Regulatory mechanism of CCN2 production by serotonin (5-HT) via 5-HT_2A and 5-HT_2B receptors in chondrocytes

Serotonin (5-hydroxytryptamine: 5-HT) is recognized as a neurotransmitter in the central nerve system and as a regulator of systemic blood pressure in the peripheral tissues. Recently, it was reported that 5-HT₂ receptors (5-HT₂Rs) were expressed in cartilage tissues lacking both vessels and neurons, suggesting possible novel functions of 5-HT during cartilage development and regeneration. Our previous data indicated that CCN family protein 2/connective tissue growth factor (CCN2/CTGF) plays a central role in cartilage development and regeneration. Therefore, the aim of this study was to investigate the effect of 5-HT on the production of CCN2 in chondrocytes. Firstly, we showed that the mRNAs of 5-HT₂R subtypes 5-HT_2AR and 5-HT_2BR, were expressed in a human chondrocytic cell line, HCS-2/8; however, 5-HT_2CR mRNA was not detected. In addition, exogenously added 5-HT did not affect the 5-HT_2AR and 5-HT_2BR expressions. Next, we demonstrated that CCN2 production was increased by treatment with a 5-HT_2AR agonist and the combination of 5-HT and 5-HT_2BR antagonist. In contrast, treatment with a 5-HT_2BR agonist and the combination of 5-HT and 5-HT_2AR antagonist decreased CCN2 production. Furthermore, we showed that phosphorylation of Akt and p38 MAPK were increased by treatment with 5-HT_2AR agonist, and that phosphorylation of PKCε, PKCζ, ERK1/2 and JNK were increased by treatment with 5-HT_2BR agonist. Finally, we found that 5-HT_2AR was localized in the growth plate, whereas 5-HT_2BR was localized in the articular cartilage. These findings suggest that 5-HT promotes CCN2 production through the 5-HT_2AR in growth plates, and that it represses CCN2 production through the 5-HT_2BR in articular cartilage for harmonized development of long bones.