We reviewed recent literature (1993-2011) of novel treatment approaches to ovarian cancer stem cells. As the focus of ovarian cancer investigation has centered on the cancer stem cell model and the complexities that it presents in the development of effective treatments, the future of treating ovarian cancer lies in utilizing individualized treatment systems that include enhancing existing treatments, aiming for novel therapy targets, managing the plasticity of stem cells to induce cellular differentiation, and regulating oncogenic signaling pathways.

Aldehyde dehydrogenase activity as a functional marker for lung cancer

Aldehyde dehydrogenase (ALDH) activity has been implicated in multiple biological and biochemical pathways and has been used to identify potential cancer stem cells. Our main hypothesis is that ALDH activity may be a lung cancer stem cell marker. Using flow cytometry, we sorted cells with bright (ALDH(br)) and dim (ALDH(lo)) ALDH activity found in H522 lung cancer cell line. We used in vitro proliferation and colony assays as well as a xenograft animal model to test our hypothesis. Cytogenetic analysis demonstrated that the ALDH(br) cells are indeed a different clone, but when left in normal culture conditions will give rise to ALDH(lo) cells. Furthermore, the ALDH(br) cells grow slower, have low clonal efficiency, and give rise to morphologically distinct colonies. The ability to form primary xenografts in NOD/SCID mice by ALDH(br) and ALDH(lo) cells was tested by injecting single cell suspension under the skin in each flank of same animal. Tumor size was calculated weekly. ALDH1A1 and ALDH3A1 immunohistochemistry (IHC) was performed on excised tumors. These tumors were also used to re-establish cell suspension, measure ALDH activity, and re-injection for secondary and tertiary transplants. The results indicate that both cell types can form tumors but the ones from ALDH(br) cells grew much slower in primary recipient mice. Histologically, there was no significant difference in the expression of ALDH in primary tumors originating from ALDH(br) or ALDH(lo) cells. Secondary and tertiary xenografts originating from ALDH(br) grew faster and bigger than those formed by ALDH(lo) cells. In conclusion, ALDH(br) cells may have some of the traditional features of stem cells in terms of being mostly dormant and slow to divide, but require support of other cells (ALDH(lo)) to sustain tumor growth. These observations and the known role of ALDH in drug resistance may have significant therapeutic implications in the treatment of lung cancer.

Arachidonic acid suppresses growth of human lung tumor A549 cells through down-regulation of ALDH3A1 expression

Expression of aldehyde dehydrogenase 3A1 (ALDH3A1) in certain normal and tumor cells is associated with protection against the growth inhibitory effect of reactive aldehydes generated during membrane lipid peroxidation. We found that human lung tumor (A549) cells, which express high levels of ALDH3A1 protein, were significantly less susceptible to the antiproliferative effects of 4-hydroxynonenal compared to human hepatoma HepG2 or SK-HEP-1 cells that lack ALDH3A1 expression. However, A549 cells became susceptible to lipid peroxidation products when they were treated with arachidonic acid. The growth suppression of A549 cells induced by arachidonic acid was associated with increased levels of lipid peroxidation and with reduced ALDH3A1 enzymatic activity, protein, and mRNA levels. Furthermore, arachidonic acid treatment of the A549 cells resulted in an increased expression of peroxisome proliferator-activated receptor gamma (PPARgamma), whereas NF-kappaB binding activity was inhibited. Blocking PPARgamma using a selective antagonist, GW9662, prevented the arachidonic acid-mediated reduction of ALDH3A1 expression as well as the growth inhibition of A549 cells, suggesting the central role of PPARgamma in these phenomena. The increase in PPARgamma and the reduction in ALDH3A1 were also prevented by exposing cells to vitamin E concomitant with arachidonic acid treatment. In conclusion, our data show that the arachidonic acid-induced suppression of A549 cell growth is associated with increased lipid peroxidation and decreased ALDH3A1 expression, which may be due to activation of PPARgamma.

RNAi-mediated knockdown of aldehyde dehydrogenase class-1A1 and class-3A1 is specific and reveals that each contributes equally to the resistance against 4-hydroperoxycyclophosphamide

PURPOSE

Aldehyde dehydrogenases class-1A1 (ALDH1A1) and class-3A1 (ALDH3A1) have been associated with resistance to cyclophosphamide (CP) and its derivatives. We have previously reported the downregulation of these enzymes by all-trans retinoic acid (ATRA).

METHODS

In this study, we used siRNA duplexes as well as retrovirally expressed siRNA to knockdown one or both enzymes together in A549 lung cancer cell line in order to investigate the role of each one in mediating the resistance and the effect of the addition of ATRA.

RESULTS

The results show that significant and specific knockdown of each enzyme can be achieved and that each one contributes similarly to cell resistance to 4-hydroperoxycyclophosphamide (4-HC), an active derivative of CP. Added effects were seen when both enzymes were inhibited. The addition of ATRA also exhibited additional inhibitory effects on ALDH activity and increased 4-HC toxicity when added to single siRNA aimed at one of the enzymes. On the other hand, ATRA had minimal and insignificant additional inhibitory effects on ALDH enzyme activity when added to a combination of siRNAs against both enzymes, but still increased 4-HC toxicity beyond that seen with RNAi-mediated inhibition of both enzymes together.

CONCLUSIONS

We conclude that both enzymes, ALDH1A1 and ALDH3A1 will need to be blocked in order to achieve the highest sensitivity to 4-HC. Furthermore, ATRA increases 4-HC toxicity even when added to a combination of siRNAs against both enzymes, thus suggesting additional mechanisms by which ATRA can increase drug toxicity.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

Role of human aldehyde dehydrogenases in endobiotic and xenobiotic metabolism

The human genome contains at least 17 genes that are members of the aldehyde dehydrogenase (ALDH) superfamily. These genes encode NAD(P)(+)-dependent enzymes that oxidize a wide range of aldehydes to their corresponding carboxylic acids. Aldehydes are highly reactive molecules that are intermediates or products involved in a broad spectrum of physiologic, biologic, and pharmacologic processes. Aldehydes are generated during retinoic acid biosynthesis and the metabolism of amino acids, lipids, carbohydrates, and drugs. Mutations in several ALDH genes are the molecular basis of inborn errors of metabolism and contribute to environmentally induced diseases.

Retinoic Acid Down-Regulates Aldehyde Dehydrogenase and Increases Cytotoxicity of 4-Hydroperoxycyclophosphamide and Acetaldehyde

Multiple prior studies have identified aldehyde dehydrogenases (ALDH) that are capable of oxidizing retinal to retinoic acid. In this study, we test the hypothesis that the accumulation of intracellular retinoic acid may lead to the suppression of ALDH expression and thus increase cytotoxicity to 4-hydroperoxycyclophosphamide (4-HC) in vitro. Mainly A549, but also other lung cancer cell lines, were used in our experiments, with the former having high levels of two ALDH isozymes expressed. Dose-response and time-course experiments were performed by incubating the cells with all-trans retinoic acid (ATRA) as well as other commercially available retinoids. The results show that incubation of A549 cells with any of the retinoids at pharmacologic doses for > or =48 h results in a significant decrease in ALDH-1A1 and ALDH-3A1 enzyme activity and protein levels but not the corresponding mRNAs. Such a decrease in ALDH activity was seen in all cell lines tested and results in a significant increase in toxicity of 4-HC and acetaldehyde, both of which are substrates for the enzymes. Prior incubation with ATRA also results in increased cytotoxicity, although to a lesser degree, of phenylketophosphamide and melphalan, neither of which is a substrate for ALDHs. These results suggest a post-translational mechanism through which retinoids decrease both ALDH expression, which results in increased cytotoxicity of 4-HC and acetaldehyde, although other previously described effects of these retinoids may contribute to the slight increase in cytotoxicity seen with other chemotherapy agents. These results may have clinical implications in regard to the use of retinoids in lung cancer prevention and treatment.

Antisense oligonucleotides against aldehyde dehydrogenase 3 inhibit hepatoma cell proliferation by affecting MAP kinases

The increased activity of enzymes that eliminate anti-tumour drugs or their metabolites is one of the important limiting factors in therapeutic protocols. Among these enzymes, aldehyde dehydrogenase 3 (ALDH3) is considered a mechanism by which tumour cells evade the cytotoxic effects exerted by cyclophosphamide and drugs acting by free radical generation. It is also important in metabolising cytostatic aldehydes derived from lipid peroxidation. Therefore, ALDH3 may play a role in regulating cell proliferation in tumour cells with high activity of this enzyme. We previously reported that antisense oligonucleotides (AS-ODN) against ALDH3 strongly inhibit hepatoma cell growth, suggesting that this effect could be due to the accumulation of cytostatic aldehydes in the cells. In this research we demonstrate that AS-ODN against ALDH3 increase the quantity of malondialdehyde in the cells, and inhibit cell proliferation by affecting the MAPK pathway: a reduction of pRaf-1 and pERK1,2 was observed. These results confirm the importance of aldehydes derived from lipid peroxidation and of ALDH3 in regulating hepatoma proliferation. Moreover, the results indicate the use of AS-ODN against ALDH3 as a possible strategy to reduce growth in tumours overexpressing this enzyme.

Novel competitive irreversible inhibitors of aldehyde dehydrogenase (ALDH1): restoration of chemosensitivity of L1210 cells overexpressing ALDH1 and induction of apoptosis in BAF(3) cells overexpressing bcl(2)

4-Amino-4-methyl-pent-2-ynthioc acid S-methyl ester (ampal thiolester: ATE) was used as a lead compound to synthesise new amino-substituted derivatives of alpha, beta acetylenic thiolester compounds as inhibitors of aldehyde dehydrogenase 1, (ALDH1). Of these compounds, the dimethyl derivative (DIMATE) was a competitive irreversible inhibitor (K(i) approximately 280 microM) of baker's yeast ALDH1 in vitro showing 80% inhibition at 400 microM when preincubated with the enzyme for 30min, whereas the trimethyl ammonium and the morpholine derivatives showed only 15% inhibition at 600 microM even after 60min preincubation. ATE inhibited ALDH1 activity in ALDH1-transfected L1210 T cells resistant to hydroperoxycyclophosphamide (HCPA) and inhibited growth synergistically in the presence of HCPA. In non-transfected L1210 counterparts ATE did not potentiate growth inhibition by HCPA. DIMATE was a 30-100-fold more effective growth inhibitor than ATE. Endogenous ALDH1 activities of BAF(3) cells over-expressing different levels of bcl(2) (0-100%) were similar (16-20mU/mg protein) and were all inhibited by DIMATE, reaching 20-30% at 4 microM. Up to 4 microM no apoptosis, as measured by DNA-fragmentation was observed, but at 8 and 10 microM DIMATE, DNA-fragmentation increased concomitantly with ALDH1 inhibition. No DNA-fragmentation was observed with ALDH1 irreversible inhibitors devoid of a thiolester group or with thiolesters which were not inhibitors of ALDH1. It was seen only with competitive irreversible inhibitors having the methanethiol and enzyme-inhibitory moieties. The methanethiol putatively released from DIMATE by ALDH1 esterase activity plays a role, albeit undefined, in lowering intramitochondrial glutathione levels which decreased by 47% as DNA-fragmentation increased.

Inhibition of cytosolic class 3 aldehyde dehydrogenase by antisense oligonucleotides in rat hepatoma cells

Aldehyde dehydrogenases (ALDHs) are a superfamily of several isoenzymes widely expressed in bacteria, yeast, plant and animals. Three major classes of ALDHs have been traditionally identified, classes 1, 2 and 3. Both exogenous and endogenous aldehydes, including aldehydes derived from lipid peroxidation, are oxidized by the ALDH superfamily. Several changes in ALDH isoenzyme expression take place in hepatoma cells, in particular cytosolic class 3 ALDH (ALDH3), not expressed in normal hepatocytes, appears and increases with the degree of deviation. It has been demonstrated that cytosolic ALDH3 is important in determining the resistance of tumor cells to antitumor drugs, such as cyclophosphamide. Moreover, hepatoma-associated ALDH3 seems to be important in metabolizing aldehydes derived from lipid peroxidation, and in particular the cytostatic aldehyde 4-hydroxynonenal (4-HNE). We demonstrated previously that restoring endogenous lipid peroxidation in hepatoma cells by enriching them with arachidonic acid causes a decrease of mRNA, protein and enzyme activity of ALDH3 and that this decrease reduces cell growth and/or causes cell death, depending on basal class 3 ALDH activity. To confirm the correlation between inhibition of class 3 ALDH and reduction of cell proliferation, we exposed hepatoma cells to antisense oligonucleotides (ODNs) against ALDH3. In JM2 hepatoma cell line, with high ALDH3 activity, the exposure to antisense ODNs significantly decreases mRNA and enzyme activity (90%). At the same time, cell growth was reduced by about 70%. The results confirm that in hepatoma cells ALDH3 expression is closely related with cell growth, and that its inhibition is important in reducing the proliferation of hepatoma cells overexpressing ALDH3.

Structure-activity relationships for growth inhibition and induction of apoptosis by 4-hydroxy-2-nonenal in raw 264.7 cells

4-Hydroxy-2-nonenal (HNE) is a highly reactive lipid aldehyde byproduct of the peroxidation of cellular membranes. The structure of HNE features three functional groups, a C1 aldehyde, a C2==C3 double bond, and a C4- hydroxyl group, each of which may contribute to the toxicity of the compound. In addition, the length of the aliphatic chain may influence toxic potency by altering lipophilicity. Using analogous compounds that lacked one or more of the structural moieties, the role of each of these structural motifs in the cytotoxicity of HNE was examined in a mouse alveolar macrophage cell line (RAW 264.7) by a cell survival and growth assay. The importance of these functional groups in the potency of HNE for induction of apoptosis was also examined. The rank order of effects on toxicity was C1---aldehyde >/= C2==C3 double bond >> C4---hydroxyl, with parallel results in both the survival/growth inhibition and apoptosis induction assays. The chain length also influenced toxicity in a series of alpha,beta-unsaturated alkenyl aldehydes, with increasing chain length yielding increasing toxicity. To confirm the importance of the aldehyde moiety, and to examine the role of metabolic detoxification in cellular defenses against HNE toxicity, a RAW 264.7 cell line overexpressing human aldehyde dehydrogenase-3 (hALDH3) was generated. This cell line exhibited nearly complete protection against HNE-protein adduct formation as well as HNE-induced apoptosis. These results illustrate the comparative significance of key structural features of HNE in relation to its potent toxicity and induction of apoptosis.

Inhibition of class-3 aldehyde dehydrogenase and cell growth by restored lipid peroxidation in hepatoma cell lines

Hepatoma cells have a below-normal content of polyunsaturated fatty acids; this reduces lipid peroxidation and the production of cytotoxic and cytostatic aldehydes within the cells. In proportion to the degree of deviation, hepatoma cells also show an increase in the activity of Class-3 aldehyde dehydrogenase, an enzyme important in the metabolism of lipid peroxidation products and also in that of several drugs. When hepatoma cells with different degrees of deviation were enriched with arachidonic acid and stimulated to peroxidize by ascorbate/iron sulphate, their growth rate was reduced in proportion to the quantity of aldehydes produced and to the activity of aldehyde dehydrogenase. Therefore, 7777 cells, less deviated and with low Class-3 aldehyde dehydrogenase activity, were more susceptible to lipid peroxidation products than JM2 cells. It is noteworthy that repeated treatments with prooxidant also caused a decrease in mRNA and activity of Class-3 aldehyde dehydrogenase, contributing to the decreased growth and viability. Thus, Class-3 aldehyde dehydrogenase could be considered relevant for the growth of hepatoma cells, since it defends them against cell growth inhibiting aldehydes derived from lipid peroxidation.

Dependence of aldehyde dehydrogenase-mediated oxazaphosphorine resistance on soluble thiols: importance of thiol interactions with the secondary metabolite acrolein

Acrolein is a highly reactive and cytotoxic by-product released during activation of oxazaphosphorine (OAP) anticancer alkylating agents. Previously, we demonstrated that transfected human aldehyde dehydrogenase (ALDH, EC 1.2.1.3) isozymes (class 1 or 3) protect V79/SD1 cells from mafosfamide (MAF) cytotoxicity, but protection from 4-hydroperoxy-cyclophosphamide (4-hpCPA) was weaker. Acrolein, an ALDH inhibitor, may be detoxified by conjugation with the nucleophilic thiol 2-mercaptoethanesulfonate (MESNA), which is released from MAF but not from 4-hpCPA. We examined the effect of acrolein or acrolein/thiol conjugates on ALDH activity in vitro. We found that both ALDH isozymes were inhibited by acrolein, with IC50 values of 35 and 144 microM for ALDH-1 or ALDH-3, respectively. Both isozymes were partially protected by NAD+ cofactor, being at least five-fold more sensitive to acrolein if added before cofactor. In contrast, thiol conjugates of acrolein did not inhibit ALDH-3 activity, but were substrates only for ALDH-1. Further, acrolein was shown to be oxidized by ALDH-3, but not by ALDH-1. The effect of acrolein on ALDH-mediated resistance to OAP agents in intact cells was also examined. In control cells (without ALDH expression), acrolein and 4-hpCPA rapidly depleted intracellular GSH levels, whereas the effect of MAF was much less. Depletion of GSH by preincubation of V79/SD1 cells with a low concentration of acrolein (2 microM) before MAF exposure caused a two-fold reduction in ALDH-mediated resistance. Conversely, protection from 4-hpCPA cytotoxicity was enhanced by the addition of thiols (GSH, 2-mercaptoethanesulfonate, or N-acetylcysteine) during the drug exposure. These results suggest 1) that thiol content is an important determinant of the OAP resistance conferred by ALDH isoenzymes; and 2) a new mechanism whereby thiol modulation could increase the therapeutic index of OAP chemotherapy.

Enhanced expressions of glucose-6-phosphate dehydrogenase and cytosolic aldehyde dehydrogenase and elevation of reduced glutathione level in cyclophosphamide-resistant human leukemia cells

Elevation of activity and mRNA level of a cytosolic aldehyde dehydrogenase-1 (ALDH1), which oxidizes aldophosphamide, was previously observed in a cyclophosphamide-resistant murine leukemia cell line. However, changes in other enzyme(s) which may detoxify the drug or produce anti-alkylating agent(s), have not been examined. The human leukemia cell line, K562, was made 30-fold resistant against 4-hydroperoxycyclophosphamide (4HC) by exposing the cells to increasing concentrations of the drug. Resistance against cisplatin was also increased by about 3-fold. Activities of glucose-6-phosphate dehydrogenase (G6PD) and ALDH1 were elevated more than 7-fold in the resistant cells. The mRNA level of the two enzymes was also proportionally elevated. The concentration of reduced glutathione (GSH) was higher in the resistant cells (i.e., 21.1 versus 4.68 nmole per 10(6) cells), while activities of gamma-glutamylcysteine synthetase and glutathione synthetase, and the expressions of other human ALDH genes were not increased in the resistant cells. These findings suggest that the acquired resistance against 4HC is a consequence of transcriptional activation of two genes, i.e., one encoding the G6PD, a major enzyme regenerating anti-alkylating GSH, and the other encoding ALDH1, which has a high activity for oxidation of aldophosphamide derived from 4HC.

Inhibition of human class 3 aldehyde dehydrogenase, and sensitization of tumor cells that express significant amounts of this enzyme to oxazaphosphorines, by chlorpropamide analogues

In some cases, acquired as well as constitutive tumor cell resistance to a group of otherwise clinically useful antineoplastic agents collectively referred to as oxazaphosphorines, e.g. cyclophosphamide and mafosfamide, can be accounted for by relatively elevated cellular levels of an enzyme, viz. cytosolic class 3 aldehyde dehydrogenase (ALDH-3), that catalyzes their detoxification. Ergo, inhibitors of ALDH-3 could be of clinical value since their inclusion in the therapeutic protocol would be expected to sensitize such cells to these agents. Identified in the present investigation were two chlorpropamide analogues showing promise in that regard, viz. (acetyloxy)[(4-chlorophenyl)sulfonyl]carbamic acid 1,1-dimethylethyl ester (NPI-2) and 4-chloro-N-methoxy-N-[(propylamino)carbonyl]benzenesulfonamide (API-2). Each inhibited NAD-linked benzaldehyde oxidation catalyzed by ALDH-3s purified from human breast adenocarcinoma MCF-7/0/CAT cells (IC50 values were 16 and 0.75 microM, respectively) and human normal stomach mucosa (IC50 values were 202 and 5 microM, respectively). The differential sensitivities of stomach mucosa ALDH-3 and breast tumor ALDH-3 to each of the two inhibitors can be viewed as further evidence that the latter is a subtle variant of the former. Human class 1 (ALDH-1) and class 2 (ALDH-2) aldehyde dehydrogenases were much less sensitive to NPI-2; IC50 values were >300 microM in each case. API-2, however, did not exhibit a similar degree of specificity; IC50 values for ALDH-1 and ALDH-2 were 7.5 and 0.08 microM, respectively. Each sensitized MCF-7/0/CAT cells to mafosfamide; the LC90 value decreased from >2 mM to 175 and 200 microM, respectively. Thus, the therapeutic potential of combining NPI-2 or API-2 with oxazaphosphorines is established.

Retroviral coexpression of thymidylate synthase and dihydrofolate reductase confers fluoropyrimidine and antifolate resistance

Retroviral gene transfer of dominant selectable markers into hematopoietic cells can be used to select genetically modified cells in vivo or to attenuate the toxic effects of chemotherapeutic agents. We show that retroviral gene transfer of thymidylate synthase (TS) confers resistance to TS directed anticancer agents and that co-expression of TS and dihydrofolate reductase (DHFR) confers resistance to TS and DHFR cytotoxic agents. Retroviral vectors encoding Escherichia coli TS, human TS, and the Tyr-to-His at residue 33 variant of human TS (Y33HhTS) were constructed and fibroblasts transfected with these vectors conferred comparable resistance to the TS-directed agent fluorodeoxyuridine (FdUrd, approximately 4-fold). Retroviral vectors that encode dual expression of Y33HhTS and the human L22Y DHFR (L22YhDHFR) variants conferred resistance to FdUrd (3- to 5-fold) and trimetrexate (30- to 140-fold). A L22YhDHFR-Y33HhTS chimeric retroviral vector was also constructed and transduced cells were resistant to FdUrd (3-fold), AG337 (3-fold), trimetrexate (100-fold) and methotrexate (5-fold). These results show that recombinant retroviruses can be used to transfer the cDNA that encodes both TS and DHFR and dual expression in transduced cells is sufficiently high to confer resistance to TS and DHFR directed anticancer agents.

Coding region-specific destabilization of mRNA transcripts attenuates expression from retroviral vectors containing class 1 aldehyde dehydrogenase cDNAs

Class 1 aldehyde dehydrogenases (ALDH-1) function as drug resistance gene products by catalyzing the irreversible conversion of aldophosphamide, an active metabolite of cyclophosphamide, to an inert compound. Because the dose-limiting toxicity of cyclophosphamide is myelosuppression, retrovirus-mediated transfer of ALDH-1 to bone marrow cells has been proposed as a protective strategy. Here we show that expression of ALDH-1 vectors was problematic due to low levels of ALDH-1 mRNA accumulation. A number of vectors containing several different ALDH-1 cDNAs were introduced into a variety of different cell lines either by transfection or transduction. Detectable ALDH-1 protein and enzyme activity was only seen in one transfected cell clone. Cells transduced with ALDH-1 retroviral vectors had no detectable protein expression and very low levels of ALDH-1 mRNA. Analogous vectors containing other drug resistance cDNAs led to much higher levels of steady-state mRNA. The mRNA half-life from ALDH-1 vectors was less than 2 hr suggesting that vector-derived mRNAs were destabilized by ALDH-1 coding sequences. These results suggest that methods which increase the stability of ALDH-1 mRNAs will be important for increased drug resistance in retrovirally transduced hematopoietic cells.

Reduced folate carrier gene (RFC1) expression and anti-folate resistance in transfected and non-selected cell lines

Methotrexate transport deficiency due to decreased reduced folate carrier (RFC) activity has been observed in several cell lines selected for resistance to methotrexate (MTX). Since MTX resistance is multifactorial, however, it is difficult to quantify the relative importance of changes in RFC activity in selected cell lines and even more so to determine the relative contribution of naturally occurring RFC activity in the MTX sensitivity of non-selected cell lines. We examined the role of RFC in MTX resistance by studying a transport-deficient cell line transfected with the gene for human RFC, RFC1, and by correlating relative RFC1 expression with MTX and trimetrexate (TMTX) growth inhibition (GI50) in a panel of cell lines used in the NCI Anticancer Drug Screen. Clones of transport-deficient, MTX-resistant ZR-75-1 human breast cancer cells (MTX(R) ZR-75-1) transfected with RFC1 were 250-fold more sensitive to MTX and 300-fold more resistant to TMTX than control cell clones, showing that restoration of RFC activity has a significant impact on MTX and TMTX cytotoxicity. We also surveyed 40 of the 60 cell lines in the NCI drug screen panel for RFCI RNA levels by a quantitative RT-PCR assay. RFCI RNA levels varied over a range of 15-fold, with only 1 cell line found to be null in expression. Using data from the 6-day drug exposure assay, RFC1 correlated positively with MTX and negatively with TMTX cytotoxicity. As predicted by transfection studies, the calculated difference between MTX and TMTX potency was even more strongly correlated with RFC1 RNA levels of the cell lines. In addition, compounds in the NCI Anticancer Drug Screen database with cytotoxicity profiles which correlated with RFC1 RNA levels or with the calculated difference in MTX-TMTX potency were examined for MTX uptake inhibition and cytotoxicity in the RFC1-transfected MTX(R) ZR-75-1 cell line. Overall, our data demonstrate the importance of RFC1 in MTX resistance both as a transgene and as a constitutively expressed gene in non-selected cell lines.

Downexpression of aldehyde dehydrogenase 1 in murine lung tumors

We analyzed the expression of the cytosolic aldehyde dehydrogenase 1 (Aldh1) gene in mouse lung tumors by northern blotting and immunocytochemical analysis. Aldh1 was abundantly expressed in normal lung tissue, with a predominant cellular localization on bronchiolar cells. However, expression of Aldh1 was strongly reduced (more than tenfold) in lung tumors. As aldehyde dehydrogenases metabolize some antitumor alkylating drugs to inactive compounds, the low expression of Aldh1 in lung tumors may account for the drug sensitivity of these tumors to chemotherapeutic agents.

Over-expression of glutathione S-transferases, DT-diaphorase and an apparently tumour-specific cytosolic class-3 aldehyde dehydrogenase by Warthin tumours and mucoepidermoid carcinomas of the human parotid gland

Cytosolic class-3 aldehyde dehydrogenase (ALDH-3) may help to protect organisms from certain environmental aldehydes by catalysing their detoxification. Consistent with this notion are the reports that relatively high levels of this enzyme are present in tissues, e.g. stomach mucosa and lung, that are so-called ports of entry for such agents. Further, it is found in human saliva. The present investigation revealed that small amounts of this enzyme are also present in human salivary glands; mean values for ALDH-3 activities (NADP-dependent enzyme-catalysed oxidation of benzaldehyde) in cytosolic fractions prepared from submandibular and parotid glands were 52 (range: 29-92) and 44 (range: 13-73) mIU/g tissue, respectively. Essentially identical or slightly lower levels of this enzyme activity were found in pleomorphic adenomas, an undifferentiated carcinoma, and an adenocystic carcinomas, of the parotid gland. On the other hand, Warthin tumours, and mucoepidermoid carcinomas of the parotid gland exhibited relatively elevated levels of ALDH-3 activity; mean values were 1200 (range: 780-1880) and 810 (range: 580-1200) mIU/g tissue, respectively. The ALDH-3 found in normal salivary glands was, as judged by physical, immunological and kinetic criteria, identical to human stomach mucosa ALDH-3 whereas the ALDH-3 present in Warthin tumours, and mucoepidermoid carcinomas, of the parotid gland appeared to be a subtle variant thereof. Qualitatively paralleling the relatively elevated ALDH-3 levels in mucoepidermoid carcinomas and Warthin tumours were relatively elevated levels of glutathione S-transferase (alpha and pi) and DT-diaphorase. As was the case with ALDH-3 levels, glutathione S-transferase (alpha and pi) and DT-diaphorase levels were not elevated in pleomorphic adenomas. Glutathione S-transferase mu was not detected in the two normal parotid gland samples, or in the single pleomorphic adenoma sample, tested. It was found in the single mucoepidermoid carcinoma sample, and in one of the two Warthin tumour samples tested. Cellular levels of ALDH-3, glutathione S-transferases and/or DT-diaphorase could be useful criteria when the decision to be made is whether a salivary gland tumour is a mucoepidermoid carcinoma. ALDH-3 and glutathione S-transferases are known to catalyse the detoxification of two agents that are used to treat salivary gland tumours, viz. cyclophosphamide and cisplatin, respectively. Thus, elevated levels of these enzymes in the mucoepidermoid carcinomas must account for, or at least contribute to, the relative ineffectiveness of these agents when used to treat this tumour.

De novo expression of transfected human class 1 aldehyde dehydrogenase (ALDH) causes resistance to oxazaphosphorine anti-cancer alkylating agents in hamster V79 cell lines. Elevated class 1 ALDH activity is closely correlated with reduction in DNA interstrand cross-linking and lethality

Human class 1 aldehyde dehydrogenase (hALDH-1) can oxidize aldophosphamide, a key aldehyde intermediate in the activation pathway of cyclophosphamide and other oxazaphosphorine (OAP) anti-cancer alkylating agents. Overexpression of class 1 ALDH (ALDH-1) has been observed in cells selected for survival in the presence of OAPs. We used transfection to induce de novo expression of human ALDH-1 in V79/SD1 Chinese hamster cells to clearly quantitate the role of hALDH-1 expression in OAP resistance. Messenger RNA levels correlated well with hALDH-1 protein levels and enzyme activities (1.5-13.6 milliunits/mg with propionaldehyde/NAD+ substrate, compared to < 1 milliunit/mg in controls) in individual clonal transfectant lines, and slot blot analysis confirmed the presence of the transfected cDNA. Expressed ALDH activity was closely correlated (r = 0.99) with resistance to mafosfamide, up to 21-fold relative to controls. Transfectants were cross-resistant to other OAPs but not to phosphoramide mustard, ifosfamide mustard, melphalan, or acrolein. Resistance was completely reversed by pretreatment with 25 microM diethylaminobenzaldehyde, a potent ALDH inhibitor. Alkaline elution studies showed that expression of ALDH-1 reduced the number of DNA cross-links commensurate with mafosfamide resistance, and this reduction in cross-links was fully reversed by the inhibitor. Thus, overexpression of human class 1 ALDH alone is sufficient to confer OAP-specific drug resistance.

Protection by transfected rat or human class 3 aldehyde dehydrogenase against the cytotoxic effects of oxazaphosphorine alkylating agents in hamster V79 cell lines. Demonstration of aldophosphamide metabolism by the human cytosolic class 3 isozyme

Expression of class 3 aldehyde dehydrogenase (ALDH-3) has been associated with acquired or inherent resistance to oxazaphosphorine (OAP) antineoplastic alkylating agents (eg. cyclophosphamide). We previously demonstrated that expression of transfected rat ALDH-3 can confer OAP-specific resistance in human MCF-7 cells (Bunting, K. D., Lindahl, R., and Townsend, A. J. (1994) J. Biol. Chem. 269, 23197-23203). However, the aldophosphamide intermediate inactivated by human class 1 ALDH (hALDH-1) has not proven to be a good substrate for the purified hALDH-3. We have examined the ability of transfected human or rat ALDH-3 to confer OAP resistance in V79/SDl cells. Clones expressing elevated human (386-5938 milliunits/mg) or rat (4-597 milliunits/mg, benzaldehyde/NADP+ substrate) ALDH-3 activity were 1.3- to 12-fold resistant to mafosfamide relative to control cells (<1 milliunit/mg). Resistance was correlated with hALDH-3 activity, and was reversed by pretreatment with the ALDH inhibitor diethylaminobenzaldehyde. Transfectants were cross-resistant to 4-hydroperoxycyclophosphamide and 4-hydroperoxyifosfamide but not to phosphoramide mustard, ifosfamide mustard, melphalan, or acrolein. DNA interstrand cross-links were reduced commensurately with the fold resistance to mafosfamide in the highest activity clone. A key finding was the detection of a metabolite, most likely carboxyphosphamide, that is formed only by cytosols from cells expressing either class 3 or class 1 ALDH.

Retrovirus-mediated gene transfer of rat glutathione S-transferase Yc confers in vitro resistance to alkylating agents in human leukemia cells and in clonogenic mouse hematopoietic progenitor cells

Recently, we have reported that N2Yc, a Moloney-based retrovirus vector expressing the Yc isoform of rat glutathione S-transferase (GST-Yc), conferred resistance to alkylating agents in mouse NIH-3T3 fibroblasts. In this report, we address the feasibility of using rat GST-Yc somatic gene transfer to confer chemoprotection to the hematopoietic system. Human chronic myelogenous leukemia K-562 cells were efficiently transduced with the N2Yc retrovirus vector and showed a significant increase in the 50% inhibitory concentration of chlorambucil (3.2- to 3.3-fold), mechlorethamine (4.7- to 5.3-fold), and melphalan (2.1- to 2.2-fold). In addition, primary murine clonogenic hematopoietic progenitor cells transduced with the N2Yc vector were significantly more resistant to alkylating agents in vitro than cells transduced with the antisense N2revYc vector. The survival of Yc-transduced hematopoietic colonies at 400 nM mechlorethamine and 4 mu M chlorambucil was 39.4% and 42.6%, respectively, compared to 27.2% and 30.4% for N2revYc-transduced cells. Future experiments will determine the level of chemoprotection achievable in vivo, following transplantation of N2Yc-transduced hematopoietic cells in mice.