John C. Jennings

Primary structure of human skeletal growth factor: homology with human insulin-like growth factor-II.

Human skeletal growth factor (human SGF) extracted from human bone has been purified to homogeneity by hydroxyapatite chromatography and gel filtration under dissociative conditions followed by FPLC heparin-Sepharose affinity chromatography and reverse phase HPLC. Human SGF was homogeneous except that in each preparation about 30% of SGF molecules lacked the N-terminal alanine. 75% of the human SGF sequence has been determined. The amino acid sequences of the N-terminal 20 amino acids and of several tryptic fragments were identical to the corresponding sequences of human insulin-like growth factor-II (IGF-II) purified from serum. However, since the C-peptide (variable region) of human SGF has not yet been sequenced, we cannot conclude that SGF is identical to IGF-II. Comparison of the amino acid sequence of human SGF with that of IGF-II variants that have been described in the literature revealed that human SGF is not one of the known IGF-II variants. IGF-I was also found in human bone extract but was several-fold less abundant than SGF/IGF-II. The relative abundance of SGF/IGF-II and IGF-I in bone corresponded to the relative rates of production of these two mitogens by human bone cells in vitro. Regarding the physiological significance of IGF-II in bone, previous studies on the biological actions of SGF in vitro suggest that this growth factor can have both paracrine and autocrine functions on cells of the osteoblast line. In addition, we have proposed the concept that SGF is a mediator of the coupling of bone formation to bone resorption, an important bone volume regulatory mechanism. In as much as SGF is very similar (if not identical) to IGF-II, it seems likely that these proposed regulatory functions of SGF in bone are attributable to IGF-II.

Characterization of mitogenic activities extracted from bovine bone matrix

The mitogenic activity in the unfractionated mixture of proteins released from adult bovine bone matrix during demineralization with ethylenediaminetetraacetate (EDTA) has been examined. Bovine bone extract (BBE) from 1 to 25 micrograms protein per ml stimulated proliferation of chick embryo calvaria bone cells, newborn mouse bone cells, and osteoblastlike cell lines MMB-1 and ROS 17/2.8. BBE also stimulated DNA synthesis in cells from chick embryo cartilage, skin and skeletal muscle tissues and fibroblastlike BALB/c 3T3 and NRK cells. BBE contained beta transforming growth factor (TGF) activity (NRK cell colony formation in soft agar in the presence of epidermal growth factor EGF). The cell specificity results suggest that BBE contains more than one growth factor, including a beta TGF and a factor that is not specific for bone cells, and all of the bone derived growth factor activities that have been described previously, including SGF, are apparently present in BBE. Maximal stimulation of chick embryo calvarial cell DNA synthesis by BBE was equal to or exceeded maximal stimulation by nonosseous growth factors that have been reported to stimulate DNA synthesis in bone organ cultures (EGF, fibroblast growth factor, platelet-derived growth factor, insulinlike growth factor I, and multiplication stimulating activity). Combinations of BBE with maximally stimulatory concentrations of each growth factor stimulated DNA synthesis to a greater magnitude than did each growth factor alone. These results suggest that combinations of bone derived and systemic factors can coordinately stimulate bone cell proliferation.

Isolation and purification of a low-molecular-weight skeletal growth factor from human bones

A low-molecular-weight potent bone cell mitogen termed human skeletal growth factor (human SGF) was purified to homogeneity from human bone matrix. Extraction and initial purification steps were done under dissociative conditions to separate human SGF from high-molecular-weight complexes of bone matrix proteins. SGF activity was extracted from human femoral heads by demineralization with 10% EDTA in the presence of 4 M guanidine-HCl and proteinase inhibitors and was purified by hydroxyapatite, HPLC gel-filtration and HPLC reverse-phase chromatography. Human SGF thus purified was homogeneous by HPLC reverse-phase chromatography and SDS-polyacrylamide gel electrophoresis. The relative molecular mass of human SGF purified under dissociative conditions was 11,000. Human SGF stimulated bone cell proliferation ([3H]thymidine incorporation and cell number) at picomolar concentrations, with half maximum activity at 2-3 ng/ml (180-270 pM). Human SGF constitutes 0.00024% of organic bone matrix by weight.

Chemical and biological characterization of low-molecular-weight human skeletal growth factor

Skeletal growth factor (SGF) activity was extracted from human bone matrix by demineralization and purified under dissociative conditions using hydroxyapatite, HPLC gel-filtration and HPLC reverse-phase chromatography. Human SGF thus purified was characterized chemically and biologically. Purified human SGF stimulated chick embryo bone cell proliferation at picomolar concentrations (half maximum at 2-3 ng/ml) and had little or no activity on other cell types tested (mouse 3T3 and normal rat kidney fibroblasts, embryonic chick intestinal and human placental cells). Human SGF did not displace 125I-labeled epidermal growth factor binding to normal rat kidney cells and did not stimulate normal rat kidney cell colony formation in soft agar. Human SGF activity was sensitive to trypsin, chymotrypsin, papain, dithiothreitol and performic acid but was resistant to heat (upto 70 degrees C), pH (3-10), cyanogen bromide, alkaline phosphatase and neuraminidase and did not bind jack bean concanavalin A or kidney bean lectin. From our chemical and biological studies it appears that human SGF is different from other known polypeptide growth factors: epidermal growth factor, fibroblast growth factor, insulin, insulin-like growth factor-I, platelet-derived growth factor and transforming growth factor.

[27] Purification of bovine skeletal growth factor

Publisher Summary Unlike most of the organ systems, the specific cellular components of bone—osteoblasts, osteocytes, and osteoclasts—account for only a minor portion (5–10%) of tissue weight. The major component of bone is matrix (90–95%), which consists of a mineral phase (60–65% of noncellular bone weight) and a protein phase (35–40%). The largest protein component (90% of matrix protein) is type I collagen. The noncollagenous proteins (10%) are heterogeneous in origin; some appear to be produced by bone cells, while others are incorporated from or are concentrated from serum. This chapter discusses the chick embryo calvarial cell bioassay that is used to monitor the purification of skeletal growth factor (SGF) because a sensitive assay procedure is needed for purification. To obtain greater quantities of SGF, bovine bone is used because large quantities of fresh bone are readily available for the extraction of matrix proteins.

Quantitation of growth factors in ossein-mineral-compound

Study was undertaken to identify polypeptide factors in the commercially available ossein-mineral-compound and to see if they are present in a biologically relevant quantity. Using the guanidine-EDTA extraction, 35.7 +/- 0.1 mg proteins were obtained from 1 g of the ossein-mineral-compound. At concentration 1 micrograms/ml, guanidine-EDTA-extractable proteins stimulated the incorporation of thymidine into DNA by human bone cells to 581 +/- 122% (p less than 0.001) of that by bovine serum albumin-treated control cells, decreasing thereafter. Similarly, it stimulated the activity of alkaline phosphatase in the human bone cells. Growth factors IGF-I, IGF-II, and TGF-beta were identified in the ossein-mineral-compound. This leads to speculation regarding possible role of growth factors in explaining the beneficial effects of the compound in retarding bone loss in patients with osteoporosis.

Bovine skeletal growth factor stimulates protein phosphorylation of chicken bone cells in vitro.

1. Bovine skeletal growth factor (SGF), a potent bone cell mitogen, stimulated protein phosphorylation in cultured chicken calvarial cells. 2. SDS-PAGE followed by autoradiographic analysis of the cellular proteins indicated that [32P] incorporation was enhanced in several proteins in response to 10 ng/ml of SGF (the maximum stimulatory mitogenic dose for these cells). 3. Under conditions favoring tyrosine kinases, SGF stimulated phosphorylation of at least 6 proteins in crude calvarial cell membrane fraction, and caused a time-dependent stimulation of phosphorylation of angiotensin II by crude calvarial cell membrane fractions. 4. Thus, our data demonstrate that SGF stimulates protein phosphorylation in bone cells, and suggest that at least some of the protein phosphorylation involves tyrosine residues.