Convergent Synthesis of Novel Muramyl Dipeptide Analogues: Inhibition of Porphyromonas gingivalis-Induced Pro-inflammatory Effects by High Doses of Muramyl Dipeptide

Porphyromonas gingivalis (P.g.)-induced TNF-α can be affected by muramyl
dipeptide
(MDP) in a biphasic concentration-dependent manner. We found that
in P.g.-exposed macrophages, treatment with 10 μg/mL
of MDP (MDP-low) up-regulated TNF-α by 29%, while 100 μg/mL
or higher (MDP-high) significantly decreased it (16% to 38%). MDP-high
was found to affect the ubiquitin-editing enzyme A20 and activator
protein 1 (AP1). An AP1 binding site was found in the promoter region
of A20. A20 promoter activity was up-regulated after transfection
of AP1 cDNA in cells. Four analogues of MDP (3–6) were prepared through a convergent strategy involving the
synthesis of two unique carbohydrate fragments, 7a and 7b, using the peptide coupling reagents, EDCI and HOAt. Analogue 4 improved MDP function and P.g.-induced
activities. We propose a new signaling pathway for TNF-α induction
activated after exposing macrophages to both P.g. and MDP-high or analogue 4.