Regulation of insulin-like growth factor (IGF)-binding protein-4 availability in normal human osteoblast-like cells: role of endogenous IGFs.
Insulin-like growth factor-binding protein-4 (IGFBP-4) is an important regulator of insulin-like growth factor-I (IGF-I) anabolic activity in bone. Although cultured human osteoblast-like (hOB) cells have been reported to secrete IGFBP-4, we could not detect IGFBP-4 protein in 8 of 27 individual donor-derived hOB-cell conditioned medium (hOB-CM) samples examined by Western ligand blotting. Nonetheless, this subset of hOB cells had normal IGFBP-4 messenger ribonucleic acid expression and protein secretion. Regulation of IGFBP-4 levels in hOB cultures appeared to occur extracellularly. hOB cells produce an IGFBP-4 proteinase that requires the presence of IGF for cleavage of the IGFBP-4 molecule into 2 fragments of approximately 18 and 14 kilodaltons. These fragments are not detected by Western ligand blotting. Our data indicate that elevated endogenous levels of IGF can activate IGFBP-4 proteolysis, because in hOB cultures lacking detectable IGFBP-4 protein 1) basal IGF messenger ribonucleic acid expression was increased; 2) IGF-II peptide levels were elevated; 3) IGF-neutralizing antibodies added to hOB-CM attenuated the proteolysis of exogenous IGFBP-4; and 4) recombinant human IGFBP-4 was proteolyzed into 2 immunoreactive fragments of approximately 18 and 14 kilodaltons during cell-free incubations in these hOB-CM without the addition of exogenous IGF. In conclusion, elevated IGF expression and secretion can contribute to enhanced proteolysis of endogenous and exogenous IGFBP-4 via a proteinase secreted by cultured hOB cells. Levels of endogenous IGF peptide may determine IGFBP-4 availability in the bone microenvironment and, thus, modulate the local cell response to IGF-I.
Durham
S K
SK
Endocrine Research Unit, Mayo Clinic, Rochester, Minnesota 55905.
De León
D D
DD
Okazaki
R
R
Riggs
B L
BL
Conover
C A
CA
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001682:Biological Availability; D002352:Carrier Proteins / Q000378:metabolism*; D002478:Cells, Cultured; D005260:Female; D015870:Gene Expression; D006801:Humans; D018974:Insulin-Like Growth Factor Binding Protein 4; D008297:Male; D008875:Middle Aged; D010006:Osteoblasts / Q000378:metabolism*; D010447:Peptide Hydrolases / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D012016:Reference Values; D013002:Somatomedins / Q000235:genetics / Q000502:physiology*
D002352:Carrier Proteins; D018974:Insulin-Like Growth Factor Binding Protein 4; D012333:RNA, Messenger; D013002:Somatomedins; D010447:Peptide Hydrolases