Nucleotide oligomerization domains 1 and 2: Regulation of expression and function in preadipocytes

Translocation of bacteria into the mesenteric fat during intestinal inflammation and the expression of functional TLR1-9 in murine preadipocytes and adipocytes suggest an active role for these cells in innate immunity. The present study focuses on nucleotide oligomerization domains I and 2 representing intracellular pattern recognition receptors that sense motifs derived from bacterial peptidoglycans. On mRNA level nucleotide oligomerization domain I was found to be constitutively expressed in the preadipocyte cell line 3T3L1 and in primary preadipocytes isolated from murine mesenteric fat, while nucleotide oligomerization domain 2 was only weakly expressed by these cells. Treatment with lactyl-tetra-diaminopimelic acid, muramyl dipeptide, LPS, IL-1 beta, and TNF-alpha did not affect cellular nucleotide oligomerization domain 1 mRNA amounts. Except muramyl dipeptide, all factors significantly increased nucleotide oligomerization domain 2 mRNA in mesenteric fat preadipocytes after 4 h. However, specific stimulation of nucleotide oligomerization domain I induced IL-6 synthesis in preadipocytes from wild-type or TLR2/4-deficient mice. Confirming nucleotide oligomerization domain I specificity, transfection of nucleotide oligomerization domain I-specific small interfering RNA significantly blocked the effect of lactyl-tetra-diaminopimelic acid on IL-6 production. With specific inhibitors and a NF-kappa B reporter plasmid, nucleotide oligomerization domain 1-mediated activation of NF-kappa B was shown to be responsible for the induction of IL-6 in preadipocytes. In addition, expression of functional nucleotide oligomerization domain I could be confirmed in primary human preadipocytes. In summary, we here identified preadipocytes as a novel cell population expressing nucleotide oligomerization domains 1 and 2. Not regulated on transcriptional level, nucleotide oligomerization domain I in preadipocytes serves as a sensor for bacterial degradation products and triggers proinflammatory effector responses. Thus, our results further strengthen the allocation of the mesenteric fat and especially of preadipocytes to the innate immune system.