title: Cell density mediated pericellular hypoxia leads to induction of HIF-1alpha via nitric oxide and Ras/MAP kinase mediated signaling pathways.
abstract: 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 different pericellular oxygen levels. However, pericellular hypoxia can be induced by increasing cell density, which can exert profound influences on the target cell lines and may explain a number of findings previously attributed to normoxic cell-cell interactions. Thus, we first 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 affect 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 HIF-1a 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 diffusible 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, HIF-1a via pericellular diffusion 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.
authors.affiliation: Department of Molecular Physiology and Biological Physics, University of Virginia Health Sciences Center, Charlottesville, Virginia, USA.
authors.forename: E A
authors.initials: EA
authors.lastname: Sheta
authors.forename: H
authors.initials: H
authors.lastname: Trout
authors.forename: J J
authors.initials: JJ
authors.lastname: Gildea
authors.forename: M A
authors.initials: MA
authors.lastname: Harding
authors.forename: D
authors.initials: D
authors.lastname: Theodorescu
mesh_terms: D015082:2,2'-Dipyridyl / Q000494:pharmacology; D002452:Cell Count; D015687:Cell Hypoxia; D006801:Humans; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D007502:Iron Chelating Agents / Q000494:pharmacology; D048369:MAP Kinase Kinase 1; D008297:Male; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors; D052247:Nitric Oxide Synthase Type II; D011471:Prostatic Neoplasms; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D020218:Response Elements; D015398:Signal Transduction; D014157:Transcription Factors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D014407:Tumor Cells, Cultured; D048051:p38 Mitogen-Activated Protein Kinases; D018631:ras Proteins / Q000378:metabolism*
chemical_list: C497442:HIF1A protein, human; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D007502:Iron Chelating Agents; D014157:Transcription Factors; D009569:Nitric Oxide; D015082:2,2'-Dipyridyl; C496319:NOS2 protein, human; D019001:Nitric Oxide Synthase; D052247:Nitric Oxide Synthase Type II; D017346:Protein Serine-Threonine Kinases; D020928:Mitogen-Activated Protein Kinases; D048051:p38 Mitogen-Activated Protein Kinases; D048369:MAP Kinase Kinase 1; C482125:MAP2K1 protein, human; D020929:Mitogen-Activated Protein Kinase Kinases; D018631:ras Proteins