Bioinactive ACTH causing glucocorticoid deficiency

CONTEXT

A 4-year-old girl and a 4-month-old boy presented with hypoglycemia, normal electrolytes, low cortisol, and high ACTH. A diagnosis of primary adrenal insufficiency was made and initial treatment was with glucocorticoids and mineralocorticoids. The genes known to cause ACTH resistance were normal. Whole exome sequencing revealed that the girl was compound heterozygous for POMC mutations: one previously described null allele and one novel p.R8C mutation in the sequence encoding ACTH and α-MSH. The boy was homozygous for the p.R8C mutation.

HYPOTHESIS

The p.R8C ACTH mutant is immunoreactive, but the mutant peptides, ACTH-R8C and α-MSH-R8C, are bioinactive.

METHODS

Methods included whole exome sequencing, Sanger sequencing, peptide synthesis, ACTH immunoradiometric assay, hormone binding, and activation assays in cells expressing melanocortin receptors.

RESULTS

ACTH-R8C was immunoreactive but failed to bind and activate cAMP production in melanocortin-2 receptor (MC2R)-expressing cells, and α-MSH-R8C failed to bind and stimulate cAMP production in MC1R- and MC4R-expressing cells.

CONCLUSION

These are the first documented cases of glucocorticoid deficiency due to the secretion of an ACTH molecule that lacks biological bioactivity but conserves immunoreactivity. POMC mutations should thus be considered in patients presenting with apparent ACTH resistance. Our findings also highlight a limitation to immunoassay-based diagnostics and demonstrate the value of genetic analysis. Establishing the molecular etiology of the disorder in our patients allowed cessation of the unnecessary mineralocorticoids. Finally, discovery of this mutation indicates that in humans, the amino acid sequence His(6)Phe(7)Arg(8)Trp(9) is important not only for cAMP activation but also for ACTH binding to MC2R.

Alpha2-adrenoceptor activation increases a cationic conductance and spontaneous GABAergic synaptic activity in dopaminergic neurones of the rat substantia nigra

Noradrenaline (NA) plays an important role in compensating for the loss in dopaminergic (DA) function following lesions of the DA neurones of the substantia nigra (SN). Alpha2-adrenoceptors are largely expressed in these neurones, but the cellular response to their activation is unknown. Whole-cell patch-clamp recordings were made from DA neurones of rat SN. At a holding potential of -60 mV, bath application of NA (50 microM) induced an inward current (-20.3+/-10.0 pA) in 50% of the recorded neurones. This effect was mimicked by UK-14304 (50 microM), a specific alpha2-adrenoceptor agonist, whereas alpha1-adrenoceptor and beta-adrenoceptor agonists failed to induce a response. Surprisingly, alpha2-adrenoceptor antagonists (idazoxan, RX-811059, SKF-86466 and yohimbine) also induced an inward current that could occlude the one induced by UK-14304, suggesting that they may act as alpha2-adrenoceptor agonists. The inward current results from an increase in cationic conductance identical to the one previously described in these neurones, as neurotensin (1 microM), known to activate it, occluded the inward current induced by UK-14304. In addition, GABAergic miniature inhibitory postsynaptic current frequency was increased by activation of presynaptic alpha2-adrenoceptors. We conclude that the effects of NA on alpha2-adrenoceptors can contribute to the previously described composite action of NA on DA neurone firing and can be pharmacologically differentiated from the effect of NA on DA and neighbouring neurones known to be mediated through alpha1-adrenoceptors.

[The synaptology of 2 types of neurons in the sympathetic ganglia of the frog]

This work is based on the selective section of the B or C preganglionic fibres innervating respectively the B or C neurons of the last two abdominal ganglia of the sympathetic chain in the frog. A quantitative study of three morphological features of their synapses was performed for each of the two neuronal types. Significant differences were observed in the mean size of the preganglionic ending sections; on the contrary there were no differences in the mean length of the active zones. The subsynaptic apparatus, which is fairly frequent in this material, is not restricted to one neuronal type, although much more frequent in the synapses of B neurons. The modifications of the innervation of ganglionic neurons after selective section of one type of preganglionic fibres were followed measuring the synaptic density, using a synaptic index and an index of simple contacts. It appeared that, after selective degeneration of preganglionic C fibres, their regeneration being prevented, the preganglionic B fibres were able to from functional synapses on C neurons with a short latency, normal values of synaptic density being reached within 2 months. On the contrary, after degeneration of the preganglionic B fibres, the preganglionic C fibres reinnervated B neurons with a limited efficiency, in such a way that the synaptic density remained after several months much lower than normal and that electrophysiological recordings of synaptic transmission were limited to 44% of the impaled neurons. Possible interpretations of these data are discussed.

Reinnervation of frog sympathetic ganglia after selective denervation of B or C neurons

Selective transection of the B or C preganglionic nerve fibres respectively innervating the B and C sympathetic neurons was carried out on the last two ganglia of the sympathetic chain of the frog Rana esculenta. At different times thereafter, the cross-reinnervation of one type of denervated neuron by nerve endings sprouting within the ganglia from intact fibres innervating the other type was investigated by both the quantitative morphology of the synaptic contacts and related structures and electrophysiological recordings of ganglionic transmission. As there are no fine ultrastructural criteria for distinguishing B from C neurons, the overall density of synapse, simple contact, and 'vacated' postsynaptic differentiation profiles was measured in the two cases of selective section and compared with the values for normal ganglia, therefore permitting the progress of cross-reinnervation with time for each type of neuron to be followed. At ten days after section of the C preganglionic fibres, immunocytochemistry showed that there were no anti-LH-RH-like peptide containing fibres within the ganglia. The B myelinated preganglionic fibres were able to reinnervate the denervated C neurons, with return to normal values of synaptic density and fully efficient transmission at two months in all tested C neurons. However, the latency of orthodromic action potentials was close to that of normally innervated B neurons. In contrast, the C non-myelinated preganglionic fibres reinnervated the denervated B neurons with limited efficiency, the synaptic density being two-thirds the normal value after five months, while subthreshold excitatory postsynaptic potentials or action potentials were only recorded in 44% of the tested B neurons. The latency of these orthodromic responses was close to that of normally innervated C neurons. It is postulated that the poor cross-reinnervation of B neurons could be due to insufficient sprouting of C fibres and/or lack of 'affinity' between C fibres and B neurons. In addition, these experiments demonstrated that the subsynaptic apparatus, fairly characteristic of frog ganglionic synapses, is present in both types of sympathetic neurons, although predominantly in B neurons.

Synaptic organization of amphibian sympathetic ganglia

The synaptic organization of the amphibian sympathetic ganglia was studied, especially in the last two abdominal paravertebral ganglia of the frog. These ganglia appear to form a monosynaptic relay, not containing interneurons. They consist of two systems working in parallel: the principal neurons, by far the most numerous, and a small number of chromaffin (i.e., SIF) cells, usually arranged in clusters. Each principal neuron is innervated by a preganglionic branch forming a set of cholinergic synapses which exhibit classical ultrastructure. The only peculiarity is the presence of a subsynaptic apparatus in a variable percentage of synaptic complexes. Electrophysiological studies have demonstrated that synaptic transmission is due to ACh release and involves several postsynaptic potentials. Moreover, the principal neurons are of two types, B and C, whose preganglionic axons and their own axons have different conduction velocities. C neurons tend to be small in diameter, and B neurons are larger, but the size distribution of the two populations overlaps. More recently, it was demonstrated that these two neuronal systems have different immunocytochemical features. The C preganglionic fibers contain an LHRH-like peptide, which is responsible for late synaptic events. The B preganglionic fibers contain CGRP, whose role has not yet been established. The principal neurons all contain adrenaline, but neuropeptide Y is also present in C neurons and could be a second transmitter at peripheral junctions. SP-containing fibers also pass through the ganglia, but give rise to intraganglionic synapses only rarely, except in the celiac plexus. Galanin can coexist with neuropeptide Y in certain C neurons. Numerous principal neurons are immunoreactive for VIP. Chromaffin cells contain noradrenaline and metenkephalin, and some contain SP or LHRH; they are endocrine cells controlled by preganglionic fibers and can have a modulatory effect on principal neurons endowed with appropriate receptors. The accessibility of frog abdominal ganglia and the anatomical separation of B and C preganglionic fiber pathways provide interesting systems in which to carry out experimentation on the stability and specificity of synaptic contacts. After postganglionic axotomy, the majority of synapses disappear by disruption of synaptic contacts. There is a certain discrepancy between the recovery of synaptic transmission and the reappearance of morphologically identifiable synapses, suggesting that a certain amount of transmission is possible at contacts devoid of synaptic complexes. The selective deafferentation of B or C neurons showed that the subsynaptic apparati are mainly found at B neuron synapses. The course of reinnervation following selective deafferentation reveals the existence of different specificities at B and C synapses: C neurons are easily reinnervated by B preganglionic fibers, whereas C fibers appear fairly ineffective at reinnervating B neurons, even after a long interval. Attempts were made to reinnervate ganglionic neurons with somatic motor nerve fibers. Reinnervation was achieved only rarely, and it is concluded that the ganglionic synapses in the frog have a higher specificity and lower plasticity than in mammals.

Toxicity of the antiandrogen flutamide in isolated rat hepatocytes

The hepatotoxicity of flutamide, an antiandrogen that produces hepatitis in some human recipients, was studied in isolated rat hepatocytes. Flutamide (1 mM) led to the covalent binding of reactive electrophilic metabolites to male rat hepatocyte proteins. It decreased the reduced glutathione (GSH)/glutathione disulfide ratio and total protein thiols. This was associated with an early increase in phosphorylase a activity (a Ca(++)-dependent enzyme) and a decrease in cytoskeleton-associated protein thiols, the formation of plasma membrane blebs, the release of lactate dehydrogenase (LDH) and a loss of cell viability. Both covalent binding and LDH release were decreased by piperonyl butoxide (an inhibitor of cytochrome P450) and increased by dexamethasone pretreatment (which induces cytochrome P450 3A). The toxicity was increased by beta-naphthoflavone (which induces cytochrome P450 1A). Hepatocytes from female rats (which lack cytochrome P450 3A2) exhibited lower covalent binding and lower LDH release. The addition of cystine (a GSH precursor) increased hepatocellular GSH and decreased LDH release in male hepatocytes. The administration of a diet deficient in sulfur-containing amino acids had the opposite effects; it produced toxicity with 100 microM flutamide. Flutamide (50 microM) markedly inhibited respiration (mainly at the level of complex I) in isolated male rat liver mitochondria and flutamide (1 mM) decreased ATP levels in isolated male rat hepatocytes. It was concluded that flutamide is toxic to rat hepatocytes as a result of the cytochrome P450 (3A and also 1A)-mediated formation of electrophilic metabolites, whose damaging effects are further aggravated by the inhibitory effect of flutamide on mitochondrial respiration and ATP formation.

Metabolic activation of the nitroaromatic antiandrogen flutamide by rat and human cytochromes P-450, including forms belonging to the 3A and 1A subfamilies

The in vitro metabolic activation of flutamide, a nitroaromatic antiandrogen which produces hepatitis in a few recipients, was first studied with male rat liver microsomes. There was no electron spin resonance evidence for the reduction of flutamide by reduced nicotinamide adenine dinucleotide phosphate (NADPH)-cytochrome P-450 reductase into a nitro anion free radical. In contrast, flutamide was oxidatively transformed by cytochrome P-450 into reactive metabolite(s) that covalently bound to microsomal proteins. Covalent binding required oxygen and NADPH, and was decreased by the nucleophile glutathione and by the cytochrome P-450 inhibitors SKF 525-A, piperonyl butoxide and troleandomycin (an inhibitor of the cytochrome P-450 3A subfamily). Covalent binding was increased markedly by pretreatment with dexamethasone (an inducer of the cytochrome P-450 3A subfamily) and moderately by pretreatment with beta-naphthoflavone (an inducer of the 1A family). Covalent binding was immunoinhibited markedly by anticytochrome P-450 3A immunoglobulin G and moderately by anticytochrome P-450 1A immunoglobulin G. Covalent binding was much lower with liver microsomes from female rats (not expressing P-450 3A2). Covalent binding of flutamide also occurred with human liver microsomes (where it was inhibited by troleandomycin), and with yeast microsomes expressing human liver cytochromes P-450 1A1, 1A2 or 3A4. We concluded that flutamide was oxidatively transformed into chemically reactive metabolite(s) by rat and human cytochromes P-450, including forms belonging to the 3A and 1A subfamilies.

Mechanism for the hepatotoxicity of the antiandrogen, nilutamide. Evidence suggesting that redox cycling of this nitroaromatic drug leads to oxidative stress in isolated hepatocytes

The nitroaromatic drug nilutamide has been shown previously to undergo redox cycling in aerobic rat liver microsomes, being reduced by NADPH-cytochrome P-450 reductase to a nitro anion-free radical which reacts with oxygen, to regenerate the parent drug, and form a superoxide anion dismuted to hydrogen peroxide. In the present study, the effects of nilutamide on isolated rat hepatocytes have been determined. After 6 and 8 hr of incubation with 0.5 mM nilutamide, lactate dehydrogenase was released in the incubation medium, and cell viability was decreased markedly. Consistent with a redox cycle producing reactive oxygen species, nilutamide increased nonmitochondrial (cyanide-resistant) oxygen consumption; the toxicity of nilutamide occurred sooner and was more extensive in the presence of sodium azide (an inhibitor of catalase). Consistent with an oxidative stress, the toxicity of nilutamide was associated with depletion of reduced glutathione, increased levels of glutathione disulfide, increased Ca(++)-dependent phosphorylase a activity, oxidation and accumulation of cytoskeleton-associated proteins and formation of blebs; toxicity was prevented by glutathione precursors, thiol reductants and/or antioxidants, such as L-cystine, L-cysteine, N-acetyl-L-cysteine, dithiothreitol, N,N'-diphenyl-p-phenylene-diamine and alpha-tocopherol. Feeding the animals with a diet supplemented with 2% L-cystine increased the initial glutathione stores of hepatocytes and prevented nilutamide toxicity. It is concluded that nilutamide is toxic to isolated rat hepatocytes, as a probable consequence of an oxidative stress due to the redox cycling of this nitroaromatic compound.

Effects of axotomy on synaptic transmission and structure in frog sympathetic ganglia

Axotomy was carried out on frog sympathetic neurons of the two last lumbar chain ganglia. At different times thereafter, synaptic transmission was analysed electrophysiologically by intracellular microelectrodes and compared with synaptic density, measured by electron microscopy in the same ganglia. For this purpose, modifications in synaptic transmission were estimated first, by the numbers of B and C sympathetic neurons exhibiting subthreshold excitatory postsynaptic potentials in response to 10 Hz orthodromic stimulation of preganglionic fibres, and second, by the amplitude and number of excitatory postsynaptic potentials occurring over 5-10 s periods of 10 Hz stimulation. By distinguishing two types of morphological relationships between the pre- and postsynaptic elements, two contact indices were defined: a synaptic index (ratio of the number of synapses encountered to the number of perikarya explored) and a simple contact index corresponding to the same type of contacts, but without any membrane differentiation. Both the electrophysiological and morphological results showed that the first effects were detectable 4 days after axotomy, and that the main alterations in synaptic transmission and density occurred at 2 weeks. In addition, while in normal ganglia the excitatory postsynaptic potentials of B and C neurons reached the threshold for action potential generation in response to 10 Hz stimulation, about 29% of the axotomized neurons had subthreshold excitatory postsynaptic potentials 1 week after section. At 2 weeks, this proportion reached 65%, and the synaptic and simple contact indices, at 90% and 60% respectively, were significantly lower than the control ganglion indices. At longer times after axotomy, there was a discrepancy between the morphological and electrophysiological results: at 1 month, the synaptic index seemed to rise as the decline in the efficacy of synaptic transmission became more marked. The amplitude of the subthreshold excitatory postsynaptic potentials recorded in B neurons was 5.5 +/- 2.8 mV (mean +/- SD, n = 18); this value was significantly lower by about 50% than that measured 1 week after axotomy. In addition, the number of excitatory postsynaptic potentials in B neurons reached an average maximum of 83 +/- 29 for 100 stimuli applied at 10 Hz. Similar results were obtained for C neurons. Two months after axotomy, the physiological and morphological parameters of synaptic efficacy began to recover and return to normal values, but had not reached them by 4 months. These observations show that some synaptic transmission remains possible, even with a much reduced number of synaptic complexes. It is suggested that after axotomy, simple contacts also might be involved in synaptic transmission.