Insulin-like growth factor II signaling in neoplastic proliferation is blocked by transgenic expression of the metalloproteinase inhibitor TIMP-1

Insulin-like growth factor (IGF) II is overexpressed in many human cancers and is reactivated by, and crucial for viral oncogene (SV40 T antigen: [TAg ])-induced tumorigenesis in several tumor models. Using a double transgenic murine hepatic tumor model, we demonstrate that tissue inhibitor of metall oproteinase 1 (TIMP-1) blocks liver hyperplasia during tumor development, d espite TAg-mediated reactivation of IGF-II. Because the activity of IGFs is controlled by IGF-binding proteins (IGFBPs), we investigated whether TIMP- 1 overexpression altered the IGFBP status in the transgenic liver. Ligand b lotting showed that IGFBP-3 protein levels were increased in TIMP-1-overexp ressing double transgenic littermates, whereas IGFBP-3 mRNA levels were not different, suggesting that TIMP-1 affects IGFBP-3 at a posttranscriptional level. IGFBP-3 proteolysis assays demonstrated that IGFBP-3 degradation wa s lower in TIMP-1-overexpressing livers, and zymography showed that matrix metalloproteinases (MMPs) were present in the liver homogenates and were ca pable of degrading IGFBP-3, As a consequence of reduced IGFBP-3 proteolysis and elevated IGFBP-3 protein levels, dissociable IGF-II levels were signif icantly lower in TIMP-1-overexpressing animals. This decrease in bioavailab le IGF-II ultimately resulted in diminished IGF-I receptor signaling in viv o as evidenced by diminished receptor kinase activity and decreased tyrosin e phosphorylation of the IGF-I receptor downstream effecters, insulin recep tor substrate 1 (IRS-1), extracellular signal regulatory kinase (Erk)-1, an d Erk-2. Together, these results provide evidence that TTMP-1 inhibits live r hyperplasia, an early event in TAg-mediated tumorigenesis, by reducing th e activity of the tumor-inducing mitogen, IGF-II. These data implicate the control of MMP-mediated degradation of IGFBPs as a novel therapy for contro lling IGF bioavailability in cancer.