TRANSCRIPTIONAL ACTIVATION OF THE HEME OXYGENASE-1 GENE BY CGMP VIA ACAMP RESPONSE ELEMENT ACTIVATOR PROTEIN-1 ELEMENT IN PRIMARY CULTURESOF RAT HEPATOCYTES

The expression of the rate-limiting enzyme of haem degradation, haem o xygenase-l (HO-1), can be induced by various stimuli, including lipopo lysaccharide, tumour necrosis factor alpha and NO. The NO signal can b e transmitted by cGMP, therefore this study was aimed at testing the a ctivation of the HO-1 gene by cGMP. In primary cultures of rat hepatoc ytes, both HO-1 mRNA and protein were induced by the NO donor sodium n itroprusside and 8-bromo-cGMP. The HO-1 mRNA induction by cGMP was pre vented by the specific protein kinase G inhibitor KT5823. The cGMP-dep endent HO-1 mRNA induction was dose-dependent and transcriptionally re gulated, as determined by studies with actinomycin D and a nuclear run -on assay. Cycloheximide lowered the cGMP-dependent induction of HO-1 mRNA to about one half. Luciferase reporter constructs driven by about 800 bp of the 5'-flanking region of the rat HO-I gene were transientl y transfected into primary rat hepatocytes; 8-bromo-cGMP caused a 6-fo ld induction, which was obliterated by deletion and mutation of the cA MP response element/activator protein-1 (CRE/AP-1) (-665/-654) site. T hus HO-1 induction by cGMP appears to be stimulated by the protein kin ase G pathway and may be mediated mainly via a CRE/AP-1 element in the rat HO-1 promoter.