Growth hormone receptor activity is stimulated by insulin-like growth factor binding protein 5 in rat osteosarcoma cells.
Osteoblast-like UMR-106.01 rat osteosarcoma cells express high affinity growth hormone (GH) receptors (GHRs). Because osteoblasts secrete insulin-like growth factor binding protein-5 (IGFBP-5), we evaluated whether it also modulates GH binding and GHR expression in UMR cells. Human recombinant intact IGFBP-5 stimulated 125I-hGH binding in a dose-dependent manner (dose range 300-3000 ng/ml), inducing an increase to 193.6 +/- 2.1% of control binding at 3000 ng/ml (P < 0.001). Carboxy-truncated IGFBP-5 also stimulated GH binding but with less potency (125 +/- 2.7% of control at 3000 ng/ml, P < 0.01). GHRs identified by chemical crosslinking of 125I-hGH to cell monolayers increased after treatment with IGFBP-5 and decreased in response to insulin-like growth factor-I (IGF-I). GHR mRNA levels, as quantitated by a solution hybridization RNAse protection assay, increased up to 3 to 7-fold in a time-dependent manner by intact IGFBP-5 but not by carboxy-truncated IGFBP-5. An antiserum to IGFBP-5 reduced basal GH binding to 56.7 +/- 4.3% of control value at a concentration of 0.5% (P < 0.001), showing that IGFBP-5 produced by the cells is a strong regulator of GH binding. IGFBP-5 antiserum also decreased GH binding to 85.9 +/- 0.9% of IGFBP-5 stimulated value (P < 0.001), showing the specificity of IGFBP-5 stimulation. To determine whether the GHR upregulation was physiologically significant, cell proliferation was evaluated after coincubation of IGFBP-5 with low, non-stimulatory concentrations of GH. IGFBP-5 (1000 ng/ml) induced cell proliferation to 116.2 +/- 3.2% of control levels, and coincubation with hGH at 10 ng/ml induced an increase to 133.3 +/- 0.1% of control levels. We conclude that exogenous and endogenous IGFBP-5 upregulate GHR mRNA levels and GH binding and this interaction potentiates GH-stimulated mitogenesis in osteoblastic cells.
Slootweg
M C
MC
Research Institute for Endocrinology, Reproduction and Metabolism, Free University Hospital, Amsterdam, The Netherlands.
Ohlsson
C
C
van Elk
E J
EJ
Netelenbos
J C
JC
Andress
D L
DL
D000818:Animals; D001859:Bone Neoplasms / Q000378:metabolism*; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D003432:Cross-Linking Reagents; D013006:Growth Hormone / Q000494:pharmacology; D006801:Humans; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000494:pharmacology*; D007457:Iodine Radioisotopes; D007553:Isotope Labeling; D012516:Osteosarcoma / Q000378:metabolism*; D011869:Radioligand Assay; D051381:Rats; D011986:Receptors, Somatotropin / Q000187:drug effects*; D012260:Ribonucleases / Q000378:metabolism; D014407:Tumor Cells, Cultured
D003432:Cross-Linking Reagents; D018975:Insulin-Like Growth Factor Binding Protein 5; D007457:Iodine Radioisotopes; D011986:Receptors, Somatotropin; D013006:Growth Hormone; D012260:Ribonucleases