Regulation of Cyclooxygenase by the Heme-Heme Oxygenase System in Microvessel Endothelial Cells

Abstract

Heme oxygenase (HO) is a microsomal enzyme that oxidatively cleaves heme to form biliverdin, with the release of iron and carbon monoxide (CO). HO not only controls the availability of heme for the synthesis of heme proteins but also is responsible for the generation of CO, which binds to the heme moiety of heme proteins thus affecting their enzymatic activity. Cyclooxygenase (COX) is a heme protein that catalyzes the conversion of arachidonic acid to prostaglandin H 2 , the precursor of prostanoids that participate in the regulation of vascular function. The goal of the present study was to determine whether the heme-HO system regulates COX enzyme expression and activity in vascular endothelial cells. Endothelial cells stably transfected with the human HO-1 gene exhibited a severalfold increase in human HO-1 mRNA levels, which was accompanied by an increase in HO activity and a marked decrease in prostaglandin (PG) E 2 and 6-keto-PGF 1␣ levels. Exposure of cells to CoCl 2 , an inducer of HO-1 gene expression, resulted in increases in HO-1 protein levels and HO activity. The increase in HO activity was associated with a subsequent decrease in COX activity, which returned to normal levels following normalization of HO activity. The addition of heme resulted in an increase in COX activity with an increase in PGE 2 and 6-keto-PGF 1␣ levels. The degree of HO-1 expression and, consequently, the level of cellular heme, were directly related to COX activity. These results demonstrate that the heme-HO system can function as a cellular regulator of the expression of vascular COX, thus influencing the generation of prostanoids, PGE 2 and PGI 2 , known to play a role in vascular homeostasis.

Heme Oxygenase-2 Deficiency Contributes to Diabetes-Mediated Increase in Superoxide Anion and Renal Dysfunction

“…Upregulation of HO-1 inhibits the inflammatory response, presumably by a decrease in cellular heme-mediated oxidative stress (50) and vasoconstrictors (46,(51)(52)(53)(54)(55). Consistent with these observations, mice and humans who are deficient in HO-1 experience progressive chronic inflammation and are sensitive to stressful injury, presumably as a result of elevated cellular heme and iron (56,57).…”

Potential role of heme oxygenase-1 in the progression of rat adjuvant arthritis

“…40 Nevertheless, in systems where high levels of HO-1 expression can be induced, this enzyme activity would be able to deplete cellular heme leading to decreased COX activity and PG production, as reported in microvessel endothelial cells transfected with the HO-1 gene or treated with CoCl 2 . 41 Joint tissues are subject to a complex network of regulatory mediators. ROS production in rheumatoid arthritis affects signaling pathways and may cause damage to matrix components through reduction of synthesis, induction of apoptosis or activation of metalloproteinases.…”

Heme Oxygenase-2 Is a Critical Determinant for Execution of an Acute Inflammatory and Reparative Response

“…33,34 The activity of both enzymes is regulated by HO. 32,35 Analyses of corneas by immunohistochemistry indicated exaggerated COX-2 protein expression in macrophages and stromal keratinocytes of HO-2-null mice, 2 and 4 days after injury, when directly compared with WT mice ( Figure 4A). COX-2 immunostaining was not detected in uninjured corneas of WT or HO-2-null mice ( Figure 4A).…”