Increased content of glucocorticoid receptors in mouse muscular dystrophy

Expression patterns of regulatory RNAs, including lncRNAs and tRNAs, during postnatal growth of normal and dystrophic (mdx) mouse muscles, and their response to taurine treatment

Post-natal skeletal muscle growth in mice is very rapid and involves complex changes in many cells types over the first 6 weeks of life. The acute onset of dystropathology also occurs around 3 weeks of age in the mdx mouse model of the human disease Duchenne Muscular Dystrophy (DMD). This study investigated (i) alterations in expression patterns of regulatory non-coding RNAs (ncRNAs) in vivo, including miRNAs, lncRNAs and tRNAs, during early growth of skeletal muscles in normal control C57Bl/10Scsn (C57) compared with dystrophic mdx mice from 2 to 6 weeks of postnatal age, and revealed inherent differences in vivo for levels of 3 ncRNAs between C57 and mdx muscles before the onset of dystropathology. Since the amino acid taurine has many benefits and reduces disease severity in mdx mice, this study also (ii) determined the impact of taurine treatment on these expression patterns in mdx muscles at the onset of dystropathology (3 weeks) and after several bouts of myonecrosis and regeneration (6 weeks). Taurine treatment of mdx mice only altered ncRNA levels when administered from 18 days to 6 weeks of age, but a deficiency in tRNA levels was rectified earlier in mdx skeletal muscles treated from 14 days to 3 weeks. Myogenesis in tissue culture was also used to (iii) compare ncRNA expression patterns for both strains, and (iv) the response to taurine treatment. These analyses revealed intrinsic differences in ncRNA expression patterns during myogenesis between strains, as well as increased sensitivity of mdx ncRNA levels to taurine treatment.

Hormonal receptors in skeletal muscles of dystrophic mdx mice

Introduction. Several evidences show that muscles have an endocrine function. Glucocorticoid, estrogen, progesterone, and testosterone receptors have already been found in normal skeletal muscles, but not in dystrophic muscles. Methods. The gene expression of hormone receptors was compared between dystrophic and healthy muscles in mdx and C57BL6 mice strains. Results. The mdx mice showed a significant increase in the steroid receptors mRNA when compared to the C57BL6 mice: levels of androgen(s) receptors in the heart, estrogen receptors alpha in the EDL, and estrogen receptors beta in the quadriceps were increased. In addition, significant lowered levels of some other hormone receptors were found: corticosteroid receptors in the EDL and estrogen receptors alpha in the quadriceps. Conclusion. Dystrophic muscles bear significant differences in the expression of hormone receptors when compared to the C57BL6 mice strain. The importance of such differences is yet to be better understood.

Differential effects of methimazole and dexamethasone in avian muscular dystrophy

We showed previously that thyroid antagonists and glucocorticoids partially alleviated the impaired righting ability and abnormally high levels of plasma creatine kinase activity in genetically dystrophic chicks. The goals of the present study were: (1) to ascertain whether the beneficial effects of methimazole (MMI; thyroid antagonist) on muscle function and plasma creatine kinase (CK) activity in dystrophic chickens are correlated with significant reduction in plasma triiodothyronine (T3) and thyroxine (T4); (2) to assess whether the MMI-induced thyroid changes are accompanied by increased plasma corticosterone level and/or changes in muscle glucocorticoid receptors which might account partially for the beneficial effects of MMI; and (3) to determine if plasma T3 and T4 are reduced in dexamethasone (DEX) treated dystrophic chickens which might account at least partially for the beneficial effects of DEX (a potent glucocorticoid) on avian dystrophy. The data show that beneficial effects of MMI are associated with reduced plasma levels of thyroid hormones and increased circulating levels of corticosterone. In addition, DEX actually increases plasma T3 levels. These differential effects indicate that reduced plasma thyroid hormone levels do not represent a common mechanism of beneficial drug effects in avian muscular dystrophy. On the other hand, elevated plasma glucocorticoid levels accompany the beneficial effects of both severe hypothyroidism and DEX treatment. The data also show that MMI induces down-regulation of muscle cytosolic glucocorticoid receptors which are higher than normal in dystrophic muscles.

Influence of compensatory overload on glucocorticoid receptors in rat skeletal muscle

The present study was undertaken to evaluate the state of glucocorticoid cytosol receptor binding in muscle hypertrophied by compensatory overload in normal, testosterone-treated, and castrated male rats. Compensatory hypertrophy in plantaris muscles was induced by myomectomy of the gastrocnemius and the soleus of 100 male Sprague-Dawley rats. The controlateral sham-operated limb was used as control. The rats were divided in 3 groups: control injected with peanut oil, castrated injected with peanut oil, and castrated injected with testosterone propionate (1 mg/rat). Five days later the animals were sacrificed and the muscles obtained for glucocorticoid receptor binding studies using [3H]dexamethasone as ligand. Receptor concentrations (Bmax) were significantly increased in hypertrophied muscle and the increase was not affected either by castration or by androgen treatment. These results tend to suggest that gonadal steroids do not participate in hypertrophy-induced proliferation of glucocorticoid receptors.

Hormonal control of muscle growth

In muscle of whole animals, pituitary growth hormone, the thyroid hormones, and insulin are major growth-promoting hormones, and the glucocorticoids have significant catabolic actions. At the cellular level the primary anabolic hormones for cultured myoblasts are the somatomedins (insulin-like growth factors) and fibroblast growth factor. In these cells physiological concentrations of growth hormone, thyroid hormones, and insulin have no growth-promoting effect; some of the reported actions of insulin probably result from cross-reaction with the somatomedin receptor. Results with purified proteins do not support the view that mitogens block myoblast differentiation; transforming growth factor-beta and interferon are nonmitogenic proteins that inhibit differentiation, insulin-like growth factors are mitogens that stimulate differentiation, and fibroblast growth factor is the only purified mitogen that inhibits differentiation. At least six serum-free media have now been devised for the growth of various kinds of muscle cells under closely defined conditions.

Evaluation of the endogenous glucocorticoid hypothesis of denervation atrophy

We studied the effects of oral administration of RU38486, a potent and selective glucocorticoid antagonist, on muscle weight, non-collagen protein content, and selected enzyme activities (choline acetyltransferase, glucose-6-phosphate dehydrogenase, and glutamine synthetase) following denervation of rat skeletal muscle. Neither decreases in muscle weight, protein content, and choline acetyltransferase activity, nor increases in the activities of glucose-6-phosphate dehydrogenase and glutamine synthetase were affected by RU38486. These data do not support the hypothesis that denervation atrophy results from enhanced sensitivity of muscle to endogenous glucocorticoids.

Hormonal regulation of dipeptidyl-aminopeptidase I activity in cultured human fibroblasts

Human male fibroblasts, cell line GM2987, were grown in 10% Nu-Serum or fetal bovine serum. Dipeptidyl-aminopeptidase I (DAP-I) activity was higher in cells grown with Nu-Serum and cells grown in 10% fetal bovine serum purchased from Grand Island Biological Company (GIBCO) and lower in cells grown in 10% fetal bovine serum obtained from Sterile Systems, Inc. (Hyclone). The addition of 0.3 microM cortisol to all three types of sera resulted in cells that had similar levels of DAP-I activity (maximum of 800-900 nmol of beta-naphthylamine released from glycyl-L-phenylanine-beta-naphthylamine per hour per milligram of cellular protein). The addition of cortisol to Hyclone fetal bovine serum increased the DAP-I levels by up to threefold with a half-maximal response occurring at 30 nM cortisol. Triiodothyronine also could increase DAP-I levels, but only between 1.5- and 2.0-fold. Testosterone propionate increased DAP-I levels by 1.4-fold. These changes in growth media and hormones had little effect on other lysosomal enzymes or the growth characteristics of the cells.

Increased glucocorticoid/androgen receptor ratios in denervated striated muscle

Cytosolic glucocorticoid (GR) and androgen receptors (AR) were evaluated at varying time intervals (1-28 days) post-denervation in rat bulbocavernosus/levator ani muscles and the receptor content in the unilaterally denervated muscles compared to the contralateral innervated muscles. Rats were adrenalectomized 3 days prior to GR or castrated 24 h prior to AR determinations. Receptor analyses were performed using charcoal adsorption. Lowered AR were noted in the denervated muscles during very early time periods post-denervation while increased GR were not observed until later. During later post-denervation intervals relative increases or decreases in receptors were dependent on the basis of data expression (cytosolic protein or DNA). The GR to AR ratio was consistently higher in denervated muscles regardless of the basis of comparison, however. We conclude that decreased responsiveness to anabolic stimuli may play a role in the atrophic process.

A possible role for endogenous glucocorticoids in orchiectomy-induced atrophy of the rat levator ani muscle: studies with RU 38486, a potent and selective antiglucocorticoid

We employed RU 38486, a potent and selective antiglucocorticoid, to study a possible role for endogenous glucocorticoids in atrophy of the levator ani muscle secondary to castration of male rats. RU 38486 was shown to block [3H]triamcinolone acetonide binding to cytosol from levator ani muscle. Daily oral administration of RU 38486 to castrated rats partially prevented atrophy of the levator ani muscle, as well as a decrease in RNA concentration. In a control group receiving RU 38486 alone, the levator ani underwent significant (20%) hypertrophy. Administration of exogenous dexamethasone also caused pronounced atrophy of the levator ani muscle. This atrophy was prevented, to a significant degree, by simultaneous oral administration of RU 38486. It is concluded that endogenous glucocorticoids, the actions of which are blocked by RU 38486, may be involved in regulation of the mass of the levator ani muscle in intact rats.

The pathogenesis of Duchenne muscular dystrophy: significance of experimental observations

The pathogenesis of Duchenne muscular dystrophy (DMD) remains elusive, but as this is an inherited condition the primary manifestation of the disease is assumed to be in the regulatory control or biosynthesis of a protein. Current hypotheses attribute the pathological state of skeletal muscle in DMD to a defect in the nerve supply, or in the vasculature, or in the muscle itself. However, various tissues other than skeletal muscle are also affected; thus the current view of DMD requires reevaluation. The following possibilities should be considered: That the primary lesion is expressed in one of the major communication systems (nervous, vascular, or endocrine). That the primary lesion is expressed in a specific tissue: a) if in skeletal muscle, alterations in non-muscular tissues must be due to the release of muscle constituents into extracellular fluid; b) if in a non-muscular tissue, this might produce too much (or too little or none) of a constituent normally secreted into the extracellular fluid, or produce a "toxic" agent. That the primary lesion is expressed in a wide variety of tissues: the effect on a particular tissue will depend entirely upon the degree of requirement of the altered protein for function.