Simian virus 40 (SV40) large
tumor antigen (LT) triggers oncogenic transformation by inhibition of key
tumor suppressor proteins, including p53 and members of the
retinoblastoma family. In addition, SV40 transformation requires binding of LT to
Cullin 7 (CUL7), a core component of
Cullin-RING
E3 ubiquitin ligase 7 (CRL7). However, the pathomechanistic effects of LT-CUL7 interaction are mostly unknown. Here we report both in vitro and in vivo experimental evidence that SV40 LT suppresses the
ubiquitin ligase function of CRL7. We show that SV40 LT, but not a CUL7 binding-deficient mutant (LT(Δ69-83)), impaired
26S proteasome-dependent proteolysis of the CRL7 target
protein insulin receptor substrate 1 (IRS1), a component of the
insulin and
insulin-like growth factor 1 signaling pathway. SV40 LT expression resulted in the accumulation and prolonged half-life of IRS1. In vitro, purified SV40 LT reduced CRL7-dependent IRS1 ubiquitination in a concentration-dependent manner. Expression of SV40 LT, or depletion of CUL7 by RNA interference, resulted in the enhanced activation of IRS1 downstream signaling pathways
phosphatidylinositol-3-kinase/AKT and Erk
mitogen-activated pathway
kinase, as well as up-regulation of the downstream target gene c-fos. Finally, SV40 LT-positive
carcinoma of
carcinoembryonic antigen 424/SV40 LT transgenic mice displayed elevated IRS1
protein levels and activation of downstream signaling. Taken together, these data suggest that SV40 LT protects IRS1 from CRL7-mediated degradation, thereby sustaining high levels of promitogenic IRS1 downstream signaling pathways.