Roles of monocarboxylate transporter subtypes in promotion and suppression of osteoclast differentiation and survival on bone

Monocarboxylate transporters(MCTs) provide transmembrane transport of monocarboxylates such as lactate and pyruvate. The present results showed that α-cyano-4- hydroxycinnamic acid(CHC), an inhibitor of MCTs, promoted osteoclastdifferentiation from macrophages at lower concentrations (0.1–0.3 mM) and suppressed that at a higher concentration (1.0 mM). On the other hand, CHC reduced the number of mature osteoclastson the surface of dentin in a concentration-dependent manner. Additionally, macrophages and osteoclastswere found to express the Mct1, Mct2, and Mct4 genes, with Mct1 and Mct4 expression higher in macrophages, and that of Mct2 higher in osteoclasts. Although Mct1 gene knockdownin macrophages enhanced osteoclastformation induced by RANKL, Mct2 gene knockdownsuppressed that. Finally, Mct2 gene silencingin mature osteoclastsdecreased their number and, thereby, bone resorption. These results suggest that MCT1 is a negative regulator and MCT2 a positive regulator of osteoclastdifferentiation, while MCT2 is required for bone resorptionby osteoclasts.