CYCLIC-AMP AND FATTY-ACIDS INCREASE CARNITINE PALMITOYLTRANSFERASE-I GENE-TRANSCRIPTION IN CULTURED FETAL-RAT HEPATOCYTES

In the rat, the gene for liver mitochondrial carnitine palmitoyltransf erase I (CPT I), though dormant prior to birth, is rapidly activated p ostnatally. We sought to elucidate which hormonal and/or nutritional f actors might be responsible for this induction. In cultured hepatocyte s from 20-day-old rat fetus, the concentration of CPT I mRNA, which in itially was very low, increased dramatically in a dose-dependent manne r after exposure of the cells to dibutyryl cAMP (Bt(2)cAMP). Similar r esults were obtained when long-chain fatty acids (LCFA), but not mediu m-chain fatty acids, were added to the culture medium. The effects of Bt(2)cAMP and LCFA were antagonized by insulin, also dose dependently. In contrast, CPT II gene expression, which was already high in fetal hepatocytes, was unaffected by any of the above manipulations. Bt(2)cA MP stimulated CPT I gene expression even when endogenous triacylglycer ol breakdown was suppressed by lysosomotropic agents suggesting that t he actions of cAMP and LCFA were distinct. Moreover, half-maximal conc entrations of Bt(2)cAMP and linoleate produced an additive effect on C PT I mRNA accumulation. While linoleate and Bt,cAMP stimulated CPT I g ene transcription by twofold and fourfold, respectively, the fatty aci d also increased the half-life of CPT I mRNA (50%). When hepatocytes w ere cultured in the presence of 2-bromopalmitate, (which is readily co nverted by cells into its non-metabolizable CoA ester) CPT I mRNA accu mulation was higher than that observed with oleate or linoleate. Simil arly, the CPT I inhibitor, tetradecylglycidate, which at a concentrati on of 20 I-IM did not itself influence the CPT I mRNA level, enhanced the stimulatory effect of linoleate. The implication is that induction of the CPT I message by LCFA does not require mitochondrial metabolis m of these substrates; however, formation of their CoA esters is a nec essary step. Unlike linoleate, the peroxisome proliferator, clofibrate , increased both CPT I and CPT II mRNA levels and neither effect was o ffset by insulin. It thus appears that the mechanism of action of LCFA differs from that utilized by clofibrate, which presumably works thro ugh the peroxisome proliferator activated receptor. We conclude that t he rapid increase in hepatic CPT I mRNA level that accompanies the fet al to neonatal transition in the rat is triggered by the reciprocal ch ange in circulating insulin and LCFA concentrations, coupled with elev ation of the liver content of cAMP.