RANKL enhances the expression of PEPT1/SLC15A1 and PEPT2/SLC15A2 in RAW264.7 cells

Muramyl dipeptide (MDP), a bacterial cell wall component, is recognized by NOD2 and vital in innate immune responses, including inflammatory cytokine production. MDP is transported into the cells via PEPT1/SLC15A1 and PEPT2/SLC15A2, which are members of the proton-coupled oligopeptide transporter family within the SLC15 solute carrier group. Although the effects of RANKL stimulation on certain transporters are known, its effects on the SLC15 family remain unclear. This study aimed to clarify the effects of RANKL stimulation on PEPT1/SLC15A1 and PEPT2/SLC15A2 in RAW264.7 cells and determine their role in osteoclast differentiation.

RAW264.7 cells were stimulated with RANKL and MDP. Expression levels of NOD2, PEPT1/SLC15A1, PEPT2/SLC15A2, cathepsin K, and NFATc1 were analyzed via Western blotting. Osteoclast differentiation was evaluated using a tartrate-resistant acid phosphatase (TRAP) activity assay.

RANKL stimulation increased NOD2, PEPT1/SLC15A1, and PEPT2/SLC15A2 expression in RAW264.7 cells. Colistin and polymyxin B, which are PEPT1 and PEPT2 inhibitors, respectively, did not affect the stimulation of cells with RANKL alone. However, RANKL and MDP co-stimulation suppressed the RANKL/MDP-induced increase in TRAP activity and cathepsin K and NFATc1 expression.

RANKL stimulation increased PEPT1/SLC15A1 and PEPT2/SLC15A2 levels in RAW264.7 cells, suggesting an increase in the intracellular uptake of MDP. This may promote osteoclast differentiation, potentially through NOD2activation or NOD2-independent mechanisms.