Clinical background
Since the successful use of steroids in 1948 in the suppression of clinical manifestations of rheumatoid arthritis, numerous compounds with glucocorticoid activity have been synthesized. Today, they represent standard therapy for the reduction of immune activation in asthma and inflammation associated with allergies, rheumatoid arthritis, inflammatory bowel syndromes, and other systemic diseases, plus they are used in ocular infections and allotransplantation. The therapeutic usage of glucocorticoids has risen continuously in recent years. Each year 10 million new prescriptions are written just for oral corticosteroids in the USA. Overall, the total market size is considered to reach about 10 billion US dollars per year. Glucocorticoids are used in almost all medical specialties for systemic as well as topical therapies.
Despite its therapeutic efficacy, glucocorticoid therapy is associated with metabolic and toxic effects. The side-effects (summarized in Table 33.1 ) occur with different prevalence, in different organs, and after different durations of therapy. Ocular complications of glucocorticoid therapy include cataract, glaucoma, infection, and corneal epithelial healing ( Table 33.2 ).
| Skin |
| Atrophy, delayed wound healing |
| Erythema, teleangiectasia, hypertrichosis |
| Skeleton and muscle |
| Muscle atrophy |
| Osteoprosis, bone necrosis |
| Central nervous system |
| Mood, behavior, memory, and cognitive disturbances |
| Cerebral atrophy |
| Eye |
| Electrolyte, metabolism, endocrine system |
| Increased Na + retention and K + secretion |
| Diabetes mellitus |
| Cushing’s syndrome |
| Growth retardation |
| Adrenal atrophy |
| Hypogonadism, delayed puberty |
| Cardiovascular system |
| Hypertension |
| Thrombosis |
| Vasculitis |
| Gastrointestinal |
| Bleeding, peptic ulcer |
| Pancreatitis |
| Immune system |
| Infection |
|
Cataracts: background
Cataracts are a clouding of the ocular lens of the eye which is most often associated with age-related oxidation and insolubilization of lens proteins. Cataracts are a major public health problem, affecting almost 50% of adults aged >65 years, and cataract extraction is the most common surgical procedure carried out in the USA. Furthermore, it has been estimated that, by delaying the development of cataract formation by 10 years, 45% of these extractions would be avoided. The development of cataract and its subsequent progression are influenced by multiple factors, including age, gender, diabetes, ultraviolet light exposure, smoking, a diet low in antioxidants, and steroid use.
Epidemiology of the steroid cataracts
The incident of cataracts has considerably increased in recent years with widespread use of steroid therapy. Steroids typically induce cataract formation in the cortex of the posterior region of the lens, called posterior subcapsular cataract (PSC). In 1966, the incident of cataracts was 0.2% and 0.6% in young adults and those in their fifth and sixth decades, respectively. In recent years, chronic use of steroids had become one of the risk factors for development of cataracts, surpassed only by diabetes, myopia, and glaucoma. The incidence of steroid-mediated development of cataracts is expected to accelerate due to aging of the population and the associated increase of conditions that require steroid treatment. Therefore, better understanding of the mechanism of action of steroid-mediated cataracts may lead to their prevention, which would have an impact on the quality of life in the elderly.
Characteristics of steroid-induced cataract
The link between steroid use and the development of PSC was first reported in 1960 by Black and colleagues. They examined 72 patients suffering from rheumatoid arthritis. In this study, 42% of the patients developed PSC whereas none of the individuals in the control group developed PSC. All cataracts were bilateral. Two subsequent papers by the same group provided a more detailed picture of glucocorticoid-mediated PSC. The cataract which developed in these patients was characterized by “black spots or thread-like opacities” seen against the red fundus and located in the lens cortex immediately adjacent to the posterior lens capsule in the center of the field of vision. Other investigators reported “small yellow-white, highly refractile” cataracts with “small, scattered punctate vacuoles” that form a granular conglomerate. Since then, numerous other studies have described an association between the use of steroids such as dexamethasone, beclomethasone, prednisone, and triamcinolone and development of cataract regardless of their route of administration.
Steroid cataracts which present with superficial cortical vacuoles in posterior subcapsular region are referred to as “vacuolated PSC.” However other conditions, such as age-related PSC, diabetes PSC, and retinitis pigmentosa-associated PSC, are classified as vacuolated PSC. With the exception of diabetes PSC, these other forms of vacuolated PSC appear to be clinically different in the early stage of their development. It is not clear why diabetes PSC and steroid-induced PSC share common features. The mechanism of steroid-induced cataracts has been actively investigated ( Box 33.1 ).
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Steroid cataracts which present with superficial cortical vacuoles in posterior subcapsular region are referred to as “vacuolated posterior subcapsular cataract (PSC)”
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Other conditions, such as age-related PSC, diabetes PSC, and retinitis pigmentosa-associated PSC, are also classified as vacuolated PSC
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With the exception of diabetes PSC, these other forms of vacuolated PSC appear to be clinically different in the early stage of their development
In early studies Black et al found that the incidence of steroid-induced cataract increased in proportion to the steroid dose. However, the dose-dependent nature of steroid-induced cataracts formation is controversial. Some investigators confirm the observation of Black et al whereas others have observed no direct relationship between steroid dose and PSC severity. It is possible that, while steroid dose is important, other factors such as age, susceptibility of the individual, genetic effects, co-medication, and duration of treatment may influence the development of PSC.
Epidemiology of route of steroid therapy and development of cataract
Systemic administration of glucocorticoids is associated with PSC development. However, topical application and intravitreal injection of glucocorticoids can also induce cataract. Intravitreal glucocorticoid therapy is used for the treatment of many allergic and inflammatory disorders of the eye, such as uveitis, choroiditis, optic neuritis, and allergic conjunctivitis.
Inhaled steroids taken by asthma patients have also been related to the development of PSC. Most recent data show that inhaled corticosteroid therapy increased significantly the risk of cataract even at low doses of beclomethasone (<500 µg/day) and reached a maximum level of 44% at doses of 1500–2000 µg/day among elderly patients. The risk of cataract increased with increased dose of inhaled and nasal corticosteroid treatment. Therefore, these results have important implications for the treatment of asthma and chronic obstructive pulmonary disease in the elderly. For asthma, significant effort should be devoted to reducing the dose of inhaled corticosteroid as much as possible, perhaps by using inhaled corticosteroid in combination with long-acting bronchodilators or antileukotrienes. Steroid therapy of children should be monitored more closely since PSC development occurs at a faster rate and at lower doses. Shiono et al reported that 33 (26.2%) of 126 children receiving long-term corticosteroid therapy developed PSC. Some of these patients manifest lens changes in under 6 months. In the majority of cases, adverse effects of steroids were irreversible. Complete reversal of lens opacification is highly unlikely once vision is affected.
Mechanism of action of glucocorticoid-induced cataract
Steroids
Steroids are derived from cholesterol ( Figure 33.1 ). They comprise five groups: progestagens, androgens, estrogens, mineralocorticoids, and glucocorticoids ( Figure 33.2 ). They are produced in different tissues and have different functions. Progestagens are produced by corpus luteum, whereas androgens and estrogens are produced by testis and ovaries, respectively. Mineralocorticoids and glucocorticoids are produced by the adrenal cortex. Steroid effects on normal cell function are complex and not fully understood. Steroid actions may be plasma membrane-initiated, intracellular receptor-mediated (genomic), and/or posttranscriptional.
Nongenomic rapid glucocorticoid signaling
From early studies on the mechanism of actions of steroids, it became apparent that all classes of steroid hormones can induce effects that occur in a very short time frame, within minutes or even seconds of their application. These changes have been well documented in vitro on intracellular signaling pathways, and in vivo, in a wide array of human and animal models used to study biological functions such as oogenesis, vasoregulation, response to stress, and neurobehavioral changes ( Box 33.2 ). These rapid events do not fit the classical “genomic” model for steroid action. Nongenomic effect of glucocorticoids may contribute to insulin resistance. Reduced kinase activities of the insulin receptor (INSR) and several downstream INSR signaling intermediates (i.e., p70S6K, adenosine monophosphate-activated protein kinase, glycogen synthase kinase-3, and Fyn) were detected in adipocytes and T lymphocytes due to short-term treatment with dexamethasone. Several membrane-associated proteins are involved with rapid nongenomic action of glucocorticoids. Glucocorticoid-dependent phosphorylation of caveolin and protein kinase B/Akt was reported to inhibit lung epithelial cell growth. Disruption of caveolae led to dissociation of glucocorticoid action, with impaired induction of Akt activation. Glucocorticoids rapidly inhibit high-voltage-activated calcium currents in several different cell types in a G protein-dependent manner.
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From early studies of the mechanism of action of steroids, it became apparent that all classes of steroid hormones can induce effects that occur in a very short time frame, within minutes or even seconds of their application
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These changes have been well documented in vitro on intracellular signaling pathways, and in vivo, in a wide array of human and animal models used to study biological functions such as oogenesis, vasoregulation, response to stress, and neurobehavioral changes
The classical corticosteroid receptor, a cytosolic protein, has also been found localized to the plasma membrane, suggesting the possibility that rapid effects of glucocorticoids may be mediated by the classical intracellular receptors associated with the cell membrane. In addition, glucocorticoid signaling to the nucleus may be mediated via the membrane glucocorticoid receptor. Glucocorticoids cause rapid activation of several mitogen-activated protein kinases (MAPK), including p38, c-Jun NH 2 -terminal kinase (JNK), and ERK1/2 in cultured hippocampal neurons and PC12 cells. Activation of ERK by glucocorticoids occurs in a 1–3 hour time frame, which is not consistent with the nongenomic action of steroids. Regardless, the activation and nuclear translocation of MAPK by either, or both, transcriptional and nontranscription mechanisms provide the potential for epigenetic regulation of gene expression, because MAPK can cause histone phosphorylation that can lead to modification of chromatin structure. Transcriptional and nontranscriptional signaling by MAPK in human lens is not defined. It will be interesting in the future to show how these potentially distinct mechanisms of accessing the genome might interact, either positively or negatively, to formulate the resulting net regulation of gene expression by glucocorticoids in human lens.
Receptor-mediated glucocorticoid action
Classical genomic action of steroids is mediated through their receptors, ligand-regulated transcription factors that belong to the superfamily of nuclear receptors. Glucocorticoid initiates a process culminating in dimerization of glucocorticoid receptor and translocation of the ligand–receptor complex to the nucleus via the microtubule network ( Figure 33.3 ). Once in the nucleus, the activated glucocorticoid receptor associates with unique DNA sequences termed glucocorticoid response elements (GRE) in the promoter or enhancer region of target genes. The ligand–glucocorticoid receptor may modulate the expression of the target genes ( Box 33.3 ). The glucocorticoid receptor can be recruited to target genes either through direct DNA binding or with other DNA-bound transcription factors. Once bound to GREs as a homodimer, glucocorticoid receptor serves as scaffold for the assembly of distinct macromolecular complexes that activates transcription of target gene (transactivation). The macromolecular complex includes coactivator proteins, chromatin remodeling factors and other factors that directly or indirectly engage the transcriptional machinery. The genes mainly controlled by glucocorticoid receptor transactivation are involved in metabolic regulation; for example, increasing blood glucose levels, gluconeogenesis, and mobilization of amino acids and fatty acids. The reduction of transcription (transrepression) by glucocorticoid receptor occurs by different mechanisms. One mechanism resembles transactivation, but the receptors bind to DNA sequences distinct from positive GREs (i.e., negative GRE sites or nGREs).
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Classical genomic action of steroids is mediated by nuclear receptors
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Glucocorticoid initiates dimerization of glucocorticoid receptor and translocation of the ligand–receptor complex to the nucleus
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Once in the nucleus, the activated glucocorticoid receptor associates with unique DNA sequences termed glucocorticoid response elements (GRE) in the promoter or enhancer of target genes
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Glucocorticoid receptor can be recruited to target genes either through direct DNA binding or with other DNA-bound transcription factors
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Ligand–glucocorticoid receptor promotes formation of macromolecular complex formation on DNA molecule
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The macromolecular complex includes coactivator proteins, chromatin remodeling factors, and other factors that directly or indirectly engage the transcriptional machinery
Glucocorticoid receptor also triggers transcriptional repression through a mechanism that does not involve its direct DNA binding but rather a tethering to other DNA-bound transcription factors such as AP-1 and NF-κB. Anti-inflammatory actions of glucocorticoid receptors are through modulation of AP-1 and NF-κB transcriptional activity ( Box 33.4 ).
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Once bound to glucocorticoid response elements as a homodimer, glucocorticoid receptor serves as scaffold for the assembly of distinct macromolecular complexes that activates transcription of target gene (transactivation)
- •
The genes mainly controlled by glucocorticoid receptor transactivation are involved in metabolic regulation; for example, increasing blood glucose levels, gluconeogenesis, and mobilization of amino acids and fatty acids
- •
The reduction of transcription (transrepression) by glucocorticoid receptor occurs by different mechanisms. One mechanism resembles transactivation, but the receptors bind to DNA sequences distinct from positive glucocorticoid response elements
Glucocorticoid receptors share common structural organization with other steroid receptors consisting of several modulatory domains with two highly conserved, zinc-finger DNA binding domains (DBD), a less-conserved carboxy terminal ligand binding domain (LBD), and a divergent amino terminal domain (NTD) ( Figure 33.4 ). A region rich in acidic amino acids in the NTD domain known as activation factor-1 (AF-1) is a transcriptional regulator that can be ligand-independent. Disruption of the AF-1 region reduces gene expression. A second region in the LBD domain, known as AF-2, undergoes conformational changes modulating transcriptional activator or repressor activity of glucocorticoid receptor. LBD promotes receptor dimerization and contains the sequence for heat shock protein 90 (HSP90) interaction. Maturation of proper folding of glucocorticoid receptor is provided by HSP90 interaction ( Figure 33.5 ). In the absence of ligand, inactive glucocorticoid receptor in the cytoplasm interacts with HSP90. In addition to HSP90, other heat shock proteins (HSP70, HSP40) and co-chaperones p23, immunophilins FKBP52 and Cyp40 play key roles in the proper folding and function of glucocorticoid receptor. For example, functional disruption of p23 gene produces a phenotype that is distinctly similar to that of mouse glucocorticoid receptor knockout. Since glucocorticoid receptor is well characterized in the lens, understanding the role of chaperones, co-chaperones, and immunophilins may provide key information on the mechanism of corticosteroid-induced PSC.
