REGULATIN OF HEPATIC HEME METABOLISM

Univ Of Massachusetts Med Sch Worcester, Worcester MA

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Abstract

Heme is essential to the function of nearly all cells and organisms, serving chiefly as a prosthetic group of oxygen focused on the regulation of, and inter-relationships between, the rate-controlling enzymes of hematic heme synthesis (ALA synthase) and breakdown (heme oxygenase [HO]). These studies established a central role of HO in physiologic heme breakdown and in regulation of activity of ALA synthase and the processes of hepatic porphyrin roles, functioning as antioxidants (biliverdin, bilirubin) or as a neurotransmitter (carbon monoxide. Although three forms of HO have been identified, only isoform 1 (HO-1) is inducible by heme, certain other metalloporphyrins, and transition metals such as cadmium and cobalt. During the last funding period, the HO-1 gene, including 7.1 kb of its 5'- untranslated region, was cloned and characterized. Selection portions of the HO-1 Promoter were linked to reporter hepatocellular carcinoma. Key regulatory elements, including consensus AP-1 binding sites and metal- responding element near the transcription start point, were identified and their importance confirmed by electromobility shift assays and by analyses of site-directed mutations in these elements. A major role for the mitogen- activated kinases ERK and p38 in mediating responses of HO-1 to sodium arsenite, a strong inducer of the HO-1 gene, were established. The long term goal of this research program is to understand the regulation of hepatic porphyrin and heme metabolism and inter-relationships of this metabolism with cellular hemoproteins. To achieve this goal, the prior accomplishments will serve as the foundation for the achievement of the following specific aims: (1) Define the key elements in the HO-1 gene promoters; (2) Define thje role of Jun-B and other AP-1 proteins in the regulation of HO-1 gene expression; (3) Define the role(s) of specific MAP kinase component in regulation expression of the HO-1 gene in intact mammals; (4) Elucidate the role of protein phosphatase(s) in the regulation of HO-1 gene expression. A major working hypothesis is that heme (and perhaps other metalloporphyrins) binds to protein(s) that then interact selectively and specifically with key regulatory elements that control transcription and/or post-transcriptional activities of HO-1 mRNA. Because of the importance of heme metabolism and hemoproteins in the liver, the results of the proposed studies will provide fundamental new insights into normal biochemistry, molecular biology, and physiology and into key metabolic alterations that occur in disorders of heme metabolism and the stress response.