RT Journal Article
SR Electronic
T1 Impaired Dexamethasone-Mediated Induction of Tryptophan 2,3-Dioxygenase in Heme-Deficient Rat Hepatocytes: Translational Control by a Hepatic eIF2α Kinase, the Heme-Regulated Inhibitor
JF Journal of Pharmacology and Experimental Therapeutics
JO J Pharmacol Exp Ther
FD American Society for Pharmacology and Experimental Therapeutics
SP 979
OP 989
DO 10.1124/jpet.107.124602
VO 323
IS 3
A1 Mingxiang Liao
A1 Michael K. Pabarcus
A1 YongQiang Wang
A1 Colleen Hefner
A1 David A. Maltby
A1 Katalin F. Medzihradszky
A1 Saida Patricia Salas-Castillo
A1 James Yan
A1 Jacquelyn J. Maher
A1 Maria Almira Correia
YR 2007
UL http://jpet.aspetjournals.org/content/323/3/979.abstract
AB Tryptophan 2,3-dioxygenase (TDO), a liver-specific cytosolic hemoprotein, is the rate-limiting enzyme in l-tryptophan catabolism and thus a key serotonergic determinant. Glucocorticoids transcriptionally activate the TDO gene with marked enzyme induction. TDO is also regulated by heme, its prosthetic moiety, as its expression and function are significantly reduced after acute hepatic heme depletion. Here we show in primary rat hepatocytes that this impairment is not due to faulty transcriptional activation of the TDO gene but rather due to its posttranscriptional regulation by heme. Accordingly, in acutely heme-depleted hepatocytes, the de novo synthesis of TDO protein is markedly decreased (>90%) along with that of other hepatic proteins. This global suppression of de novo hepatic protein syntheses in these heme-depleted cells is associated with a significantly enhanced phosphorylation of the α-subunit of the eukaryotic initiation factor eIF2 (eIF2α), as monitored by the phosphorylated eIF2α/total eIF2α ratio. Heme supplementation reversed these effects, indicating that heme regulates TDO induction by functional control of an eIF2α kinase. A cDNA was cloned from heme-depleted rat hepatocytes, and DNA sequencing verified its identity to the previously cloned rat brain heme-regulated inhibitor (HRI). Proteomic, biochemical, and/or immunoblotting analyses of the purified recombinant protein and the immunoaffinity-captured hepatic protein confirmed its identity as a rat heme-sensitive eIF2α kinase. These findings not only document that a hepatic HRI exists and is physiologically relevant but also implicate its translational shut-off of key proteins in the pathogenesis and symptomatology of the acute hepatic heme-deficient conditions clinically known as the hepatic porphyrias. The American Society for Pharmacology and Experimental Therapeutics