A role for glucocorticoids in the polyphosphoinositide second messenger system

Corticosteroids inhibit the delivery of short-term activational pulses of phorbol ester and calcium ionophore to human peripheral T cells

Although there is evidence that corticosteroids inhibit receptor-ligand-induced phospholipid hydrolysis, the immunosuppressive effects of these agents downstream of protein kinase C (PK-C) activation and cytosolic Ca2+ mobilization is unclear. Previous studies indicated that T cell proliferative activation could be achieved with simultaneous short-term (e.g., 15-120 min) exposure to agents activating PK-C and elevating cytosolic Ca2+. In the studies reported here, similar procedures were utilized for determining whether corticosteroids alter T cell activation signals downstream of second messenger events. Dexamethasone interfered with T cell activation induced by short-term exposure to phorbol 12,13-dibutyrate (PDBu) and the calcium ionophore, ionomycin. The inhibitory effect was evident with as little as 15 min of exposure to dexamethasone and T cell activating agents, making mechanisms involving de novo protein synthesis unlikely. Dexamethasone's effects in this system were blocked by the steroid receptor antagonist RU-486, indicating that the inhibition was mediated through the glucocorticoid receptor. The inclusion of recombinant interleukin-2 (IL-2) only partially overcame the dexamethasone inhibitory effect. Long-term (i.e., 48 hr) direct stimulation of PK-C with either PDBu or the non-tumor-promoting PK-C activator, bryostatin 1, also substantially overcame dexamethasone's effects, resulting in a recovery of IL-2 production and significant restoration of the T-cell proliferative response. These observations suggest that treatment with a PK-C-activating agent such as bryostatin 1 could reduce glucocorticosteroid-induced immunosuppression.

Characterization of glucocorticoid inhibition of antigen-induced inositolphosphate formation by rat basophilic leukemia cells: possible involvement of phosphatases

The suppressive effect of glucocorticoids (GC) upon antigen-induced phosphatidylinositol phospholipase C (PI-PLC) activity and inositol phosphate formation by rat basophilic leukemia cells (RBL-2H3) has been characterized. Addition of antigen for a period of 1-30 min enhanced production of [3H]inositol monophosphate (IP1), inositol 1,4-bisphosphate (IP2) and inositol 1,4,5-trisphosphate (IP3) by about 5-10-fold. Pretreatment with hydrocortisone (HC) reduced formation of the various inositol phosphates (IPs) and degradation of phosphatidylinositol 4,5-bisphosphate (PIP2) by an average of 50%. Maximal inhibition of hydrolysis of PIP2 and reduction in stimulation of IP3 formation was reached after 4 h of preincubation with 2.10(-6) M of HC. Cycloheximide and RU486, a GC receptor antagonist, completely prevented the inhibitory effect of HC on IP formation. Other GC, dexamethasone (DEX) and triamcinolone (each at 2.10(-7) M) markedly suppressed antigen induced IP3 production, while aldosterone and sex steroids such as estradiol and progesterone (each at 2.10(-6) M) were virtually inactive. Antigen-stimulated phosphorylation of a 18 kDa and other proteins was inhibited by about 60% following pretreatment with the GC. This inhibition was in turn prevented by cycloheximide. DEX also doubled the activity of cellular acid phosphatase activity. The results suggest that the inhibitory effect of GC is specific, receptor-mediated, dependent on protein synthesis and possibly mediated by protein phosphatase activity.

The phospholipid/arachidonic acid second messenger system: its possible role in physiology and pathophysiology of metabolism

The phospholipid/arachidonic acid second messenger system is a signaling system by which systemic regulators (hormones) and local mediators (tissue factors) control certain aspects of tissue metabolism. In vivo and in vitro evidence indicates that these effectors activate phospholipolytic enzymes in cellular membranes. The products of these enzymatic reactions (such as inositol phosphates or arachidonic acid metabolites) can serve as second messengers that can potentially influence glucose, lipid and protein metabolism at the cellular level. Alterations in this second messenger system could be involved in metabolic changes associated with some pathologic conditions as well as certain drug treatments, and thus, a better understanding of the system could reveal new possibilities for therapeutic interventions.

Psychiatric implications of altered limbic-hypothalamic-pituitary-adrenocortical activity

Hormones of the limbic-hypothalamic-pituitary-adrenocortical (LHPA) system are much involved in central nervous system regulation. The major LHPA neuropeptides, corticotropin-releasing hormone (CRH), vasopressin (AVP) and corticotropin (ACTH) do not only coordinate the neuroendocrine response to stress, but also induce behavioral adaptation. Transcription and post-translational processing of these neuropeptides is regulated by corticosteroids secreted from the adrenal cortex after stimulation by ACTH and other proopiomelanocortin derived peptides. These steroids play a key role as regulators of cell development, homeostatic maintenance and adaptation to environmental challenges. They execute vitally important actions through genomic effects resulting in altered gene expression and nongenomic effects leading to altered neuronal excitability. Since excessive secretory activity of this particular neuroendocrine system is part of an acute stress response or depressive symptom pattern, there is good reason to suspect that central actions of these steroids and peptides are involved in pathophysiology determining the clinical phenotype, drug response and relapse liability. This overview summarizes the clinical neuroendocrine investigations of the author and his collaborators, while they worked at the Department of Psychiatry in Mainz. The major conclusions from this work were: (1) aberrant hormonal responses to challenges with dexamethasone, ACTH or CRH are reflecting altered brain physiology in affective illness and related disorders; (2) hormones of the LHPA axis influence also nonendocrine behavioral systems such as sleep EEG; (3) physiologically significant interactions exist between LHPA hormones, the thyroid, growth hormone, gonadal and other neuroendocrine systems; (4) hormones of the LHPA axis constitute a bidirectional link between immunoregulation and brain activity; and (5) future psychiatric research topics such as molecular genetics of affective disorders, familial risk studies, drug response analysis and neurobiology of aging will benefit from extended knowledge of neural corticosteroid effects at a clinical, cellular, and molecular level.