Thyroid Hormone Regulation of Transcription Factors Involved in Malic Enzyme Gene Expression

The mechanisms by which triiodothyronine (T3) regulates gene transcription are inadequately understood. In order to examine the effect of T3 on transcriptional mechanisms, we utilized several techniques to characterize trans-acting factors which interact with cis-regulatory elements of the promoter for the T3-responsive rat malic enzyme gene. Transcription of deletion mutants of the promoter by HeLa extracts revealed the existence of three cis-regulatory elements at -144 to -123, -70 to -50, and -30 to +5 (relative to the cap site at +1). DNase I footprinting disclosed the presence of proteins which bound to each of these regions. Gel mobility shift assays showed that specific binding of liver nuclear proteins to two of these regions (-144/-114 and -76/-47) was diminished in hypothyroid and elevated in hyperthyroid rats. Similar but less marked effects of T3 on the binding of kidney (a T3-responsive tissue) nuclear proteins to the -76/-47 and cap site regions were observed while binding of testis (a T3-nonresponsive tissue) nuclear proteins was unaffected by T3 treatment. Changes in the magnitude and time course of binding of liver proteins to this promoter correlated closely with previously documented T3-induced increases in the hepatic malic enzyme gene transcription rate. These results are consistent with a model in which the regulation of malic enzyme gene transcription by T3 in responsive tissues is mediated in part by hormone-induced increases in the binding of trans-acting factors to cis-regulatory elements.