Human and mouse hepatic aldehyde dehydrogenases important in the biotransformation of cyclophosphamide and the retinoids

Oxidative Stress-Related Mechanisms in Melanoma and in the Acquired Resistance to Targeted Therapies

Melanoma is a highly aggressive cancer with the poorest prognosis, representing the deadliest form of skin cancer. Activating mutations in BRAF are the most frequent genetic alterations, present in approximately 50% of all melanoma cases. The use of specific inhibitors towards mutant BRAF variants and MEK, a downstream signaling target of BRAF in the MAPK pathway, has significantly improved progression-free and overall survival in advanced melanoma patients carrying BRAF mutations. Nevertheless, despite these improvements, resistance still develops within the first year of therapy in around 50% of patients, which is a significant problem in managing BRAF-mutated advanced melanoma. Understanding these mechanisms is one of the mainstreams of the research on BRAFi/MEKi acquired resistance. Both genetic and epigenetic mechanisms have been described. Moreover, in recent years, oxidative stress has emerged as another major force involved in all the phases of melanoma development, from initiation to progression until the onsets of the metastatic phenotype and chemoresistance, and has thus become a target for therapy. In the present review, we discuss the current knowledge on oxidative stress and its signaling in melanoma, as well as the oxidative stress-related mechanisms in the acquired resistance to targeted therapies.

Peroxisome Proliferator-Activated Receptors (PPARs) and Oxidative Stress in Physiological Conditions and in Cancer

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone receptor superfamily. Originally described as “orphan nuclear receptors”, they can bind both natural and synthetic ligands acting as agonists or antagonists. In humans three subtypes, PPARα, β/δ, γ, are encoded by different genes, show tissue-specific expression patterns, and contribute to the regulation of lipid and carbohydrate metabolisms, of different cell functions, including proliferation, death, differentiation, and of processes, as inflammation, angiogenesis, immune response. The PPAR ability in increasing the expression of various antioxidant genes and decreasing the synthesis of pro-inflammatory mediators, makes them be considered among the most important regulators of the cellular response to oxidative stress conditions. Based on the multiplicity of physiological effects, PPAR involvement in cancer development and progression has attracted great scientific interest with the aim to describe changes occurring in their expression in cancer cells, and to investigate the correlation with some characteristics of cancer phenotype, including increased proliferation, decreased susceptibility to apoptosis, malignancy degree and onset of resistance to anticancer drugs. This review focuses on mechanisms underlying the antioxidant and anti-inflammatory properties of PPARs in physiological conditions, and on the reported beneficial effects of PPAR activation in cancer.

Aldehyde dehydrogenase 1A1: friend or foe to female metabolism?

In this review, we summarize recent advances in understanding vitamin A-dependent regulation of sex-specific differences in metabolic diseases, inflammation, and certain cancers. We focus on the characterization of the aldehyde dehydrogenase-1 family of enzymes (ALDH1A1, ALDH1A2, ALDH1A3) that catalyze conversion of retinaldehyde to retinoic acid. Additionally, we propose a “horizontal transfer of signaling” from estrogen to retinoids through the action of ALDH1A1. Although estrogen does not directly influence expression of Aldh1a1, it has the ability to suppress Aldh1a2 and Aldh1a3, thereby establishing a female-specific mechanism for retinoic acid generation in target tissues. ALDH1A1 regulates adipogenesis, abdominal fat formation, glucose tolerance, and suppression of thermogenesis in adipocytes; in B cells, ALDH1A1 plays a protective role by inducing oncogene suppressors Rara and Pparg. Considering the conflicting responses of Aldh1a1 in a multitude of physiological processes, only tissue-specific regulation of Aldh1a1 can result in therapeutic effects. We have shown through successful implantation of tissue-specific Aldh1a1^−/− preadipocytes that thermogenesis can be induced in wild-type adipose tissues to resolve diet-induced visceral obesity in females. We will briefly discuss the emerging role of ALDH1A1 in multiple myeloma, the regulation of reproduction, and immune responses, and conclude by discussing the role of ALDH1A1 in future therapeutic applications.

Role of pharmacogenetics in busulfan/cyclophosphamide conditioning therapy prior to hematopoietic stem cell transplantation

Hematopoietic stem cell transplantation (HSCT) is a curative treatment for several malignant and nonmalignant disorders. Busulfan (Bu) and cyclophosphamide (Cy) are the most commonly used alkylators in high-dose pretransplant conditioning for HSCT; a treatment that is correlated with drug-related toxicity and relapse. Pharmacogenetic investigations have shown that CYP450, as well as aldehyde dehydrogenase, are clearly involved with Cy metabolism and are associated with altered treatment response, Cy metabolism and the unique stem-cell sparing capacity. Moreover, glutathione-S-transferase isoenzymes have been associated with cellular outward transport of various alkylating agents, including Cy metabolites, melphalan, Bu and chlorambucil. A shift from genetic-based studies to whole-genome-based investigations of Cy- and Bu-associated markers may contribute to personalizing the conditioning therapy and enhancing the clinical outcome of HSCT.

A point mutation produced a class 3 aldehyde dehydrogenase with increased protective ability against the killing effect of cyclophosphamide

Cyclophosphamides are pro-drugs whose killing agent is produced from an aldehyde that is formed by the action of a P450 oxidation step. The mustard from the aldehyde can destroy bone marrow cells as well as the tumor. Aldehyde dehydrogenase (EC 1.2.1.3) can oxidize the aldehyde and hence inactivate the cytotoxic intermediate but bone marrow has little, if any, of the enzyme. Others have shown that over-expression of the enzyme can afford protection of the marrow. A T186S mutant of the human stomach enzyme (ALDH3) that we developed has increased activity against the aldehyde compared to the native enzyme and HeLa cells transformed with the point mutant are better protected against the killing effect of the drug. It took threefold more drug to kill 90% of the cells transformed with the mutant compared to the native enzyme (15.8 compared to 5.1mM of a precursor of the toxic aldehyde). Analysis of molecular models makes it appear that removing the methyl group of threonine in the T186S mutant allows the bulky aldehyde to bind better. The mutant was found to be a poorer enzyme when small substrates such as benzaldehyde derivatives were investigated. Thus, the enzyme appears to be better only with large substrates such as the one produced by cyclophosphamide.

The role of corneal crystallins in the cellular defense mechanisms against oxidative stress

The refracton hypothesis describes the lens and cornea together as a functional unit that provides the proper ocular transparent and refractive properties for the basis of normal vision. Similarities between the lens and corneal crystallins also suggest that both elements of the refracton may also contribute to the antioxidant defenses of the entire eye. The cornea is the primary physical barrier against environmental assault to the eye and functions as a dominant filter of UV radiation. It is routinely exposed to reactive oxygen species (ROS)-generating UV light and molecular O(2) making it a target vulnerable to UV-induced damage. The cornea is equipped with several defensive mechanisms to counteract the deleterious effects of UV-induced oxidative damage. These comprise both non-enzymatic elements that include proteins and low molecular weight compounds (ferritin, glutathione, NAD(P)H, ascorbate and alpha-tocopherol) as well as various enzymes (catalase, glucose-6-phosphate dehydrogenase, glutathione peroxidase, glutathione reductase, and superoxide dismutase). Several proteins accumulate in the cornea at unusually high concentrations and have been classified as corneal crystallins based on the analogy of these proteins with the abundant taxon-specific lens crystallins. In addition to performing a structural role related to ocular transparency, corneal crystallins may also contribute to the corneal antioxidant systems through a variety of mechanisms including the direct scavenging of free radicals, the production of NAD(P)H, the metabolism and/or detoxification of toxic compounds (i.e. reactive aldehydes), and the direct absorption of UV radiation. In this review, we extend the discussion of the antioxidant defenses of the cornea to include these highly expressed corneal crystallins and address their specific capacities to minimize oxidative damage.

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.

Human aldehyde dehydrogenases: potential pathological, pharmacological, and toxicological impact

Aldehyde dehydrogenases catalyze the pyridine nucleotide-dependent oxidation of aldehydes to acids. Seventeen enzymes are currently viewed as belonging to the human aldehyde dehydrogenase superfamily. Summarized herein, insofar as the information is available, are the structural composition, physical properties, tissue distribution, subcellular location, substrate specificity, and cofactor preference of each member of this superfamily. Also summarized are the chromosomal locations and organization of the genes that encode these enzymes and the biological consequences when enzyme activity is lost or substantially diminished. Broadly, aldehyde dehydrogenases can be categorized as critical for normal development and/or physiological homeostasis (1). even when the organism is in a friendly environment or (2). only when the organism finds itself in a hostile environment. The primary, if not sole, evolved raison d'être of first category aldehyde dehydrogenases appears to be to catalyze the biotransformation of a single endobiotic for which they are relatively specific and of which the resultant metabolite is essential to the organism. Most of the human aldehyde dehydrogenases for which the relevant information is available fall into this category. Second category aldehyde dehydrogenases are relatively substrate nonspecific and their evolved raison d'être seems to be to protect the organism from potentially harmful xenobiotics, specifically aldehydes or xenobiotics that give rise to aldehydes, by catalyzing their detoxification. Thus, the lack of a fully functional first category aldehyde dehydrogenase results in a gross pathological phenotype in the absence of any insult, whereas the lack of a functional second category aldehyde dehydrogenase is ordinarily of no consequence with respect to gross phenotype, but is of consequence in that regard when the organism is subjected to a relevant insult.

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.

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.

Kinetic properties of the human liver cytosolic aldehyde dehydrogenase for retinal isomers

Retinoic acid exerts pleiotropic effects by acting through two families of nuclear receptors, RAR and RXR. All-trans and 9-cis retinoic acid bind RARs, whereas 9-cis retinoic acid binds and activates only the RXRs. To understand the role of human liver cytosolic aldehyde dehydrogenase (ALDH1) in retinoic acid synthesis, we examined the ability of ALDH 1 to catalyze the oxidation of the naturally occurring retinal isomers. ALDH1 catalyzed the oxidation of all-trans, 9-cis, and 13-cis retinal with equal efficiency. However, the affinity to all-trans retinal (Km = 2.2 microM) was twofold higher than to 9-cis (Km = 5.5 microM) and 13-cis (Km = 4.6 microM) retinal. All-trans retinol was a potent inhibitor of ALDH1 activity, and inhibited all-trans retinal oxidation uncompetitively. Comparison of the kinetic properties of ALDH1 for retinal isomers with those of previously reported rat kidney retinal dehydrogenase showed distinct differences, suggesting that ALDH1 may play a different role in retinal metabolism in liver.

The cytosolic aldehyde dehydrogenase gene (Aldh1) is developmentally expressed in Leydig cells

Cytosolic aldehyde dehydrogenase, ALDH1, participates in the oxidation of different aldehydes including that of all-trans retinal to retinoic acid. The accumulation of mouse Aldh1 transcripts is characterized by having different patterns in different tissues. This paper reports the greatest expression of Aldh1 in testis and liver. It was demonstrated that in testis, Aldh1 is specifically expressed in Leydig cells and is under developmental regulation. In vitro studies of cultured Leydig TM3 cells confirmed these results though such gene expression was found not to be mediated by LH regulation. Previous investigations have associated androgen receptors, and hence the androgen insensitivity syndrome in man, with the presence of ALDH1 in genital skin fibroblasts. However, this relationship was not established in a functional cell type, as is reported here for Leydig cells. These results could suggest a model for a molecular pathway from androgen receptor to retinoic acid biogenesis in Leydig cells via the mediation of ALDH.

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.

Sensitivity of aldehyde dehydrogenases in murine tumor and hematopoietic progenitor cells to inhibition by chloral hydrate as determined by the ability of chloral hydrate to potentiate the cytotoxic action of mafosfamide

Several murine aldehyde dehydrogenases, most notably AHD-2, are known to catalyze the detoxification of cyclophosphamide, mafosfamide, and other oxazaphosphorines. Thus, cellular sensitivity to these agents decreases as the relevant aldehyde dehydrogenase activity increases, and vice versa. Chloral hydrate is a sedative/hypnotic agent that is sometimes administered to patients being treated with cyclophosphamide. It is known to inhibit some, but not all, aldehyde dehydrogenases. Murine (CFU-S, CFU-GEMM and CFU-Mk) and human (CFU-Mix, CFU-GM, BFU-E and CFU-Mk) hematopoietic progenitor cells, as well as murine oxazaphosphorine-resistant (L1210/OAP and P388/CLA) tumor cells, are known to contain the relevant aldehyde dehydrogenase activity but the identity of the specific enzyme present in the normal cells is unknown and may be different than that, namely AHD-2, present in neoplastic cells. In that event, the potential exists to inhibit the detoxification of the oxazaphosphorines in tumor cells without inhibiting this event in normal cells; the net effect of such a selective inhibition would be to increase the margin of safety of the oxazaphosphorines. In ex vivo experiments, chloral hydrate markedly potentiated the antitumor activity of mafosfamide against oxazaphosphorine-resistant L1210/OAP and P388/CLA cells. It did not potentiate the cytotoxic action of mafosfamide against any of the murine or human hematopoietic cells tested, even at concentrations which fully restored the sensitivity of the resistant tumor cell lines to this agent. One explanation for these observations is that hematopoietic progenitor, and the resistant tumor, cells express different relevant aldehyde dehydrogenases and that these aldehyde dehydrogenases differ in their sensitivity to inhibition by chloral hydrate. Consistent with this notion were the observations that AHD-2 was exquisitely sensitive to inhibition by chloral hydrate, whereas two other aldehyde dehydrogenases that also catalyze the detoxification of aldophosphamide, namely AHD-12a, b and AHD-13, were relatively unaffected.