Expression and enhancement of FABP4 in septoclasts of the growth plate in FABP5-deficient mouse tibiae

In our previous study, fatty acid-binding protein 5 (FABP5) was expressed in septoclasts with long processes which are considered to resorb uncalcified matrix of the growth plate (GP) cartilage, and no apparent abnormalities were detected in the histo-architecture of the GP of FABP5-deficient (FABP5^−/−) mice. Those finding lead us to hypothesize that another FABP can compensate the deletion of FABP5 in septoclasts of its gene-mutant mice. Based on the hypothesis, the present study examined the expression levels of several other FABPs in septoclasts and their morphology in FABP5^−/− mouse tibiae. Processes of FABP5^−/− septoclasts tend to be shorter than wild septoclasts. FABP4-positive septoclasts in FABP5^−/− mice were more numerous than those cells in wild mice. Peroxisome proliferator-activated receptor (PPAR) γ was expressed in FABP4-positive septoclasts of FABP5^−/− mice as well as mice administered with GW1929, a PPARγ agonist, suggesting that the occurrence of PPARγ induces an increase of FABP4-positive septoclasts. The present finding suggests that the functional exertion of FABP5 in septoclasts is supplemented by FABP4 in normal and FABP5^−/− mice, and that the expression of FABP4 is up-regulated in accompany with PPARγ in FABP5^−/− for maintenance of resorptive activity in the GP.