Regulation of transcription in rat thymus cells by glucocorticoids

Massive telomere loss and telomerase RNA expression in dexamethasone-induced apoptosis in mouse thymocytes

Apoptosis is a well-recognized process of cell death occurring under several physiological and pathological conditions and represents the principal mechanism involved in cell selection in the thymus. Glucocorticoids are well known to stimulate apoptosis in rat thymocytes. However, it is unclear whether the same changes occur after in vivo glucocorticoid treatment in mice. Chromosomal stability and cell viability require a proficient telomeric end-capping function. Cells with critical telomere shortening and telomerase dysfunction undergo increased apoptosis. In turn, the change in telomere function in cells undergoing apoptosis is not fully characterized. In order to investigate this, we studied the changes in thymocytes after dexamethasone administration in BALB/c mice. The loss of normal thymocytes coincided with the appearance of small dense cells with characteristic features of apoptosis including condensed chromatin, internucleosomal DNA cleavage, and a "hypodiploid" peak on flow cytometry, which suggested that dexamethasone-induced thymocyte apoptosis in BALB/c mice could be considered a well-defined experimental model for studying apoptotic processes. Dexamethasone-treated thymocytes exhibited rapid and dynamic loss of telomeric sequences and up-regulation of telomerase RNA as an early event in the apoptotic process. Telomerase activity was unchanged in this event. Thereafter, telomere gain associated with an increase in telomerase activity occurred in the regenerative process of the thymus. These results suggest a role of telomere loss and up-regulation of telomerase RNA as key apoptosis sensors.

Changes in nuclear protein pattern by glucocorticoid treatment of lymphoid cells

Glucocorticoids initiate a cytolytic process in lymphoid cells. The ultimate response is preceded by several phenomena. It is generally accepted that these are mediated through messenger proteins. The induction of these proteins is considered the primary effect of glucocorticoids. However, as yet specific gene products have not been identified. In electrophoretic assays, we observed an increased concentration of 6 nuclear proteins within a few hours of exposure of lymphoid cells to glucocorticoids. These proteins displayed prominent DNase activity. However, further studies showed: (1) that the proteins concerned are histones, (2) that histones are more easily extracted after glucocorticoid-induced alterations of lymphoid cells, and (3) that basic proteins in general express nuclease activity under certain experimental conditions. This nuclease activity is, however, artifactual. Therefore, though the changes observed are certainly related to glucocorticoid-induced effects, these do not reflect the induction of specific proteins. The results of the study indicate that glucocorticoid-induced changes in the concentration of cellular proteins should be interpreted with caution.

Effects of glucocorticoid and cycloheximide on the activity and amount of RNA polymerase I in nuclei of rat liver

The activity of the template-engaged form of RNA polymerase I from livers of adrenalectomized rats was about 50-60% of that of normal control rats, and increased about 2-fold at 6 h after the administration of dexamethasone. However, no change was found in the activity of the 'free' form of RNA polymerase I or the template-engaged form of RNA polymerase II. Immunochemical studies using guinea-pig anti-(RNA polymerase I) serum disclosed that the total number of RNA polymerase I molecules did not vary during the treatment with dexamethasone. Cycloheximide caused a rapid decrease in the template-engaged form of RNA polymerase I activity in normal rats and in dexamethasone-treated (6 h) adrenalectomized rats, to the value in adrenalectomized rats, but affected it only slightly in adrenalectomized rats. The elongation rate of rRNA-precursor synthesis in liver nuclei was not affected by a change in the concentration of circulating dexamethasone. From these results, it is concluded that about half the rRNA-precursor synthesis in rat liver is regulated by glucocorticoids, probably through the synthesis of short-lived protein(s) which may play a role in conversion of the 'dormant' form of RNA polymerase I into the 'engaged' form.

Chromatin-bound and free RNA polymerase A activities in rat thymus cells following glucocorticoid treatment

Treatment of rat thymus cell suspensions with dexamethasone resulted in inhibition of engaged RNA polymerase A, without significant change in free pool activity. Studies with the re-initiation inhibitor, rifamycin AF/0-13, and measurements of numbers of RNA polymerase A molecules and of elongation rates showed that the inhibition of pre-rRNA synthesis resulted from a decrease in elongation rate. This effect was selectively abolished by mild proteolysis of nuclei. It is concluded that glucocorticoid treatment of rat thymus cells suppresses 45 S rRNA synthesis primarily by decreasing the polyribonucleotide elongation rate, rather than by effecting a change in enzyme redistribution or concentration.

Chromatin cleavage in apoptosis: association with condensed chromatin morphology and dependence on macromolecular synthesis

In glucocorticoid-treated rat thymocytes and the murine lymphoid cell lines L5178 and S49 the morphology of apoptosis is associated with chromatin cleavage. The cleavage is at internucleosomal sites, apparently through activation of an endogenous endonuclease. In variants of the cell lines selected for resistance to glucocorticoid, neither apoptosis nor chromatin cleavage were observed after steroid treatment, and steroid receptors were undetectable. In thymocytes, both the morphological changes of apoptosis and chromatin cleavage were inhibited by cycloheximide and actinomycin D. The calcium-magnesium ionophore A23187 induced apoptosis and chromatin cleavage in thymocytes, and these effects were also inhibited by cycloheximide. The data confirm that the condensed chromatin which characterizes apoptosis morphologically consists of endogenously digested chromatin fragments. They also provide support for the view that at least some cells enter apoptosis by a process dependent upon macromolecular synthesis.

Hormonal regulation of androgen production by the Leydig cell

The control of androgen production by the Leydig cell is dependent upon the episodic secretion of hormone (LH), which is released from the anterior pituitary gland in pulses of high biological activity. This mode of episodic LH secretion supports steroidogenic enzyme activity in the testis through interaction with LH receptors and stimulation of the adenylate cyclase/protein kinase sequence, leading to phosphorylation of key intermediates in the steroid biosynthetic pathway. The plasma membrane events that are rapidly activated by the specific interaction of LH or hCG with Leydig cell receptors include increased binding of guanyl nucleotide, and stimulation of cAMP-independent, Ca2+-dependent phosphorylation of a 44,500 Mr protein, with the characteristics of the adenylate cyclase nucleotide regulatory unit. Hormonal activation of adenylate cyclase is affected by Ca2+ with the same concentration-dependence, suggesting that nucleotide-induced phosphorylation is related to activation of the catalytic cyclase unit. In addition to the characteristic increases in pregnenolone synthesis and androgen production, gonadotropin-stimulated Leydig cells show prominent changes in LH receptor content and steroidogenic activity that modify their subsequent responses to hormonal signals. Thus, after exposure to increased LH and hCG levels in vivo and in vitro, LH receptors show an initial transient increase (up-regulation) followed by a marked decrease (down-regulation) and a prolonged depletion of LH receptor sites. Large doses of hCG cause "early" (prior to pregnenolone) and "late" steroidogenic lesions (17 alpha-hydroxylase, 17-20 desmolase) that are independent of receptor loss. The early lesion is partly due to reduced activity of HMG CoA reductase, and is mainly attributable to the increased activity of an inhibitory protein factor that modulates the activity of cholesterol side chain cleavage enzyme in Leydig cell mitochondria. In contrast, the late steroidogenic lesion is related to the nuclear actions of E2 produced during hormonal action. After hCG stimulation, an increase in nuclear E2 binding was accompanied by an early rise of RNA polymerase activities within 45 min coincident with the maximal increases in circulating testosterone and estradiol levels. These events were followed by the emergence of an E2-induced protein of Mr 27,000 at 3-6 h, and by reduction in the activity of 17 alpha-hydroxylase/17-20 desmolase, and a decrease in microsomal cytochrome P-450.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies on the mechanism of glucocorticoid hormone induced alterations in rat thymic transcription--I. Evidence from reconstituted cross-over transcription assays that sequential increases and decreases in transcription are due to changes in the activity of RNA polymerase II rather than in the activity of chromatin template

In experiments to determine the mechanism of glucocorticoid induced decreases in thymic transcription, adrenalectomized rats were injected with hydrocortisone (50 mg/kg) or vehicle. Thymic nuclei were used to prepare chromatins and soluble nuclear extracts containing RNA polymerase II for cross-over experiments. With calf thymus DNA or rat thymic chromatins as templates limiting RNA polymerase II from rats treated with hydrocortisone 3 h previously had 130% of the [3H]UMP incorporating activity of RNA polymerase II from control vehicle treated rats. In contrast, limiting RNA polymerase II from rats treated with hydrocortisone 12 h previously had 40-50% of the [3H]UMP incorporating activity of RNA polymerase II from controls. When limiting calf thymus DNA or rat thymic chromatins were used in 12 h cross-over experiments. Individual RNA polymerases II produced equal [3H]UMP incorporations, but RNA polymerase II activity from hydrocortisone treated rats was again only 50% of control values. Thus with template saturation, RNA polymerase II from hydrocortisone treated rats could not transcribe rat thymic chromatin templates to the level achieved by RNA polymerase II from control rats. This suggests that the activity, rather than the amount, of RNA polymerase II from hydrocortisone treated rats is reduced. Double reciprocal plots of [3H]UMP incorporation on rat chromatins with increasing concentrations of RNA polymerases II were made at 12 h. The apparent Km for RNA polymerase II from animals treated with hydrocortisone was identical to that of RNA polymerase II from controls, but the Vmax of RNA polymerase II from hydrocortisone treated animals was reduced. These data suggest the presence of an inhibitor of transcription or an RNA polymerase II defective in its capacity to initiate and/or elongate RNA transcripts. Further experiments demonstrated that these effects were not due to steroid induced changes in ribonuclease or protease activities.

The role of protein metabolism in glucocorticoid-induced prolongation of G1 phase in human NHIK 3025 cells

It has previously been found that human NHIK 3025 cells have a glucocorticoid-sensitive restriction point in mid-G1 phase of the cell cycle. When these cells were synchronized by mitotic selection and exposed to dexamethasone before the restriction point, G1 phase was prolonged whereas the rest of the cell cycle was unperturbed by the hormone. These observations were confirmed by flowcytometric measurements of synchronized cells in the present study. Cells that received dexamethasone (10(-6) M) just after mitotic selection had a 4 hour prolongation of both G1 and the total cell cycle. However, the general rates of both protein synthesis and protein degradation were found not to be altered by the hormone, i.e., the rate of protein accumulation in dexamethasone exposed cells was equal to that of control cells. Dexamethasone exposed NHIK 3025 cells were found to be larger than control cells at the time of cell division. This is a direct consequence of a prolonged cell cycle duration with no change in general protein metabolism. It thus appears that the dexamethasone-induced prolongation of G1 phase is the result of a steroid-regulated G1 specific process(es) leading toward DNA replication, a process that does not alter general protein accumulation.

An electrophoretic characterization of the glucocorticoid response of the Fu5-5 rat hepatoma cell line

In this study we have further characterized the response of liver-derived cells to glucocorticoid treatment using the rat hepatoma, Fu5-5. Using two-dimensional electrophoresis we have examined changes in the synthetic rates of cytosol proteins following glucocorticoid administration by using [35S]methionine. We have also demonstrated changes in the incorporation of 32P by several cytosol proteins after hormone treatment. One of these changes occurs within 1 h of hormone treatment. We were unable to detect any changes in the nuclear protein content of Fu5-5 cells after glucocorticoid treatment. We then compared the glucocorticoid-regulated changes in cytosol protein synthesis in Fu5-5 cells with those of the H35 rat hepatoma, monolayer cultures of rat hepatocytes and the response of rat liver in vivo. This examination revealed that although nearly all of the eighteen hormonally responsive proteins appeared to be present in all of the cells types, only three were responsive to glucocorticoid in more than one system. Moreover, the response patterns were often reversed in different cell types, the same protein being induced in one cell type and repressed in another.