Studies on the Mechanisms by Which Insulin-like Growth Factor (IGF) Binding Protein-4 (IGFBP-4) and IGFBP-5 Modulate IGF Actions in Bone Cells
The growth potentiating effects of the insulin-like growth factor (IGF)-I and IGF-II are modulated by a family of six insulin-like growth factor binding proteins (IGFBPs). Despite the similarity in amino acid sequences of the IGFBPs, their effects on the growth of bone cells differ. Studies on the molecular mechanisms for IGFBP-4 actions revealed that coincubation of bone cells with IGFBP-4 and 125I-IGF-I or 125I-IGF-II decreased the binding of both of these ligands in a dose-dependent manner. In addition, IGFBP-4 decreased the binding of IGF-I tracer to purified type I IGF receptor. These data in conjunction with data showing that IGFBP-4 had no effect on cell proliferation induced by analogs of IGF-I or IGF-II, which exhibited >100-fold reduced affinity for binding to IGFBP-4 suggest that IGFBP-4 may inhibit IGF action by preventing the binding of ligand to its membrane receptor. In contrast to IGFBP-4, IGFBP-5 treatment increased the binding of IGF tracer to bone cells but did not increase the binding of 125I-IGF-I to type I IGF receptor. Studies on the mechanism by which IGFBP-5 increased the binding of 125I-IGF tracer to bone cells suggest that IGFBP-5 could facilitate IGF binding by a mechanism in which IGFBP-5 has cell surface binding sites independent of IGF receptors. These data in conjunction with the findings that IGFBP-5 potentiated cell proliferation even in the presence of those same IGF analogs that exhibited >200-fold reduced affinity for binding to IGFBP-5, suggest that IGFBP-5 may in part stimulate bone cell proliferation by an IGF-independent mechanism involving IGFBP-5-specific cell surface binding sites. The growth potentiating effects of the insulin-like growth factor (IGF)-I and IGF-II are modulated by a family of six insulin-like growth factor binding proteins (IGFBPs). Despite the similarity in amino acid sequences of the IGFBPs, their effects on the growth of bone cells differ. Studies on the molecular mechanisms for IGFBP-4 actions revealed that coincubation of bone cells with IGFBP-4 and 125I-IGF-I or 125I-IGF-II decreased the binding of both of these ligands in a dose-dependent manner. In addition, IGFBP-4 decreased the binding of IGF-I tracer to purified type I IGF receptor. These data in conjunction with data showing that IGFBP-4 had no effect on cell proliferation induced by analogs of IGF-I or IGF-II, which exhibited >100-fold reduced affinity for binding to IGFBP-4 suggest that IGFBP-4 may inhibit IGF action by preventing the binding of ligand to its membrane receptor. In contrast to IGFBP-4, IGFBP-5 treatment increased the binding of IGF tracer to bone cells but did not increase the binding of 125I-IGF-I to type I IGF receptor. Studies on the mechanism by which IGFBP-5 increased the binding of 125I-IGF tracer to bone cells suggest that IGFBP-5 could facilitate IGF binding by a mechanism in which IGFBP-5 has cell surface binding sites independent of IGF receptors. These data in conjunction with the findings that IGFBP-5 potentiated cell proliferation even in the presence of those same IGF analogs that exhibited >200-fold reduced affinity for binding to IGFBP-5, suggest that IGFBP-5 may in part stimulate bone cell proliferation by an IGF-independent mechanism involving IGFBP-5-specific cell surface binding sites. INTRODUCTIONStudies in our and other laboratories provide strong support to the concept that insulin-like growth factors (IGFs) 1The abbreviations used are: IGFinsulin-like growth factorIGFBPinsulin-like growth factor binding proteinrhrecombinant humanBSAbovine serum albuminDMEMDulbecco's modified Eagle's mediumHPLChigh pressure liquid chromatographyhBDIGFBPhuman bone-derived IGFBP. function in an autocrine/paracrine manner to regulate the proliferative and differentiative functions of bone cells(1Mohan S. Baylink D.J. Clin. Ortho. 1991; 263: 30-48PubMed Google Scholar, 2Baylink D.J. Finkelman R.D. Mohan S. J. Bone Miner. Res. 1993; 8: S565-S572Crossref PubMed Scopus (331) Google Scholar, 3Canalis E. Pash J. Varghese S. Crit. Rev. Eukaryotic Gene Expression. 1993; 3: 155-166PubMed Google Scholar). The findings that support this concept are as follows. First, IGF-I and IGF-II have been found in bone extracts of several different species, and IGFs are the most abundant growth factors stored in bone(4Bautista C. Mohan S. Baylink D.J. Metabolism. 1990; 39: 96-100Abstract Full Text PDF PubMed Scopus (115) Google Scholar, 5Canalis E. McCarthy T. Centrella M. Calcif. Tissue Int. 1988; 43: 346-351Crossref PubMed Scopus (61) Google Scholar). Second, IGFs have been shown to be one of the most abundant mitogens produced by bone cells derived from both fetal and adult bone, and the bone cell production of IGFs is regulated by both local and systemic effectors of bone metabolism(1Mohan S. Baylink D.J. Clin. Ortho. 1991; 263: 30-48PubMed Google Scholar, 2Baylink D.J. Finkelman R.D. Mohan S. J. Bone Miner. Res. 1993; 8: S565-S572Crossref PubMed Scopus (331) Google Scholar, 3Canalis E. Pash J. Varghese S. Crit. Rev. Eukaryotic Gene Expression. 1993; 3: 155-166PubMed Google Scholar). Third, the exogenous addition of IGFs have been shown to stimulate both proliferative and differentiative functions in bone cells(1Mohan S. Baylink D.J. Clin. Ortho. 1991; 263: 30-48PubMed Google Scholar, 2Baylink D.J. Finkelman R.D. Mohan S. J. Bone Miner. Res. 1993; 8: S565-S572Crossref PubMed Scopus (331) Google Scholar, 3Canalis E. Pash J. Varghese S. Crit. Rev. Eukaryotic Gene Expression. 1993; 3: 155-166PubMed Google Scholar). Fourth, there is recent evidence that demonstrates that IGFs increase bone formation parameters in vivo in both rats and humans (6Mueller K. Cortesi R. Modrowski D. Marie P.J. Am. J. Physiol. 1994; 267: E1-E6Crossref PubMed Google Scholar, 7Johansson A.G. Lindh E. Ljunghall S. Lancet. 1992; 339: 1619Abstract PubMed Scopus (97) Google Scholar, 8Ebeling P.R. Jones J.D. O'Fallon W.M. Janes C.H. Riggs B.L. J. Clin. Endocrinol. & Metab. 1993; 77: 1384-1387Crossref PubMed Scopus (0) Google local of IGFs in bone and other is not on the of IGFs produced but by a family of proteins that to IGF binding proteins different from the IGF have been purified from or of Scholar, Res. Full Text PDF PubMed Scopus Google Scholar, S. Res. 1991; Full Text PDF Scopus Google Scholar, Rev. Physiol. 1993; Scopus Google Scholar, S. 1993; 3: Google Scholar, 1993; PubMed Scopus Google Scholar). In have shown that bone cells in a of of which the IGFBP-4 is one of the produced by bone Studies on the actions of IGFBP-4 revealed that IGFBP-4 both and cell proliferation in cells and S. C. Baylink D.J. S. PubMed Scopus Google Scholar). In addition, IGFBP-4 has been shown to inhibit the growth of in Mohan S. Baylink D.J. J. Bone Miner. Res. 1993; 8: PubMed Scopus Google Scholar). The effect of IGFBP-4 on cell proliferation not to be to bone cells S. 1991; PubMed Scopus Google have shown that IGFBP-4 cell proliferation in cell and and M. Res. 1991; Google have shown that the cell growth purified from of the cell is to contrast to IGFBP-4, on the actions of IGFBP-5, the stored in bone, revealed that IGFBP-5 is a of bone cell Baylink D.J. Mohan S. Res. 1991; PubMed Scopus Google Scholar). potentiating effects of IGFBP-5 purified from cell have been by and Res. 1991; PubMed Scopus Google Scholar, J. 1992; 267: Full Text PDF PubMed Google Scholar). In addition, Jones C. J. 1993; PubMed Scopus Google have that IGFBP-5 in cell potentiated the growth effects of IGF-I on C. M. E. K. P.J. J. J. 1992; 267: Full Text PDF PubMed Google have that IGFBP-5 produced in in from the same have shown that IGFBP-5 in M. J. J. Bone Miner. Res. 1993; 8: Scholar). the findings that IGF actions or and that the production of is regulated by of Scholar, Res. Full Text PDF PubMed Scopus Google Scholar, S. Res. 1991; Full Text PDF Scopus Google Scholar, Rev. Physiol. 1993; Scopus Google Scholar, S. 1993; 3: Google Scholar, 1993; PubMed Scopus Google support the that the the and of the and of in as IGFBP-4 and IGFBP-5, which with effects on bone cell is the molecular mechanisms by which these binding proteins their effects on bone cell is data in other cell for and which have been shown to both stimulate and inhibit IGF actions C. J. J. J. Full Text PDF PubMed Google Scholar, Res. 1988; PubMed Scopus Google Scholar, 1992; PubMed Scopus Google Scholar, D. and of and Google Scholar). C. J. J. J. Full Text PDF PubMed Google have shown that exogenous addition of a of induced by IGF-I in In of with to the addition of IGF-I in the of IGF-I Res. 1988; PubMed Scopus Google Scholar). cell surface of which to has been to be an mechanism for the of IGF-I action by a of other mechanisms have been 1992; PubMed Scopus Google Scholar, D. and of and Google Scholar). The to the data for the by the effects of IGFBP-4 and IGFBP-5 on and on bone cells and by IGFBP-4 and IGFBP-5 their actions on bone cell proliferation by different These data that the molecular mechanisms by which IGFBP-4 and IGFBP-5 bone cells are different and that IGFBP-5 may bone cells by an IGF-independent cell from cells S. S. J. PubMed Scopus Google and had been by M. cell as S. T. Metabolism. 1991; Full Text PDF PubMed Scopus Google and had been by J. bone cells derived from bone as Baylink D.J. PubMed Scopus Google and had been by T. IGFBP-5 and and of IGFBP-5 produced in and purified by R. in and Scholar). IGFBP-4 purified from bone cell as and the of the by and amino acid S. C. Baylink D.J. S. PubMed Scopus Google Scholar, S. Baylink D.J. 1991; Google Scholar, S. Baylink D.J. PubMed Scopus Google Scholar). of IGF-I or IGF-II found in the purified of IGFBP-4 and IGF-I and IGF-II from and by for Tissue and IGF-I a from of modified Eagle's from serum from serum from and from and and other used and from of proteins as S. C. S. Baylink D.J. 1990; PubMed Scopus Google Scholar). The of IGFs and of that of and of of IGFs stored and used for IGF of as Baylink D.J. Mohan S. Res. 1991; PubMed Scopus Google with to and and IGF-II affinity the to a and the with a of to a of and in a and of the used for ligand and and molecular molecular of both and reduced and an of as S. C. Baylink D.J. S. PubMed Scopus Google Scholar, S. Baylink D.J. 1991; Google Scholar, S. Baylink D.J. PubMed Scopus Google cells in with serum and by the cells with cell in with and the cells by the cells with bone cells in with serum and as S. C. S. Baylink D.J. 1990; PubMed Scopus Google proliferation by the of cells in of serum in and for of or of the and the cells for an The cells for with of The and the cells with The cells with of and the to and with of used for of on IGF-I or IGF-II to in in serum in as for the the with and of of of and the with IGF-I or IGF-II for The and the with IGF-I or IGF-II to cells with and as S. Baylink D.J. PubMed Scopus Google Scholar, S. C. S. Baylink D.J. 1990; PubMed Scopus Google to Bone in in serum in as for the with and of of or to and the cells for with of the of the and the with to cells with and as S. Baylink D.J. PubMed Scopus Google Scholar, S. C. S. Baylink D.J. 1990; PubMed Scopus Google to IGF-I in the of of IGF binding to IGF-I by type I IGF that purified as S. K. J. Full Text PDF PubMed Google Scholar). of of in of of 125I-IGF-I and of IGFBP-4 or The in the or presence of IGF-I to the of the of affinity of the ligand by of in of the by the addition of of of the for and to the of the and to of 125I-IGF-I to or by of the with of of IGFBP-4 or IGFBP-5 with of IGF-I or IGF-II for in and 125I-IGF by of serum and of and S. C. Baylink D.J. S. PubMed Scopus Google Scholar). binding by in the presence of IGF from of IGFs or their of amino acid for the to but not to the sequences from S. M. J. 1991; Full Text PDF PubMed Google and J. Res. 1991; PubMed Scopus Google the to or a of IGFBP-5, bone as to to by affinity on and on IGF-II, by a of these one with an molecular of that exhibited IGF binding on ligand not acid of the of and of reduced and of the and of these sequences with the of which has been 1992; Google that the of the is to IGFBP-5 The of a of which had to IGFBP-5 of IGFBP-4 and IGFBP-5 on and Bone of the cell decreased to IGFBP-4 cell proliferation in the of exogenous growth factors by cells to The of proliferation with to IGF-I decreased by IGFBP-4 had effect on the proliferation with to which exhibited >100-fold reduced affinity for IGFBP-4 with IGF-I IGFBP-4 a dose-dependent in the of proliferation with to IGF-II but has effect on the of proliferation with to of IGF-II, exhibited >100-fold reduced affinity for IGFBP-4 with IGF-II of IGFBP-4 and IGFBP-5 on cell proliferation as by the of are as a of the the of the by the the effect of IGFBP-4 on or cell proliferation in the of exogenous IGFBP-4 and in the presence of IGFBP-4 the effect of IGFBP-4 on cells with IGF-II and or and the effect of the and the on or cell IGFBP-5 on cell proliferation and IGFBP-5 on cell proliferation and IGFBP-5 on cell proliferation and IGFBP-5 on cell proliferation and on cell proliferation and IGFBP-5 on cell proliferation and in from to with for binding of 125I-IGF-I or 125I-IGF-II to IGFBP-4 and IGFBP-5 by the IGFs and their for binding of 125I-IGF-I to IGFBP-4 and to IGFBP-5 by IGF-I and and IGF-II and and and the for binding of 125I-IGF-II to IGFBP-4 and to IGFBP-5 by IGF-I and and IGF-II and and and the of from contrast to IGFBP-4, of to IGFBP-5 in an increased proliferation of cells both in the and in the presence of that cell proliferation increased by in a dose-dependent manner. cell proliferation increased to of In addition to cell cell proliferation in the presence of IGF-I or IGF-II in a dose-dependent manner. The and of IGFBP-5 on cells or on cells to IGF-I or IGF-II the the not the of to proliferation and exhibited in cell and increased cell proliferation by and contrast to IGFBP-4, which did not inhibit cell proliferation induced by or IGFBP-5 potentiated cell proliferation even in the presence of those IGF analogs that exhibited >200-fold reduced affinity for binding to and IGF-I cell proliferation to and of which increased by and with IGFBP-5 and by and with IGFBP-5 treatment in cell proliferation induced by not effect of IGFBP-4 and IGFBP-5 on cell proliferation bone cells in that cell proliferation by and in bone cells derived from and by In addition, cell proliferation by the exogenous addition of IGFBP-4 in bone cells derived from both and In contrast to IGFBP-4 exogenous addition of cell proliferation by in bone cells derived from and in different not increase in cell proliferation not to be an of in the presence of IGFBP-5 data the of not In addition, both and cell proliferation increased by and in the presence of in a in a IGFBP-4 and IGFBP-5 on the of IGFs to Bone our on the mechanism of IGFBP-4 and IGFBP-5 the that IGFBP-4 and IGFBP-5 regulate IGF actions in bone cells by the binding of IGFs to their receptors. this the effect of IGFBP-4 and IGFBP-5 on IGF binding to cells by cell binding IGF-I or These data revealed that IGFBP-4 produced a dose-dependent in the binding of both IGF-I and IGF-II IGFBP-5 in different a dose-dependent increase in the binding of both of and IGF-II, of binding in IGF-I and IGF-II binding treatment of and cells with IGFBP-4 and IGFBP-5 not of on the binding of 125I-IGF-I or 125I-IGF-II to are the of from as the of the in the of for the effect of IGFBP-4 on the binding of 125I-IGF-I and 125I-IGF-II and for the effect of IGFBP-5 on the binding of 125I-IGF-I and 125I-IGF-II of IGFBP-4 and IGFBP-5 to the IGF-I and the type I IGF-I with and a in which the type I IGFBP-4 or affinity and found in the IGF-I which by IGF-I the of IGFBP-4 from to in a in the of to type I and a increase in the of to These are with the that IGFBP-4 binding to its ligand may the affinity of the ligand that IGFs no to the receptors. In contrast to IGFBP-4, IGFBP-5 or had no effect on the of to the type I receptor. In other IGFBP-5 in a in the of tracer to the type I and a increase in the of to IGFBP-5 not The increase in 125I-IGF-I binding in cells by IGF-I but not by or which to IGF-I but not to IGFBP-5 not of IGFBP-4 and IGFBP-5 on the binding of 125I-IGF-I to type I IGF receptor. type I IGF with 125I-IGF-I in the presence or of of of purified IGFBP-4 or IGFBP-5 to the as in the The to in a of as and and IGFBP-4 to Bone the of IGFBP-4 and IGFBP-5 to to cells in revealed that IGFBP-4 not to cells IGFBP-4 in the or presence of IGFBP-5 to and this binding is not by IGFBP-4, but is in a dose-dependent manner by IGFBP-5 binding to cells decreased by the addition of IGFBP-5 but not evidence for the presence of IGFBP-5 but not IGFBP-4 binding sites have been and cells in not of on the binding of to to with of IGFBP-5 and IGFBP-4 as are the of the binding in as the of the binding IGFBP. in the of cell binding to the effect of on the binding of to cells on the evidence that to D. and of and Google Scholar). These data revealed that treatment of cells with decreased the binding of in a dose-dependent manner. the binding to cells decreased to of In contrast to treatment of cells with an in an increase in the binding of to cells of increase in binding of to cells by the addition of IGFBP-5 but not In addition, the increased binding of to cells in the presence of decreased to of by with effect of IGF-I and IGF-II on IGFBP-5 binding to cells by cell binding in in which to In these IGF-I IGF-II had effect on the binding of to cells and binding in the presence of IGF-I and IGF-II, with These data are with the IGF binding with purified IGF-I and suggest that the increased binding of 125I-IGF-I to cells in the presence of IGFBP-5 is not to IGF findings of this that purified IGFBP-4 and IGFBP-5 exhibited effects on bone cell proliferation IGFBP-4 IGFBP-5 both and bone cell The of cell proliferation by IGFBP-5 be by of IGFBP-5 with IGFs or other proteins both bone-derived and IGFBP-5 exhibited and no of IGF-I or IGF-II found in of these The effects of IGFBP-4 and IGFBP-5 are not to effects bone These binding IGFBP-4 and IGFBP-5, exhibited effects on the binding of IGF ligand to bone cells in IGFBP-4 decreased the binding of both IGF-I and IGF-II in a IGFBP-5 treatment increased IGF-I binding that of IGF-II binding to cells in The for in IGF-I binding with IGF-II in the presence of IGFBP-5 is purified IGFBP-4 decreased the binding of IGF-I to the type I receptor. In IGFBP-5 did not an increase in the binding of IGF-I to type I receptor. the findings that and IGFBP-4 but not IGFBP-5 decreased the binding of IGF-I ligand to type I are with the that the of IGFBP-5 and IGF-I may to the type I are to this findings of this that the increase in 125I-IGF-I binding to bone cells be on the of increased IGF-I binding to its receptors. In addition, increase in 125I-IGF-I binding in cells by IGF-I but not by or which to IGF-I but not to These data suggest that IGFBP-5 to bone cells independent of IGF and the binding of IGF-I to bone on IGFBP-5 binding to bone cells are to with IGFBP-5 by and J. 1992; 267: Full Text PDF PubMed Google and with 1992; PubMed Scopus Google Scholar). In to the of binding sites for IGFBP-5, the findings that IGFBP-5 binding to bone cells is not by of IGF-I or IGF-II suggest that IGFBP-5 to sites independent of IGF receptors. In addition, Jones C. J. 1993; PubMed Scopus Google have shown that IGFBP-5 to proteins and and IGFBP-5 not an which has been shown to be in the binding of proteins to J. 1994; Full Text PDF PubMed Google the findings of this that an IGFBP-5 binding to bone cells with the that IGFBP-5 to is with the that IGFBP-5 may to binding to proteins as on the findings of this and other have as shown in the mechanisms by which IGFBP-4 and IGFBP-5 may IGF actions in bone to these IGFBP-4 IGF binding to the by binding to IGFs or the binding The findings that IGFBP-4 the binding of IGF-I to purified IGF-I and IGF-II to the in cells S. C. S. Baylink D.J. 1990; PubMed Scopus Google support the that IGFBP-4 bone cell proliferation by preventing IGF binding to its receptors. with this of IGFBP-4 action are the findings that IGFBP-4 had no effect on the of or which has >100-fold reduced affinity for IGFBP-4 IGF-I or IGFBP-4 may which has been shown to inhibit action by the binding of to its T. T. J. 1992; Scopus Google for IGFBP-4 and IGFBP-5 actions in bone cells for IGFBP-4 IGF binding to IGF by binding IGFs or the binding to IGFBP-5, are In the of to IGF receptors. In IGFBP-5 to bone cell surface independent of IGF and may stimulate cell proliferation IGF independent In IGFBP-5 binding sites in bone cell surface may IGFs the surface of IGF the ligand to be by IGF molecular mechanisms by which IGFBP-5 bone cell proliferation to be those of is that IGFBP-5 to IGF a from that of IGFBP-4 and that the of IGFBP-5 and IGF to the with affinity IGFs be to a mechanism the affinity of to its is increased binding to D. J. 1992; PubMed Scopus Google Scholar). IGFBP-5 action may be by IGFBP-5 binding to bone cells independent of IGF receptors. In this IGFBP-5 may to proteins as and may and IGFs IGF receptors. on the findings that the IGFBP-5 has affinity for the ligand IGFBP-5, Jones C. J. 1993; PubMed Scopus Google have that IGFBP-5 could facilitate of IGF to IGF-I receptors. mechanism of action has been for by and 1991; PubMed Scopus (115) Google Scholar). In these has been that and IGFBP-5 may inhibit IGF actions in affinity stimulate IGF actions their affinity is and the affinity of the IGF-I receptor. IGFBP-5 IGF actions the of IGFs to their IGFBP-5 to be in the presence of IGF-I in the presence of which has >200-fold reduced affinity IGF-I for this not the These findings with the findings that the of the of IGF-I or IGF-II increased by treatment with IGFBP-5 are with the that IGFBP-5 may of its actions binding to its that have is from other T. Endocrinol. 1993; PubMed Scopus Google have shown that the growth of the in not to those in that the of has an effect on cell which is IGF 1992; Scopus Google have shown evidence for of cell growth by exogenous in cells an IGF-independent In addition, J. C. 1994; PubMed Scopus Google have found in the effects and and that has and has a function different from its function to these findings are with the that may of their effects independent of IGFs binding to their binding involving the of these and of the and effect binding to its and actions are to this of action for The of affinity has been by bone cell production of a of abundant proteins to which IGFBP-5 have that IGFBP-4 IGFBP-5 bone cell proliferation IGFBP-4 the binding of IGF-I tracer to the type I IGF and IGFBP-4 may its effects on bone cell proliferation by and IGFBP-5 to bone cells and IGFBP-5 may its effects on bone cell proliferation an IGF-independent INTRODUCTIONStudies in our and other laboratories provide strong support to the concept that insulin-like growth factors (IGFs) 1The abbreviations used are: IGFinsulin-like growth factorIGFBPinsulin-like growth factor binding proteinrhrecombinant humanBSAbovine serum albuminDMEMDulbecco's modified Eagle's mediumHPLChigh pressure liquid chromatographyhBDIGFBPhuman bone-derived IGFBP. function in an autocrine/paracrine manner to regulate the proliferative and differentiative functions of bone cells(1Mohan S. Baylink D.J. Clin. Ortho. 1991; 263: 30-48PubMed Google Scholar, 2Baylink D.J. Finkelman R.D. Mohan S. J. Bone Miner. Res. 1993; 8: S565-S572Crossref PubMed Scopus (331) Google Scholar, 3Canalis E. Pash J. Varghese S. Crit. Rev. Eukaryotic Gene Expression. 1993; 3: 155-166PubMed Google Scholar). The findings that support this concept are as follows. First, IGF-I and IGF-II have been found in bone extracts of several different species, and IGFs are the most abundant growth factors stored in bone(4Bautista C. Mohan S. Baylink D.J. Metabolism. 1990; 39: 96-100Abstract Full Text PDF PubMed Scopus (115) Google Scholar, 5Canalis E. McCarthy T. Centrella M. Calcif. Tissue Int. 1988; 43: 346-351Crossref PubMed Scopus (61) Google Scholar). Second, IGFs have been shown to be one of the most abundant mitogens produced by bone cells derived from both fetal and adult bone, and the bone cell production of IGFs is regulated by both local and systemic effectors of bone metabolism(1Mohan S. Baylink D.J. Clin. Ortho. 1991; 263: 30-48PubMed Google Scholar, 2Baylink D.J. Finkelman R.D. Mohan S. J. Bone Miner. Res. 1993; 8: S565-S572Crossref PubMed Scopus (331) Google Scholar, 3Canalis E. Pash J. Varghese S. Crit. Rev. Eukaryotic Gene Expression. 1993; 3: 155-166PubMed Google Scholar). Third, the exogenous addition of IGFs have been shown to stimulate both proliferative and differentiative functions in bone cells(1Mohan S. Baylink D.J. Clin. Ortho. 1991; 263: 30-48PubMed Google Scholar, 2Baylink D.J. Finkelman R.D. Mohan S. J. Bone Miner. Res. 1993; 8: S565-S572Crossref PubMed Scopus (331) Google Scholar, 3Canalis E. Pash J. Varghese S. Crit. Rev. Eukaryotic Gene Expression. 1993; 3: 155-166PubMed Google Scholar). Fourth, there is recent evidence that demonstrates that IGFs increase bone formation parameters in vivo in both rats and humans (6Mueller K. Cortesi R. Modrowski D. Marie P.J. Am. J. Physiol. 1994; 267: E1-E6Crossref PubMed Google Scholar, 7Johansson A.G. Lindh E. Ljunghall S. Lancet. 1992; 339: 1619Abstract PubMed Scopus (97) Google Scholar, 8Ebeling P.R. Jones J.D. O'Fallon W.M. Janes C.H. Riggs B.L. J. Clin. Endocrinol. & Metab. 1993; 77: 1384-1387Crossref PubMed Scopus (0) Google local of IGFs in bone and other is not on the of IGFs produced but by a family of proteins that to IGF binding proteins different from the IGF have been purified from or of Scholar, Res. Full Text PDF PubMed Scopus Google Scholar, S. Res. 1991; Full Text PDF Scopus Google Scholar, Rev. Physiol. 1993; Scopus Google Scholar, S. 1993; 3: Google Scholar, 1993; PubMed Scopus Google Scholar). In have shown that bone cells in a of of which the IGFBP-4 is one of the produced by bone Studies on the actions of IGFBP-4 revealed that IGFBP-4 both and cell proliferation in cells and S. C. Baylink D.J. S. PubMed Scopus Google Scholar). In addition, IGFBP-4 has been shown to inhibit the growth of in Mohan S. Baylink D.J. J. Bone Miner. Res. 1993; 8: PubMed Scopus Google Scholar). The effect of IGFBP-4 on cell proliferation not to be to bone cells S. 1991; PubMed Scopus Google have shown that IGFBP-4 cell proliferation in cell and and M. Res. 1991; Google have shown that the cell growth purified from of the cell is to contrast to IGFBP-4, on the actions of IGFBP-5, the stored in bone, revealed that IGFBP-5 is a of bone cell Baylink D.J. Mohan S. Res. 1991; PubMed Scopus Google Scholar). potentiating effects of IGFBP-5 purified from cell have been by and Res. 1991; PubMed Scopus Google Scholar, J. 1992; 267: Full Text PDF PubMed Google Scholar). In addition, Jones C. J. 1993; PubMed Scopus Google have that IGFBP-5 in cell potentiated the growth effects of IGF-I on C. M. E. K. P.J. J. J. 1992; 267: Full Text PDF PubMed Google have that IGFBP-5 produced in in from the same have shown that IGFBP-5 in M. J. J. Bone Miner. Res. 1993; 8: Scholar). the findings that IGF actions or and that the production of is regulated by of Scholar, Res. Full Text PDF PubMed Scopus Google Scholar, S. Res. 1991; Full Text PDF Scopus Google Scholar, Rev. Physiol. 1993; Scopus Google Scholar, S. 1993; 3: Google Scholar, 1993; PubMed Scopus Google support the that the the and of the and of in as IGFBP-4 and IGFBP-5, which with effects on bone cell is the molecular mechanisms by which these binding proteins their effects on bone cell is data in other cell for and which have been shown to both stimulate and inhibit IGF actions C. J. J. J. Full Text PDF PubMed Google Scholar, Res. 1988; PubMed Scopus Google Scholar, 1992; PubMed Scopus Google Scholar, D. and of and Google Scholar). C. J. J. J. Full Text PDF PubMed Google have shown that exogenous addition of a of induced by IGF-I in In of with to the addition of IGF-I in the of IGF-I Res. 1988; PubMed Scopus Google Scholar). cell surface of which to has been to be an mechanism for the of IGF-I action by a of other mechanisms have been 1992; PubMed Scopus Google Scholar, D. and of and Google Scholar). The to the data for the by the effects of IGFBP-4 and IGFBP-5 on and on bone cells and by IGFBP-4 and IGFBP-5 their actions on bone cell proliferation by different These data that the molecular mechanisms by which IGFBP-4 and IGFBP-5 bone cells are different and that IGFBP-5 may bone cells by an IGF-independent