Metabolic inhibitors accentuate the anti-tumoral effect of HDAC5 inhibition

The US FDA approval of broad-spectrum histone deacetylase (HDAC) inhibitors has firmly laid the cancer community to explore HDAC inhibition as a therapeutic approach for cancer treatment. Hitting one HDAC member could yield clinical benefit but this required a complete understanding of the functions of the different HDAC members. Here we explored the consequences of specific HDAC5 inhibition in cancer cells. We demonstrated that HDAC5 inhibition induces an iron-dependent reactive oxygen species (ROS) production, ultimately leading to apoptotic cell death as well as mechanisms of mitochondria quality control (mitophagy and mitobiogenesis). Interestingly, adaptation of HDAC5-depleted cells to oxidative stress passes through reprogramming of metabolic pathways towards glucose and glutamine. Therefore, interference with both glucose and glutamine supply in HDAC5-inhibited cancer cells significantly increases apoptotic cell death and reduces tumour growth in vivo; providing insight into a valuable clinical strategy combining the selective inhibition of HDAC5 with various inhibitors of metabolism as a new therapy to kill cancer cells.

The antitrypanosomal drug melarsoprol competitively inhibits thiamin uptake in mouse neuroblastoma cells

Melarsoprol is the main drug used for the treatment of late-stage sleeping sickness, although it causes severe side-effects such as encephalopathy and polyneuropathy leading to death in some patients. Recent data suggest that melarsoprol and its active metabolite melarsenoxide interfere with thiamin transport and metabolism in E. coli and yeast, but there are no data concerning their possible effects on thiamin metabolism in mammalian cells. We tested both drugs on thiamin transport in cultured mouse neuroblastoma cells using (14)C-labeled thiamin. Melarsoprol, competitively inhibits high-affinity thiamin transport in mouse neuroblastoma cells with a K(i) of 44 micromol/L. However, the active compound melarsenoxide has no inhibitory effect. This suggests that the side effects of melarsoprol treatment are unlikely to be due to inhibition of thiamin transport by melarsenoxide, its main metabolite in the brain.

Neuronal localization of the 25-kDa specific thiamine triphosphatase in rodent brain

Thiamine triphosphate (ThTP) is found in small amounts in most organisms from bacteria to mammals, but little is known about its physiological role. In vertebrate tissues, ThTP may act as a phosphate donor for the phosphorylation of certain proteins; this may be part of a new signal transduction pathway. We have recently characterized a highly specific 25-kDa thiamine triphosphatase (ThTPase) that is expressed in most mammalian tissues. The role of this enzyme may be the control of intracellular concentrations of ThTP. As the latter has been considered to be a neuroactive form of thiamine, we have studied the distribution of ThTPase mRNA and protein in rodent brain using in situ hybridization and immunohistochemistry. With both methods, we found the strongest staining in hippocampal pyramidal neurons, as well as cerebellar granule cells and Purkinje cells. Some interneurons were also labeled and many ThTPase mRNA-positive and immunoreactive cells were distributed throughout cerebral cortical gray matter and the thalamus. White matter was not significantly labeled. ThTPase immunoreactivity seems to be located mainly in the cytoplasm of neuronal perikarya. Immunocytochemical data using dissociated cultured cells from hippocampal and cerebellum showed that the staining was more intense in neurons than in astrocytes. The protein was rather uniformly located in the perikarya and dendrites, suggesting that ThTP and ThTPase may play a general role in neuronal metabolism rather than a specific role in excitability. There was no apparent correlation between ThTPase expression and selective vulnerability of certain brain regions to thiamine deficiency.

Thiamine triphosphate and thiamine triphosphatase activities: from bacteria to mammals

In most organisms, the main form of thiamine is the coenzyme thiamine diphosphate. Thiamine triphosphate (ThTP) is also found in low amounts in most vertebrate tissues and can phosphorylate certain proteins. Here we show that ThTP exists not only in vertebrates but is present in bacteria, fungi, plants and invertebrates. Unexpectedly, we found that in Escherichia coli as well as in Arabidopsis thaliana, ThTP was synthesized only under particular circumstances such as hypoxia (E. coli) or withering (A. thaliana). In mammalian tissues, ThTP concentrations are regulated by a specific thiamine triphosphatase that we have recently characterized. This enzyme was found only in mammals. In other organisms, ThTP can be hydrolyzed by unspecific phosphohydrolases. The occurrence of ThTP from prokaryotes to mammals suggests that it may have a basic role in cell metabolism or cell signaling. A decreased content may contribute to the symptoms observed during thiamine deficiency.

Specific phosphorylation of Torpedo 43K rapsyn by endogenous kinase(s) with thiamine triphosphate as the phosphate donor

43K rapsyn is a peripheral protein specifically associated with the nicotinic acetylcholine receptor (nAChR) present in the postsynaptic membrane of the neuromuscular junction and of the electrocyte, and is essential for its clustering. Here, we demonstrate a novel specific phosphorylation of 43K rapsyn by endogenous protein kinase(s) present in Torpedo electrocyte nAChR-rich membranes and identify thiamine triphosphate (TTP) as the phosphate donor. In the presence of Mg(2+) and [gamma-(32)P]-TTP, 43K rapsyn is specifically phosphorylated with a (32)P-half-maximal incorporation at approximately 5-25 microM TTP. The presence of TTP in the cytosol and of 43K rapsyn at the cytoplasmic face of the postsynaptic membrane, together with TTP-dependent phosphorylation of 43K rapsyn without added exokinases, suggests that TTP-dependent-43K-rapsyn phosphorylation may occur in vivo. In addition, phosphoamino acid and chemical stability analysis suggests that the residues phosphorylated are predominantly histidines. Inhibition of phosphorylation by Zn(2+) suggests a possible control of 43K rapsyn phosphorylation state by its zinc finger domain. Endogenous kinase(s) present in rodent brain membranes can also use [gamma-(32)P]-TTP as a phosphodonor. The use of a phosphodonor (TTP) belonging to the thiamine family but not to the classical (ATP, GTP) purine triphosphate family represents a novel phosphorylation pathway possibly important for synaptic proteins.

Low thiamine diphosphate levels in brains of patients with frontal lobe degeneration of the non-Alzheimer's type

We compared the thiamine and thiamine phosphate contents in the frontal, temporal, parietal, and occipital cortex of six patients with frontal lobe degeneration of the non-Alzheimer's type (FNAD) or frontotemporal dementia with five age-, postmortem delay-, and agonal status-matched control subjects. Our results reveal a 40-50% decrease in thiamine diphosphate (TDP) in the cortex of FNAD patients, whereas thiamine monophosphate was increased 49-119%. TDP synthesizing and hydrolyzing enzymes were unaffected. The activity of citrate synthase, a mitochondrial marker enzyme, was decreased in the frontal cortex of patients with FNAD, but no correlation with TDP content was found. These results suggest that decreased contents of TDP, which is essentially mitochondrial, is a specific feature of FNAD. As TDP is an essential cofactor for oxidative metabolism and neurotransmitter synthesis, and because low thiamine status (compared with other species) is a constant feature in humans, a nearly 50% decrease in cortical TDP content may contribute significantly to the clinical symptoms observed in FNAD. This study also provides a basis for a trial of thiamine, to improve the cognitive status of the patients.

Reversibility of thiamine deficiency-induced partial necrosis and mitochondrial uncoupling by addition of thiamine to neuroblastoma cell suspensions

Culture of neuroblastoma cells in the presence of low thiamine concentration (16 nM) and of the transport inhibitor amprolium leads to the appearance of signs of necrosis: the chromatin condenses, the oxygen consumption decreases and is uncoupled, the mitochondrial cristae are disorganized, the thiamine diphosphate-dependent dehydrogenase activities are impaired. When 10 microM thiamine are added to these cells, the basal respiration increases, the coupled respiration is restored and mitochondrial morphology is recovered within 1 h. Addition of succinate, which is oxidized via a thiamine diphosphate-independent dehydrogenase, to digitonin-permeabilized cells immediately restores a coupled respiration. Our results suggest that the slowing of the citric acid cycle is the cause of the biochemical lesion induced by severe thiamine deficiency and that part of the mitochondria remain functional.

A non-cofactor role of thiamine derivatives in excitable cells?

Thiamine diphosphate (TDP) is an important cofactor of pyruvate (PDH) and alpha-ketoglutarate (KGDH) dehydrogenases and transketolase. Thiamine deficiency leads to reversible and irreversible brain lesions due to impaired oxidative metabolism. A specific non-cofactor role for thiamine has also been proposed in excitable cells and thiamine triphosphate (TTP) might be involved in the regulation of ion channels. Thiamine is taken up by neuroblastoma cells through a high affinity transporter. Inside the cells, it is rapidly phosphorylated to TDP. This high turnover TDP pool is the precursor for TTP. Most of the TDP however has a low turnover and is associated with PDH and KGDH in mitochondria. In excised inside-out patches from neuroblastoma cells, TTP, at a concentration of 1 microM, activates chloride channels of large unitary conductance, the so-called maxi-Cl- channels. These channels are inhibited by oxythiamine from the outide. In addition to the role of TTP in the regulation of chloride channels, thiamine itself, or a presently unknown analog, may have trophic effects on neuronal cells.

Brain levels of thiamine and its phosphate esters in Friedreich's ataxia and spinocerebellar ataxia type 1

Decreased blood and cerebrospinal fluid levels of thiamine have been reported in patients with spinocerebellar ataxia disorders. To determine whether a thiamine deficiency is present in the brain, we measured levels of thiamine and its phosphate esters thiamine monophosphate (TMP) and thiamine diphosphate (TDP), in postmortem cerebellar and cerebral cortices of patients with Friedreich's ataxia (FA) and spinocerebellar ataxia type 1 (SCA1). Brain levels of free (nonphosphorylated) thiamine, TMP, TDP, and total thiamine in FA and SCA1 were, on average, not significantly different from control values. However, a nonsignificant trend was observed for slightly reduced levels of TDP and total thiamine in cerebellar cortex of the SCA1 patients, a finding that might be related to the severe neuronal damage in this brain area. We conclude that in FA, brain thiamine concentrations are normal, whereas in SCA1 the levels are, at most, only slightly reduced.

Brain thiamine, its phosphate esters, and its metabolizing enzymes in Alzheimer's disease

Clinical data suggest that high-dose thiamine (vitamin B1) may have a mild beneficial effect in some patients with Alzheimer's disease (AD). Since this action could be related to a brain thiamine deficiency, we measured directly levels of free (nonphosphorylated) thiamine and its phosphate esters, thiamine monophosphate and thiamine diphosphate (TDP), and activities of three TDP-metabolizing enzymes (thiamine pyrophosphokinase, thiamine diphosphatase, and thiamine triphosphatase) in autopsied cerebral cortex of 18 patients with AD and 20 matched controls. In the AD group, mean levels of free thiamine and its monophosphate ester were normal, whereas levels of TDP were significantly reduced by 18 to 21% in all three cortical brain areas examined. Activities of the TDP-metabolizing enzymes were normal in the AD group, suggesting that decreased TDP is not due to altered levels of these enzymes. The TDP decrease could be explained by a cerebral cortical deficiency in AD of ATP, which is needed for TDP synthesis. Although the magnitude of the TDP reduction is slight, a chronic subclinical TDP deficiency could contribute to impaired brain function in AD and might provide the basis for the modest improvement by thiamine in cognitive status of some patients with AD.

Brain protein and alpha-ketoglutarate dehydrogenase complex activity in Alzheimer's disease

To determine whether the reduction in brain alpha-ketoglutarate dehydrogenase complex activity in Alzheimer's disease (AD) is associated with an abnormality in one of its three constituent enzyme subunits, we measured protein levels of alpha-ketoglutarate dehydrogenase (El), dihydrolipoamide succinyltransferase (E2), and dihydrolipoamide dehydrogenase (E3), in postmortem brain of 29 patients with AD (mean age, 73 years; age range of onset, 50-78 years) and 29 control subjects. In the AD group protein levels of all three subunits were significantly reduced by 23 to 41% in the temporal cortex, whereas in the parietal cortex (El: -28%; E3: -32%) and hippocampus (E3: -33%) significant changes were limited to El and E3. alpha-Ketoglutarate dehydrogenase complex activities were more markedly reduced (by 46-68%) and did not correlate with protein levels, suggesting that decreased enzyme activity cannot be primarily explained by loss of alpha-ketoglutarate dehydrogenase complex protein. We did not find two E2 immunoreactive forms in the brain of any patient, as has been reported in fibroblasts of patients with very-early-onset chromosome 14-linked AD. We conclude that brain protein and activity levels of alpha-ketoglutarate dehydrogenase complex are reduced in patients with AD who have onset after 50 years and suggest that these changes, which are also observed in other human brain disorders, may represent a nonspecific consequence of different neurodegenerative processes. Nevertheless, reduced levels of this rate-limiting enzyme of the Krebs cycle could contribute to the brain neurodegenerative mechanisms of AD.

Immunoreactive levels of alpha-ketoglutarate dehydrogenase subunits in Friedreich's ataxia and spinocerebellar ataxia type 1

Enzyme activities of a alpha-ketoglutarate dehydrogenase complex (alpha KGDHC) and one of its constituent subunits, dihydrolipoamide dehydrogenase (E3), are reported to be reduced in non-CNS tissues of some patients with Friedreich's ataxia (FA); however, the results are highly conflicting. To determine whether an enzyme abnormality occurs in brain, we measured immunoreactive levels of the three alpha KGDHC subunits, namely, alpha-ketoglutarate dehydrogenase (E1), dihydrolipoamide succinyltransferase (E2) and E3 in postmortem frontal, occipital and cerebellar cortices of 18 control subjects, 9 patients with FA and, for comparison, 12 patients with spinocerebellar ataxia type 1 (SCA1). Decreased (-20 to -31%) levels of E3 were observed in all three examined areas of the patients with FA with the changes statistically significant in cerebellar and frontal cortices. The E3 reduction could be explained by a loss of alpha KGDHC or other dehydrogenase complexes (e.g. pyruvate dehydrogenase complex) which utilize this subunit. In SCA1, enzyme changes were limited to E2 in cerebellar (-26%) and frontal (-19%) cortices. Although the E3 and E2 reductions are only slight, and may represent secondary events, the changes in this key Krebs cycle enzyme could exacerbate degenerative processes in both of the spinocerebellar ataxia disorders.

Paradoxical sleep deprivation increases the content of glutamate and glutamine in rat cerebral cortex

We investigated the influence of the sleep/waking cycle, the effects of paradoxical sleep deprivation (PSD) and of the vigilance-promoting drug modafinil on the amino acid contents of rat brain cortex. No significant nycthemeral variations in amino acid levels could be detected. PSD (12-24 hours), using the water tank method, significantly increased the levels of glutamate and glutamine. The increase was still observed after the sleep rebound period. gamma-Aminobutyric acid (GABA) levels did not change significantly during the instrumental sleep deprivation but increased during the rebound period. Control experiments indicate that the increase in glutamate and glutamine levels is due to PSD rather than to the stress associated with the experimental procedure. The increase in glutamate content cannot arise only from transamination reactions, because the levels of other amino acids (such as aspartate) did not decrease. Modafinil treatment did not significantly modify the brain cortex content of any of the amino acids tested.

Thiamine, thiamine phosphates, and their metabolizing enzymes in human brain

Total thiamine (the sum of thiamine and its phosphate esters) concentrations are two- to fourfold lower in human brain than in the brain of other mammals. There were no differences in the total thiamine content between biopsied and autopsied human brain, except that in the latter, thiamine triphosphate was undetectable. The main thiamine phosphate-metabolizing enzymes could be detected in autopsied brain, and the kinetic parameters were comparable to those reported in other species. Thiamine diphosphate levels were lowest in hippocampus (15 +/- 4 pmol/mg of protein) and highest in mammillary bodies (24 +/- 4 pmol/mg of protein). Maximal levels of thiamine and its phosphate ester were found to be present at birth. In parietal cortex and globus pallidus, mean levels of total thiamine in the oldest age group (77-103 years) were, respectively, 21 and 26% lower than those in the middle age group (40-55 years). Unlike cerebral cortex, the globus pallidus showed a sharp drop in thiamine diphosphate levels during infancy, with concentrations in the oldest group being only approximately 50% of the levels present during the first 4 months of life. These data, consistent with previous observations conducted in blood, suggest a tendency toward decreased thiamine status in older people.

Thiamine deficiency--induced partial necrosis and mitochondrial uncoupling in neuroblastoma cells are rapidly reversed by addition of thiamine

Culture of neuroblastoma cells in a medium of low-thiamine concentration (6 nM) and in the presence of the transport inhibitor amprolium leads to the appearance of overt signs of necrosis; i.e., the chromatin condenses in dark patches, the oxygen consumption decreases, mitochondria are uncoupled, and their cristae are disorganized. Glutamate formed from glutamine is no longer oxidized and accumulates, suggesting that the thiamine diphosphate-dependent alpha-ketoglutarate dehydrogenase activity is impaired. When thiamine (10 microM) is added to the cells, the O2 consumption increases, respiratory control is restored, and normal cell and mitochondrial morphology is recovered within 1 h. Succinate, which is oxidized via the thiamine diphosphate-independent succinate dehydrogenase, is also able to restore a normal O2 consumption (with respiratory control) in digitonin-permeabilized thiamine-deficient cells. Our results therefore suggest that the slowing of the citric acid cycle is the main cause of the biochemical lesion induced by thiamine deficiency as observed in Wernicke's encephalopathy.

Thiamine deficiency in cultured neuroblastoma cells: effect on mitochondrial function and peripheral benzodiazepine receptors

When neuroblastoma cells were transferred to a medium of low (6 nM) thiamine concentration, a 16-fold decrease in total intracellular thiamine content occurred within 8 days. Respiration and ATP levels were only slightly affected, but addition of a thiamine transport inhibitor (amprolium) decreased ATP content and increased lactate production. Oxygen consumption became low and insensitive to oligomycin and uncouplers. At least 25% of mitochondria were swollen and electron translucent. Cell mortality increased to 75% within 5 days. [3H]PK 11195, a specific ligand of peripheral benzodiazepine receptors (located in the outer mitochondrial membrane) binds to the cells with high affinity (KD = 1.4 +/- 0.2 nM). Thiamine deficiency leads to an increase in both Bmax and KD. Changes in binding parameters for peripheral benzodiazepine receptors may be related to structural or permeability changes in mitochondrial outer membranes. In addition to the high-affinity (nanomolar range) binding site for peripheral benzodiazepine ligands, there is a low-affinity (micromolar range) saturable binding for PK 11195. At micromolar concentrations, peripheral benzodiazepines inhibit thiamine uptake by the cells. Altogether, our results suggest that impairment of oxidative metabolism, followed by mitochondrial swelling and disorganization of cristae, is the main cause of cell mortality in severely thiamine-deficient neuroblastoma cells.

Thiamine homeostasis in neuroblastoma cells

We recently showed that thiamine uptake by neuroblastoma cells is mediated by two saturable transport system: the first with high affinity for thiamine (Km = 35 nM) is blocked by veratridine; the other, with low affinity is blocked by Ca2+. The driving force for thiamine uptake is its phosphorylation to thiamine diphosphate (TDP) by thiamine pyrophosphokinase and subsequent binding of this cofactor to apoenzymes. Our results suggest that cells of neuronal origin possess mechanisms regulating the intracellular concentration of thiamine. At low external thiamine, the vitamin is taken up by a high-affinity transporter and pyrophosphorylated in thiamine diphosphate (TDP): this is the TDP pool of slow turnover. An intraover extracellular concentration gradient of free thiamine is observed at low external concentration of the vitamin. At higher external thiamine concentration, TDP accumulation is limited by the binding capacity to the apoenzymes and unbound TDP (i.e. a small pool of fast turnover) is quickly hydrolyzed. Thiamine is slowly released by the cells by at least two different mechanisms. The first, accounting for a maximum of 50% of total thiamine release, is stimulated by external thiamine and is blocked by veratridine, suggesting that it is a self-exchange mechanism catalyzed by the high affinity thiamine transporter. The remaining thiamine efflux is neither sensitive to veratridine nor to Ca2+ and its mechanism is unknown. About 25% of intracellular thiamine is not released, even after treatment of the cells with digitonin, thus maintaining an apparent gradient. This suggests a binding or sequestration in intracellular compartments.(ABSTRACT TRUNCATED AT 250 WORDS)

An atypical anion transporter functioning at acid pH in neuroblastoma cells

At pH 7.4, 36Cl- uptake by neuroblastoma cells was Na(+)-independent, saturable and blocked by submicromolar concentrations of DIDS. This suggests that at this pH, Cl- transport is mediated by an exchanger analogous to erythroid band 3. At pH 6.2, 36Cl- uptake was markedly activated by external carboxylate anions such as acetate. Acetate-stimulated 36Cl- uptake was blocked by DIDS (IC50 = 0.15 microM). Saturation by external 36Cl- was observed with K0.5 = 8 mM. K0.5 was not modified by acetate. As 36Cl efflux is also activated by acetate, we suggest the presence, in neuroblastoma cells, of an anion exchanger activated by carboxylic anions. This exchanger is active when the extracellular pH is 6.0-6.5.

Chloride permeability of rat brain membrane vesicles correlates with thiamine triphosphate content

Incubation of rat brain homogenates with thiamine or thiamine diphosphate (TDP) leads to a synthesis of thiamine triphosphate (TTP). In membrane vesicles subsequently prepared from the homogenates, increased TTP content correlates with increased 36Cl- uptake. A hyperbolic relationship was obtained with a K0.5 of 0.27 nmol TTP/mg protein. In crude mitochondrial fractions from the brains of animals previously treated with thiamine or sulbutiamine, a positive correlation between 36Cl- uptake and TTP content was found. These results, together with other results previously obtained with the patch-clamp technique, suggest that TTP is an activator of chloride channels having a large unit conductance.

Subcellular localization and compartmentation of thiamine derivatives in rat brain

The subcellular distribution of thiamine derivatives in rat brain was studied. Thiamine diphosphate content was highest in the mitochondrial and synaptosomal fractions, and lowest in microsomal, myelin and cytosolic fractions. Only 3-5% of total thiamine diphosphate was bound to transketolase, a cytosolic enzyme. Thiamine triphosphate was barely detectable in the microsomal and cytosolic fraction, but synaptosomes were slightly enriched in this compound compared to the crude homogenate. Both myelin and mitochondrial fractions contained significant amounts of thiamine triphosphate. In order to estimate the relative turnover rates of these compounds, the animals received an intraperitoneal injection of either [14C]thiamine or [14C]sulbutiamine (isobutyrylthiamine disulfide) 1 h before decapitation. The specific radioactivities of thiamine compounds found in the brain decreased in the order: thiamine > thiamine triphosphate > thiamine monophosphate > thiamine diphosphate. Incorporation of radioactivity into thiamine triphosphate was more marked with [14C]sulbutiamine than with [14C]thiamine. The highest specific radioactivity of thiamine diphosphate was found in the cytosolic fraction of the brain, though this pool represents less than 10% of total thiamine diphosphate. Cytosolic thiamine diphosphate had a twice higher specific radioactivity when [14C]sulbutiamine was used as precursor compared with thiamine though no significant differences were found in the other cellular compartments. Our results suggest the existence of two thiamine diphosphate pools: the bound cofactor pool is essentially mitochondrial and has a low turnover; a much smaller cytosolic pool (6-7% of total TDP) of high turnover is the likely precursor of thiamine triphosphate.

The compartmentation of phosphorylated thiamine derivatives in cultured neuroblastoma cells

Thiamine transport in cultured neuroblastoma cells is mediated by a high-affinity carrier (KM = 40 nM). In contrast, the uptake of the more hydrophobic sulbutiamine (isobutyrylthiamine disulfide) is unsaturable and its initial transport rate is 20-times faster than for thiamine. In the cytoplasm, sulbutiamine is rapidly hydrolyzed and reduced to free thiamine, the overall process resulting in a rapid and concentrative thiamine accumulation. Incorporation of radioactivity from [14C]thiamine or [14C]sulbutiamine into intracellular thiamine diphosphate is slow in both cases. Despite the fact that the diphosphate is probably the direct precursor for both thiamine monophosphate and triphosphate, the specific radioactivity increased much faster for the latter two compounds than for thiamine diphosphate. This suggests the existence of two pools of thiamine diphosphate, the larger one having a very slow turnover (about 17 h); a much smaller, rapidly turning over pool would be the precursor of thiamine mono- and triphosphate. The turnover time for thiamine triphosphate could be estimated to be 1-2 h. When preloading the cells with [14C]sulbutiamine was followed by a chase with the same concentration of the unlabeled compound, the specific radioactivities of thiamine and thiamine monophosphate decreased exponentially as expected, but labeling of the diphosphate continued to increase slowly. Specific radioactivity of thiamine triphosphate increased first, but after 30 min it began to slowly decrease. These results show for the first time the existence of distinct thiamine diphosphate pools in the same homogeneous cell population. They also suggest a complex compartmentation of thiamine metabolism.

Mechanism of thiamine transport in neuroblastoma cells. Inhibition of a high affinity carrier by sodium channel activators and dependence of thiamine uptake on membrane potential and intracellular ATP

Nerve cells are particularly sensitive to thiamine deficiency. We studied thiamine transport in mouse neuroblastoma (Neuro 2a) cells. At low external concentration, [14C]thiamine was taken up through a saturable high affinity mechanism (Km = 35 nM). This was blocked by low concentrations of the Na+ channel activators veratridine (IC50 = 7 +/- 4 microM) and batrachotoxin (IC50 = 0.9 microM). These effects were not antagonized by tetrodotoxin and were also observed in cell lines devoid of Na+ channels, suggesting that these channels are not involved in the mechanism of inhibition. At high extracellular concentrations, thiamine uptake proceeds essentially via a low affinity carrier (Km = 0.8 mM), insensitive to veratridine but blocked by divalent cations. In both cases, the uptake was independent on external sodium, partially inhibited (10-35%) by depolarization and sensitive to metabolic inhibitors. A linear relationship between the rate of thiamine transport and intracellular ATP concentration was found. When cells grown in a medium of low thiamine concentration (6 nM) were exposed to 100 nM extracellular thiamine, a 3-fold increase in intracellular thiamine diphosphate was observed after 2 h while the concomitant increase in intracellular free thiamine was barely significant. These data suggest a secondary active transport of thiamine, the main driving force being thiamine phosphorylation rather than the sodium gradient.

Thiamine triphosphate activates an anion channel of large unit conductance in neuroblastoma cells

In neuroblastoma cells, the intracellular thiamine triphosphate (TTP) concentration was found to be about 0.5 microM, which is several times above the amount of cultured neurons or glial cells. In inside-out patches, addition of TTP (1 or 10 microM) to the bath activated an anion channel of large unit conductance (350-400 pS) in symmetrical 150 mM NaCl solution. The activation occurred after a delay of about 4 min and was not reversed when TTP was washed out. A possible explanation is that the channel has been irreversibly phosphorylated by TTP. The channel open probability (Po) shows a bell-shaped behavior as a function of pipette potential (Vp). Po is maximal for -25 mV < Vp < 10 mV and steeply decreases outside this potential range. From reversal potentials, permeability ratios of PCl/PNa = 20 and PCl/Pgluconate = 3 were estimated. ATP (5 mM) at the cytoplasmic side of the channel decreased the mean single channel conductance by about 50%, but thiamine derivatives did not affect unit conductance; 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (0.1 mM) increased the flickering of the channel between the open and closed state, finally leading to its closure. Addition of oxythiamine (1 mM), a thiamine antimetabolite, to the pipette filling solution potentiates the time-dependent inactivation of the channel at Vp = -20 mV but had the opposite effect at +30 mV. This finding corresponds to a shift of Po towards more negative resting membrane potentials. These observations agree with our previous results showing a modulation of chloride permeability by thiamine derivatives in membrane vesicles from rat brain.

Metabolism of thiamine triphosphate in rat brain: correlation with chloride permeability

Our results show that a net synthesis of thiamine triphosphate (TTP) can be demonstrated in vitro using rat brain extracts. The total homogenate was preincubated with thiamine or its diphosphate derivative (TDP), centrifuged, and washed twice. With TDP (1 mM) as substrate, a 10-fold increase in TTP content was observed in this fraction (nuclear fraction, membrane vesicles). A smaller, but significant, increase was observed in the P2 fraction (mitochondrial/synaptosomal fraction). In view of the low TTP content of our fractions, it was carefully assessed that authentic TTP was being formed. Incorporation of radioactivity from [beta-32P]TDP and [gamma-32P]ATP in TTP suggests that these two compounds are its precursors. Furthermore, TTP synthesis was inhibited by ADP and relatively low concentrations of Zn2+. These results suggest that TTP synthesis is catalyzed by an ATP:TDP transphosphorylase rather than by the cytoplasmic adenylate kinase that may be present in the vesicles. After osmotic lysis of the vesicles at alkaline pH, TTP was recovered in protein-bound form. Concomitantly, a soluble thiamine triphosphatase, with alkaline pH optimum, was also released from the vesicles. No net synthesis could be obtained in the cytosolic fraction or in detergent-solubilized systems. Like TTP synthesis, chloride permeability of the vesicles was increased when the homogenate had been incubated with thiamine and particularly with TDP. Our results suggest a regulatory role of TTP on chloride permeability, but the target remains to be characterized.

Thiamin and derivatives as modulators of rat brain chloride channels

Several membrane fractions were prepared from rat brain by differential and sucrose density gradient centrifugation. Most fractions took up 36Cl- rapidly at a rate linear with time during the first 30-60 s, then the rate progressively slowed down. The lowest rate of uptake was found in the mitochondrial fraction. Oxythiamin partially inhibited 36Cl- uptake in all fractions. In P2 (crude synaptosomal fraction), oxythiamin decreased the initial rate of uptake by 32%, the apparent Ki being 1.5 mM. Thiamin and amprolium were less effective as inhibitors. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid (0.1-1 mM) inhibited 36Cl- uptake by 40-50%. In the presence of this compound at a concentration > or = 5 x 10(-4) M, oxythiamin became ineffective. 36Cl- uptake was increased by GABA (0.1 mM) and this effect was antagonized by picrotoxin as expected, but not by oxythiamin. The rate of 36Cl- uptake did not appreciably depend on the external chloride concentration and was unaffected by bumetanide or by replacement of external Na+ by choline. Taken together, these data suggest that the oxythiamin-sensitive 36Cl- influx is essentially diffusional and is not related to the GABA receptor or the Na:K:2Cl co-transport. Partial replacement of external Na+ by K+ or treatment with 0.1 mM veratridine (which should both result in membrane depolarization) increased 36Cl- uptake by 50 and 30% respectively; the inhibitory effect of oxythiamin was enhanced to the same proportion.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of thiamin and its phosphate esters in cultured neurons and astrocytes using an ion-pair reversed-phase high-performance liquid chromatographic method

A sensitive method, based on fluorescence detection, for the determination of thiamin derivatives after precolumn derivatization is described. The separation is achieved on a PRP-1 column using ion-pair reversed-phase HPLC. This method is especially well adapted to the detection of thiamin triphosphate in complex mixtures such as tissue extracts. The detection limit for TTP is 50 fmol. The contents of thiamin derivatives were determined in primary cultures of rat cerebellar granule neurons and cerebral astrocytes. The amount of TTP is about five times higher in neurons than in astrocytes. Thus in rat brain TTP seems to be essentially associated with neurons and the intracellular concentration is estimated to be about 0.2 microM. Our results suggest the existence, in nerve cells, of specific regulatory mechanisms not related to the blood-brain barrier and responsible for the maintenance of thiamin homeostasis in brain.

Application of high-performance liquid chromatography to the study of thiamine metabolism and in particular thiamine triphosphatase

Thiamine triphosphate can be found in most tissues at very low levels, but its role is unknown. Organs and muscles that generate electrical impulses are particularly rich in this compound. This paper describes a thiamine triphosphatase from the electrical organ of Electrophorus electricus. The activity of this enzyme, as measured by a high-performance liquid chromatographic method, is closely anion-regulated. Furthermore, thiamine triphosphate increases chloride uptake in membrane vesicles prepared from rat brain. Our results suggest that this compound could play an important role in the regulation of chloride permeability.

Solubilization of thiamine triphosphatase from the electric organ of Electrophorus electricus

The membrane-associated, anion-regulated thiamine triphosphatase from Electrophorus electricus electric organ can be solubilized by various neutral detergents. Polyoxyethylene ethers are the most effective. Anionic detergents readily inactivate the enzyme. A 6.4-fold increase in specific activity is obtained by successive treatment of crude membranes with octanoyl-N-methylglucamide, which solubilized other proteins, and Lubrol-PX with releases 60% of the thiamine triphosphatase (TTPase) activity. Solubilization by Lubrol-PX rapidly modifies kinetic parameters. The Km, Vmax and pH optimum are decreased. However, the solubilized TTPase may be kept at 0 degrees C for many hours without further change in specific activity. At 35 degrees C, the half-life is still 83 min at pH 5.0, but denaturation becomes rapid at pH greater than or equal to 7. Solubilization modifies anion effects on TTPase activity. The activating effect of nitrate is nearly lost, while inhibition by sulfate is no longer time-dependent.

Regulation of ion uptake in membrane vesicles from rat brain by thiamine compounds

We examined the effects of thiamine derivatives on ion uptake in rat brain membrane vesicles. Thiamine triphosphate (1 mM) and pyrithiamine (0.1 mM) increase chloride uptake. Preincubation of crude homogenate with thiamine or pyrithiamine increases chloride uptake while oxythiamine has the reverse effect. Thiamine and oxythiamine also affect 22Na+ and 86Rb+ uptake in the same way as for 36Cl- but to a lesser extent. Thiamine-dependent 36Cl- uptake is activated by sodium bicarbonate (10 mM) and partially inhibited by bumetanide (0.1 mM) and 2,4-dinitrophenol (0.1 mM). Preincubation with thiamine increases the thiamine triphosphate content of the vesicles. The hypothesis that TTP is the activator of a particular chloride uptake mechanism is discussed.

Thiamine triphosphatase in the membranes of the main electric organ of Electrophorus electricus: substrate-enzyme interactions

The main electric organ of Electrophorus electricus is particularly rich in thiamine triphosphate (TTP). Membrane fractions prepared from this tissue contain a thiamine triphosphatase that is strongly activated by anions and irreversibly inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), an anion transport inhibitor. Kinetic parameters of the enzyme are markedly affected by the conditions of enzyme preparation: In crude membranes, the apparent Km is 1.8 mM and the pH optimum is 6.8, but trypsin treatment of these membranes or their purification on a sucrose gradient decreases both the apparent Km (to 0.2 mM) and the pH optimum (to 5.0). Anions such as NO3- (250 mM) have the opposite effect, i.e., even in purified membranes, the pH optimum is now 7.8 and the Km is 1.1 mM; at pH 7.8, NO3- increases the Vmax 24-fold. TTP protects against inhibition by DIDS, and the KD for TTP could be estimated to be 0.25 mM, a value close to the apparent Km measured in the same purified membrane preparation. Thiamine pyrophosphate (0.1 mM) did not protect against DIDS inhibition. At lower (10(-5)-10(-6) M) substrate concentrations, Lineweaver-Burk plots of thiamine triphosphatase activity markedly deviate from linearity, with the curve being concave downward. This suggests either anticooperative binding or the existence of binding sites with different affinities for TTP. The latter possibility is supported by binding data obtained using [gamma-32P]TTP. Our data suggest the existence of a high-affinity binding site (KD of approximately 0.5 microM) for the Mg-TTP complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylated thiamine derivatives and cortical activity in the baboon Papio papio: effect of intermittent light stimulation

The effect of intermittent light stimulation (ILS) on the distribution of thiamine derivatives in three brain areas (occipital, motor, and premotor) was compared in photosensitive and nonphotosensitive baboons. ILS induces paroxysmal discharges in the motor and premotor areas of photosensitive animals only. In baboons submitted to ILS, thiamine triphosphate (TTP) decreases in both photosensitive and nonphotosensitive animals; thiamine monophosphate (TMP) increases in photosensitive animals, which present ILS-induced paroxysmal discharges, whereas it is unaffected in nonphotosensitive animals. The variations are the most significant in the occipital (visual) cortex. A consumption of TTP may result from electrical activity induced by light stimulation in the occipital area. No correlation between ILS-induced paroxysmal activity and a decrease in TTP contents was found. However, photosensitive animals are affected differently from nonphotosensitive animals, as their content of TMP in the cerebral cortex increases on stimulation. However, as long as the exact role of thiamine compounds in relation to membrane excitability in the nervous system remains unknown, it is impossible to conclude whether the differences observed in the metabolism of thiamine compounds are the cause or the consequence of the photosensitivity in the baboon Papio papio.

Solubilization of sodium channel from human brain

[3H]Tetrodotoxin binds to a single class of receptor sites in homogenates of human brain with a KD of 9.1 nM at 0 degree C and a maximal binding capacity of 5.9 pmol/mg of protein. This tetrodotoxin receptor has been solubilized, and several parameters influencing the efficiency of this critical step have been studied. Treatment of brain membranes with 2% (wt/vol) Nonidet P-40 solubilizes up to 38% of the tetrodotoxin receptor sites. The duration of this solubilization step must not exceed 15 min at an optimal pH of 6.8. The binding activity is most stable when exogenous phosphatidylcholine is added to the soluble receptor with a phosphatidylcholine/detergent ratio of 1:5.

Thiamine triphosphatase from Electrophorus electric organ is anion-dependent and irreversibly inhibited by 4,4'-diisothiocyanostilbene-2,2'disulfonic acid

Thiamine triphosphatase (TTPase) from membranes isolated from the main electric organ of E. electricus is activated about 8 fold by NO3-, I- and SCN- while SO42- is inhibitory. Activating anions shift the pH optimum of the enzyme from 5.0 to 8.0. The enzyme is irreversibly inactivated by low concentrations of 4,4'-diisothiocyano-2,2' disulfonic acid (DIDS), an inhibitor of anion transport. Anions protect from DIDS inactivation. These and other results suggest that the membrane-bound TTPase activity is tightly controlled, possibly through mechanisms involving anion transport.

Synthesis of [gamma-32P]thiamine triphosphate

We developed a novel chemical synthesis of thiamine triphosphate which allows us to incorporate 32P in the gamma position. The reaction is based on the condensation of [32P]orthophosphoric acid and thiamine diphosphate in the presence of ethyl chloroformate. After purification by two ion-exchange purification steps, the thiamine derivative has a specific radioactivity of 10 Ci/mmol. The average final yield synthesis is about 10%.

Thiamine triphosphate and membrane-associated thiamine phosphatases in the electric organ of Electrophorus electricus

The main electric organ of Electrophorus electricus is particularly rich in thiamine triphosphate, which represents 87% of the total thiamine content in this tissue. The thiamine pyrophosphate concentration, however, is very low in the eel electric organ and skeletal muscle as compared with other eel or rat tissues. Furthermore, electroplax membranes contain a whole set of enzymes responsible for the dephosphorylation of thiamine tri-, pyro- and monophosphate. Thiamine triphosphatase has a pH optimum of 6.8 and is dependent on Mg2+. The real substrate of the enzyme is probably a 1:1 complex of Mg2+ and thiamine triphosphate. Thiamine pyrophosphatase is activated by Ca2+. The apparent Km for thiamine triphosphate and Vmax are found to be, respectively, 1.76 mM and 5.95 nmol/mg of protein/min. Thiamine triphosphatase activity is inhibited at physiological K+ concentrations (up to 90 mM) and increasing Na+ concentrations (50% inhibition at 300 mM). ZnCl2 (10 mM) inhibits 90% of the enzyme activity. ATP and ITP are also strongly inhibitory. No significant effect of neurotoxins is seen. Membrane-associated thiamine triphosphatase is affected differently by proteolytic enzymes and is partially inactivated by pretreatment with phospholipase C and neuraminidase. The physiological significance of thiamine triphosphatase is discussed in relation to a specific role of thiamine in the nervous system.

Phosphate derivatives of thiamine and Na+ channel in conducting membranes

The results show that thiamine derivatives are copurified with the specific proteins forming the Na+ channel in conducting membranes. Therefore, thiamine derivatives could well play a specific role in the molecular aspects of bioelectrogenesis , an interpretation that could help explain the neurological symptoms observed in human pathology as well as in animals experimentally rendered deficient in vitamin B1.

Determination of thiamine and thiamine phosphates in excitable tissues as thiochrome derivatives by reversed-phase high-performance liquid chromatography on octadecyl silica

The analysis of thiamine and thiamine phosphates by high-performance liquid chromatography owes its high sensitivity to the fluorescent derivatives or thiochromes obtained by chemical oxidation in alkaline medium. The possibility of performing precolumn oxidation with potassium ferricyanide instead of using the hazardous cyanogen bromide has been investigated. The derivatization step has been optimized with respect to the following parameters: concentration of alkali and oxidant, presence of methanol and stability of the thiochromes . A gradient separation with 25 mM phosphate buffer (pH 8.4) and methanol as mobile phase components and an octadecyl silica column as stationary phase has been set up. The analytical run takes 14 min with the following elution order: thiochrome triphosphate, thiochrome pyrophosphate, thiochrome monophosphate and thiochrome. The minimum detectable amount is 0.05 pmol. The method was found suitable for the determination of thiamine compounds in excitable tissues such as nerves and electric organs as well as in proteins extracted from membranes of these organs. It may be useful to study the role of thiamine in the electrical activity of these tissues at the molecular level.