[Corticosteroid-induced diabetes: Novelties in pathophysiology and management]

Diabetes frequently occurs during corticosteroid treatment, sometimes necessitating urgent therapeutic management, with insulin for example. Corticosteroids induce insulin resistance in the liver, adipocytes and skeletal muscle, and have direct deleterious effects on insulin secretion. The development of insulin resistance during corticosteroid treatment, and the insufficient adaptation of insulin secretion, are key elements in the pathophysiology of corticosteroid-induced diabetes. The capacity of pancreatic β-cells to increase insulin secretion in response to insulin resistance is partly genetically determined. A familial history of type 2 diabetes is, therefore, a major risk factor for diabetes development on corticosteroid treatment. Corticosteroid treatments are usually initiated at a fairly high dose, which is subsequently decreased to the lowest level sufficient to achieve disease control. Pharmacological management of diabetes is needed in patients with blood glucose levels exceeding 2.16 g/l (12 mmol/l) and insulin therapy can be started when blood glucose levels are higher than 3.6 g/l (20 mmol/l) with clinical symptoms of diabetes. Insulin can then be replaced with oral hypoglycemic compounds when both blood glucose levels and corticosteroid dose have decreased. Patient education is essential, particularly for the management of hypoglycemia when corticosteroids are withdrawn or their dose tapered.

The gut microbiota and diabetic cardiomyopathy in humans

Type 2 diabetes (DT2) increases the risk of cardiovascular events and cardiac insufficiency. This insufficiency is mostly post-ischaemic in nature, but other aetiologies are possible in this high-risk population. In patients with DT2, diabetic cardiomyopathy is a recognized cause of cardiac insufficiency secondary to chronic hyperglycaemia and myocardial lipotoxicity, which promotes cardiomyocyte hypertrophy (and, frequently, apoptosis of these cells), interstitial fibrosis and a decrease in myocardial contractile performance. Several studies have shown that diabetic cardiomyopathy is associated with modifications to the intestinal microbiota, and changes in the synthesis of bacterial metabolites and their diffusion into the host, some of which appear to have direct deleterious effects on cardiac contractility. These findings open up new perspectives for pathophysiological studies by establishing the presence of a 'microbiota-myocardium' axis and raising the possibility of innovative new treatments. Correction of intestinal dysbiosis in patients with cardiac insufficiency could, therefore, constitute an innovative therapeutic approach to cases of this disease with a poor prognosis.

Do white matter hyperintensities mediate the association between brain iron deposition and cognitive abilities in older people?

BACKGROUND AND PURPOSE

Several studies have reported associations between brain iron deposits (IDs), white matter hyperintensities (WMHs) and cognitive ability in older individuals. Whether the association between brain IDs and cognitive abilities in older people is mediated by or independent of total brain tissue damage represented by WMHs visible on structural magnetic resonance imaging (MRI) was examined.

METHODS

Data from 676 community-dwelling individuals from the Lothian Birth Cohort 1936, with Mini-Mental State Examination scores >24, who underwent detailed cognitive testing and multimodal brain MRI at mean age 72.7 years were analysed. Brain IDs were assessed automatically following manual editing. WMHs were assessed semi-automatically. Brain microbleeds were visually counted. Structural equation modelling was used to test for mediation.

RESULTS

Overall, 72.8% of the sample had IDs with a median total volume of 0.040 ml (i.e. 0.004% of the total brain volume). The total volume of IDs, significantly and negatively associated with general cognitive function (standardized β = -0.17, P < 0.01), was significantly and positively associated with WMH volume (std β = 0.13, P = 0.03). WMH volume had a significant negative association with general cognitive function, independent of IDs (std β = -0.13, P < 0.01). The association between cognition and IDs in the brain stem (and minimally the total brain iron load) was partially and significantly mediated by WMH volume (P = 0.03).

CONCLUSIONS

The negative association between brain IDs and cognitive ability in the elderly is partially mediated by WMHs, with this mediation mainly arising from the iron deposition load in the brain stem. IDs might be an indicator of small vessel disease that predisposes to white matter damage, affecting the neuronal networks underlying higher cognitive functioning.

[Primary hyperparathyroidism revealed by pseudomyopathia]

We report the case of an 11-year-old child with delayed development who developed signs of exercise-induced pain in the lower limb muscles after an acute attack of appendicitis. He had difficulty standing up from the sitting position and ascending and descending stairs. The physical examination revealed increased reflex activity in the lower limbs. Initially, blood tests, MRI and EMG were normal. Serum phosphorus and calcium were not assayed. Eight months later, the boy's condition worsened (myopathy gait, hyperlordosis) leading to the possible diagnosis of muscle disease. After muscle biopsy, blood tests revealed hypercalcemia at 3.5 mmol/l (normal 2.2-2.6), hypercalciuria, and hypophosporemia. The diagnosis of primary hyperparathyroidism was confirmed by the abnormal level of parathormone initially (19 ng/ml) and later (156 ng/ml) with hypercalcemia. Medical treatment failed and surgery was performed to remove three and a half parathyroid glands. After removal, blood tests returned to normal in six days and the physical examination in three years. The diagnosis of principal cell hyperplasia was retained at the pathology examination. We found no evidence of hypercalcemia or other endocrinopathy such as multiple endocrine neoplasia (MEN 1 or 2a). Study of the menine gene did not reveal any mutation. Muscle dysfunction suggest possible abnormal phosphocalcium regulation. A normal parathormone level with hypercalcemia reveals inappropriate synthesis and secretion.

Polyomavirus large T antigen mutants affected in viral DNA replication

We have characterized two polyomavirus large T antigen mutants with different properties in viral DNA replication. dl-97, a mutant active in immortalization, exerts a dominant negative effect in viral DNA replication. 13val, which is defective in both immortalization and viral DNA replication, has a lesion in the putative DnaJ domain affecting the block of Rb function.

Expression of a carrot invertase gene in tobacco suspension cells cultivated in batch and continuous culture conditions

Plant cells (Nicotiana tabacum) were genetically modified to produce an heterologous protein, the acidic invertase from carrot, and invertase production from suspension tobacco cells was investigated. Suspension cultures were grown in shake flasks and stirred bioreactor. Total invertase activity was growth related. A 75 d continuous culture in 10 l bioreactor was performed. Our study demonstrates the high potential of plant cell cultures for long term production of heterologous protein.

Introns enable the polyomavirus middle and small T antigens to stimulate the growth of primary rat embryo fibroblasts

We constructed spliceable vectors that separately encode polyomavirus MT and ST. The addition of an intron enables MT to transform and to immortalize more efficiently and ST to transiently stimulate the growth of primary rat embryo fibroblasts.

Gene targeting in rat embryo fibroblasts promoted by the polyomavirus large T antigen

We used the recombination-promoting activity of the polyomavirus large T antigen (T-ag) to increase the frequency of gene targeting in rat fibroblasts. We constructed a cell line carrying a functional polyomavirus replication origin and a transformation-defective middle T-ag oncogene. The structure of the locus was such that homologous recombination with the targeting DNA reconstituted a functional transforming gene and converted the cells from the normal to the transformed state. Introduction of the large T-ag with the targeting DNA promoted recombinational events that corrected the mutation in either the target locus or the targeting DNA. The frequency of recombination was not substantially influenced by the extent of homology between the recombining sequences. However, it was reduced when the replication origin was inactivated in the targeting DNA, and was reduced further when the origin was inactivated in the target locus.

Elements of the polyomavirus replication origin required for homologous recombination mediated by large T antigen

We introduced various elements of the polyomavirus origin of DNA replication into the genome of rat cells, and we analyzed their capacity to elicit rearrangements within the integrated sequences when exposed to large T antigen. The cis-acting sequences required for homologous recombination were those that make up a functional replication origin.

Intrachromosomal recombination mediated by the polyomavirus large T antigen

We used a spleen necrosis virus-based retroviral vector to introduce the polyomavirus replication origin into rat cells and developed a system to analyze homologous recombination events that do not reconstitute a selectable marker. Introduction of the gene coding for the polyomavirus large T antigen into the cell lines by DNA transfection promoted high-frequency recombination between the two retroviral LTRs, leading to amplification and excision of DNA sequences. To analyze homology requirements, we constructed cell lines carrying only the replication origin without exogenous repeats. Most of the cell lines sustained high-frequency recombination, presumably by undergoing homologous recombination between repetitive DNA lying in the vicinity of the integrated origin. Our results indicate that homologous recombination promoted by large T antigen does not require recombination hot spots in the viral genome other than the replication origin and they explain the cytotoxicity observed in some cell types when large T antigen is expressed in the presence of a functional origin.

Transcription activation mediated by chromosomal inversion in rat cells

To investigate the relationship between the local configuration of a gene and its level of expression, we constructed a rat cell line, Hy5, carrying a mutant polyomavirus middle T oncogene (pmt) whose overexpression converted the cells to the transformed state. The structure of the transgene was such that pmt was able to undergo chromosomal inversion at a relatively high rate by a cross-over in flanking pBR322 sequences. Hy5 cells became spontaneously transformed at a rate of 10(-5) per cell generation and all of the transformants analysed had sustained pmt inversion. CpG sequences were partially methylated in the Hy5 insert but appeared demethylated in transformants. In two subclones derived from untransformed Hy5 cells, the pmt insert was densely methylated, transcriptionally inactive and unable to undergo homologous recombination. Our results suggest that DNA repair associated with recombinational events leads to a heritable hypomethylation of the locus which is responsible for its activation.

Amplification mediated by polyomavirus large T antigen defective in replication

The polyomavirus large T antigen promotes homologous recombination at high rates when expressed in rat cells carrying the viral replication origin and two repeats of viral DNA sequences stably integrated into the cellular genome. Recombination consists of both reciprocal and nonreciprocal events and is promoted by mutants defective in the initiation of viral DNA synthesis (L. St-Onge, L. Bouchard, and M. Bastin, J. Virol. 67:1788-1795, 1993). We have extended our studies to a rat cell line undergoing amplification of the viral insert. We show that large T antigen promotes amplification independently of its replicative function but that its origin-specific DNA binding activity is not sufficient to promote homologous recombination.

High-frequency recombination mediated by polyomavirus large T antigen defective in replication

We investigated the mechanism by which the large T antigen (T-Ag) of both polyomavirus and simian virus 40 (SV40) promotes homologous recombination in mammalian cells. To this end, we constructed a rat cell line, designated Hy5, that carries two mutated copies of the polyomavirus middle-T-Ag (pmt) oncogene lying as direct repeats on the same chromosome. The structure of the viral insert was devised so that intrachromosomal recombination between the pmt repeats reconstitutes wild-type pmt and yields cell populations amenable to selection for the transformed phenotype. Correction of pmt by gene conversion occurred spontaneously at a rate of ca. 1.7 x 10(-7) per cell generation and was masked by another recombination event that also led to the transformation of the Hy5 cell line. This event was identified as chromosomal inversion and overexpression of the upstream pmt copy as a result of homologous recombination between adjacent pBR322 sequences. Both events were promoted by the polyomavirus large T-Ag by several orders of magnitude, as well as by mutants defective in the initiation of viral DNA synthesis. Large T-Ag also promoted reconstitution of wild-type pmt by unequal exchange between sister chromatids, yielding structures compatible with some of the chromosomal aberrations commonly observed in transformed cells. Our data indicate that large T-Ag has a recombination-promoting activity that can be dissociated from its replicative function.

A mathematical model of homologous recombination in cultured cells

This work presents a model describing the rate of recombination between homologous segments of DNA stably integrated into the genome of cultured cells. The model has been applied to rat cell lines carrying the polyomavirus middle T oncogene and a functional origin of viral DNA replication. Introduction of the gene coding for the polyoma large T antigen or the SV40 large T antigen into cells by DNA transfection promotes homologous recombination in the resident viral inserts with rates varying between 0.1 x 10(-3) and 3.7 x 10(-1) per cell generation.

Amplification of polyomavirus DNA sequences stably integrated in rat cells

To investigate the mechanism by which the polyomavirus large T antigen (T-Ag) promotes amplification of integrated viral sequences, we constructed a rat cell line, Hy2-ts5, carrying two different inserts of polyomavirus DNA. The first insert, designated the middle T (pmt) locus, was devised to analyze homologous recombination between two defective copies of pmt lying 3.3 kb apart on the same chromosome. Reconstitution of a functional pmt by spontaneous recombination occurred at a rate of about 2 x 10(-7) per cell generation. The second locus contained the polyomavirus large T (plt) gene carrying a temperature-sensitive mutation and producing a nonfunctional large T-Ag at 39 degrees C. A shift to the permissive temperature for as little as 24 h induced the production of a functional large T-Ag which, in turn, promoted homologous recombination in the pmt locus at a rate close to 1.0 per cell generation. The particularity of this system is that it allowed recombination products to be analyzed as early as a single cell doubling following the initial recombinational event. Amplification occurred by successive duplications of a discrete sequence in the viral insert. Unequal sister chromatid exchange was ruled out as the recombination mechanism promoted by large T-Ag. Instead, we proposed a model of nonconservative recombination involving mispairing between homologous sequences.

Polyomavirus large T mutants affected in retinoblastoma protein binding are defective in immortalization

To clarify the relationship between the various activities of the polyomavirus large T antigen and the contribution of this oncogene to neoplastic transformation, we constructed a series of mutants with small deletions or single-amino-acid substitutions in two separate regions of the protein. These sequences were targeted because they showed considerable similarity to conserved regions 1 and 2 of adenovirus E1A which are thought to be binding sites for the retinoblastoma gene product (pRB). The pRB-binding properties of the large T mutants were assessed with an in vitro coimmunoprecipitation assay. pRB binding was readily detected with wild-type large T, but coprecipitation was completely abolished by as little as a single amino acid substitution (Asp-141----Glu or Glu-146----Asp) in region 2 of the polyomavirus large T antigen. Mutants defective in pRB binding were unable to immortalize primary rat embryo fibroblasts, suggesting that association with pRB is an important component of immortalization mediated by polyomavirus large T. The mutations in region 1 affected pRB binding only marginally, yet some of them severely impaired immortalization, indicating that pRB binding may be essential but not sufficient for immortalization.

Intrachromosomal recombination mediated by papovavirus large T antigens

To investigate the mechanism by which the large T antigen (T-Ag) of polyomavirus and simian virus 40 can promote recombination in mammalian cells, we analyzed homologous recombination events occurring between two defective copies of the polyomavirus middle T (pmt) oncogene lying in close proximity on the same chromosome in a rat cell line. Reconstitution of a functional pmt gene by spontaneous recombination occurred at a rate of about 2 x 10(-7) per cell generation. Introduction of the polyomavirus large T (plt) oncogene into the cell line by DNA transfection promoted recombination very efficiently, with rates in the range of 10(-1) to 10(-2) per cell generation. Recombination was independent of any amplification of viral sequences and could even be promoted by the large T-Ag from simian virus 40, which cannot activate polyomavirus DNA replication. To explain the role of large T-Ag, we propose a novel mechanism of nonconservative recombination involving slipped-strand mispairing between the two viral repeats followed by gap repair synthesis.

Mutations in polyomavirus large T affecting immortalization of primary rat embryo fibroblasts

To clarify the relationship between various functions of the polyomavirus large T antigen and the contribution of this oncogene toward neoplastic transformation, we have analyzed the properties of mutants with in-frame deletions in the second large T exon. dl45, dl96, and dl97 have retained the ability to immortalize primary rat embryo fibroblasts and to trans-activate viral promoters. dl8, dl23, and dl300, which are deficient immortalization, are also deficient in transactivation. However, a newly constructed mutant, designated dl141, which is deficient in immortalization, is still able to trans-activate both the polyoma and SV40 late promoters. This indicates that the ability to trans-activate promoters is not sufficient to confer on the large T antigen the ability to immortalize primary cells.

Effect of CpG-rich sequences in transformation and tumorigenesis by polyomavirus

To address the question of the role of CpG-rich sequences in gene expression, we investigated the effect of an HTF island on the activity of the polyomavirus middle T (pmt) oncogene. pmt is less transformant and less tumorigenic when it is introduced into cultured cells or newborn rats in the presence of an HTF island. Transformed cells carrying pmt in the vicinity of the HTF island have a propensity to revert at a relatively high rate of about 2 X 10(-3) per cell per generation by a mechanism probably involving methylation of some CpG sites in the island. Our results suggest that HTF islands can function as transcriptional silencers.

Mutations in polyomavirus middle T antigen affecting tumorigenesis

P155 is a polyomavirus mlt mutant with normal transforming ability but impaired tumorigenic potential. The mutation, a 12-bp deletion (nucleotides 1348-1359), removes amino acids 372 to 375 from middle T and affects its ability to function in tumorigenesis (C. Gelinas, S. Masse, and M. Bastin, 1984, J. Virol. 51, 242-246). We used deletion loop mutagenesis to introduce point mutations within the wild-type sequence spanned by the P155 deletion. A mutant phenotype resembling that of P155 could be produced by as little as one alanine to valine substitution at residue 373. The mutants were impaired in their ability to induce tumors in rats but they could still transform established cell lines or primary fibroblasts in culture. To define the biochemical defect, we examined the mutant middle T antigen both for association with pp60c-src, the cellular src gene product, as well as its pattern of phosphorylation. No obvious differences explaining the phenotype were observed. The mutant middle T associated with, and activated pp60c-src, but exhibited a slightly altered pattern of phosphorylation, presumably because of additional sites on the middle T protein.

Polyoma large T can activate middle T expression by a hit-and-run mechanism

We studied the activation of polyoma middle T expression in revertant cells carrying transcriptionally inactive copies of the middle T (pmt) oncogene. Introduction of polyoma large T with neo into a rat cell line containing multiple copies of pmt stably integrated into the genome corrected the transformation defect in some of the transfected cells by activating the resident pmt gene. However, once the cells were transformed, continuous expression of the large T protein was not required for the maintenance of pmt expression and hence the maintenance of the transformed state. Transformants arising spontaneously as well as those induced by large T exhibited frequent rearrangements of the pmt inserts. Our results suggest that large T activated pmt expression by a hit-and-run mechanism involving recombination of sequences in the viral insert.

Transfer of mlt mutations into polyomavirus intronless genomes by intramolecular recombination in bacteria

We describe a modification of the procedure of Weber and Weissmann [Nucl. Acids Res. 11 (1983) 5661-5669] for the formation of hybrid genes by in vivo recombination to introduce two separate mutations into the same gene. The mutants of interest are inserted as head-to-tail tandems in a bacterial plasmid in such a way that the 5'-proximal mutation is located upstream from the mutant with the more distal mutation. Propagation of the plasmid in a rec+ strain of Escherichia coli allows recombination between homologous sequences in the insert. DNA with the size expected for the recombinant plasmid is isolated by agarose gel electrophoresis, cloned in a recA strain, and characterized by restriction endonuclease mapping. Using this procedure, we have transferred the deletion from polyomavirus mutant dl-8 into other mutant genomes lacking the intervening sequences for either middle T or large T.

Site-directed mutagenesis of the polyomavirus genome: replication-defective large T mutants with increased immortalization potential

We used the deletion loop mutagenesis procedure to direct point mutations into a small region of the polyomavirus genome, at 0.97 map units, affecting the structure of both the middle and large T antigens. We describe the construction of six middle T mutants which have retained the ability to transform rat cells in culture and four large T mutants, three of which are deficient in viral DNA replication and capable of immortalizing primary rat embryo fibroblasts very efficiently.

High-frequency changes in transcriptional activity in polyomavirus-transformed cell lines

We applied the Luria and Delbruck fluctuation test to analyze high-frequency changes in the phenotype of rat cells transformed by a plasmid carrying the polyomavirus middle T (pmt) gene. All of the transformed cell lines analyzed were capable of switching to the normal state with rates ranging from 10(-3) to 10(-2) per cell per generation. Analysis of both middle T antigen and middle T transcripts indicated that the reversion occurred by a mechanism involving a transcriptional block of the pmt locus. Cell lines containing two separate loci reverted with a lower rate, suggesting that phenotypic switching in these cells involved two independent events affecting each locus. The flat revertants mutated to the transformed state with rates in the range of 10(-5) to 5 X 10(-5) per cell per generation. To determine whether changes in pmt expression would affect neighboring sequences, we transfected a hybrid plasmid carrying pmt linked to the neo marker and selected either for morphological transformants or for G418-resistant cells. Although their coordinate regulation was not absolute, both genes were usually subject to the same changes, reflected by loss and reacquisition of transcriptional activity.

Effect of ACTH on cholesterol and steroid synthesis in adrenocortical tissues

Previous studies have established that under normal conditions, adrenal HMG-CoA reductase activity is higher in hamsters than in rats and humans. The hamster reductase activity follows a diurnal rhythm corresponding to that of plasma ACTH and glucocorticoids [Endocrinology 107 (1980) 215] but not to that of aldosterone. ACTH treatments to hamsters increased reductase activity after a latency of 60 min; this enhancement was prevented by cycloheximide [J. steroid Biochem. 24 (1986) 325]. Immunotitration and immunoblotting studies confirmed that ACTH caused an increase in reductase protein synthesis. In rats, long-term (1-9 days) and short-term (3 h) treatments with ACTH also induced increase in adrenal HMG-CoA reductase activity and reductase protein. In the presence of iodoacetamide and inhibitors of proteolytic enzyme, a main specific band of enzyme was evinced in the area of 102 +/- 6 kDaMr, by Western blotting, for both hamster homogenate and microsomal preparations (Endocrinology, 120 (1987]. Similarly Mr values were found with rat adrenal preparations. The concentration of mRNA, analyzed using the c-DNA pRed-10 coding for the Chinese hamster ovary reductase, was increased in adrenals of hamsters treated with ACTH. The reductase mRNA levels also fluctuated during the day in parallel with those of reductase activity and reductase protein. In conclusion, these results indicate that ACTH and other conditions inducing a change in hamster adrenal HMG-CoA reductase activity provoke parallel changes in reductase mRNA and reductase protein content. ACTH acts on the adrenal reductase of species synthesizing large as well as small quantities of cholesterol, thus indicating the general importance of this hormonal control.

Expression of the malignant phenotype in rat fibroblasts transfected with the polyomavirus transforming genes

As a step toward understanding the molecular mechanism of cooperation between viral and cellular genes in oncogenic transformation, we examined various properties of rat cells transfected with the polyomavirus transforming genes without selecting for a neoplastic phenotype. The cell lines displayed a phenotype ranging from nontumorigenic (flat) to fully transformed (tumorigenic). In the established FR3T3 cell line, acquisition of the fully transformed phenotype correlated with effective expression of the polyomavirus middle T (pmt) antigen. Flat cells carrying silent copies of pmt mutated spontaneously to the fully transformed state with a frequency of 2 to 6 X 10(-5) per cell per generation. In unestablished rat fibroblasts, simultaneous transfer of either pmt and small T or pmt and large T in the presence of the neo marker conferred only a partially transformed phenotype to most of the cell lines. The same results were obtained when wild-type genomic DNA was cotransfected with pSV2-neo. The flat transformants progressively acquired properties characteristic of fully transformed cells with passage in culture. However, in contrast to FR3T3 cells, the generation of fully transformed variants from the flat, unestablished fibroblasts was not caused by activation of pmt expression. This indicates that the functions conferred by the large and small T antigens, alone or in combination with each other, cannot substitute for all the functions expressed by the FR3T3 cell line as a result of in vitro establishment. Thus, polyomavirus-mediated transformation may require additional cellular alterations beyond the acquisition of the three viral oncogenes.

Ability of a T-antigen transport-defective mutant of simian virus 40 to immortalize primary cells and to complement polyomavirus middle T in tumorigenesis

The oncogenic potential of polyomavirus in newborn rats could not be expressed by a genome encoding only the middle T antigen but required the presence of one of the other two viral early genes, small T or large T. The tumorigenicity defect could also be complemented by other viral or cellular genes that are known to be implicated in immortalization and establishment functions. The simian virus 40(cT)-3 mutant (R. E. Lanford and J. S. Butel, Cell 37:801-813, 1984), which fails to localize to the nucleus, has the capacity to complement polyomavirus middle T in tumorigenesis and to immortalize primary rat embryo fibroblasts when it was cotransfected in the presence of pSV2-neo. Our data suggested that under the conditions of DNA-mediated tumor induction and cotransfection with a dominant selection marker, the cellular alterations achieved by nonnuclear oncogenes such as polyomavirus small T and simian virus 40(cT)-3 were sufficient to complement polyomavirus middle T in transformation and tumorigenesis.

Mutation in the polyomavirus genome that activates the properties of large T associated with neoplastic transformation

We have constructed a polyomavirus mutant genome which exhibits an increased immortalization potential when transfected into primary rat embryo fibroblasts. The mutation is a 30-base-pair deletion (nucleotides 1367 through 1396) that inactivates the transforming potential of middle T but activates some of the properties of large T associated with neoplastic transformation. Unlike the wild-type large T, the mutant large T can fully complement polyoma middle T in the tumorigenic process in vivo as well as in the transformation of primary cells in vitro. The activity of the mutant can be explained by its inability to replicate in cells and, hence, its inability to exert a cytopathic effect after gene transfer at high multiplicity. A recombinant which encodes the middle and small T antigens, but not the large T antigen, can also elicit a fully transformed phenotype when introduced into primary rat fibroblasts. These results confirm previous observations from this laboratory indicating that two, and not three, viral gene functions are required for polyomavirus-mediated oncogenic transformation.

Sequences from polyomavirus and simian virus 40 large T genes capable of immortalizing primary rat embryo fibroblasts

We developed a procedure to evaluate quantitatively the capacity of subgenomic fragments from polyomavirus and simian virus 40 (SV40) to promote the establishment of primary cells in culture. The large T antigen from both of these viruses can immortalize primary rat embryo fibroblasts. Both antigens have amino-terminal domains that retain biological activity after deletion of other parts of the polypeptide chain. However, this activity varies considerably among various mutants, presumably because of alterations in the stability or conformation of the truncated polypeptides. The polyomavirus middle T gene alone immortalizes at a low efficiency, which indicates that this oncogene can have both immortalization and transformation potentials depending on the assay system chosen. We generated deletions in the polyomavirus and SV40 large T genes to localize more precisely the functional domains of the proteins involved in the immortalization process. Our results show that the region of the SV40 large T antigen involved in immortalization is localized within the first 137 amino acid residues. This region is encoded by the first large T exon and a small portion from the second exon which includes the SV40 large T nuclear location signal. The polyomavirus sequence involved in immortalization comprises a region from the second large T exon, mapping between nucleotides 1016 and 1213, which shares no homology with SV40 and is thought to be of cellular origin. We suggest that this region of the polyomavirus large T gene functions either as a nuclear location signal or as part of the large T protein sequence involved in DNA binding.

Malignant transformation of rat cells by the polyomavirus middle T gene

To gain an insight into the molecular mechanism of cooperation between the polyomavirus middle T gene and cellular genes in the tumorigenic process, we have examined various properties of rat cell lines transformed by middle T alone. Middle T transformants display a phenotype ranging from nontumorigenic (flat) to fully transformed (tumorigenic) and the phenotype of a given cell line correlates very well with its cellular level of middle T antigen. Highly transformed, tumorigenic variants arise spontaneously in the flat cells during their growth with a mutation rate of 2.2 X 10(-5) per cell per generation. These variants contain elevated levels of both middle T antigen and middle T transcripts, suggesting that fully transformed cells arise as a consequence of an efficient mode of viral gene expression.

mlt Mutation in the polyomavirus genome impairing a function of the middle T protein

The DNA from polyomavirus mlt mutant P155 transforms cells in culture as efficiently as wild-type DNA but has a much lower tumorigenic potential when injected into newborn rodents. The mutant has a 12-base-pair deletion between nucleotides 1347 and 1360, i.e., in a region which encodes parts of the middle and large T antigens (G elinas et al., J. Virol. 43:1072-1081, 1982). To determine which of the two viral gene functions was affected by the mutation, we transferred the latter into a modified polyomavirus genome encoding exclusively the middle T protein. Our results show that the P155 mutation alters a function of the polyomavirus middle T protein required for the induction of the tumorigenic process in vivo. Beside the 12-base-pair deletion at 96.3 map units, there is no other alteration in the coding sequence of P155 middle T with respect to that of P16, the wild-type parental strain. We conclude, therefore, that the deletion is the lesion affecting the tumorigenic potential of mutant P155 .

Tumorigenic activity of polyoma virus and SV40 DNAs in newborn rodents

A procedure has been developed whereby the oncogenicity of the DNA from polyoma (Py) virus and Simian virus 40 (SV40) can be tested directly by injecting recombinant DNA into newborn rodents. Injection of 0.2-2.0 micrograms of linear DNA induced the development of subcutaneous liposarcomas and fibrosarcomas at the site of inoculation. Coinjection of high-molecular-weight rat DNA as carrier had little or no effect on tumor formation but plasmids pBR322, pAT153 , and pML2 behaved as strong inhibitors. Tumor induction by injecting DNA into newborn rodents provides an in vivo equivalent to a transformation assay but appears to be a more stringent and rigorous criterion of oncogenic transformation. The oncogenic potential of Py virus in newborn hamsters could be expressed by a recombinant encoding only the middle T protein, although with average tumor latencies 5-10 times longer than those observed with wild-type Py DNA. Py middle T required the cooperation from small T to induce tumors in newborn rats. SV40 DNA was tumorigenic only in newborn hamsters. delta 2005 DNA which is unable to produce the SV40 small T antigen was much less active and required a latent period about twice that of wild-type SV40 DNA. However, its tumorigenic potential was restored by addition of the Py small T antigen gene. This indicates that Py and SV40 small T antigens are interchangeable and that they probably play an identical role in malignant transformation. Finally, evidence was provided that intermolecular recombination or recombination between DNA fragments can occur in vivo.

Polyoma middle T antigen requires cooperation from another gene to express the malignant phenotype in vivo

The oncogenic potential of polyomavirus in newborn hamsters can be expressed by a recombinant encoding only the middle T protein. However, polyoma middle T requires the cooperation from small T to induce tumors in newborn rats. Similar complementary functions such as cocarcinogens or tumor promotors can be exerted by the simian virus 40 T antigens as well as by one or several products of the early region 1A of adenovirus 2.

Role of the three polyoma virus early proteins in tumorigenesis

A modified polyoma virus genome which can encode the middle T protein but not the large or small T proteins transforms rat cells in culture with an efficiency about 20% that of the wild-type genome. Although middle T-transformed cells grow as tumors when transplanted into nude mice or syngeneic rats, the middle T gene alone is totally inactive when used in a more stringent and rigorous assay for tumorigenicity such as the injection of DNA into newborn rats. Thus, functions other than those expressed by middle T antigen are required for the elaboration of all the properties associated with tumorigenesis. To assess whether a complementary function could be exerted by the large or the small T antigen, we constructed plasmids containing two modified early regions which independently encoded middle T and one of the two other proteins. Both recombinants were tumorigenic in newborn rats. Cell lines derived by transfer of these plasmids under no special selective conditions did not acquire the property of growth in low-serum medium but exhibited the same tumorigenic properties as wild-type polyoma DNA-transformed cells. Furthermore, a recombinant which encoded the middle and small T antigens, but not the large T antigen, was tumorigenic in newborn rats. Although the small T antigen provides a complementary function for tumorigenicity, it cannot complement the middle T antigen for an efficient induction of transformation of cultured cells. This suggests that the complementary function exerted by the small T antigen is different from that of the N-terminal fragment of the large T protein.

Polyoma virus mutant with normal transforming ability but impaired tumorigenic potential

Cloned DNA from the P155 mutant of polyoma virus transforms cells in culture as efficiently as wild-type DNA, but has a much lower tumorigenic potential when injected into newborn rats. Like cells transformed by wild-type DNA, cells transformed by the mutant DNA grow in low serum concentrations, form colonies in agar suspension, and grow to high saturation densities compared with untransformed cells. They are, however, much less tumorigenic since they transplant 100- to 2,000-fold less efficiently than cells transformed by wild-type DNA. Substitution of the region between 89.7 and 1.8 map units by the corresponding region of P155 DNA decreased the tumorigenicity of wild-type DNA. When this region was isolated from wild-type DNA and substituted in P155 DNA, the tumorigenicity of the latter increased to values comparable to those of wild-type DNA. This showed that the lesion affecting tumorigenicity occurred between 89.7 and 1.8 map units on the polyoma virus genome. Sequence analysis in this region revealed a 12-base-pair deletion between nucleotides 1,347 and 1,360. This identified P155 as an mlt mutant, i.e., a mutant with a deletion from a region which encodes parts of the large and middle T antigens.

Transformation of rat fibroblasts by cloned defective polyoma DNA

Defective polyoma DNA molecules isolated from mouse cells infected with high-multiplicity-passaged virus were cloned in pBR322, and the recombinant plasmids were tested for their capacity to transform Fischer rat 3T3 cells in culture. Recombinants carrying an intact proximal portion of the early region, i.e., the region coding for both small and middle T antigens, were able to induce the transformed phenotype. A recombinant plasmid containing a defective polyoma genome with a deletion of about 300 base pairs in the region coding for the C-terminal segment of middle T antigen failed to transform.

Molecular cloning in plasmid pBR322 giving altered expression of the tetracycline resistance gene

The two HindIII fragments of polyoma virus DNA were cloned in the HindIII site of plasmid pBR322, a site located in the RNA polymerase promoter involved in the expression of tetracycline resistance. Although insertion of foreign DNA into this site did not always result in the complete loss of tetracycline resistance, Escherichia coli K12 strain chi 1776 harbouring recombinant plasmids exhibited reduced growth properties in liquid culture with tetracycline and could easily be differentiated from bacteria transformed by non-recombinant plasmids. The formation of plasmid multimers increased the resistance to tetracycline at the level of the induction period, presumably as a result of a gene dosage effect.

Biological properties of polyoma DNA fragments cloned in plasmid pBR322

The two HindIII fragments of polyoma virus DNA were cloned in the HindIII site of plasmid pBR322, and the biological activity of the recombinant plasmids was tested in tissue culture cells. A mixture of recombinant plasmids containing the HindIII-A and HindIII-B fragments was infectious, but only after cleavage with HindIII. Recombinant plasmids containing the HindIII-A fragment, but not those containing the HindIII-B fragment, induced the transformation of Fischer rat 3T3 cells. These findings indicate that about half of the early region of polyoma virus DNA is not essential for the initiation of the maintenance of transformation.

Interaction of elongation factor 1 with aminoacylated brome mosaic virus and tRNA's

Tyrosylated Brome mosaic virus RNA was found to interact with a binary complex of wheat germ, elongation factor 1 and [3H]GTP. Increasing amounts of the aminoacylated viral RNA proportionately reduced radioactivity bound to a nitrocellulose filter, as has previously been noted by others for the charged forms of tobacco mosaic virus, turnip yellow mosaic virus, and tRNA's. However, Sephadex chromatography of the products showed that instead of forming the ternary complex elongation factor-GTP-aminoacyl RNA, the viral RNA caused release of GTP from its complex with elongation factor. Acetylated tyrosyl Brome mosaic virus RNA did not react with the binary complex,and only a slight degree, if any, of stabilization of tyrosine bound to viral RNA was observed after interaction with elongation factor 1. Although such interactions are similar to the reaction of elongation factor with aminoacyl-tRNA , the release of GTP is different and accentuates the possible role for aminoacylation in transcription rather than in translation events.