PTEN-induced apoptosis in U251 glioma cells is associated with decreased expression of insulin-like growth factor binding protein-2

Abstract

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PTEN is a tumor suppressor gene whose loss of function is observed in ∼40-50% of human cancers. Although insulin-like growth factor binding protein-2 (IGFBP-2) was classically described as a growth inhibitor, multiple recent reports have shown an association of overexpression and/or high serum levels of IGFBP-2 with poor prognosis of several malignancies, including gliomas. Using an inducible PTEN expression system in the PTEN-null glioma cell line U251, we demonstrate that PTEN-induction is associated with reduced proliferation, increased apoptosis, and a substantial reduction of the high levels of IGFBP-2 expression. The PTEN-induced decrease in IGFBP-2 expression could be mimicked with the PI3-kinase inhibitor LY294002, indicating that the lipid phosphatase activity of PTEN is responsible for the observed effect. However, the rapamycin analog CCI-779 did not effect IGFBP-2 expression, suggesting that the PTEN-induced decrease in IGFBP-2 expression is not attributable to decreased mTOR signalling. Treatment with IGFBP-2 siRNA induced apoptosis in U251 cells, indicating that IGFBP-2 acts as an anti-apoptotic agent in this system. Recombinant human IGFBP-2 was unable to rescue cells from the antiproliferative effects of PTEN expression or IGFBP-2 siRNA, suggesting a growth stimulatory/anti-apoptotic role of intracellular IGFBP-2. Our results provide evidence that the clinical data linking IGFBP-2 expression to poor prognosis may arise, at least in part, because IGFBP-2 expression simply serves as a marker for loss of function of PTEN. However, the possibility that IGFBP-2 acts as an anti-apoptotic molecule cannot be excluded, because IGFBP-2 siRNA was observed to induce apoptosis in U251 cells. Our results motivate translational research regarding the relationship between IGFBP-2 expression and loss of function of PTEN.