Biomedical subjects

T L Benjamin

Publications and source records attributed to T L Benjamin.

At least 37 records · Page 2Linked to original sources

PubMed1993

Pyvs: a dominantly acting gene in C3H/BiDa mice conferring susceptibility to tumor induction by polyoma virus.

Results in a previous report established the presence of dominant genetic factor(s) in C3H/BiDa mice which contributed to the unusually high susceptibility of this strain to polyoma virus-induced tumors (Freund et al. (1992) Virology 191, 724-731). Here we show that C57BR/cdJ mice, while identical to C3H/BiDa at the H-2 locus, are almost completely lacking in susceptibility. Analysis of crosses between these two H-2k strains has shown that susceptibility is due to a single dominant autosomal gene, designated Pyvs. On an H-2k background, Pyvs acts in a highly penetrant manner to confer susceptibility to induction of a broad range of tumors by the virus. Analysis of F1 mice between C3H/BiDa and the highly resistant C57BL/6 (H-2b) strain shows that Pyvs can partially overcome the immunologically mediated resistance associated with an H-2b haplotype. Pyvs does not encode cell receptors for the virus, nor does it affect levels of virus replication or anti-viral humoral responses in the host. Studies with early passage embryo fibroblasts in culture show that Pyvs does not affect intracellular events essential for either productive infection or cell transformation by the virus. Pyvs thus determines a generalized susceptibility of the host to polyoma-induced tumors but apparently does not act at the level of target cells for tumor induction.

3T3 Cells↗

PubMed1992

Polyoma virus middle T is essential for virus replication and persistence as well as for tumor induction in mice.

A mutant strain of polyoma virus encoding a truncated middle T protein has been studied for its ability to replicate and induce tumors following inoculation into newborn mice. Virus replication in the acute period prior to expected onset of tumors as well as persistence of virus in older animals were followed. The mutant virus proved to be defective in replication and persistence and failed to induce tumors. These results demonstrated that middle T plays an essential role in productive viral infection in the animal. Since the mutant virus encodes normal large and small T proteins, the results also indicate that functions associated with these T antigens, including large T binding of the retinoblastoma tumor suppressor gene product and the ability to immortalize, are insufficient to cause development of tumors in this system.

Aging↗

PubMed1992

Heterogeneity in state and expression of viral DNA in polyoma virus-induced tumors of the mouse.

We have examined the state and expression of polyoma viral DNA in representative epithelial and mesenchymal tumors, using a combination of biochemical and in situ methods. Results showed wide variations among tumor types and also in different regions within individual tumors, with respect to copy number of viral DNA, presence or absence of deletions, and expression of early and late viral proteins. Epithelial tumors showed the greatest heterogeneity. High copy free viral DNA, frequently with deletions, was found in all such tumors. A portion of free viral DNA was recoverable as transcriptionally active minichromosomes. Three distinct subpopulations of cells were distinguished by in situ analyses. Type 1 cells showed high copy free viral DNA and expressed the major viral capsid protein VP1; these cells appeared to be at various stages of productive (lytic) viral infection. Some productively infected cells were able to undergo mitosis; in a portion of these cells, VP1 was found in close association with the mitotic spindle. Type 2 cells contained high copy free DNA but did not express VP1; by some unknown mechanism, these cells manifest a post-replication block to late gene expression and lytic infection. Type 3 cells contained only low copy, presumably integrated, viral DNA and expressed no VP1; they thus resemble cells transformed in vitro by the virus. Epithelial tumors contained variable mixtures of these subpopulations, while mesenchymal tumors were composed of Type 3 cells only. Differences in virus-cell interactions are discussed in terms of their possible implications in tumor development.

Animals↗

PubMed1992

Functional asymmetry of the regions juxtaposed to the membrane-binding sequence of polyomavirus middle T antigen.

The functional importance of the two clusters of positively charged amino acids which flank the hydrophobic membrane-anchoring sequence of polyomavirus middle T (mT) protein has been investigated by using site-directed mutagenesis. A clear asymmetry was apparent. No effect on transformation was seen following multiple alterations or complete removal of the cluster at the carboxyl end of the protein. In contrast, a single substitution replacing the first arginine amino terminal to the hydrophobic stretch with glutamic acid, but not with lysine, histidine, or methionine, produced a partially transformation-defective mutant with a novel phenotype. This mutant failed to confer anchorage-independent growth on F111 established rat embryo fibroblasts but induced foci with altered morphology compared with wild-type mT. Biochemical studies on this mutant revealed that F111 clones expressing levels of mutant mT equivalent to those of wild-type controls showed a 65% reduction in pp60c-src activation and an 87% reduction in mT-associated phosphatidylinositol 3-kinase activity. However, F111 clones expressing seven times more mutant mT than did wild-type controls showed equal or greater levels of kinase activities yet remained incompletely transformed. Possible mechanisms involving this transformation-sensitive region of mT are discussed.

Animals↗

PubMed1992

Separation of immortalization from tumor induction with polyoma large T mutants that fail to bind the retinoblastoma gene product.

The mouse polyomavirus encodes a tumor-suppressor gene inactivator in its large T protein and a proto-oncogene activator in its middle T protein. We have used site-directed mutagenesis to selectively inactivate the former function without affecting the latter. Two mutant viruses were constructed to encode altered large T proteins that fail to bind the retinoblastoma tumor-suppressor gene product pRB, along with normal small and middle T proteins. The pRB-binding mutants proved to be defective in immortalization of primary rat embryo fibroblasts by a variety of tests. Yet they proved capable of transforming both primary and established fibroblasts in culture. Most importantly, the inability of these mutants to bind pRB had little effect on their ability to induce tumors in mice. We conclude that induction of multiple tumor types in this system does not depend on large T-pRB interactions but rather on middle T-dependent pathways. In addition, the ability of this virus to immortalize cells in culture is not essential to its ability to induce tumors in the animal.

Animals↗

PubMed1992

Changes in frequency, morphology, and behavior of tumors induced in mice by a polyoma virus mutant with a specifically altered oncogene.

Alterations in the tumor-inducing ability of a polyoma virus mutant encoding a partially defective middle T oncogene have been investigated. The mutant middle T associates with and activates the tyrosine protein kinase pp60c-src normally but does not promote binding of a second enzyme, phosphatidyl-inositol 3-kinase. Compared with the wild type virus, this mutant shows an altered and reduced ability to induce tumors after inoculation into newborn mice, as judged by the following criteria: lower frequency of tumors, reduced morbidity and increased survival times of host mice, changes in the spectrum of tumor types, and altered morphologic properties of tumors at several target organ sites. These results indicate an important role of changes in 3-phosphoinositide metabolism in induction of a variety of tumors in this experimental system.

Animals↗

PubMed1991

Structure of simian virus 40 at 3.8-A resolution.

The crystallographically determined structure of simian virus 40 shows that the 72 pentamers of viral protein VP1, which form the outer shell, have identical conformations except for the C-terminal arms of their subunits. Five arms emerge from each pentamer and insert into neighbouring pentamers. This tying together of standard building blocks allows for the required variability in packing geometry without sacrificing specificity.

Amino Acid Sequence↗

PubMed1991

Polyomavirus tumor induction in mice: effects of polymorphisms of VP1 and large T antigen.

By testing recombinants between "high tumor" (inducing a high incidence of tumors) and "low tumor" (inducing a low incidence of tumors) strains of polyomavirus, we have previously shown that the key determinant(s) for induction of a high tumor profile resides in coding regions of the high tumor strain (R. Freund, G. Mandel, G. G. Carmichael, J. P. Barncastle, C. J. Dawe, and T. L. Benjamin, J. Virol. 61:2232-2239, 1987). Three single-amino-acid differences between the PTA (high tumor) and RA (low tumor) virus strains have now been identified by DNA sequencing, one each in the large T antigen, in the region common to the middle and small T antigens, and in the major capsid protein VP1. Further tests of appropriate recombinants and oligonucleotide-induced mutants show that VP1 of PTA is the major determinant for induction of a high tumor profile, including all tumors of epithelial origin. The differential effect of the VP1s of PTA and RA on the tumor profile is discussed in terms of a likely contribution of the polymorphic region of VP1 to binding of receptors and infection of different cell types in the animal. The polymorphism in the large T antigen has a more restricted action, which is seen only when tested in virus carrying the VP1 type of PTA; the PTA large T antigen then promotes more rapid growth of tumors of salivary gland and thymus than the RA large T antigen.

Antigens, Polyomavirus Transforming↗

PubMed1991

Polyomavirus replication in mice: influences of VP1 type and route of inoculation.

Patterns of polyomavirus replication and spread have been studied following inoculation of virus into newborn mice. Levels of virus replication in different tissues were followed in situ by using whole mouse section blots and immunoperoxidase staining for the major capsid protein VP1, as well as by tissue extraction and direct quantitation of viral DNA and infectious virus. Patterns of replication and spread were compared between the "high tumor" strain (inducing a high incidence of tumors) PTA and and the "low tumor" strain (inducing a low incidence of tumors) RA, following different routes of inoculation. The ability to induce a high tumor profile correlated with the ability to establish disseminated productive infection, with the kidney as a major site of amplification. Furthermore, results with PTA-RA recombinant viruses and site-directed mutants showed that the VP1 specificity of PTA, demonstrated earlier to be a critical determinant for induction of a high tumor profile (R. Freund, A. Calderone, C. J. Dawe, and T. L. Benjamin, J. Virol. 65:335-341, 1991), is also critical for amplification in the kidney and for establishment of disseminated infections.

Animals↗

PubMed1991

A single-amino-acid substitution in polyomavirus VP1 correlates with plaque size and hemagglutination behavior.

The plaque size and hemagglutination characteristics of five cloned wild-type strains of polyomavirus were determined. The strains fell into two groups, those with large or small plaques, each with distinctive hemagglutination behavior at different temperatures and pHs. The nucleotide sequence of VP1, the major capsid protein of the virus, was determined for each of the viral strains. The PTA (large-plaque) and RA (small-plaque) strains differed only at residue 92 of VP1, where there is a glutamic acid or glycine, respectively (R. Freund, A. Calderone, C. J. Dawe, and T. L. Benjamin, J. Virol. 65:335-341, 1991). The same amino acid difference in VP1 correlated with plaque size and hemagglutination properties of the other sequenced viruses. Mutagenesis converting amino acid 92 from glutamic acid to glycine converted the plaque size and hemagglutination behavior of the large-plaque PTA strain to that of a small-plaque strain. Furthermore, PTA and RA VP1 proteins produced in Escherichia coli behaved as their parental viruses did in hemagglutination assays. These results demonstrate that amino acid residue 92 of VP1 is involved in determining the plaque size and hemagglutination behavior of polyomavirus and strongly suggest that this region of the VP1 polypeptide interacts directly with cell receptors.

Amino Acid Sequence↗

PubMed1991

Regulation of pp60c-src expression in rat and mouse fibroblasts by an inducible antisense gene: effects on serum regulation of growth and polyoma virus middle T function.

Expression of antisense c-src RNAs in rat and mouse fibroblasts had a dramatic effect on the function of polyoma virus middle T (mT). Antisense c-src RNA decreased the amount of mT:pp60c-src complexes in de novo virus-infected cells and prevented expression of the transformed phenotype in rat F111 cells. Expression of antisense c-src RNA in infected NIH3T3 cells also reduced the formation of mT:pp60c-src complexes but did not affect the ability of polyoma virus to carry out a productive infection. Further analysis of the effects of antisense c-src RNA in uninfected cells revealed that pp60c-src is required for cell growth. When pp60c-src synthesis was reduced, F111 cells stopped proliferating and showed decreased S6 phosphorylation in response to serum. However, F111 cells expressing reduced pp60c-src could be efficiently transformed by v-rasHa, even in the presence of low serum. Thus, pp60c-src appears to function as a component of a signal transduction pathway which regulates cell proliferation in response to serum.

Animals↗

PubMed1990

T-cell lymphomas emerging as epineoplasms in mice bearing transplanted polyoma virus-induced salivary gland tumors.

A subset of salivary epithelial tumors induced by mouse polyoma virus (PyV) has been designated lymphoepithelioma on the basis of a prominent lymphocytic component. Serial transplantation of this variant has previously been observed to result in lymphoma development. A recent repetition of this phenomenon allowed us to characterize the lymphoma cell populations with regard to phenotypic markers and PyV content. Lymphomas emerged in recipients of the third, fifth, sixth, and seventh transplant generations of the lymphoepithelioma. Most lymphomas were widely disseminated in hematopoietic and lymphoreticular tissues, and other sites as well. Flow cytometric analysis of lymphocyte populations from lymphomas in six recipients revealed that, while all lymphomas expressed phenotypic markers of immature cortical thymocytes, i.e., Thy-1, Pgp-1, Jlld, and CD5, they were not uniform with regard to other T-cell markers, notably CD4 and CD8. Varying levels of T-cell receptor markers CD3 and alpha/beta, as well as interleukin 2 receptor, were also noted. DNA blot analysis failed to detect PyV in lymphoma cells at a sensitivity level capable of detecting less than one intact copy per cell. It appears improbable the lymphoma was directly induced by PyV. Hypotheses invoking other mechanisms of lymphoma development are outlined.

Animals↗

PubMed1990

Inositol trisphosphate levels in cells expressing wild-type and mutant polyomavirus middle T antigens: evidence for activation of phospholipase C via activation of pp60c-src.

The transforming protein of polyomavirus, middle T (mT), forms a complex with two cellular enzymes: the protein tyrosine kinase pp60c-src and a phosphatidylinositol (PtdIns) 3-kinase. A mutant virus, Py1178T, encodes an mT protein which associates with and activates pp60c-src to the same extent as the wild type but fails to associate with PtdIns 3-kinase. To investigate relationships between activation of pp60c-src, association of PtdIns 3-kinase, and cellular levels of the second messenger inositol 1,4,5-trisphosphate (InsP3), we examined the effects of wild-type and mutant mT proteins on inositol metabolism in rat and mouse fibroblasts. Expression of either wild-type or 1178T mT caused a 300 to 500% increase in the InsP3 level. Cells transformed by Rous sarcoma virus also showed similar increases in InsP3 levels. Mutant mT proteins which failed to activate pp60c-src (NG59 and 1387T) had no effect on InsP3 levels. Pulse-chase experiments with [3H]inositol showed that the turnover of phosphoinositides was increased in cells transformed by either wild-type polyomavirus or Py1178T as compared with the normal parent cell line. The turnover of inositol phosphates was unchanged upon transformation. These data indicate that cells expressing either wild-type or mutant 1178T mT or pp60v-src exhibit elevated levels of InsP3 because of activation of phospholipase C. This activation appears to depend, directly or indirectly, upon activation of pp60src protein kinase activity. Activation of pp60c-src and elevation of InsP3 content are not sufficient for full transformation. Full transformation also requires the association of mT-pp60c-src complexes with PtdIns 3-kinase.

Animals↗

PubMed1989

Phosphorylation of middle T by pp60c-src: a switch for binding of phosphatidylinositol 3-kinase and optimal tumorigenesis.

Substitution of phenylalanine for tyrosine 315 of the polyoma virus middle T (mT) protein lowers the incidence and limits the spectrum of tumors induced following inoculation of the virus into newborn mice. This substitution removes the major site of phosphorylation by pp60c-src without altering the ability of mT to associate with or to activate pp60c-src. The mutant mT fails to show binding of a phosphatidylinositol 3-kinase (Ptdlns 3-kinase) activity that is normally present in wild-type mT complexes. Furthermore, an anti-peptide antiserum that specifically recognizes mT lacking phosphate at tyrosine 315 precipitates binary (mT-pp60c-src) but not ternary (mT-pp60c-src-Ptdlns 3-kinase) complexes from wild-type infected cell extracts. Reprecipitation with either anti-pp60c-src or anti-mT serum brings down ternary complexes containing mT phosphorylated on tyrosine 315. Phosphorylation of mT by pp60c-src in vivo is therefore a critical event for binding of Ptdlns 3-kinase and for expression of the full tumorigenic potential of the virus.

Amino Acid Sequence↗

PubMed1989

Phosphatidylinositol 3-phosphate is present in normal and transformed fibroblasts and is resistant to hydrolysis by bovine brain phospholipase C II.

The transforming protein of polyoma virus, middle T antigen, associates with two cellular enzymes, pp60c-src, a protein tyrosine kinase, and a phosphatidylinositol kinase that forms phosphatidylinositol 3-phosphate. The formation of a ternary complex of these proteins is essential for complete transformation and maximal tumor induction by the virus. A mutant virus encoding an altered middle T protein that activates pp60c-src but fails to bind phosphatidylinositol kinase is partially defective in transformation. We have confirmed, using an enzymological method, that the product of the in vitro reaction catalyzed by middle T-pp60c-src-phosphatidylinositol kinase complexes is phosphatidylinositol 3-phosphate (PtdIns(3)P), as previously reported (Whitman, M., Downes, C. P., Keeler, M., Keller, T., and Cantley, L. (1988) Nature 332, 644-646). PtdIns(3)P is present in normal as well as virus-infected and transformed cells at levels ranging from 0.6 to 2.6% of the major phosphatidylinositol phosphate isomer, phosphatidylinositol 4-phosphate (PtdIns(4)P). Steady-state levels of PtdIns(3)P do not appear to be affected by the expression of middle T in cells. PtdIns(3)P is not hydrolyzed by bovine brain phospholipase C II, which readily cleaves PtdIns(4)P and other phosphatidylinositols. This result underscores the likelihood that the metabolism of PtdIns(3)P is distinct from that of PtdIns(4)P and raises further questions regarding a possible role of PtdIns(3)P in normal and neoplastic cell growth.

Animals↗

PubMed1989

Separation of host range from transformation functions of the hr-t gene of polyomavirus.

hr-t mutants of polyomavirus are defective in virus growth as well as in cell transformation, and have genetic alterations that invariably affect both the middle and small T proteins. We have examined the growth properties of three site-directed mutants that either eliminate or alter the middle T without affecting the small T protein. Mutant 808A encodes large and small T proteins but no middle T; it grew poorly in NIH 3T3 cells. In contrast, mutants 1387T and 1178T which express altered middle T along with normal large and small T proteins grew nearly as well as wild-type virus. Thus, although the altered middle T proteins encoded by 1387T and 1178T are defective for cell transformation, they retained the ability to induce expression of a cellular permissivity factor(s) required for virus production. At the biochemical level, the induction of permissivity by middle T was manifested primarily in terms of phosphorylation of VP1 on threonine and in efficient encapsidation of viral DNA to form infectious virus. The natural role of middle T involves regulation of phosphorylation events, and can be enacted, at least in part, independently of interactions with pp60c-src.

Animals↗

PubMed1988

Duplication of noncoding sequences in polyomavirus specifically augments the development of thymic tumors in mice.

A 40-base-pair duplication of noncoding sequences in polyomavirus specifically augmented the development of thymic epitheliomas following inoculation of virus into newborn mice. Virus strains carrying only one copy of this sequence induced a full spectrum of tumors except for overt thymic tumors. This 40-base-pair repeat, on the early side of the replication origin, constituted a tissue-specific regulatory determinant for tumor induction.

Animals↗

PubMed1988

Polyomavirus middle T antigen induces ribosomal protein S6 phosphorylation through pp60c-src-dependent and -independent pathways.

Phosphorylation of ribosomal protein S6 is elevated in polyomavirus-infected cells. This elevation results only in part from activation of S6 kinase activity. These effects appear to reflect independent activities of wild-type middle T antigen. Hr-t mutant NG59, encoding a defective middle T protein, and mutant Py808A, encoding no middle T protein, were unable to induce S6 kinase activity or elevate S6 phosphorylation. Two other site-directed mutants encoding altered middle T proteins did elevate S6 phosphorylation while only weakly stimulating S6 kinase activity. These results suggest at least two independent pathways leading to elevation of S6 phosphorylation. One pathway leads to induction of S6 kinase activity following activation of pp60c-src by transformation-competent middle T antigen. Another pathway operates independently of S6 kinase induction and can be regulated by transformation-defective middle T mutants such as Py1387T. This mutant, encoding a truncated middle T protein that failed to associate with the plasma membrane and to activate pp60c-src, caused increased levels of S6 phosphorylation without detectably increasing S6 kinase activity. The ability of mutants such as Py1387T to induce S6 phosphorylation correlated with their ability to increase phosphorylation of VP1, an event linked to maturation of infectious virions.

Animals↗