TRANSLATIONAL REGULATION OF LIPOPROTEIN-LIPASE BY THYROID-HORMONE IS VIA A CYTOPLASMIC REPRESSOR THAT INTERACTS WITH THE 3'-UNTRANSLATED REGION

To better characterize the increase in lipoprotein lipase (LPL) transl ation hy hypothyroidism, adipocytes were prepared from control and hyp othyroid rats. Whereas LPL synthesis was higher in hypothyroid adipocy tes, with no change in mRNA levels, there was no increase in hormone-s ensitive lipase (HSL) synthesis. To determine whether a transacting tr anslation regulatory factor was present a cytoplasmic fraction was pre pared from control and hypothyroid adipocytes, and added to an in vitr o translation system containing the hLPL mRNA. The hypothyroid cell fr action from adipose and heart yielded an increase in LPL translation, when compared to control extracts. Further experiments determined that the control adipocyte extract contained a translation-inhibitory fact or that was 8-fold lower in activity in the hypothyroid extract. Using different LPL mRNA constructs in the in vitro translation reaction, t he region that controlled translation was localized to nucleotides 159 9 to 1638 (proximal 3' untranslated region (UTR)). To confirm rile pre sence of a transacting factor, a sense RNA strand corresponding to thi s region was added to the in vitro translation reaction. This sense st rand competed for the transacting factor in the control cell extract, yet had no effect on the hypothyroid cell extract. Thus, there is a tr anslation repressor factor in the cytoplasm of rat adipocytes, and thi s factor is greatly reduced in activity in hypothyroid rat adipocytes. Because a similar mechanism of LPL regulation occurs in response to e pinephrine, the absence of the translation repressor mar; be a mechani sm for the loss of sensitivity of hypothyroid cells for catecholamines .