Cyclosporin A and FK506 are potent activators of proopiomelanocortin-derived peptide secretion without affecting corticotrope glucocorticoid receptor function

Changes in Corticotrope Gene Expression Upon Increased Expression of Peptidylglycine α-Amidating Monooxygenase

Throughout evolution, secretion has played an essential role in the ability of organisms and single cells to survive in the face of a changing environment. Peptidylglycine α-amidating monooxygenase (PAM) is an integral membrane monooxygenase, first identified for its role in the biosynthesis of neuroendocrine peptides released by the regulated secretory pathway. PAM was subsequently identified in Chlamydomonas reinhardtii, a unicellular green alga, where it plays an essential role in constitutive secretion and in ciliogenesis. Reduced expression of C. reinhardtii PAM resulted in significant changes in secretion and ciliogenesis. Hence, a screen was performed for transcripts and proteins whose expression responded to changes in PAM levels in a mammalian corticotrope tumor cell line. The goal was to identify genes not previously known to play a role in secretion. The screen identified transcription factors, peptidyl prolyl isomerases, endosomal/lysosomal proteins, and proteins involved in tissue-specific responses to glucose and amino acid availability that had not previously been recognized as relevant to the secretory pathway. Perhaps reflecting the dependence of PAM on molecular oxygen, many PAM-responsive genes are known to be hypoxia responsive. The data highlight the extent to which the performance of the secretory pathway may be integrated into a wide diversity of signaling pathways.

60 YEARS OF POMC: Biosynthesis, trafficking, and secretion of pro-opiomelanocortin-derived peptides

Pro-opiomelanocortin (POMC) is a prohormone that encodes multiple smaller peptide hormones within its structure. These peptide hormones can be generated by cleavage of POMC at basic residue cleavage sites by prohormone-converting enzymes in the regulated secretory pathway (RSP) of POMC-synthesizing endocrine cells and neurons. The peptides are stored inside the cells in dense-core secretory granules until released in a stimulus-dependent manner. The complexity of the regulation of the biosynthesis, trafficking, and secretion of POMC and its peptides reflects an impressive level of control over many factors involved in the ultimate role of POMC-expressing cells, that is, to produce a range of different biologically active peptide hormones ready for action when signaled by the body. From the discovery of POMC as the precursor to adrenocorticotropic hormone (ACTH) and β-lipotropin in the late 1970s to our current knowledge, the understanding of POMC physiology remains a monumental body of work that has provided insight into many aspects of molecular endocrinology. In this article, we describe the intracellular trafficking of POMC in endocrine cells, its sorting into dense-core secretory granules and transport of these granules to the RSP. Additionally, we review the enzymes involved in the maturation of POMC to its various peptides and the mechanisms involved in the differential processing of POMC in different cell types. Finally, we highlight studies pertaining to the regulation of ACTH secretion in the anterior and intermediate pituitary and POMC neurons of the hypothalamus.

FK506-binding protein 51 regulates nuclear transport of the glucocorticoid receptor beta and glucocorticoid responsiveness

PURPOSE

A spliced variant of the human glucocorticoid receptor GRbeta has been implicated in glucocorticoid responsiveness in glaucoma. Over-expression of the FK506-binding immunophilin FKBP51 also causes a generalized state of glucocorticoid resistance. In the present study, the roles of FKBP51 in the nuclear transport of GRbeta and glucocorticoid responsiveness were investigated.

METHODS

Human trabecular meshwork cells (GTM3 and TM5) and HeLa cells were treated with dexamethasone (DEX) and FK506 and transfected with GRbeta and FKBP51 expression vectors. Coimmunoprecipitation and Western blot analyses were performed to study interactions of FKBP51 and FKBP52 with GRalpha, GRbeta, Hsp90, or dynein. The cells were transfected with a GRE-luciferase reporter to evaluate the effects of DEX and FK506 and the overexpression of GRbeta and FKBP51 on glucocorticoid-mediated gene expression.

RESULTS

FKBP51 was involved in constitutive nuclear transport of both GRalpha and -beta in the absence of ligands. FKBP52 appeared to be solely responsible for the nuclear transport of ligand-activated GRalpha. DEX stimulated the translocation of GRalpha but not GRbeta. Overexpression of either GRbeta or FKBP51 stimulated GRbeta translocation and reduced DEX-induced luciferase in HeLa cells. FK506 did not alter DEX-induced translocation of GRalpha. However, FK506 increased the association of FKBP51 with GRbeta and stimulated DEX-induced translocation of GRbeta in normal TM cells, but not in glaucoma TM cells. Increased nuclear GRbeta significantly inhibited glucocorticoid responsiveness in TM cells.

CONCLUSIONS

Nuclear transport of GRbeta represents a novel mechanism through which FKBP51 alters GC sensitivity. GRbeta and FKBP51 may be responsible for increased responsiveness in steroid-induced ocular hypertensive individuals as well as in patients with glaucoma.

Induction of antinociception and increased met-enkephalin plasma levels by cyclosporine and morphine in rats: implications of the combined use of cyclosporine and morphine and acute posttransplant neuropsychosis

BACKGROUND

Cyclosporine A (CsA) and morphine have neurotoxic and psychiatric side effects, respectively. Endogenous opiatelike peptides can elicit a number of behavioral responses that mimic the symptoms of psychiatric illness. The purpose of this study was to quantitiate the changes of Met-enkephalin (ME) and beta-endorphin (BE) after administration of CsA and morphine in surgery and to assess the antinociceptive effect.

PATIENTS AND MATERIALS

Pain sensitivity, an antinociceptive indicator in rats, was determined with the hotplate test. Plasma ME and BE levels were measured with radioimmunoassays.

RESULTS

In normal unoperated rats, CsA induced a profound analgesic effect concomitant with an increased plasma ME level on day 1. Morphine produced an analgesic effect on days 1 and 2, with decreased ME levels on days 2 and 3. Coadministration of CsA and morphine prolonged the analgesia from days 1 to 4 and increased the plasma ME level on day 1. No change in plasma BE level was found. In surgically operated rats, CsA induced an analgesic effect and higher ME levels than those in unoperated rats. Interestingly, the combined use of CsA and morphine prolonged the analgesia and increased plasma ME levels from days 1 to 4, with no significant change in plasma BE levels.

CONCLUSIONS

Our results showed that CsA can induce antinociception and increase plasma ME levels. This induction can be potentiated by the addition of morphine. Acute neuropsychiatric manifestations in the early posttransplant period might, therefore, be due to induction of ME after coadministration of CsA and morphine.

Overexpression of the FK506-binding immunophilin FKBP51 is the common cause of glucocorticoid resistance in three New World primates

Many New World primates have high circulating levels of cortisol to compensate for the expression of glucocorticoid receptors (GRs) with low activity. Recent work in squirrel monkeys has suggested that this may be due to either the expression of GRs that are transcriptionally incompetent or the expression of an FK506-binding immunophilin that inhibits GR binding. The goal of this study was to resolve this controversy by determining the molecular basis of glucocorticoid resistance not only in species of squirrel monkeys but also in other glucocorticoid-resistant New World primates. First, the transcriptional activity of the GR from the Bolivian squirrel monkey was compared to that of the human GR. Incubation of COS-7 cells transfected with the squirrel monkey GR with 10 nM dexamethasone resulted in a robust stimulation of MMTV-luciferase activity (up to 260-fold), which was similar in magnitude to that achieved with the human GR. Second, the effect of FK506 on GR binding was determined in cytosol from cells from two species of squirrel monkeys as well as glucocorticoid-resistant cotton-top tamarins and owl monkeys. Incubation with 10 microM FK506 increased GR binding by at least 4-fold in cytosol from cells of each of the New World primates but had no effect on GR binding in cytosol from human WI-38 VA13 cells. Third, Western blots showed elevated expression of FKBP51 in New World primate cells and liver samples from two squirrel monkey species. On the other hand, the levels of FKBP52 were significantly lower in cells and liver from New World primates. The sequences of FKBP51 from the cotton-top tamarin, owl monkey and squirrel monkey are closely related and share differences from the human, rhesus monkey, mouse, and lemur FKBP51 sequences in the same 18 positions. Fourth, the relative activities of FKBP51 from the cotton-top tamarin, owl monkey and squirrel monkey were determined in cytosol mixing and GR transactivation studies and showed that FKBP51 from each of these primates was a potent inhibitor of GR activity. These results indicate that the elevated expression of FKBP51 contributes to glucocorticoid resistance in three New World primate genera.

Decreased cortisol secretion by adrenal glands perfused with the P-glycoprotein inhibitor valspodar and mitotane or doxorubicin

The aim of this study was to investigate the role of P-glycoprotein (P-gp) in the adrenal gland. It has been presumed that P-gp, rather than being involved in physiological cortisol secretion, plays a role in protecting the adrenacortical cells from xenobiotics. To explore this a study was performed on perfused bovine adrenal glands. Individual experimental groups were perfused with either a selective P-gp blocker (valspodar) alone, with a xenobiotic (mitotane or doxorubicin) alone or with both valspodar and a xenobiotic. The cumulative amounts of cortisol secreted in each individual group were calculated and the two-sample t-test was used to compare the mean values of cumulative amounts. The mean value of cortisol secreted from the group of adrenals perfused with the P-gp blocker was not significantly different from that of the control group. Treatment with either mitotane or doxorubicin decreased the amount of cortisol secreted but not significantly when compared to the amount of cortisol secreted in basal conditions. However, treatment with the P-gp blocker valspodar in addition to either mitotane or doxorubicin significantly decreased cortisol secreted compared to the amount of cortisol secreted by the glands treated with either mitotane (p=0.009) or doxorubicin (p=0.017) alone. The regressive changes discovered in all experimental groups were most prominent when valspodar was used with either mitotane or doxorubicin. We found that P-gp blockade increases by xenobiotic (mitotane and doxorubicin)-induced damage of adrenocortical cells, which points to a role of P-gp in the protection of adrenal gland from xenobiotics.