t-butyl-4-hydroxyanisole as an inhibitor of tumor cell respiration

FR58P1a; a new uncoupler of OXPHOS that inhibits migration in triple-negative breast cancer cells via Sirt1/AMPK/β1-integrin pathway

Highly malignant triple-negative breast cancer (TNBC) cells rely mostly on glycolysis to maintain cellular homeostasis; however, mitochondria are still required for migration and metastasis. Taking advantage of the metabolic flexibility of TNBC MDA-MB-231 cells to generate subpopulations with glycolytic or oxidative phenotypes, we screened phenolic compounds containing an ortho-carbonyl group with mitochondrial activity and identified a bromoalkyl-ester of hydroquinone named FR58P1a, as a mitochondrial metabolism-affecting compound that uncouples OXPHOS through a protonophoric mechanism. In contrast to well-known protonophore uncoupler FCCP, FR58P1a does not depolarize the plasma membrane and its effect on the mitochondrial membrane potential and bioenergetics is moderate suggesting a mild uncoupling of OXPHOS. FR58P1a activates AMPK in a Sirt1-dependent fashion. Although the activation of Sirt1/AMPK axis by FR58P1a has a cyto-protective role, selectively inhibits fibronectin-dependent adhesion and migration in TNBC cells but not in non-tumoral MCF10A cells by decreasing β1-integrin at the cell surface. Prolonged exposure to FR58P1a triggers a metabolic reprograming in TNBC cells characterized by down-regulation of OXPHOS-related genes that promote cell survival but comprise their ability to migrate. Taken together, our results show that TNBC cell migration is susceptible to mitochondrial alterations induced by small molecules as FR58P1a, which may have therapeutic implications.

Small structural changes on a hydroquinone scaffold determine the complex I inhibition or uncoupling of tumoral oxidative phosphorylation

Mitochondria participate in several distinctiveness of cancer cell, being a promising target for the design of anti-cancer compounds. Previously, we described that ortho-carbonyl hydroquinone scaffold 14 inhibits the complex I-dependent respiration with selective anti-proliferative effect on mouse mammary adenocarcinoma TA3/Ha cancer cells; however, the structural requirements of this hydroquinone scaffold to affect the oxidative phosphorylation (OXPHOS) of cancer cells have not been studied in detail. Here, we characterize the mitochondrial metabolism of TA3/Ha cancer cells, which exhibit a high oxidative metabolism, and evaluate the effect of small structural changes of the hydroquinone scaffold 14 on the respiration of this cell line. Our results indicate that these structural changes modify the effect on OXPHOS, obtaining compounds with three alternative actions: inhibitors of complex I-dependent respiration, uncoupler of OXPHOS and compounds with both actions. To confirm this, the effect of a bicyclic hydroquinone (9) was evaluated in isolated mitochondria. Hydroquinone 9 increased mitochondrial respiration in state 4o without effects on the ADP-stimulated respiration (state 3ADP), decreasing the complexes I and II-dependent respiratory control ratio. The effect on mitochondrial respiration was reversed by 6-ketocholestanol addition, indicating that this hydroquinone is a protonophoric uncoupling agent. In intact TA3/Ha cells, hydroquinone 9 caused mitochondrial depolarization, decreasing intracellular ATP and NAD(P)H levels and GSH/GSSG ratio, and slightly increasing the ROS levels. Moreover, it exhibited selective NAD(P)H availability-dependent anti-proliferative effect on cancer cells. Therefore, our results indicate that the ortho-carbonyl hydroquinone scaffold offers the possibility to design compounds with specific actions on OXPHOS of cancer cells.

Antiproliferative and uncoupling effects of delocalized, lipophilic, cationic gallic acid derivatives on cancer cell lines. Validation in vivo in singenic mice

Tumor cells principally exhibit increased mitochondrial transmembrane potential (ΔΨ(m)) and altered metabolic pathways. The therapeutic targeting and delivery of anticancer drugs to the mitochondria might improve treatment efficacy. Gallic acid exhibits a variety of biological activities, and its ester derivatives can induce mitochondrial dysfunction. Four alkyl gallate triphenylphosphonium lipophilic cations were synthesized, each differing in the size of the linker chain at the cationic moiety. These derivatives were selectively cytotoxic toward tumor cells. The better compound (TPP(+)C10) contained 10 carbon atoms within the linker chain and exhibited an IC50 value of approximately 0.4-1.6 μM for tumor cells and a selectivity index of approximately 17-fold for tumor compared with normal cells. Consequently, its antiproliferative effect was also assessed in vivo. The oxygen consumption rate and NAD(P)H oxidation levels increased in the tumor cell lines (uncoupling effect), resulting in a ΔΨ(m) decrease and a consequent decrease in intracellular ATP levels. Moreover, TPP(+)C10 significantly inhibited the growth of TA3/Ha tumors in mice. According to these results, the antineoplastic activity and safety of TPP(+)C10 warrant further comprehensive evaluation.

Survival of TNF toxicity: dependence on caspases and NO

Tumor necrosis factor (TNF) is an endogenous pro-inflammatory cytokine, implicated in pathologies such as rheumatoid arthritis and septic shock. It was originally discovered as a factor with extraordinary antitumor activity, but its shock-inducing properties still prevent its systemic use in cancer. Clinical trials revealed hypotension as the major dose-limiting factor of TNF toxicity. When administered to mice, TNF provokes a lethal shock syndrome, where cardiovascular collapse is centrally orchestrated by nitric oxide (NO). Nevertheless, NO synthase (NOS) inhibition in animal models and septic shock patients could not improve and even aggravated outcome, suggesting a bivalent role for NO. Lymphocyte and enterocyte apoptosis has been described in septic, endotoxemic, or TNF-treated animals, as well as in septic patients. In this review, we describe our recent studies on the role of NO and caspases in TNF-induced shock in mice. In summary, we have found that both NO and caspases may exert unexpected and dual functions during TNF shock. Whereas excessive NO production provokes lethal hypotension, it also has an important anti-oxidant function, protecting organs from oxidative stress and lipid peroxidation. In addition, our results also indicate that caspases may exert an important endogenous negative feedback on oxidative stress as well.

The superoxide dismutase inhibitor diethyldithiocarbamate has antagonistic effects on apoptosis by triggering both cytochrome c release and caspase inhibition

Tumor necrosis factor-alpha (TNF-alpha) and etoposide both trigger a large and rapid production of reactive oxygen species (ROS) in HeLa cells. This occurs before translocations of the proapoptotic Bax and cytochrome c proteins, the loss of mitochondrial membrane potential (DeltaPsim), and apoptosis. We have used diethyldithiocarbamate (DDC), a well-known inhibitor of Cu, Zn superoxide dismutase to study the role of ROS in this system. We report that DDC strongly inhibits caspase activation, loss of DeltaPsim, and cell death induced by TNF-alpha or etoposide. Surprisingly, DDC does not inhibit Bax and cytochrome c translocations. On the contrary, we have observed that DDC can trigger the translocations of these proteins by itself, without altering DeltaPsim. Here, we report that DDC has at least two antagonistic apoptosis regulation functions. First, DDC triggers ROS-dependent Bax and cytochrome c translocations, which are potentially proapoptotic, and second, DDC inhibits caspase activation and activity, loss of DeltaPsim, and cell death, in a ROS-independent manner. Our results suggest an interesting model in which ROS-dependent Bax and cytochrome c translocations can be studied without interference from later apoptotic events.

Comparative cytotoxicity of alkyl gallates on mouse tumor cell lines and isolated rat hepatocytes

Alkyl esters of gallic acid inhibited the respiration rate of mouse sarcoma 786A and mouse mammary adenocarcinoma TA3 cell lines and its multiresistant variant TA3-MTX-R more effectively than gallic acid, both in the absence and in the presence of the uncoupler CCCP. The order of inhibition of the respiration rate by gallates in intact cells was n-octyl- approximately iso-amyl- approximately n-amyl- approximately iso-butyl->n-butyl->iso-propyl->n-propyl-gallate>>gallic acid. Sarcoma 786A was significantly more susceptible to all seven esters than the TA3 cell line. Respiration rates of the TA3-MTX-R cell line showed almost the same sensitivity to these esters as the TA3 cell line. However, hepatocytes were significantly less sensitive than all tumor cells tested. These alkyl gallates blocked mitochondrial electron flow, mainly at the NADH-CoQ segment, preventing ATP synthesis, which would lead to cellular death. These esters also inhibited, in the same order of potencies as respiration, the growth of 786A, TA3 and TA3-MTX-R cells in culture. In mice carrying TA3 or TA3-MTX-R tumor cells, an important decrease of the tumor growth rate and an increase of survival were observed when mice were treated with iso-butyl gallate alone or in combination with doxorubicin. These results indicate that alkyl gallates are selectively cytotoxic to tumor cells, which may be due to the mitochondrial dysfunctions of these cells.

Photosensitizers and antioxidants: a way to new drugs?

Photodynamic therapy (PDT) is a relatively new modality of treatment of diseases involving uncontrolled cell proliferation. It is based on the production of reactive species upon illumination of a photosensitizer in the presence of oxygen. Antioxidants are primarily reducing agents prone to scavenge reactive species in one way or another. Their presence in photodynamic reactions usually reduces the efficacy of PDT. Some antioxidants like ascorbic acid, alpha-tocopherol or butyl-4-hydroxyanisole, however, when added to cells at adequate concentrations may enhance the photodamaging activity of PDT. The presence of transition metals and precise timing of antioxidant administration may also be important factors in increasing the efficacy of PDT. Antioxidant carrier sensitizers have been designed, synthesised and tested for their antibacterial PDT activity. The promising results raise the question whether the introduction of antioxidant moieties into sensitizer molecules would lead to the synthesis of highly effective new drugs.

Molecular Mechanism of Cell Death Induced by the Antioxidant tert-Butylhydroxyanisole in Human Monocytic Leukemia U937 Cells

A phenolic antioxidant 3-tert-butyl-4-hydroxyanisole (BHA) is a widely used food additive. BHA had cytotoxicity in human monocytic leukemia U937 cells. BHA at 0.75 mM caused nuclear condensation and fragmentation, structural damage in mitochondria, decrease in mitochondrial transmembrane potential, and internucleosomal DNA cleavage. It induced the activities of caspase-3 and/or -7, -6, -8 and -9, especially high when DEVD-MCA was the substrate (caspase-3 and/or -7). DEVDase activity increased in time- and dose-dependent manner and high activity was observed in lysates of cells treated for 3 h at 0.75 mM. Addition of GSH (reduced glutathione) during the treatment of cells with BHA inhibited the induction of DEVDase activity, and the intracellular GSH level decreased as the concentration of BHA was raised. Intracellular ATP levels decreased in time- and dose-dependent manner when the cells were treated with BHA in the presence or absence of glucose. Enzyme activities involved in the respiratory chain were assayed with the mitochondrial fraction prepared from U937 cells. BHA distinctly inhibited NADH-ubiquinone oxidoreductase (complex I) and cytochrome c oxidase (complex IV) at low concentrations. Succinate-ubiquinone oxidoreductase (complex II) was also inhibited, but to somewhat less extent. Without mitochondrial enzymes, BHA stimulated the ubiquinol-dependent reduction of cytochrome c (complex III), but it might have some detrimental effects on the mitochondrial enzyme reaction of complex III. The inhibition of mitochondrial oxidative phosphorylation might corroborate the mechanistic evidence for apoptosis of leukemia cells by BHA. Cell death induced by BHA is primarily ascribable to apoptosis.

Inhibitory effect of vanillin-like compounds on respiration and growth of adenocarcinoma TA3 and its multiresistant variant TA3-MTX-R

The effects of some imine and amine derivatives of vanillin on the respiration rate of mouse mammary adenocarcinoma TA3 line, its multiresistant variant TA3-MTX-R line and mouse hepatocytes, together with their respective mitochondrial fractions, are described. These derivatives inhibit respiration in both tumour cell lines more effectively than vanillin in the absence or presence of the uncoupler CCCP. Since both types of derivatives block the electron flow, mainly through the NADH-CoQ span, they behave as oxidative phosphorylation inhibitors. Thus, they prevent ATP synthesis and alter cellular processes requiring energy, which would lead to cellular death. Amine derivatives of vanillin present a similar effect on both tumour cell lines, being amine C the most efficient inhibitor. Moreover, mouse hepatocytes are about 4-fold less sensitive to amine C than tumour cells. These amine derivatives are better inhibitors than the corresponding imines; probably because they should interact better with the respiratory chain reaction site.

Effects of 4,4-dimethyl-5,8-dihydroxynaphtalene-1-one and 4,4-dimethyl-5,8-dihydroxytetralone derivatives on tumor cell respiration

A set of structurally related compounds incorporating a carbonyl group in the ortho position with regard to a phenol function were tested against the TA3 mouse carcinoma cell line and its multidrug-resistant variant TA3-MTX-R. The series consists of 2'-hydroxyacetophenone, 4'-hydroxyacetophenone 2',5'-dihydroxyacetophenone, 4-acetyl-3,3-dimethyl-5-hydroxy-2-morpholino-2,3-dihydrobenzobfuran, five 4,4-dimethyl-5,8-dioxygenated naphtalene-1-ones and three 4,4-dimethyl-5,8-dioxygenated tetralones. A tentative structure-activity relationship was found for this family of substances, suggesting that a coplanar ortho-carbonyl-1,4-hydroquinone motif is able to cause inhibition of cellular respiration.

Structure/Function analysis of p55 tumor necrosis factor receptor and fas-associated death domain. Effect on necrosis in L929sA cells

Tumor necrosis factor (TNF) induces a typical apoptotic cell death program in various cell lines by interacting with the p55 tumor necrosis factor receptor (TNF-R55). In contrast, triggering of the fibrosarcoma cell line L929sA gives rise to characteristic cellular changes resulting in necrosis. The intracellular domain of TNF-R55 can be subdivided into two parts: a membrane-proximal domain (amino acids 202-325) and a C-terminal death domain (DD) (amino acids 326-413), which has been shown to be necessary and sufficient for apoptosis. Structure/function analysis of TNF-R55-mediated necrosis in L929sA cells demonstrated that initiation of necrotic cell death, as defined by swelling of the cells, rapid membrane permeabilization, absence of nuclear condensation, absence of DNA hypoploidy, and generation of mitochondrial reactive oxygen intermediates, is also confined to the DD. The striking synergistic effect of the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone on TNF-induced necrosis was also observed with receptors solely containing the DD. TNF-R55-mediated necrosis is not affected by the dominant negative deletion mutant of the Fas-associated death domain (FADD-(80-205)) that lacks the N-terminal death effector domain. Moreover, overexpression of FADD-(80-205) in L929sA is cytotoxic and insensitive to CrmA, while the cytotoxicity due to overexpression of the deletion mutant FADD-(1-111) lacking the DD is prevented by CrmA. These results demonstrate that the death domain of FADD can elicit an active necrotic cell death pathway.

Enhancement of the efficiency of photodynamic therapy of tumours by t-butyl-4-hydroxyanisole

The effects of photodynamic therapy (PDT) alone and in combination with 3(2)-t-butyl-4-hydroxyanisole (BHA) on Ehrlich ascites carcinoma (EAC) cells have been investigated. BHA, a widely used food antioxidant, administered to the cells prior to light exposure is found to cause concentration-dependent alterations of the haematoporphyrin derivative (HpD)-based PDT. BHA (0.15 mM) causes a small (about 10%) inhibition in the rate of HpD-photosensitized injury of EAC cells. In contrast, upon increasing the concentration of BHA from 0.15 to 0.5 mM, a 1.3-fold enhancement in HpD-PDT efficiency is achieved. The cytotoxic effect on the cells treated with HpD-PDT and a higher concentration of BHA (0.5 mM) is additive. When BHA (0.5 mM) is given immediately after HpD-PDT, the combination is found to be three to four times more effective than when BHA is added to EAC cells before phototherapy. In this treatment regimen BHA acts synergistically with HpD-PDT. Such a difference in the action of BHA on the efficiency of HpD-PDT might be explained by the ability of BHA to inhibit the HpD-photosensitized destruction of some biomolecules. An enhancing action of BHA on the intensity of HpD-photosensitized death of tumour cells is also observed in vivo. Even a single dose of BHA (0.6 mM kg-1, 15 min after irradiation) causes (in an additive manner) an approximately two-fold increase in the efficiency of HpD-PDT of mice bearing Ehrlich ascites tumour (intraperitoneal transplantation). The results obtained indicate that the potentiating effect of BHA on the HpD-PDT could be caused by the impairment of the mitochondrial respiration, since there is a good correspondence between the concentration of BHA that increases the efficiency of PDT and the concentration that inhibits the oxygen consumption and dehydrogenase activity of EAC cells. The influence of BHA on the efficiency of PDT does not depend on the nature of the photosensitizer used; the effects with chlorin-e6 trimethyl ester are similar to that seen for HpD.

Effects and mode of action of 1,4-naphthoquinones isolated from Calceolaria sessilis on tumoral cells and Trypanosoma parasites

The naphthoquinones 2-hydroxy-3-(1,1-dimethylallyl)-1,4-naphthoquinone (CS-1), (-)-2,3,3-trimethyl-2-3-dihydronaphtho[2,3-b]furan-4,9-quinone (CS-3), and 2-acetoxy-3-(1,1-dimethylallyl)-1,4-naphthoquinone (CS-5) isolated from Calceolaria sessilis were tested against Trypanosoma cruzi epimastigotes, the TA3 tumor cell line and the methotrexate-resistant subline TA3-MTX-R. Naphthoquinone CS-3 was the most active; the 50% culture growth inhibition (I50) on T. cruzi (Tulahuén and LQ strain and DM28c clone) was at concentrations ranging from 2.1 to 5.2 mumolar. Also CS-3 inhibited TA3 and TA3-MTX-R culture growth with an I50 of 2.1 and 3.8 mumolar, respectively. Naphthoquinone CS-3 inhibited the respiration of the tumor cells by interfering with the electron transport at some point between NADH and ubiquinone. The respiration of T. cruzi was not inhibited by naphthoquinone CS-3. Naphthoquinone CS-3 produced a temporary increase of oxygen consumption in T. cruzi and tumor cells, suggesting the generation and participation of free radicals.

Inhibition of tumoral cell respiration and growth by nordihydroguaiaretic acid

The effects of nordihydroguaiaretic acid (NDGA), best known as an inhibitor of lipoxygenase activities, on the culture growth, oxygen consumption, ATP level, viability, and redox state of some electron carriers of intact TA3 and 786A ascites tumor cells have been studied. NDGA inhibited the respiration rate of these two tumor cell lines by preventing electron flow through the respiratory chain. Consequently, ATP levels, cell viability and culture growth rates were decreased. NDGA did not noticeably inhibit electron flow through both cytochrome oxidase and ubiquinone-cytochrome b-c1 complex. Also, the presence of NDGA changed to redox state of NAD(P)+ to a more reduced level, and the redox states of ubiquinone, cytochrome b and cytochromes c + c1 changed to a more oxidized level. These observations suggest that the electron transport in the tumor mitochondria was inhibited by NDGA at the NADH-dehydrogenase-ubiquinone level (energy-conserving site 1). As a consequence, mitochondrial ATP synthesis would be interrupted. This event could be related to the cytotoxic effect of NDGA.

On the mechanisms of 3-tert-butyl-4-hydroxyanisole- and its metabolites-induced cytotoxicities in isolated rat hepatocytes

The cytotoxic effects of 3-tert-butyl-4-hydroxyanisole (BHA) and its metabolites, 3-tert-butylhydroquinone (tBHQ) and 3-tert-butyl-4,5-dihydroxyanisole (BHA-OH), were investigated in freshly isolated rat hepatocytes. These compounds caused a time-dependent cell death accompanied by loss of intracellular ATP, glutathione (GSH) and protein thiols at concentration of 0.5 mM. Supplementation of the hepatocyte suspension with 5 mM N-acetylcysteine, a precursor of intracellular GSH, significantly delayed the onset of cytotoxicity induced by BHA-OH and tBHQ; the loss of intracellular ATP, GSH and protein thiols was also prevented. Although N-acetylcysteine did not affect BHA disappearance in the cell suspension, disappearance of tBHQ and formation of tBHQ-GSH conjugate were stimulated by N-acetylcysteine. In addition, N-acetylcysteine prevented BHA-OH disappearance and 3-tert-butyl-5-methoxy-1,2-benzoquinone (BHA-Q) formation. In isolated hepatic mitochondria, BHA, tBHQ and BHA-OH impaired respiration related to oxidative phosphorylation; tert-butylquinone (tBQ) and BHA-Q, quinones derived from tBHQ and BHA-OH, resulted in the significant inhibition of mitochondrial respiration. These results indicate that BHA-OH is the most cytotoxic followed by tBHQ and BHA and that protein thiols and mitochondrial respiratory system are important targets for BHA and its intermediates.

Interaction of butylated hydroxyanisole with mitochondrial oxidative phosphorylation

The antioxidant, butylated hydroxyanisole (BHA), has a number of effects on mitochondrial oxidative phosphorylation. In this study we apply the novel approach developed by Brand (Brand MD, Biochim Biophys Acta 1018: 128-133, 1990) to investigate the site of action of BHA on oxidative phosphorylation in rat liver mitochondria. Using this approach we show that BHA increases the proton leak through the mitochondrial inner membrane and that it also inhibits the delta p (proton motive force across the mitochondrial inner membrane) generating system, but has no effect on the phosphorylation system. This demonstrates that compounds having pleiotypic effects on mitochondrial oxidative phosphorylation in vitro can be analysed and their many effects distinguished. This approach is of general use in analysing many other compounds of pharmacological interest which interact with mitochondria. The implications of these results for the mechanism of interaction of BHA with mitochondrial oxidative phosphorylation are discussed.

Effect of butylated hydroxyanisole on electron transport in rat liver mitochondria

The effects of butylated hydroxyanisole (BHA), a commonly used food antioxidant, on oxygen consumption, ATPase activity, and the redox state of some electron carriers of rat liver mitochondria have been studied. It was observed that BHA slightly stimulated state 4 respiration but strongly inhibited ADP- and uncoupler-stimulated respiration on NAD(+)- and FAD-linked substrates. ATPase activity and vectorial H+ ejection were affected only slightly by BHA, suggesting that BHA predominantly inhibits mitochondrial electron flow. Experiments to determine its site of action showed that BHA did not noticeably affect electron flow through cytochrome oxidase; in contrast, NADH:duroquinone reductase activity and electron flow through ubiquinone-cytochrome b-cytochrome c complex were inhibited strongly because the oxidation of duroquinol was affected markedly. The BHA block of electron transport was bypassed by both N,N,N',N'-tetramethyl-p-phenylenediamine and 2,6-dichlorophenolindophenol. Also, the presence of BHA changed the redox state of cytochrome b and c1 to a more oxidized level. These observations suggest that electron transport is inhibited by BHA at the NADH-ubiquinone and at the ubiquinone-cytochrome b levels. From Hill plots, it is clear that more than one binding site is involved in complete inhibition; in addition, available evidence suggests that there may be two sites at the substrate side of ubiquinone and another two sites at the oxygen side of ubiquinone. Consequently, mitochondrial ATP synthesis would be interrupted. This event could be related to the toxicity of BHA.