Regulation by retinoic acid of acylation-stimulating protein and complement C3 in human adipocytes

Acylation-stimulating protein (ASP), a product of complement C3, stimulates triacylglycerol synthesis in adipocytes. Previous studies have identified transthyretin, associated with chylomicrons, as a stimulator of C3 and ASP production. Since both transthyretin and chylomicrons transport retinyl ester/retinol, our goal was to investigate whether retinoic acid (RA) could be a potential hormonal mediator of the effect. Inhibitors of protein synthesis and protein secretion eliminated the stimulatory effects of chylomicrons on both C3 and ASP production in human differentiated adipocytes, suggesting that de novo protein synthesis and secretion are both required. Incubation with chylomicrons increased C3 mRNA levels (37±1.5%). RA alone or with chylomicrons had a stimulatory effect on C3 production (29-fold at 16.6nM RA) and ASP production. An RA receptor antagonist blocked stimulation of C3 mRNA and C3 secretion by both RA and chylomicrons. Finally, RA and chylomicrons activated a 1.8kb C3-promoter–luciferase construct transfected into 3T3-F442 and 3T3-L1 cells (by 41±0.2% and 69±0.3% respectively), possibly via RA receptor half-sites identified by sequence analysis. This is the first evidence documenting stimulation by RA of the C3 gene. Thus we propose RA as a novel cellular trigger in chylomicrons that subsequently results in increased ASP production by adipocytes after a meal.