Cell type-specific roles of histone deacetylase in TR ligand-independent transcriptional repression.

Recent evidence indicates that corepressor protein with histone deacetylase (HDAC) activity mediates thyroid hormone receptor (TR) transcriptional repression. In order to examine the physiological relevance of HDAC in ligand-independent TR-mediated repression, we studied the effect of trichostatin A (TSA), a specific HDAC inhibitor, in transient transfection studies with natural reporters, and assessed the expression of TR-regulated endogenous genes. Luciferase-coupled DR4-, F2-, PAL- or GH-TREs and TRbeta1 expression vectors were cotransfected in CV-1 and GH(3) cells. We did not observe any effect of TSA on TR-induced basal repression in CV-1 cells. Instead, TSA was able to induce an increase in transcription without T(3) on all TREs tested in GH(3) cells. This increase was >7-fold on F2-, >4-fold on DR4-, and 3-fold on GH-TREs. The cotransfection of a TRbeta1 mutant that exhibits decreased affinity with N-CoR (AHT) reduced the TSA effect in GH(3) cells, demonstrating a primary role for TR/N-CoR/Sin3/HDAC complex. Next, we examined the effects of TSA on endogenous GH mRNA production in GH(3) cells by Northern blot analysis. We observed an increase of 50-70% of GH mRNA in cells treated with TSA in hypothyroid medium, and an increase of GH mRNA in T(3)-treated cells after TSA treatment. Our results show that TSA can increase the expression of endogenous genes that are susceptible to TR regulation. These results support an active role of HDAC in transcriptional repression by ligand-independent TR.

Cholinergic modulation of spontaneous hypothalamic-pituitary-adrenal activity and its circadian variation in man.

Controversy still exists regarding the role of cholinergic pathways in the regulation of the hypothalamic-pituitary-adrenal axis in man. We studied the effects of the administration of placebo, pyridostigmine (PD); 120 mg, orally), and the combination of PD and pirenzepine (PZP; 100 mg, orally) on ACTH, cortisol, and GH secretion at 0730 and 2230 h in seven normal males. PD induced a clear decrease in ACTH levels at both times of the day compared to treatment with placebo, producing higher suppression in the nocturnal period (34.4 +/- 5.8% vs. 21.8 +/- 10.7%). The combination PD and PZP prevented the inhibitory action of PD on ACTH secretion in the morning, but not in the evening, when ACTH values showed a decrease similar to that seen after giving PD alone (38.1 +/- 5.6% vs. 34.4 +/- 5.8%, respectively). Cortisol values declined only when the association PD plus PZP was given in the evening. GH levels had a significant increase after PD administration in the morning (4.1 +/- 1.2 ng/mL) and in the evening (10.2 +/- 1.6 ng/mL), confirming that cholinergic stimulation was taking place, whereas the addition of PZP to PD induced a significant attenuation of these responses. It is concluded that cholinergic pathways have a inhibitory role in ACTH secretion in man. M1 muscarinic receptors seem to be involved in the diurnal inhibition of PD, whereas our observations are consistent with the mediation of another type of cholinergic receptors as an explanation for the nocturnal effect of PD on ACTH secretion. PD did not alter the circadian variation in the hypothalamic-pituitary-adrenal axis, whereas the association of PD and PZP increased the differences between diurnal and nocturnal ACTH values, suggesting a modulatory effect of the cholinergic system on the circadian rhythm of ACTH secretion.

[Clinical usefulness of the glucagon test in the study of pancreatic islet cell function]. / Utilidad clínica del test de glucagón en el estudio de la función insular pancreática.

In an attempt to assess the utility of glucagon test as an index of beta cell function, we have studied the C-Peptide response to intravenous injection of 1 mg of glucagon in 77 patients with type I diabetes, 27 type II diabetics and 14 normal subjects. A significant difference between basal and glucagon-stimulated C-Peptide levels as well as a good relationship between both values were observed in all groups (r values 0.91, 0.80 and 0.89 respectively). Both basal and stimulated C-Peptide concentrations showed significant relationship with the body mass index and total cholesterol levels, whereas both parameters were negatively related to insulin requirements in type I diabetic group. Absolute C-Peptide increment was different in all groups, despite it was not related to basal C-Peptide levels. These findings confirm the usefulness of glucagon test as an investigative tool to assess the secretory capacity of beta-cell, suggesting that estimation of C-Peptide increment represents the best parameter to evaluate beta-cell reserve.