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Monokines produced by macrophages stimulate the growth of osteoblasts
1Department of Medicine Research, Jewish Hospital, Washington University Medical Center, St. Louis, Missouri 63110.
Abstract:
We have previously reported that the J774A.1 macrophage-like tumor cell line produces two potent monokines which stimulate the growth of osteoblasts and chondrocytes. These growth factors, which have an affinity for heparin-agarose, have been termed HEP I (a 30 Kd PDGF-like molecule) and HEP II (an approximately 20 Kd molecule), respectively, based on their elution profile. Unlike HEP I, HEP II does not stimulate the growth of fibroblasts. Extensive biological and chromatographic studies disclosed that HEP II appears to be a unique bone cell mitogen unlike any known growth factor, including the FGFs, IL-1s, and TNFs, EGF, IGF-I and -II, TGF-beta, beta 2 microglobulin, G-CSF, CSF-1 and GM-CSF. To characterize more fully the effects of the macrophage-derived monokines on osteoblast growth and function, clones were derived from calvaria explant cultures. Two clones, SDFRC-2.05 and SDFRC-3, were developed and found to exhibit osteoblastic characteristics, including high levels of alkaline phosphatase, synthesis of type I but not type III collagen, and an increased intracellular cAMP production in response to PTH. The SDFRC-3 cells exhibited a polygonal morphology like that of the explant-derived cells while SDFRC-2.05 cells exhibited a more fibroblastic morphology. When tested on the explant cultures and clones, HEP I and HEP II were found to stimulate DNA synthesis and increase protein per culture, but decreased alkaline phosphatase activity. Clone SDFRC-3 was found to be more responsive to HEP II than clone SDFRC-2.05. Both monokines were found to be more potent mitogens for bone cells than TGF-beta. HEP II, but not HEP I or TGF-beta, induced a transformation of bone cells from a polygonal to a fibroblastic morphology, suggesting the induction of migration prior to proliferation. Thus, macrophages may be responsible not only for bone repair but also for ensuring the linkage of bone formation to resorption during physiological remodeling.
Insights
Macrophages produce two growth factors, HEP I and HEP II, that stimulate bone cell growth. HEP II is a unique bone cell mitogen that may play a role in bone remodeling by influencing cell migration and proliferation.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- J774A.1 macrophage cell line produces two heparin-binding monokines (HEP I and HEP II) that stimulate osteoblast and chondrocyte growth.
- HEP I is a 30 kDa PDGF-like molecule, while HEP II is an approximately 20 kDa molecule with unique bone cell mitogenic properties.
- HEP II does not stimulate fibroblast growth and is distinct from known growth factors like FGFs, IL-1s, TNFs, EGF, IGFs, and TGF-beta.
Purpose of the Study:
- To further characterize the effects of macrophage-derived monokines (HEP I and HEP II) on osteoblast growth and function.
- To investigate the role of these monokines in bone remodeling processes.
Main Methods:
- Derivation and characterization of osteoblast clones (SDFRC-2.05 and SDFRC-3) from calvaria explant cultures.
- Assessing osteoblastic characteristics such as alkaline phosphatase activity, collagen synthesis, and cAMP production.
- Evaluating the mitogenic effects of HEP I and HEP II on bone cell cultures and clones.
- Observing morphological changes in bone cells induced by HEP II.
Main Results:
- Both HEP I and HEP II stimulated DNA synthesis and increased protein in bone cell cultures and clones, but decreased alkaline phosphatase activity.
- Clone SDFRC-3, exhibiting polygonal morphology and osteoblastic characteristics, was more responsive to HEP II than clone SDFRC-2.05, which had a fibroblastic morphology.
- HEP II, but not HEP I or TGF-beta, induced a morphological transformation from polygonal to fibroblastic, suggesting induction of migration prior to proliferation.
- Both monokines were more potent mitogens for bone cells than TGF-beta.
Conclusions:
- Macrophages produce unique bone cell mitogens (HEP I and HEP II) that influence osteoblast proliferation and function.
- HEP II may play a critical role in bone remodeling by inducing cell migration and proliferation, linking bone formation to resorption.
- These findings highlight the significant role of macrophages in bone repair and physiological remodeling.