Characterization of high affinity and low affinity dexamethasone binding sites on male rat liver nuclear envelopes

Characterization of cyclic nucleotide phosphodiesterase isoforms associated to isolated cardiac nuclei

The identity and location of nuclear cyclic nucleotide phosphodiesterases (PDE) has yet to be ascertained. Intact cardiac nuclei and subnuclear fractions from ovine hearts were isolated to determine cAMP-specific PDE activity which was 3-fold greater than that of cGMP PDE, the latter being insensitive to Ca-calmodulin and zaprinast. Specific hydrolytic activities of the cardiac nuclear envelopes (NE) were similar to those measured in the corresponding intact nuclei, thus suggesting that most PDE activity is associated with the nuclear membrane. Moreover, the main hydrolytic activities in cardiac nuclei were attributed to PDE4 (56%) and PDE3 (44%). The pharmacological sensitivity of each isoform in terms of IC(50), K(m) and K(i) values was typical of previously characterized cardiac PDE 3 and 4 isoforms. PDE2 (cGMP-stimulated PDE) represented a minor component (8-9%) of total hydrolytic activity. Solubilization of nuclear envelopes and HPLC separation also yielded rolipram-sensitive PDE activities. Upon 1% Triton X-100 extractions, the presence of PDE3 and PDE4 was revealed in a low speed, nucleopore complex-enriched, P1 pellet. In addition, Western blot analysis demonstrated the presence of PDE4B and PDE4D subtypes in the nuclei as well as enrichment in NE. However, in the same preparations, the presence of PDE4A could not be ascertained. Altogether, these results suggest an intrinsic and predominant association of these nuclear PDEs with the NE and much likely with nucleopore complexes.

Does the nuclear envelope contain two types of ligand-gated Ca2+ release channels?

The nuclear envelope is composed of two membranes deliminating a perinuclear space which displays functional properties similar to those of a Ca2+-storing compartment. ATP-driven Ca2+ uptake and InsP3-induced Ca2+ release processes have been described in isolated nuclei. Recently, it was reported that cADP-ribose and InsP3 can trigger a nucleoplasmic Ca2+ increase. It was hypothesized that the inner nuclear membrane possesses Ca2+ channels that are regulated by ryanodine or InsP3. Radio-ligand binding assays and Western blot experiments were performed in order to investigate their presence in sheep cardiac and rat liver nuclear envelopes. Ryanodine receptors (RyR) were not detected in liver nuclear envelopes by either binding assay or Western blot analysis. However, cardiac nuclear envelopes were found to retain a very low level of specific ryanodine binding, which was not detected on immuno-blots obtained with three types of isoform-specific RyR antibodies. In contrast, nuclear InsP3-binding sites were consistently detected in both cardiac and liver nuclear envelopes. Altogether, these results provide evidence for the major contributor InsP3-gated Ca2+ channels in control of Ca2+ release from the perinuclear space in liver and cardiac cells.

Reconstitution of ionic channels from inner and outer membranes of mammalian cardiac nuclei

Recent reports suggest that the nuclear envelope possesses specific ion transport mechanisms that regulate the electrolyte concentrations within the nucleoplasm and perinuclear space. In this work, intact nuclei were isolated from sheep cardiac cells. After chromatin digestion, the nuclear envelopes were sonicated and four nuclear vesicle populations were separated by sucrose step gradients (SF1-SF4). These fractions were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and their protein content was analyzed by Western blot, using lamin and SEC 61 antibodies. The lamins, which are associated with the inner nuclear membrane, were present in three fractions, SF2, SF3, and SF4, with a lower amount in SF2. The SEC 61 protein, a marker of the rough endoplasmic reticulum, was detected in small amounts in SF1 and SF2. Upon fusion of vesicles into bilayers, the activities of nuclear ionic channels were recorded in 50 mM trans/250 mM cis KCl or CsCl, pH 7.2. Two types of Cl- selective channels were recorded: a large conducting 150-180-pS channel displaying substates, and a low conducting channel of 30 pS. They were both spontaneously active into bilayers, and their open probability was poorly voltage dependent at negative voltages. Retinoic acid (10(-8) M) increases the po of the large Cl- conducting channel, whereas ATP modifies the kinetics of the low conductance anion selective channel. Our data also suggest that this anionic channel is mainly present in the SF3 and SF4 population. The presence of a 181 +/- 10 pS cation-selective channel was consistently observed in the SF2 population. The behavior of this channel was voltage dependent in the voltage range -80 to +60 mV. Furthermore, we report for the first time the activity of a channel exclusively present in the SF3 and SF4 fractions, shown to contain mainly inner membrane vesicles. This cation selective channel displays a 75-pS conductance in 50 mM trans/250 mM cis K-gluconate. It is concluded that the bilayer reconstitution technique is an attractive approach to studying the electrophysiological properties of the inner and outer membranes of the nuclear envelope.

5 alpha-Reductase type 1 is localized to the outer nuclear membrane

The subcellular distribution of the two isozymes of 5 alpha-reductase has been controversial. To resolve this issue which could provide clues about the respective functions of the two isozymes, two antisera were generated, one which was specific for the Type 1 5 alpha-reductase and one which recognized both isozymes. In COS cells transfected separately with the Type 1 or Type 2 cDNA, both isozymes were detected on Western blots at an M(r) of 26,000. Subfractionation of the COS cells resulted in the partitioning of both isozymes between the crude nuclear and cytosolic fractions, while cytoimmunofluorescence localized both reductases to the nuclear periphery. In rat liver homogenate, the 5 alpha-reductase was also detected at M(r) 26,000. The 5 alpha-reductase immunoreactivity was increased after castration of the animals with no further effect when castrated animals were treated with androgens. Although the rat liver expresses only the Type 1 5 alpha-reductase, the 5 alpha-reductase was distributed about equally between crude nuclear and cytosolic subfractions; this distribution could be shifted to the cytosolic fractions with harsher homogenization procedures. Further extensive subfractionation and extraction studies identified the rat liver Type 1 5 alpha-reductase as an integral membrane protein present in the outer nuclear membrane of the nuclear envelope and in rough endoplasmic reticulum. Thus, the subfractionation and cytoimmunofluorescence studies are consistent with the localization of the Type 1 5 alpha-reductase to the outer nuclear membrane of the nuclear envelope which is continuous with and indistinguishable from the endoplasmic reticulum. This study is the first to localize rat liver Type 1 5 alpha-reductase to the nuclear envelope to which the prostatic 5 alpha-reductase activity previously had been localized. We conclude that, contrary to previous tissue distribution studies, but consistent with investigations in transfected cells, both isozymes are similarly localized to the nuclear periphery.

EGF causes hyperproliferation and apoptosis in T51B cells: involvement of high and low affinity EGFR binding sites

We have recently described an in vitro model for liver homeostasis which involves growing T51B rat liver epithelial cells with 1 nM epidermal growth factor (EGF). There is an initial period of hyperplasia, lasting about 3 days, which is followed by an increase in apoptosis. The cell density returns to around the confluent control level 5 days after EGF addition. The dose response of T51B cells to EGF shows three distinct growth patterns. We have carried out EGF binding studies that suggest that the occupancy of the low affinity binding site of the EGF receptor, is responsible for the hyperproliferation seen when the cells are grown with high doses of EGF. These studies also suggest that the apoptosis could be triggered by down-regulation of the receptor, in a manner analogous to the removal of a trophic hormone in other systems.

Nuclear and nuclear envelope binding proteins of the glucocorticoid receptor nuclear localization peptide identified by crosslinking

The molecular mechanisms underlying the nuclear entry of steroid receptors and possible regulation of steroid hormone action during receptor passage across the nuclear envelope have not been elucidated. A nuclear localization signal has been identified in the hinge region of the glucocorticoid receptor. A synthetic peptide corresponding to this sequence was radio-iodinated and incubated with high salt- and detergent-extracted rat liver nuclei or nuclear envelope in the presence of crosslinker. After SDS-PAGE, two nuclear polypeptides of 60 and 76 kDa which had been specifically crosslinked were identified by autoradiography. A 60 kDa polypeptide was also crosslinked in the nuclear envelope fraction. ATP and elevated temperatures enhanced the crosslinking of both nuclear peptides. Finally, we showed that the pattern of crosslinking of the simian virus 40 large tumour antigen nuclear localization signal was identical to that of the glucocorticoid receptor signal to the nuclear polypeptides. The crosslinked peptides are good candidates for nuclear importers of the glucocorticoid receptor. In addition, the data suggest that these binding sites may be part of a general mechanism for nuclear entry of proteins.

Structural requirements for the binding of dexamethasone to nuclear envelopes and plasma membranes

The specificity of dexamethasone binding sites on nuclear envelopes (NE) and plasma membranes (PM) was determined in competition studies with natural and synthetic steroids. The binding affinities for nuclear envelopes and plasma membranes were then correlated with the three-dimensional structures of the ligands. Three major factors are implicated in the ability of the steroid to bind to the membrane sites: (1) the separation between the terminal oxygen atoms substituted at atoms C3 and C17, or attached to the substituent at C17, is found to be longer than 10 A for the medium and high affinity steroids; (2) the beta-orientation of the oxygen atom in the C17-substituent to the D-ring is favored over alpha-orientation; and (3) bulky substituents and nontypical configurations are not accepted by the binding sites. A nearly linear correlation between the O3...O (substituted at C17) distance and the binding affinity of the tested steroids is observed; explanations for the lack of linear correlation of some steroids are given. A preliminary model for the interaction of steroids with these membrane sites is proposed which requires two hydrogen bonding regions that interact with the 2 oxygen atoms and some steric restriction sites that prevent the binding of steroids with large substituents. The hydrophobicities of the steroids do not correlate with binding affinities to the dexamethasone binding sites; hydrophobicity seems to play a minor role in these steroid-membrane interactions. Comparisons of the specificity of the dexamethasone binding sites on membranes to the specificity of various steroid receptors are also presented.