Vitamin A deficiency increases hepatic apolipoprotein A-I mRNA expression in both euthyroid and hypothyroid rats

We have examined the effects of chronic thyroid hormone deficiency, vitamin A deficiency, and subsequent acute repletion with thyroid hormone (T₃) and/or retinoic acid (RA) on the expression of hepatic apolipoprotein (ape) A-I mRNA. Chronically hypothyroid and/or retinoid-deficient male Lewis rats were treated once or three times with T₃, all-trans-RA, or both hormones. The hypothyroid state significantly reduced (P < 0.001) and the vitamin A-deficient state significantly elevated (P < 0.0001) the expression of apo A-I mRNA. A single injection of T₃ (10 μg/100 gm body weight) significantly elevated apo A-I mRNA in hypothyroid rats (P < 0. 001). A single injection of RA (20 μg/rat) reduced apo A-I mRNA by about 20 to 25% and daily injection of RA for three days reduced apo A-I mRNA abundance further in hypothyroid (P < 0.05), but not euthyroid rats. There was a significant interaction between T₃ hormone treatment and vitamin A status of the rats, as well as between RA and T₃ treatment and the thyroid status of the rats. These in vivo results may indicate that nuclear retinoid receptors and thyroid hormone receptors interact in the regulation of the apo A-I gene in liver.