Glucocorticoid-regulated compartmentalization of cell surface-associated glycoproteins in rat hepatoma cells: evidence for an independent response that requires receptor function and de novo RNA synthesis

Role of glucocorticoids in the maturation of the rat renal Na+/H+ antiporter (NHE3)

BACKGROUND

Neonates have a lower Na+/H+ antiporter activity on the apical membrane of proximal tubule than that of adults. The maturational increase in Na+/H+ antiporter activity occurs at the time when there is a rise in serum glucocorticoid levels in rats. The purpose of the present study was to examine whether glucocorticoids are responsible for the postnatal increase in Na+/H+ antiporter activity.

METHODS

Nine-day-old Sprague-Dawley rats were compared with rats studied at 30 days of age who had either a sham operation or adrenalectomy (ADX) at nine days of age and with rats that had an adrenalectomy and physiologic corticosterone replacement (ADX-Cort) to determine whether glucocorticoid deficiency prevented the maturational increase in Na+/H+ antiporter activity. Na+/H+ antiporter activity was measured in proximal convoluted tubules perfused in vitro by the change in cell pH (pHi) following luminal sodium removal. NHE3 mRNA abundance was measured using Northern blot analysis, and NHE3 protein abundance was measured by immunoblot.

RESULTS

Na+/H+ antiporter activity was 93.8 +/- 17.7, 157.0 +/- 18.0, 356.7 +/- 29.9, and 402.5 +/- 14.5 pmol/mm. min in nine-day-old, ADX, ADX-Cort, and sham control groups, respectively. The ADX-Cort and sham control were higher than the 9-day-old and the 30-day-old ADX group (P < 0.05). Brush-border membrane NHE3 protein abundance in the nine-day-old and ADX groups were sixfold less than ADX-Cort and sham control groups (P < 0.001). Nine-day-old neonates had fivefold less renal cortical NHE3 mRNA than the ADX, ADX-Cort, and sham-operated control groups (P < 0.01).

CONCLUSIONS

These data demonstrate that glucocorticoids play a role in the postnatal maturation of the proximal tubule Na+/H+ antiporter activity and brush-border membrane NHE3 protein abundance. Glucocorticoid deficiency does not completely prevent the maturational increase in Na+/H+ antiporter activity and does not affect NHE3 mRNA abundance.

Estrogen regulation of peptidylglycine alpha-amidating monooxygenase messenger ribonucleic acid levels by a nuclear posttranscriptional event

Peptidylglycine alpha-amidating monooxygenase (PAM; EC 1.14.17.3) is a bifunctional protein containing two enzymes that act sequentially to catalyze the conversion of glycine-extended peptides into COOH-terminal amidated peptides. We have previously shown that PAM messenger RNA (mRNA) levels in the anterior pituitary of intact cycling adult female rats showed changes inversely related to the physiological variations of plasma estrogen levels during the estrous cycle. Chronic treatment of ovariectomized (OVX) rats with 17beta-estradiol was accompanied by a 4.5 +/- 0.5-fold decrease in total PAM mRNA and a 2-fold decrease in PAM activity in the anterior pituitary gland. To investigate the cellular site at which 17beta-estradiol acts to affect the PAM mRNA, we made parallel measurements of the relative levels of PAM mRNA and nuclear precursor RNA and the relative rate of gene transcription after treatments designed to alter the estrogen status. The transcription rate experiments indicated that these 17beta-estradiol effects were not due to reduced PAM gene activity, suggesting that a posttranscriptional mechanism was involved. The most common mechanism of posttranscriptional regulation affects cytoplasmic mRNA stability. Primary rat pituitary cell cultures from OVX and OVX-17beta-estradiol-treated rats in the presence of actinomycin D showed that 17beta-estradiol treatment decreased the half-life of PAM mRNA from 15-16 h to 8-9 h. There was no effect of 17beta-estradiol on PAM mRNA poly(A) tail length or site of polyadenylation. However, in this study the down-regulation of PAM was identified as a nuclear event. Analysis of nuclear RNA with probes specific for PAM intron sequences shows that decreased PAM expression after 17beta-estradiol treatment was largely due to intranuclear destabilization of the primary transcript. The levels of nuclear precursor RNA were decreased roughly 5- to 6-fold in OVX + 17beta-estradiol compared with OVX rats. The decrease in PAM mRNA is blocked by cycloheximide, indicating that its requires new protein synthesis. Mechanisms that would generate such an effect include altered stability of unprocessed message in the nucleus. The proportional changes observed in the nuclear precursor and mRNA levels suggest that the site of control is at the level of stability of the nuclear precursor RNA for PAM mRNA.

Anti-idiotypic antibody to the V3 domain of gp120 binds to vimentin: a possible role of intermediate filaments in the early steps of HIV-1 infection cycle

Although the CD4 molecule is the major cellular receptor for human immunodeficiency virus (HIV), several lines of evidence suggest participation of additional molecules that are engaged after the binding of HIV to the CD4 receptor and that may facilitate viral entry into the target cell. Some of the post-CD4 binding, perfusion events involve the third hypervariable region (V3 loop) of the viral envelope protein gp120. To identify cellular proteins that interact with the V3 loop, we chose as a probe an antiidiotypic monoclonal antibody (MAb), anti-id2, which was prepared against the neutralizing MAb 110.4 that binds the V3 domain in the envelope glycoprotein gp120 of the LAI isolate of HIV-1. Anti-id2 reacted specifically with a 55- to 60-kDa protein in human T cell and monocytoid cell lines, and in a mouse melanoma cell line. This protein was identified immunologically and by protein sequence analysis as vimentin, an intermediate filament protein of lymphoid and other cells of mesodermal origin. Antiserum raised against vimentin inhibited nuclear translocation of HIV-1 DNA following infection of monocytes and CD4+ T cells with live virus, and reduced the amount of HIV-1 gag-specific RNA in the nuclei of monocytes following inoculation with HIV-1 pseudovirions. These data suggest that vimentin may participate in the early steps of HIV-1 replication, perhaps during the uptake of HIV-1 preintegration complexes into the nuclear compartment.

The antiglucocorticoid action of mifepristone

Glucocorticoid hormones influence the physiological activity of almost all cell types in the mammal. This is accomplished via a soluble receptor that, in the presence of an appropriate steroid, modifies the activity of RNA polymerase by binding to the site where different factors assemble for the initiation of cell transcription. The development of antiglucocorticoids has permitted the molecular elucidation of a number of underlying events. Contrary to the classical view, it is now clear that the affinity, stability and activability of the glucocorticoid receptor in the presence of a steroid are cell- and/or tissue-dependent events. The antiglucocorticoid RU 38486 can even activate transcription by binding to sites distinct from those that process transactivation by the agonist. Furthermore, glucocorticoids can sometimes activate the mineralocorticoid receptor, whereas mineralocorticoids can bind the glucocorticoid receptor. Since mifepristone is devoid of adverse toxicity, it has been used for the paraclinical diagnosis of the hypothalamus-pituitary-adrenal axis in normal volunteers, subjects with disorders of the behaviour, and the treatment of Cushing's disease. However, the whole spectrum of cell-specific processes that are antagonized by RU 38486 suggests wide ranging possibilities in the eventual application of antigluco-corticoids.

Selectivity of cell cycle regulation of glucocorticoid receptor function

The restricted expression of some genes to distinct stages of the cell cycle is often brought about through alterations in the activity and/or abundance of specific transcription factors. Many cells have been shown to be unresponsive to glucocorticoid hormone action during the G2 phase of the mammalian cell cycle, suggesting that some activities of the glucocorticoid receptor (GR), a ligand-activated transcription factor, are subjected to cell cycle control. We show here that GR insensitivity in G2 is selective, affecting receptor-mediated transactivation from a simple glucocorticoid response element, but not repression from a composite glucocorticoid response element. Since glucocorticoid-dependent down-regulation of GR protein levels is also unaffected in G2, distinct activities of the receptor that participate in this homologous down-regulation must be operating as effectively in G2-synchronized cells as in asynchronous cells. Finally, the phosphorylation state of the GR is altered in G2-synchronized cells reflecting, in part, both site-specific phosphorylation and dephosphorylation events. These results suggest that, while GR may be a target for cell cycle regulated kinases and phosphatases, the resulting changes in receptor phosphorylation have an impact only on selected GR functions.

Differential transport and processing of variant mouse mammary tumor virus glycoproteins

The transport and proteolytic processing of two individual gene isolates of the mouse mammary tumor virus (MMTV) glycoprotein were compared in transfected rat HTC hepatoma cells. Plasmids were constructed such that the MMTV glycoprotein genes were constitutively expressed from the promoter within the Rous Sarcoma Virus 5' Long Terminal Repeat in the absence of other MMTV proteins. An isolate of the GR strain MMTV glycoprotein was efficiently transported and processed resulting in the localization of MMTV glycoproteins at the cell surface and in the extracellular environment. Moreover, the kinetics of acquisition of endoglycosidase H resistant oligosaccharide side chains and the rate of endoproteolytic cleavage of the glycosylated polyprotein expressed in transfected cells were virtually identical to that observed in viral-infected rat hepatoma cells. In contrast, a natural variant of the C3H strain MMTV glycoprotein expressed in transfected cells was retained in an intracellular compartment by a heavy chain binding protein (BiP)-independent pathway in an endoglycosidase H sensitive and uncleaved form. This MMTV glycoprotein isolate was retained early in the exocytic pathway and displayed a half-life of approximately 45 min in transfected cells. Only a minor fraction of the expressed C3H variant glycoprotein was detected at the cell surface but was not externalized. Our results suggest that the variant C3H MMTV glycoprotein contains one or more mutations that preclude its efficient transport through the exocytic pathway.

The cytoplasmic tail of HIV-1 gp160 contains regions that associate with cellular membranes

The HIV-1 envelope glycoprotein gp160 associates with cellular membranes via a discrete transmembrane domain. Unlike other retroviral envelope proteins, however, gp160 also forms a secondary association with the lipid bilayer mediated by one or more regions located in the cytoplasmic tail. We have expressed the full cytoplasmic tail sequence of gp160, as a fusion protein with the HSV-1 glycoprotein D signal sequence, transiently in a human embryonic kidney cell line. Our results show that in the absence of any defined transmembrane domain or stop transfer sequence, the protein corresponding to the cytoplasmic tail of HIV-1 gp160 formed stable interactions with cellular membranes that mediated its export to the cell surface.

Human immunodeficiency virus-like, nonreplicating, gag-env particles assemble in a recombinant vaccinia virus expression system

We report the assembly of human immunodeficiency virus (HIV)-like particles in African green monkey kidney cells coinfected with two recombinant vaccinia viruses, one carrying the HIV-1 gag and protease genes and the other the env gene. Biochemical analysis of particles sedimented from culture supernatants of doubly infected cells revealed that they were composed of gag proteins, primarily p24, as well as the env proteins gp120 and gp41. Thin-section immunoelectron microscopy showed that these particles were 100 to 120 nm in diameter, were characterized by the presence of cylindrical core structures, and displayed the mature gp120-gp41 complexes on their surfaces. Furthermore, thin-section immunoelectron microscopy analysis of infected cells showed that particle assembly and budding occurred at the plasma membrane. Nucleic acid hybridization suggested that the particles packaged only the gag mRNA but not the env mRNA. Therefore, the system we present is well suited for studies of HIV virion maturation. In addition, the HIV-like particles provide a novel and attractive approach for vaccine development.

Glucocorticoid responsiveness of mouse mammary tumor virus (MMTV) promoters in a down-transcription hepatoma tissue culture (HTC) variant

Complement-mediated cytolysis of the mouse mammary tumor virus (MMTV)-infected rat hepatoma (HTC) cell line, M1.54, resulted in recovery of a mutant derivative, designated CR5, in which the magnitude of both basal and dexamethasone-induced proviral MMTV RNA expression was selectively reduced. Variant CR5 cells were transfected with a plasmid containing the glucocorticoid-regulated MMTV promoter linked to the neomycin resistance gene (pLNL). Half-maximal resistance to G418 killing was glucocorticoid inducible in both pLNL-transfected CR5 and M1.54 cells and was dependent on glucocorticoid receptor occupancy. The down-transcription of MMTV provirus sequences cannot be conferred to transfected genes driven by the same viral promoter suggesting that CR5 cells are defective in cis acting factors. Consistent with this notion, indirect immunofluorescence of transient heterokaryons revealed that uninfected wild-type HTC cells failed to complement the defect in CR5 while CR5 cells did not suppress the wild-type phenotype of M1.54 cells.

Genetic evidence that the steroid-regulated trafficking of cell surface glycoproteins in rat hepatoma cells is mediated by glucocorticoid-inducible cellular components

The biological control of posttranslational maturation and compartmentalization reactions that operate upon proteins during transport to their final cellular destinations is crucial for normal cellular function. Using the expression of mouse mammary tumor virus (MMTV) glycoproteins as sensitive probes in the viral-infected rat hepatoma cell line M1.54, we have discovered and documented a novel glucocorticoid-regulated trafficking pathway that controls the cell surface localization of MMTV glycoproteins. One complement-selected derivative of M1.54 cells, CR4, failed to compartmentalize cell surface MMTV glycoproteins in the presence of dexamethasone. To test genetically if this glycoprotein trafficking pathway is mediated by cellular or viral gene products, CR4 cells were fused with uninfected Fu5 rat hepatoma cells. Indirect immunofluorescence of CR4 X Fu5 heterokaryons revealed that Fu5 complemented the defect in CR4 only after exposure to 1 microM dexamethasone. The glucocorticoid inhibition of Fu5 proliferation was exploited to recover the receptor-deficient uninfected derivative EDR3 that expressed a 100-fold lower level of [3H]dexamethasone binding activity. Analysis of CR4 X EDR3 cell fusions by indirect immunofluorescence revealed that EDR4 cells complemented CR4 in a dexamethasone-dependent manner, suggesting that EDR3 supplied a functinal trafficking component while CR4 provided a functional glucocorticoid receptor to the heterokaryons. Taken together, our results demonstrate that cellular-encoded glucocorticoid-inducible components mediate the regulated trafficking of cell surface MMTV glycoproteins.