Mechanisms of cell transformation induced by polyomavirus

Polyomavirus is a DNA tumor virus that induces a variety of tumors in mice. Its genome encodes three proteins, namely large T (LT), middle T (MT), and small T (ST) antigens, that have been implicated in cell transformation and tumorigenesis. LT is associated with cell immortalization, whereas MT plays an essential role in cell transformation by binding to and activating several cytoplasmic proteins that participate in growth factor-induced mitogenic signal transduction to the nucleus. The use of different MT mutants has led to the identification of MT-binding proteins as well as analysis of their importance during cell transformation. Studying the molecular mechanisms of cell transformation by MT has contributed to a better understanding of cell cycle regulation and growth control

Mecanismos de oncogénesis viral / Mechanisms of viral oncogenesis

Actualmente se sabe que el 20 por ciento de los cánceres humanos están asociados con virus oncogénicos. El virus papiloma humano con cáncer anogenital, los virus de la hepatitis B y C con carcinoma hepatocelular, el virus Epstein Barr con carcinomas nasofaríngeos y linfomas, el virus de la leucemia-linfoma T con leucemias en el adulto. Un rasgo común en todos los tumores asociados con infección viral es el largo período de latencia entre la infección y la aparición de la neoplasia y la baja proporción de individuos infectados que desarrollan un tumor maligno. Estas observaciones indican que los virus oncogénicos son necesarios pero no suficientes para inducir cáncer, otros factores podrían estar involucrados. Esta actualización resume informaciones recientes acerca de los mecanismos de carcinogénesis viral, en particular, la interacción de oncoproteínas virales y proteínas supresoras tumorales. La inactivación de estas proteínas supresoras podría representar una estrategia común a través de la cual los virus tumorales pueden contribuir a la transformación maligna de la célula

Polyomavirus-induced malignant transformation: comparative analysis of wild type and mutant middles T-overexpressing cell lines

Polyomavirus, a DNA tumor virus, expresses three viral oncoproteins (large, middle and small T antigens), causes malignant transformation in cell culture and induces multiple tumors in vivo. The middle T (MT) antigen seems to play an essential role in transformation and tumori-genicity. The observation that MT-overexpressing cell lines are able to grow in the absence of PDGF (platelet-derived growth factor) led several laboratories to study the mechanism underlying MT-induced growth deregulation and the signal transduction pathway used by this viral oncoprotein. A number of cellular proteins were shown to be common to both the normal PDGF mitogenic pathway and the MT transforming pathway. The expression of some PDGF primary response genes (fos, jun, myc, JE, KC) was shown to be rendered constitutive by MT overexpression. Using MT mutants, important domains for binding and activation of cytoplasmic proteins were mapped. Wild type and mutant MT cell lines are used in our laboratory to analyze the expression and activity of the PDGF early response genes during cell transformation and correlate them with activation of specific cytoplasmic proteins. In addition to abrogating the PDGF requirement for growth, activation of cellular proteins caused by MT results in cell lines that have an altered morphology and are able to form colonies in agarose. These changes may be due to alterations in connexin 43 and other cell surface proteins.

Molecular genetic approach to cell proliferation control and neoplasia

A number of gene products involved in the control of cell proliferation fall into one of two classes: oncogenes and tumor suppressor genes. The same gene products have also been associated with malignant growth (tumors) caused by radiation, chemicals and tumor viruses. Here we describe our attempts to elucidate the molecular mechanisms underlying polyomavirus-induced cell transformation and the anti-tumor activity of glucocorticoid hormones. Wild type and mutant polyomavirus middle T (MT) overexpressing cell lines, generated with retroviral vector constructs, were used to investigate the role played by peptide growth factor primary response genes (fos, jun, myc, JE, KC) in viral transformation and to map the transduction pathway of the mitogenic signal of MT. Overexpression of MT leads to increased AP-1 (Fos/Jun) transcriptional complex activity. Transformation defective mutant analysis allowed the identification of sites in the MT molecule that are crucial for this activity. Two different approaches were used to investigate the molecular basis for glucocorticoids anti-tumor activity, namely: blind cloning of cDNAs and analysis of growth control genes in C6 glioma cell variants that are either hypersensitive (C6/ST1) or unresponsive to glucocorticoids (C6/P7). Four different glucocorticoid-regulated cDNA sequences were isolated using differential hybridization. A number of differentially expressed sequences were isolated from glucocorticoid-treated C6/ST1 cells by differential display (DDRT-PCR) and are currently being characterized. Expression of known growth control genes in C6/ST1 cells allowed the identification of important candidates for glucocorticoid hormone targets.

Bases moleculares de la carcinogénesis viral de papiloma y polioma / The molecular bases of papilomavirus and polyomavirus carcinogenesis

El virus de polioma es capaz de inducir tumores en sus hospederos naturales y transformar células en cultivo. Por otro lado, el virus de papiloma humano se ha relacionado con diversos tipos de neoplasias; de manera particular con lesiones anogenitales humanas. No se conoce con exactitud el mecanismo a través del cual estos virus inducen transformación y tumorigénesis. El presente trabajo muestra algunas de las características de los mecanismos que utilizan los virus mencionados para participar en la transformación y tumorigénesis. Además, se ha encontrado que ciertos aspectos de la infección por el virus de polioma son parecidos a la infección del virus del papiloma (ambos pertenecen a la misma familia Papovaviridae), por lo que se consideran algunas semejanzas y diferencias entre los mismos

Polyomavirus is able to induce tumors in its natural host as well as to transform cells in cultures. On the other hand, human papillomavirus has been involved in several types of neoplasias such as anogenital lesions. Little is known about the mechanisms through which these viruses induce both transformation and tumorigenesis. The present work shows some characteristics of the mechanisms that papillomavirus and polyomavirus use to participate in tumorigenesis. It has also been noticed that the infection caused by polyomavirus resembles that performed by papillomaviruses (which belong to the same Papovaviridae family). Some similarities and differences between these viruses are considered.

Elimination of polyomavirus origin of replication in constructions with two regulatory regions

Diseñamos un genoma con dos regiones reguladoras (no codificantes) del virus del polima, para estudiar secuencias específicas que tengan alguna función en la expresión genética. Uno de los elementos funciona como "cassette", puede ser escindido de la molécula recombinante, mutado y reinsertado, permitiendo evaluar el efecto de las mutaciones. El fragmento clonado contiene las secuencias correspondientes del nucleótido 5090 al 85 y contiene el ORI, la mayoría de los sitios 5' de iniciación de la transcipción y las secuencias que se sospecha deben conformar el promotor tardío (careciendo del líder). En este estudio demostramos que el sistema funciona si ambas regiones se sitúan en repetición directa, ya que si se localizan en orientación opuesta el virus no es viable en células de ratón. También demostramos que cuando se presenta una mutación en el tramo rico en timinas, se logra la replicación, pero el genoma es inestable ya que elimina la porción de DNA reinsertada. Concluimos de estos estudios que se puden insertar secuencias no codificadoras de Py por duplicado en la región del potenciador sin que se altere la viabilidad viral; esto es, las clonas que presentan ambas regiones reguladoras dispuestas en tándem son infectivas, por lo que consideramos que este sistema puede ser útil en la evaluación de secuencias mutadas detectando si dicha unidad alterada es eliminada