Evidence for ADP-ribosylation in the mechanism of rapid thyroid hormone control of mitochondria

NAD and ADP-ribose metabolism in mitochondria

Mitochondrial metabolism is intimately connected to the universal coenzyme NAD. In addition to its role in redox reactions of energy transduction, NAD serves as substrate in regulatory reactions that lead to its degradation. Importantly, all types of the known NAD-consuming signalling reactions have been reported to take place in mitochondria. These reactions include the generation of second messengers, as well as post-translational protein modifications such as ADP-ribosylation and protein deacetylation. Therefore, the availability and redox state of NAD emerged as important factors in the regulation of mitochondrial metabolism. Molecular mechanisms and targets of mitochondrial NAD-dependent protein deacetylation and mono-ADP-ribosylation have been established, whereas poly-ADP-ribosylation and NAD-derived messenger generation in the organelles await in-depth characterization. In this review, we highlight the major NAD-dependent reactions occurring within mitochondria and describe their metabolic and regulatory functions. We also discuss the metabolic fates of the NAD-degradation products, nicotinamide and ADP-ribose, and how the mitochondrial NAD pool is restored.

Direct thyroid hormone signalling via ADP-ribosylation controls mitochondrial nucleotide transport and membrane leakiness by changing the conformation of the adenine nucleotide transporter

Addition of triiodothyronine at 10 pM in vitro to hypothyroid rat liver mitochondria doubles the rate of the adenine nucleotide transporter at low ADP concentrations. Nicotinamide abolishes this effect in parallel with its inhibition of the ADP-ribosylation of an inner membrane protein identical in size to the transporter. Nicotinamide also renders euthyroid preparations indistinguishable from hypothyroid ones. A mechanism is offered to explain these findings in which it is proposed that the adenine nucleotide transporter is a true allosteric protein and that its covalent modification by ADP-ribosylation increases the stability of the less favoured externally-facing C-conformation and thus increases the proportion of transporters in this orientation: although the C-conformation is significantly more leaky to cations than the tight matrix-facing M-conformation, this enhances ADP import. This model is shown to offer an explanation not only for the transport effects of T3 but also for those of oxidative stress and ADP-ribosylation inhibitors on Ca2+, H+ and K+ transfer across the mitochondrial inner membrane. Ca2+ at 30 nM appears to stabilize the M-conformation of the transporter by a mechanism other than ADP-ribosylation.

Thyroid hormones and the creatine kinase system in cardiac cells

The paper reviews the current evidence on the role of thyroid hormones in regulating the creatine kinase energy transfer system at multiple structures in cardiac cells. 1) Thyroid hormones modulate the overall synthesis of phosphocreatine (PCr) by increasing the rate of mitochondrial oxidative phosphorylation. 2) Thyroid hormones regulate the total activity of creatine kinase and its isoenzyme distribution. In comparison with normal thyroid state (euthyroidism), hypothyroidism is characterized by decreased total creatine kinase activity owing to diminished fraction of creatine kinase. On the other hand, hyperthyroidism, while causing no change in total creatine kinase activity, leads to increased fractions of neonatal isoforms of creatine kinase, and, in case of prolonged hyperthyroidism, to decreased fraction of mitochondrial creatine kinase. The latter change is associated with partial uncoupling between mitochondrial creatine kinase and adenine nucleotide translocase reflected by decreased PCr/O ratio. 3) Hyperthyroidism leads to increased passive sarcolemmal permeability due to which the leakage of creatine along its concentration gradient occurs. As a result of (i) increased sarcolemmal permeability for creatine, (ii) uncoupling of mitochondrial PCr synthesis, and (iii) increased energy utilization rate the steady state intracellular PCr content decreases under hyperthyroidism which, in turn, increases the myocardial susceptibility to hypoxic damage. Thyroid state also modulates the protective effects of exogenous PCr on energetically depleted myocardium.

The control of cellular adenine nucleotides: the discovery and metabolism of an oligomeric derivative of ATP and phosphoglyceric acid in mitochondrial intermembrane space

The observation of large systematic variations in the purine nucleotides in perfused rat hearts inexplicable by known metabolic transformations led us to propose the existence of some unknown major form of nucleotide. Radiolabelling studies located a rapidly metabolized TCA/methanol-insoluble species in heart, kidney and liver which selective digestion and chromatography allied to 31P-nmr and mass spectrometry suggested to be a tetraphosphate derivative of adenosine and phosphoglyceric acid of the form p3Gri1p[pp5'p3Gri1p]npp5'A, oligo-phosphoglyceroyl-ATP. Partial chemical synthesis has confirmed this basic structure. We have shown that substantial amounts of oPG-ATP are located in liver mitochondrial intermembrane space tightly bound to a specific 3' phosphodiesterase which releases the monomer p3Gri1ppp5'A. Here we show that oPG-ATP is remarkably stabilized by Mg2+ ions and describe the 300-fold purification of the stable substrate-free form of the 3' phosphodiesterase; it has an apparent M(r) = 165k with possibly up to 4 subunits of M(r) = 42.5k, 41.9k, 40.5k and 38.9k. The potential function of oPG-ATP is briefly discussed.

The rapid response of isolated mitochondrial particles to 0.1 nM-tri-iodothyronine correlates with the ADP-ribosylation of a single inner-membrane protein

Under defined conditions liver mitochondria from hypothyroid rats show an apparent lowering of the ADP/O ratio, which can be corrected by addition in vitro of 0.1 nM-tri-iodothyronine (T3). Nicotinamide prevents this restoration by hormone, lowers the ADP/O ratio of euthyroid-rat mitochondria to hypothyroid-rat values and induces T3-sensitivity in euthyroid-rat mitoplasts indistinguishable from that found with hypothyroid-rat preparations. Incorporation into the trichloroacetic-acid insoluble fraction of mitoplasts and hypothyroid-rat mitochondria of radiolabel from [adenine-14C]-NAD+ was stimulated by T3: this stimulation was abolished by nicotinamide. The findings strongly suggest that this incorporation occurs external to the matrix. Confirming the work of others, PAGE of radiolabelled mitoplasts shows alkali-labile modification of a major species of approx. 30 kDa: both nicotinamide and T3 abolish this modification. By contrast, T3 promotes incorporation of label into a single major 11 kDa species: this incorporated label is somewhat acid-labile, and the incorporation is abolished by nicotinamide. Comparative electrophoresis of purified sub-mitoplast fractions show that the 11 kDa species is in the inner membrane and absent from the matrix. The findings are consistent with a receptor-mediated ADP-ribosylation mechanism for the rapid action of T3 on mitochondria.

The characterization of a new enzyme from rat liver mitochondria, oligophosphoglyceroyl-ATP 3'-phosphodiesterase

By using an assay based on the precipitation of intact 14C-labelled substrate, an activity has been located in the mitochondrial fraction of rat liver which selectively hydrolyses the 3' ester link in the fairly recently discovered oligomeric tetraphosphate derivative of ATP and glyceric acid for which the structure 3-phospho[glyceroyl-gamma-triphospho-5'-adenosine-3'-3-phospho]n-glyceroyl- gamma-triphospho-5'-adenosine has been proposed [Hutchinson, Morris & Mowbray (1986) Biochem. J. 234, 623-627]. This enzyme activity (Mr 85,000) has been purified approx. 30-fold from washed mitochondria by (NH4)2SO4 precipitation and f.p.l.c. The apparent Km for substrate (adenosine equivalents) is around 35 microM. The recovery of total activity is about 20%, and this, allied to the relatively low Vmax. found in contrast with the rapid turnover of oligomer seen in post-ischaemic tissues, suggests that some activating factors have been lost in purification. Percoll-gradient studies confirm that the activity is mitochondrial and not lysosomal or endoplasmic-reticular. The activity is latent in intact mitochondria; it is not, however, associated with intact inner-membrane vesicles but released during their preparation, implying an intermembrane-space location. The product of the enzyme is proposed to be the monomeric unit 3-phosphoglyceroyl-gamma-triphospho-5'-adenosine, from which digestion with snake-venom phosphodiesterase releases ADP.

Reversal and inhibition of cholera toxin-induced secretion in isolated rabbit ileum

1. Cholera toxin (1 microgram/ml) abolished net fluid absorption by everted sacs of rabbit ileum, leading to net fluid secretion. This action occurred via the toxin-catalysed ADP ribosylation of the stimulatory GTP-binding protein Gs which is linked to adenylate cyclase. Nicotinamide (10 mM), a reaction product of ADP ribosylation, reversed cholera toxin-induced secretion, restoring absorption. Lower concentrations of nicotinamide induced partial reversal. 2. Nicotinamide (1 mM) blocked the secretory action of cholera toxin applied to ileal sacs. This inhibitory action was more effective in the presence of methionine (1 mM). 3. Other inhibitors of ADP ribosylation, benzamide and adenine, blocked the secretory action of cholera toxin. Hypoxanthine, an analogue and metabolite of adenine, was similarly effective. 4. Nicotinamide was not, however, effective in blocking or reversing the secretory action of theophylline (10 mM) or of heat-stable E. coli enterotoxin STa. This indicates that nicotinamide has a highly specific action against ADP ribosylating toxins. 5. It is proposed that nicotinamide reverses the secretory action of cholera toxin by reversing ADP ribosylation, simply by the law of mass action. This counters the established idea that the effects of cholera and other ADP-ribosylating toxins are irreversible under physiological conditions.

Thyroid hormone effects on the proton permeability of rat liver mitochondria

The effect of thyroid status on passive permeability of liver mitochondrial inner membrane was studied in euthyroid (EU), hyperthyroid (HYPER), hypothyroid (HYPO) rats and hypothyroid rats treated with a single dose of 3,3',5-triiodothyronine (HYPO + T3). Respiration was titrated with uncoupler and plotted against uncoupler concentration to calculate proton permeability. The passive permeability was the same in EU, HYPER, and HYPO + T3 rats, but was significantly lower in HYPO mitochondria. The State IV respiration rates were the same as the oligomycin-inhibited rats for EU, HYPO and HYPO + T3 rats, but in HYPER rats the oligomycin rate was less than the State IV rate. The data show that hypothyroid mitochondria have decreased permeability of the inner membrane which can be normalized by a single dose of T3. Hyperthyroid mitochondria show increased oligomycin-sensitive ATPase activity in State IV, a possible mechanism of the enhanced respiratory rate.

On the cytotoxicity of chrysotile asbestos fibers toward pulmonary alveolar macrophages. II. Effects of nicotinamide on the cell metabolism

Since it was recently shown that the addition of nicotinamide (NAM) to pulmonary alveolar macrophages (PAM) cell monolayers significantly altered their ATP pools (Nadeau and Lane, 1988), the effects of the vitamin on the metabolism of the cells, exposed or not to very short chrysotile asbestos fibers (VSF), were evaluated. First, it was found that the addition of NAM to the culture medium caused a dose-dependent (5-30 mM) decrease in the extracellular liberation of lactate and pyruvate by PAM. This is suggestive of a direct effect of NAM on the metabolism of glucose. A decrease in extracellular lactate was also observed when control PAM were exposed to 50 micrograms of VSF asbestos. This latter effect was however progressively abolished when the NAM was added to the asbestos-exposed cell monolayers. Second, contrary to the lactic acid production, the exposure to chrysotile caused an increase in the extracellular liberation of pyruvate by PAM. This cell response to the asbestos fibers could represent an antioxidative defense mechanism. Yet, interestingly enough, this effect of the VSF on PAM was not suppressed by the presence of the vitamin. The NAM also induced a dose-dependent decrease in the total lactate dehydrogenase content of PAM monolayers. By comparison, 3-aminobenzamide (up to 5 mM) did not appreciably modify these parameters. After an 18-hr incubation period with 20 mM NAM, the NAD+ pools of control PAM increased by approximately 300% comparatively to a approximately 40% increase for the NADP+ content. The exposure to the VSF asbestos caused a dose-dependent depletion of the cellular NAD+ and NADH pools. However, for the latter, the vitamin prevented the depletion effect of the asbestos fibers. Comparatively, the NADP(H) pools increased. This shift toward the phosphorylated pyridine nucleotide forms could also represent a defense of the cell against the oxygen radicals produced during the ingestion of the fibers. Overall, it is shown that changes in the energy metabolism could be implicated in the toxicity of chrysotile asbestos fibers toward PAM, and that the cells seem to be able to respond to an oxidant stress. Although not fully elucidated at the present time, these data tend nonetheless to point out that the protective effect of NAM could involve some modifications of the host defenses against prooxidants.