Kenji Hiromatsu

Fatty acid–binding protein 4 (FABP4) is a critical immune-metabolic modulator, mainly expressed in adipocytes and macrophages, secreted from adipocytes in association with lipolysis, and plays essential pathogenic roles in cardiovascular and metabolic diseases. We previously reported Chlamydia pneumoniae infecting murine 3T3-L1 adipocytes and causing lipolysis and FABP4 secretion in vitro. However, it is still unknown whether C. pneumoniae intranasal lung infection targets white adipose tissues (WATs), induces lipolysis, and causes FABP4 secretion in vivo. In this study, we demonstrate that C. pneumoniae lung infection causes robust lipolysis in WAT. Infection-induced WAT lipolysis was diminished in FABP4−/− mice or FABP4 inhibitor–pretreated wild-type mice. Infection by C. pneumoniae in wild-type but not FABP4−/− mice induces the accumulation of TNF-α– and IL-6–producing M1-like adipose tissue macrophages in WAT. Infection-induced WAT pathology is augmented by endoplasmic reticulum (ER) stress/the unfolded protein response (UPR), which is abrogated by treatment with azoramide, a modulator of the UPR. C. pneumoniae lung infection is suggested to target WAT and induce lipolysis and FABP4 secretion in vivo via ER stress/UPR. FABP4 released from infected adipocytes may be taken up by other neighboring intact adipocytes or adipose tissue macrophages. This process can further induce ER stress activation and trigger lipolysis and inflammation, followed by FABP4 secretion, leading to WAT pathology. A better understanding of the role of FABP4 in C. pneumoniae infection–induced WAT pathology will provide the basis for rational intervention measures directed at C. pneumoniae infection and metabolic syndrome, such as atherosclerosis, for which robust epidemiologic evidence exists.