Metabolic acidosis stimulates RANKL RNA expression in bone through a cyclo-oxygenase-dependent mechanism.
Metabolic acidosis inhibits osteoblastic bone formation and stimulates osteoclastic resorption. To determine whether acidosis alters expression of RNA for the osteoclastic differentiation factor RANKL, mouse calvariae were incubated in neutral or physiologically acidic media. Acidosis resulted in a significant cyclo-oxygenase-dependent increase in RANKL RNA levels, which would be expected to induce the associated increase in bone resorption.


BACKGROUND
Metabolic acidosis increases net calcium efflux from bone, initially through physicochemical mechanisms and later through predominantly cell-mediated mechanisms. Acidosis decreases osteoblastic bone formation and increases osteoclastic resorption. The growth and maturation of osteoclasts, derived from hematopoietic precursors in the monocyte/macrophage lineage, are dependent on the interplay of a number of factors. Commitment of pre-osteoclasts to osteoclasts is induced by the interaction of the osteoclastic cell-surface receptor RANK with a ligand expressed by osteoblasts, RANKL. The RANK/RANKL interaction not only initiates a differentiation cascade that culminates in mature bone-resorbing osteoclasts but also increases osteoclastic resorptive capacity and survival.


METHODS
To test the hypothesis that metabolic acidosis increases expression of RANKL, we cultured neonatal mouse calvariae in acidic (initial medium pH approximately 7.1 and [HCO3-] approximately 11 mM) or neutral (initial medium pH approximately 7.5 and [HCO3-] approximately 25 mM) medium for 24 and 48 h. We determined the relative expression of RANKL RNA by reverse transcriptase-polymerase chain reaction (RT-PCR) and quantitated the expression by Northern analysis.


RESULTS
In this model of metabolic acidosis, there was significantly increased expression of RANKL RNA at both 24 (2-fold) and 48 h (5-fold) compared with respective controls. Net calcium efflux from bone was also increased in acidic medium compared with control medium. At 48 h, net calcium efflux correlated directly with RANKL expression (r = 0.77, n = 15, p < 0.001). Inhibition of prostaglandin synthesis with indomethacin blocked the acid-induced increase in RANKL RNA as well as the increased calcium efflux.


CONCLUSIONS
Metabolic acidosis induces osteoblastic prostaglandin synthesis, followed by autocrine or paracrine induction of RANKL. This increase in RANKL would be expected to augment osteoclastic bone resorption and help explain the increase in cell-mediated net calcium efflux.
Frick
Kevin K
KK
Department of Medicine, Nephrology Unit, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA. Kevin_Frick@URMC.Rochester.edu
Bushinsky
David A
DA
D000138:Acidosis / Q000201:enzymology* / Q000235:genetics*; D000818:Animals; D001842:Bone and Bones / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002245:Carbon Dioxide / Q000032:analysis; D002352:Carrier Proteins / Q000235:genetics*; D016861:Cyclooxygenase Inhibitors / Q000494:pharmacology; D005786:Gene Expression Regulation; D006863:Hydrogen-Ion Concentration; D007213:Indomethacin / Q000494:pharmacology; D008562:Membrane Glycoproteins / Q000235:genetics*; D051379:Mice; D011451:Prostaglandin-Endoperoxide Synthases / Q000378:metabolism*; D053245:RANK Ligand; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D053246:Receptor Activator of Nuclear Factor-kappa B; D012886:Skull / Q000254:growth & development; D013997:Time Factors
D002352:Carrier Proteins; D016861:Cyclooxygenase Inhibitors; D008562:Membrane Glycoproteins; D053245:RANK Ligand; D012333:RNA, Messenger; D053246:Receptor Activator of Nuclear Factor-kappa B; C505687:Tnfrsf11a protein, mouse; C505689:Tnfsf11 protein, mouse; D002245:Carbon Dioxide; D011451:Prostaglandin-Endoperoxide Synthases; D002118:Calcium; D007213:Indomethacin