Glucocorticoid Therapy for Immune-Mediated Diseases: Basic and Clinical Correlates

Dr. Dimitrios T. Boumpas (Kidney Disease Section, National Institute of Diabetes and Digestive and Kidney Diseases [NIDDK], National Institutes of Health [NIH], Bethesda, Maryland): Since 1949, when Hench and colleagues first introduced cortisone for the treatment of rheumatoid arthritis, glucocorticoids have revolutionized the treatment of immunologically mediated diseases. Although substantial complications associated with glucocorticoids have tempered enthusiasm for their use, they have remained the cornerstone of therapy for virtually all immunologically mediated diseases. In recent years, an explosion of new information has occurred relevant to both basic and clinical aspects of glucocorticoid therapy. We describe the molecular mechanisms, sites of action, and effects of glucocorticoids on various cells involved in inflammatory and immunologically mediated reactions. Treatment principles are also provided with examples of specific glucocorticoid regimens in prototypical conditions. We also review selective complications of glucocorticoid therapy and discuss recent information about their pathogenesis and management. Mechanisms of Action Dr. George P. Chrousos (Chief, Pediatric Endocrinology Section, Developmental Endocrinology Branch, National Institute of Child Health and Human Development, NIH, Bethesda, Maryland): Glucocorticoids exert most of their effects through specific, ubiquitously distributed intracellular receptors [1]. The classic model of glucocorticoid action was described more than two decades ago and is briefly updated here (Figure 1, panel A). Glucocorticoids circulate in blood, is either in the free form or in association with cortisol-binding globulin. The free form of the steroid can readily diffuse through the plasma membrane and can bind with high affinity to cytoplasmic glucocorticoid receptors (the role of receptors primarily residing in the nucleus is controversial). The formation of the ligand-receptor complex is followed by its activation (that is, translocation into the nucleus and binding to what are called acceptor sites). The bound complex modulates transcription of specific genes that encode proteins responsible for the action of glucocorticoids. Figure 1. Mechanisms of glucocorticoid action. Panel A. Panel B. Glucocorticoid Receptors In 1985, the complementary DNA of the human glucocorticoid receptor was cloned [2]; it contains three main functional domains Figure 1, panel B): first, the DNA-binding domain in the center of the molecule that recognizes specific sequences of the DNA called hormone-responsive elements; second, the ligand-binding domain in the carboxyl terminal region that interacts with the specific steroid; and third, the immunogenic domain in the amino terminal region. The nonactivated glucocorticoid receptor resides in the cytosol in the form of a hetero-oligomer with other highly conserved proteins [3]. This molecular complex comprises receptor, heat-shock proteins, and immunophilin (Appendix Table 1) [4]. The binding of the receptor to the heat-shock protein 90 facilitates its interaction with the ligand [5]. When the ligand binds, the receptor dissociates from the rest of the hetero-oligomer and translocates into the nucleus. Before or after the translocation, the receptor forms homodimers through sequences present in the DNA and ligand-binding domains [6]. Appendix Table 1. Glossary of Genetic Terms Gene Regulation After specific interaction with pore-associated proteins, the hormone-receptor complexes enter the nucleus through the nuclear pores [7]. The interaction is facilitated by two nuclear localization sequences in the receptor, both in the ligand-binding domain. Inside the nucleus, the hormone-receptor complexes bind to specific glucocorticoid responsive elements within DNA [8]. The complexes modulate the transcription rates of the corresponding glucocorticoid-responsive genes [9], apparently by stabilizing the initiation complex, composed of RNA polymerase II and its ancillary factors A through F. The hormone-receptor complex may interact directly with factor IIB [10], but it also interacts with other nuclear proteins to produce the conditions necessary for effective transcription [11]. These proteins may be able to relax the DNA away from the nucleosome and thus make it easier for the polymerase to exert its effects. In addition, glucocorticoid receptors may interact with DNA-binding proteins that are associated with different regulatory elements of the DNA [12, 13]. At least two such proteins have been described: One is the glucocorticoid modulatory element-binding protein and the other is the CACCC-box-binding protein. Both of these transcription factors potentiate the modulatory effects of glucocorticoids after transcription of specific genes. Transcription appears to be important in the regulation of genes involved in growth and inflammation. Glucocorticoid response elements can act both positively and negatively on transcription, depending on the gene on which the complex acts [14, 15]. One major way by which glucocorticoids exert down-modulatory effects on transcription is through noncovalent interaction of the activated hormone-receptor complex with the c-Jun/c-Fos heterodimer [16-18], which binds to the activator protein (AP)-1 site of genes of several growth factors and cytokines. The glucocorticoid-receptor complex prevents the c-Jun/c-Fos heterodimer from stimulating the transcription of these genes. Another mechanism by which glucocorticoids may suppress gene transcription is by an interaction between the hormone-receptor complex and glucocorticoid response elements that are in close proximity to responsive elements for other transcription factors [19]. Thus, the promoter region of the glycoprotein hormone- subunit, which is stimulated by cyclic AMP through the cyclic AMP-responsive element, contains a glucocorticoid response element in close proximity, so that when the receptor dimer binds to its own element, it hinders the cyclic AMP-binding protein from exerting its stimulatory effect on that gene. Post-Transcriptional Effects In addition to modulating transcription, glucocorticoids also have effects on later cellular events, including RNA translation, protein synthesis, and secretion. They can alter the stability of specific messenger RNAs of several cytokines and other proteins, thereby altering the intracellular steady-state levels of these molecules [20, 21]. This may occur through modulation of transcription of still unknown proteins that bind RNA and alter its translation and degradation rates. Also, glucocorticoids influence the secretion rates of specific proteins through mechanisms that have not yet been defined. Finally, the receptor itself has guanylate cyclase activity, and glucocorticoids can rapidly alter the electrical potential of some cells [22, 23]. Anti-inflammatory and Immunosuppressive Effects Dr. Dimitrios T. Boumpas: Although the cause and pathogenesis of many immunologically mediated diseases are not completely understood, it is known that the localization of leukocytes at sites of inflammation, their subsequent activation, and the generation of secretory products contribute to tissue damage, as shown in Figures 2 and 3 [24-26]. Glucocorticoids inhibit the access of leukocytes to inflammatory sites, interfere with their function and the function of fibroblasts and endothelial cells at those sites, and suppress the production and the effects of humoral factors. In general, leukocyte traffic is more susceptible to alteration by glucocorticoids than is cellular function; in turn, cellular immunity is more susceptible than humoral immunity to these agents. Figure 2. Models of the pathogenesis of inflammation and immune injury. Panel A. Panel B. Figure 3. Cellular adhesion molecules. Even though the effects of glucocorticoids on the different types of inflammatory cells will be discussed separately, each cell type is actually involved in complex interactions with other cells. Glucocorticoids affect many, if not all, the cells and tissues of the body, thus provoking a wide range of changes that involve several cell types concurrently. Effects on Nonlymphoid Inflammatory Cells Dr. Ronald L. Wilder (Chief, Inflammatory Joint Diseases Section, Arthritis and Rheumatism Branch, National Institute of Arthritis and Musculoskeletal and Skin Diseases, NIH, Bethesda, Maryland): Glucocorticoids are among the most potent anti-inflammatory agents available in clinical medicine. Pharmacologic doses of glucocorticoids dramatically inhibit exudation of plasma and accumulation of leukocytes at sites of inflammation. Several factors influence the magnitude of these effects, including the dose and route of administration of the glucocorticoids used, as well as the type and differentiation state of the target cell population [27]. Several host variables also modify the anti-inflammatory response to glucocorticoids. For example, some persons (those with active systemic lupus erythematosus) appear to have an accelerated rate of glucocorticoid catabolism [28]. Various levels of target tissue resistance may exist in some patients with systemic lupus erythematosus and rheumatoid arthritis [29]. These factors, alone or in combination, may explain the observation that different patients and diseases have variable therapeutic responses to glucocorticoids [30, 31]. Macrophages Glucocorticoids antagonize macrophage differentiation and inhibit many of their functions [27]. These agents 1) depress myelopoiesis and inhibit expression of class II major histocompatibility complex antigens induced by interferon-; 2) block the release of numerous cytokines, such as interleukin-1, interleukin-6, and tumor necrosis factor-; 3) depress production and release of proinflammatory prostaglandins and leukotrienes; and 4) depress tumoricidal and microbicidal activities of activated macrophages. Neutrophils The major effect of glucocorticoids on neutrophil