Hexachlorobenzene, a dioxin type compound, increase Malic Enzyme gene transcription through a mechanism involving the thyroid hormone response element

ANDREA LOAIZA; MARIA TERESA SEISDEDOS; DIANA KLEIMAN DE PISAREV; HORACIO SANCOVICH; ANDREA RANDI; ANA FERRAMOLA DE SANCOVICH; PILAR SANTISTEBAN

ENDOCRINOLOGY

ENDOCRINE SOC

Año: 1999 vol. 140 p. 4142 - 4151

0013-7227

Hexachlorobenzene (HCB) is a dioxin-type chemical that acts mainly through the aryl hydrocarbon receptor. Chronic exposure of rats to HCB increases the activity of malic enzyme (ME). In this report, we show that this increase is correlated with an induction of ME messenger RNA (mRNA) levels, with the maximal HCB effectachieved after 9 days of intoxication. This effect is specific for ME, as other liver enzymes, such as glyceraldehyde-3-phosphate dehydrogenase, phosphoenol pyruvate carboxykinase, and mitochondriala-glycerol-3-phosphate dehydrogenase, are not affected by HCB. The induction of ME mRNA levels is accompanied by an increase in ME promoter activity, as demonstrated by transient transfection exper-iments performed in rat hepatoma H35 cells. In an attempt to identify the cis-regulatory elements responsible for the HCB effect, different promoter deletions and mutations were used. The results obtainedlocalize the responsive region between positions 2315 and 2177. This region does not contain either consensus xenobiotic response or activating protein-1 elements, the two main mediators of dioxin compounds described to date. In contrast, a thyroid hormone response element (TRE) is located between 2281 to 2261. Deletions and mutations of the TRE element do not respond to HCB, demonstrating that this element mediates the response of this dioxin-type compound. As ME gene expression is regulated mainly by thyroid hormones, wenext investigated the role of T3 receptor (T3 R) in the ME gene transcriptional induction mediated by HCB. Using Scatchard analysis, we show that neither T 3 R binding features for its ligand nor a1 or b1T3RmRNA levels are changed with the toxic. In gel shift assays, however, we observed that protein/DNA complexes formed on TRE from the ME promoter were induced by HCB. Using an oligonucleotide with amutation that eliminates the TRE function, we demonstrate a loss of the induced protein/DNA complexes. Together, these data suggest that the dioxin-type compound HCB increases ME gene transcriptionby modulating the levels of still unidentified nuclear proteins that bind to the TRE element of the ME promoter.