Abstract

Thyroid hormone [triiodothyronine (T3)] positively regulates NADPH cytochrome P450 reductase (P450R) mRNA expression in rat liver, with P450R transcription initiation being a key regulated step. T3 is presently shown to have significant post-transcriptional effects on P450R expression. T3 increased the size of cytoplasmic P450R mRNA by ∼105 nucleotides 12 h after T3 treatment, followed by a return to basal levels at 24 h. Primer extension analysis and Northern hybridization with 5′-untranslated region probes revealed no change in P450R mRNA 5′ structure with T3 treatment. By contrast, RNase H analysis revealed a transient, T3-induced increase in P450R mRNA poly(A) tail, from ∼100 to ∼205 A. This increase in P450R polyadenylation, detectable in the nucleus 8 h after T3 treatment and in the cytoplasm at 12 h, was transient and was reversed by 16 h, when the T3-induced accumulation of cytoplasmic P450R mRNA was near maximal. Actinomycin D blocked the increase in P450R poly(A) tail and the induction of P450R mRNA, indicating a requirement for ongoing gene transcription for both T3 responses. T3 treatment destabilized P450R mRNA in rat liver in vivo, as shown by the T3-dependent 6-fold decrease in cytoplasmic P450R mRNA half-life, from a basal value of ≥16 h in uninduced liver to ∼2.5 h, measured 24 h after T3 administration. These findings demonstrate that T3 increases nuclear polyadenylation of P450R RNA as a transient, early regulatory response and that this response is temporally dissociated from the subsequent decrease in cytoplasmic P450R mRNA stability.