The adipokine C1q/TNF-related protein-3 (CTRP-3) inhibits Toll-like receptor (TLR)-induced expression of Cathelicidin antimicrobial peptide (CAMP) in adipocytes.

Background And Aim: CAMP (Cathelicidin antimicrobial peptide) expression in adipocytes is regulated by Toll-like receptor (TLR) agonists. Secreted adipokines such as CTRP-3 have been suggested to participate in innate immune signaling in adipose tissue (AT). This study investigates whether TLR-induced CAMP expression in adipocytes is antagonized by CTRP-3.

Methods: 3T3-L1 adipocytes were co-stimulated with TLR agonists (LPS, MALP-2, Pam3CSK4, pI:C) and recombinant CTRP-3. In a SIRS model, C57BL/6 wild-type mice were intraperitoneally (ip) injected with recombinant CTRP-3 prior to LPS. CAMP expression was analyzed by real-time PCR in AT of wild-type mice and in AT and primary adipocytes from transgenic mice lacking adipocyte CTRP-3 expression. Comparative transcriptome analysis by RNA seq. was applied in CTRP-3 KO adipocytes.

Results: In vitro, CTRP-3 antagonized TLR4- and TLR1/2-induced CAMP expression in adipocytes whereas TLR3- and TLR2/6-mediated induction of CAMP was not affected. in vivo, application of exogenous CTRP-3 dose-dependently antagonized LPS-induced CAMP expression in intra-abdominal AT. CAMP expression in total AT and in primary adipocytes of subcutaneous and intra-abdominal AT did not differ between wild-type mice and transgenic mice lacking adipocyte CTRP-3 expression.

Conclusions: The study suggests a hypothetical role of CAMP in host defense not only against Gram-positive bacteria sensed by TLR1/2 and TLR2/6 but also against Gram-negative bacteria sensed by TLR4 and potentially against viruses sensed by TLR3. The machinery of TLR-mediated pro-inflammatory activation of the CAMP gene in adipocytes seems to be partly modulated by secreted adipokines belonging to the growing family of C1q/TNF-related proteins such as CTRP-3.