Growth in
neuroblastoma cells is regulated by
insulin-like growth factors (IGFs) whose action is modulated by
IGF binding proteins (IGFBPs). In this study, SK-N-SH
neuroblastoma cells were shown to produce
IGF-II,
IGFBP-2,
IGFBP-4 and small quantities of
IGFBP-6. We have studied the effects of a natural morphogen,
retinoic acid (RA), on growth and
IGFBP expression in these cells. In all experiments, cells were cultured in serum-free medium and treated with 1 mumol/l RA for 12 h. Cell number increased by almost 50% during the first 24 h after the beginning of treatment. This stimulation was inhibited by 80% or more in the presence of the anti-type 1 IGF receptor antibody alpha-IR3 and anti-
IGF-II antibody. The
IGF-II concentrations in the
culture media, measured after acidic gel filtration, increased about 1.5-fold and Northern blotting showed a concomitant increase in
IGF-II mRNA levels. The mitogenic effect of RA therefore reflects its stimulation of
IGF-II production. The availability of
IGF-II to the cells may also be enhanced because of the proteolysis of
IGFBP-2 to which it is bound. After this initial phase, proliferation ceased despite continued
IGF-II production between 24 and 72 h. Both
IGFBP-2 and
IGFBP-4 production decreased, whereas that of
IGFBP-6 increased. These changes appeared both in the
protein quantities and in their mRNAs.
Insulin-like growth factor binding protein 6 has a strong affinity for
IGF-II, 5-10 times that of
IGFBP-2 and at least 10 times that of the type I IGF receptor, and the arrested proliferation may result, at least in part, from sequestration by
IGFBP-6 of the
IGF-II secreted.