NOD2 and Cowden disease: Since numerous studies strongly suggest an involvement of bacterial components in CD pathogenesis and since the intracellular receptor for bacterial wall component muramyl dipeptide (MDP) [29], NOD2, is associated with a fistulizing disease course in CD [30], we further assessed the effects of MDP, but also the bacterial component lipopolysaccharide LPS on DKK-1 expression.