Rhabdomyosarcoma is the most common
soft-tissue sarcoma of childhood.
Rhabdomyosarcoma 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 cells in soft
agar in a dose-dependent manner (p < 0.01). Anchorage-independent growth of RD cells on polyhydroxyethylmethacrylate-coated plates was decreased to a minimum of 48% of control
after treatment with
IGFBP-6 (p < 0.001). In this system,
IGFBP-6 increased apoptosis 4-fold (p < 0.001).
IGF-II partially reversed the IGFBP-6-induced decrease in growth and increase in apoptosis. 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 independent 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
rhabdomyosarcoma in vitro and dramatically inhibits xenograft growth in vivo, at least in part by inhibiting
IGF-II. Low expression of
IGFBP-6 may therefore contribute to
rhabdomyosarcoma growth and
metastasis.