THE RAT HEME OXYGENASE-1 GENE IS TRANSCRIPTIONALLY INDUCED VIA THE PROTEIN-KINASE A SIGNALING PATHWAY IN RAT HEPATOCYTE CULTURES

Heme oxygenase-1 (HO-1) is the inducible form of the rate-limiting enz yme of heme degradation; it regulates the cellular content of heme. To investigate the role of the cAMP-dependent protein kinase (PKA) signa ling pathway on hepatic HO-1 gene expression, primary rat hepatocyte c ultures were treated with the PKA-stimulating agents dibutyryl-cAMP (B t(2)cAMP), forskolin, and glucagon. HO-1 mRNA levels were induced by t hese agents in a time-dependent manner with a transient maximum after 6 hr of treatment. The induction of HO-1 was dose dependent, reaching a maximum at concentrations of 250 mu M Bt(2)cAMP and 50 nM glucagon, respectively. The accumulation of HO-1 mRNA correlated with increased levels of HO-1 protein as determined by Western blot analysis. The Bt( 2)cAMP-dependent induction of HO-1 mRNA expression was prevented by pr etreatment with the PKA inhibitor KT5720 but not with the protein kina se G inhibitor KT5823. HO-1 mRNA induction by CdCl2 and heme was diffe rentially affected by Bt(2)cAMP. Upregulation of the HO-1 gene by Bt(2 )cAMP occurred on the transcriptional level as determined by nuclear r un-off assay and blocking of the Bt(2)cAMP-dependent induction of HO-1 mRNA by actinomycin D, Treatment with Bt(2)cAMP increased the half-li fe of HO-1 mRNA from 4.7 to 5.5 hr. Taken together, the results of the current study demonstrate that HO-1 gene expression is induced by act ivation of the cAMP signal transduction pathway via a transcriptional mechanism in primary rat hepatocyte cultures.