SYNTHESIS AND ACCUMULATION OF A RECEPTOR REGULATORY PROTEIN ASSOCIATED WITH LIPID DROPLET ACCUMULATION IN 3T3-L1 CELLS

Synthesis and accumulation of the recently identified prostaglandin F- 2 alpha receptor regulatory protein (FPRP) was found to correlate clos ely with lipid droplet accumulation by 3T3-L1 preadipose cells. FPRP, a transmembrane glycoprotein, has been shown to regulate the binding o f ligand to certain seven-transmembrane receptors, Anti-FPRP immunohis tochemistry, Western blotting, and metabolic labeling/immunoprecipitat ion experiments demonstrated that FPRP was not detectable in undiffere ntiated 3T3-L1 cells. Interestingly, low levels of FPRP mRNA were dete cted in the undifferentiated 3T3-L1 cells. After induction of adipose differentiation, FPRP mRNA increased similar to 3 fold whereas FPRP sy nthesis increased similar to 50 fold. Differentiation induction with e ither dexamethasone/insulin/isobutylmethylxanthine or the thiazolidine dione derivative ADD 4743 were both effective at inducing FPRP accumul ation and accumulation of lipid droplets. By co-immunohistochemical an d lipid staining, greater than 99% of the cells accumulating lipid dro plets possessed FPRP, FPRP mRNA and protein are also found in rat adip ose tissue, Treatment of 3T3-L1 cells with an FPRP anti-sense oligonuc leotide during differentiation decreased FPRP accumulation and resulte d in a decrease in lipid droplets without altering the level of induct ion of a late marker of adipocyte differentiation, glycerol-3-phosphat e dehydrogenase activity. Transient expression of an FPRP cDNA in undi fferentiated 3T3-L1 cells was insufficient to induce lipid droplet acc umulation.