AN AMINO-TERMINAL FIBRONECTIN FRAGMENT STIMULATES THE DIFFERENTIATIONOF ST-13 PREADIPOCYTES

Differentiation of ST-13 preadipocytes into adipocytes was inhibited a lmost completely by addition of rat plasma fibronectin (FN) (approxima tely 100 mg/mL), but was reversed by GRGDSP cell recognition peptide ( 1.5 mM) and anti-alpha5beta1. On the contrary, the thermolysin digest of FN stimulated adipocyte differentiation in a dose-dependent manner, in which remarkable increases in the values of the differentiation in dexes, the number of adipocytes (8-fold above the control), glyceropho sphate dehydrogenase (GPD) activity (12-fold), and triacylglycerol con tent (5-fold), were observed by inclusion of the thermolysin digest (1 00 mug/mL). The increase in GPD activity by the thermolysin digest was inhibited remarkably (about 70% inhibition) by an antibody directed t o the amino-terminal fibrin-binding (Fib 1) domain of FN and slightly (about 15%) by an antibody directed to the central cell-binding (Cell) domain, but not by anti-gelatin-binding domain and anti-carboxy-termi nal fibrin-binding domain. Treatment of ST-13 cells by a purified 24K fragment (100 mug/mL) derived from the Fib 1 domain caused an over 20- fold augmentation of the GPD activity, accounting for a major part of the differentiation stimulatory activity of the thermolysin digest. Th e differentiation stimulatory effect of the 24K Fib 1 fragment was not affected by either GRGDSP peptide or anti-alpha5beta1). Thus, FN can regulate adipose development of ST-13 cells by its two antipodal, inhi bitory and stimulatory, activities, the latter of which is expressed o nly upon fragmentation. Proteolytic cleavage of FN may play an importa nt role in controlling the action of FN on adipocyte differentiation.