Xuexun Fang

Date of Graduation


Document Type



Stimulation of malic enzyme transcription by triiodothyronine (T3) is robust ({dollar}>{dollar}60-fold) in chick embryo hepatocytes, weak (5-fold) in chick embryo fibroblasts that stably overexpress the nuclear T3 receptor-{dollar}\\alpha,{dollar} and still weaker (1-fold) in chick embryo fibroblasts which contain nuclear T3 receptor levels that are similar to those of CEH. Directed by the DNase I hypersensitive regions in the malic enzyme gene, functional transfection and in vitro DNA-binding analyses were performed and four cis-acting elements were identified in the malic enzyme 5{dollar}\\sp\\prime{dollar}-flanking DNA. These four cis-acting elements confer differences in nuclear T3 receptor activity between chick embryo hepatocytes and chick embryo fibroblasts, they are located at {dollar}-{dollar}3895/{dollar}-{dollar}3890, {dollar}-{dollar}3761/{dollar}-{dollar}3744, {dollar}-{dollar}3703/{dollar}-{dollar}3686, and {dollar}-{dollar}3474/{dollar}-{dollar}2715 bp and overlap with DNase I hypersensitive sites that are observed in chromatin of chick embryo hepatocytes. Each element enhances T3 responsiveness of the malic enzyme promoter in chick embryo hepatocytes but has no effect on T3 responsiveness in chick embryo fibroblasts. Three of the cell-specific regulatory elements flank a previously identified DNA fragment {dollar}-{dollar}3889 to {dollar}-{dollar}3769 bp (Hodnett et. al., 1996) that contains one major and four minor T3 response elements. The cell-specific regulatory element at {dollar}-{dollar}3703/{dollar}-{dollar}3686 bp binds to the liver-enriched factor, CCAAT/enhancer-binding protein-{dollar}\\alpha,{dollar} whereas, cell-specific regulatory elements at {dollar}-{dollar}3895/{dollar}-{dollar}3890 and {dollar}-{dollar}3761/{dollar}-{dollar}3744 bp bind proteins of unknown identity. These data suggest that cell-specific differences in T3 responsiveness of the malic enzyme gene is mediated in large part by non-receptor proteins that augment the transcriptional activity of the nuclear T3 receptor in hepatocytes.