Glucocorticoid regulation of mouse mammary tumor virus gene expression

The WNKs: atypical protein kinases with pleiotropic actions

WNKs are serine/threonine kinases that comprise a unique branch of the kinome. They are so-named owing to the unusual placement of an essential catalytic lysine. WNKs have now been identified in diverse organisms. In humans and other mammals, four genes encode WNKs. WNKs are widely expressed at the message level, although data on protein expression is more limited. Soon after the WNKs were identified, mutations in genes encoding WNK1 and -4 were determined to cause the human disease familial hyperkalemic hypertension (also known as pseudohypoaldosteronism II, or Gordon's Syndrome). For this reason, a major focus of investigation has been to dissect the role of WNK kinases in renal regulation of ion transport. More recently, a different mutation in WNK1 was identified as the cause of hereditary sensory and autonomic neuropathy type II, an early-onset autosomal disease of peripheral sensory nerves. Thus the WNKs represent an important family of potential targets for the treatment of human disease, and further elucidation of their physiological actions outside of the kidney and brain is necessary. In this review, we describe the gene structure and mechanisms regulating expression and activity of the WNKs. Subsequently, we outline substrates and targets of WNKs as well as effects of WNKs on cellular physiology, both in the kidney and elsewhere. Next, consequences of these effects on integrated physiological function are outlined. Finally, we discuss the known and putative pathophysiological relevance of the WNKs.

Early auxin-regulated polyadenylylated mRNA sequences in pea stem tissue

Polyadenylylated mRNA from etiolated pea stem segments treated with or without 20 muM indoleacetic acid (IAA) for various periods of time was assayed by translating it in a wheat germ extract containing [(35)S]methionine and separating the translation products by two-dimensional gel electrophoresis. Within 2 hr IAA causes at least five mRNA sequences to increase in translational activity, relative to initial levels and to simultaneous controls; three of these rise significantly within 20 min after exposure of tissue to IAA but are apparently not elevated at 10 min, whereas the others begin to increase at successive times later than 30 min, and still others begin to change only later than 2 hr. These observations indicate an early, highly selective IAA regulation of mRNA amounts or activities, becoming progressively more extensive with time. The earliest detected enhancement seems close to the primary action of IAA but appears not to be rapid enough to be responsible for auxin induction of cell enlargement.

Anaerobiosis and plant growth hormones induce two genes encoding 1-aminocyclopropane-1-carboxylate synthase in rice (Oryza sativa L.)

The plant hormone ethylene is believed to be responsible for the ability of rice to grow in the deepwater regions of Southeast Asia. Ethylene production is induced by hypoxia, which is caused by flooding, because of enhanced activity of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, the key enzyme in the ethylene biosynthetic pathway. We have cloned three divergent members, (OS-ACS1, OS-ACS2, and OS-ACS3), of a multigene family encoding ACC synthase in rice. OS-ACS1 resides on chromosome 3 and OS-ACS3 on chromosome 5 in the rice genome. The OS-ACS1 and OS-ACS3 genes are induced by anaerobiosis and indoleacetic acid (IAA) + benzyladenine (BA) + LiCl treatment. The anaerobic induction is differential and tissue specific; OS-ACS1 is induced in the shoots, whereas OS-ACS3 is induced in the roots. These inductions are insensitive to protein synthesis inhibitors, suggesting that they are primary responses to the inducers. All three genes are actually induced when protein synthesis is inhibited, indicating that they may be under negative control or that their mRNAs are unstable. The OS-ACS1 gene was structurally characterized, and the function of its encoded protein (M(r) = 53 112 Da, pI 8.2) was confirmed by expression experiments in Escherichia coli. The protein contains all eleven invariant amino acid residues that are conserved between aminotransferases and ACC synthases cloned from various dicotyledonous plants. The amino acid sequence shares significant identity to other ACC synthases (69-34%) and is more similar to sequences in other plant species (69% with the tomato LE-ACS3) than to other rice ACC synthases (50-44%). The data suggest that the extraordinary degree of divergence among ACC synthase isoenzymes within each species arose early in plant evolution and before the divergence of monocotyledonous and dicotyledonous plants.

Tissue-specific regulation of avian glutamine synthetase expression during development and in response to glucocorticoid hormones

We have isolated a glutamine synthetase cDNA clone derived from chicken retinal RNA. The clone detects a 3.2-kilobase RNA in chicken retina, liver, and brain, based on Northern blotting analysis. The dramatic developmental rise observed for the retinal enzyme, assayed as glutamyl transferase activity, is accompanied by a corresponding rise in this RNA. Injection of hydrocortisone 21-phosphate into the yolk sac of day 10 embryos produces an increase in retinal glutamine synthetase mRNA and glutamyl transferase activity, assayed 4 days after injection. An increase in glutamine synthetase mRNA is also observed within 2 h of incubation of retinal organ cultures with hydrocortisone. Moreover, incubation of these cultures with cycloheximide at a concentration that inhibits protein synthesis by 93% affects neither the basal level nor the hydrocortisone-mediated induction of glutamine synthetase mRNA. Although expression of this RNA is developmentally regulated in the brain, steroid hormone injection does not result in a substantial induction. Hepatic glutamine synthetase mRNA is expressed constitutively between embryonic day 10 and 6 days after hatching and is also not hormone inducible. Southern blotting data with chicken DNA digested with EcoRI, HindIII, and BamHI are best interpreted in terms of the cDNA clone detecting only one gene. If so, several cell-type-specific regulatory mechanisms must function to modulate expression of this gene during development.

Tissue-specific regulation of avian glutamine synthetase expression during development and in response to glucocorticoid hormones

We have isolated a glutamine synthetase cDNA clone derived from chicken retinal RNA. The clone detects a 3.2-kilobase RNA in chicken retina, liver, and brain, based on Northern blotting analysis. The dramatic developmental rise observed for the retinal enzyme, assayed as glutamyl transferase activity, is accompanied by a corresponding rise in this RNA. Injection of hydrocortisone 21-phosphate into the yolk sac of day 10 embryos produces an increase in retinal glutamine synthetase mRNA and glutamyl transferase activity, assayed 4 days after injection. An increase in glutamine synthetase mRNA is also observed within 2 h of incubation of retinal organ cultures with hydrocortisone. Moreover, incubation of these cultures with cycloheximide at a concentration that inhibits protein synthesis by 93% affects neither the basal level nor the hydrocortisone-mediated induction of glutamine synthetase mRNA. Although expression of this RNA is developmentally regulated in the brain, steroid hormone injection does not result in a substantial induction. Hepatic glutamine synthetase mRNA is expressed constitutively between embryonic day 10 and 6 days after hatching and is also not hormone inducible. Southern blotting data with chicken DNA digested with EcoRI, HindIII, and BamHI are best interpreted in terms of the cDNA clone detecting only one gene. If so, several cell-type-specific regulatory mechanisms must function to modulate expression of this gene during development.

A distinct glucocorticoid hormone response regulates phosphoprotein maturation in rat hepatoma cells

Glucocorticoid hormone-dependent maturation of the mouse mammary tumor virus (MMTV) phosphorylated polyprotein (Pr74) allows experimental access to certain posttranslational regulatory circuits under steroid control in M1.54 cells, an MMTV-infected rat hepatoma cell line. Pulse-chase experiments revealed that [35S]methionine-labeled Pr74 synthesized in uninduced cells could be converted posttranslationally into p24, a stable phosphorylated maturation product, only after 4 h of exposure to 1 microM dexamethasone, a synthetic glucocorticoid. This regulated processing could be prevented by prior exposure, during the chase period, to inhibitors of RNA (actinomycin D) or protein (cycloheximide or puromycin) synthesis. Moreover, half-maximal production of p24 occurred at 10 nM dexamethasone, a concentration that approximated half-maximal receptor binding and stimulation of MMTV transcript synthesis. Kinetic, hormonal, and genetic evidence suggest that p24 expression did not require or result from the overall glucocorticoid-dependent increase in polyprotein concentration. First, 20 h after dexamethasone withdrawal, Pr74 maturation was completely deinduced, whereas the absolute level of this MMTV precursor remained 10-fold over its basal level. Second, progesterone, which competes with dexamethasone for receptor binding, facilitated the regulated production of p24 but prevented the steroid-mediated accumulation of functional MMTV mRNA. Lastly, certain glucocorticoid-responsive variants, derived from M1.54 cells by resistance to complement cytolysis, expressed p24 in the presence or absence of glucocorticoid-induced levels of Pr74. Taken together, our results suggest that the glucocorticoid-regulated maturation of MMTV phosphopolyproteins resulted from an independent hormone response that required normal receptor function and de novo RNA and protein synthesis.

Hepatitis B virus DNA contains a glucocorticoid-responsive element

It has recently been shown that hepatitis B virus (HBV) contains a transcriptional enhancer element. In order to determine whether this enhancer responds to glucocorticoids, a series of derivatives of plasmid pA10CAT2 was constructed containing the HBV enhancer and variable lengths of further upstream sequences. Transient expression of chloramphenicol acetyltransferase (CAT) was determined after introduction of these plasmids into PLC/PRF/5, Hep 3B, Hep G2, HeLa, and mouse L cells. Highest CAT activity was noted in the human hepatocellular carcinoma line PLC/PRF/5, which contains integrated HBV DNA sequences. Dexamethasone augmented CAT expression in all cell lines tested with 40% of maximal induction at 10 nM and maximum stimulation (3- to 8-fold) at 1 microM dexamethasone. Dexamethasone augmentation of CAT expression was observed only when constructs contained HBV DNA sequences residing upstream to map position 735 from the EcoRI site. This indicates that the glucocorticoid-responsive region is distinct from the previously defined HBV enhancer sequence located at map position 1080-1234. These studies suggest that HBV DNA contains a glucocorticoid-responsive element, which may mediate expression of HBV genes in infected mammalian cells.

A distinct glucocorticoid hormone response regulates phosphoprotein maturation in rat hepatoma cells

Glucocorticoid hormone-dependent maturation of the mouse mammary tumor virus (MMTV) phosphorylated polyprotein (Pr74) allows experimental access to certain posttranslational regulatory circuits under steroid control in M1.54 cells, an MMTV-infected rat hepatoma cell line. Pulse-chase experiments revealed that [35S]methionine-labeled Pr74 synthesized in uninduced cells could be converted posttranslationally into p24, a stable phosphorylated maturation product, only after 4 h of exposure to 1 microM dexamethasone, a synthetic glucocorticoid. This regulated processing could be prevented by prior exposure, during the chase period, to inhibitors of RNA (actinomycin D) or protein (cycloheximide or puromycin) synthesis. Moreover, half-maximal production of p24 occurred at 10 nM dexamethasone, a concentration that approximated half-maximal receptor binding and stimulation of MMTV transcript synthesis. Kinetic, hormonal, and genetic evidence suggest that p24 expression did not require or result from the overall glucocorticoid-dependent increase in polyprotein concentration. First, 20 h after dexamethasone withdrawal, Pr74 maturation was completely deinduced, whereas the absolute level of this MMTV precursor remained 10-fold over its basal level. Second, progesterone, which competes with dexamethasone for receptor binding, facilitated the regulated production of p24 but prevented the steroid-mediated accumulation of functional MMTV mRNA. Lastly, certain glucocorticoid-responsive variants, derived from M1.54 cells by resistance to complement cytolysis, expressed p24 in the presence or absence of glucocorticoid-induced levels of Pr74. Taken together, our results suggest that the glucocorticoid-regulated maturation of MMTV phosphopolyproteins resulted from an independent hormone response that required normal receptor function and de novo RNA and protein synthesis.

Cell anchorage determines whether mammary tumor virus glycoproteins are processed for plasma membranes or secretion

The subcellular localization of mouse mammary tumor virus (MMTV) glycoproteins was analyzed in infected and cloned rat hepatocarcinoma cells cultured with the MMTV transcriptional inducer dexamethasone. When reacted with protein A-coated erythrocytes in the presence of antisera specific for viral glycoproteins or with fluorescent antisera, only some of the cells acquired surface label. This diversity was dependent on cell anchorage to the substratum. In general, the more rounded, less adherent cells contained the MMTV glycoproteins on their surfaces, whereas the flatter, more adherent cells did not. After a change in adherence, a delay preceded complete remodeling of the plasma membranes. Fluorescent antibody studies of fixed cells and analyses of viral glycoprotein synthesis and shedding using L-[35S]methionine indicated that the different expression of MMTV glycoproteins in round versus flat cells is caused by a switch in posttranslational processing. In round cells, the MMTV-encoded precursor glycoprotein is proteolytically cleaved and then transported to plasma membranes as a complex of two subunits, the smaller being the membrane anchor. In flat adherent cells, the smaller subunit is rapidly degraded in an intracellular organelle and the larger is then secreted into the medium. As indicated by labeling of cells with 125I, the concentrations of several host-encoded plasma membrane components are also influenced by cell anchorage. We propose that this switch in cell surfaces and in secretions dependent upon cell-substratum attachments may be a common control mechanism important for embryogenesis, wound healing, and cancer.

Mouse mammary tumor virus-related sequences in mouse lymphocytes are inducible by 12-O-tetradecanoyl phorbol-13-acetate

cDNA libraries from EL-4 cells treated with 12-O-tetradecanoyl phorbol-13-acetate (TPA) were screened for TPA-inducible sequences by differential hybridization. The most abundant inducible species was a sequence similar to that of mouse mammary tumor virus (MMTV). Induction of the mRNA corresponding to the MMTV-related sequences was already evident 30 min after TPA treatment, whereas the maximum accumulation occurred after 20 h of exposure to TPA. TPA also increased levels of MMTV-related RNA in the normal spleen cells of BALB/c and C57BL/6 mice. The level of RNA expression corresponding to MMTV-related sequences, however, was markedly elevated in EL-4 cells as compared with normal spleen cells. Southern blots of EL-4 cell DNA showed that the MMTV-related sequences were inserted into multiple locations of the EL-4 genome. Sequence analysis revealed that the MMTV-related cDNA clones included a part of the env gene and the right long terminal repeat of MMTV. However, the cDNA sequences were substantially different from published MMTV proviral sequences, most notably because of a contiguous deletion of 491 base pairs in the open reading frame within the U3 region.

Functional analysis of the steroid hormone control region of mouse mammary tumor virus

Gene fusions between the mouse mammary tumor virus long terminal repeat and the E. coli lacZ gene have been shown to exhibit hormone dependent expression of beta-galactosidase activity. These constructions were used in transient expression experiments to assess the effects of specific modifications introduced into the region upstream of the transcription initiation site. 5' deletions demonstrate that sequences sufficient for wild-type promoter function are contained downstream of residue -64 relative to the initiation site. Other deletions define a region of approximately 80 base pairs between -220 and -140 which contains sequences essential for hormonal control. Between this control region and the promoter lie sequences dispensable for both functions.

Dexamethasone-stimulated expression of a proviral copy of mouse mammary tumor virus env mRNA

The kinetics of dexamethasone induction of mouse mammary tumor virus RNAs in a cell line (designated line 8) which contains a deleted proviral DNA has been examined. This DNA is an apparent reverse transcription product of the env mRNA. A detectable increase in the 3.8-kilobase RNA (mouse mammary tumor virus env mRNA) was present within 15 min after hormone addition. The 2.5-kilobase RNA detected previously during hormone induction studies was also observed in these cells. Both RNAs increased in concentration up to about 4 h after hormone addition and thereafter attained the steady-state level. Pr73env was detected by immunoprecipitation and appeared to be identical to bona fide Pr73env isolated from wild-type mouse mammary tumor virus-infected cells. An additional 5.7 kilobases of RNA was detected in augmented quantities after hormone addition, but the structure of this RNA is not known.

Asymmetric transcription of mouse mammary tumor virus genes in vivo and in vitro

Mouse mammary tumor virus (MMTV) DNA fragments were cloned into M13 bacteriophage, and the single-stranded recombinant phage DNAs were used as strand-specific nucleic acid hybridization probes to measure synthesis of plus (genomic) and minus strands of MMTV RNA in cultured cell lines and in cell-free preparations of nuclei. Pulse-labeling studies showed that synthesis of MMTV RNA in three different cell lines was highly asymmetric. In nuclear preparations from a cloned line of MMTV-infected rat hepatoma cells, elongation of nascent MMTV RNA chains and initiation of new MMTV RNA chains with nucleoside (beta-S)triphosphates were also highly asymmetric.

Glucocorticoid regulation of transcription at an amplified, episomal promoter

The mouse mammary tumor virus long terminal repeat (MMTV LTR) has been introduced into cultured murine cells, using the 69% transforming fragment of bovine papilloma virus type 1 (BPV). Transformed cells contain up to 200 copies of the chimeric molecules per diploid genome. The restriction endonuclease map of the acquired recombinants, as well as the physical structure of the DNA, indicates that the LTR-BPV molecules present in these cells occur exclusively as unintegrated, extrachromosomal episome. When a 72-base pair direct repeat "enhancer" element (derived from the Harvey sarcoma retrovirus) was included in the MMTV LTR-BPV chimeric plasmids, DNA acquired through transfection, with a single exception, was integrated or rearranged or both. The transcriptional potential of the episomal MMTV promoter present in these cells was tested in two ways. First, steady-state levels of MMTV-initiated RNA were measured by quantitative S1 mapping. Second, the relative number of transcription complexes initiated in vivo was determined by using a subnuclear fraction highly enriched for MMTV-BPV minichromosomes in an in vitro transcription extension assay. Both approaches showed that the MMTV LTR present in the episomal state was capable of supporting glucocorticoid hormone-regulated transcription. We have therefore demonstrated the hormone response for the first time in a totally defined primary sequence environment. Significant differences both in the basal level of MMTV-initiated transcription and in the extent of glucocorticoid induction were observed in individual cell lines with similar episomal copy numbers. These phenotypic variations suggest that epigenetic structure is an important component of the mechanism of regulation.

Glucocorticoid regulation of transcription at an amplified, episomal promoter

The mouse mammary tumor virus long terminal repeat (MMTV LTR) has been introduced into cultured murine cells, using the 69% transforming fragment of bovine papilloma virus type 1 (BPV). Transformed cells contain up to 200 copies of the chimeric molecules per diploid genome. The restriction endonuclease map of the acquired recombinants, as well as the physical structure of the DNA, indicates that the LTR-BPV molecules present in these cells occur exclusively as unintegrated, extrachromosomal episome. When a 72-base pair direct repeat "enhancer" element (derived from the Harvey sarcoma retrovirus) was included in the MMTV LTR-BPV chimeric plasmids, DNA acquired through transfection, with a single exception, was integrated or rearranged or both. The transcriptional potential of the episomal MMTV promoter present in these cells was tested in two ways. First, steady-state levels of MMTV-initiated RNA were measured by quantitative S1 mapping. Second, the relative number of transcription complexes initiated in vivo was determined by using a subnuclear fraction highly enriched for MMTV-BPV minichromosomes in an in vitro transcription extension assay. Both approaches showed that the MMTV LTR present in the episomal state was capable of supporting glucocorticoid hormone-regulated transcription. We have therefore demonstrated the hormone response for the first time in a totally defined primary sequence environment. Significant differences both in the basal level of MMTV-initiated transcription and in the extent of glucocorticoid induction were observed in individual cell lines with similar episomal copy numbers. These phenotypic variations suggest that epigenetic structure is an important component of the mechanism of regulation.

A small region of the mouse mammary tumor virus long terminal repeat confers glucocorticoid hormone regulation on a linked heterologous gene

Expression of mouse mammary tumor virus (MMTV) proviruses is transcriptionally regulated by glucocorticoid hormones. We have linked the MMTV long terminal repeat (LTR) to the coding region of the herpes simplex virus thymidine kinase gene and used this construction to characterize sequences within the LTR that are involved in glucocorticoid regulation. Our results show that 290 base pairs (bp) of the MMTV LTR, including 190 bp upstream from the start site for transcription, are sufficient to confer regulation on the downstream gene. Deletion of an additional 50 bp, leaving sequences from position -140 to +100, eliminates the response. However, the constitutive level of expression is maintained even after deletion of sequences upstream from position -80, indicating that sequences required for the hormone response can be distinguished from those required for basal expression. We also have shown, by making a 4-bp insertion or a 20-bp deletion around position -107, that the distance between the putative signal for hormone response and the start site of transcription can be varied without affecting regulation. Furthermore, replacement of MMTV sequences from position -59 to +100 with analogous sequences from the Rous sarcoma virus LTR does not change the regulation.

Transcription initiation of transfected mouse mammary tumor virus LTR DNA is regulated by glucocorticoid hormones

A chimeric gene, recombined in vitro, containing a long terminal repeat (LTR) sequence from the proviral DNA of mouse mammary tumor virus and the thymidine kinase (tk) gene of Herpes Simplex Virus was introduced into L tk- cells. No transcription of LTR RNA was observed in transfected cells when glucocorticoid hormones were absent from the growth medium. Accumulation of LTR initiated RNA was measured upon hormone addition by the single strand specific nuclease RNA mapping procedure. The accumulation was rapid (detectable after 7.5 minutes), independent of simultaneous protein synthesis and mediated by a functional glucocorticoid receptor complex. Glucocorticoid hormones affect LTR transcription at the level of initiation. The rate of initiation (1.8 X 10(-2) molecules/cell/sec) and a half life of about 30 minutes could be calculated for LTR RNA. The half life of LTR RNA is independent of the presence of hormone.

Glucocorticoids and chromosomal position modulate murine mammary tumor virus transcription by affecting efficiency of promoter utilization

The rate of transcription of murine mammary tumor virus (MTV) sequences in MTV-infected rat hepatoma tissue culture cells is strongly affected by both glucocorticoid hormones and the chromosomal position of provirus integration. We have characterized MTV RNAs produced in J2.17 and M1.54, independent isolates containing, respectively, 1 and 10 proviruses integrated at distinct chromosomal loci. M1.54, but not J2.17, synthesized MTV RNA in the absence of glucocorticoids; the rate of hormone-stimulated viral gene transcription in M1.54 was 50- to 100-fold higher than in J2.17. In each case in which MTV genes were expressed (J2.17 induced, M1.54 basal and induced), the viral RNAs produced were indistinguishable. RNA blotting revealed accumulation of two transcripts, 7.8 and 3.8 kilobases; the latter was likely produced from the former by RNA splicing. Sites used for transcription initiation, polyadenylation, and splicing have been identified from the sizes of end-labeled hybridization probes protected from digestion with mung bean nuclease; the unique initiation and polyadenylation sites were both encoded within the MTV long-terminal-repeat sequence. The efficiencies of splicing and of utilization of the polyadenylation signal did not appear to vary as functions of chromosomal position or hormonal stimulation. Differences in rates of viral gene transcription were reflected in the differential accumulation of the 5'-terminal 136 nucleotides of MTV RNA. Thus, glucocorticoids and chromosomal position appeared to affect solely the efficiency of utilization of the MTV promoter, leaving unchanged the sites of initiation, splicing, and polyadenylation, as well as the efficiencies of the latter two processes.

Mouse mammary tumour virus related sequences are present in human DNA

MuMTV-related sequences have been identified in the DNA of human breast cancer cells using the Southern transfer technique and hybridisation with cloned MuMTV DNA under conditions in which partially mismatched sequences form stable hybrids. Hybridisation with cloned fragments of the MuMTV genome showed that the gag-pol region shares the most homology (estimated to be greater than 80%) with the human MuMTV-related sequences, however, DNA fragments partially homologous to the MuMTV LTR, gag ad env regions were also detected. Analysis of several human DNA samples suggests that the majority of the human MuMTV-related sequences are genetically transmitted but additional Eco R1 fragments were detected in the DNA of one out of three breast cancer cell lines, MCF7. These sequences are potential probes for the human MuMTV-related retroviral sequences and will allow their possible role in human breast cancer to be evaluated.

Regulation of endogenous murine mammary tumor virus expression in C57BL mouse lactating mammary glands: transcription of functional mRNA with a block at the translational level

Expression of endogenous murine mammary tumor viruses (MuMTVs) in various mouse strains is regulated in different ways, and in the absence of exogenous MuMTV, this regulation influences the incidence of spontaneous mammary tumors. Two mouse strains with low mammary tumor incidence, BALB/c and C57BL, control endogenous MuMTV expression at different stages. Neither of the strains had any detectable MuMTV polypeptides in its lactating mammary glands (LMG). However, in C57BL LMG, substantial amounts of MuMTV RNA were present, whereas very little viral RNA was detected in BALB/c LMG. By determining MuMTV RNA levels in LMG of hybrids and backcrosses of BALB/c and C57BL mice, we found that there are three unlinked, independently segregating genetic loci in C57BL mice that are responsible for the presence of moderately high amounts of MuMTV RNA in LMG. The viral RNA in C57BL LMG was processed and transported to the cytoplasm where it was found to cosediment with EDTA-sensitive polysomes. No viral proteins were detected in run-off reactions that permit completion of nascent polypeptide synthesis with polysomes from C57BL LMG, and sensitive radioimmunoassays failed to detect any MuMTV proteins in these tissues. In contrast, MuMTV mRNA purified from C57BL LMG did direct the synthesis of both gag and env MuMTV polypeptides when added to a heterologous rabbit reticulocyte lysate cell-free translation system. We propose that MuMTV mRNA in C57BL LMG, for unknown reasons, is blocked at the translational level.

Two classes of mutant mammary tumor virus-infected HTC cell with defects in glucocorticoid-regulated gene expression

We have isolated mutant derivatives of M1.54 (a mammary tumor virus [MTV]-infected rat hepatoma [HTC] cell line containing multiple integrated proviruses) that fail to express hormone-inducible cell surface viral glycoproteins. In wild-type M1.54, the synthetic glucocorticoid dexamethasone selectively stimulates the rate of synthesis of MTV RNA. In addition, dexamethasone is essential for posttranslational maturation of three of the four cell surface viral glycoproteins processed from the MTV glycosylated precursor polyprotein; the fourth mature species is produced constitutively. Two mutant phenotypes are described; each contains glucocorticoid receptors that are indistinguishable from the wild-type receptor with respect to hormone affinity, intracellular concentration, nuclear translocation efficiency, DNA-cellulose chromatography, and sedimentation rate. In one class, represented by the mutant line CR1, dexamethasone fails to stimulate the low basal rate of MTV gene transcription; surprisingly, hormonal regulation of tyrosine aminotransferase activity is also defective in CR1, whereas several other cellular responses to dexamethasone are normal. In the second class of mutants, represented by CR4, dexamethasone stimulates synthesis of MTV transcripts indistinguishable from those produced in M1.54, but only the constitutive cell surface viral glycoprotein is expressed. Thus, these mutants define two distinct and novel aspects of glucocorticoid regulated gene expression in HTC cells: CR4 contains a defect in a hormone inducible protein maturation pathway that acts on specific viral (and presumably cellular) precursor polypeptides, whereas the lesion in CR1 appears to affect the expression of a subset of the gene products normally under glucocorticoid control in M1.54.

Glucocorticoids and chromosomal position modulate murine mammary tumor virus transcription by affecting efficiency of promoter utilization

The rate of transcription of murine mammary tumor virus (MTV) sequences in MTV-infected rat hepatoma tissue culture cells is strongly affected by both glucocorticoid hormones and the chromosomal position of provirus integration. We have characterized MTV RNAs produced in J2.17 and M1.54, independent isolates containing, respectively, 1 and 10 proviruses integrated at distinct chromosomal loci. M1.54, but not J2.17, synthesized MTV RNA in the absence of glucocorticoids; the rate of hormone-stimulated viral gene transcription in M1.54 was 50- to 100-fold higher than in J2.17. In each case in which MTV genes were expressed (J2.17 induced, M1.54 basal and induced), the viral RNAs produced were indistinguishable. RNA blotting revealed accumulation of two transcripts, 7.8 and 3.8 kilobases; the latter was likely produced from the former by RNA splicing. Sites used for transcription initiation, polyadenylation, and splicing have been identified from the sizes of end-labeled hybridization probes protected from digestion with mung bean nuclease; the unique initiation and polyadenylation sites were both encoded within the MTV long-terminal-repeat sequence. The efficiencies of splicing and of utilization of the polyadenylation signal did not appear to vary as functions of chromosomal position or hormonal stimulation. Differences in rates of viral gene transcription were reflected in the differential accumulation of the 5'-terminal 136 nucleotides of MTV RNA. Thus, glucocorticoids and chromosomal position appeared to affect solely the efficiency of utilization of the MTV promoter, leaving unchanged the sites of initiation, splicing, and polyadenylation, as well as the efficiencies of the latter two processes.

Two classes of mutant mammary tumor virus-infected HTC cell with defects in glucocorticoid-regulated gene expression

We have isolated mutant derivatives of M1.54 (a mammary tumor virus [MTV]-infected rat hepatoma [HTC] cell line containing multiple integrated proviruses) that fail to express hormone-inducible cell surface viral glycoproteins. In wild-type M1.54, the synthetic glucocorticoid dexamethasone selectively stimulates the rate of synthesis of MTV RNA. In addition, dexamethasone is essential for posttranslational maturation of three of the four cell surface viral glycoproteins processed from the MTV glycosylated precursor polyprotein; the fourth mature species is produced constitutively. Two mutant phenotypes are described; each contains glucocorticoid receptors that are indistinguishable from the wild-type receptor with respect to hormone affinity, intracellular concentration, nuclear translocation efficiency, DNA-cellulose chromatography, and sedimentation rate. In one class, represented by the mutant line CR1, dexamethasone fails to stimulate the low basal rate of MTV gene transcription; surprisingly, hormonal regulation of tyrosine aminotransferase activity is also defective in CR1, whereas several other cellular responses to dexamethasone are normal. In the second class of mutants, represented by CR4, dexamethasone stimulates synthesis of MTV transcripts indistinguishable from those produced in M1.54, but only the constitutive cell surface viral glycoprotein is expressed. Thus, these mutants define two distinct and novel aspects of glucocorticoid regulated gene expression in HTC cells: CR4 contains a defect in a hormone inducible protein maturation pathway that acts on specific viral (and presumably cellular) precursor polypeptides, whereas the lesion in CR1 appears to affect the expression of a subset of the gene products normally under glucocorticoid control in M1.54.

Polyadenylylated RNA complementary to a mouse retrovirus-like multigene family is rapidly and specifically induced by epidermal growth factor stimulation of quiescent cells

Complementary DNA probes prepared from total polysomal poly(A)+RNA populations were used to identify clones of mouse DNA containing sequences whose expression is specifically enhanced after epidermal growth factor (EGF) stimulation of quiescent mouse embryo cells in culture. Three such clones were isolated and used to study changes in the levels of clone-specific poly(A)+RNA in the polysomes of cells after mitogenic stimulation by EGF. RNA complementary to sequences present in these clones increased approximately equal to 10-fold as a fraction of the total poly(A)+RNA by 6 hr after stimulation. All three clones were found by hybridization criteria to contain sequences related to the class of mouse retrovirus or transposon-like elements termed VL30. These VL30-related sequences were further found to be complementary to EGF-inducible poly(A)+RNAs and enhanced expression was detectable as early as 1 hr after EGF stimulation. In contrast, nine additional clones, including an AKR-type murine leukemia provirus DNA clone, contained no detectable VL30 sequence elements and were complementary to poly(A)+RNA species whose relative concentration was essentially constant in quiescent and EGF-stimulated cells. Therefore, VL30 sequence elements appear distinct in that they encompass members whose expression is specifically regulated in response to a defined peptide growth factor.

Characterization of endogenous and exogenous mouse mammary tumor virus proviral DNA with site-specific molecular clones

Restriction fragments of the mouse mammary tumor virus (MMTV) proviral DNA were obtained by molecular cloning procedures. A 4-kilobase fragment delimited by two PstI sites was isolated from unintegrated, linear MMTV DNA and amplified in the pBr322 plasmid vector. EcoRI fragments of proviral DNA, integrated into the genome of a GR mammary tumor cell line, were isolated as lambda recombinant molecules. Five different recombinant phages which contained the 3' region of the MMTV proviral DNA and adjacent host DNA sequences were isolated. Heteroduplex analysis and S1 nuclease digestion suggested that there is no extensive sequence homology in the host DNA flanking the different proviral genes. The cloned DNA was fractionated into site-specific restriction fragments which served as molecular probes in the analysis of the endogenous MMTV proviral copies of C3H, GR, BALB/c, and feral mice. This allowed the correlation of MMTV-specific EcoRI fragments obtained from genomic DNA of these strains with the 5' and 3' ends of the proviral gene. Restriction fragments of two clones which contained the proviral sequences adjacent to the flanking host DNA as well as 1 to 2 kilobases of host DNA were used as hybridization probes, and the results allow the following conclusions: the proviral DNA of both clones contains nucleotide sequences complementary to the 5' and 3' ends of proviral DNA; and the host DNA flanking one clone belongs to the unique class of genomic DNA, whereas the DNA flanking the second clone is reiterated at least 15 times within the mouse genome.