Overexpression of insulin-like growth factor binding protein-6 inhibits rhabdomyosarcoma growth in vivo

Rhabdomyosarcoma is the most common soft-tissue sarcoma of childhood. Rhabd omyosarcoma cell lines overexpress insulin-like growth factor-II (IGF-II), an autocrine growth factor that is inhibited by insulin-like growth factor binding protein-6 (IGFBP-6). IGFBP-6 is associated with myoblast quiescence , and expression in rhabdomyosarcoma cells is low. The effect of IGFBP-6 on 2 rhabdomyosarcoma cell lines, RD and Rh30, was studied. IGFBP-6 inhibited anchorage-dependent growth of RD and Rh30 cells in a dose-dependent manner (p<0.0001). IGFBP-6 also inhibited anchorage-independent growth of RD cell s in soft agar in a dose-dependent manner (p<0.01). Anchorage-independent g rowth of RD cells on polyhydroxyethylmethacrylate-coated plates was decreas ed to a minimum of 48% of control after treatment with IGFBP-6 (p<0.001). I n this system, IGFBP-6 increased apoptosis 4-fold (p<0.001). IGF-II partial ly reversed the IGFBP-6-induced decrease in growth and increase in apoptosi s. Rh30 cells were stably transfected with an IGFBP-6 cDNA and subcutaneous xenografts established in BALB/c nude mice. After 18 days, sizes of 2 inde pendent clones of IGFBP-6-overexpressing Rh30 cells were reduced to 12% and 26% of vector control-transfected tumors (p=0.0006 and 0.002, respectively ). IGFBP-6 therefore inhibits proliferation and promotes apoptosis of rhabd omyosarcoma in vitro and dramatically inhibits xenograft growth in vivo, at least in part by inhibiting IGF-II. Low expression of IGFBP-6 may therefor e contribute to rhabdomyosarcoma growth and metastasis. (C) 2001 Wilely-Lis s, Inc.