We have isolated and characterized in our laboratory a lipocalin specifically binding unsaturated long chain fatty acids (Ex-FABP). In developing chicken embryo long bones, Ex-FABP first appears at the boundary of the cone of hypertrophic cartilage. 'In vitro' EX-FABP is highly expressed by differentiating hypertrophic chondrocytes. Ex-FABP is expressed also in the forming myotubes both 'in vivo' and 'in vitro'. In cultured chondrocytes, Ex-FABP expression is strongly induced by treatment with inflammatory agents such as the bacterial liposaccharide LPS or interleukin-6. The possible mechanism for this induction was investigated. Expression of Ex-FABP was studied in other stress conditions.
To investigate a possible mechanism for Ex-FABP induction by LPS or interleukin-6, we have cultured the cells in the presence of either hydrogen peroxide or the NO donor SNAP (S-nitrosil-acetil-D, L-penicillamine), two agents known to produce cellular stresses through the activation of specific signalling pathways. To investigate Ex-FABP expression in other stress conditions, chondrocytes were cultured for 3 days in the presence of alpha,alpha-dipyridyl, an agent inhibiting prolyl hydroxylase activity and collagen secretion. Supplement of this agent to the culture medium results in an impairment of collagen secretion and assembly and the consequent altered interaction of the cell with the surrounding extracellular matrix. In addition Ex-FABP expression was studied also in chondrocytes cultured in the absence of serum, a stress condition activating cell defence mechanisms.
We have excluded that induction of Ex-FABP expression by inflammatory agents is mediated by oxidative stress or NO production. Ex-FABP expression was induced also by changes in the hypertrophic chondrocyte microenvironment, considered either as extracellular matrix surrounding the cell in culture or as nature and concentration of growth factor in the culture medium.
No definitive data are so far available on the possible role of Ex-FABP when induced by cellular stresses. The capacity of the protein to specifically bind and transport unsaturated long chain fatty acids suggests that lipid metabolism and fatty acid utilization by the cells may be involved. Based on literature data the NRL/N-GAL (neu-related lipocalin/neutrophil gelatinase-associated lipocalin) protein was proposed as a possible mammal counterpart of the chick Ex-FABP. We have suggested that Ex-FABP and NRL/NGAL expression in forming bones and muscles is part of a 'physiological' acute phase response. Interestingly the expression of Ex-FABP and NRL/NGAL is also activated in osteoarthritic cartilage and in the case of NRL/N-GAL during neoplastic transformation of chondrogenic lineage cells.