Identification of the thyroid hormone-responsive messenger RNA spot 11 as apolipoprotein-A1 messenger RNA and effects of the hormone on the promoter

Abstract

The induction of rat hepatic mRNA S11 by L-T3 (T3) is a useful model for studying the mechanisms of thyroid hormone action. Although numerous reports have examined the response of mRNA S11 to various physiological and hormonal manipulations, the role of S11 protein in cellular metabolism remains unknown. In this study we show that mRNA S11 is abundantly expressed and regulated by T3 only in liver and small intestine. High levels of the mRNA are present at birth, but drop sharply between 30-60 days of age. These and other features of the S11 gene product were similar to those of rat apolipoprotein-A1 (Apo-A1). The sequence of S11 cDNA was identical to a portion of the Apo-A1 mRNA, thus confirming identity of the S11 mRNA. To examine whether DNA sequences immediately adjacent to the transcription start site mediate the effects of thyroid hormone, we measured the activity of an Apo-A1 gene fragment, U-1 (-474 to -7) using a transient transfection assay. The activity of the full-length U-1 DNA in HuH-7 hepatoma cells was 2- to 2.5-fold higher in the presence of thyroid hormone. This finding closely matched previous results using the in vitro nuclear run-on assay. Internal deletion of a motif that resembles a thyroid hormone response element from U-1 DNA not only abolished the induction by T3, but suppressed promoter activity by 3- to 4-fold in response to the hormone. We conclude that the products of genes S11 and Apo-A1 are the same and that T3-regulated expression of the gene is mediated in part by a thyroid hormone response element positioned at the 5′-end of the gene.