HEPATIC LECITHIN - RETINOL ACYLTRANSFERASE ACTIVITY IS INDUCED IN-VIVO BY RETINOIC ACID, BUT NOT BY TRIIODOTHYRONINE, IN VITAMIN-A-DEFICIENT, HYPOTHYROID RATS

The activity of the enzyme lecithin: retinol acyltransferase (LRAT) is extremely low in the liver of vitamin A-deficient rats, but is rapidl y induced after administration of retinoic acid (RA). The nuclear rece ptors for RA are closely related to the receptors for thyroid hormone, and molecular cross-talk between these receptors has been observed in vitro and in cultured cells. Therefore we have examined whether retin oid status and thyroid hormone status interact in the regulation of he patic LRAT activity in vivo. Vitamin A-deficient male Lewis rats, eith er euthyroid or made hypothyroid by treatment with propylthiouracil, w ere treated once or three times with 20 mu g of all-trans-RA, 10 mu g/ 100 gm body weight of triiodothyronine (T-3), or both hormones. Hepati c LRAT activity, which was negligible in all retinoid-deficient rats, was induced by RA (P < 0.0001) regardless of the animal's thyroid horm one status. T-3 by itself had no ability to induce hepatic LRAT activi ty (P = 0.42), nor did co-administration of T-3 with RA increase or de crease the response to RA (P = 0.13). In retinoid-sufficient rats, hyp othyroidism did not alter hepatic LRAT activity; however, LRAT activit y was reduced by half (P < 0.05) after three treatments with T-3. Ther efore we conclude that thyroid hormone status alone does not regulate LRAT, nor does thyroid status affect the ability of RA to induce hepat ic LRAT in retinoid-deficient animals. However, in retinoid-sufficienc y, repeated treatment with T-3 may reduce the hepatic esterification o f retinol by LRAT. (C) Elsevier Science Inc. 1997.