Serotonin transporter and receptor expression in osteocytic MLO-Y4 cells

Neurotransmitter regulation of bone metabolism has been a subject of increasing interest and investigation. We reported previously that osteoblastic cells express a functional serotonin (5-HT) signal transduction system, with mechanisms for responding to and regulating uptake of 5-HT. The clonal murine osteocytic cell line, MLO-Y4, demonstrates expression of the serotonin transporter (5-HTT), and the 5-HT_1A, and 5-HT_2A receptors by real-time RT-PCR and immunoblot analysis. Immunohistochemistry using antibodies for the 5-HTT, and the 5-HT_1A and 5-HT_2A receptors reveals expression of all three proteins in both osteoblasts and osteocytes in rat tibia. 5-HTT binding sites were demonstrated in the MLO-Y4 cells with nanomolar affinity for the stable cocaine analog [¹²⁵I]RTI-55. Imipramine and fluoxetine, antagonists with specificity for 5-HTT, show the highest potency to antagonize [¹²⁵I]RTI-55 binding in the MLO-Y4 cells. GBR-12935, a relatively selective dopamine transporter antagonist, had a much lower potency, as did desipramine, a selective norepinephrine transporter antagonist. The maximal [³H]5-HT uptake rate in MLO-Y4 cells was 2.85 pmol/15 min/well, with a K_m value of 290 nM. Imipramine and fluoxetine inhibited specific [³H]5-HT uptake with IC₅₀ values in the nanomolar range. 5-HT rapidly stimulated PGE₂ release from MLO-Y4 cells; the EC₅₀ for 5-HT was 0.1 μM, with a 3-fold increase seen at 60 min. The rate-limiting enzyme for serotonin synthesis, tryptophan hydroxylase, is expressed in MLO-Y4 cells as well as osteoblastic MC3T3-E1 cells. Thus, osteocytes, as well as osteoblasts, are capable of 5-HT synthesis, and express functional receptor and transporter components of the 5-HT signal transduction system.