REGULATION OF LIPOPROTEIN-LIPASE TRANSLATION BY EPINEPHRINE IN 3T3-L1CELLS - IMPORTANCE OF THE 3'-UNTRANSLATED REGION

Lipoprotein lipase (LPL) is a central enzyme in lipoprotein metabolism and is in part responsible for adipocyte lipid accumulation. Catechol amines are known to decrease the activity of LPL in adipocytes, and we have previously demonstrated that this inhibition occurs posttranscri ptionally, with a prominent inhibition of LPL translation. To better c haracterize the inhibition of LPL translation, 3T3-L1 cells were diffe rentiated into adipocytes, and exposed to epinephrine. Epinephrine ind uced a dose-dependent decrease in LPL synthesis using [S-35]methionine incorporation, with no change in LPL mRNA levels, demonstrating trans lational regulation of LPL in this cell line. The poly A-enriched RNA from epinephrine-treated cells was translated well in vitro, and there was no difference in the polysome profiles from control and epinephri ne-treated cells, suggesting that epinephrine did not affect mRNA edit ing, and did not induce an inhibition of translation initiation. To ob tain evidence for the presence of an inhibitory factor, a cytoplasmic extract from control, and epinephrine-treated adipocytes was human. Wh en compared to the control cell extract, the epinephrine-treated cell extract sharply inhibited LPL translation in vitro, yet had no effect on the translation of other mRNAs. Epinephrine-treated cells had fourf old more of this inhibitor activity than control cells, and this trans lation inhibition was partially reversed by heat treatment. To determi ne what region of the LPL mRNA was involved in the translation inhibit ion, different LPL constructs were synthesized. The inhibitory effect of the epinephrine-treated cell extract was dependent on the presence of the first 40 nucleotides of the 3' (untranslated region UTR) (nucle otides 1599-1638), whereas deletion of the 5' UTR and other areas of t he 3' UTR had no effect on translation inhibition. When a sense RNA st rand corresponding to this region was added to the in vitro translatio n reaction, it restored translation towards normal, suggesting that th e sense strand was competing for a transacting binding protein. Thus, epinephrine-treated adipocytes produced a transacting factor, probably a protein, that interacted with a region on the LPL mRNA between nucl eotides 1599 and 1638, resulting in an inhibition of translation. Thes e studies add new insight into the hormonal regulation of LPL.