Simian Virus 40 Small Tumor Antigen Induces Deregulation of the Actin Cytoskeleton and Tight Junctions in Kidney Epithelial Cells

Abstract

There is increasing evidence that the transforming DNA tumor virus simian virus 40 (SV40) is associated with human malignancies. SV40 small tumor antigen (small t) interacts with endogenous serine/threonine protein phosphatase 2A (PP2A) and is required for the transforming activity of SV40 in epithelial cells of the lung and kidney. Here, we show that expression of SV40 small t in epithelial MDCK cells induces acute morphological changes and multilayering. Significantly, it also causes severe defects in the biogenesis and barrier properties of tight junctions (TJs) but does not prevent formation of adherens junctions. Small t-induced TJ defects are associated with a loss of PP2A from areas of cell-cell contact; altered distribution and reduced amounts of the TJ proteins ZO-1, occludin, and claudin-1; and marked disorganization of the actin cytoskeleton. Small t-mediated F-actin rearrangements encompass increased Rac-induced membrane ruffling and lamellipodia, Cdc42-initiated filopodia, and loss of Rho-dependent stress fibers. Indeed, these F-actin changes coincide with elevated levels of Rac1 and Cdc42 and decreased amounts of RhoA in small t-expressing cells. Notably, these cellular effects of small t are dependent on its interaction with endogenous PP2A. Thus, our findings provide the first evidence that, in polarized epithelial cells, expression of small t alone is sufficient to induce deregulation of Rho GTPases, F-actin, and intercellular adhesion, through interaction with endogenous PP2A. Because defects in the actin cytoskeleton and TJ disruption have been linked to loss of cell polarity and tumor invasiveness, their deregulation by PP2A and small t likely contributes to the role of SV40 in epithelial cell transformation.A majority of human tumors arise from epithelial cells. Invasion is characterized by loss of polarity, deregulation of cell adhesion, and cytoskeletal disruption. There is increasing evidence that the transforming DNA tumor virus simian virus 40 (SV40) is associated with human malignancies (17, 33). Inactivation of retinoblastoma proteins following expression of the SV40 large T antigen in MDCK cells results in an epithelialmesenchymal conversion and invasiveness (21). Studies with transgenic mice have shown that SV40 small tumor antigen (small t) cooperates with the large T antigen in inducing tumors in slowly dividing epithelial cells of the lung and kidney. Small t is also absolutely required for SV40-mediated formation of lung and kidney adenocarcinoma (2). Moreover, it promotes serum-independent cell growth in mammary gland epithelial cells and induces breast cancer formation in transgenic mice (9).Following SV40 infection (23, 30) or ectopic expression (35), small t specifically associates with endogenous serine/threonine protein phosphatase 2A (PP2A). PP2A is believed to exist in vivo predominantly as a heterotrimeric complex comprising a core enzyme containing the catalytic C and structural A subunits, which is bound to a regulatory B subunit. Significantly, distinct regulatory B ...

Signaling and Transcriptional Changes Critical for Transformation of Human Cells by Simian Virus 40 Small Tumor Antigen or Protein Phosphatase 2A B56γ Knockdown

“…The observations that we report here suggest that ST may achieve this function in part by repression of components of junctional adhesion complexes such as ␤-catenin, plakoglobin, JAM1, and protocadherin family members. Our conclusions are consistent with reports that overexpression of plakoglobin can suppress tumorigenicity of SV40-transformed cells (58) and that ST can alter distribution of and reduce levels of tight junction proteins such as occludin and claudin in polarized epithelial cells (59).…”

Regulation of cell adhesion by PP2A and SV40 small tumor antigen: An important link to cell transformation

“…Thus, ABα C negatively regulates TJ assembly, probably at the level of the PKCζ / Par3/Par6 signaling complex. In further support for an important role of ABα C in TJ regulation, expression of SV40 ST is associated with altered distribution and reduced expression levels of ZO-1, occludin and claudin-1 [20]. These effects also correlate with deregulation of cell growth.…”

The role of actin bundling proteins in the assembly of filopodia in epithelial cells

“…[55][56][57] Viruses are often associated with the dense microvilli of polarized epithelial cells but also with the filopodia of both polarized and nonpolarized cells. 56,[58][59][60][61] Transport of viruses along filopodia during the attachment and entry process has been documented for a number of viruses. 55,[62][63][64] Filopodia have been shown to act as cellular tentacles that serve to increase the efficiency of uptake of pathogens.…”