Dietary sucrose enhances processing of mRNA-S14 nuclear precursor

The rapid response of rat hepatic mRNA-S14 to hormonal or dietary manipulation makes it an excellent model to study the control of lipogenic enzyme mRNA. The mechanism of regulation of this mRNA by triiodothyronine (T3) or sucrose remains controversial. Although initial studies suggested that T3 stabilized the nuclear precursor, subsequent studies suggest that T3 acts by increasing the transcriptional rate of this gene. More recently, the induction of mRNA-S14 by sucrose administration was shown to be associated with an increase in transcriptional "run-on" activity. Because T3 and carbohydrate feeding synergistically regulate this mRNA, we studied the response to short and long term high carbohydrate feeding in hypothyroid and euthyroid rats. We found the response to the lipogenic diet was rapid in hypothyroid rats, with maximal levels of mRNA-S14 attained by 4 h (2.2 +/- 0.6 chow fed versus 13.5 +/- 2.5 pg/micrograms RNA on lipogenic diet). The rapid induction by the lipogenic diet contrasts with the diminished response to sucrose by gastric gavage (4.6 +/- 1.2 pg/micrograms RNA) over the same time interval. Despite the large increase in the mature mRNA induced by the lipogenic diet, the rise in the nuclear precursor was small and not different from that observed after sucrose gavage (0.14 +/- 0.01 chow, 0.26 +/- 0.03 sucrose gavage, 0.25 +/- 0.04 pg/micrograms RNA lipogenic diet). The molar ratio of the mature to precursor mRNA-S14 showed progressive increases with the smallest level in the fasting rat, an intermediate level in the chow-fed and sucrose gavaged rats, and the highest level in the animals fed a lipogenic diet (2.1, 16.5, 16.3, 62.7, respectively). Based on the previously reported half-life for the mature mRNA-S14, these data show that feeding sucrose by gavage or by a lipogenic diet leads to enhanced fractional conversion of precursor to mature mRNA-S14 with a simultaneous stabilization of the precursor mRNA-S14.