The neuronal nitric oxide synthase (nNOS) has been successfully overexpressed in Escherichia coli, with average yields of 125-150 nmol (20-24 mg) of enzyme per liter of cells. The cDNA for nNOS was subcloned into the pCW vector under the control of the tac promotor and was coexpressed with the chaperonins groEL and groES in the protease-deficient BL21 strain ofE. coli. The enzyme produced is replete with heme and flavins and, after overnight incubation with tetrahydrobiopterin, contains 0.7 pmol of tetrahydrobiopterin per pmol of nNOS. nNOS is isolated as a predominantly high-spin heme protein and demonstrates spectral properties that are identical to those of nNOS isolated from stably transfected human kidney 293 cells. It binds N'0-nitroarginine dependent on the presence of bound tetrahydrobiopterin and exhibits a Kd of 45 nM. The enzyme is completely functional; the specific activity is 450 nmol/min per mg. This overexpression system will be extremely useful for rapid, inexpensive preparation of large amounts of active nNOS for use in mechanistic and structure/function studies, as well as for drug design and development.

The nitric oxide synthases (NOS-I, neuronal, NOS-II, inducible, and NOS-III, endothelial) are the most recent additions to the large number of heme proteins that contain cysteine thiolate-liganded protoporphyrin IX heme prosthetic groups. This group of oxygenating enzymes also includes one of the largest gene families, that of the cytochromes P450, which have been demonstrated to be involved in the hydroxylation of a variety of substrates, including endogenous compounds (steroids, fatty acids, and prostaglandins) and exogenous compounds (therapeutic drugs, environmental toxicants, and carcinogens). The substrates for cytochromes P450 are universally hydrophobic while the physiological substrate for the nitric oxide synthases is the amino acid L-arginine, a hydrophilic compound. This review will discuss the approaches being used to study the structure and mechanism of neuronal nitric oxide synthase in the context of its known prosthetic groups and regulation by Ca(2+)-calmodulin and/or tetrahydrobiopterin (BH4).

Environmental signals in the cellular milieu such as hypoxia, growth factors, extracellular matrix (ECM), or cell-surface molecules on adjacent cells can activate signaling pathways that communicate the state of the environment to the nucleus. Several groups have evaluated gene expression or signaling pathways in response to increasing cell density as an in vitro surrogate for in vivo cell-cell interactions. These studies have also perhaps assumed that cells grown at various densities in standard in vitro incubator conditions do not have dierent pericellular oxygen levels. However, pericellular hypoxia can be induced by increasing cell density, which can exert profound in¯uences on the target cell lines and may explain a number of ®ndings previously attributed to normoxic cell-cell interactions. Thus, we ®rst sought to test the hypothesis that cell-cell interactions as evaluated by the surrogate approach of increasing in vitro cell density in routine normoxic culture conditions results in pericellular hypoxia in prostate cancer cells. Second, we sought to evaluate whether such interactions aect transcription mediated by the hypoxia response element (HRE). Thirdly, we sought to elucidate the signal transduction pathways mediating the induction of HRE in response to cell density induced pericellular hypoxia in routine normoxic culture conditions. Our results indicate that paracrine cell interactions can induce nuclear localization of HIF1a protein and this translocation is associated with strong stimulation of the HRE-reporter activity. We also make the novel observation that cell density-induced activity of the HRE is dependent on nitric oxide production, which acts as a diusible paracrine factor secreted by densely cultured cells. These results suggest that paracrine cell interactions associated with pericellular hypoxia lead to the physiological induction of HRE activity via the cooperative action of Ras, MEK1, HIF1a via pericellular diusion of nitric oxide. In addition, these results highlight the importance of examining pericellular hypoxia as a possible stimulus in experiments involving in vitro cell density manipulation even in routine normoxic culture conditions. Oncogene (2001) 20, 7624 ± 7634.

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