Therapeutic implications of non-genomic glucocorticoid activity

COMMENTARY

exemplify the importance of carefully integrating VCT activities into the existing health systems according to local well-identified priorities. Since such strategies have been shown to be successful for condom promotion and other means of prevention, social marketing of a VCT package has also been suggested, but it has not been evaluated. One of the more difficult challenges may well be to convince health professionals that VCT is their job. The next decade should see the integration of care and support for all HIV-infected individuals worldwide. A broad role for health-care providers, to include coverage of the psychosocial aspects of health, with accessible and affordable health-care services of an acceptable quality, will be instrumental. Philippe Van de Perre Centre Muraz, Organisation de Coordination et de Coopération pour la lutte contre les Grandes Endémies (OCCGE), Bobo-Dioulasso, Burkina Faso 1 2

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Foster S, Buvé A. Benefits of HIV screening of blood transfusions in Zambia. Lancet 1995; 346: 225–27. Anglaret X, Chene G, Attia A, et al. Early chemoprophylaxis with trimethoprim-sulphamethoxazole for HIV-1-infected adults in Abidjan, Cote d'Ivoire: a randomised trial. Cotrimo-CI Study Group. Lancet 1999; 353: 1463–68. Wiktor SZ, Sassan-Morokro M, Grant AD,et al. Efficacy of trimethoprim-sulphamethoxazole prophylaxis to decrease morbidity and mortality in HIV-1-infected patients with tuberculosis in Abidjan, Côte d'Ivoire: a randomised controlled trial. Lancet 1999; 353: 1469–75. Guay LA, Musoke P, Fleming T, et al. Intrapartum and neonatal single-dose nevirapine compared with zidovudine for prevention of mother-to-child transmission of HIV-1 in Kampala, Uganda: HIVNET 012 randomised trial. Lancet 1999; 354: 795–802. Van de Perre P. Breast-feeding transmission of HIV-1: how can it be prevented ? J Infect Dis 1999; 179: S405–S407. Cartoux M, Meda N, Van de Perre P, Newell ML, de Vicenzi I, Dabis F and the Ghent International Working Group on Motherto-Child Transmission of HIV. Acceptability of voluntary HIV testing by pregnant women in developing countries: an international survey. AIDS 1998; 12: 2489–93. Meda N, Gautier-Charpentier L, Soudré RB, et al. Serological diagnosis of human immunodeficiency virus (HIV) in Burkina Faso through reliable, practical and less expensive strategies using commercial test kits. WHO Bull 1999; 77: 731–39. Mertens TE, Smith GD, Van Praag E. Home testing for HIV. Lancet 1994; 343: 1293.

Therapeutic implications of non-genomic glucocorticoid activity The glucocorticoids are an important class of drug when potent anti-inflammatory or immunosuppressive activity is required. In cases in which direct topical therapy is not possible, high doses of glucocorticoid are commonly given systemically to initiate antiinflammatory and immunosuppressive activity—for example, in acute vasculitis. Glucocorticoids are recognised to exhibit their pharmacological effects via classic genomic mechanisms—ie, the lipophilic glucocorticoid passes across the cell membrane, attaches to the cytosolic glucocorticoid receptor and heatshock protein, then binds to glucocorticoid-responsive elements on genomic DNA and interacts with nuclear transcription factors.1 Consequently, when acting through genomic action, glucocorticoids take at least 30 min, but often several hours, to start to exert their effects. Glucocorticoids can start to act within seconds to minutes via non-genomic mechanisms, which occur either

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via specific receptor-mediated activity or via non-specific physicochemical activity.2 Recently, Stefanie Sanden and colleagues,3 studying ex-vivo peripheral-blood mononuclear cells, found that pulsed glucocorticoid therapy downregulated glucocorticoid receptors in a doserelated manner in patients with rheumatic disease compared with controls not receiving any steroid therapy. This finding suggests that non-genomic activity might become relatively more important in mediating the therapeutic effects of high-dose pulsed glucocorticoid activity. Such non-genomic activity has been recognised for other steroid hormones such as neurosteroids (eg, pregnanolone), mineralocorticoids (eg, aldosterone), gonadal steroids (eg, progesterone), vitamin D3, and triiodothyronine. The specific receptor-mediated nongenomic effects of steroid hormones have been reviewed in detail elsewhere.4 The mechanism for non-specific non-genomic effects has been elucidated by Frank Buttgereit and colleagues in a series of elegant studies to investigate the immediate effects of glucocorticoids on cellular energy metabolism in rat thymocytes stimulated by concanavalin-A. They have shown that clinically relevant concentrations of methylprednisolone inhibit cycling of sodium and calcium across the plasma membrane, with a consequent decrease in the concentration of intracellular free calcium, but no significant effect on protein synthesis, and presumably no immediate effect on gene transcription.5–7 Glucocorticoids such as methylprednisolone probably dissolve in cell membranes, which influences their physicochemical properties and alters the regulation of membraneassociated proteins. Inhibition of calcium and sodium entry across the cell membrane would lead to a drop in cytosolic free calcium concentrations and a decrease in ATP use, whereas a direct effect on mitochondrial inner membranes would increase proton permeability and uncoupling of oxidative phosphorylation.8,9 Such intracellular processes, which are essential for immediate and sustained activation of immune cells such as lymphocytes, may be associated with concentrations of glucocorticoids used in high-dose bolus therapy.7 The response to high-dose pulsed methylprednisolone therapy may therefore be biphasic, with an early, rapid nongenomic effect and a delayed and more sustained classic genomic effect. Assessment of the relative in-vitro potencies of different glucocorticoids in their propensity for exerting nongenomic versus genomic activity is important. Potency has been investigated by calculating the equivalent doses for non-specific non-genomic effects of glucocorticoids on respiration in rat thymocytes stimulated by concanavalinA.10 Indexation of drug potency to that of prednisolone gave the following rank order of potency in rat thymocytes: prednylidene (7·4), dexamethasone (2·9), methylprednisolone (2·5), prednisolone (1·0), betamethasone (0·6). The order for non-specific non-genomic activity differs completely from that for classic genomic activity (figure). Hence, in rat thymocytes prednylidene exhibits a 8·5 fold greater potency for non-genomic versus genomic activity, whereas betamethasone has an 11·1 fold greater potency for genomic than non-genomic activity. The hierarchy for relative potency of non-genomic glucocorticoid activity in human peripheral-blood mononuclear cells is the same as that in rat thymocytes: prednylidene (8·1), dexamethasone (5·1), methylprednisolone (3·4), prednisolone (1·0), betamethasone 87

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COMMENTARY

Relative genomic and non-genomic potencies for different glucocorticoids, indexed to prednisolone 8

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Genomic activity Non-specific non-genomic activity

Relative potency

6·25

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6 5 4 2·9

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2 1 0

0·88

1·0 1·0

1·25 0·56

Prednyl

Prednis

MePrednis

Dexa

Beta

Prednis=prednisolone Prednyl=prednylidene MePrednis=methylprednisolone Dexa=dexamethasone Beta=betamethasone Adapted from Biochem Pharmacol 1999; 58: 363–68.

(0·7).10 For high-dose systemic glucocorticoid therapy, a close balance between genomic versus non-genomic potency is preferable, so methylprednisolone or dexamethasone (figure) is usually used for this purpose. There is uncertainty about the precise dose-response relation between genomic and non-genomic effects, although it is known that the former tends to occur at lower doses (ie, <60 mg prednisone-equivalent per day) and the latter at higher doses (ie, >250 mg prednisoneequivalent per day). Furthermore, downregulation of glucocorticoid receptors occurs rapidly in response to exogenous glucocorticoid therapy. Data from peripheralblood mononuclear cells show downregulation to be dose related.3,11 Pulsed administration of high doses of glucocorticoids would oversaturate as well as downregulate receptors, so the relative contribution of the non-genomic glucocorticoid effects would be predicted to become more prominent with such high doses of corticosteroids. Thus, when a rapid onset of response is wanted, it might be logical to use a glucocorticoid with high non-genomic potency (such as prednylidene) in conjunction with one with high genomic potency (dexamethasone or betamethasone). Whether non-genomic glucocorticoid activity contributes as much as classic genomic activity to long-term efficacy is unclear. If it does, perhaps a combination of glucocorticoids based on differences in mechanism of action might produce the same overall anti-inflammatory or immunosuppressive effects as therapy with an agent having genomic-predominant effect, but without causing the dose-dependent genomic type of systemic adverse effects. In-vivo clinical studies are required to try and tease out the doseresponse relations for relative genomic versus nongenomic effects in different diseases. However, it is important to remember that the choice of glucocorticoid for therapeutic use will be influenced also by other pharmacokinetic and pharmacodynamic factors, such as bioavailability, volume of distribution, clearance, tissue-receptor-binding affinity, and residency time. 88

Might all topically active glucocorticoids be similar in their profile for relative genomic versus non-genomic activity, or might some of the newer more potent ones, such as fluticasone propionate or mometasone furoate, exhibit a higher degree of non-genomic than genomic activity? This issue could be important because the very high local tissue concentrations reached with topically delivered therapy (eg, in allergic airway or skin diseases) could mean that the drug will be exerting non-genomic effects as well as potent genomic effects. It may be possible in the future to tailor the molecular structure of new glucocorticoid compounds to achieve a desirable mix of genomic and non-genomic activities to suit the disease and route of delivery. An example of the potential for tailoring is the five-fold difference in relative potency for non-genomic activity between dexamethasone and betamethasone, which is presumably due to the beta position of the 16-methyl group in betamethasone as compared with the alpha position in dexamethasone; by contrast their genomic potencies are identical (figure). Buttgereit and colleagues have proposed a new graded classification for glucocorticoid activity based on the doseresponse relation, mechanism of action, and onset of activity.3 Level 1 is activity occurring within hours at a prednisone-equivalent concentration of more than 10–12 mol/L by a genomic action; level 2 occurs within minutes at a predisone-equivalent concentration of more than 10–9 mol/L by an additional non-genomic specific-receptor-mediated action; and level 3 occurs within seconds at a prednisone-equivalent concentration of more than 10–4 mol/L via an additional non-genomic non-specific physiochemical action. Increments in glucocorticoid dose from that in level 1 to that in level 3 would be required to treat increasing disease severity, depending on the relative potency for genomic versus non-genomic activity. Further in-vitro and in-vivo research into the relative genomic and non-genomic components of glucocorticoid action will help to refine how this important class of drug is used. Brian J Lipworth Asthma and Allergy Research Group, Department of Clinical Pharmacology & Therapeutics, Ninewells Hospital and Medical School, University of Dundee, Dundee DD1 9SY, UK 1

Barnes PJ. Anti-inflammatory actions of glucocorticoids: molecular mechanisms. Clin Sci 1998; 94: 557–72. 2 Buttgereit F, Wehling M, Burmester GR. A new hypothesis of modular glucocorticoid actions. Glucocorticoid treatment of rheumatic diseases revisited. Arthritis Rheum 1998; 41: 761–67. 3 Sanden S, Tripmacher R, Weltrich R, et al. Glucocorticoid dosedependent down-regulation of glucocorticoid receptors in patients with rheumatic diseases. J Rheumatol 2000; 27: 1265–70. 4 Wehling M. Specific, non-genomic steroid action. Annu Rev Physiol 1997; 59: 365–93. 5 Buttgereit F, Brand ND, Müller M. Effects of methylprednisolone on the energy metabolism of quiescent and concanavalin-A stimulated thymocytes of the rat. Biosci Rep 1993; 13: 41–52. 6 Buttgereit F, Brand MD. A hierarchy of ATP consuming processes in mammalian cells. Biochem J 1995; 312: 163–67. 7 Buttgereit F, Krauss, Brand MD. Methylprednisolone inhibits uptake of Ca2+ and Na+ into concanavalin A stimulated thymocytes. Biochem J 1997; 326: 329–32. 8 Buttgereit F, Grant A, Müller M, Brand MD. The effects of methylprednisolone on oxidative phosphorylation in concanavalin A stimulated thymocytes: top-down elasticity analysis and control analysis. Eur J Biochem 1994; 223: 513–19. 9 Martens ME, Peterson PL, Lee CO. In vitro effects of glucocorticoid on mitochondrial energy metabolism. Biochim Biophys Act 1991; 1058: 152–60. 10 Buttgereit F, Brand MD, Burmester GD. Equivalent doses and

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COMMENTARY relative drug potencies for non-genomic glucocorticoid effects: a novel glucocorticoid hierarchy. Biochem Pharmacol 1999; 58: 363–68. 11 Silva CM, Powell-Oliver FE, Jewell CM, Sar M, Allgood VE, Cidlowski JA. Regulation of the human glucocorticoid receptor by long-term and chronic treatment with glucocorticoid. Steroids 1994; 59: 436–42.

Mono-ocular occlusion for treatment of dylexia Can an inexpensive and apparently simple measure, such as the occlusion of one eye, albeit for several months, help dyslexic children improve their reading? This debate was started in The Lancet 15 years ago.1 With the publication of a controlled study of the effects of occlusion of the left eye for 9 months on binocular stability and reading,2 how close is this contentious issue to being resolved? The participants in this study2 were children referred to an eye clinic and selected on the basis of unstable binocular fixation and a reading age 2 SD below that predicted by IQ (a definition of dyslexia). The monoocclusion group and the controls were matched overall for age, IQ, and reading age. Both groups wore spectacles with clear yellow lenses while reading. Binocular stability and reading ability were assessed before, during, and after the study. After the first 3 months, the proportion of children who achieved stable binocular fixation was greater in the occlusion than the control group. Among those in the occlusion group whose binocular fixation became stable, reading ability improved by 1·8 months per month over the 9 months, nearly double the rate of progress among those whose binocular fixation remained unstable. Although the findings indicate that occlusion of the left eye improves reading ability, many questions need to be answered—about how the measures taken exert their effect, how they compare with other treatments, and whether the findings are in keeping with the theoretical origins of dyslexia. Why does the wearing of the plano yellow lenses contribute to improvements in reading age? Is it because these lenses reduce an adverse effect of glare on reading,3 or is it because yellow is the peak of the broadband absorption spectrum of the magnocellular system, which is one of the major visual pathways linking the posterior parietal cortex, the cerebellum, and the superior colliculus and which is important for timing visual events and for the control of eye movements?2 Does occlusion of one eye contribute to development of stability of binocular fixation? Stein and colleagues’ study indicates that it does, but by the end of the study, only six more children in the occlusion than in the control group had achieved stable binocular fixation (a non-significant difference), so is occlusion really worthwhile? Moreover, the proportion of children attaining stable fixation in the control group was nearly three times that expected with the plano yellow lenses (54% vs 20%), and no information is provided on whether the successful children in the occlusion group were slightly older or more intelligent than the rest. Whether occlusion helps those whose binocular fixation does not become stable is not known. To establish whether the data support the stable binocular fixation view of reading, according to which

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the frequency of letter transpositions would decline sharply with occlusion or spontaneous attainment of stability of binocular fixation,4 the individual reading data and the letter-error data are needed. In addition, improvement in reading ability should be greater among children who achieved stable fixation than among children who did not, irrespective of whether or not they underwent mono-ocular occlusion. Although data from a broader sample of people are needed to check whether occlusion would be generally useful for dyslexic individual, the likelihood is small because only around 10–25% of children with dyslexia present with visual problems. However, 45% of the original sample in this study, who had both reading and visual difficulties, were excluded because their binocular fixation was stable, and whether any of them was dyslexic is not known. Moreover, as Stein and colleagues, acknowledge, only a minority of children with dyslexia have unstable fixation (and in their group only 20% had unstable fixation and dyslexia, even though all had been referred via an eye clinic). Furthermore, occlusion is beneficial in only a limited age-range. Unstable fixation would also need to be measured with an alternative to the Dunlop test, the results of which are difficult to replicate and which is not suitable for administration by non-expert personnel.5 How does occlusion compare with other therapeutic measures? The cheapness of occlusion is attractive but should be weighed against the potential costs of, for example, the effect that 9 months’ occlusion might have on self-concepts, which in turn might affect compliance with therapy. Phonological deficits are the commonest deficits in dyslexic patients, and some patients have both visual and phonological deficits,4 so the most useful comparison is with phonological interventions, even though, unlike instability of binocular fixation, spontaneous improvements in phonology are rare. Phonological intervention is effective, more so when combined with reading instruction, and especially so when fluency is targeted as well,6 because automaticity is thought to be key to the development of reading.7 For a traditional school-intervention study of phonology and reading, costs would include the demands on teachers’ time, which can be reduced to some extent by use of short-term small-group interventions. Data needed to conduct a comparative evaluation of different therapeutic techniques include the mean scores for reading, the standard deviations, and the number of teacher hours of intervention undertaken. What does Stein and colleagues’ study suggest about the origins of dyslexia? Their interpretation that the instability of binocular fixation is indicative of magnocellular deficits is at variance with views that the visual magnocellular pathway is important primarily for the detection of low-contrast stimuli or slow coherent movement. The cerebellar deficit hypothesis,8 by contrast, accounts not only for the deficits that make up the criteria for dyslexia (deficits in reading, spelling, and writing), but indicates that instability of binocular fixation is a cerebellar-mediated deficiency in a learned eye-movement skill. However, it should be noted that the cerebellar and magnocellular hypotheses are not mutually exclusive. The cerebellum’s main input is from magnocellular systems, which is why it plays such an important part in timing, both for the control of eye and 89

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