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) gamma was expressed in FABP4-positive septoclasts of FABP5(-/-) mice as well as mice administered with GW1929, a PPAR gamma agonist, suggesting that the occurrence of PPAR gamma 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 PPAR gamma in FABP5(-/-) for maintenance of resorptive activity in the GP.