Serotonergic 5-HT2B Receptor Controls Tissue-nonspecific Alkaline Phosphatase Activity in Osteoblasts via Eicosanoids and Phosphatidylinositol-specific Phospholipase C

Abstract

In previous studies, we observed that mice knocked out for the serotonin-2B receptor (5-HT 2B R) show defects in bone homeostasis. The present work focuses on the downstream targets relaying the anabolic function of this receptor in osteoblasts. A functional link between the 5-HT 2B R and the activity of the tissue-nonspecific alkaline phosphatase (TNAP) is established using the C1 osteoprogenitor cell line. During C1 osteogenic differentiation, both 5-HT 2B R and TNAP mRNA translations are delayed with respect to extracellular matrix deposition. Once the receptor is expressed, it constitutively controls TNAP activity at a post-translational level along the overall period of mineral deposition. Indeed, pharmacological inhibition of the 5-HT 2B R intrinsic activity or shRNA-mediated 5-HT 2B R knockdown prevents TNAP activation, but not its mRNA translation. In contrast, agonist stimulation of the receptor further increases TNAP activity during the initial mineralization phase. Building upon our previous observations that the 5-HT 2B R couples with the phospholipase A2 pathway and prostaglandin production at the beginning of mineral deposition, we show that the 5-HT 2B R controls leukotriene synthesis via phospholipase A2 at the terminal stages of C1 differentiation. These two 5-HT 2B R-dependent eicosanoid productions delineate distinct time windows of TNAP regulation during the osteogenic program. Finally, prostaglandins or leukotrienes are shown to relay the post-translational activation of TNAP via stimulation of the phosphatidylinositol-specific phospholipase C. In agreement with the above findings, primary calvarial osteoblasts from 5-HT 2B R-null mice exhibit defects in TNAP activity.Imbalances in bone formation or resorption frequently result in pathological situations, such as osteoporosis, osteopenia, or osteomalacia. Proper skeletal development and bone homeostasis notably necessitate a tight regulation of mineralization. Over the past few years, growing attention has been paid to the involvement of serotonin (5-hydroxytryptamine, 5-HT) 2 in bone biology (1). Indeed, beyond its role as a neurotransmitter in brain (2), platelet-stored 5-HT broadly participates to the homeostasis of various tissues. For instance, 5-HT regulates cardiovascular, smooth muscle, and endocrine functions (for review, see Ref.3).In the periphery, 5-HT is synthesized exclusively by enterochromaffin cells of the gut and stored by platelets. Thus, because they express the serotonin transporter and diverse serotonergic receptors (4 -6), osteoblasts may be directly influenced by circulating 5-HT. In agreement with this idea, inhibition of the serotonin transporter reduces bone formation (7, 8). Moreover, patients receiving selective serotonin reuptake inhibitor antidepressants appear to be at risk for osteoporosis (9). Five types of serotonergic receptors have been depicted in osteoblast primary cultures or osteoblastic cell lines: 5-HT 1A , 5-HT 1B , 5-HT 1D , 5-HT 2A , and 5-HT 2B (4, 6, 10, 11). For instance, focusing on the 5-HT ...

The Expression of Groups IIE and V Phospholipase A2 is Associated with an Increased Expression of Osteogenic Molecules in Human Calcified Aortic Valves

“…Another report showed that PGE2 stimulates the osteogenic differentiation of cultured tendon stem cells via BMP-2 induction 18) . Taken together, these previous reports 2,17,18) appear to support the hypothesis that eicosanoids produced via the PLA2 pathway contribute to the process of ectopic osteogenesis in cardiac valves.…”

“…In line with the present findings, a previous report showed that the 5-LOX expression is increased in calcified human aortic valves and that the incubation of myofibroblast cultures with LTC4, a 5-LOX metabolite, induces calcium nodule formation 2) . In cultures of tendon stem cells, serotonergic receptor signals have been shown to stimulate tissuenonspecific ALP through the actions of eicosanoids produced via the PLA2 pathway 17) . Another report showed that PGE2 stimulates the osteogenic differentiation of cultured tendon stem cells via BMP-2 induction 18) .…”

Modulation of osteoblast differentiation and bone mass by 5-HT2A receptor signaling in mice

“…5-HT 2B receptor expression during osteoblast differentiation in vitro was shown to be increased in primary cultured mouse osteoblasts (Hirai et al, 2009), and the ablation of the 5-HT 2B receptor in female mice led to osteopenia due to impaired bone formation (Collet et al, 2008). In addition, 5-HT 2B receptor signaling in osteoblasts was found to regulate cellular proliferation, differentiation, and functions (Baudry et al, 2010;Locker et al, 2006;Li et al, 2013). Previous studies e confirmed the pattern of expression for the 5-HT receptor in osteoblastic cells (Hirai et al, 2009).…”

FOXO1 orchestrates the bone-suppressing function of gut-derived serotonin

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