Chlamydia pneumoniae infection–induced endoplasmic reticulum stress causes fatty acid–binding protein 4 secretion in murine adipocytes

Fatty acid–binding protein 4 (FABP4) is predominantly expressed in adipocytes and macrophages and regulates metabolic and inflammatory pathways. FABP4 is secreted from adipocytes during lipolysis, and elevated circulating FABP4 levels are associated with obesity, metabolic disease, and cardiac dysfunction. We previously reported that the bacterial respiratory pathogen Chlamydia pneumoniae infects murine adipocytes and exploits host FABP4 to mobilize fat and replicate within adipocytes. However, whether C. pneumoniae induces FABP4 secretion from adipocytes has not been determined. Here, we show that FABP4 is actively secreted by murine adipocytes upon C. pneumoniae infection. Chemical inhibition of lipase activity and genetic deficiency of hormone-sensitive lipase blocked FABP4 secretion from C. pneumoniae–infected adipocytes. Mechanistically, C. pneumoniae infection induced endoplasmic reticulum (ER) stress and the unfolded protein response (UPR), resulting in elevated levels of mitochondrial reactive oxygen species and cytosolic Ca2+. Of note, exposure to a mitochondrial reactive oxygen species–specific scavenger, MitoTEMPO, reduced FABP4 release from C. pneumoniae–infected adipocytes. Furthermore, treatment with azoramide, which protects cells against ER stress, decreased FABP4 release from C. pneumoniae–infected adipocytes. Using gene silencing of CHOP (C/EBP homologous protein), a central regulator of ER stress, we further validated the role of C. pneumoniae infection–induced ER stress/UPR in promoting FABP4 secretion. Overall, these results indicate that C. pneumoniae infection robustly induces FABP4 secretion from adipocytes by stimulating ER stress/UPR. Our findings shed additional light on the etiological link between C. pneumoniae infection and metabolic syndrome.