Toshihiko Suzuki

Expression and regulation of nuclear receptor coactivators in glucocorticoid action

Nuclear receptor coactivators are involved in receptor-mediated transcriptional activation of target genes in a hormone-sensitive manner, and the mechanism of their transactivation has been studied in recent years. The glucocorticoid receptor (GR) interacts with several coactivators, including steroid receptor coactivator-1 (SRC-1) family and CREB-binding protein (CBP). Since coactivators function as transcription amplifiers, subtle changes in expression levels of coactivators in certain cells would markedly intensify receptor-mediated transcriptional activity. The regulation of coactivators by glucocorticoid action, however, has not yet been clarified. In this study, we have shown that one of the coactivators interacting with GR, SRC-1, is downregulated by dexamethasone (DEX) both in vivo and in vitro. In experiments on Sprague-Dawley rats in vivo, the downregulation of SRC-1 was observed in heart, stomach, kidney, liver, and cerebrum, and in experiments on two types of kidney-derived cells in vitro, similar downregulation of SRC-1 was demonstrated in both types of cells. DEX-mediated downregulation of SRC-1 mRNA recovered in 4-8 h, while the downregulation of SRC-1 protein lasted for 12 h and its levels returned to the basal level, 24 h after DEX treatment. Other coactivators examined in this study showed no remarkable changes in expression by DEX treatment, implying that ligand-mediated downregulation of SRC-1 has a pivotal role in the physiology of glucocorticoid action.

Risk Factors Associated with Persistent Postoperative Hypertension in Cushing's Syndrome

The aim of the present study was to assess the long-term results of adrenalectomy and to evaluate potential risk factors for the persistence or recurrence of hypertension. Forty-five patients with Cushings syndrome caused by benign cortisol-producing adrenocortical adenomas were evaluated before and for a period of 1 year after surgical cure. When the patients were classified into two groups according to whether their preoperative BP was more (HBP group) or less (NBP group) than 140/90 mmHg, the BP level was found to be continuously higher in the HBP group than in the NBP group during the year after surgery. This finding suggests that the preoperative BP level in Cushings syndrome may be a determinant factor for persistent hypertension after surgery (P<0.05). In addition, a correlation was found between postoperative BP level and duration of hypertension (P<0.05), but no relationships were found between postoperative BP levels and other factors, including age, BMI, tumor size, serum cortisol, aldosterone, potassium, total cholesterol, or glucose levels. The above findings indicate that intensive control of preoperative BP to maintain it below 140/90 mmHg with antihypertensive medication is a very important means of improving prognosis for postoperative BP. Immediate diagnosis and surgical treatment to reduce the duration of hypertension are also crucial for the long-term BP prognosis.

Differential expression of an orphan receptor coup-tfi and corepressors in adrenal tumors

Recently, it has been shown that CYP17 gene transcription is activated by SF-1 (Steroidogenic Factor-1) binding to a cyclic AMP-responsive sequence within the promoter region of the gene, whereas it is inhibited by COUP-TF (Chicken Ovalbumin Upstream Promoter-Transcription Factor) binding to the sequence. We have shown that transcriptional repression by COUP-TFI is mediated by corepressors, N-CoR (nuclear receptor corepressor) and SMRT (silencing mediator for retinoid and thyroid hormone-receptor). Therefore, we compared the expression of COUP-TFI, N-CoR and SMRT in non-hyperfunctioning adrenocortical adenomas and normal adrenal glands. We found significantly higher expression of COUP-TFI mRNA in non-hyperfunctioning adenomas (n=5, 227+/-18%) than in normal adrenals (n=5, 96+/-4%). Interestingly, the pattern of N-CoR and SMRT expression was different compared with COUP-TFI expression. These data suggest that COUP-TFI, N-CoR, and SMRT may play a differential role in steroid biosynthesis of non-hyperfunctioning adenomas.

Transcriptional regulation of steroid receptor coactivator-1 (SRC-1) in glucocorticoid action

Diverse mechanisms of steroid receptor action have been clarified in recent years, as a consequence of the discovery of multiple coactivators. Among them, steroid receptor coactivator-1 (SRC-1) is a member of the p160 coactivator families, which are 160 kDa proteins that interact with steroid receptors in a hormone-sensitive manner. Since coactivators function as transcriptional power boosters, subtle changes in coactivator expression levels in certain cells markedly change of receptor-mediated transcriptional activity. Expression of the glucocorticoid receptor (GR) has been shown to be autoregulated in glucocorticoid action, i.e., GR is downregulated by its cognate ligand, indicating that this autoregulation of GR may protect target cells against excessive hormone action. In the present study, we examined whether coactivator expression levels are also regulated by glucocorticoids. Among several coactivators, the SRC-1 mRNA level was downregulated by dexamethasone treatment in rat tissues, such as liver, heart, kidney, stomach, and cerebrum, in vivo We also demonstrated dexamethasone-mediated downregulation of SRC-1 mRNA and its protein levels in rat renal mesangial cells in vitro These results suggest that ligand-mediated downregulation of SRC-1 is crucial in the physiology of glucocorticoid action.

The possible role of apoptosis-suppressing genes, BCL-2 and MCL-1/EAT in human adrenal tumors

The expression levels of bcl-2, mcl-1/EAT, and bax were examined by Northern blot analysis and semi-quantitative RT-PCR method in 25 adrenal tumors, including seven adrenal pheochromocytomas (PHE), seven aldosterone-producing adenomas (APA), four adrenal cortisol-producing adenomas (CS), one deoxycorticosterone-producing adenoma (DOC) and six non-hyperfunctioning adrenal cortical adenomas (NF). Northern blot analysis revealed both bcl-2 and mcl-1/EAT mRNAs in all of the adrenal tumors. The expression levels differed greatly among the tumor samples. Mcl-1/EAT mRNA levels were enhanced in APA and CS compared with those in NF. In contrast, bcl-2 levels in NF were higher than in other tumors. Our results suggest that deregulation of such apoptosis-suppressing genes may contribute to the multistep tumorigenesis of human adrenal tumors and that mcl-1/EAT, one of the bcl-2 family genes, has a different role from that of bcl-2 in such pathways.

Orphan receptors COUP-TF and DAX-1 as targets in disordered CYP17 expression in adrenocortical tumors.

CYP17 gene transcription is activated by SF-1 binding to a cyclic AMP-responsive sequence within the promoter region of the gene, and its transcription is inhibited by COUP-TF binding to the sequence. Another orphan receptor, DAX-1, is shown to act as a suppressor of SF-1-mediated transcription. We examined the expression level of these orphan receptors in adrenocortical tumors and compared the results with CYP17 expression. CYP17 was highly expressed in cortisol-producing adenomas, whereas COUP-TF and DAX-1 expression levels were very low. In deoxycorticosterone-producing adenomas, on the other hand, CYP17 expression was extremely low, whereas DAX-1 was highly expressed and SF-1 expression was slightly decreased. In conclusion, the reciprocal expression of CYP17 and the transcriptional repressors COUP-TF and DAX-1 indicates that these orphan receptors have a pathophysiologic role in the excessive hormone production in cortisol- and deoxycorticosterone-producing adrenocortical tumors.

Expression and regulation of BCL-2 family genes in human adrenocortical adenomas in comparison with adrenal hyperplasia of Cushing's disease.

The significance of increases in the expression of apoptosis-suppressing genes such as bcl-2 and mcl-1/EAT in human adrenal tumors has not yet been fully elucidated. Furthermore the roles of these genes in cell proliferation may involve interaction with steroidogenesis in the tumors via intracellular second messengers. Cyclic AMP (cAMP) caused human adrenocortical H295R cells to overexpress hCYP17 resulting in hypersecretion of cortisol. At the same time, however, expression of bcl-2, which has a cAMP response element (CRE), was not affected. Furthermore, in vivo Bcl-2 protein analysis showed its down-regulation in adrenal hyperplasia of Cushings disease despite ACTH stimulation. Exogenous addition of glucocorticoid did not affect the expression of bcl-2 family genes. Expressions of McL-1/EAT and Bax did not differ markedly among human adrenal glands affected by various pathologies. In conclusion the down-regulation of Bcl-2 in Cushings disease did not agree with no induction of this gene by cAMP in H295R cells, suggesting that expression of Bcl-2 protein was not regulated mainly by cAMP-protein kinase (PKA) pathways in human adrenal hyperplasia.

Cardiac hypertrophy in chronic kidney disease—role of Aldosterone and FGF23

Cardiac hypertrophy is a life-threatening disorder and is frequently observed in patients with chronic kidney disease (CKD). Much attention has been focused on the derangement in hormonal factors, including aldosterone and FGF23, as novel causes of cardiac hypertrophy in CKD. Plasma aldosterone concentrations are elevated as renal function declines. Although aldosterone antagonists are available for the treatment of hypertension with cardiac hypertrophy, concern remains regarding the possible occurrence of serious hyperkalemia. Alternatively, certain types of calcium channel blockers suppress aldosterone synthesis or exert blocking action for mineralocorticoid receptors and could halt the progression of cardiac dysfunction. Recently, FGF23 is shown to be elevated as CKD progresses and may be responsible for the development of cardiac hypertrophy and heart failure. Furthermore, FGF23 not only inhibits the renal expression of angiotensin converting enzyme 2 but also enhances renin gene transcription, both of which could accelerate renin-angiotensin-aldosterone system. Although the increase in serum phosphate concentrations is a pivotal stimulus for FGF23 production, recent studies suggest that reduced iron status and elevated aldosterone levels, frequently seen in patients with CKD or on dialysis, might also contribute to the elevation in serum FGF23 levels. Conversely, phosphate binders and appropriate iron status could reduce serum FGF23, potentially leading to the alleviation of cardiac hypertrophy and heart failure. In conclusion, novel therapeutic strategies associated with aldosterone and FGF23 may confer a benefit in the management of cardiac disorders in CKD.