Human spot 14 glucose and thyroid hormone response: Characterization and thyroid hormone response element identification

Spot 14 is a 17-kDa protein expressed in lipogenic tissues and is postulated to play a role in thyroid hormone stimulation of lipogenesis. To further our understanding of Spot 14 regulation in humans, our laboratory recently cloned the human Spot 14 gene. The gene is highly homologous to the rat Spot 14 ortholog and located on a chromosomal region implicated in human obesity. Because our understanding of Spot 14 transcriptional regulation is derived from rat promoter studies, we assessed the thyroid hormone responsivity of the human Spot 14 promoter. These studies revealed a significantly greater thyroid hormone response for the human promoter, compared with the rat. Deletional studies of the human Spot 14 promoter reveal a 774-bp region at approximately position 2700, which is both necessary and sufficient for the thyroid hormone response. EMSAs with subfragments from this region identify a 146-bp DNA fragment capable of binding a TRbeta1-retinoid X receptor heterodimer. Site-directed mutagenesis confirmed the identity of a candidate DR-4 thyroid hormone response element within this fragment that is similar, but not identical, to the two rat Spot 14 thyroid hormone response elements. We hypothesize that the difference in thyroid hormone response between the orthologous promoters may allow a selective advantage to each species based on their different nutritional and physiological niches.