Regulation of Cathelicidin Antimicrobial Peptide (CAMP) Gene Expression by TLR9 in Adipocytes and in Adipose Tissue

Abstract

Understanding the complex interactions between metabolism and the immune system (“meta-flammation”) is crucial for the identification of key immunomodulatory factors as potential therapeutic targets in obesity and in cardiovascular diseases. Cathelicidin antimicrobial peptide (CAMP) is an important factor of innate immunity and is expressed in adipocytes. CAMP therefore might play a role as an adipokine in metaflammation and adipose inflammation. Toll-like receptor (TLR) 9 is another component of the innate immune system that is expressed and functionally active in adipocytes. The aim of the present study was to investigate the impact of TLR9 signaling on CAMP expression in adipocytes and in adipose tissue.
 CAMP gene expression in murine 3T3-L1 and human SGBS adipocytes and in murine and human adipose tissues was quantified by real-time PCR. TLR9 knockdown was applied in murine 3T3-L1 adipocytes via siRNA transfection. Adipocyte inflammation was induced in vitro by TNFα stimulation. Serum CAMP concentrations in TLR9 knockout (KO) and in wildtype mice were quantified by ELISA. 
 CAMP gene expression was considerably increased in 3T3-L1 adipocytes during differentiation. TNFα significantly induced CAMP gene expression in mature adipocytes, which was antagonized by inhibitors of NF-κB and PI3K signaling. Cell-free nucleic acids (cfDNA) as endogenous TLR9 ligand significantly impaired CAMP gene expression, whereas synthetic agonistic and antagonistic TLR9 ligands had no effect. Cellular TLR9 knockdown reduced adipocyte CAMP gene expression in vitro and male TLR9 knockout mice exhibited lower systemic CAMP concentrations than wildtype mice. CAMP and TLR9 gene expression were correlated positively in murine and human subcutaneous but not in intraabdominal/visceral adipose tissues.
 These findings suggest a regulatory role of TRL9 in adipocytic CAMP expression as a novel putative molecular mechanism in adipose tissue innate immunity.