Role of the nucleus in the glutamine-dependent degradation of glutamine synthetase in cultured hepatoma cells

Hepatocellular expression of glutamine synthetase: an indicator of morphogen actions as master regulators of zonation in adult liver

Glutamine synthetase (GS) has long been known to be expressed exclusively in pericentral hepatocytes most proximal to the central veins of liver lobuli. This enzyme as well as its peculiar distribution complementary to the periportal compartment for ureogenesis plays an important role in nitrogen metabolism, particularly in homeostasis of blood levels of ammonium ions and glutamine. Despite this fact and intensive studies in vivo and in vitro, many aspects of the regulation of its activity on the protein and on the genetic level remained enigmatic. Recent experimental advances using transgenic mice and new analytic tools have revealed the fundamental role of morphogens such as wingless-type MMTV integration site family member signals (Wnt), beta-catenin, and adenomatous polyposis coli in the regulation of this particular enzyme. In addition, novel information concerning the structure of transcription factor binding sites within regulatory regions of the GS gene and their interactions with signalling pathways could be collected. In this review we focus on all aspects of the regulation of GS in the liver and demonstrate how the new findings have changed our view of the determinants of liver zonation. What appeared as a simple response of hepatocytes to blood-derived factors and local cellular interactions must now be perceived as a fundamental mechanism of adult tissue patterning by morphogens that were considered mainly as regulators of developmental processes. Though GS may be the most obvious indicator of morphogen action among many other targets, elucidation of the complex regulation of the expression of the GS gene could pave the road for a better understanding of the mechanisms involved in patterning of liver parenchyma. Based on current knowledge we propose a new concept of how morphogens, hormones and other factors may act in concert, in order to restrict gene expression to small subpopulations of one differentiated cell type, the hepatocyte, in different anatomical locations. Although many details of this regulatory network are still missing, and an era of exciting new discoveries is still about to come, it can already be envisioned that similar mechanisms may well be active in other organs contributing to the fine-tuning of organ-specific functions.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Subcellular compartmentalization of MCF-7 estrogen receptor synthesis and degradation

Turnover of the estrogen receptor protein was studied by using enucleation of human breast cancer-derived MCF-7 cells, to examine receptor synthesis and receptor degradation in the separated cytoplasmic compartment (cytoplasts) and nuclear compartment (nucleoplasts). Cytoplasts synthesized estrogen receptors as measured by both hormone-binding and immunoassay, while estrogen receptors (but not progesterone or glucocorticoid receptors) were rapidly degraded in nucleoplasts with a half-life of 3-4 h. Little or no degradation of estrogen receptors in cytoplasts was observed under several conditions. Interestingly, MCF-7 cytoplasts contained approximately 15% of the cell's estrogen receptors, which were not 'translocated' by treatment with 17 beta-estradiol before enucleation. We conclude that the estrogen receptor can be synthesized at least to a hormone binding form in the cytoplasm alone without requiring processing in the nucleus, while the nucleus (or perinuclear cytoplasm) is the primary site of degradation of the estrogen receptor protein. In addition, the presence of a population of estrogen receptors that is cytoplasmic but nontranslocatable may need to be considered in the subcellular localization and actions of steroid receptors.

Effects of centrifugation on the degradation of short-lived proteins in exponentially growing cultured cells

The degradation mechanisms of short-lived proteins in cultured cells are unknown, probably due to the lack of procedures which specifically affect the degradation of these proteins. We found that centrifugation of cultured cells, growing either in monolayer or in suspension, between 5000 and 25,000g for 30 min, inhibits (more than 50%) the degradation of short-lived proteins but not of long-lived proteins. Protein synthesis or cell viability is not affected. Centrifugation also disorganizes the Golgi apparatus, as checked by routine electron microscopy, and inhibits the degradation of endocytosed proteins (a lysosomal process which is controlled by the Golgi apparatus). Using different centrifugation speeds, a good correlation was found between alteration of the Golgi apparatus and inhibition of protein degradation.

Degradation of short-lived proteins is decreased by centrifugation

We have examined the effects of enucleation and of inhibitors of mRNA synthesis (actinomycin D and cordycepin) on protein turnover of HeLa cells. Enucleation markedly inhibited the rate of protein degradation for short-lived proteins. However, cells centrifuged in the absence of cytochalasin B at the speed required to obtain cytoplasts showed protein degradation rates identical to those of cytoplasts, while inhibitors of mRNA synthesis did not affect the process. Although enucleation may affect degradation of specific proteins, these results suggest that centrifugation is largely responsible for the inhibition of protein degradation in cytoplasts.

Derepression of the glutamine synthetase in neuroblastoma cells at low concentrations of glutamine

Regulation of the biosynthesis of glutamine synthetase was studied in neuroblastoma cells (Neuro-2A) by use of a recently developed, sensitive radioisotopic assay. The removal of glutamine from the culture medium of these cells for 24 h resulted in a 10-fold increase in glutamine synthetase specific activity (15-fold after 2 weeks) compared with the basal level found in cells grown in the presence of 2 mM glutamine. Following the growth of these cells for 2 weeks in the presence of various concentrations of glutamine, a negative linear correlation was observed between the specific activity of glutamine synthetase (from 1.7 to 0.14 unit/mg) and the concentration of glutamine in the growth medium (from 0.5 to 2 mM). Cycloheximide or actinomycin D blocked the increase in glutamine synthetase activity observed in the absence of glutamine. These results suggest that the removal of glutamine led to the induction of glutamine synthetase by stimulating new enzyme synthesis. The enzyme was not degraded, but only diluted, by growth upon readdition of glutamine to the medium. The influence of glutamine depletion is also reported for C-6 glioma cells and glial cells in primary cultures.