Adult Liver Nuclei Research Articles

Ontogeny and subcellular localization of rat liver mitochondrial branched chain amino-acid aminotransferase.

Branched chain amino-acid aminotransferase (BCAT) activity is present in fetal liver but the developmental pattern of mitochondrial BCAT (BCATm) expression in rat liver has not been studied. The aim of this study was to determine the activity, protein and mRNA concentration of BCATm in fetal and postnatal rat liver, and to localize this enzyme at the cellular and subcellular levels at both developmental stages. Maximal BCAT activity and BCATm mRNA expression occurred at 17 days' gestation in fetal rat liver and then declined significantly immediately after birth. This pattern was observed only in liver; rat heart showed a different developmental pattern. Fetal liver showed intense immunostaining to BCATm in the nuclei and mitochondria of hepatic cells and blood cell precursors; in contrast, adult liver showed mild immunoreactivity located only in the mitochondria of hepatocytes. BCAT activity in isolated fetal liver nuclei was 0.64 mU x mg(-1) protein whereas it was undetectable in adult liver nuclei. By Western blot analysis the BCATm antibody recognized a 41-kDa protein in fetal liver nuclei, and proteins of 41 and 43 kDa in fetal liver supernatant. In adult rat liver supernatant, the BCATm antibody recognized only a 43-kDa protein; however, neither protein was detected in adult rat liver nuclei. The appearance of the 41-kDa protein was associated with the presence of the highly active form of BCATm. These results suggest the existence of active and inactive forms of BCAT in rat liver.

Glucocorticoid receptors in developing rat brain and liver

Dexamethasone receptors were measured by conventional equilibrium steroid binding studies in rat liver and brain cytosol, during late prenatal and postnatal development, Receptor binding could be detected in both cytosol preparations as early as the 17th day of gestation. Receptor levels in the cytosol from intact animals reached adult values by the 1st day after birth in both tissues. Using adrenalectomized animals an increase which reached adult values was observed during the first postnatal week for liver and the second postnatal week for brain. At physiological concentrations of endogenous glucocorticoids depletion of receptor from the cytosol of intact animals was minimal at postnatal day 1 and reached adult levels by day 7. chromatographie analysis in DEAE-Sephadex A50 minicolumns of unactivated and activated receptor constituents revealed the same pattern as that of adult animals. Glucocorticoid receptor complex from developing liver and brain was shown to be capable of binding to isolated adult liver nuclei after in vitro activation. However full capacity, for nuclear binding was observed in vivo, after injection of inducing doses of [ 3H]dexamethasone: By the end of the first week after birth adult nuclear binding capacity was observed in experiments in vivo while values peaked during the second week, in both tissues studied.

Binding of glucocorticoids to liver nuclei and chromatin of fetal, immature and adult rats

The interaction of dexamethasone with nuclei and chromatin was investigated following incubation of liver slices from fetal, immature (6-day-old) and adult rats with the labeled steroid at 37°. The number of specific binding sites for dexamethasone in purified liver nuclei increases with the age of the animal in a manner similar to that previously reported for the cytoplasmic receptor. The high affinity nuclear binding approaches saturation at 40 and 500 nM dexamethasone in fetal and adult liver, respectively. In comparison with dexamethasone, the relative efficiency of corticosterone to accumulate in the nucleus is 9 percent in fetal liver and only 1 percent in adult liver. Specifically bound dexamethasone in adult nuclei exists in at least three forms; a Tris-soluble, a KCl-soluble, and a residual (non-extractable with KC1 or DNase) form. Part of the Tris-soluble steroid is associated with macromolecules sedimenting at about 4 S both in the presence and absence of 0.4 M KCl. This form of the receptor was not detected in fetal liver nuclei. In liver chromatin, bound dexamethasone exists in a KCl-soluble and a residual form, the latter comprising the major fraction of steroid associated with chromatin from both fetal and adult tissue (60 and 75 percent, respectively). Treatment with Triton X-100 releases about 20 percent of the radioactivity in adult liver nuclei, but has no effect on fetal liver nuclei. In contrast with the above observations in the intact tissue, the major fraction of steroid bound to chromatin in cell-free systems is KCl- and DNase-soluble, only 30 percent remaining in the residual pellet.

Early events in the action of glucocorticoids in developing tissues

The interaction of glucocorticoids with developing tissues has been examined. Saturable, high affinity, glucocorticoid-specific binding sites (receptors) have been detected in several fetal and mature tissues of the rabbit and other species. Extensive studies in fetal rabbit lung indicate that nuclear uptake and retention of glucocorticoids in the tissue involves a multi-step mechanism in which the steroid first associates with an extranuclear receptor protein. Subsequently, the steroid-receptor complex is activated to a form which can enter the nucleus where it interacts with acceptor sites in the chromatin. The nuclear acceptor sites appear to involve DNA and do not seem to be specific for target tissues. Fetal lungs of various species contain glucocorticoid receptors long before the normal appearance of surfactant in alveolar spaces. In some species the pulmonary receptor could be detected only during fetal life suggesting a maturation-dependent responsiveness of lung to glucocorticoids. In addition to its ability to bind and retain glucocorticoids, the fetal rabbit lung is very efficient in activating cortisone to cortisol. Inactivation of cortisol to cortisone is a minor reaction. The failure of glucocorticoids to induce tyrosine aminotransferase in fetal rat liver does not appear to be due to the absence of receptors in this tissue. Glucocorticoid receptors are present in fetal rat liver and their concentration increases after birth reaching mature levels by the 5th postnatal day. A complex pattern of glucocorticoid interaction with adult liver nuclei was observed. Both high affinity and low affinity binding sites were detected. The high affinity sites do not appear to be homogeneous since only a small fraction is extractable with 0·4 M KCl. In addition, a fraction of these sites is released by Triton X-100 suggesting an interaction of glucocorticoids with nuclear membranes. In contrast, fetal rat liver nuclei appear to contain only a single class of high affinity sites and no evidence for the presence of low affinity sites or for glucocorticoid interaction with nuclear membranes was obtained. Comparative studies suggest that the glucocorticoid receptors of fetal and adult rat liver may not be identical. Evidence for this includes differences in the relative affinity of cortisol and corticosterone for the binding sites of fetal and adult liver cytosol as well as differences in the dissociation constants and sedimentation coefficients of the steroid-receptor complexes.

Cytonucleoproteins in cleaving eggs of Xenopus lœvis

ABSTRACT The cytoplasm of unfertilized eggs of Xenopus laevis induces DNA synthesis in a high proportion of adult and embryonic nuclei introduced into it (Graham, Arms & Gurdon, 1966). This induction of DNA synthesis is not due merely to the fact that the egg cytoplasm supplies precursors of DNA synthesis which might be absent from the nuclei, since adult liver nuclei, at least, of those nuclei which are induced to synthesize DNA after their injection into eggs, do not do so if incubated in vitro with DNA precursors (Arms, 1968). One of the ways in which the cytoplasm of unfertilized eggs might induce DNA synthesis is by supplying other molecules, such as various enzymes, to the introduced nuclei.

Production of replicable persistent changes in zygote chromosomes of Rana pipiens by injected proteins from adult liver nuclei

An investigation was carried out to test the hypothesis that cellular differentiation is based upon the acquisition of specific macromolecules by the chromosomes. Very small amounts of various protein fractions from adult Rana pipiens liver cells were injected into zygotes of the same species. The albumin fraction, and to a lesser extent the histones, when injected cause a highly reproducible cessation of cell division and arrested development in the late blastula stage. Nuclei from arrested blastulae were serially transplanted for seven generations without showing any recovery. It is concluded that the effect is primarily on the chromosomes and is replicated during cell division. Cytological examination of blastulae obtained from such nuclear transplantations shows the presence of normal and abnormal chromosome complements in the same individual. This indicates that at the time of transplantation the nuclei were normal, and that the abnormalities occurred later as a consequence of the arrest. The original chromosomal changes induced by the injected macromolecules must therefore be below the level of microscopic detection.