Carbon monoxide (CO) is a product of the enzyme heme oxygenase (HO; EC 1.14.99.3). In vascular smooth muscle cells, exogenously administered CO increases cyclic guanosine 3',5'-monophosphate (cGMP), which is an important regulator ofvessel tone. We report here that smooth muscle cells produce CO via HO and that it regulates cGMP levels in these cells. Hypoxia, which has profound effects on vessel tone, significantly increased the transcriptional rate of the HO-1 gene resulting in corresponding increases of its mRNA and HO enzymatic activity. In addition, under the same conditions, rat aortic and pulmonary artery smooth muscle cells accumulated high levels of cGMP following a similar time course to that of HO-1 production. The increased accumulation of cGMP in smooth muscle cells required the enzymatic activity of HO, since it was abolished by a specific HO inhibitor, tin protoporphyrin. In contrast, NO-nitro-Larginine, a potent inhibitor ofnitric oxide (NO) synthesis, had no effect on cGMP produced by smooth muscle cells, indicating that NO is not responsible for the activation of guanylyl cyclase in this setting. Furthermore, conditioned medium from hypoxic smooth muscle cells stimulated cGMP production in recipient cells and this stimulation was completely inhibited by tin protoporphyrin or hemoglobin, an inhibitor of CO production and a scavenger of CO, respectively. This report shows that HO-1 is expressed by vascular smooth muscle cells and that its product, CO, may regulate vascular tone under physiologic and pathophysiologic (such as hypoxic) conditions.Regulation of blood vessel tone is critical to maintaining adequate tissue oxygenation and perfusion. This phenomenon involves a delicate balance between vasodilators and vasoconstrictors. Hypoxia, for example, has profound effects on blood vessel tone, principally through the release or inhibition of vasoactive mediators from endothelial cells (1, 2). One endothelium-derived mediator is nitric oxide (NO), a potent vasodilator that helps maintain normal vascular tone by stimulating guanylyl cyclase in smooth muscle cells (SMCs) and elevating cGMP levels. Endothelial NO was shown to be suppressed by a hypoxic state resulting in low cGMP levels (3). NO is normally produced by the body and serves as an important chemical messenger not only in the regulation ofvessel tone, but also in neuronal transmission. Like NO, carbon monoxide (CO) is an endogenously produced gas molecule that activates guanylyl cyclase (4). Although a role for CO has been suggested in neuronal signal transduction (5), it is not known whether CO plays a physiologic role in the vasculature.There are at least two endogenous sources of CO production, one of which is from the oxidation of organic molecules, but the predominant source is from the degradation of heme (6). Heme is metabolized to biliverdin and CO by hemeThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S...

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Inactivation of glycogen synthase kinase 3␤ (GSK3␤) and the resulting stabilization of free ␤-catenin are critical steps in the activation of Wnt target genes. While Akt regulates GSK3␣/␤ in the phosphatidylinositide 3-OH kinase signaling pathway, its role in Wnt signaling is unknown. Here we report that expression of Wnt or Dishevelled (Dvl) increased Akt activity. Activated Akt bound to the Axin-GSK3␤ complex in the presence of Dvl, phosphorylated GSK3␤ and increased free ␤-catenin levels. Furthermore, in Wnt-overexpressing PC12 cells, dominant-negative Akt decreased free ␤-catenin and derepressed nerve growth factor-induced differentiation. Therefore, Akt acts in association with Dvl as an important regulator of the Wnt signaling pathway.

Although hyperhomocysteinemia has been recognized recently as a prevalent risk factor for myocardial infarction and stroke, the mechanisms by which it accelerates arteriosclerosis have not been elucidated, mostly because the biological effects of homocysteine can only be demonstrated at very high concentrations and can be mimicked by cysteine, which indicates a lack of specificity. We found that 10 -50 M of homocysteine (a range that overlaps levels observed clinically) but not cysteine inhibited DNA synthesis in vascular endothelial cells (VEC) and arrested their growth at the G 1 phase of the cell cycle. Homocysteine in this same range had no effect on the growth of vascular smooth muscle cells (VSMC) or fibroblasts. Homocysteine decreased carboxyl methylation of p21 ras (a G 1 regulator whose activity is regulated by prenylation and methylation in addition to GTP-GDP exchange) by 50% in VEC but not VSMC, a difference that may be explained by the ability of homocysteine to dramatically increase levels of S-adenosylhomocysteine, a potent inhibitor of methyltransferase, in VEC but not VSMC. Moreover, homocysteine-induced hypomethylation in VEC was associated with a 66% reduction in membrane-associated p21 ras and a 67% reduction in extracellular signal-regulated kinase 1/2, which is a member of the mitogen-activated protein (MAP) kinase family. Because the MAP kinases have been implicated in cell growth, the p21 ras