Hepatic translation control in the late-gestation fetal rat

Abstract

We have investigated the regulation of translation during the period of rapid liver growth that occurs at the end of gestation in the rat. This work was based on our prior observation that fetal hepatocyte proliferation is resistant to the inhibitory effects of rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), a nutrient-sensing kinase that controls ribosome biogenesis and protein translation. We hypothesized that translation control in late-gestation fetal liver differs from that in adult liver. We first examined the ability of rapamycin to inhibit the translation of mRNAs encoding ribosomal proteins. Consistent with the effect of rapamycin on proliferation, the activation of adult liver 5′-terminal oligopyrimidine tracts (5′-TOP) translation that occurred during refeeding after food deprivation was sensitive to rapamycin. Fetal liver 5′-TOP translation was insensitive. We went on to examine the eukaryotic initiation factor (eIF) 4F cap-binding complex that controls global protein synthesis. The molecular weights of the multiple eIF4G1 isoforms present in fetal and adult liver eIF4F complexes differed. In addition, fetal liver expressed the eIF4A1 form of the eIF4A helicase, whereas adult liver contained eIF4A1 and eIF4A2. Rapamycin administration before refeeding in adult rats inhibited formation of the preinitiation complex to a much greater degree than rapamycin administration to fetal rats in situ. We conclude that there are major structural and functional differences in translation control between late-gestation fetal and adult liver. These differences may confer differential sensitivity to the growth inhibitory effects of rapamycin.

Regulation of fetal liver growth in a model of diet restriction in the pregnant rat

“…Profiling of the fetal translatome in our present studies did not disclose an IUGR-associated effect on 5=TOP translation. Our previous studies on mechanisms of hepatic translation control in the late-gestation fetal rat (28) showed marked fetal-adult differences in the composition of the cap-binding translation initiation complex. These included the identification of isoforms of the cap-binding complex scaffolding protein, eIF4G, that were specific to fetal liver and fetal-adult isoform differences in the eIF4A helicase.…”

Profiling of the fetal and adult rat liver transcriptome and translatome reveals discordant regulation by the mechanistic target of rapamycin (mTOR)

“…RNA was further purified using the RNeasy Mini Kit (Qiagen, Valencia, CA). Translating polysomes were prepared and isolated by sucrose density centrifugation (14). RNA was extracted from pooled polysome preparations by acid-phenol:chloroform extraction followed by precipitation in ethanol plus ammonium acetate.…”

“…Several years ago, we made the observation that late gestation fetal hepatocytes in vivo were resistant to the growth inhibitory effects of rapamycin (5). We went on to examine the hepatic translation control apparatus in the late gestation fetus (14). The results of our studies indicated resistance to the growth inhibitory effect of rapamycin at the level of G1 cell-cycle progression (13) and translation of 5=TOP mRNAs (12,14).…”

“…We went on to examine the hepatic translation control apparatus in the late gestation fetus (14). The results of our studies indicated resistance to the growth inhibitory effect of rapamycin at the level of G1 cell-cycle progression (13) and translation of 5=TOP mRNAs (12,14). We have also had a long-standing interest in the regulation of hepatic gene expression by mTOR (13,19,21).…”

The physiology and pathophysiology of rapamycin resistance

“…This hypothesis was tested using the model of direct administration of rapamycin to late gestation (ED19) fetal rats in situ. 32 We first examined the ability of rapamycin to inhibit the translation of mRNAs encoding ribosomal proteins; that is, those mRNAs containing 5'-terminal oligopyrimidine (5'TOP) tracts. As expected, the activation of hepatic translation of 5'TOP mRNAs that occurred during refeeding after food deprivation was sensitive to rapamycin.…”