Serotonin regulates osteoclast differentiation through its transporter.
5-HTT mediates antidepressant-sensitive clearance of 5-HT after its release into neural synapses. We found increased expression of 5-HTT in RANKL-induced osteoclast-like cells. Fluoxetine, an inhibitor of 5-HTT, reduced osteoclast differentiation but not activation. Reserpine, an inhibitor of 5-HT intracellular transport, potentiated differentiation. These results indicate a role for 5-HTT in osteoclast function and suggest that commonly used antidepressive agents may affect bone mass.


BACKGROUND
Interactions between the serotonergic and skeletal systems are suggested by various clinical observations but are poorly understood.


METHODS
Using gene microarrays, we found that the serotonin transporter (5-HTT) was strongly expressed in RANKL-induced osteoclasts. Using RANKL stimulation of RAW264.7 cells and mouse bone marrow cells as a model system for osteoclast differentiation, we studied the possible role/s of the different components of the serotonin (5-HT) system on the differentiation process.


RESULTS
Osteoclast 5-HTT exhibited typical 5-HT uptake activity that was inhibitable by fluoxetine (Prozac). Fluoxetine reduced osteoclast differentiation but did not inhibit the activation of preformed osteoclasts, whereas the addition of 5-HT itself enhanced differentiation. Fluoxetine-treated osteoclast precursors had reduced NF-kappa B activation and elevated inhibitory protein kappa B alpha (I kappa B alpha) levels compared with untreated cells. 5-HT, on the other hand, resulted in activation of NF-kappa B. Reserpine inhibition of intracellular transport of 5-HT into cytoplasmic vesicles potentiated RANKL-induced osteoclast formation, suggesting the importance of intracellular 5-HT in regulating osteoclast differentiation. Reserpine also modestly enhanced the expression of the osteoclast marker TRACP in the absence of RANKL.


CONCLUSIONS
Taken together, these data suggest that the 5-HT system plays an important role in bone homeostasis through effects on osteoclast differentiation and implies that commonly used antidepressive agents may affect bone mass.
Department of Cytokine Biology, The Forsyth Institute, Boston, Massachusetts 02115, USA.
Ricardo
R
Battaglino
Jia
J
Fu
Ulrike
U
Späte
Ulku
U
Ersoy
Martha
M
Joe
Leela
L
Sedaghat
Philip
P
Stashenko
D000818:Animals; D001854:Bone Marrow Cells / Q000166:cytology; D002352:Carrier Proteins / Q000378:metabolism; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D005473:Fluoxetine / Q000494:pharmacology; D008264:Macrophages / Q000502:physiology; D008562:Membrane Glycoproteins / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000378:metabolism*; D051379:Mice; D016328:NF-kappa B / Q000037:antagonists & inhibitors / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D010010:Osteoclasts / Q000166:cytology* / Q000502:physiology*; D053245:RANK Ligand; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D053246:Receptor Activator of Nuclear Factor-kappa B; D011985:Receptors, Serotonin / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism*; D012110:Reserpine / Q000494:pharmacology; D012701:Serotonin / Q000378:metabolism* / Q000494:pharmacology; D050486:Serotonin Plasma Membrane Transport Proteins
D002352:Carrier Proteins; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D016328:NF-kappa B; D009419:Nerve Tissue Proteins; D053245:RANK Ligand; D012333:RNA, Messenger; D053246:Receptor Activator of Nuclear Factor-kappa B; D011985:Receptors, Serotonin; D011994:Recombinant Proteins; D050486:Serotonin Plasma Membrane Transport Proteins; C493231:Slc6a4 protein, mouse; C505687:Tnfrsf11a protein, mouse; C505689:Tnfsf11 protein, mouse; D005473:Fluoxetine; D012701:Serotonin; D012110:Reserpine