Spectral analysis of cytochromes in rat heart myocytes: transient and steady-state photodiode array spectrophotometry measurements

The redox state of cytochrome c modulates resistance to methotrexate in human MCF7 breast cancer cells

Background

Methotrexate is a chemotherapeutic agent used to treat a variety of cancers. However, the occurrence of resistance limits its effectiveness. Cytochrome c in its reduced state is less capable of triggering the apoptotic cascade. Thus, we set up to study the relationship among redox state of cytochrome c, apoptosis and the development of resistance to methotrexate in MCF7 human breast cancer cells.

Results

Cell incubation with cytochrome c-reducing agents, such as tetramethylphenylenediamine, ascorbate or reduced glutathione, decreased the mortality and apoptosis triggered by methotrexate. Conversely, depletion of glutathione increased the apoptotic action of methotrexate, showing an involvement of cytochrome c redox state in methotrexate-induced apoptosis. Methotrexate-resistant MCF7 cells showed increased levels of endogenous reduced glutathione and a higher capability to reduce exogenous cytochrome c. Using functional genomics we detected the overexpression of GSTM1 and GSTM4 in methotrexate-resistant MCF7 breast cancer cells, and determined that methotrexate was susceptible of glutathionylation by GSTs. The inhibition of these GSTM isoforms caused an increase in methotrexate cytotoxicity in sensitive and resistant cells.

Conclusions

We conclude that overexpression of specific GSTMs, GSTM1 and GSTM4, together with increased endogenous reduced glutathione levels help to maintain a more reduced state of cytochrome c which, in turn, would decrease apoptosis, thus contributing to methotrexate resistance in human MCF7 breast cancer cells.

Tetramethylphenylenediamine protects the isolated heart against ischaemia-induced apoptosis and reperfusion-induced necrosis

BACKGROUND AND PURPOSE

Cytochrome c when released from mitochondria into cytosol triggers assembly of the apoptosome resulting in caspase activation. Recent evidence suggests that reduced cytochrome c is unable to activate the caspase cascade. In this study, we investigated whether a chemical reductant of cytochrome c, N,N,N',N'-tetramethylphenylene-1,4-diamine (TMPD), which we have previously shown to block cytochrome c-induced caspase activation, could prevent ischaemia-induced apoptosis in the rat perfused heart.

EXPERIMENTAL APPROACH

The Langendorff-perfused rat hearts were pretreated with TMPD and subjected to stop-flow ischaemia or ischaemia/reperfusion. The activation of caspases (measured as DEVD-p-nitroanilide-cleaving activity), nuclear apoptosis of cardiomyocytes (measured by dUTP nick end labelling assay), mitochondrial and cytosolic levels of cytochrome c (measured spectrophotometrically and by elisa), and reperfusion-induced necrosis (measured as the activity of creatine kinase released into perfusate) were assessed.

KEY RESULTS

We found that perfusion of the hearts with TMPD strongly inhibited ischaemia- or ischaemia/reperfusion-induced activation of caspases and partially prevented nuclear apoptosis in cardiomyocytes. TMPD did not prevent ischaemia- or ischaemia/reperfusion-induced release of cytochrome c from mitochondria into cytosol. TMPD also inhibited ischaemia/reperfusion-induced necrosis.

CONCLUSIONS AND IMPLICATIONS

These results suggest that TMPD or related molecules might be used to protect the heart against damage induced by ischaemia/reperfusion. The mechanism of this protective effect of TMPD probably involves electron reduction of cytochrome c (without decreasing its release) which then inhibits the activation of caspases.

Control of cell respiration by nitric oxide in Ataxia Telangiectasia lymphoblastoid cells

Ataxia Telangiectasia (AT) patients are particularly sensitive to oxidative-nitrosative stress. Nitric oxide (NO) controls mitochondrial respiration via the reversible inhibition of complex IV. The mitochondrial response to NO of AT lymphoblastoid cells was investigated. Cells isolated from three patients and three intrafamilial healthy controls were selected showing within each group a normal diploid karyotype and homogeneous telomere length. Different complex IV NO-inhibition patterns were induced by varying the electron flux through the respiratory chain, using exogenous cell membrane permeable electron donors. Under conditions of high electron flux the mitochondrial NO inhibition of respiration was greater in AT than in control cells (P< or =0.05). This property appears peculiar to AT, and correlates well to the higher concentration of cytochrome c detected in the AT cells. This finding is discussed on the basis of the proposed mechanism of reaction of NO with complex IV. It is suggested that the peculiar response of AT mitochondria to NO stress may be relevant to the mitochondrial metabolism of AT patients.

Mitochondrial regulation of caspase activation by cytochrome oxidase and tetramethylphenylenediamine via cytosolic cytochrome c redox state

Cytochrome c release from mitochondria induces caspase activation in cytosols; however, it is unclear whether the redox state of cytosolic cytochrome c can regulate caspase activation. By using cytosol isolated from mammalian cells, we find that oxidation of cytochrome c by added cytochrome oxidase stimulates caspase activation, whereas reduction of cytochrome c by added tetramethylphenylenediamine (TMPD) or yeast lactate dehydrogenase/cytochrome c reductase blocks caspase activation. Scrape-loading of cells with this reductase inhibited caspase activation induced by staurosporine. Similarly, incubating intact cells with ascorbate plus TMPD to reduce intracellular cytochrome c strongly inhibited staurosporine-induced cell death, apoptosis, and caspase activation but not cytochrome c release, indicating that cytochrome c redox state can regulate caspase activation. In homogenates from healthy cells cytochrome c was rapidly reduced, whereas in homogenates from apoptotic cells added cytochrome c was rapidly oxidized by some endogenous process. This oxidation was prevented if mitochondria were removed from the homogenate or if cytochrome oxidase was inhibited by azide. This suggests that permeabilization of the outer mitochondrial membrane during apoptosis functions not just to release cytochrome c but also to maintain it oxidized via cytochrome oxidase, thus maximizing caspase activation. However, this activation can be blocked by adding TMPD, which may have some therapeutic potential.

Exogenous nitric oxide reduces oxygen consumption of isolated ventricular myocytes less than other forms of guanylate cyclase stimulation

We tested the hypothesis that increasing cyclic GMP with nitric oxide (NO) would reduce cardiac myocyte metabolism less than other forms of guanylate cyclase stimulation. The steady state O2 consumption (VO2) of a suspension of ventricular myocytes in 2.0 mM Ca2+ isolated from hearts of New Zealand white rabbits was measured in a glass chamber using Clark-type oxygen electrode. The cellular cyclic GMP levels, determined by radioimmunoassay, were increased by (1) adding 3-morpholinosydnonimine (SIN-1, 10(-8)-10(-5) M) and nitroprusside (10(-8)-10(-5) M), NO donors-soluble guanylate cyclase stimulators; (2) carbon monoxide (CO, 1.5 x 10(-8)-1.5 x 10(-6) M), soluble guanylate cyclase stimulator and (3) guanylin (10(-8)-10(-5) M), particulate guanylate cyclase stimulator. The baseline myocyte cyclic GMP level was 86 +/- 13 fmol/10(5) myocytes with a corresponding VO2 of 268 +/- 21 nl O2/min per 10(5) myocytes. An inverse relationship between cellular cyclic GMP levels and VO2 existed in these myocytes. The regression equations for the four treatments were: VO2 = -0.45 x [cyclic GMP] + 294.4, r = 0.94 for SIN-1; VO2 = -1.46 x [cyclic GMP] + 444.7, r = 0.96 for CO; VO2 = -1.25 x [cyclic GMP] + 389.1, r = 0.84 for guanylin and VO2 = -0.55 x [cyclic GMP] + 322.8. r = 0.79 for nitroprusside. The regression lines of the two NO donors were parallel. A similar result was also evident for the regressions of CO and guanylin. However, the slopes of both the SIN-1 and nitroprusside regression line were significantly less steep than that of either the CO or guanylin lines. Therefore, VO2 is reduced less for a similar increase in cyclic GMP with NO donors compared to direct stimulation with CO or guanylin. These results suggest that NO has metabolic effects on myocytes in addition to its stimulatory effects on cellular cyclic GMP.

Lonidamine-mediated respiratory changes in rat heart myocytes: a re-examination of the functional response of mitochondrial cytochrome c oxidase

Respiratory activity of intact cardiac myocytes isolated from rats treated with lonidamine (LND) has been examined under conditions where cytochrome oxidase turns over at its maximal rate. Compared to myocytes isolated from control rat hearts, those treated with LND displayed a 60% increase in the cytochrome oxidase-dependent rate of respiration; electron microscopy revealed, in agreement with the literature, that the membrane structure of the mitochondrion had become disorganized. The increase in the rate of oxygen consumption was correlated with the (partial) impairment of the membrane ability to maintain the proton electrochemical potential gradient which normally inhibits oxidase activity. Results are discussed with reference to previous reports showing no effect of LND on cytochrome c oxidase activity. The evidence reported better clarifies the contribution of cytochrome oxidase to the demonstrated energetic failure displayed by cells treated with LND.

The oxygen reactive species of cytochrome-c-oxidase: an alternative view

In a recent review article Babcok and Wikström (Nature, 1992, 356, 301-309) proposed that the species of cytochrome-c-oxidase which binds molecular oxygen during turnover is the so-called mixed valence enzyme, in which the binuclear center cytochrome a3-CuB is reduced, while the cytochrome a/CuA sites are oxidized. This proposal is based on earlier work (Morgan and Wikström, Biochemistry 1991, 30, 948-958) in which it was found that the steady-state reduction levels of cytochrome c and cytochrome a in respiring rat liver mitochondria (sustained by ascorbate and TMPD) are quite different, the latter being much more oxidized than the former; evaluation of the steady-state reduction levels demanded a large correction due to the optical contribution of oxidized TMPD+ which overlaps with the cytochromes. We report below that application of transient spectroscopy and SVD analysis to respiring rat heart myocytes, under conditions in which the contribution of TMPD+ is very small or absent, allows to show that the steady-state reduction levels of cytochrome c and cytochrome a are comparable at all times accessible to measurement in the rapid-scanning stopped-flow spectrophotometer. Our conclusion, in agreement with previous results, is that mixed valence cytochrome-c-oxidase as defined above is not the prevailing oxygen binding species of cytochrome-c-oxidase, unless electron donation to cytochrome c becomes rate limiting.