Human osteoblast precursors produce extracellular adenosine, which modulates their secretion of IL-6 and osteoprotegerin.
We showed that human osteoprogenitor cells produced adenosine and expressed ecto-5'-nucleotidase and all four adenosine receptor subtypes. Adenosine stimulated IL-6 but inhibited osteoprotegerin secretion, suggesting that adenosine is a newly described regulator of progenitor cell function.


BACKGROUND
Maintaining skeletal homeostasis relies on there being a balance between bone formation and resorption; an imbalance between these processes can lead to diseases such as osteoporosis and rheumatoid arthritis. Recent reports showed that locally produced ATP, acting through P2 receptors, has pronounced effects on bone formation. However, ATP can be enzymatically cleaved to adenosine that has little or no activity at P2 receptors but mediates its action through the P1 family of receptors. We studied whether adenosine may also have an important role in controlling bone cell differentiation and function.


METHODS
Extracellular adenosine levels were analyzed by high-performance liquid chromatography in HCC1 and bone marrow stromal (BMS) cells. Ecto-5'-nucleotidase (CD73) expression and activity was determined by RT-PCR, immunocytochemistry, and the cleavage of etheno-AMP to ethenoadenosine. Adenosine receptor expression and activity were determined by RT-PCR and cAMP measurements. The effects of adenosine receptor agonists on IL-6, osteoprotegerin (OPG), and RANKL expression were determined by ELISA and QRT-PCR.


RESULTS
HCC1 and BMS cells produce adenosine and express CD73 and all four adenosine receptor subtypes. The A2b receptor was shown to be functionally dominant in HCC1 cells, as determined by cAMP production and in its stimulation of IL-6 secretion. Adenosine receptor agonism also inhibited OPG secretion and OPG but not RANKL mRNA expression.


CONCLUSIONS
Our findings show that HCC1 and primary BMS cells produce adenosine, express CD73 and all four adenosine receptor subtypes. In HCC1 cells, adenosine has a potent stimulatory action on IL-6 secretion but an inhibitory action on OPG expression. These data show for the first time that adenosine may be an important regulator of progenitor cell differentiation and hence an important local contributor to the regulation of bone formation and resorption.
Evans
Bronwen A J
BA
Department of Child Health, Cardiff University, Cardiff, United Kingdom.
Elford
Carole
C
Pexa
Annette
A
Francis
Karen
K
Hughes
Alis C
AC
Deussen
Andreas
A
Ham
Jack
J
D015720:5'-Nucleotidase / Q000378:metabolism; D000241:Adenosine / Q000819:agonists / Q000096:biosynthesis*; D000243:Adenosine Deaminase / Q000378:metabolism; D000248:Adenosine Kinase / Q000378:metabolism; D001854:Bone Marrow Cells / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D002454:Cell Differentiation; D006023:Glycoproteins / Q000378:metabolism*; D006801:Humans; D015850:Interleukin-6 / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism; D010006:Osteoblasts / Q000166:cytology* / Q000378:metabolism; D053244:Osteoprotegerin; D021381:Protein Transport; D058906:Purinergic P1 Receptor Agonists; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D018047:Receptors, Purinergic P1 / Q000235:genetics / Q000378:metabolism; D018124:Receptors, Tumor Necrosis Factor / Q000378:metabolism*; D013234:Stem Cells / Q000166:cytology / Q000378:metabolism*; D017154:Stromal Cells / Q000378:metabolism
D002352:Carrier Proteins; D006023:Glycoproteins; D015850:Interleukin-6; D008562:Membrane Glycoproteins; D053244:Osteoprotegerin; D058906:Purinergic P1 Receptor Agonists; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; D018160:Receptors, Cytoplasmic and Nuclear; D018047:Receptors, Purinergic P1; D018124:Receptors, Tumor Necrosis Factor; C505686:TNFRSF11A protein, human; C506030:TNFRSF11B protein, human; C505688:TNFSF11 protein, human; D000248:Adenosine Kinase; D015720:5'-Nucleotidase; D000243:Adenosine Deaminase; D000241:Adenosine