Stimulation ofS14mRNA andlipogenesis inbrownfatby hypothyroidism, coldexposure, andcafeteria feeding: Evidence supporting ageneral roleforS14inlipogenesis andlipogenesis in themaintenance ofthermogenesis

Inliver, thyroid hormone rapidly induces S14 mRNA,which encodes asmall acidic protein. This sequence is abundantly expressed only inlipogenic tissues andisthought to havesomefunction infatmetabolism. Intheeuthyroid rat, we measured 20-fold higher levels ofS14mRNAininterscapular brownadipose tisste thanliver. Furthermore, whereas inliver orepididymal fat, hypothyroidism resulted inan80%fall in S14mRNA,inbrownfat thelevel ofthis sequence increased a further 3-fold. Inall three tissues, theexpression ofS14mRNA correlated well with lipogenesis, asassessed by3H20incorpo- ration. Physiological activation ofbrownfatbychronic cold exposure orcafeteria feeding increased theconcentration of S14mRNAinthis tissue andagain this wasaccompanied bya greater rate offatty acid synthesis. Overall, inliver andwhite andbrownadipose tissue, S14mRNAandlipogenesis were well correlated andstrongly suggest afunction oftheS14protein related tofatsynthesis. Thesestudies suggest thattheS14 protein andlipogenesis maybeimportant forthyroid hormone- induced andbrownadipose tissue thermogenesis andthat stimulation ofthese functions inhypothyroid brownfatisa consequence ofdecreased thyroid hormone-induced thermo- genesis elsewhere.