INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-2 AND PROTEIN-3 STIMULATE GROWTH-HORMONE RECEPTOR-BINDING AND MITOGENESIS IN RAT OSTEOSARCOMA CELLS

GH exerts its biological actions an osteoblasts through a specific hig h affinity receptor expressed on these cells. GH receptor binding is p ositively modulated by a number of factors, including retinoic acid an d dexamethasone, whereas fetal calf serum strongly decreases the bindi ng. To identify responsible factors in serum, components of serum, the insulin-like growth factors (IGFs)-I and -II, and IGF binding protein s (IGFBPs)-2 and -3 were tested for a possible negative modulatory rol e. IGF-I and -II decreased [I-125]hGH binding at an optimal concentrat ion of 30 ng/ml for IGF-I and 100 ng/ml IGF-II, reducing the binding t o 51% and 55%, respectively, of control values. A stimulation of [I-12 5]hGH binding was observed with IGFBP-2 as well as IGFBP-3, inducing a n increase to 148% and 151% of control binding at an optimal concentra tion of 3000 ng/ml for both peptides. The effects of all peptides were dependent on the incubation time, being significantly increased after 8 h of incubation and reaching the full effect thereafter. The effect s were declined at 24 h compared with 16 h for IGFBP-2 and -3 but not for IGF-I and -II. Coincubation of the cells with TGF-I and -II and IG FBP-2 and -3 neutralized the effects of the factors alone. In conclusi on, these results show that IGF-I and -II on the one hand and IGFBP-2 and -3 on the other hand exert opposite actions on [I-125]hGH binding, IGFBP-5 and -3 exerting probably an IGF-independent effect. Further, IGF-I and -II decreased GH receptor messenger RNA (mRNA) levels, as qu antified by a solution hybridization ribonuclease protection assay, fr om 8.65 +/- 1.78 attomoles (amol)/mu g DNA (control) to 2.4 +/- 0.68 a nd 2.16 +/- 0.92 amol/mu g DNA, respectively. IGFBP-5 increased GH rec eptor mRNA levels from 5.26 +/- 1.17 (control) to 18.19 +/- 3.48. Incu bation with IGFBP-3 did not result in stimulation of GH receptor mRNA levels (8.59 +/- 2.91 amol/mu g DNA). This shows that the mechanism of regulation of the GH receptor is, except for IGFBP-3, at least in par t on the mRNA level. Lastly, IGFBP-2 and IGFBP-3 are mitogenic for UMR -106.01 rat osteosarcoma cells, inducing an increase in cell number to 125% and 142% of control cell counts after 48 h of incubation with 10 00 ng/ml IGFBP-2 and -3, whereas IGF-I, IGF-II and Long R(3) IGF-I did not stimulate proliferation. IGFBP-2 and -3 potentiate hGH induced mi togenesis at low hGH concentrations of both factors, whereas at higher concentrations no such effect is observed. IGF-I and -II inhibit hGHs action. The physiological relevance of GH receptor modulation is a su bject of investigation at the present time.