Glucocorticoid Production and Regulation in Thymus: Of Mice and Birds

The thymus is the key immunological organ for the maturation of T cells in mammals. Enlargement of the thymus gland in patients with Addison’s disease and in adrenalectomized rats was recognized a century ago (1, 2). Elevation of glucocorticoids (GC) due to chronic stress or therapeutic administration causes involution of the thymus. T cells, especially immature thymocytes, are particularly sensitive to apoptosis induced by GC (3, 4). GC act through the GC receptor (GR), a member of the steroid receptor superfamily of ligand-activated transcription factors. Upon binding to GC, GR located in the cytosol are translocated to the nucleus where they exert transcriptional effects. They also modulate gene transcription indirectly, without binding to DNA, by interacting with other transcription factors, including activation protein-1, nuclear factorB, and signal transducer and activator of transcription proteins (5, 6). This type of protein-protein interaction mediates the cross talk important for the regulation of the immune system (5, 7). GC are strong inducers of apoptosis in thymocytes and play a significant role in the development, differentiation, homeostasis, and function of T cells (6, 8). Immature double-negative thymocytes (CD4 CD8 ) proliferate and differentiate in the thymus, undergoing extensive genetic and phenotypic alteration to generate a double-positive (CD4 CD8 ) cell population. Most CD4 CD8 thymocytes undergo apoptosis; the surviving double-positive cells differentiate into single-positive CD4 or CD8 cells that populate the peripheral lymphoid tissues (4, 9–11). According to the mutual antagonism model, thymic selection is controlled by the cross talk of activation-induced and GC-dependent cell death of double-positive thymocytes (4, 11). Studies using transgenic and knockout (KO) models addressing GR function clearly demonstrate GRinduced apoptosis but have been equivocal in addressing the role of GC in T cell development (4, 10, 12). GC are produced primarily in the adrenal gland but are also produced in other organs including the brain (13, 14), intestinal tract (15), skin (16–18), and thymic epithelial and immune cells (19–25) and express the necessary steroidogenic enzymes for the synthesis of GC, which apparently act in an autocrine or paracrine fashion. The thymus has endocrine properties and expresses various hormones and receptors of the hypothalamic-pituitary-adrenal axis, corticotropin-releasing factor (26), ACTH (27), and ACTH receptors (28, 29) including melanocortin receptor subtype 2 (MC2R) and MC5R (30) in thymus or T cells (28, 31). Thymus epithelial cells and thymocytes express mRNA for all the necessary steroidogenic components including the steroidogenic acute regulator (StAR), CYP11A1, 3 -hydroxysteroid dehydrogenase, CYP21, and CYP11B1 enzymes and can synthesize GC (19–22, 32–37). StAR, CYP11A1, and CYP11B1 are expressed in both thymus epithelial cells and thymocytes of 4-wk-old mice; at 14 wk, they are significantly increased in thymocytes but decreased in thymus epithelial cells. The CYP17 enzyme is expressed at very low levels in both the thymus and adrenal gland of mice (22). Measurement of individual enzymatic activity with exogenous substrate demonstrated that the enzymes are functional (19, 21). However, because the availability of substrate relative to the kinetic requirements for optimal functioning of the enzymes and mRNA for steroidogenic enzymes in comparison with the adrenal are both very low, the inherent synthesis of GC by thymocytes was not certain. The de novo synthesis of GC was elegantly demonstrated using a