REGULATION OF THE MURINE ADIPOCYTE FATTY-ACID TRANSPORTER GENE BY INSULIN

A cDNA encoding a novel fatty acid transport protein (FATP) was identi fied recently using expression cloning methodologies. We have studied the expression of FATP in differentiating 3T3-L1 cells and adipose tis sue in vivo. When 313-L1 preadipocytes are treated with a combination of methylisobutylxanthine, dexamethasone, and insulin to induce differ entiation, the abundance of FATP mRNA decreases within 24 h to less th an one-third that of preadipocytes and increases subsequently, such th at mature adipocytes have 5-7 times more FATP mRNA than fibroblastic p recursors. In fully differentiated 3T3-L1 adipocytes, insulin alone is sufficient to down-regulate FATP mRNA levels 10-fold. The concentrati on of insulin necessary for half-maximal repression (I-0.5) is similar to 1 nM and is specific for insulin; insulin-like growth factor I (IG F-I) has little effect at similar concentrations. Kinetic analysis ind icates that the reduction in expression of FATP mRNA by insulin is rap id (t1/2 = similar to 4 h) and reversible upon withdrawal of insulin. The half-lives of FATP mRNA are 2.9 h and 4.4 h in the absence and pre sence of insulin, respectively. The insulin-mediated decrease in FATP steady state mRNA level correlates with a decrease in its transcriptio n rate as measured by nuclear run-on transcription assay. To determine whether physiological conditions that alter insulin concentration in vivo affect adipose FATP levels, feeding/fasting studies are employed. Fasting of C57BL/6J mice for 48 h results in an Ii-fold up-regulation of FATP mRNA expression in adipose tissue. Refeeding of fasted animal s for 72 h results in a return of FATP mRNA to basal levels. In sum, t hese results indicate that the expression of FATP gene is negatively r egulated by insulin at the transcriptional level in cultured adipocyte s and that transporter mRNA expression in murine adipose tissue is alt ered in a manner consistent with insulin being a negative regulator of gene activity.