Roles of Tetrahydrobiopterin in Promoting Tumor Angiogenesis

Abstract

Nitric oxide (NO), which is derived from endothelial NO synthase (eNOS), provides crucial signals for angiogenesis in the tumor microenvironment. Tetrahydrobiopterin (BH4) is an absolute requirement for eNOS activity. In this study, we investigated whether this activation is both maintained by a wild-type Ras/ phosphatidylinositol 3-kinase (PI3K)/Akt-positive feedback loop in endothelial cells and affects tumor angiogenesis. We found that supplementation of BH4 (via the pterin salvage pathway with Sep) increased Akt/eNOS phosphorylation in both human eNOS-transfected COS-7 cells and endothelial cells concomitant with increases in NO production, cell proliferation, migration, and tube formation. This augmentation was abrogated by a PI3K inhibitor. Sepiapterin (Sep) also increased GTP-bound wild-type Ras and PI3K/Akt/ eNOS activation, which was prevented by the eNOS inhibitor, N-Nitro-L-arginine methyl ester (L-NAME). Growth of new blood vessels from pre-existing ones, ensuring oxygen and nutrient supplies, is a crucial step in tumor cell proliferation, growth, and metastasis. This pathological angiogenesis is maintained by overproduction of proangiogenic factors, which act via a variety of signaling pathways in tumor stroma.1 Recent evidence suggests that nitric oxide (NO) provides crucial signals for angiogenesis in the tumor microenvironment, where moderate levels of NO are proangiogenic, 2,3 chiefly increasing DNA synthesis, cell proliferation and migration of endothelial cells to promote tumor angiogenesis. 4NO is a gaseous free radical, resulting from the conversion of L-arginine to citrulline by three distinct forms of NO synthase (NOS): endothelial NOS (eNOS), inducible NOS, and neuronal NOS in different cell types and tissues.5 eNOS is the principal isoform in tumor vascular endothelial cells. Under pathological conditions, eNOSderived NO is the dominant stimulus to angiogenesis in B16 melanomas.6 Inhibition of basal eNOS by caveolin-1, a protein that sequesters eNOS in caveolae, attenuates NO-dependent blood flow in tumors and delays their growth as xenografts.7 Similarly, tumor growth is enhanced in caveolin-1-deficient mice because of increased microvascular permeability and angiogenesis. 8 Inhibiting eNOS with L-arginine analogs (N-Nitro-L-arginine methyl ester [L-NAME], NG-Methyl-L-arginine [L-NMMA] and N-Nitro-L-arginine [L-NNA]) decreases both tumor blood flow and microvascular permeability in breast cancer xenografts.

Inhibitory effects of sepiapterin on vascular endothelial growth factor-a-induced proliferation and adhesion in human umbilical vein endothelial cells

“…Many investigations have demonstrated that biological and pharmacological effects of BH 4 on various pathophysiological conditions such as vascular diseases and inflammation are dependent on the restoration of normal NO biosynthesis and the reduction of other reactive oxygen species generation (Gao et al, 2009;Katusic et al, 2009). In addition, sepiapterin, a stable form of BH 4 precursor, improves endothelial dysfunction in small mesenteric arteries from diabetic (db/db) mice, and induces angiogenesis via increase of endothelial cell proliferation and migration (Pannirselvam et al, 2005;Chen et al, 2010). However, recent studies have demonstrated that BH 4 can inhibit the proliferation and migration in rat aortic smooth muscle cells, independently of its ability to regulate NO synthase activity (Jiang et al, 2010).…”

Inhibition of GTP cyclohydrolase attenuates tumor growth by reducing angiogenesis and M2‐like polarization of tumor associated macrophages

“…An imbalance increases the formation of reactive oxygen and nitrogen species 14, 17. Within the tumor environment BH4 and NO are likely to regulate the blood flow, the formation of new blood vessels18, 19 and functions of immune cells 20, 21…”

Design, Synthesis, and Activity Evaluation of a New 5-fluorouracil Prodrug Containing an Asn-Gly-Arg(NO2)COOCH3 Tripeptide

“…Also, in designing the structure of our 5-FU prodrug, we followed the lead of other reports that suggest the -nitro- L-arginine methyl ester (L-NAME) is an effective nitric oxide synthases(NOS) inhibitor and thus has an anti-angiogenesis effect [14,15,16,17], allowing it to act as an adjuvant agent of other anti-cancer drugs. As such, the protective nitro group of guanidine in our prodrug was not eliminated.…”