INSULIN-LIKE-GROWTH-FACTOR (IGF)-BINDING PROTEIN-3 INDUCES APOPTOSIS AND MEDIATES THE EFFECTS OF TRANSFORMING GROWTH-FACTOR-BETA-1 ON PROGRAMMED CELL-DEATH THROUGH A P53-INDEPENDENT AND IGF-INDEPENDENT MECHANISM

Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) is known to block IGF action and inhibit cell growth, IGFBP-3 is thought to act by sequestering free IGFs or, possibly, act via a novel IGF-independe nt mechanism. Supporting its role as a primary growth inhibitor, IGFBP -3 production has been shown to be increased by cell growth-inhibitory agents, such as transforming growth factor-beta (TGF-beta), and the t umor suppressor gene p53. In this paper, we demonstrate, for the first time, a novel function of IGFBP-3 as an apoptosis-inducing agent and show that this action is mediated through an IGF.IGF receptor independ ent pathway. In the p53 negative prostate cancer cell line, PC-3, the addition of recombinant IGFBP-3 resulted in a dose-dependent induction of apoptosis, I-125-IGFBP-3 bound with high affinity to specific prot eins in PC-3 cell lysates and plasma membrane preparations. These memb rane associated mole cules may serve as receptors that mediate the dir ect effect of IGFBP-3 on apoptosis, In addition, in an IGF receptor-ne gative mouse fibroblast cell line, treatment with recombinant IGFBP-3 as well as transfection of the IGFBP-3 gene induced apoptosis, suggest ing that neither IGFs nor IGF receptors are required for this action. Furthermore, treatment with TGF-beta 1, a known apoptosis-inducing age nt, resulted in the induction of IGFBP-3 expression 6-12 h before the onset of apoptosis. This effect of TGF-beta 1 was prevented by co-trea tment with IGFBP-3-neutralizing antibodies or IGFBP-3-specific antisen se thiolated oligonucleotides. These findings suggest that IGFBP-3 ind uces apoptosis through a novel pathway independent of either p53 or th e IGF.IGF receptor-mediated cell survival pathway and that IGFBP-3 med iates TGF-beta 1 induced apoptosis in PC-3 cells.