Glucocorticoid-dependent maturation of viral proteins in mouse lymphoma cells: isolation of defective and hormone-independent cell variants

Abundant authentic MMTV-Env production from a recombinant provirus lacking the major LTR promoter

As for all retroviruses, the env mRNA is thought to be a singly spliced product of the full-length transcript from the P1 promoter in the MMTV provirus. However, we show that envelope proteins can be produced in an inducible manner in the absence of the P1 promoter from an otherwise complete provirus. Furthermore, we demonstrate in both reporter assays and the proviral context that the R region is necessary for protein production in transiently transfected cells and in a number of independent, stably transfected cell clones. Using 5' RACE, we show that a sequence within the R region functions as a TATA less initiator. The most distal part of the 5' LTR (first 804 bases of the U3 region) is required for the activity of the R-initiator element only when the provirus is integrated. Transfection with a full-length proviral DNA carrying a deletion of P1 in the 5' LTR resulted in the establishment of stable cell clones able to produce Env in a dexamethasone-dependent manner but not infectious virions. We therefore conclude that in the absence of P1, R can drive transcription of the spliced env mRNA but not genomic viral RNA.

Cofilin 1 is revealed as an inhibitor of glucocorticoid receptor by analysis of hormone-resistant cells

Significant knowledge about glucocorticoid signaling has accumulated, yet many aspects remain unknown. We aimed to discover novel factors involved in glucocorticoid receptor regulation that do not necessarily require direct receptor interaction. We achieved this by using a functional genetic screen: a stable cell line which cannot survive hormone treatment was engineered, randomly mutated, and selected in the presence of glucocorticoid. A hormone-resistant clone was analyzed by two-dimensional gel electrophoresis. Differentially expressed proteins were identified and tested as candidates for regulation of the glucocorticoid receptor. An unexpected candidate, cofilin 1, inhibited receptor activity. Cofilin is known to promote actin depolymerization and filament severing. Several experiments suggest that this feature of cofilin is involved in its inhibitory action. Both its actin depolymerization activity and its inhibitory action on the receptor are dependent on its phosphorylation state. Treatment of cells with a cytoskeleton-disrupting agent decreased receptor activity, as did overexpression of actin, particularly a mutant actin that does not polymerize. In addition, overexpression of cofilin and actin as well as chemical cytoskeleton disruption changed the subcellular receptor distribution and upregulated c-Jun, which could constitute the inhibitory mechanism of cofilin. In summary, cofilin represents a novel factor that can cause glucocorticoid resistance.

The order of processing events in mouse mammary tumor virus envelope protein maturation: implications for the location of the glucocorticoid-regulated step

Treatment of the W7MG1 mouse T lymphoma cell line with glucocorticoid stimulates directly or indirectly two observable steps in the processing of mouse mammary tumor virus (MMTV) envelope glycoprotein precursor Pr74: cleavage of Pr74 to yield the mature glycoprotein products gp52 and gp33, and processing of the N-linked oligosaccharides to endoglycosidase H (endo H)-resistant forms found on the mature products but not on the precursor. Therefore, the primary hormone-regulated event in this pathway must occur at or before the point where MMTV envelope proteins become endo H resistant. Pulse-chase analyses identified a novel endo H-resistant 80-kDa species (designated gp80) as a processing intermediate. Therefore, in contrast to conclusions drawn for the envelope proteins of several other retroviruses, proteolytic cleavage of MMTV envelope proteins occurs after acquisition of endo H resistance. Also, proteolytic cleavage cannot be the primary hormone-regulated step. Second, inhibition of mannosidase II by the drug swainsonine did not prevent Pr74 from being proteolytically processed, thus demonstrating that conversion of oligosaccharide chains from endo H-sensitive to -resistant forms was not a prerequisite for proteolytic cleavage. Therefore, the requisite hormone-regulated event in MMTV glycoprotein processing must precede both acquisition of endo H resistance and proteolytic cleavage. This places the regulated event in the endoplasmic reticulum or early Golgi.

Prolactin regulation of mouse mammary tumor virus (MMTV) expression in normal mouse mammary epithelium

The hormonal regulation of mouse mammary tumor virus (MMTV) RNA in normal mouse mammary epithelium was studied in an explant system. In tissue from parous mice, physiological concentrations of prolactin stimulated MMTV expression, while only pharmacological concentrations of cortisol were effective. Regulation in explants from virgin mice was similar to that in parous animals except that the former were less sensitive to prolactin; this relative unresponsiveness may explain why uninduced tissue from virgin mice does not express MMTV RNA, while that from parous mice does exhibit some basal production. These results suggest that prolactin plays a major role in MMTV expression in normal mammary epithelium and that glucocorticoids may only have a permissive effect or may act through an indirect mechanism requiring high concentrations. These data also suggest that the greater susceptibility of parous mice to MMTV-induced tumorigenesis may reflect the greater prolactin sensitivity in the glands from these animals.

Temperature-sensitive transport of glycoproteins to the surface of a variant mouse lymphoma cell line

The expression of mouse mammary tumor virus (MMTV) glycoproteins on the surface of stably infected mouse lymphoma cell line W7MG1 is dramatically increased by glucocorticoid hormones. A variant cell line, W7M.TS1, was selected from W7MG1 for its lack of expression of MMTV glycoproteins on the cell surface in response to treatment with glucocorticoid. Hormonal stimulation of MMTV RNA levels and hormone-induced cytolysis occurred normally in the variant cells. Furthermore, the rates of production of the precursor and mature forms of MMTV glycoproteins in the presence of glucocorticoid were similar in variant and wild-type cells. However, the accumulation of MMTV glycoproteins on the cell surface after hormone treatment was delayed by about 8 h in the variant relative to wild-type cells. The steady-state level of a constitutively expressed cellular protein, T200, on the variant cell surface was comparable to that on wild-type cells. However, in pulse-chase experiments, the appearance of newly synthesized T200 on the cell surface was delayed in the variant compared with wild-type cells. Another glucocorticoid hormone response, removal of H-2 class I antigens from the cell surface, was also delayed in the variant relative to wild-type cells, suggesting that turnover or internalization of cell surface glycoproteins may also be affected in the variant. The defects in the variant cell line were observed at 37 degrees C, but not at 31 degrees C; the variant cells grew normally at both temperatures. This variant phenotype defines a new genetic entity that is important for transport of glycoproteins between internal microsomal compartments and the cell surface.

Glucocorticoid-resistant lymphoma cell variants that contain functional glucocorticoid receptors

A mouse T-lymphosarcoma cell line stably infected with mouse mammary tumor virus (MMTV) was used as the parent line for a genetic analysis of two glucocorticoid hormone responses, hormone-induced cytolysis and stimulation of viral gene expression. Variants were selected for survival and elevated expression of MMTV proteins in the presence of the steroid. The MMTV marker provided a sensitive test for glucocorticoid receptor (GR) function in the hormone-resistant variants. This strategy resulted in the isolation of two novel types of hormone-resistant variants. One type of variant with only about 25% of the level of GR found in the parent line was resistant to the cytolytic effects of glucocorticoid but produced increased levels of MMTV gene products in response to the hormone. This variant phenotype demonstrated that the MMTV response requires fewer GR than the cytolytic response. Another variant, which required approximately 100-fold higher concentrations of hormone than the wild-type cells for both responses, apparently contained GR with altered hormone-binding properties.

Temperature-sensitive transport of glycoproteins to the surface of a variant mouse lymphoma cell line

The expression of mouse mammary tumor virus (MMTV) glycoproteins on the surface of stably infected mouse lymphoma cell line W7MG1 is dramatically increased by glucocorticoid hormones. A variant cell line, W7M.TS1, was selected from W7MG1 for its lack of expression of MMTV glycoproteins on the cell surface in response to treatment with glucocorticoid. Hormonal stimulation of MMTV RNA levels and hormone-induced cytolysis occurred normally in the variant cells. Furthermore, the rates of production of the precursor and mature forms of MMTV glycoproteins in the presence of glucocorticoid were similar in variant and wild-type cells. However, the accumulation of MMTV glycoproteins on the cell surface after hormone treatment was delayed by about 8 h in the variant relative to wild-type cells. The steady-state level of a constitutively expressed cellular protein, T200, on the variant cell surface was comparable to that on wild-type cells. However, in pulse-chase experiments, the appearance of newly synthesized T200 on the cell surface was delayed in the variant compared with wild-type cells. Another glucocorticoid hormone response, removal of H-2 class I antigens from the cell surface, was also delayed in the variant relative to wild-type cells, suggesting that turnover or internalization of cell surface glycoproteins may also be affected in the variant. The defects in the variant cell line were observed at 37 degrees C, but not at 31 degrees C; the variant cells grew normally at both temperatures. This variant phenotype defines a new genetic entity that is important for transport of glycoproteins between internal microsomal compartments and the cell surface.

Glucocorticoid-resistant lymphoma cell variants that contain functional glucocorticoid receptors

A mouse T-lymphosarcoma cell line stably infected with mouse mammary tumor virus (MMTV) was used as the parent line for a genetic analysis of two glucocorticoid hormone responses, hormone-induced cytolysis and stimulation of viral gene expression. Variants were selected for survival and elevated expression of MMTV proteins in the presence of the steroid. The MMTV marker provided a sensitive test for glucocorticoid receptor (GR) function in the hormone-resistant variants. This strategy resulted in the isolation of two novel types of hormone-resistant variants. One type of variant with only about 25% of the level of GR found in the parent line was resistant to the cytolytic effects of glucocorticoid but produced increased levels of MMTV gene products in response to the hormone. This variant phenotype demonstrated that the MMTV response requires fewer GR than the cytolytic response. Another variant, which required approximately 100-fold higher concentrations of hormone than the wild-type cells for both responses, apparently contained GR with altered hormone-binding properties.