Transformation by the simian virus 40 T antigen is regulated by IGF-I receptor and IRS-1 signaling

Abstract

Previous work has shown that the Simian Virus 40 T antigen (T antigen) cannot transform mouse embryo fibroblasts (MEFs) that do not express the type 1 insulinlike growth factor receptor (IGF-IR). We have now investigated the mechanism(s) by which the transforming activity of T antigen is affected by IGF-IR signaling. We demonstrate that transformation by T antigen of MEFs and several other cell lines requires an insulin receptor substrate-1 (IRS-1) phosphorylated on tyrosines. If IRS-1 is not expressed, or is serine phosphorylated or otherwise inactive, T antigen fails to transform cells in culture. For instance, while T antigen cannot transform 32D myeloid cells (that do not express IRS-1), its transforming activity is restored by the expression of a wild-type IRS-1, but not of an IRS-1 mutated at the PI3K binding sites. The importance of IRS-1 activation of PI3K in T-antigen transformation is supported by the finding that a constitutively activated p110 subunit of PI3K, a target of IRS-1, overcomes the inability of T antigen to transform MEFs with a serine phosphorylated IRS-1. Taken together, these results indicate that the IRS-1/PI3K signaling is one of the mechanisms regulating transformation by the SV40 T antigen. We propose that the requirement for a tyrosylphosphorylated IRS-1 provides a mechanism to explain the failure of T antigen to transform MEFs with deleted IGF-IR genes.

Adenovirus 5 E1A is responsible for increased expression of insulin receptor substrate 4 in established adenovirus 5-transformed cell lines and interacts with IRS components activating the PI3 kinase/Akt signalling pathway

“…Transformation by SV40T requires IRS-1 to be phosphorylated on tyrosine residues to allow the activation of PI3 kinase, such that the absence or inactivation of IRS-1 renders cells refractory to transformation (DeAngelis et al, 2006). Whether there is a requirement for IRS-1 for Ad-mediated transformation is not clear at present, but the similarities between AdE1A and SV40T in relation to IRS proteins are apparent.…”

Growth inhibition by microRNAs that target the insulin receptor substrate-1

“…9,15,16 IRS-1 expression is often increased in human cancers 22 and increased or ectopic expression of IRS-1 causes cell transformation, i.e., ability to form colonies in soft agar and tumors in mice. 15,[23][24][25][26][27][28][29][30] Conversely, downregulation of IRS-1 (by antisense or siRNA) reverses the transformed phenotype. [30][31][32][33] One intriguing aspect of IRS-1 downregulation is that cells that do not express IRS-1 survive, although they have a tendency to differentiate (see above).…”

Nuclear insulin receptor substrate-1 activates promoters of cell cycle progression genes

“…IRS-1 is translocated to the nuclei in cells that express the SV40T antigen or its human equivalent (Lassak et al, 2002). However, T antigen cannot transform MEFs unless IRS-1 is tyrosyl phosphorylated (DeAngelis et al, 2006). We, therefore, tested the phosphorylation status of IRS-1 using an antibody to IRS-1 phosphorylated on tyrosine 608 on nuclear lysates of parental R12, R12/IRS1/NLS and R þ cells Figure 7a shows that LY294002 and wortmannin do not inhibit the activity of the c-myc promoter in R þ or R12/NLS/IRS1 cells.…”

“…When IRS-1 levels are decreased by experimental procedures, cancer cells lose their transformed phenotype (D'Ambrosio et al, 1995;del Rincon et al, 2004). Over-expression or ectopic expression of IRS-1 causes cell transformation, including the ability to form colonies in soft agar and tumors in mice (Cristofanelli et al, 2000;Valentinis et al, 2000;DeAngelis et al, 2006;Dearth et al, 2006). IRS-1 is often over-expressed in human cancer (Chang et al, 2002).…”